Recent research has focused on advanced architectures for reconfigurable intelligent surfaces (RIS) incorporating sensing capabilities. A notable development is the hybrid simultaneously sensing and reflecting RIS (HRIS), which integrates both sensing and reflecting meta-atoms. This design endows HRIS with signal processing abilities to tackle the channel estimation challenge. This work develops a closed-form semi-blind receiver pair for HRIS-assisted multiple-input multiple-output wireless communications, leveraging HRIS for joint symbol detection and channel estimation under a tensor space-time coding. By transmitting information symbols instead of pilots during the CE stage, our data-aided method reduces decoding delay while estimating symbols at a low computational cost. Simulation results show competitive performance in terms of normalized mean square error and symbol error rates compared to existing solutions, highlighting the effectiveness of enabling semi-blind estimation at both the HRIS and BS. Furthermore, data estimation directly at the HRIS opens new opportunities and use cases for HRIS/RISassisted communications.

Despite the rapid advancements in 5G technology, accurately assessing the energy consumption of its Radio Ac-cess Networks (RANs) remains a challenge due to the diverse range of applicable technologies and implementation solutions. Designing a versatile power model for estimating the 5G RAN-specific power consumption requires extensive data collection and experimental studies to capture the diverse range of technolo-gies and implementation solutions. The objective is to outline a versatile energy model capable of estimating RAN-specific energy consumption, encompassing both mobile terminals and the physical layer (PHY) of base stations. In this paper, we focus on the computational complexity of the baseband part of the model. The developed (part of the) model is compared with the estimation of the number of cycles (and energy per cycle) used by a specific implementation (here a Matlab code ported on an Intel target), enabling the assessment of the model with the estimation of energy consumed on a real target. The study's results show a good agreement between the model and the implementation, even if some parts need to be refined to take specific algorithms into account. The key contribution is the development of an initial flexible energy model with finer granularity, enabling comparisons of energy use across various applications and contexts, and offering a comprehensive tool for optimizing 5G network energy consumption.

Recently, dual-polarized antenna has attracted strong attention in Millimeter wave (mmWave) systems. It provides an additional degree-of-freedom in wireless communication, yielding higher throughput. Nowadays, with the development of antenna technology, dual-polarized large scale antenna arrays can be realized inexpensively. However, several challenges must be addressed when using dual-polarized antennas in practical transmission such as mobile phone rotation and non-ideal polarization isolation. We address these challenges in the frame of dual-polarized hybrid beamformers. In this paper, we analyze the performance of dual-polarized beamforming based on two popular techniques: Beam steering and Orthogonal Matching Pursuit. Three categories of dual-polarized beam steering are analyzed: (1) same ray: the two polarizations are sent on the same ray, (2) different ray - same polarization: the two polarizations are sent on different pathes, and the receiver uses the same polarization as the emitter, (3) different ray - different polarization: the receiver uses the orthogonal polarization w.r.t. the emitter. An algorithm to choose the rays and polarizations to use, taking the mobile rotation into account, is also developed. Moreover, we develop a hybrid beamforming algorithm inspired by Orthogonal Matching Pursuit that approaches the fully digital beamforming data-rate and outperforms beam steering. Its developed version - Orthogonal based Matching Pursuit - is also introduced to reduce the computational complexity and overcome the unavailability of the Angle of Arrival.

One of the biggest challenges in operating massive multiple-input multiple-output systems is the acquisition of accurate channel state information at the transmitter. To take up this challenge, time division duplex is more favorable thanks to its channel reciprocity between downlink and uplink. However, while the propagation channel over the air is reciprocal, the radio-frequency front-ends in the transceivers are not. Therefore, calibration is required to compensate the RF hardware asymmetry. Although various over-the-air calibration methods exist to address the above problem, this paper offers a unified representation of these algorithms, providing a higher level view on the calibration problem, and introduces innovations on calibration methods. We present a novel family of calibration methods, based on antenna grouping, which improves accuracy and speeds up the calibration process compared to existing methods. We then provide the Cramér-Rao bound as the performance evaluation benchmark and compare maximum likelihood and least squares estimators. We also differentiate between the coherent and non-coherent accumulation of calibration measurements, and point out that enabling non-coherent accumulation allows the training to be spread in time, minimizing the impact to the data service. Overall, these results have special value in allowing the design of reciprocity calibration techniques that are both accurate and resource-effective.

For time division duplexing (TDD) systems, the physical channel in the air is reciprocal for uplink (UL) and downlink (DL) within the channel coherence time. However when the transceivers' radio frequency (RF) hardware is taken into consideration, TDD channel reciprocity no longer holds because of the non-symmetric characteristics of RF transmit and receive chains. Relative calibration has been proposed to compensate this hardware impairment with a multiplicative matrix. In this paper we perform hardware measurements on this calibration matrix which gives a direct insight on the physical phenomenon of TDD transceivers. Especially, we inspect the assumption that this calibration matrix is diagonal, which is widely adopted in literature but has never been verified by experiments. This work can be regarded as an experimental base for TDD calibration or for theoretical analysis of non-perfect channel reciprocity of TDD systems.

Continuous phase modulation (CPM) has gained increasing attention due to its favorable trade-off between power and bandwidth efficiency. Multi-h CPM appeared as a generalization of single-h schemes so as to decrease further the need for bandwidth expansion over the wireless channel. Despite the interesting characteristics of CPM, the decoding of the received signal is particularly difficult in a multi-path wireless environment with no diversity. To provide some level of diversity, several authors have proposed to combine CPM with Space-Time Block Coding (STBC). A new family of codes for CPM, based on L2-orthogonality was recently introduced in [8]. These full rate codes achieve full diversity and a low decoding complexity. In this paper, we detail a non trivial extension of these L2-orthogonal Space-Time codes using multi-h signaling schemes. These new codes achieve full diversity and a better spectral compactness by utilizing the available communication bandwidth more efficiently. Also, the decoding complexity is greatly decreased by using only one correlation filter bank for the detection of all transmitted signals.

Cognitive radio (CR) aims at enabling the coexistence between legacy systems called primary users (PU) and secondary users (SU), provided the SU do not (significantly) cause interference on PU links. Massive MIMO systems on the other hand yield significant performance and reliability enhancement. In this paper, we investigate the capacity of both PU and SU systems using multi-user (MU) massive MIMO and time division duplex (TDD). TDD indeed permits to estimate the channel state information (CSI) at the PU/SU transmitters to perform linear precoding, taking advantage of the channel reciprocity. The capacity is derived considering the CSI imperfections, the non-reciprocal radio frequency front-ends as well as antenna coupling effects. These impairments can be compensated using reciprocity calibration techniques we recently developed. Extending these techniques to MU massive MIMO, we evaluate interference cancellation precoders. Simulations reveal performance improvements using the massive MIMO reciprocity-based precoder in a CR approach.

Non-linear modulations, like the Continuous phase modulation (CPM) with its constant envelope property, have been appealing for energy efficient communication systems. In parallel, linear orthogonal Space-Time block codes (STBC) have emerged as a simple way of achieving spectral efficiency by full diversity and simple decoupled maximum-likelihood decoding. However, linear codes rely on pointwise orthogonality which leads to a well-known degradation of data rate for more than two antennas. In this paper, we introduce the concept of L2-orthogonality for non-linear Space-Time codes (STC). Our approach generalizes code design based on pointwise orthogonality. Namely, we are able to derive new codes with the same advantages as pointwise orthogonal STBC, i.e. low decoding complexity and diversity gain. At the same time, we are no longer limited by the restrictions of pointwise orthogonal codes, i.e. the reduction in data rate. Actually, we show how to construct full rate codes for any arbitrary number of transmit antennas. More precisely, a family of codes for continuous phase modulation (CPM) is detailed. The L2-orthogonality of these codes is ensured by a bank of phase correction functions which maintains the phase continuity but also introduces frequency offsets. We prove that these codes achieve full diversity and have full rate. Moreover, these codes don't put any restriction on the CPM parameters.

Blind deterministic receivers for DS-CDMA systems based on the PARAFAC model have been proposed in several papers since their conception in. In many cases, the transmitted signals can be considered uncorrelated. Hence, we develop PARAFAC receivers for uncorrelated signals. We introduce several numerical algorithms for orthogonality constrained PARAFAC on which receivers for uncorrelated signals can be based. Simulation results show an increase in performance when the PARAFAC receiver takes the uncorrelatedness of the transmitted signals into account.

In current DSL systems, crosstalk is a major source of performance degradation. In the case of in-domain crosstalk and AWGN, it has been shown that the crosstalk can be effectively mitigated using a linear zero-forcing canceler. Furthermore, the complexity can be reduced by only canceling the crosstalk from major crosstalkers on each tone, which is reffered to as partial cancellation. However, such approach does not work e.g. in the case of out-of-domain (alien) crosstalk. As alien crosstalk is spatially correlated, the zero-forcing canceler performs very poorly, hence an alternative linear canceler has to be examined. In this paper, we demonstrate that an MMSE crosstalk canceler provides improved performance compared to the zero-forcing canceler in this scenario and additionally, we also present an efficient algorithm to perform MMSE partial cancellation.

Mobile users with single antennas can still take advantage of spatial diversity through cooperative space-time encoded transmission. In this paper, we consider a scheme in which a relay chooses to cooperate only if its source-relay channel is of an acceptable quality and we evaluate the usefulness of relaying when the source acts blindly and ignores the decision of the relays whether they may cooperate or not. In our study, we consider the regenerative relays in which the decisions to cooperate are based on a signal-to-noise ratio (SNR) threshold and consider the impact of the possible erroneously detected and transmitted data at the relays. We derive the end-to-end bit-error rate (BER) expression and its approximation for binary phase-shift keying modulation and look at two power allocation strategies between the source and the relays in order to minimize the end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide well with our analytical results.

In this paper, we present a novel iterative receiver for MIMO-OFDM systems with synchronous interferers. The receiver is derived based on the Kullback-Leibler divergence minimization framework, and combines channel estimation, interference cancellation and residual noise estimation in an iterative manner. By using both the pilot and data symbols, the channel estimator improves the accuracy of the estimates in each iteration, which leads to a more effective interference cancellation and data detection process. A performance evaluation based on Monte-Carlo simulations shows that the proposed scheme can effectively mitigate the effect of interferers, and operates very close to the single-user performance even in severe interference scenarios.

In this paper we present an alternative separable implementation of L2-orthogonal space-time codes (STC) for continuous phase modulation (CPM). In this approach, we split the STC CPM transmitter into a single conventional CPM modulator and a correction filter bank. While the CPM modulator is common to all transmit antennas, the correction filter bank applies different correction units to each antenna. Thereby desirable code properties as orthogonality and full diversity are achievable with just a slightly larger bandwidth demand. This new representation has three main advantages. First, it allows to easily generalize the orthogonality condition to any arbitrary number of transmit antennas. Second, for a quite general set of correction functions that we detail, it can be proved that full diversity is achieved. Third, by separating the modulation and correction steps inside the receiver, a simpler receiver can be designed as a bank of data independent inverse correction filters followed by a single CPM demodulator. Therefore, in this implementation, only one correlation filter bank for the detection of all transmitted signals is necessary. The decoding effort grows only linearly with the number of transmit antennas.

In this paper, we present a novel approach for pilot-aided channel estimation in OFDM systems with synchronous co-channel interference. The estimator is derived based on the Kullback-Leibler divergence minimization framework. The obtained solution iteratively updates both the desired user's and the interferer's channels, using a combination of linear minimum mean squared-error (LMMSE) filtering and interference cancellation, avoiding the complex matrix inversions involved in the full LMMSE channel estimation approach. Estimation of the noise variance is also included in the iterative algorithm, accounting for Gaussian noise and residual interference after each iteration. The estimates of both channels are used at the equalizer to reject the interfering signal, thus mitigating the degradation due to co-channel interference. Simulation results show that the receiver using the proposed estimator performs as good as the one employing the full LMMSE estimator and very closely to a receiver having perfect knowledge of the channel coefficients.

Space-time block codes can be represented by tensors, like for example the Kathri-Rao space time codes introduced by Sidiropoulos. In this paper, we introduce a parallel factor analysis 2 (PARAFAC2) model for orthogonal space-time block codes (OSTBC). This model, along with a small modification at the transmitter and the use of tensor diagonalisation designed for orthogonal tensors at the receiver, leads us to develop a generic parallel factor analysis (PARAFAC) blind receiver for OSTBC. Unlike most tensor based blind receivers, our channel estimator is based on a data tensor rather than on a cumulant tensor.

The paper deals with turbo detection techniques for Single User Multiple-Input-Multiple-Output (SU MIMO) antenna schemes. The context is on the uplink of the upcoming Long Term Evolution - Advanced (LTE-A) systems. Iterative approaches based on Parallel Interference Cancellation (PIC) and Successive Interference Cancellation (SIC) are investigated, and a low-complexity solution allowing to combine interstream interference cancellation and noise enhancement reduction is proposed. Performance is evaluated for Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Frequency Division Multiplexing (SC-FDM) as candidate uplink modulation schemes for LTE-A. Simulation results show that, in a 2times2 antenna configuration, the turbo processing allows a consistent improvement of the link performance, being SC-FDM the one having higher relative gain with respect to linear detection. The turbo receiver's impact is however much reduced for both modulation schemes in a 2times4 configuration, due to the higher diversity gain provided by the additional receive antennas.

To combine the power efficiency of continuous phase modulation (CPM) with enhanced performance in fading environments, some authors have suggested to use CPM in combination with space-time codes (STC). Recently, we have proposed a CPM ST-coding scheme based on L2-orthogonality for two transmitting antennas. In this paper we extend this approach to the three antennas case. We analytically derive a family of coding schemes which we call parallel code (PC). This code family has full rate and we prove that the proposed coding scheme achieves full diversity as confirmed by accompanying simulations. We detail an example of the proposed ST codes that can be interpreted as a conventional CPM scheme with different alphabet sets for the different transmit antennas which results in a simplified implementation. Thanks to L2-orthogonality, the decoding complexity, usually exponentially proportional to the number of transmitting antennas, is reduced to linear complexity.

Per-tone pulse shaping has been proposed as an alternative to time domain spectral shaping for discrete multi-tone (DMT) transmitters, e.g. VDSL modems. This enables the transmitter to use more tones without violating the power spectral density (PSD) mask constraint for data transmission. The computational complexity of the per-tone pulse shaping and transmit power is evenly distributed over tones, however, resources (computational complexity and power) can be better exploited by using different filter lengths for different tones and by resorting to power loading. For a fixed pulse shaping filter length, the contribution of a particular tone to the stop band energy depends on the power allocated to the tone and on the distance of the tone from the band edges. The use of high order pulse shaping filters for the tones at the band edges (as well as the use of lower power) will reduce their contribution to the out of band PSD, whereas for the tones at the middle of the band these factors will have less effect on the out of band PSD. Therefore, the combination of both power loading and a variable length pulse shaping filter can be used to achieve a high data rate under resource and PSD constraints. In this paper we present an algorithm to optimally allocate the resources i.e. power and filter taps, using a dual problem formulation. This solves the problem of optimally distributing power and filter taps over tones for a given PSD mask constraint, with a relatively low complexity.

Per-tone pulse shaping has been proposed as an alternative to time domain spectral shaping for DMT transmitters, e.g. VDSL modems. It shapes the spectrum of individual tones such that the stop band energy of each tone can be minimized. This in particular enables transmitter to use more tones without violating the PSD mask constraint for data transmission. In per-tone pulse shaping based DMT transmitters a fixed length pulse shaping filter is typically used for every tone. The tones in the middle of the pass band however, contribute less to the overall stop band energy, so that using a high order pulse shaping filter for these tones does not result in a significant reduction of the stop band energy. As a result a significant number of pulse shaping filter taps are wasted on the tones in the middle of the pass band and do not bring any performance gain. Using a variable length pulse shaping filter which is designed such that the PSD mask constraint is not violated can then significantly reduce the total number of pulse shaping filter taps without compromising performance. In this paper, a resource allocation technique is presented for variable length pulse shaping filter design using a dual problem formulation. This optimally solves the problem of pulse shaping filter tap distribution over tones for given PSD mask constraints, with a relatively low complexity.

The technical realization of modern wireless receivers yields significant interfering IQ-imbalances, which have to be compensated digitally. To cancel these IQ-imbalances, we propose an algorithm using iterative blind source separation (IBSS) as well as information about the modulation scheme used (hence the term semi-blind). The novelty of our approach lies in the fact that we match the nonlinearity involved in the IBSS algorithm to the probability density function of the source signals. Moreover, we use approximations of the ideal non-linearity to achieve low computational complexity. For severe IQ-mismatch, the algorithm leads to 0.2 dB insertion loss in an AWGN channel and with 16-QAM modulation.

Mobile users with single antennas can still take advantage of spatial diversity through cooperative space-time encoded transmission. In this paper, we consider a scheme in which the relay chooses to cooperate only if the source-relay channel is of an acceptable quality and we evaluate the usefulness of relaying when the source acts blindly and ignores the decision of the relay whether it may cooperate or not. In our study, we consider a regenerative relay in which the decision to cooperate is based on a signal-to-noise ratio (SNR) threshold and consider the impact of the possible erroneously detected and transmitted data at the relay. We derive the end-to-end bit-error rate (BER) for binary phase-shift keying modulation and look at two power allocation strategies between the source and the relay in order to minimize the end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide with our analytical results.

In this paper, we investigate different multiple-input multiple-output (MIMO) receiver structures based on MMSE filtering and sequential interference cancellation (SIC) for the downlink of the 3GPP long term evolution (LTE) system. We divide them into two approaches: symbol-SIC receivers, in which the detection and interference cancellation is done independently for each subcarrier, and codeword-SIC structures, in which the processing is carried out for each independently-coded stream by including the turbo-decoder inside the feedback loop. The results show that symbol-SIC receivers need to take into account the propagation of errors in the interference cancellation to provide the turbo decoder with reliable soft bit values. However, these are clearly outperformed by codeword-SIC schemes, due to the error correction capabilities of the turbo-decoder inside the feedback loop. We show that the best tradeoff between computational complexity and receiver performance is achieved by only cancelling the interference of a codeword when this has been successfully decoded.

The paper proposes an innovative system to multiplex digital information over a cable network. The key principle is to artificially create multiple reflections in a controlled way and use multi-path propagation. At the transmitter side, the information is modulated by letting termination impedances vary. At the receiver side, network scattering parameters are measured for different frequencies and used by supervised classification techniques to recover the digital streams of the transmitters. The system is optimized through the joint design of four parts: the transmitter modulation scheme, the cable network characteristics, the measured scattering parameters and the receiver algorithm. We particularly investigate the network topology, the measurement frequencies, and the distance metrics. A low-complexity receiver algorithm is presented.

A novel method to multiplex digital information over cable networks has recently been proposed [1]. This new method baptized ScDMA - standing for Scattering Division Multiple Access - leads to unprecedented advantages [2]. Two important differences with usual communication systems have to be highlighted. Firstly, the usual flow of physical signals carrying information is changed. Even though information is sent by the source, the source does not emit any physical signal. Indeed, the receiver reads the information of the source by sending a signal and analyzing the reflected signal. Secondly, the channel is somehow controlled in a deterministic manner, while in usual communication systems the channel is uncontrolled and considered as random. These two differences have impacts on the theoretical limit performance. In particular the usual expression of the system capacity C is modified: the maximization of the average mutual information I(X; Y) has to be done not only on the input symbol probabilities P(X) but also on the conditional probabilities P(Y|X) since the channel itself can be controlled. In addition, the case of the multi-rate multi-user capacity leads to unexpected results.

We consider a multiuser MAC fading channel with two users communicating with a common destination, where each user mutually acts as a relay for the other one as well as transmits his own information. We propose a power control-enhanced cooperative transmission scheme allowing each user to allocate a certain amount of power for his own transmitted data while the rest is devoted to relaying. The underlying protocol is based on a modification of the so-called non-orthogonal amplify and forward (NAF) protocol in Azarian, K. et al, (2005). We develop capacity expressions for our scheme and derive the rate-optimum power allocation, in closed form. Our results indicate that even in a mutual cooperation setting like ours, on any given realization of the channel, one of the users will always allocate zero power to relaying the data of the other one, and thus act selfishly.

This paper studies common linear frequency direction pilot- symbol aided channel estimation algorithms for orthogonal frequency division multiplexing in a UTRA long term evolution context. Three deterministic algorithms are analyzed: the maximum likelihood (ML) approach, the noise reduction algorithm (NRA) and the robust Wiener (RW) filter. A closed form mean squared error is provided for these three algorithms. Analytical and simulation results show that, in the presence of virtual subcarriers, the ML can suffer large performance degradation due to ill-conditioned matrix issues. A solution to this problem is to use the Tikhonov regularization method giving the NRA. The equivalence between the NRA and the RW is proved analytically. A practical implementation of the NRA and RW is proposed based on partial-input partial-output FFT, leading to 6 to 8 times lower complexity than the reference implementation.

Mobile users with single antennas can use spatial transmission diversity through cooperative space-time encoded transmission. This paper presents an end-to-end performance analysis of two-hop asynchronous cooperative diversity with regenerative relays over Rayleigh block-flat-fading channels. We present a preceding frame-based scheme with packet-wise encoding which enables best synchronization and channel estimation. We derive the bit-error rate and the end-to-end bit-error rate expressions for binary phase-shift keying. We present the performance of the frame-error rate and the end-to-end frame-error rate. Finally, comparisons between three system configurations, differing by the amount of cooperation, are presented. The influence of the amount of cooperation is small (about 2dB), Furthermore, simulations show that the analytical results are correct at all SNRs.

The design of spectrally efficient and robust schemes is vital to overcome the harsh wireless environment when delivering high data rate services. In the present paper, the performance of a constant power and discrete rate adaptation scheme is evaluated in a multiple antenna scenario. Such a technique has low complexity, and needs a low rate feedback channel to adapt the transmission rate. However, the feedback can become outdated when the channel is rapidly changing. In the light of the feedback delay, the authors propose to analyze the effect of outdated channel information. Closed form expressions are obtained for the bit-error-rate (BER) performance for MIMO diversity, and spatial multiplexing (SM) schemes. Through simulation results, we analyze the impact of feedback delay on the BER. The feedback delay proves to have different impact on each of the MIMO scheme when coupled to a rate adaptive system. We demonstrate that antenna selection suffers more from feedback delay than other diversity schemes. The results can be used to assess the performance of different MIMO techniques, and may be applied to a switching mechanism, where, at each instant, the channel behavior is analyzed and the best fit MIMO mode is selected

To approach the potential multiple-input multiple-output (MIMO) capacity while optimizing the system bit-error rate (BER) performance, the joint transmit and receive minimum mean squared error (joint Tx/Rx MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams p as well as fixed modulation and coding across these spatial streams. However, the number of spatial streams has been arbitrarily chosen and fixed, which may lead to an inefficient power allocation strategy and a poor BER performance. In this paper, we relax the constraint of fixed number of streams p and optimize this value for the current channel realization, under the constraints of fixed average total transmit power P/sub T/ and fixed rate R, what we refer to as mode selection . Based on the observation of the existence of a dominant optimal number of streams value for the considered Rayleigh flat-fading MIMO channel model, we further propose an "average" mode selection that avoids the per-channel adaptation through using the latter dominant value for all channel realizations. Finally, we exhibit the significant BER improvement provided by our mode selection over the conventional joint Tx/Rx MMSE design. Such significant improvement is due to the better exploitation of the MIMO spatial diversity and the more efficient power allocation enabled by our mode selection.

The combination of space-time block coding (STBC) and direct-sequence code division multiple access (DS-CDMA) has the potential to increase the performance of multiple users in a cellular environment. However, if not carefully designed, the resulting communication scheme suffers from increased multi-user interference (MUI), which dramatically deteriorates the performance. To tackle this MUI problem in the downlink, we combine two specific CDMA and STBC techniques, namely single-carrier block-spread (SCBS) CDMA and time-reversal (TR) STBC. The resulting transceiver allows for deterministic maximum likelihood (ML) user separation through low-complexity code-matched filtering as well as deterministic transmit stream separation through linear processing. These properties guarantee maximum diversity gains of N/sub T/N/sub R/(L+1) for every user in the system, irrespective of the system load, where N/sub T/ is the number of transmit antennas, N/sub R/ the number of receive antennas and L the order of the underlying multipath channels. Moreover, it turns out that a low-complexity linear receiver based on frequency-domain equalization comes close to extracting the full diversity in reduced as well as full load settings.

Today a lot of attention is spent on developing inexpensive OFDM receivers. Especially, zero-IF receivers are very appealing, because they avoid costly IF filters. However, this implies IQ demodulation at RF, which therefore cannot be done digitally and thus introduces IQ mismatch. Unfortunately, OFDM is very sensitive to receiver IQ imbalance. Therefore, we developed a new compensation scheme to combat the IQ imbalance at baseband. In this paper, we describe the algorithm and represent the performance results. Our compensation scheme eliminates the IQ imbalance almost perfect. This leads to tremendous improvements, especially in multi-path channels (up to 10 dB performance gain), and enables low-cost zero-IF receivers.

The combination of space-time block coding (STBC) and direct-sequence code-division multiple access (DS-CDMA) has the potential to increase the performance of multiple users in a cellular network. However, if not carefully designed, the resulting transmission scheme suffers from increased multiuser interference (MUI), which dramatically deteriorates the performance. To tackle this MUI problem in the downlink, we combine two specific DS-CDMA and STBC techniques, namely single-carrier block transmission (SCBT) DS-CDMA and time-reversal STBC. The resulting transmission scheme allows for deterministic maximum-likelihood (ML) user separation through low-complexity code-matched filtering, as well as deterministic ML transmit stream separation through linear processing. Moreover, it can achieve maximum diversity gains of N/sub T/N/sub R/(L+1) for every user in the system, irrespective of the system load, where N/sub T/ is the number of transmit antennas, N/sub R/ the number of receive antennas, and L the order of the underlying multipath channels. In addition, it turns out that a low-complexity linear receiver based on frequency-domain equalization comes close to extracting the full diversity in reduced, as well as full load settings. In this perspective, we also develop two (recursive) least squares methods for direct equalizer design. Simulation results demonstrate the outstanding performance of the proposed transceiver compared to competing alternatives.

Orthogonal frequency-division multiplexing with cyclic prefix enables low-cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum-likelihood channel estimator. In addition to being optimal (up to the modeling error), this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth which necessitates a high spectral efficiency. A combined orthogonal frequency division multiplexing (OFDM)/spatial division multiple access (SDMA) approach can efficiently tackle both obstacles and paves the way for cheap, high-capacity wireless indoor networks. The channel distortion due to multipath propagation is efficiently mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. However, to keep the cost of an indoor wireless network comparable to its wired counterpart's cost, low-complexity SDMA processors with good performance are of special interest. In this paper, we propose a new multiuser SDMA detector which is designed for constant modulus signals. This constrained least squares (CLS) receiver, which deterministically exploits the constant modulus nature of the subcarrier modulation to achieve better separation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receiver. Additionally, since the CLS detector relies on reliable channel knowledge at the receiver, we propose a strategy for estimating the multiple input multiple output (MIMO) channels. Simulations for a Hiperlan II-based case-study show that the CLS detector significantly outperforms the ZF detector and comes close to the performance of the MMSE detector for QPSK. For higher order M-PSK, the CLS detector outperforms the MMSF detector. Furthermore, the estimation complexity for the CLS detector is substantially lower than that for the MMSE detector which additionally requires estimation of the noise power.

Single-carrier block transmission (SCBT) DS-CDMA, also known as chip-interleaved block-spread (CIBS) CDMA, is an interesting transmission technique for future broadband cellular systems because it inherits the benign properties of both SCBT and CDMA. By zero padding (ZP) each chip block, the orthogonality of the spreading codes is retained regardless of the underlying multipath channel which allows for deterministic maximum likelihood (ML) user separation employing low-complexity code-matched filtering. In this paper, we focus on downlink single-carrier block transmissions with known symbol padding (KSP) (as opposed to ZP) which pad each chip block with a postfix of known symbols that can be used for training purposes at the receiver. Specifically, we propose three methods for direct equalizer estimation that all exploit the presence of the known symbol postfix but differ in the amount of additional a-priori information they assume to determine the equalizer coefficients. Simulation results demonstrate the outstanding performance of the semi-blind joint CDMP/KSP-trained method, that additionally assumes knowledge of a code division multiplexed pilot (CDMP) and the multiuser code correlation matrix.

In this paper, we extend space-time block coding techniques, originally proposed for point-to-point communication links, to point-to-multipoint communication links, thereby taking into account the multiple access technique in the design of the transmission scheme. In specific, we propose two per-tone linear space-time chip equalizers for a space-time coded MC-DS-CDMA downlink with linear precoding. Both the training-based and the semi-blind chip equalizer exploit the presence of a continuous code-multiplexed pilot in the transmitted signal but differ in the amount of a-priori information they assume to estimate their coefficients. With Mttransmit antennas at the base-station, Mrreceive antennas at the mobile station and L the order of the multipath channel, they come close to extracting the full diversity of order Mt· Mr· (L + 1) in reduced as well as full load settings.

In the downlink of WCDMA systems, multi-path propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Chip-level equalization can restore the orthogonality and suppress the MUI. However, adaptive implementations of the chip-level equalizer receiver that can track time-varying multi-path channels are hard to realize in practice for two reasons: loss of spectral efficiency when using a training sequence and loss of performance when using pure blind techniques. We propose new training-based and semi-blind space-time chip-level equalizer receivers for the downlink of WCDMA systems employing long spreading codes and a continuous code-multiplexed pilot. The proposed receivers exploit the presence of common pilot symbols in the so-called Common PIlot CHannel (CPICH) of the Universal Terrestrial Radio Access (UTRA) for 3G systems. Moreover, they can simultaneously track multiple base-station signals, whenever the mobile station enters soft handover mode. For both receivers, we derive a recursive least squares (RLS) algorithm for adaptive processing. The proposed receivers are compared with the conventional space-time RAKE receiver and the ideal space-time chip-level equalizer receiver both in terms of performance and complexity.

In the downlink of traditional single input single output (SISO) DS-CDMA systems, multipath propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Chip equalization can restore the orthogonality and suppress the MUI. In the context of multiple input multiple output (MIMO) DS-CDMA systems, spatial multiplexing emits independent data streams from the M/sub T/ transmit antennas which allows one to significantly enhance the spectral efficiency. However, combined with DS-CDMA, spatial multiplexing also leads to an M/sub T/-fold increase of the MUI. Linear chip equalization only is not able to cope with the induced MUI. In this paper, we therefore propose a new combined linear and non-linear multi-user receiver for the spatially multiplexed DS-CDMA (SM DS-CDMA) downlink. The proposed receiver consists of an initial linear chip equalization stage and possibly multiple non-linear parallel interference cancellation (PIC) stages with RAKE combining. With a single PIC/RAKE stage, it achieves a 5.1 dB gain given a BER of 10/sup -3/, compared to the linear MMSE receiver only. A second PIC/RAKE stage brings an additional 0.9 dB gain at the expense of increased complexity.

In the downlink of WCDMA systems, multipath propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Chip-level equalization can restore the orthogonality and suppress the MUI. However, adaptive implementations of the chip equalizer receiver that can track time-varying multipath channels are hard to realize in practice. In this paper, we propose new training-based and semi-blind space-time chip equalizer receivers for the downlink of WCDMA systems employing long spreading codes and a code-multiplexed pilot. The proposed receivers exploit the presence of common pilot symbols in the so-called Common Pilot Channel (CPICH) of the Universal Terrestrial Radio Access (UTRA) for 3G systems. Moreover, they can simultaneously track multiple base-station signals, whenever the mobile station enters soft handover mode. For both receivers, we derive a Least Squares algorithm for block processing and a Recursive Least Squares algorithm for adaptive processing. The proposed receivers are compared in terms of performance with the conventional space-time RAKE receiver and the ideal fully-trained space-time chip equalizer receiver.

Link adaptation (LA) can significantly increase the spectral efficiency and render various levels of services, both of which are highly desirable for today's high-speed communication systems. Although optimality can be ensured through careful designing for a fixed link configuration, due to changes of available power, data rate, and especially channel statistics, it is better to adapt the adaptation scheme per se in real time. We term this technique adaptive link adaptation (ALA). Based on our previous work on a general optimal link adaptation scheme for a certain configuration, we continue to investigate the problem of adapting the adaptation scheme online. This paper first recapitulates that general optimal link adaptation scheme, then gives the theoretic basis of ALA, and finally presents a simple yet working implementation of the update algorithm for ALA. Both theoretical and simulation results indicate the viability of ALA and an 18 dB gain can be achieved for the investigated simple scenario. With low complexity, optimality, and generality, the proposed ALA provides a promising technique to substantially increase the spectral efficiency of practical communication systems.

The two major obstacles towards high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth. A combined OFDM/SDMA approach efficiently tackles both obstacles and paves the way for low cost, high-capacity wireless indoor networks (see Vandenameele, P. et al., VTC Spring, p.1712-16, 1999; IEEE J. Selected Areas in Commun., vol.18, no.11, p.2312-21, 2000). Channel distortion due to multipath propagation is efficiently mitigated by orthogonal frequency division multiplexing (OFDM) while the bandwidth efficiency is increased by the use of spatial diversity multiple access (SDMA). To keep the WLAN cost low, simple SDMA processors with good performance are necessary. Hence, we propose a new, low-complexity multiuser SDMA detector, which is designed for constant modulus signals. This linear constrained least squares (CLS) receiver, which explicitly exploits the constant modulus nature of subcarrier modulation to achieve better separation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receivers. Simulations show that the CLS detector significantly outperforms the ZF detector and comes close to the performance of the MMSE detector for QPSK. For higher order MPSK, the CLS detector outperforms the MMSE detector substantially.

Orthogonal frequency-division multiplexing (OFDM) with cyclic prefix enables low cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum likelihood (ML) channel estimator. In addition to being optimal, this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

In the forward link of WCDMA systems, multipath propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Channel equalization can restore the orthogonality and suppress the MUI. However, adaptive implementations of the chip equalizer receiver that can track time-varying multipath channels are hard to realize in practice. In this paper, we propose new training-based and semi-blind space-time chip equalizer receivers for the forward link of WCDMA systems employing long spreading codes and a time-multiplexed pilot. The proposed receivers exploit the presence of user specific pilot symbols in the so-called dedicated physical control channel (DPCCH) of the Universal Terrestrial Radio Access (UTRA). Whereas the DPCCH-trained receiver only assumes knowledge of the desired user's pilot symbols and code sequence in a training-based cost function, the enhanced DPCCH-trained receiver assumes knowledge of each active user's pilot symbols and code sequence in a semi-blind cost function. For both receivers, we derive a least squares algorithm for block processing and a recursive least squares algorithm for adaptive processing. Both receivers can track time-varying multipath channels and outperform the conventional RAKE receiver with perfect channel knowledge. The enhanced DPCCH-trained chip equalizer receiver outperforms the regular DPCCH-trained chip equalizer receiver and comes close to the performance of the ideal fully-trained chip equalizer receiver.

Link adaptation (LA) can significantly increase the spectral efficiency and meantime provide services of various qualities, both of which are highly desirable in today's high-speed communication systems. However how to perform precise switching optimally in practical systems involving non-flat fading channels and several adaptive techniques is seldom treated. We therefore investigate this problem and propose a general optimal switching scheme. This paper gives a framework of such a scheme for link adaptation and a detailed designing procedure. We present a vigorous proof of the optimality condition and show that the optimal switching boundaries have a linear relationship, which is independent of coding, modulation and channels. Also, we extend this scheme to multicarrier, MIMO and multiservice systems to ensure global optimality. With low complexity, optimality, and generality the proposed general optimal switching scheme provides a promising technique to substantially increase the spectral efficiency of practical communication systems.

The two major obstacles towards high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth which necessitates a high spectral efficiency. The combination of single carrier with cyclic prefix (SC-CP) modulation and spatial division multiple access (SDMA) tackles both obstacles and paves the way for low cost, high-capacity wireless indoor networks (see Vandenameele, P. et al., ICASSP, p.3714-17, 2000). The channel distortion due to multipath propagation is elegantly mitigated with SC-CP while the bandwidth efficiency is increased with the use of SDMA. However, to keep the WLAN cost low, simple SDMA processors with good performance are necessary. Hence, we propose a new low-complexity multiuser SDMA detector, which is designed for constant modulus signals. This linear constrained least squares (CLS) receiver, which explicitly exploits the constant modulus nature of the modulation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receiver. Simulations show that the CLS detector significantly outperforms the ZF detector and comes very close to the performance of the MMSE detector. Furthermore, the estimation complexity for the CLS detector is substantially lower than for the MMSE detector, which additionally requires estimation of the noise power.

In recent years, wireless indoor networks have received a lot of scientific and industrial attention. Most systems rely on the use of orthogonal frequency division multiplexing (OFDM) because of its capability to elegantly cope with multipath interference. However, while OFDM provides a nice solution for the digital modem, its front-end requirements should be investigated as well. To that goal, we have set up a simulation environment which comprises both the digital modem and the most important front-end nonidealities. We show that for the same data rate, bandwidth and transmit power constraints, Single-Carrier with Cyclic Prefix (SC-CP) allows the design of a more power-efficient modem than OFDM and is therefore a better candidate for portable wireless terminals.

Frequency domain equalization has gained a lot of attention in the last decade, mainly pushed by the OFDM (orthogonal frequency division multiplexing) communication scheme. It has been adopted, in various flavours, for, among others, wireless LANs and xDSL. The technique is indeed leading to low complexity implementation, at the cost of a cyclic prefix, used to avoid inter block interference (IBI). The idea has been adapted to single carrier communication, almost "as is", yielding the same computational advantages and avoiding the high crest factor of OFDM. We name this scheme CP-SC (cyclic prefix-single carrier). We take a new look at this single carrier scheme, and, with a slight modification, show that the cyclic prefix can be implemented as a training sequence, and hence play two important roles: avoid IBI and help in synchronisation and channel estimation. The latter topic is of utter importance in fast fading situations (e.g. mobile). This new training aided frequency domain equalized single carrier (we name it TASC) scheme offers these advantages at the expense of only a small fraction of a dB (in terms of E/sub b//N/sub 0/). All these arguments make TASC an interesting candidate in situations where multipath and fast fading are present, while in other situations it has hardly no drawback compared to CP-SC.

We address the problem of blind multiuser multichannel identification in a spatial division multiple access (SDMA) context. Using a stochastic model for the input symbols and only second order statistics, we develop a simple algorithm, based on the generalized Schur algorithm to apply LDU decomposition of the covariance matrix of the received data. We show that this method leads to identification of the channel, up to a unitary mixture matrix. Furthermore, the identification algorithm is shown to be robust to channel length overestimation and approaches the performance of the weighted linear prediction (WLP) method, at low computational cost.