Power-constrained embedded applications often execute Fast Fourier Transforms (FFT) on ultra-low-power microcontrollers (ULP MCUs). They can integrate memory-based hardware (HW) FFT accelerators, with additional silicon area being traded for energy and latency improvements. This trade-off is modulated by the radix of the FFT accelerator. Moreover, a fixed-point representation with a fixed wordlength is generally used, which limits the signal-to-quantization-noise ratio (SQNR) and requires a data scaling method to avoid overflows. Higher accelerator radices reduce the scaling method SQNR and increase its area overhead. In this work, we explore the trade-offs between energy, area, latency and SQNR in FFT accelerators integrated within ULP MCUs. An in-depth analysis is conducted based on design implementations in a 65-nm LP CMOS technology with a configurable radix from 2 to$2^{4}$. Energy and latency improvements of radix-$2^{4}$reach a factor$2.83\times $and$3.98\times $, respectively. However, the logic area increases by$2.14\times $. We also propose a new scaling method, called index-encoded hierarchical block floating-point (IH-BFP). It provides the high SQNR of existing methods and relies on the deterministic access pattern of the FFT to achieve energy savings of 9.7 % and logic area savings of 32.7 %.

Recently, there has been a growing interest in multistatic radar configurations to improve the localization of multiple targets. Theoretically, the maximum likelihood (ML) approach enables to fuse the information provided by each radar pair to localize the different targets. However, it involves a multidimensional search process whose complexity exponentially grows with the number of targets. Consequently, heuristic methods, notably including the block orthogonal matching pursuit (BOMP), have been used in the multistatic radar context to approach the ML estimation greedily. Interestingly, the more accurate block orthogonal least squares (BOLS) method has not been studied in this context because the performance improvement is usually low in regard to its computational complexity. In this work, we investigate the application of BOLS to an angle-based localization of multiple targets using a multistatic multiple-input and multipleoutput (MIMO) radar. First, an efficient implementation of BOLS is presented reducing its computational complexity. Then, using Monte Carlo simulations, we show evidence of the significant advantage of this efficient implementation of BOLS over BOMP in this scenario featuring highly correlated signals. The impact of radar parameters on the localization root mean square error and on the computational complexity of both algorithms is studied.

The increasing energy consumption of mobile networks has emerged as a critical concern for mobile telecommunication operators, requiring measures to curb or reverse the historical upward trajectory in order to align with the sector’s decarbonization target. Meanwhile, 5G-NR is massively deployed in the networks to improve quality of service, with the hope of simultaneously improving energy efficiency thanks to enhanced power-saving features. However, up-to-date 5G-enabled base stations, that support higher bandwidths with more transceivers at higher frequencies, raise concerns about their absolute power consumption, especially at low traffic loads. Proper power models are therefore needed to identify the key levers for energy savings in mobile networks under real traffic loads. This paper addresses this challenge by first providing a parametric power consumption model applicable to commercial sub- 6 GHz cellular base stations. Then, numerical model parameters are estimated by combining on-site measurements from operators with radio equipment documentation from manufacturers. The uncertainty of model predictions is assessed to be in the range of $\mathbf{1 0 - 2 0 \%}$. Moreover, we show that the proposed average power model aligns well with measurements of equipment that use existing power-saving features. Estimates of power consumption are also provided for typical 3-sector single-band macro base stations in active mode, e.g., 1-4 kW when equipped with traditional radio units, and 2-4 kW when using active antenna units.

LoRaWAN is nowadays one of the most popular protocols for low-power Internet of Things communications. Although its physical layer, namely LoRa, has been thoroughly studied in the literature, aspects related to the synchronization of LoRa receivers have received little attention so far. The estimation and correction of carrier frequency and sampling time offsets (STOs) are, however, crucial to attain the low sensitivity levels offered by the LoRa spread-spectrum modulation. The goal of this article is to build a low-complexity, yet efficient synchronization algorithm capable of correcting both offsets. To this end, a complete analytical model of a LoRa signal corrupted by these offsets is first derived. Using this model, we propose a new estimator for the STO. We also show that the estimations of the carrier frequency and the STOs cannot be performed independently. Therefore, to avoid a complex joint estimation of both offsets, an iterative low-complexity synchronization algorithm is proposed. To reach a packet error rate of 10−3, performance evaluations show that the proposed receiver requires only 1 or 2 dB higher signal-to-noise ratio than a theoretical perfectly synchronized receiver, while incurring a very low computational overhead.

One important problem in spectrum sensing is to detect a noisy and unknown signal, while keeping the risk of detection error as low as possible. This problem may increase in mobile environments due to fast situation changes. In this paper, we consider a mobile cognitive radio scenario, and try to evaluate whether some knowledge about the environment and the mobility parameters of the user can help in improving the detection of changes in the spectrum occupancy. To do so, we assume that the mobility parameters can be summarized in some a priori knowledge on the average time of spectrum change and we use Bayesian changepoint detection methods. Considering that the power of the signal to be detected is usually unknown, a low-complexity algorithm is proposed that does not rely on this knowledge. It is then compared with the existing algorithms in the literature. Finally, a new metric is introduced to jointly evaluate the costs of interference and spectrum waste induced by the changepoint detection algorithms, in a time-limited communication context. Results reveal that the derived algorithm outperforms its non-Bayesian equivalent at low signal to noise ratio (SNR).

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 article, 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.

Secret-key generation exploiting the channel reciprocity between two legitimate parties is an interesting alternative solution to cryptographic primitives for key distribution in wireless systems as it does not rely on an access infrastructure and provides information-theoretic security. Many works in the literature assume that the eavesdropper gets no side information about the key from her observations provided that: (i) it is spaced more than a wavelength away from a legitimate party and (ii) the channel is rich enough in scattering. In this paper, we show that this condition is not always verified under realistic propagation conditions and we study the resulting secret-key capacity.

Millimeter-wave is one of the technologies powering the new generation of wireless communication systems. To compensate the high path-loss, millimeter-wave devices need to use highly directional antennas. Consequently, beam misalignment causes strong performance degradation reducing the link throughput or even provoking a complete outage. Conventional solutions, e.g. IEEE 802.11ad, propose the usage of additional training sequences to track beam misalignment. These methods however introduce significant overhead especially in dynamic scenarios. In this paper we propose a beamforming scheme that can reduce this overhead. First, we propose an algorithm to design a codebook suitable for mobile scenarios. Secondly, we propose a blind beam tracking algorithm based on particle filter, which describes the angular position of the devices with a posterior density function constructed by particles. The proposed scheme reduces by more than 80% the overhead caused by additional training sequences.

A relay network can provide the extended link capacity and coverage thanks to its good spatial diversity. In order to analyze the relay network performance, most of the works in literature assume the independent relaying channels. In practice, the received signals experience the common scatterers, hence the correlated relaying channels should be considered. In this paper, we first derive a novel and accurate probability density function (PDF) of a modulus of the sum of products of two correlated zero-mean complex Gaussian random variables (RVs), which is useful for the statistical analysis of the cascaded relaying fading channels. Based on this result, we secondly investigate the received signal-to-noise ratio (SNR) outage probability of a relay network in the presence of correlated relaying channels. It is found that the correlation magnitude is more important than the correlation phase for the analysis of power-based signal-detection of relay networks. Numerical simulations have been carried out to confirm the correctness of the derivation.

Multiple-antenna technologies provide a good spatial diversity gain for emerging communication systems. In combination to the orthogonal frequency division multiplexing (OFDM) modulation, a precoding system can be easily implemented and efficient to combat the effect of fading channels. However, the precoding system requires a perfect knowledge of the channel, which is not usually hold in practical cases. In this paper, we investigate the impact of the imperfect channel state information (CSI) on the downlink performance of multiple-input single-output (MISO) OFDM systems using a matched-filter (MF) precoder. Particularly, the exact mean-square-error (MSE) expression of the equalized received signal of the MF precoding system is derived. Numerical simulations with Rayleigh fading channels are carried out to validate the analysis. The results show that the imperfect CSI-based precoder causes a MSE plateau compared to the ideal case of using the perfect CSI.

The goal of this work is to identify the possible degradations on a Wi-Fi based Passive Radar in the presence of an interferer. We assume that the signal-of-opportunity and the interference are 802.11ax compliant. The mathematical model derived for the interference shows that in a synchronized case, the interference may yield ghost targets. When a more occasional interference scenario is considered, the range/Doppler Map accuracy decreases significantly. Furthermore, numerical results are provided to quantify certain effects of OFDM radar interference on range/Doppler Maps.

Simultaneous wireless information and power transfer (SWIPT) has recently attracted researchers and may help satisfy future technology demands. Its principle is to let wireless power transfer (WPT) and wireless information transfer (WIT) coexist based on shared resources (or sometimes even on the same signal). SWIPT can be implemented in many different ways and the literature on the topic is very wide, each solution having different obstacles. Recent studies have shown that, due to the non linearity of the rectifiers, high-PAPR waveforms provide better performance in terms of energy harvesting; hence the design of the power signal is an important issue. In addition, the presence of this power signal should consume the smallest possible amount of resources for the information transfer. In this paper, a new waveform design is proposed where the information and power signals are superposed utilizing the same frequencies at the same time. The power signal is composed of a high peak modulated rectangular wave sent during the cyclic prefix of the CP-OFDM system such that it does not interfere with the useful OFDM data symbol duration. Interference evaluation and measurements show that a good choice of signal parameters can minimize interference on the information decoder and at the same time provide good performance in terms of energy harvesting which validates the possibility of the proposed design.

Time reversal-based indoor positioning system (TRIPS) is a promising technology for the centimeter-accuracy indoor positioning, since it exploits the rich multipath propagation in indoor environments as a specific signature for each location. In TRIPS, a database is first constructed via channel probing. Well-calibrated devices are usually assumed in this process, i.e., no hardware impairments. However, a low cost terminal to be located, whose typical impairment is the I/Q imbalance (IQI) at the front-end transmitter, can significantly influence the TRIPS performance. More specifically, IQI creates an interference image of the signal that reduces the metric value used in TRIPS and hence decreases the localization accuracy. In this paper, we analytically investigate the impact of the IQI on the metric of TRIPS. A closed-form approximation of the localization metric inherent to the IQI is derived. In order to improve the TRIPS performance, an effective IQI mitigation method is proposed. Numerical simulations are carried out to validate the derived analytical expression under the IQI impact and the proposed compensation method.

Time reversal has been shown as a promising technology for the indoor positioning. Instead of mitigating the multipath channel, the time reversal based indoor positioning system (TRIPS) exploits the rich multipath propagation in indoor environments as a specific signature for each location. In order to do this, a database should be first constructed by channel probing. The devices used in this process are assumed to have no hardware impairments or be well calibrated. However, the low cost terminal to be located can exhibit impairments, such as I/Q imbalance at the front end transmitter known to create an interference image of the signal that can particularly impact the performance of time reversal. In this paper, we investigate the impact of the I/Q imbalance on the TRIPS. We analytically show that the I/Q imbalance modifies the metric used for the localization and hence reduces the spatial focusing gain of TRIPS. Numerical simulations are carried out to evaluate this observation. The results further show that the I/Q imbalance creates errors in the positioning estimation. More specifically, at the gain and phase imbalances of respectively 1.1 and 10°, the focusing gain reduction is about 0.5 dB while the estimation error rate is increased by about 5% in the specified scenario.

Offset-QAM-based filterbank multicarrier (FBMC-OQAM) modulations are known to progressively loose their orthogonality as the channel gets more selective in time and frequency. The effect of channel frequency selectivity on FBMC-OQAM systems has been extensively studied in the literature. Many compensations methods have been proposed to combat it. However, most of them have a significant implementation complexity and do not take into account the time selective nature of the channel. In this paper, we propose a MIMO equalizing structure for doubly selective channel based on a simple single-tap per-subcarrier decoding matrix. The decoding matrices are designed to minimize the mean squared error of the symbol estimate. This decoder exploits the degrees of freedom offered by the extra antennas at the receiver to compensate for the distortion induced by time and frequency selectivity. Simulation results demonstrate the performance gain of the proposed design with respect to classical designs.

This paper addresses the carrier phase recovery problem in Offset-QAM-based filterbank multicarrier (FBMC-OQAM) systems. The combined phase noise coming from the transmit and receive lasers, is known to induce a phase rotation of the demodulated symbols at the receiver. Several approaches have been proposed to recover the phase in FBMC-OQAM communication systems on optical fiber. Most of them have a significant complexity and do not make use of all information at disposal. In this paper, we propose two new estimators, obtained by minimizing a maximum likelihood (ML) and a maximum a posteriori (MAP) criteria. They use an error model formulation which allows to easily use priors on the phase noise statistics. By linearization of the error, an analytical solution is found for the phase error, which avoids the need for multiple phase tests. Simulation results demonstrate the better performance of the proposed estimators with respect to state of the art solutions in the low signal-to-noise (SNR) regime and for a small number of subcarriers.

The first standardized version of G.fast has been conceived to provide gigabit internet access from the distribution point (DP). Low transmit levels along its operational frequency range and low maximum aggregate transmit power (MAXATP) have been specified to restrict its electromagnetic emissions and enable the use of power available at customer premises to feed its access systems. Power constraints and a maximum bit-constellation size (Bmax=12 bits) limit its coverage and data rates, which could discourage service providers to deploy G.fast. This work analyzes different strategies that could be potentially pursued to enhance its coverage and improve its spectral compatibility with VDSL2 systems. We carry out an extensive simulation study to evaluate a) capacity boundaries of current G.fast systems; b benefits of increasing MAXATP, Bmaxand power spectrum density (PSD) mask levels; and c) spectral shaping as means to improve G.fast compatibility. Our simulation results show that increasing MAXATP and Bmaxboosts data rates and coverage in short loop scenarios, whereas improvements in long loops performance are only determined by MAXATP. Since such modifications generally increase the interference on legacy systems, we propose to perform power back-off (PBO) in order to improve G.fast spectrum-compatibility. As a proof of concept we adopt the PBO from VDSL2 with a fixed parameter set. Although clearly not optimized for the combination of TDD G.fast and FDD legacy systems and despite the simplicity of the adopted approach, our results indicate that spectral shaping can be an effective means to turn G.fast into a more spectrum-friendly system.

This paper investigates the optimal design of precoders or decoders under a channel inversion criterion for multi-user (MU) MIMO filterbank multicarrier (FBMC) modulations. The base station (BS) is assumed to use a single tap precoding/decoding matrix at each subcarrier in the downlink/uplink, resulting in a low complexity of implementation. The expression of the asymptotic mean squared error (MSE) for this precoding/decoding design in the case of strong channel selectivity is recalled and simplified. Optimizing the MSE under a channel inversion constraint, the expression of the optimal precoding/decoding matrix is found. It is shown that as long as the number of BS antennas is larger than the number of users, the optimized precoder and decoder can compensate for the channel frequency selectivity and even restore the system orthogonality for a large enough number of BS antennas.

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.

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.

The optimization of power allocation in DSL systems is a well studied non-convex problem. In recent years many algorithms have been proposed to solve this problem, but due to its non-convexity, researchers had to rely on heuristic, or exhaustive research methods. In our approach we make use of the similarities of the channel gain between 2 adjacent tones to overcome the non-convexity limitation of gradient methods. This allows the proposition of an efficient and a fast routine that manages to speed up the convergence of existent state of the art algorithms.

We consider the downlink of a multicell OFDM system where the frequency reuse factor is 1. Base-stations cooperate and precode the symbols intended for the mobile stations (terminals) located in different cells and operating on the same frequency, in order to precompensate the co-channel interferences (CCIs). To design the precoders, the CCI coefficients have to be estimated. We assume that the mobile stations are able to estimate their SINR that they feedback to their base-station. By properly designing a perturbation signal, it is possible for the base-station to estimate the complex valued CCI coefficients from the SINRs fed back. We investigate the maximum likelihood (ML) estimation of co-channel coefficients from SINR measurements. We obtain the associated Cramér-Rao bound and the mean square error of a practical estimator. Finally a two-cells system is considered and we investigate the impact of such a co-channel estimation on the sum rate of the system when precoding is operated based on these estimates.

In this paper we investigate the tracking performance of the downstream (DS) crosstalk (XT) channel estimation based on the reported signal to noise ratio (SNR), in particular for digital subscriber line (DSL) systems. Aiming to its simplicity, the SNR-assisted XT estimation, has been recognized in ITU as a backward compatible method that does not require any change in the very high speed digital subscriber line 2 (VDSL2) standard. This low complex algorithm can be used for XT channel estimation in dynamic spectrum management (DSM) techniques today. The algorithm as proposed, relies on sending perturbing signals on the victim lines (VLs) and reporting the SNRs by those lines to acquire the crosstalk channel from some disturber line (DL) to the VLs. We generalize this concept to include full startup, tracking and joining scenarios as well as the impact of different perturbation signal choices. Simulation results reveal that starting from no crosstalk precompensation, and updating a precoder matrix based on the DS crosstalk channel estimates, the far-end crosstalk (FEXT) free SNR can be reached in few iterations (36 SNR measurements for the four lines case).

In this paper, we present a new method to estimate the crosstalk channels in VDSL systems, allowing to implement well-known pre-compensation schemes. As opposed to previously presented methods, this estimation only requires minimal changes to the current standard and equipments. It is based on the concept of adding small, controlled, perturbations to the transmitted signal and observing the related changes to the SNR at the receiver. It is shown in this paper that, based on a limited amount of SNR measurements from the receivers, it is possible to accurately estimate the crosstalk channels, both in amplitude and phase.

An efficient and flexible air interface is the necessary condition for enabling high data rates next generation wireless systems. A practical answer to this need consists in exploiting channel state information at the transmitter for an adaptive and efficient use of the available radio link resources. This paper contributes with a novel strategy for the allocation of the sub-carrier transmit power in the case of a wireless system featuring: i) bit-interleaved coded modulation (BICM); ii) orthogonal frequency division multiplexing (OFDM); iii) multiple antenna profile (MIMO). The proposed power-allocation algorithm aims at maximizing the so-called goodput performance metric for packet transmissions employing automatic repeat request (ARQ) schemes. In addition, it is also proven to be near-optimum in maximizing the input/output mutual information of the MIMO-OFDM subchannels. Numerical simulations over typical wireless indoor scenarios confirm the effectiveness of the procedure when applied in the context of MIMO-BICM-OFDM systems.

We consider an OFDM (orthogonal frequency division multiplexing) point to point transmission scheme improved by a relay. For each carrier, symbols sent by the source may be retransmitted during a second time slot by the relay, which is assumed to be of the decode-and-forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Compared to, the second time slot is better used. The source is allowed to transmit a new symbol during this second time slot when the relay is inactive. For this improved protocol, the optimization has been conducted for a sum power constraint and reported in. The present paper is devoted to the case of individual power constraints at the source and at the relay. The theoretical analysis is illustrated by numerical results.

We consider an OFDM (orthogonal frequency division multiplexing) point to point transmission scheme improved by means of multiple relays. For each carrier, symbols sent by the source during a first time slot, may be retransmitted during a second time slot by the relays, which are assumed to be of the decode-and-forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Similarly to the protocol proposed in, the source is allowed to transmit a new symbol during the second time slot when none of the relay is assisting. The constraints of decodability at the relays are properly handled. The optimization is conducted for a constraint on the sum of powers at the source and at the relays. Next to the optimized solution, a suboptimum method based on relay selection is proposed and discussed. The theoretical analysis is illustrated by numerical results.

The optimization of power allocation in DSL systems is a well studied non-convex problem. In recent years many algorithms have been proposed to solve this problem, but due to the non-convexity, researchers had to rely on heuristic, or exhaustive search algorithms. Iterative spectrum balancing (ISB) is one of these algorithms, that relies on iterative exhaustive search over individual power to achieve the optimization. In this paper we propose to take advantage of an approximate gradient model to reduce the complexity of ISB. Furthermore techniques such as the additional starting point (ASP) and successive optimization are used to bring the result closer to the global optimum.

In this paper we consider an OFDM (orthogonal frequency division multiplexing) transmission scheme with a relay, working in decode-and-forward (DF) mode. Assuming perfect CSI (channel state information) the paper investigates the power allocation problem for rate maximization of the scheme with individual power constraints at the source and at the relay. The theoretical analysis provides a deep understanding of the structure of the solution. Based on this, an efficient iterative algorithm is proposed that exhibits a quicker convergence than classical methods. In addition, it enables to solve the convergence issues observed in some situations.

Cooperation in wireless systems can achieve spatial diversity gains as well as coverage enhancement. In the usual decode-and-forward (DF) strategy, a relay cooperates only when it can decode successfully the signal received from the source. This paper presents instead a DF strategy in which the receiver manages to take advantage of forwarded signals containing errors too. This increases the level of cooperation and is shown to offer significant gains in bit error rate.

Adaptive resource allocation algorithms based on perfect channel state information at the transmitter (CSIT) improve the performance of OFDMA systems. However, perfect CSIT is rarely possible due to channel estimation errors and channel feedback delay. In this paper, we consider practical subchannel assignment and power allocation strategies for downlink multiuser OFDMA systems, where the base station has imperfect CSI. The goal is to maximize the expected weighted sum rate while satisfying users' minimum data rate and system fairness requirements under total power constraint. We use the Lagrange dual decomposition framework to derive an algorithm that efficiently finds the optimal allocation. Simulation results show the impact of the channel estimation error on the performance of the system and illustrate the trade-off between maximized expected weighted sum rate and the constraints.

This paper deals with the power allocation problem for coded multicarrier transmission. Specifically, we focus on a bit interleaved coded modulation (BICM) packet transmission implemented with orthogonal frequency division multiplexing (OFDM) and in the presence of automatic repeat request (ARQ) protocol. Capitalizing on the binary-input output-symmetric (BIOS) nature of the BICM channel it is provided a simple upper-bound of the rate of information bits received without any error, the so called goodput. Based on this theoretical characterization, we develop a power allocation strategy among the different subcarriers so that the system goodput performance metric is maximized. The effectiveness of the proposed method is numerically testified for BICM-OFDM transmission in the context of the typical WLAN scenario.

We consider OFDM (orthogonal frequency division multiplexing) transmission helped by a relay. Symbols sent by the source may or may not be retransmitted by a relay during a second time slot. The relay is supposed to operate in Decode-and-Forward (DF) mode. For each carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem for rate maximization of the scheme. Both cases of a sum power constraint, and of individual power constraints at the source and at the relay are tackled. The theoretical analysis is illustrated by numerical results for both types of constraints.

This paper contributes with an adaptive modulation scheme intended for bit interleaved coded (BIC) orthogonal frequency division multiplexing (OFDM) packet transmission in the presence of automatic repeat request (ARQ) protocol. The basic idea consists in properly combining the power allocation strategy with a bit-loading procedure among the different subcarriers so that the rate of information bits received without any error is maximized. Numerical simulations corroborate the effectiveness of the proposed scheme when operating over typical wireless environments.

This paper applies the concept of balanced capacity, a fair trade-off between individual and global performance, to the uplink of a Gaussian frequency-selective multiuser channel with K users, where K can be much larger than 2. Individual power constraints are applied to each transmitter, which makes the computation of the balanced capacity very complex. The concept of 'equivalent channel' is used to derive an explicit expression of the optimal power allocation, that remains compatible with the individual power constraints. The computation of the balanced capacity is then formulated as a set of 2K nonlinear equations. An iterative algorithm providing the exact solution is firstly introduced. A suboptimal solution, based on the Iterative Multiuser Water-Filling (IMWF) algorithm, is then proposed and shown to be very close to the optimal solution. The idea of this alternative method, called 'balanced water-filling', is to modify the power allocation obtained with the IMWF algorithm in order to get balanced data rates. Results are provided for a wireline access network with 20 users.

In this paper, we investigate diversity gain in coding for a 2-user multiple-access-channel (MAC) with cooperating transmitters-the MAC with relaying. We propose a simple distributed space-time-frequency block coding (D-STFBC) scheme and analyze the offered diversity gain. In particular, we show that full diversity (order 3) is possible if collaboration is well enough and rigorous signal processing is assumed both at the transmitters and the receiver. Bit-error-rate (BER) analysis and curves are provided for illustrative purposes.

This paper investigates a fair resource allocation method in crosstalk limited DSL (digital subscriber line) systems. As a measure of fairness, the concept of balanced capacity (BC), introduced in T. Sartenaer et al. (2005), is used. BC corresponds to a point on the boundary of the capacity region where each user experiences the same relative loss with respect to his maximum capacity. This is especially useful in near-far situations where some users can experience a huge capacity loss if the allocation is performed according to the sum-rate criterion (or any similar criterion). Our goal is to propose efficient algorithms to compute the BC point in DSL systems for constraints on individual power spectrum densities. Results are reported for typical crosstalk limited cases and are compared to a resource allocation optimized for the sum rate. The proposed algorithm is shown to be of low complexity.

In this paper we consider UWB (ultra wide band) based positioning. We derive the CRBs (Cramer Rao bound) for AOA (angle of arrival) and for hybrid method using both AOA and TOA (time of arrival) information. Both non modulated and modulated IR-UWB (impulse-radio) waveforms are considered. The UWB channel is supposed to be of the multipath type. The AOA technique is investigated without NB (narrow band) approximation usually made. The analytical results obtained for the CRBs are illustrated by numerical results for typical scenarios.

Iterative channel estimation can improve the channel- state information (CSI) with respect to noniterative estimation. New iterative channel estimators based on the expectation-maximization (EM) algorithm are proposed in this paper. A first estimator, called the unbiased EM (UEM), is designed to unbias the EM estimates. A second estimator is then put forward, which is based on the expectation-conditional-maximization (ECM) algorithm, and its complexity is lower than that of the EM. An unbiased ECM (UECM) estimator is also proposed. Although the unbiasedness of the UEM and UECM estimators is not rigorously proved, the use of these names is explained in the paper. The new estimators are compared with well-known ones, such as the EM, the decision-directed (DD), and the data-aided (DA) estimators. Simulations are reported for a turbo receiver operating over frequency-selective multiple-input multiple-output channels. It is shown that the UEM channel estimator outperforms the EM, and that the ECM-based estimators are very close to the EM-based ones.

The performance of extensions of digital subscriber line to higher data rates (VDSL) is limited by the effects of crosstalk among the wires in a bundle. For the downstream, a precoder can compensate for this. Without knowledge of the crosstalk channel matrix, the problem is to compute a suitable precoder based on limited feedback from the receivers. In this letter, an adaptive precoder design algorithm is derived. The algorithm works on a per-tone basis and assumes that each receiver sends back the sign of the symbol error to the transmitter. The algorithm computes the precoder iteratively using this information, without the need to estimate the crosstalk channel as an intermediate step. The asymptotic and acquisition performance is verified by simulations

A multi-user downlink MIMO-OFDM scheme is considered. Perfect channel knowledge is assumed at the base station and linear processing is used at both the transmit and receive sides. The objective is to optimize the mean BER of the system while satisfying a transmit power constraint and fulfilling each user's rate. FDMA and SDMA are investigated. We show how the multi-user interference induced by SDMA can be annihilated and exhibit that this multiple access technique should be preferred to FDMA.

With increasing bandwidths and decreasing loop lengths, crosstalk becomes the main impairment in VDSL systems. For downstream communication, crosstalk precompensation techniques have been designed to cope with this issue by using the collocation of the transmitters. These techniques naturally need an accurate estimation of the crosstalk channel impulse responses. We investigate the issue of tracking these channels. Due to the lack of coordination between the receivers, and because the amplitude levels of the remaining crosstalk channels after precompensation are very low, blind estimation schemes are inefficient in this case. So some part of the upstream or downstream bit rate needs to be used to help the estimation. In this paper, we design a new algorithm to try to limit the bandwidth used for the estimation purpose by exploiting the collocation at the transmitter side. The principle is to use feedback from the receiver to the transmitter instead of using pilots in the downstream signal. It is shown by simulations that the proposed estimation is more efficient, in terms of needed overhead, than a classical scheme using pilot symbols.

This paper deals with a multi-user downlink MIMO-OFDM scheme with perfect channel knowledge at both the transmit and receive sides. The objective is to optimize the average BER while fulfilling each user's rate and satisfying a global transmit power constraint. Space Division Multiple Access (SDMA) is investigated and three new iterative schemes are proposed to compute the linear pre/decoders. The results exhibit, inter alia, that orthogonalizing the channels does not lead to the best results. © 2005 IEEE.

This paper analyzes the multiuser capacity of Gaussian frequency-selective wireline multiaccess channels. Both the uplink (multiple-access channel) and downlink (broadcast channel) capacity regions are considered. The concept of balanced capacity is introduced to characterize the multiuser channel performance. Algorithms for the computation of the balanced multiuser capacity (and the associated power allocations) are proposed for an arbitrary number of users. The optimal power allocation in a K-user memoryless Gaussian channel is analyzed in detail, and an extension to intersymbol interference channels is given with various kinds of power constraints. Results are provided for a wireline access network with 20 users.

This paper is devoted to the design and the steady-state performance analysis of a data-aided or decision-directed timing recovery scheme for filter bank-based multicarrier transmission with equalization. The symbol synchronizer is obtained from a Mueller and Muller approach extended to multiband transmission. A simple synchronizer is first proposed which can be derived from an approximate likelihood function. Then, possible improvements are discussed. Timing jitter variances are computed and discussed for high bit-rate transmission over copper wires.

Multiuser detection in DSL systems has recently gained some attention. Several methods have been proposed and they provide a significantly increased data rate. However, there are several issues that must be resolved in order to realize the asserted improvements. These issues include the frame synchronization of the different users. We analyze the issue of frame synchronization in a multiuser DSL environment. We propose a novel method to solve this issue that utilizes DSL specific characteristics to overcome obstacles that make the general problems difficult or intractable. Then, the performances of the proposed scheme are analyzed.

Multiuser transmission methods for digital subscriber line (DSL) systems have become of interest with the potential for increased data rate and loop reach. These methods often assume that the set of crosstalk interferers, called the crosstalk profile, and their associated channel responses are known. For DSL systems, the interferers are often uncoordinated, so that in a dynamic environment where DSL transmitters can energize and deenergize, the crosstalk profile cannot be transmitted to the user of interest. While the crosstalk channel estimation problem in a dynamic environment can be intractable for general transmission systems, channel and crosstalk analysis can make use of the specific DSL environment. Namely, the physical channels in a DSL system do not change rapidly, and hence estimates of the crosstalk channel can be saved for future reference. For this reason, we introduce the concept of a channel profile. We develop several algorithms to detect the crosstalk profile and investigate the asymptotic behavior of the new algorithms. Simulations show that for typical crosstalk interference scenarios, the observation time to determine the correct crosstalk profile at probability of error less than 10/sup -3/ can be less than 2 ms.

The performance degradation of filter-bank-based multicarrier transmission due to timing errors is investigated. The receiver is made of a fractionally spaced linear or decision-feedback equalizer designed for some sampling phase. The actual sampling phase is different and the impact of the difference on the performance is investigated. Sampling phase offset and jitter are considered. Besides, assuming the sampling phase error can be estimated the efficiency of various types of interpolation is investigated.

This paper examines a decision-directed timing recovery scheme for filter-bank-based multicarrier transmission. The symbol synchronizer is obtained from a maximum-likelihood approach and uses the digital equalization filters. The algorithm is first derived, the performances of the scheme (timing offset and timing jitter) are then computed, and computational results are provided for high bit-rate transmission over copper wires. Finally, different improvements are discussed.

This paper is devoted to a decision-directed timing recovery scheme for filter bank based multicarrier transmission. The symbol synchronizer is obtained from a maximum-likelihood approach and uses the equalizer outputs. The algorithm is first derived and computational results for the resulting timing error are provided for high bit rate transmission over copper wires. Finally, different improvements are discussed.

We investigate the asymptotic performance of a multi-input multi-output decision-feedback (DF) equalizer used to detect a multicarrier (MC) signal based on a filter-bank and transmitted over a linear dispersive channel. We derive the optimum DF structure for a minimum-mean square-error criterion. We basically show that with infinite length forward and feedback filters and at high signal-to-noise ratio, the geometrical mean of prediction errors does not depend on the paraunitary filter-bank used in the transmitter. Hence, it appears that uncoded filter-bank based MC transmission and single carrier transmission, both used over the same channel and with optimum DF, lead to the same achievable bit rate.

We investigate detectors for transmultiplexer-based high bit rate transmission over dispersive channels like telephone wires. The transmitter is based on a synthesis stage. At the receiver side, we design, for an MMSE criterion, multiple-input/multiple-output (MIMO) linear detectors applied after an analysis stage and upgraded with a MIMO decision-feedback (DF) section. The performance of the system is investigated for filter banks over copper wires with different noises (AWGN, FEXT and RFI). The influence of the forward section rate is investigated. The potential of these detectors is clearly demonstrated. Besides, reduced complexity solutions are also put forward.

We investigate the asymptotic performance of a multiple input/multiple output (MIMO) decision-feedback equalizer (DFE) used to detect a multicarrier signal transmitted without guard time over a linear dispersive channel. We design the MIMO DFE for a minimum mean square error criterion (MMSE) and compare its performance to that of a single carrier system also using an infinite length MMSE DFE equalizer. We demonstrate that both uncoded systems achieve the same capacity.