Journal papers

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Articles dans des revues avec comité de lecture

2024

Large-Scale Crowd Size Classification with a Wi-Fi-Based Passive Radar

We investigate the problem of large-scale crowd size classification with a Wi-Fi-based passive radar for crowds of up to 100 people, with classes corresponding to intervals of numbers of people. A Convolutional Neural Network (CNN) operating on radar range-Doppler maps (RDMs) is used as a classification algorithm. We propose a crowd simulator based on the method of moments (MoM) in electromagnetics able to generate representative RDMs for large crowds. We show that these MoM simulation data can be used to design the classification algorithms and tune their hyperparameters. We also investigate the limitations of the MoM simulation data in training the classification algorithms for subsequent application on experimental data. Crowd size classification is performed with high accuracy on real-life experimental measurements of a crowd with up to 100 people, obtained by channel estimation with 802.11ax-compliant High-Efficiency Long Training Fields transmitted by a Wi-Fi-based passive radar setup featuring two Universal Software Radio Peripherals X310.

Joint metrics for EMF exposure and coverage in real-world homogeneous and inhomogeneous cellular networks

This paper evaluates the downlink performance of cellular networks in terms of coverage and electromagnetic field exposure (EMFE), in the framework of stochastic geometry. The model is constructed based on datasets for sub-6 GHz macro cellular networks but it is general enough to be applicable to millimeter-wave networks as well. On the one hand, performance metrics are calculated for β-Ginibre point processes which are shown to faithfully model a large number of motion-invariant networks. On the other hand, performance metrics are derived for inhomogeneous Poisson point processes with a radial intensity measure, which are shown to be a good approximation for motion-variant networks. For both cases, joint and marginal distributions of the EMFE and the coverage, and the first moments of the EMFE are provided and validated by Monte Carlo simulations using realistic sets of parameters from two sub-6 GHz macro urban cellular networks, i.e., 5G NR 2100 (Paris, France) and LTE 1800 (Brussels, Belgium) datasets. In addition, this paper includes the analysis of the impact of the network parameters and discusses the achievable trade-off between coverage and EMFE.

A spatial data focusing and generalized time-invariant frequency diverse array approach for high precision range-angle-based geocasting

A novel unified frequency diverse array (FDA) and spatial data focusing (SDF) approach is proposed to simultaneously overcome time-variance and precision constraints of conventional FDA in geocasting, i.e., spatially confined broadcasting, scenarios. This paper describes a free space FDA-based SDF (FDA-SDF) system model for 2-dimensional range-angle-based focusing, including a generalized multi-purpose baseband approach for time-invariant FDA, complemented by SDF processing for improved spatial focusing precision and reduced array size. Comprehensive analytical derivations - general for any frequency offset configuration - describe the geographical FDA-SDF properties and design rules, such as geocast delivery zone steering, location, uniqueness, and size. Simulations of the proposed scheme validate theoretical derivations and demonstrate FDA-SDF's superior spatial precision and minimal design complexity. In particular, using novel alternating logarithmic frequency offsets, a 3-antenna FDA-SDF setup is shown to match the radial and azimuthal precision of its beamforming-based FDA counterpart using, respectively, 64 and 24 antennas.

2023

Comparing crowding perception and sensor counts at the Ghent festivities

Little is known about crowding perception on events and the link with objective visitor counts and visitor's behavior. This research aims to fill this gap by collecting WiFi sensor counts and comparing them with perception ratings of visitors and police and first aid data of Europe's largest outdoor urban festival: the Ghent festivities. During 48 h visitors were asked to rate the human crowding, spatial crowding and pleasure on two event plazas and one passageway within the festivities zone. The results show that spatial crowding ratings are a better indicator for visitor counts than human crowding ratings. On two locations a pleasure drop occurred each night, which cannot be linked to the visitor counts, but it can be linked to people's ratings of crowding as follows: pleasure dropped when the human crowding ratings exceeded 3 and if the spatial crowding ratings increased above 3 on a five-point rating scale during the pleasure drop. In a last step, two datasets on visitor behavior were included in the analysis, i.e., number of police interventions and first aid visits. The analysis of these datasets shows that behavior, crowding and emotional response are linked at the Ghent festivities (e.g., at Vlasmarkt most police interventions and almost exactly 50% of the first aid happened during the pleasure drop), but we cannot say anything about the causality. This research demonstrates both the need for and potential of sensing what lives within a crowd in addition to the use of visitor counting sensors.

Direct tracking of a wireless transmitter based on rao-blackwellized point mass filter

In this article, we address the problem of direct tracking of a wireless transmitter. That is, the inputs given to the Bayesian filter are the received baseband signals instead of pre-computed ranges or angles. We first propose to use the Rao-Blackwellized Point Mass Filter (RBPMF) to solve such a tracking problem. As such, the resulting tracking solution is still computationally expensive. Therefore, we propose an approach for reducing the computational cost of the RBPMF. More precisely, we replace the prediction step by the one of the Linear Kalman Filter (LKF). This combination helps to avoid expensive operations such as the weight convolution in the prediction step. In addition, it also allows complexity reductions in the correction step. As a result, the complexity is reduced by one order of magnitude compared to the original RBPMF. We compare our approach to representative direct-tracking methods, based on Iterative Extended Kalman Filter (IEKF) and Particle Filter (PF). The proposed solution has lower and comparable localization error compared to IEKF and PF, respectively. In addition, the proposed solution is of slightly less complexity than PF. However, the complexity reduction is significant compared to the conventional RBPMF.

2022

A multi-antenna super-resolution passive Wi-Fi radar algorithm

In recent years, Wi-Fi has become the main gateway that connects users to the internet. Considering the availability ofWi-Fi signals, and their suitability for channel estimation, IEEE established the Wi-Fi Sensing (WS) Task Group whose purposeis to study the feasibility of Wi-Fi-based environment sensing. However, Wi-Fi signals are transmitted over limited bandwidthswith a relatively small number of antennas in bursts, fundamentally limiting the range, Angle-of-Arrival and speed resolutions.This paper presents a super-resolution algorithm to perform the parameter estimation in a quasi-monostatic WS scenario. Theproposed algorithm, RIVES, estimates the range, Angle-of-Arrival and speed parameters with Vandermonde decomposition ofHankel matrices. To estimate the size of the signal subspace, RIVES uses a novel Model Order Selection method which eliminatesspurious noise targets based on their distance to the noise and signal subspaces. Various scenarios with multiple targets aresimulated to show the robustness of RIVES. In order to prove its accuracy, real-life indoor experiments are conducted with humantargets by using Software Defined Radios.

MAC address anonymization for crowd counting

Research has shown that counting WiFi packets called probe requests (PRs) implicitly provides a proxy for the number of people in an area. In this paper, we discuss a crowd counting system involving WiFi sensors detecting PRs over the air, then extracting and anonymizing their media access control (MAC) addresses using a hash-based approach. This paper discusses an anonymization procedure and shows time-synchronization inaccuracies among sensors and hashing collision rates to be low enough to prevent anonymization from interfering with counting algorithms. In particular, we derive an approximation of the collision rate of uniformly distributed identifiers, with analytical error bounds.

Physical layer security in an OFDM time reversal SISO communication with imperfect channel state information

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the ϵ-achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication.

Self-synchronization based localization of a time-misaligned transmitter in cellular networks

Geo-localization services are an important functionality in cellular networks. Besides,the use of Ultra Dense Networks and small cells, in current and future cellular networks, greatly increases the complexity of centralized localization approaches. Consequently, we propose a self-synchronization Positioning Estimation (SSPE) algorithm that estimates the transmitter position in a distributed fashion.The proposed SSPE algorithm reaches consensus for the posterior distribution of the transmitter position rather than on the final estimates. Such consensus ensures that the proposed SSPE algorithm converges to the centralized Direct Positioning Estimation (DPE) approach, which has the best performance of all localization approaches. We show that the proposed algorithm is related to the Iterative Positioning Estimation (IPE) algorithm, since both exploit the self-synchronization mechanism. As a result, the improvements and extensions for IPE, previously studied in other works, can be directly applied to the proposed SSPE algorithm. In addition, the proposed algorithm is able to localize the transmitter even when it is not time synchronized with the network as it is usually the case. The performance of the algorithms is numerically assessed through Monte-Carlo simulations by the mean distance error and mean range offset error. Finally, we not only show that our approach gets close to the DPE performance after a few iterations, but also that it converges for different logical network configurations.

Monitoring large crowds with WiFi: a privacy-preserving approach

This paper presents a crowd monitoring systembased on the passive detection of probe requests. The systemmeets strict privacy requirements and is suited to monitoringevents or buildings with a least a few hundreds of attendees. Wepresent our counting process and an associated mathematicalmodel. From this model, we derive a concentration inequalitythat highlights the accuracy of our crowd count estimator. Then,we describe our system. We present and discuss our sensorhardware, our computing system architecture, and an efficientimplementation of our counting algorithm—as well as its spaceand time complexity. We also show how our system ensures theprivacy of people in the monitored area. Finally, we validateour system using nine weeks of data from a public libraryendowed with a camera-based counting system, which generatescounts against which we compare those of our counting system.This comparison empirically quantifies the accuracy of ourcounting system, thereby showing it to be suitable for monitoringpublic areas. Similarly, the concentration inequality provides atheoretical validation of the system.

Near-field exposure in FM frequencies: new methodology and estimation formulas

Workers inside transmission pylons with FM antenna arrays are likely to be exposed to near‐field radiation exceeding reference levels for occupational exposure. In this study, the near-field behavior of 64 FM pylons was studied using a new methodology. Near‐field characterization was done using field metrics without taking into account field sources' size or distance from field source. The specific absorption rate (SAR) was assessed in five hundred different near‐field cases using a human phantom. Estimation formulas for both local and whole‐body SAR are provided and validated numerically. Local and whole‐body SAR are linked to electric field strength. © 2022 Bioelectromagnetics Society

2021

OFDM-based spatial data focusing for wireless physical layer geocasting in multipath channels

OFDM-based spatial data focusing (OFDM-SDF) is proposed as a novel means of performing wireless physical layer geocasting, i.e. spatially confined broadcasting. It is shown that this approach overcomes beamforming and directional modulation (DM) limitations by exhibiting higher spatial precision with a reduced number of antennas and offering uncoupled range-angle-dependent focusing. This paper describes the OFDM-SDF system model for multipath channels, including multipath robust equalization, design rules for steering phases and sidelobe mitigation, analytical geocast delivery zone derivation, and optimized symbol mapping. Using density-based clustering of the spatial bit error rate distribution, a procedure for identifying a practical geocast delivery zone and evaluating its precision and connectivity is proposed. OFDM-SDF's performance and multipath robustness are evaluated through Rice channel simulations as a function of the Rice factor. In particular, it is shown that a 2-antenna OFDM-SDF array matches the radial and angular precision of, respectively, a 6 and 12-antenna DM array in recent literature, while robustness is ensured for 5G small cell channels.

Indoor tracking of multiple individuals with an 802.11ax Wi-Fi-based multi-antenna passive radar

We investigate indoor human multi-target tracking in cartesian coordinates based on range, Doppler and Angle-of-Arrival measurements obtained with a four-antenna passive bistatic radar capturing 802.11ax Wi-Fi signals. A reference antenna selection method is described to perform angle processing correctly when dealing with target detection diversity among antennas. The tracking is performed by an Unscented Kalman Filter (UKF) to handle the non-linear relation between the measurement space and the state space. A Joint Probabilistic Data Association Filter is coupled to the UKF to handle the data association between tracks and measurements when dealing with multiple targets. Simulations are performed to determine the tracking parameters under heavy constraints and identify key scenarios. An experimental setup is built using Universal Software Radio Peripherals, featuring an over-the-air phase calibration for angle processing with an anchor antenna. It is used to validate the proposed single and multi-target tracking scheme.

A stochastic geometry approach to EMF exposure modeling

Downlink exposure to electromagnetic fields due to cellular base stations in urban environmentsis studied using the stochastic geometry framework. A two-dimensional Poisson Point Process is assumedfor the base station distribution. Mathematical expressions of statistics of exposure are derived from a simplepropagation model taking into account the height of the base stations. The error on exposure made by takinga limited number of base stations, instead of the whole set, is quantified. The relative impact of the modelparameters on the statistics of exposure is highlighted. The method is then applied and the model parametersare calibrated using experimental data obtained by drive-tests in two Brussels municipalities, in Belgium,for the 2100 MHz and 2600 MHz frequency bands. It is shown that the proposed model fits experimentalvalues, paving the way to a new methodology to assess general public exposure to electromagnetic fields,for any frequency band. An insight is given on how to apply the methodology to a real case without accessto experimental data.

Super resolution passive radars based on 802.11 ax Wi-Fi signals for human movement detection

Passive Radars based on Wi-Fi signals provide an excellent opportunity for human sensing without violating the privacy of individuals. Due to the limited integration time of Wi-Fi bursts and relatively low bandwidths, Fourier Transform-based methods do not provide the required accuracy. Herein, a Wi-Fi-based passive radar algorithm is proposed for indoor human movement detection with super resolution which relies on the ESPRIT algorithm to estimate range/speed parameters from limited number of measurements. To determine the number of targets in the environment, a new Model Order Selection (MOS) method is proposed which exploits the orthogonality between the basis vectors of signal and noise subspaces obtained from the sample covariance matrix of the measurements. The new MOS method along with the proposed algorithm are numerically analysed and compared with other existing methods. Finally, the performance of the algorithm is experimentally validated in indoor conditions.

2020

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its goodspatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabricationfurther leverage the use of TR wideband communication systems. The TR is generally carried out inthe time domain and the focusing effect of TR comes from the use of a high transmit rate back-offfactor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequencydivisionmultiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TRprecoding in combination with OFDM communication systems. Furthermore, most existing works onFD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. Inthis paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output(MISO) OFDM systems. In particular, we compare the communication performance of such system atthe intended position and that at the unintended position. Based on the analysis, we demonstrate thatincreasing the BOF and/or the number of transmit antennas significantly improves the focusing effectat the intended position. In contrast, the unintended positions receive less useful power. We deriveapproximated mean-square-error (MSE) expressions of equalized received signals at both intendedand unintended positions. The subsequent focusing gain is presented as a function of the BOF andthe number of antennas, enabling us to gain insights in the contributions of each parameter to thesystem performance. Numerical simulations with multi-path Rayleigh fading channels are carried outto validate the MSE expressions.

Crowd forecasting based on WiFi sensors and LSTM neural networks

To ensure effective management and security in largescale public events, it is imperative for the event organizers to beaware of potentially critical crowd densities. This paper, therefore,presents a solution to the above problem in terms of WiFi basedcrowd counting and LSTM neural network based forecasting.Monitoring of an actual event organized in Brussels has beendescribed, wherein crowd counts are obtained using WiFi sensorsin a privacy-preserved manner. The time-stamped crowd countsare used to develop univariate time-series, which are in-turnutilized for forecasting. Five different LSTM models are utilizedfor crowd time-series forecasting and analyzed for theirsuitability. A random walk model is used as reference forperformance assessment. Among different LSTM models,Convolutional LSTM delivered the best performance. Overallresults and analysis show that the developed system is suitable forcrowd monitoring.

Secrecy capacity of FBMC-OQAM modulation over frequency selective channel

This letter studies the Information-theoretic secrecy capacity of an Offset-QAM-based filterbank multicarrier (FBMC-OQAM) communication over a wiretap frequency selective channel. The secrecy capacity Is formulated as an optimization problem which has a closed-form solution In the high signal-to-noise ratio (SNR) regime. Two of the most common equalization strategies In FBMC-OQAM are considered, namely, single-tap and multi-tap equalization. For the sake of comparison, we also consider the secrecy capacity of a generic modulation and a cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) modulation. As a result, we find that FBMC-OQAM Is particularly competitive for medium-to-long burst transmissions.

Crowd monitoring

With the growing concerns over public safety, the importance of crowd monitoring is being realized by varioussecurity and event management agencies worldwide. Estimation of crowd dynamics can help such agencies in prevention ofany unanticipated accidents or issues. Research on crowd monitoring has been underway since the past few decades. Conventionalcrowd monitoring systems mainly rely on computer vision approach. Due to predominant use of videos/ imagesequences, the existing techniques may raise data privacy concerns. This has led to development of new crowd monitoringtechniques which are privacy preserving and require minimum public participation. This paper aims to serve as a single andsufficient source of information to the concerned researchers on various aspects of crowd monitoring and also provide futuredirections which can be helpful in developing advanced crowd monitoring techniques.

Design of an integrated platform for mapping residential exposure to Rf-Emf sources

Nowadays, information and communication technologies (mobile phones, connectedobjects) strongly occupy our daily life. The increasing use of these technologies and the complexity ofnetwork infrastructures raise issues about radiofrequency electromagnetic fields (Rf-Emf) exposure.Most previous studies have assessed individual exposure to Rf-Emf, and the next level is to assesspopulational exposure. In our study, we designed a statistical tool for Rf-Emf populational exposureassessment and mapping. This tool integrates geographic databases and surrogate models tocharacterize spatiotemporal exposure from outdoor sources, indoor sources, and mobile phones.A case study was conducted on a 100 x 100 m grid covering the 14th district of Paris to illustratethe functionalities of the tool. Whole-body specific absorption rate (SAR) values are 2.7 times higherthan those for the whole brain. The mapping of whole-body and whole-brain SAR values showsa dichotomy between built-up and non-built-up areas, with the former displaying higher values.Maximum SAR values do not exceed 3.5 and 3.9 mW/kg for the whole body and the whole brain,respectively, thus they are significantly below International Commission on Non-Ionizing RadiationProtection (ICNIRP) recommendations. Indoor sources are the main contributor to populationalexposure, followed by outdoor sources and mobile phones, which generally represents less than 1%of total exposure.

Iterative NDA Positioning Using Angle-of-Arrival Measurements for IoT Sensor Networks

Various positioning techniques have been developed to localize Internet of Things (IoT) devices accurately. Because IoT communications are often narrowband, efficient localization can be achieved by deducing the device position from the estimated signal Angle of Arrival (AOA) at multiple arrays of antennas. It has recently been shown that significant accuracy gains can further be obtained by iterating between the AOA estimation and multi-lateration steps. However, the existing method relies on the knowledge of the transmitted signal (Data-Aided (DA) estimation) which makes it impractical for narrowband communications where the preamble is short. Non-Data-Aided (NDA) estimation is recommended to improve the positioning accuracy for low capacity IoT sensors. This paper proposes an NDA iterative (NDA-It) algorithm using AOA measurements to determine the position of an IoT sensor. Simulation results show that the proposed algorithm significantly outperforms the DA-It in a Bluetooth Low Energy (BLE) context because it can use a much higher number of samples (snapshots); however, it needs more iterations to converge. The computational complexity analysis proves the competitiveness of the proposed NDA-It. The performance of the algorithms is further investigated in multipath and Non-Line-of-Sight (NLOS) propagation environments. Finally, an experimental setup is built to validate the performance of the proposed algorithms.

Forecasting Crowd Counts With Wi-Fi Systems: Univariate, Non-Seasonal Models

Recently, event organizers and researchers have advocated the development of novel technologies supporting crowd control, notably for public events. This paper presents a crowd monitoring system based on probe requests (PRs), which are Wi-Fi packets smartphones send periodically. By estimating the global rate at which nearby smartphones send PRs, Wi-Fi sensors can estimate crowd counts. The core contribution of this paper is a computationally tractable method that forecasts crowd counts up to thirty minutes in the future, with forecasts becoming available as soon as two hours of data are available. The forecasting method relies on autoregressive integrated moving average (ARIMA) models. Contributions also include two methods that compute prediction intervals associated with the forecasts, one of which is based upon generalized autoregressive conditional heteroskedasticity (GARCH) models. Recent real-world data from Winter Wonders 2018/2019 (an event that took place in Brussels, Belgium) notably demonstrate that the proposed forecasting method outperforms its immediate variations as well as baseline models (i.e., random walk models).

2019

Analysis of residual CFO impact on downlink massive MISO systems

Massive antenna technologies provide a good power focusing gain for emerging communication systems. They can easily be integrated into an orthogonal frequency-division multiplexing (OFDM) system. However, OFDM is known to be prone to carrier frequency offset (CFO) due to the loss of the orthogonality among OFDM subcarriers. In this Letter, the authors investigate the impact of residual CFO (RCFO) on the downlink performance of massive multiple-input single-output (MISO) OFDM systems using matched-filter (MF) and maximum-ratio-transmission (MRT) precoders. Particularly, the exact mean-square-error (MSE) expressions of the equalised received signal of both MF and MRT systems are derived. Numerical simulations with Rayleigh fading channels are carried out to validate the analysis. The results show that the RCFO causes a MSE plateau compared to the ideal case of no CFO.

Performance analysis of frequency domain precoding time-reversal MISO OFDM systems

Time reversal (TR) recently emerged as an interestingcommunication technology capable of providing a goodspatio-temporal signal focusing effect. New generations of largebandwidthdevices with reduced cost leverage the use of TRwideband communication systems. TR can easily be integratedinto an orthogonal frequency-division multiplexing (OFDM) systemby precoding the signal in the frequency domain. In thispaper, we first extend the frequency-domain TR precoding to rateback-off factors (BOFs) different than one. We secondly derive aclosed-form mean-square-error (MSE) expression of the receivedequalized symbols as a function of the BOFs and the numberof antennas. The derived MSE formula is validated numericallywith Rayleigh fading channels.

2018

Low complexity iterative localization of time-misaligned terminals in cellular networks

Recently, iterative localization has arisen as a promising approach to localize a Mobile Station (MS) in a cellular system. The conventional geo-location is obtained in a two-step approach: propagation delays are estimated and then the multi-lateration is responsible for the determination of the user position, based on the estimated delays. Iterative localization iterates between the two conventional steps to progressively refine delay estimates based on the position estimate available from the previous iterations. This localization scheme was seen to provide appealing performances compared to the two-step approach. It also seems to be computationally attractive with respect to direct localization that estimates the position using the digitized received signals directly. However, the iterative localization solution developed in literature relies on a strict time synchronization between MS and Base Stations (BSs). Moreover,the computational complexity of the iterative approach is not thoroughly compared to two-step and optimal solutions. This paper therefore proposes a new iterative localization method able to operate in a cellular system with time-misaligned terminals.We show by means of a detailed complexity analysis that the iterative positioning algorithm is one order of magnitude less complex than direct localization. Simulation results prove that the achievable performance after a few iterations approaches the performance of the direct localization solution.

Iterative RToF-based localization and time synchronization in WLAN-like systems

Iterative localization is currently arising as a solution to localize a Mobile Station (MS) in a cellular network. We recently showed that iterating between the conventional delay estimation and multi-lateration steps allows one to approach the performance of direct localization algorithms. Until now, the method has only been applied to the case of networks where the access points are perfectly synchronized with each other. In this letter, we present a localization and time synchronization iterative algorithm suitable for networks where access points are not synchronized. We show numerically that iterating between the two conventional steps brings a significant performance gain.

Virtual multi-antenna array for estimating the direction of a transmitter: system, bounds and experimental results

A method is proposed to estimate the direction of a radio-frequency transmitter with a mobile single-antenna receiver. By considering the received signal at several points along its trajectory, the receiver implicitly creates a virtual multiantenna array, which can be used to estimate the direction of the transmitter. Virtual arrays differ from conventional multiantenna arrays in two ways: 1) the position and orientation of each antenna in the virtual array depend on the movement of the receiver and are not known a priori; and 2) the local oscillator (LO) frequency offset between the transmitter and the receiver adds a phase offset to the signal received by each antenna of the virtual array, which must be estimated and compensated. The first problem is solved by using an inertial measurement unit, which can provide the relative position of the receiver for short time durations. The second problem is solved by estimating the LO frequency offset jointly with the direction of the transmitter by extending the MUSIC algorithm for multidimensional estimation. We investigate the Cramér-Rao lower bound of the proposed estimator, which provides some insights in the design of our system. We implement our system on a software-defined radio testbed and present some measurement results obtained in a controlled environment.

2017

Wideband off-body measurements and channel modeling at 60 GHz

A wideband indoor channel model between an external base station and a worn receiver on the user body at 60 GHz has been developed based on indoor measurements. The results are presented for three different receiver locations: head, wrist, and belt. A Saleh-Valenzuela impulse response is proposed as numerical model for this scenario. Also path loss and delay spread results are driven out in order to discuss the node location for assessing an off-body communication.

Performance of emerging multi-carrier waveforms for 5G asynchronous communications

This paper presents an extensive and fair comparison among the most promising waveform contenders for the 5G air interface. The considered waveform contenders, namely filter-bank multi-carrier (FBMC), universal-filtered multi-carrier (UFMC), generalized frequency-division multiplexing (GFDM) and resource-block filtered orthogonal frequency-division multiplexing (RB-F-OFDM) are compared to OFDM used in 4G in terms of spectral efficiency, numerical complexity, robustness towards multi-user interference (MUI) and resilience to power amplifier non-linearity. FBMC shows the best spectral containment and reveals to be almost insensitive to multi-user interference. It however suffers from its bad spectral efficiency for short bursts and from its poor multiple input multiple output (MIMO) compatibility. GFDM reveals to be the most promising contender, with the best spectral efficiency and the smallest complexity overhead compared to OFDM. It is also the most resilient to multi-user interference after FBMC and is MIMO compatible as soon as the interference can be managed. UFMC and RB-F-OFDM are finally the closest to OFDM and benefit therefore from a better compatibility with existing systems, even if their performance is generally lower than FBMC and GFDM.

Assessment of on-body skin-confined propagation for body area network

This letter assesses the potential of using surface waves to implement secure on-body communications. The propagation medium being the human skin, a transverse resonance method is used to analyze existing modes within the multilayered structure that composes the human tissues. Confinement capabilities as well as propagation losses are investigated for frequency up to 60 GHz. It is shown in particular that the 3-10 GHz band could allow us secure communications up to few tens of centimeters. This frequency band is furthermore considered as ultrawideband channels in the IEEE 802.15.6 standard dedicated to body area networks.

Input impedance of an aperture over a lossy half-space: application to on-body antenna performance at 60GHz

This paper presents a theoretical approach to compare the performance of a directive and a quasi-omnidirectional on-body antennas. Two canonical antennas, namely, a dipole and a rectangular aperture, are considered in the 60 GHz band. We first demonstrate that for this on-body configuration, the classically-defined far-field antenna gain depends on the observation distance. Consequently, we derive results in terms of radiation efficiency and link budget. To do so, the antenna input impedance computation is a preliminary step to normalize the input power to allow a fair comparison between the two antennas. The impedance over a lossy half-plane of an aperture illuminated by a TE10 mode normally polarized is therefore derived into a convenient easy-to-compute formulation, which to authors' best knowledge, is not available in the literature. In terms of link budget, it is obtained that the received power due to an aperture is generally higher than the one due to the dipole in the main lobe direction. A constant difference is observed along the distance, and this difference increases with the aperture width for antennas touching the body. Besides, it is shown that the standard aperture waveguide WR15 exhibits a slightly higher efficiency than a vertical dipole with the same vertical size when being placed at a distance less than 3mm (i.e., 0:6λ) from the body phantom surface. Above this distance, the dipole and the aperture exhibit similar efficiency in the order of 60%.

2016

Variographic analysis of public exposure to electromagnetic radiation due to cellular base stations

The spatial structure of the vertical component of the electric field emitted by base stations in the Brussels region (Belgium) is measured, and studied using the variogram. A relationship between the variogram shape and base station antenna density in each measurement area is found; the variogram range and sill level are shown to depend on cellular base stations' antenna density, following exponential laws. Bioelectromagnetics. © 2016 Wiley Periodicals, Inc.

Performance assessment of IR-UWB body area network (BAN) based on IEEE 802.15.6 standard

Performance of impulse radio-based ultra-wideband (IR-UWB) communications in wireless body area networks are investigated using the dedicated IEEE 802.15.6 standard. An IR-UWB transceiver system is implemented for both on-off keying and differential binary phase-shift keying modulations. Bit error rates are determined from measurements for different on-body links with different data rates. It is shown that that using a 25 dB-gain LNA at the receiver, reaching an uncoded BER of 10-3 was not possible for some links operating at higher data rates. Power and energy consumption issues are then addressed and results in terms of required pJ/bit to achieve a certain quality of communication are given and discussed.

Millington effect and propagation enhancement in 60 GHz body area networks

Millington effect for on-body propagation enhancement is presented in the 60-GHz band. Millington's equations are developed to describe propagation above a flat inhomogeneous surface. This study focuses on mixed paths (human skin-metallic) for on-body scenarios. It is shown that adding metallic paths on the human skin can improve the power link budget between two nodes placed on the body. Two different schemes are studied experimentally to assess the analytical model using a flat phantom with electric properties of human skin and different lengths of metallic inserts. The first scheme considers a metallic plate between the transmitting and receiving antennas, while the second scheme proposes locating the metallic plates under the antennas. It is shown that the second scheme yields a better link budget than the first one for the same length of metal. Moreover, a numerical study is performed to assess the impact of the following different parameters: the location of the metal plate, size of the plate, and the height of the antennas. Excellent agreement between numerical and experimental results has been shown. In the best cases, the presented techniques allow to improve the path loss of 10-20 dB.

2015

TDOA estimation method using 60 GHz OFDM spectrum

In the field of high data rate wireless communications, localization issues play a key role in achieving energy-efficient communication and geographic routing. time-difference of arrival (TDOA)-based localization methods present numerous advantages. In this paper, a new method of TDOA estimation is proposed. With this method, unlike conventional TDOA measurements, it is possible to perform communication and localization at the same time by using a multi-input single-output system. By transmitting ultra-wide-band orthogonal frequency-division multiplexing signals using spatial diversity, it is possible to extract TDOA from interference patterns in spectral domain. In addition, increasing the precision of localization is also studied using a multi-band approach. This whole study is made within the framework of the WiGig alliance specifications; however, it is compatible with other standards.

Sensitivity of whole body dosimetry to channel model parameters

Impact of channel model parameters on the whole-body specific absorption rate (SAR) is studied. Closed-form expressions are derived for the mean value and the standard deviation of the SAR, taking into account the random nature of the channel parameters. Total sensitivity indices of the SAR with respect to the channel parameters are derived. They are estimated using a Monte-Carlo approach. Noninfluential channel parameters are identified. They are then suppressed from channel models for dosimetry purpose.

Near-body shadowing analysis at 60GHz

A numerical model of the fading of a receiver (Rx) located near the user body at 60 GHz in an indoor environment is presented. The model is based on the indoor channel model IEEE 802.11ad. The results are presented for a-Rx located in a zone from 5 to 30 cm away from the body. With the shadowing depending on the region (front or back) with respect to the base station, the mean attenuation of the channel over the bandwidth is analyzed and modeled thanks to a two-wave diffuse power distribution model.

UWB interferometry TDOA estimation for 60GHz OFDM communication systems

A simple technique to estimate the time difference of arrival (TDOA) that necessitates only one reference device to perform 1-D positioning of a mobile device is presented. Using a multiple-input-single-output (MISO) system, this interferometric technique uses ultrawideband signals and is particularly well suited for 60-GHz orthogonal frequency-division multiplexing (OFDM) communications. The accuracy of the technique is assessed by simulation, using the IEEE 802.11ad channel, as well as by measurement.

2014

V-band velocity estimation of creeping waves around the human body

This letter investigates the phase velocity of an electromagnetic wave propagating between two sensors located on the human body surface by using the formulation of a creeping wave around a circular cylinder. The study is performed for both horizontal and vertical polarizations. The velocity is studied over the V-band. It is theoretically shown that the variation of the phase velocity is negligible over the bandwidth. Measurements are conducted on a metallic circular cylinder to assess the validity of these formulations. Then, an experimental validation of the circular cylinder assumption for the human body is performed. Deviation between the measurements and the theoretical circular model is between 2% and 3.5%.

Information spatial focusing scheme for UWB wireless communications in smart environments

This letter presents an information spatial focusing method for future smart environments using ultrawideband (UWB) wireless communications. This method allows to send high-data-rate information at predetermined specific spatial positions without localizing users, thereby protecting their privacy. The proposed approach is a combination of simplified UWB beamforming and signal processing. Compared to classical UWB beamforming, the proposed method exhibits greater information focusing capabilities by increasing greatly the peak-to-floor ratio between main and side beams.

Statistical on-body measurement results at 60 GHz

This communication studies the path loss and shadowing between two body mounted devices at 60 GHz. The temporal fading is experimentally investigated and the Doppler spectrum is characterized and modeled. Measurements have been conducted in an anechoic chamber for both horizontal and vertical polarizations.

Analytical creeping wave model and measurements for 60 GHz body area networks

The propagation of 60 GHz electromagnetic waves around a human body is studied analytically and experimentally. The body is treated here as a circular lossy cylinder, which is an approximation of the human torso. Analytical formulations based on creeping wave theory are given and discussed for both vertical and horizontal polarizations. An exact path gain expression is derived from analytical formulations and a simpler first order approximation is given. Path gain coefficients are shown for frequencies spanning the world available 60 GHz unlicensed band and for several sizes of the torso. Finally, the results of an experimental campaign conducted in an anechoic chamber to isolate the contribution of on-body propagation are reported. The measurement of the distance dependence of the received power on a brass cylinder and on a human body for both vertical and horizontal polarizations confirmed theoretical predictions. © 2014 IEEE.

Creeping wave model of diffraction of an obliquely incident plane wave by a circular cylinder at 60 GHz

Study of human exposure using kriging method

This paper develops the kriging method to calculate the whole body Specific Absorption Rate (SAR) for any angle of incidence of a plane wave on any body model using a minimum number of Finite Difference Time Domain (FDTD) simulations. Practical application of this method is to study people's exposure. Thanks to kriging method, it will enable to answer to the challenge of studying the exposure in a realistic environment. This approach develops a new tool in order to improve the field of stochastic dosimetry. The kriging method is applied to a girl body model in order to determine the variogram model, then this model is validated on a boy body model. Thanks to only 40 numerical SAR values, kriging method enables to estimate any SAR value with a mean relative error under 3%.

2013

Theoretical and experimental investigation of a 60 GHz off-body propagation model

A fast computation and accurate analytical model for off-body propagation is derived in this paper. The paper discusses the off-body model propagation from an external source to a receiver located on the body. The model is developed for normal incident plane wave by describing the human body with a circular cylinder. We show that the total received electric field around the human body can be written as a creeping wave in the shadow region and as a geometrical optics result for the lit region. It is also shown that at 60 GHz, the shadow boundary width is negligible. The model shows perfect agreement with the experimental results conducted on a perfectly conducting cylinder. Measurements of the creeping wave path gain have been also conducted on a real body to assess the validity of the cylinder assumption. The results have shown a path gain of about 5 dB/cm for the TM case and 3 dB/cm for the TE case. The standard deviation between the measurements and the cylindrical model is about 3.5 dB for both TM and TE cases. © 2013 IEEE.

Food collection and response to pheromones in an ant species exposed to electromagnetic radiation

We used the ant species Myrmica sabuleti as a model to study the impact of electromagnetic waves on social insects' response to their pheromones and their food collection. We quantified M. sabuleti workers' response to their trail, area marking and alarm pheromone under normal conditions. Then, we quantified the same responses while under the influence of electromagnetic waves. Under such an influence, ants followed trails for only short distances, no longer arrived at marked areas and no longer orientated themselves to a source of alarm pheromone. Also when exposed to electromagnetic waves, ants became unable to return to their nest and recruit congeners; therefore, the number of ants collecting food increases only slightly and slowly. After 180 h of exposure, their colonies deteriorated. Electromagnetic radiation obviously affects social insects' behavior and physiology. © Informa UK Ltd.

Dynamic resource allocation for MIMO cognitive radio networks with low control traffic and low computational complexity

Radio spectrum scarcity hampers the development of new wireless technologies and services. Cognitive radios have been proposed to enable unlicensed (or secondary) users to borrow locally idle bands of the spectrum provided that no significant interference is created for the licensed (or primary) users. Fast adaptation to the changing spectrum availability is naturally a major requirement in such systems. This adaptation consists of detecting the spectrum occupied by the primary users, computing a new resource allocation for the secondary network, and communicating this allocation through the network. In that context, we develop a resource allocation scheme for multi-input-multi-output wireless mesh networks. The proposed algorithm combines low computational complexity and light control traffic thanks to a combination of relevant approximations in the general nonpolynomial-hard allocation problem. The allocation consists of two steps. First, a centralized carrier allocation is performed at a coordinator node based on partial knowledge of the network parameters. Then, each node locally computes its power allocation through simple water-filling algorithms. Numerical results show that compared to state-of-the-art techniques, 10% of the total throughput of the network is sacrificed to reduce the computation time and the control traffic by two orders of magnitude.

Dynamic channel modeling for multi-sensor body area networks

A channel model for time-variant multi-link wireless body area networks (WBANs) is proposed in this paper, based on an extensive measurement campaign using a multi-port channel sounder. A total of 12 nodes were placed on the body to measure the multi-link channel within the created WBAN. The resulting empirical model takes into account the received power, the link fading statistics, and the link auto- and cross-correlations. The distance dependence of the received power is investigated, and the link fading is modeled by a log-normal distribution. The link autocorrelation function is divided into a decaying component and a sinusoidal component to account for the periodical movement of the limbs caused by walking. The cross-correlation between different links is also shown to be high for a number of specific on-body links. Finally, the model is validated by considering several extraction-independent validation metrics: multi-hop link capacity, level crossing rate (LCR) and average fade duration (AFD). The capacity aims at validating the path-loss and fading model, while the LCR and AFD aim at validating the temporal behavior. For all validation metrics, the model is shown to satisfactorily reproduce the measurements, whereas its limits are pointed out.

Statistical characterization and modeling of doppler spectrum in dynamic on-body channels

The time variation, i.e., time-selective fading, of on-body channels in body area networks is dominated by the body dynamics, which not only contribute to dynamic scattering, but also modify the geometry of links over time. In this letter, we experimentally investigate the Doppler spectra of narrowband on-body channels in anechoic (at 2.45 GHz) and indoor (at 4.2 GHz) environments for people walking. The body dynamics resulting from people walking are shown to directly affect the Doppler spectrum, and empirical models are proposed to describe the global and/or local Doppler spreads. © 2013 IEEE.

2012

Tri-polarized MIMO systems in real-world channels

Polarized multi-antenna systems are an effective solution for reducing inter-antenna spacing while still maintaining low inter-antenna correlation. Traditionally, only dual-polarized antenna systems are used for polarized transceivers. In this paper, tri-polarized antenna systems are investigated. Starting from the polarization mechanisms in the wireless propagation channel, it is shown that dual-polarized MIMO systems show high sensitivity to the transmitter and receiver orientation, which may be very critical in practical applications. Tri-polarized MIMO systems are introduced as a solution to obtain a robust MIMO performances, which are independent of the transmitter and receiver orientation. The performances of dual- and tri-polarized MIMO systems are evaluated on real-world measured channels, and the limits of each of these systems is highlighted.

Tri-polarized spectrum sensing based on an experimental outdoor-to-indoor cognitive-radio scenario

Compared to classical spatially separated multiple antenna system, cross-polarized co-located antenna systems are an interesting way to reduce equipment size while reducing the inter-antenna correlation. In this paper the spectrum sensing of a Cognitive Radio (CR) system taking advantage of polarization diversity under Rayleigh fading is investigated and compared to an equivalent system using spatial diversity. This analysis is based on a theoretical formulation applied to a real-world scenario. For this purpose, an outdoor-to-indoor measurement campaign at a frequency of 3.5 GHz is realized, where an indoor secondary user senses the signals received from an outdoor primary base station. The signals received at each antenna are first combined and then applied to an energy detector. The theoretical expressions are simulated in the measurement context. The detection probability behavior as a function of distance between the Primary Transmitter (PTx) and the Secondary Terminal (STE) and the inter-antenna correlation effect on the sensing performance are studied. © 2012 Elsevier B.V.

Model parametrization and validation for specular-diffuse clustered channel models

A model parameterization and validation is proposed for specular-diffuse clustered channel models. The double-directional model parameters, based on an experimental measurement campaign, are presented. The specular-diffuse model is validated with regard to the following validation metrics: mutual information, singular values, Demmel condition number and ellipticity. A good agreement is observed when comparing the model with experimental measurements. Finally, the influence of the diffuse multipath component on the performance of the model validation is evaluated. © 1963-2012 IEEE.

GSM 900 MHz radiation inhibits ants' association between food sites and encountered cues

The kinetics of the acquisition and loss of the use of olfactory and visual cues were previously obtained in six experimental colonies of the ant Myrmica sabuleti meinert 1861, under normal conditions. In the present work, the same experiments were conducted on six other naive identical colonies of M. sabuleti, under electromagnetic radiation similar to those surrounding GSM and communication masts. In this situation, no association between food and either olfactory or visual cues occurred. After a recovery period, the ants were able to make such an association but never reached the expected score. Such ants having acquired a weaker olfactory or visual score and still undergoing olfactory or visual training were again submitted to electromagnetic waves. Not only did they lose all that they had memorized, but also they lost it in a few hours instead of in a few days (as under normal conditions when no longer trained). They kept no visual memory at all (instead of keeping 10% of it as they normally do). The impact of GSM 900 MHz radiation was greater on the visual memory than on the olfactory one. These communication waves may have such a disastrous impact on a wide range of insects using olfactory and/or visual memory, i.e., on bees. © Informa Healthcare USA, Inc.

A human body model exposed to a cluster of waves

The impact of wireless channel modeling on exposure to electromagnetic radiation is studied. Two methods are developed in order to assess the statistical properties of whole body Specific Absorption Rate for exposure estimation in indoor environment. The body model is exposed to a bundle of waves, named cluster, following the wireless channel modeling approach. The first method is analytical and based on the Uncorrelated Scattering Assumption of the incident waves. The second method is a classical stochastic method. The point is to identify the parameters of Wireless Channel which led to significant SAR's variation.

Sensing time and power allocation for cognitive radios using distributed Q-learning

In cognitive radios systems, the sparse assigned frequency bands are opened to secondary users, provided that the aggregated interferences induced by the secondary transmitters on the primary receivers are negligible. Cognitive radios are established in two steps: the radios firstly sense the available frequency bands and secondly communicate using these bands. In this article, we propose two decentralized resource allocation Q-learning algorithms: the first one is used to share the sensing time among the cognitive radios in a way that maximize the throughputs of the radios. The second one is used to allocate the cognitive radio powers in a way that maximizes the signal on interference-plus-noise ratio (SINR) at the secondary receivers while meeting the primary protection constraint. Numerical results show the convergence of the proposed algorithms and allow the discussion of the exploration strategy, the choice of the cost function and the frequency of execution of each algorithm.

Correlation for multi-frequency propagation in urban environments

The multi-frequency propagation in urban environment is investigated in this letter. An experimental measurement campaign is conducted to simultaneously measure the GSM-900, GSM-1800 and UMTS band of a cellular system in a suburban environment. The shadowing and small-scale fading parameters are extracted, and the correlation of these parameters across the different frequency bands is measured. It is shown that shadowing coeffcients are highly correlated, while small-scale fading is completely uncorrelated between different frequency bands.

Uncertainty propagation and sensitivity analysis in ray-tracing simulations

Up to now, ray-tracing simulations are commonly used with a deterministic approach. Given the input parameters, the ray-tracing algorithm computes a value for the electric field. In this paper, we present a method that aims at computing the mean and standard deviation of the electric field. More precisely, we aim to obtain the probabilistic content of the electric field value and direction. We assume that this uncertainty results from input random variables which we consider uniformly distributed. Since ray-tracing computations have a high computational cost, we use spectral methods in order to optimize the number of simulations. We consider 2D electromagnetic propagation for the multi-path components, which can interact with the environment through four processes: transmission, single reflection, double reflection and diffraction. These are modelled using adequate coeffcients. In order to calculate the polynomial chaos expansion coeffcients, we use the projection method and Gauss-Legendre quadratures. These coeffcients can then be used to determine the Sobol indices of input parameters. This is done in order to neglect variables in practical computation of the uncertainties.

2011

Special issue on "Towards the connected body: advances in body communications"

Impact of the environment and the topology on the performance of hierarchical body area networks

Personal area networks and, more specifically, body area networks (BANs) are key building blocks of future generation networks and of the Internet of Things as well. In this article, we present a novel analytical framework for network performance analysis of body sensor networks with hierarchical (tree) topologies. This framework takes into account the specificities of the on-body channel modeling and the impact of the surrounding environment. The obtained results clearly highlight the differences between indoor and outdoor scenarios, and provide several insights on BAN design and analysis. In particular, it will be shown that the BAN topology should be selected according to the foreseen medical application and the deployment environment.

Link-level performance of indoor body area networks with centralized topologies

sensors networks and, more specifically, body area networks (BANs) are key building blocks of the future generation networks and the Internet of Things as well. In the last years, research has focused on channel modeling and on the design of efficient medium access control (MAC) mechanisms. Less attention has been paid to network-level performance analysis. Thereby, this paper presents a novel analytical framework for network performance analysis with star (i.e., centralized) topologies. This framework takes into account realistic channel statistics and provides several insights on BAN design and analysis. © Dricot et al.

A wideband channel model for intravehicular nomadic systems

The increase in electronic entertainment equipments within vehicles has rendered the idea of replacing the wired links with intra-vehicle personal area networks. Ultra-wideband (UWB) seems an appropriate candidate technology to meet the required data rates for interconnecting such devices. In particular, the multiband OFDM (MB-OFDM) is able to provide very high transfer rates (up to 480 MBps) over relatively short distances and low transmit power. In order to evaluate the performances of UWB systems within vehicles, a reliable channel model is needed. In this paper, a nomadic system where a base station placed in the center of the dashboard wants to communicate with fixed devices placed at the rear seat is investigated. A single-input single-output (SISO) channel model for intra-vehicular communication (IVC) systems is proposed, based on reverberation chamber theory. The model is based on measurements conducted in real traffic conditions, with a varying number of passengers in the car. Temporal variations of the wireless channels are also characterized and parametrized. The proposed model is validated by comparing model-independent statistics with the measurements.

An analytical modeling of polarized time-variant on-body propagation channels with dynamic body scattering

On-body propagation is one of the dominant propagation mechanisms in wireless body area networks (WBANs). It is characterized by near-field body-coupling and strong body-scattering effects. The temporal and spatial properties of on-body channels are jointly affected by the antenna polarization, the body posture, and the body motion. Analysis on the time variant properties of on-body channels relies on a good understanding of the dynamic body scattering, which is highly dependent on specific scenarios. In this paper, we develop an analytical model to provide a canonical description of on-body channels in both time and space domains to investigate the on-body propagation over the trunk surface of a walking human. The scattering from the arms and the trunk in different dimensions is considered with a simplified geometrical description of the body and of the body movements during the walk. A general full-wave solution of a polarized point source with multiple cylinder scattering is derived and extended by considering time evolution. The model is finally validated by deterministic and statistical comparisons to different measurements in anechoic environments. © 2011 Lingfeng Liu et al.

2010

A polarized clustered channel model for indoor multiantenna systems at 3.6 GHz

Multiple-input-multiple-output (MIMO) technologies allow high data rates to be obtained, but they suffer from interantenna correlation caused by the limits in interantenna spacing. Polarized MIMO systems resolve this problem by using colocated perpendicularly polarized antennas that have low interantenna correlation. In this paper, a polarized single-directional channel model for 2 × N MIMO systems at 3.6 GHz in an indoor environment is presented. The wireless channel is modeled as a sum of clusters, where each cluster has specular and diffuse components. The polarization of the specular component of the clusters is included by considering a per-path polarization. The diffuse component of the clusters is modeled with a FisherBingham (FB5) spectrum in the azimuthcoelevation domain and with an exponential power delay profile. Polarization is analyzed by introducing the cross-polar discrimination of the exponential power delay profile parameters. All of the parameters in the model are extracted from an experimental measurement campaign performed in an indoor environment at 3.6 GHz. Individual paths are extracted from the measurements with the space-alternating generalized expectation-maximization (SAGE) algorithm. These paths are grouped in clusters within the azimuth of arrivalelevation of arrivaldelay domains at the receiver side using automatic clustering algorithms. The specular component properties of the clusters are then determined. Finally, the diffuse components of the clusters are investigated and parameterized by applying a beamforming algorithm on the diffuse part of the impulse response. © 2006 IEEE.

Higher-order cyclostationarity detection for spectrum sensing

Recent years have shown a growing interest in the concept of Cognitive Radios (CRs), able to access portions of the electromagnetic spectrum in an opportunistic operating way. Such systems require efficient detectors able to work in low Signal-to-Noise Ratio (SNR) environments, with little or no information about the signals they are trying to detect. Energy detectors are widely used to perform such blind detection tasks, but quickly reach the so-called SNR wall below which detection becomes impossible Tandra (2005). Cyclostationarity detectors are an interesting alternative to energy detectors, as they exploit hidden periodicities present in man-made signals, but absent in noise. Such detectors use quadratic transformations of the signals to extract the hidden sine-waves. While most of the literature focuses on the second-order transformations of the signals, we investigate the potential of higher-order transformations of the signals. Using the theory of Higher-Order Cyclostationarity (HOCS), we derive a fourth-order detector that performs similarly to the second-order ones to detect linearly modulated signals, at SNR around 0 dB, which may be used if the signals of interest do not exhibit second-order cyclostationarity. More generally this paper reviews the relevant aspects of the cyclostationary and HOCS theory, and shows their potential for spectrum sensing. Copyright © 2010 Julien Renard et al.

Probabilistic co-existence and throughput of cognitive dual-polarized networks

Diversity techniques for cognitive radio networks are important since they enable the primary and secondary terminals to efficiently share the spectral resources in the same location simultaneously. In this paper, we investigate a simple, yet powerful, diversity scheme by exploiting the polarimetric dimension. More precisely, we evaluate a scenario where the cognitive terminals use cross-polarized communications with respect to the primary users. Our approach is network-centric, i.e., performance of the proposed dual-polarized system is investigated in terms of link throughput in the primary and the secondary networks. In order to carry out this analysis, we impose a probabilistic co-existence constraint derived from an information-theoretic approach, i.e., we enforce a guaranteed capacity for a primary terminal for a high fraction of time. Improvements brought about by the use of our scheme are demonstrated analytically and through simulations. In particular, the main simulation parameters are extracted from a measurement campaign dedicated to characterization of indoor-to-indoor and outdoor-to-indoor polarization behaviors. Our results suggest that the polarimetric dimension represents a remarkable opportunity, yet easily implementable, in the context of cognitive radio networks.

A comprehensive channel model for UWB multi-sensor multi-antenna body area networks

Body area networks consist of a number of biological sensors communicating over the air with a central sink placed in close proximity of the human body. A promising solution is to use multisensor multiantenna ultrawideband architecture; each sensor carries one antenna, while the central sink supports an antenna array. In this paper, a complete analytical channel model has been developed for the on-body diffracted waves mechanism. It builds on the existing IEEE 802.15.4a standard channel model and offers an innovative space-time correlation model.

Cross-layering between physical layer, medium access control, and routing in wireless ad-hoc networks

Routing is a key issue in wireless ad-hoc networks. The goal of an efficient routing strategy is to set up routes so that the overall quality of communications will be the best possible. While the Open Systems Interconnection (OSI) reference model advocates for a clear separation of routing, access, and physical layers, in this paper we show that in scenarios with strongly faded communications, cross-layer interactions have to be carefully considered. More precisely, we compare the performance of the Ad-hoc On-demand Distance Vector (AODV) routing algorithm with that of one of its physical layer-oriented variants, denoted as AODVφ. It will be clearly shown that no single routing strategy is always optimal and that an intelligent information fusion at the Medium Access Control (MAC) level can be very beneficial. Copyright © 2012 Inderscience Enterprises Ltd.

Coupled reverberation chambers for emulating MIMO channels

This article describes a new experimental set up for emulating MIMO channels, based on two mode stirred reverberation chambers (MSRC), coupled together with a waveguide. This guide allows one to control the order of the channel diversity, the chambers generating a Rayleigh environment. This set up could thus be used to perform tests of MIMO communication systems in perfectly defined environments and thus under reproducible conditions. After a brief recall of the advantages and drawbacks of using a single reverberation chamber, we describe the theoretical approach and the experimental results for the analysis of two coupled chambers. © 2009 Académie des sciences.

Polarization measurements and modeling in indoor NLOS environments

Cross-polarized antenna systems are an attractive way to reduce equipment size while maintaining low interantenna correlation. In this paper, the polarization behaviour of indoor channels is investigated. A measurement campaign has been conducted at 3.6 GHz for a dual-polarized transmitter and a tri-polarized receiver in a non-Line-of-Sight (NLOS) scenario. The spatial and delay characteristics are extracted using a pertap beamforming algorithm. Distinct paths are isolated and the polarization of each wave is expressed in terms of its spherical components. The cross-polarization discrimination (XPD) of the wave is investigated as a function of its physical propagation parameters. The XPD of the wave is shown to be sensitive to spatial characteristics, while being insensitive to delay.

Multi-polarized channel statistics for outdoorto- indoor and indoor-to-indoor channels

Compared to classical spatial MIMO wireless systems, cross-polarized MIMO systems are an interesting way to reduce equipment size while reducing the inter-antenna correlation. Cross-Polar Discrimination (XPD) and Co-Polar Ratio (CPR) are two important parameters describing multi-polarized channels. In this paper, the behavior of these parameters is investigated for different observation scales. A measurement campaign has been performed in both Outdoor-to-Indoor and Indoor-to-Indoor scenarios, at a frequency of 3.5GHz. Small-scale variations of XPD and CPR are analyzed in different spatial positions. The distance-related and large-scale variations of XPD and CPR are also investigated and a model is deduced.

2009

Multipolarized MIMO channel characteristics

MIMO technologies enable high communication data rates, but suffer from the large antenna spacing that is required to achieve sufficiently low inter-antenna correlation. Cross-polarized antenna systems resolve this problem by using perpendicular antennas. Correlation is reduced while keeping antennas co-located. Inter-antenna correlation and cross-polar discrimination (XPD) are two fundamental parameters of these polarized antenna systems. This paper proposes an analytical channel model, from which closed-form solutions for the correlation coefficient and the XPD are deduced. The environment is supposed to have a truncated Laplacian power azimuth spectrum that is widely used in standardization bodies. The receiving di- or tri-pole antenna can have random orientation. The correlation and the XPD show to be highly sensitive to receiver orientation, azimuth spread and environment depolarization behavior. Measurements have been conducted at 3.5 GHz to validate the solution obtained. Good agreement is achieved when comparing theoretical curves and experimental results for different receiver orientations, both for the correlation coefficient and the XPD. © 2009 IEEE.

Small-scale variations of cross polar discrimination in ricean fading channels

Analytical angular correlation function in a mode-stirred reverberation chamber

Cross-layering between physical layer and routing in wireless ad-hoc networks

Routing is a key issue in wireless ad-hoc networks. The goal of an efficient routing strategy is to set up routes so that the overall quality of communications will be the best possible. While the Open Systems Interconnection (OSI) reference model advocates for a clear separation of routing, access, and physical layers, in this paper we show that in scenarios with faded communications, cross-layer interactions have to be carefully considered. More precisely, we compare the performance of the Ad-hoc On-demand Distance Vector (AODV) routing algorithm with that of one of its physical layer-oriented variants, denoted as AODVφ . It will be clearly shown that no single routing strategy is always optimal and that an intelligent adaptation should be performed. © 2009 Springer-Verlag Berlin Heidelberg.

2008

Delay spread and coherence bandwidth in a reverberation chamber

2007

On-body propagation velocity estimation using ultra-wideband frequency-domain spatial correlation analyses

2006

Statistical study of NLOS-mutlipath in urban canyons

Ultra wideband channel model for communication around the human body

Using ultra-wideband (UWB) wireless sensors placed on a person to continuously monitor health information is a promising new application. However, there are currently no detailed models describing the UWB radio channel around the human body making it difficult to design a suitable communication system. To address this problem, we have measured radio propagation around the body in a typical indoor environment and incorporated these results into a simple model. We then implemented this model on a computer and compared experimental data with the simulation results. This paper proposes a simple statistical channel model and a practical implementation useful for evaluating UWB body area communication systems. © 2006 IEEE.

An ultra wideband body area propagation channel model

Body worn wireless sensors for monitoring health information is a promising new application. In developing these sensors, a communication channel model is essential. However, there are currently few measurements or models describing propagation around the body. To address this problem, we have measured electromagnetic waves near the torso and derived relevant statistics. We find that components diffracting around the body are well modeled using correlated log normal variables, and a Nakagami-m distribution can be used to incorporate the influence of arm motions. We have implement this model and evaluated it in terms of important communication metrics. This paper describes body area propagation statistics and proposes a suitable computer model implementation. © 2006 IEEE.

Electromagnetic fields estimation using spatial statistics

The spatial statistics formalism is applied to electromagnetic fields analysis. Fields are considered as realizations of a random function. Their spatial structure is studied by a method known as variographic analysis. To infer unknown values of the fields, an interpolation method called kriging is then used. It is shown how kriging can be performed on experimental or numerical data to speed up the fields estimation process.

Statistical study of NLOS-multipath in urban canyons

2005

Modeling of NLOS-multipath in urban canyons for GNSS applications

WLAN deployment strategy for best indoor performance

2004

Statistical response of antennas under uncorrelated plane wave spectrum illumination

Electromagnetic wave propagation and coupling

Channel model for wireless communication around the human body

Electromagnetic coupling to transmission lines under complex illumination

A wavelet-based approach for disturbance line identification in printed circuit boards

Disturbance lines identification in printed circuit boards by means of wavelets

Etude des performances des réseaux WLAN en milieu indoor

2003

A volume/surface potential formulation of the method of moments applied to electromagnetic scattering

Spatial correlation functions for fields in three-dimensional rayleigh channels

Starting from a continuous plane-wave representation of the electric and magnetic fields, spatial auto- and cross-correlation functions for field components and their modulus are derived in the three-dimensional Rayleigh channel case. It is shown that existing results, generally relying on two-dimensional or isotropic models, can significantly differ from those obtained thanks to a three-dimensional approach.

Spatial correlation functions for fields in three-dimensional rayleigh channels -abstract

Starting from a continuous plane-wave representation of the electric and magnetic fields, spatial auto- and cross-correlation functions for field components and their modulus are derived in the three-dimensional Rayleigh channel case. It is shown that existing results, generally relying on two-dimensional or isotropic models, can significantly difffer from those obtained thanks to a three-dimensional approach. © Journal of electromagnetic waves and applications. All Rights reserved.

2002

Indoor propagation analysis for broadband personal area networks

2001

A potential integral equation method for electromagnetic scattering by penetrable bodies

A method for computing the electromagnetic scattering by general inhomogeneous penetrable bodies is presented. The method is based on the volume equivalence principle and it uses the electromagnetic potentials as unknowns. The resulting coupled integral equations system is solved by the method of moments in combination with cubical and curvilinear meshes in the special case of purely dielectric scatterers. To show the accuracy of the method, numerical results of the transmitted and of the scattered fields are compared with existing analytical and experimental results.

2000

An extrapolated version of the Gordon's interpolation applied to the method of moments

The use of the transfinite interpolation in the method of moments applied to electromagnetic scattering by dielectric cylinders

The method of moments (MoM) solution of electromagnetic scattering presents two major numerical difficulties: the number of unknowns and the computation time necessary to calculate the matrix elements. To circumvent these problems, a MoM using the transfinite interpolation and a reduced integration scheme is presented here. The so-called h and p versions of the new method are applied to the scattering of an electromagnetic wave by an infinite dielectric cylinder (TM case) in the Richmond's formulation. The transfinite and classical methods are compared in terms of the convergence rates of the radar cross section and of the total electric field inside the dielectric. The results confirm the superiority of the new schemes as predicted by the theory.

The use of transfinite elements in the method of moments applied to electromagnetic scattering by dielectrics cylinders

Simulating Mimo systems in reverberation chambers

The use of tansfinite elements in the methods of moments applied to electromagnetic scattering by dielectric cylinders

1999

Use of transfinite elements in the methods of moments applied to electromagnetic scattering by dielectric cylinders

Many electomagnetic applications involve the analysis of the scattering of an electromagnetic wave by a dielectric body. This kind of problem is often carried out using the method of Moments (MoM) applied to volume or surface integral equations. Unfortunately, this method seems unreliable in certain cases. In this paper, to evaluate this problem, we developed a two-dimensional MoM based on the volume-surface integral equation and employing the transfinite elements theory which provides a powerful tool to study separately the effects of the geometrical modeling and those of the interpolation schemes for all the methods (cell shapes and interpolation schemes) encountered in the conventional MoM.

1998

Why are some patients with onychomycosis still not responding to the newer antifungal agents ?

see more DI-fusion

Articles dans des revues avec comité de lecture

2024

Large-Scale Crowd Size Classification with a Wi-Fi-Based Passive Radar

We investigate the problem of large-scale crowd size classification with a Wi-Fi-based passive radar for crowds of up to 100 people, with classes corresponding to intervals of numbers of people. A Convolutional Neural Network (CNN) operating on radar range-Doppler maps (RDMs) is used as a classification algorithm. We propose a crowd simulator based on the method of moments (MoM) in electromagnetics able to generate representative RDMs for large crowds. We show that these MoM simulation data can be used to design the classification algorithms and tune their hyperparameters. We also investigate the limitations of the MoM simulation data in training the classification algorithms for subsequent application on experimental data. Crowd size classification is performed with high accuracy on real-life experimental measurements of a crowd with up to 100 people, obtained by channel estimation with 802.11ax-compliant High-Efficiency Long Training Fields transmitted by a Wi-Fi-based passive radar setup featuring two Universal Software Radio Peripherals X310.

Joint metrics for EMF exposure and coverage in real-world homogeneous and inhomogeneous cellular networks

This paper evaluates the downlink performance of cellular networks in terms of coverage and electromagnetic field exposure (EMFE), in the framework of stochastic geometry. The model is constructed based on datasets for sub-6 GHz macro cellular networks but it is general enough to be applicable to millimeter-wave networks as well. On the one hand, performance metrics are calculated for β-Ginibre point processes which are shown to faithfully model a large number of motion-invariant networks. On the other hand, performance metrics are derived for inhomogeneous Poisson point processes with a radial intensity measure, which are shown to be a good approximation for motion-variant networks. For both cases, joint and marginal distributions of the EMFE and the coverage, and the first moments of the EMFE are provided and validated by Monte Carlo simulations using realistic sets of parameters from two sub-6 GHz macro urban cellular networks, i.e., 5G NR 2100 (Paris, France) and LTE 1800 (Brussels, Belgium) datasets. In addition, this paper includes the analysis of the impact of the network parameters and discusses the achievable trade-off between coverage and EMFE.

A CFO-assisted algorithm for wireless time-difference-of-arrival localization networks

Localization of wireless transmitters is traditionally done using Radio Frequency (RF) sensors that measure the propagation delays between the transmitter and a set of anchor receivers. One of the major challenges of wireless localization systems is the need for anchor nodes to be time-synchronized to achieve accurate localization of a target node. Using a reference transmitter is an efficient way to synchronize the anchor nodes Over-The-Air (OTA), but such algorithms require multiple periodic messages to achieve tight synchronization. In this paper, we propose a new synchronization method that only requires a single message from a reference transmitter. The main idea is to use the Carrier Frequency Offset (CFO) from the reference node, alongside the Time of Arrival (ToA) of the reference node messages, to achieve tight synchronization. The ToA allows the anchor nodes to compensate for their absolute time offset, and the CFO allows the anchor nodes to compensate for their local oscillator drift. Additionally, using the CFO of the messages sent by the reference nodes and the target nodes also allow us to estimate the speed of the targets. The error of the proposed algorithm is derived analytically and is validated through controlled laboratory experiments. Finally, the algorithm is validated by realistic outdoor vehicular measurements with a software-defined radio testbed.

A spatial data focusing and generalized time-invariant frequency diverse array approach for high precision range-angle-based geocasting

A novel unified frequency diverse array (FDA) and spatial data focusing (SDF) approach is proposed to simultaneously overcome time-variance and precision constraints of conventional FDA in geocasting, i.e., spatially confined broadcasting, scenarios. This paper describes a free space FDA-based SDF (FDA-SDF) system model for 2-dimensional range-angle-based focusing, including a generalized multi-purpose baseband approach for time-invariant FDA, complemented by SDF processing for improved spatial focusing precision and reduced array size. Comprehensive analytical derivations - general for any frequency offset configuration - describe the geographical FDA-SDF properties and design rules, such as geocast delivery zone steering, location, uniqueness, and size. Simulations of the proposed scheme validate theoretical derivations and demonstrate FDA-SDF's superior spatial precision and minimal design complexity. In particular, using novel alternating logarithmic frequency offsets, a 3-antenna FDA-SDF setup is shown to match the radial and azimuthal precision of its beamforming-based FDA counterpart using, respectively, 64 and 24 antennas.

2023

Direct tracking of a wireless transmitter based on rao-blackwellized point mass filter

In this article, we address the problem of direct tracking of a wireless transmitter. That is, the inputs given to the Bayesian filter are the received baseband signals instead of pre-computed ranges or angles. We first propose to use the Rao-Blackwellized Point Mass Filter (RBPMF) to solve such a tracking problem. As such, the resulting tracking solution is still computationally expensive. Therefore, we propose an approach for reducing the computational cost of the RBPMF. More precisely, we replace the prediction step by the one of the Linear Kalman Filter (LKF). This combination helps to avoid expensive operations such as the weight convolution in the prediction step. In addition, it also allows complexity reductions in the correction step. As a result, the complexity is reduced by one order of magnitude compared to the original RBPMF. We compare our approach to representative direct-tracking methods, based on Iterative Extended Kalman Filter (IEKF) and Particle Filter (PF). The proposed solution has lower and comparable localization error compared to IEKF and PF, respectively. In addition, the proposed solution is of slightly less complexity than PF. However, the complexity reduction is significant compared to the conventional RBPMF.

2022

A multi-antenna super-resolution passive Wi-Fi radar algorithm

In recent years, Wi-Fi has become the main gateway that connects users to the internet. Considering the availability ofWi-Fi signals, and their suitability for channel estimation, IEEE established the Wi-Fi Sensing (WS) Task Group whose purposeis to study the feasibility of Wi-Fi-based environment sensing. However, Wi-Fi signals are transmitted over limited bandwidthswith a relatively small number of antennas in bursts, fundamentally limiting the range, Angle-of-Arrival and speed resolutions.This paper presents a super-resolution algorithm to perform the parameter estimation in a quasi-monostatic WS scenario. Theproposed algorithm, RIVES, estimates the range, Angle-of-Arrival and speed parameters with Vandermonde decomposition ofHankel matrices. To estimate the size of the signal subspace, RIVES uses a novel Model Order Selection method which eliminatesspurious noise targets based on their distance to the noise and signal subspaces. Various scenarios with multiple targets aresimulated to show the robustness of RIVES. In order to prove its accuracy, real-life indoor experiments are conducted with humantargets by using Software Defined Radios.

MAC address anonymization for crowd counting

Research has shown that counting WiFi packets called probe requests (PRs) implicitly provides a proxy for the number of people in an area. In this paper, we discuss a crowd counting system involving WiFi sensors detecting PRs over the air, then extracting and anonymizing their media access control (MAC) addresses using a hash-based approach. This paper discusses an anonymization procedure and shows time-synchronization inaccuracies among sensors and hashing collision rates to be low enough to prevent anonymization from interfering with counting algorithms. In particular, we derive an approximation of the collision rate of uniformly distributed identifiers, with analytical error bounds.

Physical layer security in an OFDM time reversal SISO communication with imperfect channel state information

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the ϵ-achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication.

Self-synchronization based localization of a time-misaligned transmitter in cellular networks

Geo-localization services are an important functionality in cellular networks. Besides,the use of Ultra Dense Networks and small cells, in current and future cellular networks, greatly increases the complexity of centralized localization approaches. Consequently, we propose a self-synchronization Positioning Estimation (SSPE) algorithm that estimates the transmitter position in a distributed fashion.The proposed SSPE algorithm reaches consensus for the posterior distribution of the transmitter position rather than on the final estimates. Such consensus ensures that the proposed SSPE algorithm converges to the centralized Direct Positioning Estimation (DPE) approach, which has the best performance of all localization approaches. We show that the proposed algorithm is related to the Iterative Positioning Estimation (IPE) algorithm, since both exploit the self-synchronization mechanism. As a result, the improvements and extensions for IPE, previously studied in other works, can be directly applied to the proposed SSPE algorithm. In addition, the proposed algorithm is able to localize the transmitter even when it is not time synchronized with the network as it is usually the case. The performance of the algorithms is numerically assessed through Monte-Carlo simulations by the mean distance error and mean range offset error. Finally, we not only show that our approach gets close to the DPE performance after a few iterations, but also that it converges for different logical network configurations.

Monitoring large crowds with WiFi: a privacy-preserving approach

This paper presents a crowd monitoring systembased on the passive detection of probe requests. The systemmeets strict privacy requirements and is suited to monitoringevents or buildings with a least a few hundreds of attendees. Wepresent our counting process and an associated mathematicalmodel. From this model, we derive a concentration inequalitythat highlights the accuracy of our crowd count estimator. Then,we describe our system. We present and discuss our sensorhardware, our computing system architecture, and an efficientimplementation of our counting algorithm—as well as its spaceand time complexity. We also show how our system ensures theprivacy of people in the monitored area. Finally, we validateour system using nine weeks of data from a public libraryendowed with a camera-based counting system, which generatescounts against which we compare those of our counting system.This comparison empirically quantifies the accuracy of ourcounting system, thereby showing it to be suitable for monitoringpublic areas. Similarly, the concentration inequality provides atheoretical validation of the system.

2021

OFDM-based spatial data focusing for wireless physical layer geocasting in multipath channels

OFDM-based spatial data focusing (OFDM-SDF) is proposed as a novel means of performing wireless physical layer geocasting, i.e. spatially confined broadcasting. It is shown that this approach overcomes beamforming and directional modulation (DM) limitations by exhibiting higher spatial precision with a reduced number of antennas and offering uncoupled range-angle-dependent focusing. This paper describes the OFDM-SDF system model for multipath channels, including multipath robust equalization, design rules for steering phases and sidelobe mitigation, analytical geocast delivery zone derivation, and optimized symbol mapping. Using density-based clustering of the spatial bit error rate distribution, a procedure for identifying a practical geocast delivery zone and evaluating its precision and connectivity is proposed. OFDM-SDF's performance and multipath robustness are evaluated through Rice channel simulations as a function of the Rice factor. In particular, it is shown that a 2-antenna OFDM-SDF array matches the radial and angular precision of, respectively, a 6 and 12-antenna DM array in recent literature, while robustness is ensured for 5G small cell channels.

Joint in-band full-duplex communication and radar processing

In-band full-duplex (IBFD) technology has the potential to not only double the communication throughput but also enable additional capabilities. The fusion of radar and communication subsystems is an excellent example of how IBFD facilitates integration of radar functionality into a communication system. Developing and analysis of a joint IBFD radar-communication (RadCom) system is at the core of this article. This system derives the range-Doppler image from the state of an adaptive filter, which already exists in a typical IBFD transceiver. Our approach is waveform-independent and reuses the radio frequency (RF) front-end while it requires little additional digital logic. The proposed system is prototyped to assess the modem-radar coexistence in a real-world IBFD communication link budget. Employing the prototyped system, we quantify the additional required logic resources and investigate whether such a RadCom approach dictates a tradeoff between the two intended functionalities. The experimental results show that the proposed solution enables a communication system to detect targets within 20 m while maintaining an IBFD link with another communication node

Indoor tracking of multiple individuals with an 802.11ax Wi-Fi-based multi-antenna passive radar

We investigate indoor human multi-target tracking in cartesian coordinates based on range, Doppler and Angle-of-Arrival measurements obtained with a four-antenna passive bistatic radar capturing 802.11ax Wi-Fi signals. A reference antenna selection method is described to perform angle processing correctly when dealing with target detection diversity among antennas. The tracking is performed by an Unscented Kalman Filter (UKF) to handle the non-linear relation between the measurement space and the state space. A Joint Probabilistic Data Association Filter is coupled to the UKF to handle the data association between tracks and measurements when dealing with multiple targets. Simulations are performed to determine the tracking parameters under heavy constraints and identify key scenarios. An experimental setup is built using Universal Software Radio Peripherals, featuring an over-the-air phase calibration for angle processing with an anchor antenna. It is used to validate the proposed single and multi-target tracking scheme.

A stochastic geometry approach to EMF exposure modeling

Downlink exposure to electromagnetic fields due to cellular base stations in urban environmentsis studied using the stochastic geometry framework. A two-dimensional Poisson Point Process is assumedfor the base station distribution. Mathematical expressions of statistics of exposure are derived from a simplepropagation model taking into account the height of the base stations. The error on exposure made by takinga limited number of base stations, instead of the whole set, is quantified. The relative impact of the modelparameters on the statistics of exposure is highlighted. The method is then applied and the model parametersare calibrated using experimental data obtained by drive-tests in two Brussels municipalities, in Belgium,for the 2100 MHz and 2600 MHz frequency bands. It is shown that the proposed model fits experimentalvalues, paving the way to a new methodology to assess general public exposure to electromagnetic fields,for any frequency band. An insight is given on how to apply the methodology to a real case without accessto experimental data.

Super resolution passive radars based on 802.11 ax Wi-Fi signals for human movement detection

Passive Radars based on Wi-Fi signals provide an excellent opportunity for human sensing without violating the privacy of individuals. Due to the limited integration time of Wi-Fi bursts and relatively low bandwidths, Fourier Transform-based methods do not provide the required accuracy. Herein, a Wi-Fi-based passive radar algorithm is proposed for indoor human movement detection with super resolution which relies on the ESPRIT algorithm to estimate range/speed parameters from limited number of measurements. To determine the number of targets in the environment, a new Model Order Selection (MOS) method is proposed which exploits the orthogonality between the basis vectors of signal and noise subspaces obtained from the sample covariance matrix of the measurements. The new MOS method along with the proposed algorithm are numerically analysed and compared with other existing methods. Finally, the performance of the algorithm is experimentally validated in indoor conditions.

2020

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its goodspatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabricationfurther leverage the use of TR wideband communication systems. The TR is generally carried out inthe time domain and the focusing effect of TR comes from the use of a high transmit rate back-offfactor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequencydivisionmultiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TRprecoding in combination with OFDM communication systems. Furthermore, most existing works onFD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. Inthis paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output(MISO) OFDM systems. In particular, we compare the communication performance of such system atthe intended position and that at the unintended position. Based on the analysis, we demonstrate thatincreasing the BOF and/or the number of transmit antennas significantly improves the focusing effectat the intended position. In contrast, the unintended positions receive less useful power. We deriveapproximated mean-square-error (MSE) expressions of equalized received signals at both intendedand unintended positions. The subsequent focusing gain is presented as a function of the BOF andthe number of antennas, enabling us to gain insights in the contributions of each parameter to thesystem performance. Numerical simulations with multi-path Rayleigh fading channels are carried outto validate the MSE expressions.

Crowd forecasting based on WiFi sensors and LSTM neural networks

To ensure effective management and security in largescale public events, it is imperative for the event organizers to beaware of potentially critical crowd densities. This paper, therefore,presents a solution to the above problem in terms of WiFi basedcrowd counting and LSTM neural network based forecasting.Monitoring of an actual event organized in Brussels has beendescribed, wherein crowd counts are obtained using WiFi sensorsin a privacy-preserved manner. The time-stamped crowd countsare used to develop univariate time-series, which are in-turnutilized for forecasting. Five different LSTM models are utilizedfor crowd time-series forecasting and analyzed for theirsuitability. A random walk model is used as reference forperformance assessment. Among different LSTM models,Convolutional LSTM delivered the best performance. Overallresults and analysis show that the developed system is suitable forcrowd monitoring.

Secrecy capacity of FBMC-OQAM modulation over frequency selective channel

This letter studies the Information-theoretic secrecy capacity of an Offset-QAM-based filterbank multicarrier (FBMC-OQAM) communication over a wiretap frequency selective channel. The secrecy capacity Is formulated as an optimization problem which has a closed-form solution In the high signal-to-noise ratio (SNR) regime. Two of the most common equalization strategies In FBMC-OQAM are considered, namely, single-tap and multi-tap equalization. For the sake of comparison, we also consider the secrecy capacity of a generic modulation and a cyclic prefix-orthogonal frequency division multiplexing (CP-OFDM) modulation. As a result, we find that FBMC-OQAM Is particularly competitive for medium-to-long burst transmissions.

Crowd monitoring

With the growing concerns over public safety, the importance of crowd monitoring is being realized by varioussecurity and event management agencies worldwide. Estimation of crowd dynamics can help such agencies in prevention ofany unanticipated accidents or issues. Research on crowd monitoring has been underway since the past few decades. Conventionalcrowd monitoring systems mainly rely on computer vision approach. Due to predominant use of videos/ imagesequences, the existing techniques may raise data privacy concerns. This has led to development of new crowd monitoringtechniques which are privacy preserving and require minimum public participation. This paper aims to serve as a single andsufficient source of information to the concerned researchers on various aspects of crowd monitoring and also provide futuredirections which can be helpful in developing advanced crowd monitoring techniques.

Iterative NDA Positioning Using Angle-of-Arrival Measurements for IoT Sensor Networks

Various positioning techniques have been developed to localize Internet of Things (IoT) devices accurately. Because IoT communications are often narrowband, efficient localization can be achieved by deducing the device position from the estimated signal Angle of Arrival (AOA) at multiple arrays of antennas. It has recently been shown that significant accuracy gains can further be obtained by iterating between the AOA estimation and multi-lateration steps. However, the existing method relies on the knowledge of the transmitted signal (Data-Aided (DA) estimation) which makes it impractical for narrowband communications where the preamble is short. Non-Data-Aided (NDA) estimation is recommended to improve the positioning accuracy for low capacity IoT sensors. This paper proposes an NDA iterative (NDA-It) algorithm using AOA measurements to determine the position of an IoT sensor. Simulation results show that the proposed algorithm significantly outperforms the DA-It in a Bluetooth Low Energy (BLE) context because it can use a much higher number of samples (snapshots); however, it needs more iterations to converge. The computational complexity analysis proves the competitiveness of the proposed NDA-It. The performance of the algorithms is further investigated in multipath and Non-Line-of-Sight (NLOS) propagation environments. Finally, an experimental setup is built to validate the performance of the proposed algorithms.

Pilot-based detection for DVB-T passive coherent location radars

This study investigates the feasibility of pilot-based detection for passive coherent location (PCL) radars exploiting digital video broadcasting-terrestrial (DVB-T) signals. The DVB-T signal is formed by two parts: a data signal and a pilot signal. The parameters of the pilot signal are known thanks to the DVB-T standards that permit the generation (at the receiver) of the pilot signal. The pilot recovery technique, based on the known DVB-T standard, is utilised in DVB-T based PCL systems to reduce the number of the reception channels by considering a locally generated pilot signal as a reference signal. Consequently, no reference channel is required, which reduces the cost and the complexity of the resulting PCL system. In this work, the authors propose a signal processing method to achieve this goal. They consider theoretical analysis, simulations and real-data results to validate the feasibility of such a system.

In-band full-duplex radar-communication system

In-band full-duplex (IBFD) technology is a promising solution to boost the throughput of wireless networks. To bring IBFD to reality, the modem has to cancel the self-interference (SI) signal, which includes the strong direct Tx leakage signal and the weaker reflected Tx signal from the surroundings. Adaptive analog and digital SI cancelation schemes have been proposed. It becomes then interesting to understand, although, how the echoed SI could be exploited for enabling radar functionality while reusing the waveform and the already-existing hardware. This article formulates the monostatic radar system model starting from the communication system model. Beside simulation-based assessment, the performance is also evaluated by an IBFD system prototype, which consists of both analog and digital SI canceller modules, enabling >85 dB Tx-Rx isolation. The system is enhanced with Doppler radar functionality, reusing as much as possible the existing IBFD functional blocks. The experimental result shows the accuracy of the proposed system to measure the velocity of mobile objects at various speeds between 0.2 and 1 m/s while the device is simultaneously served as a node to perform in-band bidirectional communication. This ability suits the proposed system for a broad spectrum of opportunistic remote sensing applications, such as body and hand gesture detection.

Forecasting Crowd Counts With Wi-Fi Systems: Univariate, Non-Seasonal Models

Recently, event organizers and researchers have advocated the development of novel technologies supporting crowd control, notably for public events. This paper presents a crowd monitoring system based on probe requests (PRs), which are Wi-Fi packets smartphones send periodically. By estimating the global rate at which nearby smartphones send PRs, Wi-Fi sensors can estimate crowd counts. The core contribution of this paper is a computationally tractable method that forecasts crowd counts up to thirty minutes in the future, with forecasts becoming available as soon as two hours of data are available. The forecasting method relies on autoregressive integrated moving average (ARIMA) models. Contributions also include two methods that compute prediction intervals associated with the forecasts, one of which is based upon generalized autoregressive conditional heteroskedasticity (GARCH) models. Recent real-world data from Winter Wonders 2018/2019 (an event that took place in Brussels, Belgium) notably demonstrate that the proposed forecasting method outperforms its immediate variations as well as baseline models (i.e., random walk models).

CSI-based versus RSS-based Secret-Key Generation under Correlated Eavesdropping

Physical-layer security (PLS) has the potential to strongly enhance the overall system security as an alternative to or in combination with conventional cryptographic primitives usually implemented at higher network layers. Secret-key generation relying on wireless channel reciprocity is an interesting solution as it can be efficiently implemented at the physical layer of emerging wireless communication networks, while providing information-theoretic security guarantees. In this paper, we investigate and compare the secret-key capacity based on the sampling of the entire complex channel state information (CSI) or only its envelope, the received signal strength (RSS). Moreover, as opposed to previous works, we take into account the fact that the eavesdropper’s observations might be correlated and we consider the high signal-to-noise ratio (SNR) regime where we can find simple analytical expressions for the secret-key capacity. As already found in previous works, we find that RSS-based secret-key generation is heavily penalized as compared to CSI-based systems. At high SNR, we are able to precisely and simply quantify this penalty: a halved pre-log factor and a constant penalty of about 0.69 bit, which disappears as Eve’s channel gets highly correlated.

2019

Experimental demonstration of the tradeoff between chromatic dispersion and phase noise compensation in optical FBMC/OQAM communication systems

Analysis of residual CFO impact on downlink massive MISO systems

Massive antenna technologies provide a good power focusing gain for emerging communication systems. They can easily be integrated into an orthogonal frequency-division multiplexing (OFDM) system. However, OFDM is known to be prone to carrier frequency offset (CFO) due to the loss of the orthogonality among OFDM subcarriers. In this Letter, the authors investigate the impact of residual CFO (RCFO) on the downlink performance of massive multiple-input single-output (MISO) OFDM systems using matched-filter (MF) and maximum-ratio-transmission (MRT) precoders. Particularly, the exact mean-square-error (MSE) expressions of the equalised received signal of both MF and MRT systems are derived. Numerical simulations with Rayleigh fading channels are carried out to validate the analysis. The results show that the RCFO causes a MSE plateau compared to the ideal case of no CFO.

Efficient equalization of time-varying channels in MIMO OFDM systems

The orthogonality of the cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) modulation is ensured as long as the channel can be assumed constant across the duration of one CP-OFDM symbol period. Unfortunately, this assumption may not hold anymore for a large variety of emerging scenarios with mobility, high carrier frequency and multiple carrier frequency offsets. To tackle this issue, we propose a novel equalization structure. In contrast to existing works in the literature, the equalizer is obtained by considering a Taylor approximation of the ideal time-varying channel equalizer function. This results in an extremely simple implementation only consisting of per-subcarrier multiplications and FFT/IFFT operations. The general form of the equalizer is particularized to two specific cases: zero forcing and linear minimum mean squared error. Furthermore, the implementation complexity of the equalizers is computed and an analytical formula is proposed to efficiently evaluate their performance. Finally, numerical results demonstrate the efficiency of the proposed receivers as compared to the ideal one and previous works.

Performance analysis of the reference signal reconstruction for DVB-T passive radars

The present work investigates the reference signal reconstruction method for passive coherent location (PCL) radars exploiting Digital Video Broadcasting-Terrestrial (DVB-T) signals. The reference signal reconstruction improves the signal-to-noise ratio (SNR) of the reference signal, which enhances the detection probability for DVB-T based PCL systems. The existing approach for the reference signal reconstruction is performed by demodulating the received reference signal to retrieve the transmitted quadrature amplitude modulation (QAM) symbols and modulate them to obtain the reconstructed signal. For low SNR values, the QAM symbol detection is accompanied with a significant error, which leads to a mismatch between the reconstrued signal and the exact one. Consequently, the expected improvement due to the signal reconstruction is limited. In this work, we propose an optimum reconstruction method which filters the detected QAM symbol prior to the remodulation step. To validate the proposed method, we retrieved analytic expressions for the detection and false-alarm probabilities and verified them through Monte-Carlo simulations. In addition, real-data sets were used to evaluate the proposed method performances. The results show that the proposed method outperforms the conventional one by extending the feasibility of the reference signal reconstruction for low SNR values.

Performance analysis of frequency domain precoding time-reversal MISO OFDM systems

Time reversal (TR) recently emerged as an interestingcommunication technology capable of providing a goodspatio-temporal signal focusing effect. New generations of largebandwidthdevices with reduced cost leverage the use of TRwideband communication systems. TR can easily be integratedinto an orthogonal frequency-division multiplexing (OFDM) systemby precoding the signal in the frequency domain. In thispaper, we first extend the frequency-domain TR precoding to rateback-off factors (BOFs) different than one. We secondly derive aclosed-form mean-square-error (MSE) expression of the receivedequalized symbols as a function of the BOFs and the numberof antennas. The derived MSE formula is validated numericallywith Rayleigh fading channels.

2018

Low complexity iterative localization of time-misaligned terminals in cellular networks

Recently, iterative localization has arisen as a promising approach to localize a Mobile Station (MS) in a cellular system. The conventional geo-location is obtained in a two-step approach: propagation delays are estimated and then the multi-lateration is responsible for the determination of the user position, based on the estimated delays. Iterative localization iterates between the two conventional steps to progressively refine delay estimates based on the position estimate available from the previous iterations. This localization scheme was seen to provide appealing performances compared to the two-step approach. It also seems to be computationally attractive with respect to direct localization that estimates the position using the digitized received signals directly. However, the iterative localization solution developed in literature relies on a strict time synchronization between MS and Base Stations (BSs). Moreover,the computational complexity of the iterative approach is not thoroughly compared to two-step and optimal solutions. This paper therefore proposes a new iterative localization method able to operate in a cellular system with time-misaligned terminals.We show by means of a detailed complexity analysis that the iterative positioning algorithm is one order of magnitude less complex than direct localization. Simulation results prove that the achievable performance after a few iterations approaches the performance of the direct localization solution.

Iterative RToF-based localization and time synchronization in WLAN-like systems

Iterative localization is currently arising as a solution to localize a Mobile Station (MS) in a cellular network. We recently showed that iterating between the conventional delay estimation and multi-lateration steps allows one to approach the performance of direct localization algorithms. Until now, the method has only been applied to the case of networks where the access points are perfectly synchronized with each other. In this letter, we present a localization and time synchronization iterative algorithm suitable for networks where access points are not synchronized. We show numerically that iterating between the two conventional steps brings a significant performance gain.

2 × 2 MIMO FBMC-OQAM signal transmission over a seamless fiber-wireless system in the W-band

Offset-QAM-based filterbank multicarrier (FBMC-OQAM) is an attractive modulation candidate to increase the spectral efficiency of radio-over-fiber and fiber-wireless systems. In this paper, we first propose a general framework for MIMO FBMC-OQAM transmission systems, including the description of algorithms for mitigation of the channel impairments. The proposed design is scalable in the number of transmitted streams and flexible as multiple asynchronous users can be accommodated. The proposed system is experimentally validated in a 2 × 2 MIMO FBMC-OQAM signal over a seamless fiber-wireless in the W-band for the first time. Satisfactory performance is confirmed for a data rate of up to 18 Gb/s in both downlink and uplink directions. Furthermore, algorithms previously proposed by the authors for channel estimation and phase noise compensation are experimentally validated, confirming their effectiveness. Finally, we show that, for a similar error vector magnitude performance, an FBMC-OQAM system can achieve a 25% higher throughput rate than a corresponding orthogonal frequency division multiplexing-based fiber-wireless system.

Performance analysis of linear receivers for uplink massive MIMO FBMC-OQAM systems

Offset-quadratic-amplitude-modulation-based filterbank multicarrier (FBMC-OQAM) has been shown to be a promising alternative to cyclic prefix-orthogonal frequency division multiplexing. More recently, the use of FBMC-OQAM has been proposed in combination with massive MIMO communications. In this context, it is interesting to study the overall effect of massive MIMO on the FBMC-OQAM intrinsic interference and its interaction with channel frequency selectivity. In this paper, the performance of an FBMC-OQAM uplink massive MIMO system is theoretically characterized in terms of the output mean squared error (MSE) of the estimated transmitted symbols and for three types of linear receivers, namely, zero forcer, linear minimum mean squared error, and matched filter. Using random matrix theory, the output MSE of these receivers is asymptotically characterized as the number of base station antennas N and the number of users K grow large, while keeping a finite ratio N/K. The obtained expressions allow to draw many conclusions, some of which were already noticed in the literature but not yet theoretically proven. First, the MSE becomes uniform across the frequency band as a result of the channel hardening effect. Second, it is shown that a good synchronization of the users is crucial in a massive MIMO scenario. Finally, if the users are well synchronized, the different terms that compose the MSE, such as noise, inter-user interference, and the distortion caused by the channel frequency selectivity, become negligible for large values of the ratio N/K. This effect was previously referred to as “self-equalization” in the literature

Virtual multi-antenna array for estimating the direction of a transmitter: system, bounds and experimental results

A method is proposed to estimate the direction of a radio-frequency transmitter with a mobile single-antenna receiver. By considering the received signal at several points along its trajectory, the receiver implicitly creates a virtual multiantenna array, which can be used to estimate the direction of the transmitter. Virtual arrays differ from conventional multiantenna arrays in two ways: 1) the position and orientation of each antenna in the virtual array depend on the movement of the receiver and are not known a priori; and 2) the local oscillator (LO) frequency offset between the transmitter and the receiver adds a phase offset to the signal received by each antenna of the virtual array, which must be estimated and compensated. The first problem is solved by using an inertial measurement unit, which can provide the relative position of the receiver for short time durations. The second problem is solved by estimating the LO frequency offset jointly with the direction of the transmitter by extending the MUSIC algorithm for multidimensional estimation. We investigate the Cramér-Rao lower bound of the proposed estimator, which provides some insights in the design of our system. We implement our system on a software-defined radio testbed and present some measurement results obtained in a controlled environment.

2017

Performance of emerging multi-carrier waveforms for 5G asynchronous communications

This paper presents an extensive and fair comparison among the most promising waveform contenders for the 5G air interface. The considered waveform contenders, namely filter-bank multi-carrier (FBMC), universal-filtered multi-carrier (UFMC), generalized frequency-division multiplexing (GFDM) and resource-block filtered orthogonal frequency-division multiplexing (RB-F-OFDM) are compared to OFDM used in 4G in terms of spectral efficiency, numerical complexity, robustness towards multi-user interference (MUI) and resilience to power amplifier non-linearity. FBMC shows the best spectral containment and reveals to be almost insensitive to multi-user interference. It however suffers from its bad spectral efficiency for short bursts and from its poor multiple input multiple output (MIMO) compatibility. GFDM reveals to be the most promising contender, with the best spectral efficiency and the smallest complexity overhead compared to OFDM. It is also the most resilient to multi-user interference after FBMC and is MIMO compatible as soon as the interference can be managed. UFMC and RB-F-OFDM are finally the closest to OFDM and benefit therefore from a better compatibility with existing systems, even if their performance is generally lower than FBMC and GFDM.

Advanced chromatic dispersion compensation in optical fiber FBMC-OQAM systems

We report on several methods for the chromatic dispersion (CD) compensation in optical fiber offset-QAM-based filterbank multicarrier (FBMC-OQAM) systems. We show that several equalization structures, initially proposed for wireless FBMC-OQAM systems, can also be applied to optical FBMC-OQAM systems to enhance the CD tolerance. The different CD compensation algorithms are numerically validated and compared to the conventional one tap equalizer in a 30-Gbaud optical FBMC system, in terms of performance and complexity. Considering a 1-dB optical signal-to-noise ratio penalty at a bit error rate of 3.8 × 10-3 and 256 subcarriers, the results show that the maximum CD tolerance of the frequency spreading method can be enhanced by a factor 10 and 30 for 4-OQAM and 16-OQAM modulations, respectively, compared to that of the conventional one-tap equalizer, at the cost of higher complexity. Even though the other CD compensation methods provide a reduced CD tolerance compared to the frequency spreading method, they require less complexity and hence can be good alternatives.

Parallel equalization structure for MIMO FBMC-OQAM systems under strong time and frequency selectivity

Offset-QAM-based filterbank multicarrier (FBMC-OQAM) has been shown to be a promising alternative to cyclic prefix-orthogonal frequency division multiplexing for the future generation of wireless communication systems. Unfortunately, as the channel gets more selective in time and frequency, the FBMC-OQAM orthogonality is progressively destroyed and distortion appears after the demodulation process at the receiver. While channel frequency selectivity has been very widely studied in the FBMC literature, the impact of time selectivity of the channel has not received that much attention. In this paper, the effect of the two types of selectivity on a multiple-input multiple-output (MIMO) FBMC system is characterized and a parallel equalization structure that can compensate for the doubly dispersive nature of the channel is proposed. This design uses multiple analysis filterbanks and extends previous approaches that were dealing only with channel frequency selectivity. A theoretical approximation of the remaining distortion after equalization is given. The study is performed for a general MIMO system but can also be particularized to single-input single-output systems. Simulation results demonstrate the high efficiency of the proposed receiver structure and the accuracy of the theoretical approximations is verified.

Efficient chromatic dispersion compensation and carrier phase tracking for optical fiber FBMC/OQAM systems

Offset quadratic-amplitude modulation (QAM) based filterbank multicarrier (FBMC/OQAM) is an attractive candidate to improve the spectral containment of optical fiber communication systems, especially when considering a sufficiently high number of subcarriers. As for other multicarrier modulations, the chromatic dispersion (CD) compensation is simplified in FBMC/OQAM systems since it is performed in the frequency domain. Unfortunately, FBMC/OQAM systems are sensitive to the laser phase noise (PN). The PN becomes difficult to mitigate when the number of subcarriers increases due to the increased symbol period. It results in intercarrier interference and intersymbol interference due to the loss of OQAM orthogonality. In this paper, we consider the use of moderate numbers of subcarriers to allow for simpler PN tracking. Consequently, more advanced CD compensation methods are required and a trade-off between CD and PN compensations needs to be studied. In this paper, the frequency sampling equalizer is used for the CD compensation, whereas an innovative adaptive maximum likelihood estimator is used for the PN compensation. A methodology is then presented to analyze this performance trade-off between CD and PN compensations, and design the desirable system parameters such as the number of subcarriers and the equalizer length. This is illustrated in the case of a terrestrial long-haul FBMC/OQAM transmission system, with 400-kHz laser linewidth and a 1000-km optical link.

Single-tap precoders and decoders for multiuser MIMO FBMC-OQAM under strong channel frequency selectivity

The design of linear precoders or decoders for multiuser multiple-input multiple-output filterbank multicarrier (FBMC) modulations in the case of a strong channel frequency selectivity is presented. The users and the base station (BS) communicate using space division multiple access. The low complexity proposed solution is based on a single tap per-subcarrier precoding/decoding matrix at the BS in the downlink/uplink. As opposed to classical approaches that assume flat channel frequency selectivity at the subcarrier level, the BS does not make this assumption and takes into account the distortion caused by channel frequency selectivity. The expression of the FBMC asymptotic mean squared error (MSE) in the case of strong channel selectivity derived in earlier works is developed and extended. The linear precoders and decoders are found by optimizing the MSE formula under two design criteria, namely zero forcing or minimum MSE. Finally, simulation results demonstrate the performance of the optimized design. As long as the number of BS antennas is larger than the number of users, it is shown that those extra degrees of freedom can be used to compensate for the channel frequency selectivity.

2016

Simple feedforward carrier phase estimation for optical FBMC/OQAM systems

Filter-bank multi-carrier based on offset quadrature amplitude modulation (OQAM) has recently been proposed to improve the spectral efficiency of optical fiber communication systems. Unfortunately, it is prone to the laser phase noise (PN) that results in significant inter-subcarrier interference due to the loss of OQAM orthogonality. Based on the analysis of the OQAM constellation obtained at the output of the analysis filter bank, we propose a modified blind phase search algorithm working based on the observation of all subcarriers simultaneously. It efficiently compensates for the PN and incurs a low number of multiplications. The proposed method is analytically derived and numerically validated with 4-, 16-, and 64-OQAM modulations. The impact of the number of subcarriers used per optical channel is also investigated. The results show that up to 256 subcarriers can be utilized with negligible performance degradation. Moreover, the method provides a tolerated linewidth and symbol duration product of 1.4×10-4, 2.5×10-5, and 6.2×10-6 for respective 4-, 16-, and 64-OQAM modulations to limit the SNR penalty to 1 dB at a BER of 10-3.

Sequential likelihood ratio test for cognitive radios

This paper presents a sequential likelihood ratio test (SLRT) detector for spectrum sensing scenarios in cognitive radios (CRs). Similar to other CR detectors, we exploit the structure of the sample covariance matrix of the received signal to achieve detection with minimal information regarding the signal. Unlike the majority of covariance-based CR detectors, the SLRT is a sequential detector that allows for smaller detection delays, which is a prized asset in CR systems. Using methods borrowed from the theory of continuous-time diffusion processes, we derive the statistical properties of the SLRT detector and compare it with an eigenvalue-based sequential detector which has been presented in previous work for CR systems. The comparison also includes detection scenarios with non- Gaussian noise to illustrate the robustness of the proposed detector in these situations.

On the noise robustness of simultaneous orthogonal matching pursuit

In this paper, the joint support recovery of several sparse signals whose supports exhibit similarities is examined. Each sparse signal is acquired using the same noisy linear measurement process, which returns fewer observations than the dimension of the sparse signals. The measurement noise is assumed additive, Gaussian, and admits different variances for each sparse signal that is measured. Using the theory of compressed sensing, the performance of simultaneous orthogonal matching pursuit (SOMP) is analyzed for the envisioned signal model. The cornerstone of this paper is a novel analysis method upper bounding the probability that SOMP recovers at least one incorrect entry of the joint support during a prescribed number of iterations. Furthermore, the probability of SOMP failing is investigated whenever the number of sparse signals being recovered simultaneously increases and tends to infinity. In particular, convincing observations and theoretical results suggest that SOMP committing no mistake in the noiseless case does not guarantee the absence of error in the noisy case whenever the number of acquired sparse signals scales to infinity. Finally, simulation results confirm the validity of the theoretical results.

Improving the correlation lower bound for simultaneous orthogonal matching pursuit

The simultaneous orthogonal matching pursuit (SOMP) algorithm aims to find the joint support of a set of sparse signals acquired under a multiple measurement vector model. Critically, the analysis of SOMP depends on the maximal inner product of any atom of a suitable dictionary and the current signal residual, which is formed by the subtraction of previously selected atoms. This inner product, or correlation, is a key metric to determine the best atom to pick at each iteration. This letter provides, for each iteration of SOMP, a novel lower bound of the aforementioned metric for the atoms belonging to the correct and common joint support of the multiple signals. Although the bound is obtained for the noiseless case, its main purpose is to intervene in noisy analyses of SOMP. Finally, it is shown for specific signal patterns that the proposed bound outperforms state-of-the-art results for SOMP and orthogonal matching pursuit (OMP) as a special case.

Scaled largest eigenvalue detection for stationary time-series

This paper studies the performance of the scaled largest eigenvalue (SLE) detector used for the detection of stationary time-series. We focus on a single-antenna setup and a blind detection scenario (neither the signal covariance, nor the noise variance are known). The SLE detector has received much attention in the context of cognitive radios (CR) due to its simplicity, good performance and robustness to noise level uncertainties. Specifically, our goal is to analyze the detector based on the statistic Γ = λ1∑i=1 p λi, where λ1 ≥ λ2 ≥ ⋯ ≥ λp represent the ordered eigenvalues of the sample covariance matrix. We derive a large-sample-size closed-form approximation for the test statistic which allows us to derive its statistical distribution and set up the detector to achieve the required probability of false-alarm (Pfa) and probability of detection (Pd). We also study the robustness of the detector in the presence of noise uncertainty and impulsive-noise and investigate the benefits of the spatial sign filter for such scenarios.

On the exact recovery condition of simultaneous orthogonal matching pursuit

Several exact recovery criteria (ERC) ensuring that orthogonal matching pursuit (OMP) identifies the correct support of sparse signals have been developed in the last few years. These ERC rely on the restricted isometry property (RIP), the associated restricted isometry constant (RIC) and sometimes the restricted orthogonality constant (ROC). In this paper, three of the most recent ERC for OMP are examined. The contribution is to show that these ERC remain valid for a generalization of OMP, entitled simultaneous orthogonal matching pursuit (SOMP), that is capable to process several measurement vectors simultaneously and return a common support estimate for the underlying sparse vectors. The sharpness of the bounds is also briefly discussed in light of previous works focusing on OMP.

2014

Multicarrier offset-QAM for long-haul coherent optical communications

The availability of high-speed digital-to-Analog converters for coherent optical communication systems makes it possible to digitally divide the available electrical bandwidth into subchannels on each optical carrier which enables a parallel implementation of the digital signal processing. In this paper, we use electrical multicarrier offset-QAM modulations to achieve crosstalk-free modulation for subcarriers spaced at the symbol rate. We show that in coherent optical communication systems, several impairments are bound to break the subcarrier orthogonality and cause failures of payload symbol recovering. We propose a dedicated digital signal processing architecture which implements channel estimation and a new algorithm to track the phase in multicarrier offset-QAM modulation. We experimentally compare the latter with conventional multicarrier and single carrier QAM modulation so as to assess the benefits of crosstalk mitigation via offset-QAM modulation. We carry out experiments to test the transmission performance of multicarrier offset-QAM modulation in a dispersion-unmanaged long-haul transmission link.

Spectrum sensing method based on the likelihood ratio goodness of fit test under noise uncertainty

In cognitive radio, spectrum sensing is one of the most important tasks. In this article, a blind spectrum sensing method based on goodness-of-fit (GoF) test using likelihood ratio (LLR) is studied. In the proposed method, a chi-square distribution is used for GoF testing. The performance of the method is evaluated through Monte Carlo simulations. It is shown that the proposed spectrum sensing method outperforms the GoF test using Anderson Darling (AD) and the conventional energy detection (ED) in case of a limited number of received samples and low signal to noise ratio (SNR). We also evaluate the proposed method in case of a non-Gaussian noise and in case of noise uncertainty. It is shown that the GoF based spectrum sensing methods are less sensitive to both impairments, than the conventional ED. Finally, this paper investigates the influence of the number of samples on the detection performance. The performance difference between the GoF based sensing (LLR and AD) and ED increases with decreasing number of samples for sensing, which makes the proposed method very effective in CR systems with short sensing periods.

Iterative pre-distortion of the non-linear satellite channel

Digital Video Broadcasting-Satellite-Second Generation (DVB-S2) is the current European standard for satellite broadcast and broadband communications. It relies on high-order modulations up to 32 amplitude/phase-shift keying (APSK) in order to increase the system spectral efficiency. Unfortunately, as the modulation order increases, the receiver becomes more sensitive to physical-layer impairments and, notably, to the distortions induced by the power amplifier and the channelizing filters aboard the satellite. The predistortion of a nonlinear satellite channel has been studied for many years. However, the performance of existing predistortion algorithms generally becomes poor when high-order modulations are used on a nonlinear channel with long memory. In this paper, we investigate a new iterative method that predistorts the blocks of transmitted symbols to minimize the Euclidian distance between the transmitted and received symbols. We also propose approximations to relax the predistorter complexity while keeping its performance acceptable. © 1972-2012 IEEE.

2013

Spatial sign cyclic-feature detection

This paper studies the impact of the spatial sign function on the cyclic features of communication signals and thereby on the performance of cyclic-feature detectors. It extends the existing results obtained for Gaussian signals and provides new insights into the interactions between the cyclic features of a cyclostationary signal sent through a spatial sign filter, which lead to the modified cyclic features at the output. We apply these results to analytically derive the performance of the spatial sign cyclic correlation (SSCC) detector, which filters a received signal by the spatial sign function prior to its processing using a cyclic-feature detector. These results are extended to variations of the spatial sign function and we further motivate their application to general zero-memory non-linear (ZMNL) functions.

Dynamic resource allocation for MIMO cognitive radio networks with low control traffic and low computational complexity

Radio spectrum scarcity hampers the development of new wireless technologies and services. Cognitive radios have been proposed to enable unlicensed (or secondary) users to borrow locally idle bands of the spectrum provided that no significant interference is created for the licensed (or primary) users. Fast adaptation to the changing spectrum availability is naturally a major requirement in such systems. This adaptation consists of detecting the spectrum occupied by the primary users, computing a new resource allocation for the secondary network, and communicating this allocation through the network. In that context, we develop a resource allocation scheme for multi-input-multi-output wireless mesh networks. The proposed algorithm combines low computational complexity and light control traffic thanks to a combination of relevant approximations in the general nonpolynomial-hard allocation problem. The allocation consists of two steps. First, a centralized carrier allocation is performed at a coordinator node based on partial knowledge of the network parameters. Then, each node locally computes its power allocation through simple water-filling algorithms. Numerical results show that compared to state-of-the-art techniques, 10% of the total throughput of the network is sacrificed to reduce the computation time and the control traffic by two orders of magnitude.

Decision-feedback equalization of bandwidth-constrained N-WDM coherent optical communication systems

Nyquist Wavelength division multiplexing (N-WDM), is a promising scheme in order to enhance the spectral efficiency of future coherent optical communication systems. In N-WDM systems, the channel bandwidth and spacing are selected to maximize the spectral efficiency while maintaining acceptable levels of inter-carrier and inter symbol interference. To further increase the spectral efficiency, bandwidth constrained N-WDM, where baudrate is higher than channel bandwidth can be considered. We propose to combine the bandwidth-constrained N-WDM scheme with a decision feedback equalizer (DFE) designed according to the minimum mean square error (MMSE) criterion. We show that the enhanced resilience to the inter symbol interference offered by DFE provides an effective gain on spectral efficiency. We compare system benefits of DFE and maximum a posteriori sequence detection (MAP) for coherent optical receivers, and show that DFE is as efficient as MAP in mitigating inter symbol interference at a considerably lower complexity.

Dynamic channel modeling for multi-sensor body area networks

A channel model for time-variant multi-link wireless body area networks (WBANs) is proposed in this paper, based on an extensive measurement campaign using a multi-port channel sounder. A total of 12 nodes were placed on the body to measure the multi-link channel within the created WBAN. The resulting empirical model takes into account the received power, the link fading statistics, and the link auto- and cross-correlations. The distance dependence of the received power is investigated, and the link fading is modeled by a log-normal distribution. The link autocorrelation function is divided into a decaying component and a sinusoidal component to account for the periodical movement of the limbs caused by walking. The cross-correlation between different links is also shown to be high for a number of specific on-body links. Finally, the model is validated by considering several extraction-independent validation metrics: multi-hop link capacity, level crossing rate (LCR) and average fade duration (AFD). The capacity aims at validating the path-loss and fading model, while the LCR and AFD aim at validating the temporal behavior. For all validation metrics, the model is shown to satisfactorily reproduce the measurements, whereas its limits are pointed out.

Interference-free SDMA for FBMC-OQAM

Filter-bank multi-carrier (FBMC) modulations have recently been considered for the emerging wireless communication systems as a means to improve the utilization of the physical resources and the robustness to channel time variations. FBMC divides the overall frequency channel in a set of subchannels of bandwidth proportionally decreasing with the number of subchannels. If the number of subchannels is high enough, the bandwidth of each subchannel is small enough to assume that it is approximately flat. On the other hand, space-division multiple access (SDMA) is a recognized technique to support multiple access in the downlink of a multi-user system. The user signals are precoded at the base station equipped with multiple antennas to separate the users in the spatial domain. The application of SDMA to FBMC is unfortunately difficult when the channel is too frequency selective (or when the number of subchannels to too small) to assume flat subchannels. In that case, the system suffers from inter-symbol and inter-subchannel interference, besides the multi-user interference inherent to SDMA. State-of-the art solutions simply neglect the inter-symbol/subchannel interference. This article proposes a new SDMA precoder for FBMC capable of mitigating the three sources of interference. It is constructed per subchannel in order to keep an acceptable complexity and has the structure of a filter applied on each subchannel and its neighbors at twice the symbol rate. Numerical results demonstrate that the precoder can get rid of all the interference present in the system and benefit therefore from the diversity and power gains achievable with multiple antenna systems.

Dual-polarization OFDM-OQAM for communications over optical fibers with coherent detection

In order to improve the spectral efficiency of coherent optical communication systems, it has recently been proposed to make use of the orthogonal frequency-division multiplexing offset quadrature amplitude modulation (OFDM-OQAM). Multiple optical channels spaced in the frequency domain by the symbol rate can be transmitted orthogonally, even if each channel overlaps significantly in frequency with its two adjacent channels. The solutions proposed until now in the literature unfortunately only address a single polarization communication, and therefore do not benefit from the capacity gain reached when two polarizations are used to transmit independent information signals. The aim of the present paper is to propose a receiver architecture that can decouple the two polarizations. We build an equalizer per channel at twice the symbol rate and optimize it based on the minimum mean square error (MMSE) criterion. We demonstrate the efficiency of the resulting system compared to the Nyquist wavelength-division multiplexing (N-WDM) system both in terms of performance and complexity. We also assess the system sensitivity to transmit synchronization errors and show that system can even work under significant synchronization errors. © 2013 Optical Society of America.

Nonparametric cyclic polyspectrum-based spectrum sensing

The emergence of cognitive radios and their opportunistic use of the radio spectrum calls for efficient spectrum sensing algorithms, able to reliably detect signals in very low Signal-to-Noise Ratio (SNR) environments with unknown or changing statistical distributions. Cyclic-feature detectors, which make use of the cyclostationary signature of communication signals, are a promising lead to meet these requirements. This paper presents a new nonparametric cyclic-feature detector based on the Spatial Sign Cyclic Polyspectrum (SSCP) that detects single-carrier signals for the aforementioned scenarios. In addition to being nonparametric the new detector is also robust with respect to narrowband interferers.

2012

Tri-polarized MIMO systems in real-world channels

Polarized multi-antenna systems are an effective solution for reducing inter-antenna spacing while still maintaining low inter-antenna correlation. Traditionally, only dual-polarized antenna systems are used for polarized transceivers. In this paper, tri-polarized antenna systems are investigated. Starting from the polarization mechanisms in the wireless propagation channel, it is shown that dual-polarized MIMO systems show high sensitivity to the transmitter and receiver orientation, which may be very critical in practical applications. Tri-polarized MIMO systems are introduced as a solution to obtain a robust MIMO performances, which are independent of the transmitter and receiver orientation. The performances of dual- and tri-polarized MIMO systems are evaluated on real-world measured channels, and the limits of each of these systems is highlighted.

Tri-polarized spectrum sensing based on an experimental outdoor-to-indoor cognitive-radio scenario

Compared to classical spatially separated multiple antenna system, cross-polarized co-located antenna systems are an interesting way to reduce equipment size while reducing the inter-antenna correlation. In this paper the spectrum sensing of a Cognitive Radio (CR) system taking advantage of polarization diversity under Rayleigh fading is investigated and compared to an equivalent system using spatial diversity. This analysis is based on a theoretical formulation applied to a real-world scenario. For this purpose, an outdoor-to-indoor measurement campaign at a frequency of 3.5 GHz is realized, where an indoor secondary user senses the signals received from an outdoor primary base station. The signals received at each antenna are first combined and then applied to an energy detector. The theoretical expressions are simulated in the measurement context. The detection probability behavior as a function of distance between the Primary Transmitter (PTx) and the Secondary Terminal (STE) and the inter-antenna correlation effect on the sensing performance are studied. © 2012 Elsevier B.V.

Model parametrization and validation for specular-diffuse clustered channel models

A model parameterization and validation is proposed for specular-diffuse clustered channel models. The double-directional model parameters, based on an experimental measurement campaign, are presented. The specular-diffuse model is validated with regard to the following validation metrics: mutual information, singular values, Demmel condition number and ellipticity. A good agreement is observed when comparing the model with experimental measurements. Finally, the influence of the diffuse multipath component on the performance of the model validation is evaluated. © 1963-2012 IEEE.

A human body model exposed to a cluster of waves

The impact of wireless channel modeling on exposure to electromagnetic radiation is studied. Two methods are developed in order to assess the statistical properties of whole body Specific Absorption Rate for exposure estimation in indoor environment. The body model is exposed to a bundle of waves, named cluster, following the wireless channel modeling approach. The first method is analytical and based on the Uncorrelated Scattering Assumption of the incident waves. The second method is a classical stochastic method. The point is to identify the parameters of Wireless Channel which led to significant SAR's variation.

Sensing time and power allocation for cognitive radios using distributed Q-learning

In cognitive radios systems, the sparse assigned frequency bands are opened to secondary users, provided that the aggregated interferences induced by the secondary transmitters on the primary receivers are negligible. Cognitive radios are established in two steps: the radios firstly sense the available frequency bands and secondly communicate using these bands. In this article, we propose two decentralized resource allocation Q-learning algorithms: the first one is used to share the sensing time among the cognitive radios in a way that maximize the throughputs of the radios. The second one is used to allocate the cognitive radio powers in a way that maximizes the signal on interference-plus-noise ratio (SINR) at the secondary receivers while meeting the primary protection constraint. Numerical results show the convergence of the proposed algorithms and allow the discussion of the exploration strategy, the choice of the cost function and the frequency of execution of each algorithm.

Supplementary proof for "equalization algorithms in the frequency domain for continuous phase modulations"

To enable frequency domain equalization of continuous phase modulations (CPM), a block construction that ensures cyclicity over each block without disrupting the phase continuity between them was proposed in [1]. We formalize and prove the constraints that should be respected to enable the application of this technique to any CPM scheme. © 2012 IEEE.

2011

Performance analysis of distributed ZF beamforming in the presence of CFO

We study the effects of residual carrier frequency offset (CFO) on the performance of the distributed zero-forcing (ZF) beamformer. Coordinated transmissions, where multiple cells cooperate to simultaneously transmit toward one or multiple receivers, have gained much attention as a means to provide the spectral efficiency and data rate targeted by emerging standards. Such schemes exploit multiple transmitters to create a virtual array of antennas to mitigate the co-channel interference and provide the gains of multi-antenna systems. Here, we focus on a distributed scenario where the transmit nodes share the same data but have only the local knowledge of the channels. Considering the distributed nature of such schemes, time/frequency synchronization among the cooperating transmitters is required to guarantee good performance. However, due to the Doppler effect and the non-idealities inherent to the local oscillator embedded in each wireless transceiver, the carrier frequency at each transmitter deviates from the desired one. Even when the transmitters perform frequency synchronization before transmission, a residual CFO is to be expected that degrades the performance of the system due to the in-phase misalignment of the incoming streams. This paper presents the losses of the signal-to-noise ratio gain analytically and the diversity order semi-numerically of the distributed ZF beamformer for the ideal case and in the presence of a residual CFO. We illustrate our results and their accuracy through simulations.

Impact of the environment and the topology on the performance of hierarchical body area networks

Personal area networks and, more specifically, body area networks (BANs) are key building blocks of future generation networks and of the Internet of Things as well. In this article, we present a novel analytical framework for network performance analysis of body sensor networks with hierarchical (tree) topologies. This framework takes into account the specificities of the on-body channel modeling and the impact of the surrounding environment. The obtained results clearly highlight the differences between indoor and outdoor scenarios, and provide several insights on BAN design and analysis. In particular, it will be shown that the BAN topology should be selected according to the foreseen medical application and the deployment environment.

Beamforming techniques for enabling spatial-reuse in MCCA 802.11s networks

We address the problem of co-channel interference (CCI) in wireless mesh networks based on the IEEE802.11s extension. The carrier sensing mechanism deployed in those networks insufficiently addresses the CCI problem, causing the hidden and exposed node problems; consequently degrading the throughput and latency. In this paper, we show how beamforming techniques can be implemented on top of the IEEE802.11s medium access control protocol and, using the information readily available, cancel the interference to mitigate this inefficiency of carrier sense and improve the spatial-reuse gain. In addition, we propose the signal-to-jamming-noise ratio (SJNR) beamformer and show that it significantly improves the spatial-reuse gain compared to the simple zero-forcing (ZF) beamformer and the basic IEEE802.11s access scheme. We derive the ergodic capacity of the ZF beamformer and the basic IEEE802.11s access scheme and simulate the performance of the various schemes. We show that improvements of up to 85% are achieved as function of the scenario simulated and the beamforming technique used and that the SJNR scheme outperforms the standard ZF beamformer.

Link-level performance of indoor body area networks with centralized topologies

sensors networks and, more specifically, body area networks (BANs) are key building blocks of the future generation networks and the Internet of Things as well. In the last years, research has focused on channel modeling and on the design of efficient medium access control (MAC) mechanisms. Less attention has been paid to network-level performance analysis. Thereby, this paper presents a novel analytical framework for network performance analysis with star (i.e., centralized) topologies. This framework takes into account realistic channel statistics and provides several insights on BAN design and analysis. © Dricot et al.

A wideband channel model for intravehicular nomadic systems

The increase in electronic entertainment equipments within vehicles has rendered the idea of replacing the wired links with intra-vehicle personal area networks. Ultra-wideband (UWB) seems an appropriate candidate technology to meet the required data rates for interconnecting such devices. In particular, the multiband OFDM (MB-OFDM) is able to provide very high transfer rates (up to 480 MBps) over relatively short distances and low transmit power. In order to evaluate the performances of UWB systems within vehicles, a reliable channel model is needed. In this paper, a nomadic system where a base station placed in the center of the dashboard wants to communicate with fixed devices placed at the rear seat is investigated. A single-input single-output (SISO) channel model for intra-vehicular communication (IVC) systems is proposed, based on reverberation chamber theory. The model is based on measurements conducted in real traffic conditions, with a varying number of passengers in the car. Temporal variations of the wireless channels are also characterized and parametrized. The proposed model is validated by comparing model-independent statistics with the measurements.

2010

Dynamic spectrum access

A polarized clustered channel model for indoor multiantenna systems at 3.6 GHz

Multiple-input-multiple-output (MIMO) technologies allow high data rates to be obtained, but they suffer from interantenna correlation caused by the limits in interantenna spacing. Polarized MIMO systems resolve this problem by using colocated perpendicularly polarized antennas that have low interantenna correlation. In this paper, a polarized single-directional channel model for 2 × N MIMO systems at 3.6 GHz in an indoor environment is presented. The wireless channel is modeled as a sum of clusters, where each cluster has specular and diffuse components. The polarization of the specular component of the clusters is included by considering a per-path polarization. The diffuse component of the clusters is modeled with a FisherBingham (FB5) spectrum in the azimuthcoelevation domain and with an exponential power delay profile. Polarization is analyzed by introducing the cross-polar discrimination of the exponential power delay profile parameters. All of the parameters in the model are extracted from an experimental measurement campaign performed in an indoor environment at 3.6 GHz. Individual paths are extracted from the measurements with the space-alternating generalized expectation-maximization (SAGE) algorithm. These paths are grouped in clusters within the azimuth of arrivalelevation of arrivaldelay domains at the receiver side using automatic clustering algorithms. The specular component properties of the clusters are then determined. Finally, the diffuse components of the clusters are investigated and parameterized by applying a beamforming algorithm on the diffuse part of the impulse response. © 2006 IEEE.

Higher-order cyclostationarity detection for spectrum sensing

Recent years have shown a growing interest in the concept of Cognitive Radios (CRs), able to access portions of the electromagnetic spectrum in an opportunistic operating way. Such systems require efficient detectors able to work in low Signal-to-Noise Ratio (SNR) environments, with little or no information about the signals they are trying to detect. Energy detectors are widely used to perform such blind detection tasks, but quickly reach the so-called SNR wall below which detection becomes impossible Tandra (2005). Cyclostationarity detectors are an interesting alternative to energy detectors, as they exploit hidden periodicities present in man-made signals, but absent in noise. Such detectors use quadratic transformations of the signals to extract the hidden sine-waves. While most of the literature focuses on the second-order transformations of the signals, we investigate the potential of higher-order transformations of the signals. Using the theory of Higher-Order Cyclostationarity (HOCS), we derive a fourth-order detector that performs similarly to the second-order ones to detect linearly modulated signals, at SNR around 0 dB, which may be used if the signals of interest do not exhibit second-order cyclostationarity. More generally this paper reviews the relevant aspects of the cyclostationary and HOCS theory, and shows their potential for spectrum sensing. Copyright © 2010 Julien Renard et al.

Probabilistic co-existence and throughput of cognitive dual-polarized networks

Diversity techniques for cognitive radio networks are important since they enable the primary and secondary terminals to efficiently share the spectral resources in the same location simultaneously. In this paper, we investigate a simple, yet powerful, diversity scheme by exploiting the polarimetric dimension. More precisely, we evaluate a scenario where the cognitive terminals use cross-polarized communications with respect to the primary users. Our approach is network-centric, i.e., performance of the proposed dual-polarized system is investigated in terms of link throughput in the primary and the secondary networks. In order to carry out this analysis, we impose a probabilistic co-existence constraint derived from an information-theoretic approach, i.e., we enforce a guaranteed capacity for a primary terminal for a high fraction of time. Improvements brought about by the use of our scheme are demonstrated analytically and through simulations. In particular, the main simulation parameters are extracted from a measurement campaign dedicated to characterization of indoor-to-indoor and outdoor-to-indoor polarization behaviors. Our results suggest that the polarimetric dimension represents a remarkable opportunity, yet easily implementable, in the context of cognitive radio networks.

Novel block constructions using an intrafix for CPM with frequency domain equalization

To enable frequency domain equalization (FDE) for continuous phase modulation (CPM), both cyclicity of individual symbol blocks and phase continuity between different blocks have to be guaranteed. In this letter, we present new block constructions that use a subblock of data-dependent symbols, called intrafix, to satisfy both constraints for different CPM-FDE systems: using either a cyclic prefix or a training sequence (TS), both for precoded and nonprecoded CPM. The known symbols of a TS can be used to improve synchronization and channel estimation. Precoding can be applied to a certain class of CPM schemes to halve the bit error rate. © 2006 IEEE.

A comprehensive channel model for UWB multi-sensor multi-antenna body area networks

Body area networks consist of a number of biological sensors communicating over the air with a central sink placed in close proximity of the human body. A promising solution is to use multisensor multiantenna ultrawideband architecture; each sensor carries one antenna, while the central sink supports an antenna array. In this paper, a complete analytical channel model has been developed for the on-body diffracted waves mechanism. It builds on the existing IEEE 802.15.4a standard channel model and offers an innovative space-time correlation model.

Polarization measurements and modeling in indoor NLOS environments

Cross-polarized antenna systems are an attractive way to reduce equipment size while maintaining low interantenna correlation. In this paper, the polarization behaviour of indoor channels is investigated. A measurement campaign has been conducted at 3.6 GHz for a dual-polarized transmitter and a tri-polarized receiver in a non-Line-of-Sight (NLOS) scenario. The spatial and delay characteristics are extracted using a pertap beamforming algorithm. Distinct paths are isolated and the polarization of each wave is expressed in terms of its spherical components. The cross-polarization discrimination (XPD) of the wave is investigated as a function of its physical propagation parameters. The XPD of the wave is shown to be sensitive to spatial characteristics, while being insensitive to delay.

Multi-polarized channel statistics for outdoorto- indoor and indoor-to-indoor channels

Compared to classical spatial MIMO wireless systems, cross-polarized MIMO systems are an interesting way to reduce equipment size while reducing the inter-antenna correlation. Cross-Polar Discrimination (XPD) and Co-Polar Ratio (CPR) are two important parameters describing multi-polarized channels. In this paper, the behavior of these parameters is investigated for different observation scales. A measurement campaign has been performed in both Outdoor-to-Indoor and Indoor-to-Indoor scenarios, at a frequency of 3.5GHz. Small-scale variations of XPD and CPR are analyzed in different spatial positions. The distance-related and large-scale variations of XPD and CPR are also investigated and a model is deduced.

2009

Multipolarized MIMO channel characteristics

MIMO technologies enable high communication data rates, but suffer from the large antenna spacing that is required to achieve sufficiently low inter-antenna correlation. Cross-polarized antenna systems resolve this problem by using perpendicular antennas. Correlation is reduced while keeping antennas co-located. Inter-antenna correlation and cross-polar discrimination (XPD) are two fundamental parameters of these polarized antenna systems. This paper proposes an analytical channel model, from which closed-form solutions for the correlation coefficient and the XPD are deduced. The environment is supposed to have a truncated Laplacian power azimuth spectrum that is widely used in standardization bodies. The receiving di- or tri-pole antenna can have random orientation. The correlation and the XPD show to be highly sensitive to receiver orientation, azimuth spread and environment depolarization behavior. Measurements have been conducted at 3.5 GHz to validate the solution obtained. Good agreement is achieved when comparing theoretical curves and experimental results for different receiver orientations, both for the correlation coefficient and the XPD. © 2009 IEEE.

Low-complexity linear frequency domain equalization for continuous phase modulation

In this paper, we develop a new low-complexity linear frequency domain equalization (FDE) approach for continuous phase modulated (CPM) signals. As a CPM signal is highly correlated, calculating a linear minimum mean square error (MMSE) channel equalizer requires the inversion of a nondiagonal matrix, even in the frequency domain. In order to regain the FDE advantage of reduced computational complexity, we show that this matrix can be approximated by a block-diagonal matrix without performance loss. Moreover, our MMSE equalizer can be simplified to a low-complexity zero-forcing equalizer. The proposed techniques can be applied to any CPM scheme. To support this theory we present a new polyphase matrix model, valid for any block-based CPM system. Simulation results in a 60 GHz environment show that our reduced-complexity MMSE equalizer significantly outperforms the state of the art linear MMSE receiver for large modulation indices, while it performs only slightly worse for small ones.

Small-scale variations of cross polar discrimination in ricean fading channels

Diversity order of spatial multiplexing with transmit antenna correlation based precoding

Spatial Multiplexing (SM) is an effective means for enhancing the transmission data rate inMultiple-InputMultiple-Output(MIMO) systems, particularlywhen used in combinationwith precoding techniques. However, it is not always obvious to connect the performance of such a system to its number of data streams and antennas. In this paper, the diversity order of aSMMIMOsystemusing aMinimumMean SquareError (MMSE) receiver is analytically calculated when a precoder based on transmit antenna correlation is included at the transmitter. It is shown that it is given by d = Nr - N s + 1 where Nr is the number of receive antennas and Ns is the number of data streams. This result is confirmed by simulations. Although enabling a Signal-to-Noise Ratio (SNR) gain, such a precoder is thus not able to improve the diversity order with respect to a non-precoded SM MIMO system. © 2009 Springer-Verlag Berlin Heidelberg.

2008

Single-carrier FDMA or cyclic-prefix CDMA with optimized spreading sequences

To meet the data rate and quality-of-service requirements of the future cellular systems, new air interfaces are currently under development. In this paper, we compare two air interfaces of particular interest for the uplink: (1) cyclic-prefix code-division multiple access (CP-CDMA), which has been proposed in the literature as an evolution of direct-sequence code-division multiple access (DS-CDMA) because it enables the low-complexity equalization of the multipath channel in the frequency domain, and (2) single-carrier frequency-division multiple access (SC-FDMA), which has recently been proposed in the long-term evolution of the Third-Generation Partnership Project (3GPP) standard because it enables the easy separation of users in the frequency domain. We analytically demonstrate that SC-FDMA is a special case of CP-CDMA, in which the CDMA codes have been optimized to minimize symbol estimation mean square error under a constraint of received power. Numerical results confirm that SC-FDMA significantly outperforms CP-CDMA at high user loads.

Low-complexity EM-based joint acquisition of the carrier frequency offset and IQ imbalance

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. In this context, OFDM is considered as a promising air interface candidate for both indoor and outdoor communications. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct-conversion radio frequency receivers are appealing because they avoid costly intermediate frequency hardware. However, they imply analog IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on OFDM is sensitive to synchronization errors, such as CFO, and to front- end non-idealities, such as IQ imbalance. The goal of this paper is to use the iterative EM algorithm to acquire jointly the CFO and the IQ imbalance. The solution relies on a standard compliant repetitive preamble and does not require the knowledge of the propagation channel. Based on a second order approximation of the likelihood function, the complexity of the EM algorithm is significantly reduced. The algorithm is shown to perform extremely well: the estimates of the CFO and of the IQ imbalance converge to their ML estimate after less than 3 iterations. It outperforms state-of-the-art solutions significantly and suffers from a lower computational complexity. While the CFO estimate is robust against variations of the SNR, the IQ imbalance estimate accuracy is reduced at values of the SNR below 10 dB and above 35 dB.

2007

On-body propagation velocity estimation using ultra-wideband frequency-domain spatial correlation analyses

Space-time block coding for uplink single-carrier CDMA with joint detection in the frequency domain

Single-carrier code-division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the uplink of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multipath channels and the mitigation of the resulting interference possible at a low complexity. This paper proposes a new air interface that combines SC-CDMA with space-time block coding (STBC) across multiple transmit antennas in order to make the link more robust. Contrary to existing air interfaces that perform the STBC at the chip level, making them only applicable to the downlink, the STBC is performed at the symbol level, making it also applicable to the uplink. In order to optimally detect the different antenna and user signals, a linear joint detector optimized according to the minimum mean square error (MMSE) criterion is designed. By exploiting the cyclic properties of the channel matrices, the complexity of the joint detector is significantly reduced. Furthermore, it is shown analytically that the inter-antenna interference is canceled out at the output of the first stage of the linear MMSE joint detector, consisting of a matched filter. By space-time coding the signal through two antennas at each transmit mobile terminal, a significant gain in signal-to-noise-ratio can be achieved. However, the spatial diversity gain of the proposed system is limited by the multiuser interference (MUI), that is increasing with the user load. Higher complexity non-linear receivers are needed to better compensate the MUI and still benefit from the spatial diversity at high user loads.

Comparison of OQPSK and CPM for communications at 60GHz with a non-ideal front-end

Short-range digital communications at 60 GHz have recently received a lot of interest because of the huge bandwidth available at those frequencies. The capacity offered to the users could finally reach 2 Gbps, enabling the deployment of new multimedia applications. However, the design of analog components is critical, leading to a possible high nonideality of the front end (FE). The goal of this paper is to compare the suitability of two different air interfaces characterized by a low peak-to-average power ratio (PAPR) to support communications at 60 GHz. On one hand, we study the offset-QPSK (OQPSK) modulation combined with a channel frequency-domain equalization (FDE). On the other hand, we study the class of continuous phase modulations (CPM) combined with a channel time-domain equalizer (TDE). We evaluate their performance in terms of bit error rate (BER) considering a typical indoor propagation environment at 60 GHz. For both air interfaces, we analyze the degradation caused by the phase noise (PN) coming from the local oscillators; and by the clipping and quantization errors caused by the analog-to-digital converter (ADC); and finally by the nonlinearity in the PA.

2006

Impact of frequency offsets and IQ imbalance on MC-CDMA reception based on channel tracking

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. Multicarrier code-division multiple access (MC-CDMA) is seen as a promising candidate for the fourth-generation (4G) cellular communication systems because it can interestingly deal with the multipath propagation at a low processing complexity. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct conversion radio frequency (RF) receivers are appealing because they avoid costly intermediate frequency (IF) filters. However, they imply RF IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on MC-CDMA is sensitive to synchronization errors and front-end non-idealities because it uses a long symbol duration. The goal of this paper is to evaluate the impact of the carrier frequency offset, the sampling clock offset, and the IQ imbalance on the MC-CDMA downlink system performance, considering a receiver based on channel tracking designed to cope with high mobility conditions. It is demonstrated that part of the effects is compensated by the channel estimation and an expression of the variance of the remaining symbol estimation error is provided. For the cellular system and the target performance considered in this paper, specifications are defined on the non-idealities. The results are validated with bit-error rate simulations

2005

Flexible MIMO transmission scheme for 4G wireless systems with multiple antennas

New air interfaces are currently being developed to meet the high requirements of the emerging wireless communication systems. In this context, the combinations of the multicarrier (MC) and spread-spectrum (SS) technologies are promising candidates. In this paper, we propose a generic transmission scheme that allows to instantiate all the combinations of orthogonal frequency-division multiplexing (OFDM) and cyclic-prefixed single-carrier (SC) modulations with direct-sequence code-division multiple access (DS-CDMA). The generic transmission scheme is extended to integrate the space-division multiplexing (SDM) and the orthogonal space-time block coding (STBC). Based on a generalized matrix model, the linear frequency-domain minimum mean square error (MMSE) joint detector is derived. A mode selection strategy for up- and downlink is advised that efficiently trades off the cost of the mobile terminal and the achieved performance of a high-mobility cellular system. It is demonstrated that an adaptive transceiver that supports the proposed communication modes is necessary to track the changing communication conditions.

2004

MU-MIMO channel adapted precoding for MAI/ISI-free uplink burst transmission

This paper considers the uplink of a multiuser direct-sequence code-division multiple-access communication system based on burst transmission over frequency-selective channels. A new precoding method is presented that is able to take benefit from the space diversity introduced at the mobile transmitters and at the common receiver. User-specific spreading codes are designed so that filters matched to the total impulse responses can remove completely the interference and maximize the signal-and-interference-plus-noise ratios. It is analytically proved that an infinite number of solutions is available to orthogonalize the system. Three particular solutions are considered. A progressive solution is firstly proposed to provide orthogonality in a simple way. Then a near-optimum solution is proposed to orthogonalize the system while attempting to reduce the total transmitted power. The minimization of the total transmitted power is a complex problem of nonlinear optimization that can only be solved using an iterative algorithm. The third solution proposed is based on an iterative algorithm. Those three solutions are analyzed in term of their different performance for a varying number of transmit and receive antennas. A comparison is also provided with a system using a conventional set of binary codes and linear or decision-feedback minimum mean-square error joint detection. It is shown that the near-optimum and iterative solutions are the only ones that are able to fully benefit from the space diversity.

2003

CA-CDMA

This paper proposes a precoding technique for the complete elimination of both multiple-access interference and intersymbol interference in the uplink of a code-division multiple-access system based on burst transmission, with reduction of the minimum mean-square error joint detector to a simple filter matched to the total impulse responses. If the received power is fixed, this system minimizes the variance of the symbol estimation error. An infinity of solutions exists to orthogonalize the system. Two objectives are added: a possible progression in the number of users and a minimization of the average emitted power. In an ideal progressive system, users can enter or leave the system easily without recomputing the codes of the other users. The minimization of the emitted power is a complex optimization problem. An approximate solution is proposed. It is shown that the system introduced in this paper outperforms burst systems using a conventional set of codes.

2002

A comparison between chip fractional and non fractional sampling for a direct sequence CDMA receiver

This paper examines the effects of chip fractional (CF) and chip non-fractional (CNF) sampling on the performance of a CDMA uplink receiver. The impact of the receiver front end filter, which is sampling rate dependent, is investigated. Models for burst and continuous transmissions are introduced. The discrete-time equivalent channels between the various users and the receiver are assumed to be known. First, the mutual information between the emitted sequences of symbols and the received sequence is investigated. It is analytically shown that the receiver systematically loses information in case of CNF sampling. Second, we have demonstrated that the CF receiver always achieves better performance in terms of minimum mean square error (MMSE) for both linear and decision feedback (DF) structures. A closed-form expression of the gain in performance is provided for the two metrics; under consideration. The importance of the gain due to CF sampling is also illustrated by means of computations for multipath channels. For a typical system setup, a gain of 0.1 bits per emitted symbol is observed for the mutual information. Considering the geometrical mean of symbol SINRs in case of linear and DF joint detection (JD) for a roll-off factor equal to 0.3, a gain of 0.4 dB arises for the CF linear detector, and a gain of 0.2 dB arises for the CF DF detector.

Channel adapted precoding for interference-free uplink burst transmission