In user-centric cell-free massive MIMO systems, a large number of low-complexity access points jointly serves a smaller number of mobile stations in the same communication bandwidth. Employing time division duplex mode and exploiting uplink/downlink channel reciprocity, the phases of uplink training and uplink/downlink data transmission sequentially take place within each channel coherence interval. This paper tackles the problem of integrated localization and communication in user-centric cell-free massive MIMO systems and proposes using the uplink decoded data as an additional virtual pilot to improve location and channel estimation for the mobile stations. In particular, after the usual training phase and uplink data transmission, the central processing unit shares the decoded uplink data with the access points, that use it as a long virtual pilot to refine the estimate of the users' locations. Extensive computer simulation results show the effectiveness of the proposed approach in the operating region of large Signal-to-Interference plus Noise Ratios.

This paper considers the problem of integrated sensing and communications in a user-centric cell-free massive MIMO system employing OFDM modulation. It is assumed that a large number of access points is using the same frequency band in order to communicate with a certain number of mobile users and to perform surveillance against potential targets in the surrounding area. The paper develops all the needed transceiver signal processing needed to implement both the communication and radar functionality, and provides expressions for the generic user downlink SINR and for the radar SNR. Numerical results are finally presented to show the effectiveness of the proposed system and to highlight the impact on the system performance of the different power split between the communication and radar functionalities, as well as of the access points and users density in the considered area.

Ultra-high data rates with low-power consumption wireless communications and low-complexity receivers is one of the key requirements for the next 6-th Generation (6G) of communication networks. Sub-Terahertz (SubTHz) frequency bands can support Ultra-WideBand (UWB) communications and can thus offer unprecedented increase in the wireless network capacity, rendering those bands and relevant research a strong candidate technology for 6G. However, in contrast to millimeter-wave communications, the technological advances in subTHz transceivers are not yet completely mature. In this paper, we focus on the Time Reversal (TR) precoding scheme, which is a computationally simple and robust technique capable of focusing UWB waveforms on both time and space, by exploiting channel diversity when available. We design a novel experimental setup, including a THz link inside a waveguide and operating at the carrier frequency 273.6 GHz with 2 GHz transmission bandwidth, and deploy it to perform multi-user communications with simple modulation. Our results demonstrate large communication rates with only 3 mm space separation of the receiving antennas.

Time Reversal (TR) has been proposed as a competitive precoding strategy for low-complexity wireless devices relying on Ultra-WideBand (UWB) signal waveforms. However, when TR is applied for multiple access, the signals received by the multiple users suffer from significant levels of inter-symbol and inter-user interference, which requires additional processing for mitigation by each receiving user. In this paper, we present an iterative Time-Reversal Division Multiple Access (TRDMA) approach that aims to dim the latter interference levels. The performance of iterative TRDMA is evaluated experimentally in a reverberation chamber that mimics a rich scattering indoor wireless propagation environment. The improved efficiency, in terms of the number of algorithmic iterations, of the proposed approach compared to conventional TRDMA, is demonstrated. We also consider a mobile user configuration, where the position of the receiver changes between the channel estimation and data transmission steps. It is showcased, even for this experimental setup, that the proposed iterative TRDMA approach is more efficient than conventional precoding schemes.

MISO Time-Reversal (TR) communication marks a paradigm shift for ultrawideband (UWB) communications because the processing is mainly carried out by the transmitter, which is ideal for some applications. Thanks to its focusing property, TR is naturally dedicated to multiple access trans-mission (TRDMA). Previous research works have focused on the theoretical performance of TRDMA. In this paper, Multi-user MISO (MU-MISO) TRDMA transmission is evaluated experimentally for the first time to the best of our knowledge, and by simulations. An experimental setup is used to measure the TRDMA signals for different bandwidth in multi-user scenario. The experimental results are then used in a simulation to evaluate the bit error rate and the performance of the system.

Reconfigurable intelligent surfaces (RIS) technology, as the name implies, is a grid of many small intelligent surfaces that can be reconfigured. It is a new and promising concept in wireless communications, expected to help realize the requirements of the future cellular generations. Numerous studies have been done to prove its added advantages and to control these surfaces in beneficial ways. However, these control schemes come with difficulties related to efficient and practical implementation. In this paper, we propose to control multiple RISs in a multi-user scheme with an algorithm that leads to a simple implementation. We formulate a stochastic optimization problem and we propose a new method using a distributed stochastic algorithm that allows the optimization to be done locally at each RIS with low computational requirements and without the need for instantaneous channel knowledge at the RIS. Also, the signaling between the base station (BS) and each RIS is limited to the exchange of a scalar only. Simulation results prove the success and efficiency of this algorithm.

In this work we address the distance dependence of reconfigurable intelligent surfaces (RIS). As differentiating factor to other works in the literature, we focus on the array near-field, what allows us to comprehend and expose the promising potential of RIS. The latter mostly implies an interplay between the physical size of the RIS and the size of the Fresnel zones at the RIS location, highlighting the major role of the phase. To be specific, the point-like (or zero-dimensional) conventional scattering characterization results in the well-known dependence with the fourth power of the distance. On the contrary, the characterization of its near-field region exposes a reflective behavior following a dependence with the second and third power of distance, respectively, for a two-dimensional (planar) and one-dimensional (linear) RIS. Furthermore, a smart RIS implementing an optimized phase control can result in a power exponent of four that, paradoxically, outperforms free-space propagation when operated in its near-field vicinity. All these features have a major impact on the practical applicability of the RIS concept. As one contribution of this work, the article concludes by presenting a complete signal characterization for a wireless link in the presence of RIS on all such regions of operation.

Accessing the spatial domain of wireless environments through antenna arrays has become key to improve the use of energy and, in light of this, there is an increasing need for low cost multi-antenna architectures. As one alternative, the Electronically Steerable Parasitic Array Radiator (ESPAR) was proposed as a coupling-based inexpensive option; allowing to significantly reduce the amount of required radio frequency (RF) front-ends. As a caveat, due to its inherent non-linear behavior, the required computational complexity can be prohibitive. Additionally, the unavailability of precise mutual coupling and channel state information (CSI) becomes a further issue difficult to avoid in practical setups. In this regard, the current work has two main contributions: a) it deals with the computational complexity by proposing the linearization of ESPAR's system model through the truncated Taylor expansion of the admittance matrix; facilitating the spatial processing algorithm of interest. Also, the authors propose b) to face the mutual coupling and CSI unavailability issue via the joint estimation of the channel-ESPAR parameters as directly observed through the single RF front-end. Relying on the optimization of the linearized system model, the pilot-based algorithm to be introduced allows ESPAR to track the configuration that synthesizes coherent combination at a significantly low complexity. Particularly, as shown through Monte-Carlo simulation, the latter allows to obtain 4 dB of array gain with a single RF front-end via a 5-element ESPAR.

Communication based on Time Reversal (TR) exploits rich multipath radio propagation for high resolution spatiotemporal focusing. It refers to the process of transmitting a received signal in a time reversed order, profiting from channel's spatial reciprocity. Recent theoretical studies have shown that signal processing techniques for TR communication have the potential of realizing the benefits of massive antenna systems using only a single antenna base station and simple receive processing circuitry. In addition, TR can offer highly accurate localization, especially when considered for indoor wireless positioning systems. Particularly, the larger the transmission power and bandwidth are, the more observable are the multiple channel paths, hence, TR capability for localization becomes more profitable. In this paper, we implement TR wireless communication at 3.5GHz using up to 600MHz bandwidth channel sounding signals. We present extensive experimental results showcasing the concept's potential for indoor cm-level localization and signal coverage extension.

The electronically steerable parasitic array radiator (ESPAR) has been acknowledged as an inexpensive multiple antenna architecture. Nonetheless, due to the non-linear behavior on the loads used in its control, its related computational complexity can be prohibitive for practical implementations. The current paper deals with such an issue by proposing an approximation of its model via a truncated polynomial expansion of the inverse impedance matrix. The proposed approximation allows to reframe the problem of reactance optimization for beamforming, referred to as analog maximum ratio combining (A-MRC) in previous work. The method proposed in this paper provides a means to obtain a computationally efficient solution to the problem of reactance optimization and it avoids the need of an iterative-based approach. Therefore, resulting in a major computational complexity reduction with respect to related work in the literature.

The use of decode-and-forward (DAF) and amplify-and-forward (AAF) relays with the purpose of receiving and re-transmitting information has long been proposed. Nonetheless, DAF relays increase latency and AAF relays suffer from inherent stability issues due to potential echoes on the relayed signal. Alternatively, a passive relaying array (PRA) is proposed as a means of coping with both of the so-mentioned drawbacks. The role of PRA is that of capturing and scattering energy in a desired direction. The current work proposes a closed-form model for a dipole-based PRA with an electronically tunable response. The validation of the proposed model and the link budget for communication through the relay are also shown. Furthermore, given the non-linear relationship of PRA on the electronically tunable loads, the simultaneous perturbation stochastic approximation (SPSA) method is used as a means of finding the set of loads that maximizes energy transfer in a desired direction.

The electronically steerable parasitic array radiator (ESPAR) has been proposed as an inexpensive device for spatial multiplexing in transmission via the so-called beamspace modulation. On the other hand, spatial multiplexing (SM) is well known to allow a linear increase of capacity in MIMO systems with the number of antennas under rich scattering, unlike its corresponding logarithmic increase via signal to noise ratio (SNR). When it comes to ESPAR in reception for SM, nonetheless, the issue at hand is the so-called SNR degradation due to oversampling. In particular, oversampling is required given that, although all parasitic elements can be simultaneously controlled, up to a single simultaneous sample can be conveyed in reception. The counter intuitive role of the oversampling rate when seen as the number of spatial samples and its negative impact on SNR is studied in the present paper. It is shown that, from a capacity perspective, the ideal number of spatial samples depends on both the SNR and channel conditions. Furthermore, an algorithm for capacity maximization over the set of reactive loads relying on the simultaneous perturbation stochastic approximation method (SPSA) is shown. Particularly, as an extension of previous work, such an algorithm converges to analog maximum ratio combining (A-MRC) when the oversampling rate is equal to one.

5G systems will rely on the use of massive of number of antennas in the base-stations to meet the rate, latency and coverage requirements of next generation cellular networks. This will introduce challenging constraints on the front-haul links connecting the base band processing units (BBUs)s and the radio remote units (RRUs). Digital front-haul transmission strategies like CPRI will need to deliver huge data rates because of the number of antennas and the bandwidth of the radio frequency signals that are used. An alternative strategy using analog radio-over-fiber can relax these constraints in front-haul links. In particular, it allows to deploy dummy remote radio units with reduced complexity, lighter weight and lower power consumption compared to digital solutions. However, the use of radio-over-fiber transmissions introduces non-linear distortions that need to compensated in order to satisfy the requirements on adjacent leakage ratio (ACLR) that are decreed by the 3GPP standard. In order to keep minimal complexity in RRUs, this paper investigates a pre-distorsion approach that is implemented in the BBU. A feedback enabling this pre-distortion and requiring minimal complexity in the RRU is presented. An experimental validation assessing the efficiency of the proposed approach and showing its capability to satisfy 3GPP requirements is presented.

The reactively loaded parasitic array radiator (RL-ESPAR) receiver has been acknowledged due to its compactness, fabrication cost and reconfigurability; showing significant beam-forming trade-off capabilities compared to conventional multi-element receiver ends. When used on reception, one relevant problem is to find the value of the reactance loadings to obtain the best signal to noise ratio performance. In the current paper, we compare the performance of digital and analog coherent combining over RL-ESPAR. Particularly, digital combining is realized by virtually rotating such a receiver and then applying the known maximum ratio combining (MRC) technique. On the other hand, analog combining is realized by synthesizing MRC on the reactance loadings via a technique that relies on the simultaneous perturbation stochastic approximation method (SPSA) method (widely applied on ESPAR in the literature). We show that analog combining exceeds digital combining and single dipole reception by around 3 dBs and 4 dBs via the proposed technique, respectively.

High order modulations with multi-stream transmissions have been proposed recently as a promising feature to increase the available data rates in the upcoming releases of LTE systems. Increasing the modulation order requires additional processing capabilities in both infrastructure nodes and mobile terminals. However, most of decoding algorithms are based either on turbo processing strategies or successive interference cancellation schemes. For mobile terminals, such strategies introduce significant decoding latency and are not compatible with parallel computing architectures that are implemented in modern devices. This paper proposes a tunable complexity scheme for multi-stream transmissions with high modulation orders (64QAM and higher). The scheme is based on lattice reduction followed by a list based maximum likelihood detection. The proposed strategy allows the same performance as successive interference cancellation schemes and offers a better decoding latency with parallel processing architectures.

The numerous features installed in recent mobile phones opened the door to a wide range of applications involving localization, storage, photo and video taking and communication. A significant number of applications involve user generated content and require intensive processing which limits dramatically the battery lifetime of featured mobile terminals. Mobile cloud computing has been recently proposed as a promising solution allowing the mobile users to run computing-intensive and energy parsimonious applications. This new feature requires new functionalities inside the cellular network architecture and needs appropriate resource allocation strategies which account for computation and communication in the same time. In this paper we present promising options to upgrade 4G architecture to support these new features. We also present two resource allocation strategies accounting for both computation and radio resources. These strategies are devised so that to minimize the energy consumption of the mobile terminals while satisfying predefined delay constraints. We compare online learning based solutions where the network adapts dynamically to the application that is run on mobile terminals, and pre-calculated offline solutions which are employed when a certain level of knowledge about the application and the channel conditions is available at the network side. We show, that even with imperfect knowledge about the application, pre-calculated offline strategies offer better performance in terms of energy consumption of mobile terminals.

In this paper, we address computation offloading problem from mobile users to their serving small cell base stations. These base stations can be endowed with some computational capabilities providing thus users proximity access to the cloud services. We aim to jointly optimize the radio resource scheduling and computation offloading in order to minimize the average energy consumed by all the users terminals to process their mobile applications under average delay constraints tolerated by these applications. We investigate for this problem offline and online dynamic programming approaches and we devise deterministic solutions to find the optimal scheduling-offloading policy. The proposed solutions select only one user for scheduling, hence offloading, and decides for the other users either local processing or staying idle according to their application rates. We show that the offline strategy is optimal in terms of energy saving compared to the online strategy. It can benefit from prior knowledge on the channel statistics and the application properties to satisfy the users requirements.

In wireless cellular networks, multiple scenarios involve the communication between devices located in the same vicinity. These links can be offloaded to a device to device (D2D) underlay which is controlled by the cellular infrastructure. Such an option alleviates the load on the base stations and saves spectrum resources for downlink connections. In this paper, we investigate the impact of D2D communications on the capacity of a cellular network from two directions: average link capacity and coverage. We present a stochastic geometry based framework which captures both cellular and device to device links. Based on this framework, we quantify the enhancements offered by D2D underlay in terms of system coverage, average link capacity and spectrum usage.

Three dimensional (3D) beamforming (also elevation beamforming) is now gaining interest among researchers in wireless communication. The reason can be attributed to its potential for enabling a variety of strategies such as sector or user specific elevation beamforming and cell-splitting. Since these techniques cannot be directly supported by current LTE releases, the 3GPP is now working on defining the required technical specifications. In particular, a large effort is currently being made to get accurate 3D channel models that support the elevation dimension. This step is necessary as it will evaluate the potential of 3D and full dimensional (FD) beamforming techniques to benefit from the richness of real channels. This work aims at presenting the on-going 3GPP study item “study on 3D-channel model for elevation beamforming and FD-MIMO studies for LTE” and positioning it with respect to previous standardization works.

This paper studies a Gaussian two-way relay channel where two communication nodes exchange messages with each other via a relay. It is assumed that all nodes operate in half-duplex mode without any direct link between the communication nodes. A compress-and-forward relaying strategy using nested lattice codes is first proposed. Then, the proposed scheme is improved by performing layered coding: A common layer is decoded by both receivers, and a refinement layer is recovered only by the receiver that has the best channel conditions. The achievable rates of the new scheme are characterized and are shown to be higher than those provided by the decode-and-forward strategy in some regions.

Parallel two-way relay channel models a cooperative communication scenario where a relay helps two terminals to exchange their messages over independent Gaussian channels. For the single channel case, we have shown previously that lattice-based physical layer network coding achieves the same rate as compress-and-forward scheme with a random coding strategy. A direct extension of this lattice-based scheme to parallel Gaussian channel is to repeat the same strategy for each subchannel. However this approach is not scalable with the number of sub-channels since the complexity of the scheme becomes prohibitive when a large number of sub-channels is employed. In this contribution, we investigate a lattice-based physical layer network coding scheme where the relay jointly processes all the sub-channels together. We characterize the rate region allowed by our coding scheme and assess the performance penalty compared to the separate channel processing approach.

Recently, Delay-Tolerant Space-Time Codes (DT-STCs) have been designed for cooperative communications. They are optimal in synchronous transmission and preserve their full-diversity order for arbitrary delays in asynchronous case. In this paper, we investigate these DT-STCs in femtocell networks where Femtocell Access Points can assist Macrocell Base Stations to transmit data information to users. The delays can be randomly introduced by the MBSs-FAPs backhaul links. We consider concatenated DT-STCs based on previous designs and we address their optimal and sub-optimal decoding schemes. In order to reduce the complexity, we propose a new sub-optimal algorithm based on successive sub-codewords decoding. Simulation results show that the proposed decoder achieves good performance-complexity tradeoff.

Block diagonalization is a linear precoding technique which eliminates the multi-user interference at the transmitter side by creating parallel orthogonal single user links. In this technique, the transmit diversity order is fixed for each user and only power can be controlled to satisfy SINR requirements. This restricts the degrees of freedom available for resource allocation and limits the energy efficiency of the power allocation schemes performed by the base-station. In this contribution we present a new extension to block diagonalization where spatial degrees of freedom can also be adjusted in addition to power and bandwidth. With this new technique, it is possible to serve multiple users with different rate requirements by tolerating a controllable amount of interference. The amount of interference is controlled through an algebraic parameter that can be tuned in order to allow energy efficient power allocation.

This contribution provides a very high data rate UWB system design based on direct sequence spreading. In order to attain > 500Mbps for short range communication, two classical solutions are feasible: either use very short spreading sequences or very large bandwidth. These classical strategies increase intersymbol interference. In contrast, our approach consists in allocating multiple long codes to each user. Long codes allow to cope with channel impairments while the use of multiple codes maintains very high bit rates. With such high data rates the receiver obviously has to be simple by requiring a low complexity digital processing. This paper shows that the proposed strategy is compatible with simple receivers. In fact, even if the amount of interference increases, joint usage of very long codes and simple linear receiver can result in high data rate, high performance UWB system. This assertion is substantiated through comparisons with existing UWB solutions.

Direct sequence ultra wide band (DS-UWB) is a strong candidate for high data rate (HDR) wireless personal area networks (WPAN) IEEE 802.15.3a standard for achieving up to 480 Mbps. This paper presents a new system design methodology based on a multicode approach where the spreading sequences have a duration longer than the channel impulse response (CIR) with the goal of achieving higher data rates up to 1.6 Gbps. Multiple codes are allocated to one user and transmitted synchronously in a TDMA access scheme. All codes are modulated by the same QAM constellation, the best combination of number of codes and QAM order for achieving the largest range of coverage is illustrated for 4 target rates: 10, 110, 480 and 1600 Mbps. The selection methodology is based on a theoretical SNIR derivation at the output of a linear minimum mean square error equalizer using classical WPAN channels as specified by IEEE 802.15.3a. Compared to the DS-UWB proposal for IEEE 802.15.3a, the proposed system grants around 3 times range increase

Impulse radio (IR) ultra wide band systems (UWB) by nature require an accurate timing. This contribution proposes three digital algorithms for compensating the time jitter resulting from the oscillator driving the receiver sampler. The main idea is to estimate jointly useful data and a fraction of the time jitter by using coloration properties of the oscillator phase noise. A maximum likelihood (ML) estimation based algorithm, a suboptimal but with lower complexity one and corresponding adaptive versions are detailed.

This paper quantifies the gain in throughput and quality of service (QoS) provided by the EDCA MAC adopted in the current IEEE 802.11e WLAN draft compared to the traditional DCF. In that purpose, an analytical model is developed to compute the throughput in the single and multi-user cases with a non-ideal channel. This model is hacked with system simulations in realistic deployment scenarios. Results make a clear statement on the advantage of EDCA to guarantee some QoS. Final discussions provide inputs to further improve both the delivered QoS per user and the overall cell throughput for next generation of wireless LANs.

In this paper the capacity of a specific PPM (pulse position modulation) ultra wide band (UWB) system using a semi-digital demodulator architecture is evaluated. Then the impact of time desynchronization between the local pulse templates and the received signal on the capacity of the link is assessed, the goal being to quantify the jitter effect and delay estimation error on this capacity and issue design recommendations for real system construction.