Iterative semi-blind multiuser detection for coded MC-CDMA uplink system

We propose two types of iterative semi-blind receivers for coded multicarrier code-division multiple-access (MC-CDMA) uplink systems in the presence of both intracell and intercell interference. The first is based on the minimum mean-square error criterion, and the second is a hybrid scheme, consisting of parallel interference cancellation and linear multiuser detection. These iterative receivers utilize known users' information for the computation of log-likelihood ratios (LLR) while blindly suppressing unknown interference. The LLR are refined successively during the iterative process through decoding of all known users. Simulation results demonstrate that the proposed iterative semiblind methods offer substantial performance gain over conventional noniterative and nonblind iterative receivers.     

Iterative space-time processing for multiuser detection in multipath CDMA channels

Space-time processing and multiuser detection are two promising techniques for combating multipath distortion and multiple-access interference in code division multiple access (CDMA) systems. To overcome the computational burden that rises very quickly with increasing numbers of users and receive antennas in applying such techniques, iterative implementations of several space-time multiuser detection algorithms are considered here. These algorithms include iterative linear space-time multiuser detection, Cholesky iterative decorrelating decision-feedback space-time multiuser detection, multistage interference canceling space-time multiuser detection, and expectation-maximization (EM)-based iterative space-time multiuser detection. A new space-time multiuser receiver structure that allows for efficient implementation of iterative processing is also introduced. Fully exploiting various types of diversity through joint space-time processing and multiuser detection brings substantial gain over single-receiver-antenna or single-user-based methods. It is shown that iterative implementation of linear and nonlinear space-time multiuser detection schemes discussed in this paper realizes this substantial gain and approaches the optimum performance with reasonable complexity. Among the iterative space-time multiuser receivers considered in this paper, the EM-based (SAGE) iterative space-time multiuser receiver introduced here achieves the best performance with excellent convergence properties.     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

Iterative receivers for multiuser space-time coding systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called “turbo multiuser detection” technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. An iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals. Computer simulations demonstrate that the proposed iterative receiver techniques provide significant performance improvement over conventional noniterative methods in both single-user and multiuser STC systems. Furthermore, the performance of the proposed iterative multiuser receiver approaches that of the iterative single-user receiver in both STBC and STTC systems     

Iterative space-time soft interference cancellation for UMTS-FDD uplink

Two efficient pilot-aided iterative space-time interference cancellation receivers are studied in order to increase the uplink capacity of the Universal Mobile Telecommunication System (UMTS) in frequency-division duplex (FDD) mode. Both iterative schemes use low-complexity beamforming and path combining techniques associated with soft-input soft-output decoding to mitigate the multiple-access interference in space and time. The difference between the two techniques is in the way they deal with unknown channels: the addition of a space-time channel estimation in each iteration on the one hand and iterative adaptive beamforming and path combining on the other hand. Thanks to the iterative structure, the observation signal used for estimation or adaptation contains less interference from one iteration to the following, and soft estimates of coded bits are available for data-aided estimation or adaptation. A detailed complexity analysis shows that renewing beamforming and path combining in each iteration without a-priori knowledge of the channel has no significant impact on the overall complexity of one iteration. Simulations of true UMTS-FDD uplink communications over a wideband directional channel model reveal that near-single user performance can be obtained for very high system loads, whereas more conventional receivers, such as the interference canceller without beamforming and the two-dimensional RAKE receiver, fail in recovering the transmitted information.     

Group-blind multiuser detection for uplink CDMA

Previously developed blind techniques for multiuser detection in code division multiple access (CDMA) systems lead to several near-far resistant adaptive receivers for demodulating a given user's data with the prior knowledge of only the spreading sequence of that user. In the CDMA uplink, however, typically the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper, group-blind techniques are developed for multiuser detection in such scenarios. These new techniques make use of the spreading sequences and the estimated multipath channels of all known users to suppress the intracell interference, while blindly suppressing the intercell interference. Several forms of group-blind linear detectors are developed based on different criteria. Moreover, group-blind multiuser detection in the presence of correlated noise is also considered. In this case, two receiving antennas are needed for channel estimation and signal separation. Simulation results demonstrate that the proposed group-blind linear multiuser detection techniques offer substantial performance gains over the blind linear multiuser detection methods in a CDMA uplink environment     

Adaptive space-time reduced-rank estimation based on diversity-combined decimation and interpolation applied to interference suppression in CDMA systems

An adaptive low-complexity space-time reduced-rank processor is proposed for interference suppression in asynchronous DS code division multiple access (CDMA) systems based on a diversity-combined decimation and interpolation method. The novel design approach for the processor employs an iterative procedure to jointly optimise the interpolation, decimation and estimation tasks for reduced-rank parameter estimation. Joint iterative least squares design parameter estimators are described and low-complexity adaptive recursive least squares (RLS) algorithms for the proposed structure are developed. To design the decimation unit, the optimal decimation scheme based on the counting principle is presented and lowcomplexity decimation structures are proposed. Linear space-time receivers with antenna arrays based on the proposed reduced-rank processor are then presented and investigated to mitigate multi-access interference and intersymbol interference in an asynchronous DS-CDMA system uplink scenario. An analysis of the convergence properties of the proposed space-time processor is carried out and analytical expressions are derived to predict the mean squared error performance of the proposed processor with RLS algorithms. Simulations show that the proposed processor outperforms the best known reduced-rank schemes at substantially lower complexity.     

Layered space-time multiuser detection over wireless uplink systems

In this paper, we investigate the use of layered space-time (also known as the vertical Bell Laboratories layered space-time (V-BLAST) scheme) for multiuser detection in fading channels. The multiple transmit antennas in V-BLAST are treated as individual mobile station transmitters, while the base station consists of multiple receive antennas. In the proposed system, users are organized in groups and allocated a unique spreading code within the same group. Using these codes, we are able to separate the different groups, and layered space-time algorithm is then invoked to further remove the remaining interference between users. A decorrelator-type receiver-based layered space-time detection is proposed for both complex and real constellations. For the latter case, we derive our receiver after evaluating and comparing the performance of two decorrelators based on the V-BLAST scheme. It is demonstrated that a significant performance improvement and increase in system capacity is obtained with very low spreading factors. Further results are also introduced by considering reduced complexity receivers based on serial layered space-time group multiuser detection, and parallel layered space-time group multiuser detection.     

A RAKE-based iterative receiver for space-time block-coded multipath CDMA

A turbo multiuser receiver is proposed for space-time block and channel-coded code division multiple access (CDMA) systems in multipath channels. The proposed receiver consists of a first stage that performs detection, space-time decoding, and multipath combining followed by a second stage that performs the channel decoding. A reduced complexity receiver suitable for systems with large numbers of transmitter antennas is obtained by performing the space-time decoding along each resolvable multipath component and then diversity combining the set of space-time decoded outputs. By exchanging the soft information between the first and second stages, the receiver performance is improved via iteration. Simulation results show that while in some cases a noniterative space-time coded system may have inferior performance compared with a system without space-time coding in a multipath channel, proposed iterative schemes significantly outperform systems without space-time coding, even with only two iterations. Furthermore, the performance loss in the reduced-complexity receiver due to decoupling of interference suppression, space-time decoding, and multipath combining is very small for error rates of practical interest.     

Coding for MIMO systems using matrix-alamouti and multi-user detection techniques

We investigate a space-time coding scheme with four or more transmit antennas based on a matrix-Alamouti scheme in a quasi-static fading environment. The receiver consists of an Alamouti combiner followed by an iterative joint detection and decoding process. A priori probabilities fed back from the decoder are used to both get more reliable estimates of the transmitted symbols and efficiently remove inter-block interference. Interblock interference is removed using techniques known in multiuser communication. The theoretical bounds of this scheme are established through the study of its outage probability.     

Interference robustness aspects of space-time block code-based transmit diversity

Transmit diversity, which was initially developed for noise-limited environments, has been promoted as a viable candidate for improving the link quality in both existing and future systems for wireless communication. However, to ensure efficient spectrum utilization, receivers operating within wireless multiuser networks must be robust not only to fading and noise but to interference from other system users as well. This work considers interference robustness aspects when transmit diversity, in the form of space-time block coding, is used in multiuser systems. Properties of the space-time block encoded signals such as code rate, block structure, diversity order, etc., and their implications on detection and interference rejection by means of noise whitening are discussed. To handle the presence of space-time block encoded interference, a space-time processing-based extension of an interference rejection combining algorithm is proposed. Results are presented indicating that transmit diversity based on space-time block codes (STBCs) of the linear dispersion type improve robustness against interference in terms of an increased diversity advantage. This can be achieved either by increasing the number of transmit antennas or by reducing the rate of the code. It is also shown, by analysis and by simulation examples, that the performance improvements obtained by using transmit diversity in multiuser systems may rapidly subside as the signal-to-interference ratio decreases. However, by using the proposed interference rejection scheme tailored to the space-time encoded structure, performance improvements of transmit diversity are also obtained in a multiuser environment.     

空时分组码MC-CDMA系统多用户检测

本文研究了在频率选择性瑞利衰落信道中工作于时分双工(TDD)模式的多载波码分多址(MC-CDMA)系统上行链路,在用户终端处使用两个发射天线,采用基于空时分组码的发射分集。考虑对应于子载波的衰落系数是信道冲激响应的离散傅里叶变换,给出了基于空时分组码的MC-CDMA系统上行链路信号模型。采用适用于同步CDMA系统的低代价Steiner估计器来进行基于空时分组码的MC-CDMA系统的信道估计,每个用户终端的两个发射天线各分配一个midamble。研究了基于空时分组码的MC-CDMA系统的解相关多用户检测、最小均方误差(MMSE)多用户检测,进行联合的多用户检测和空时码解码。仿真结果验证了上述模型及算法的有效性。     

MIMO iterative receiver SNR estimation strategies for link to system interfacing

This paper presents link to system (L2S) interfacing technique for multiple input and multiple output (MIMO) iterative receivers. In L2S interfacing, usually the post detection signal to noise ratio (SNR)‐based frame error rate lookup tables (LUT) are used to predict the link level performance of receivers. While L2S interfacing for linear MIMO receivers can be conveniently implemented, it is more challenging for MIMO iterative receivers due to unavailability of the closed form SNR expressions. In this paper, we propose three methods for post detection SNR estimation for MIMO iterative receivers. The first is based on the QR decomposition of the channel matrix, the second relies on the residual noise calculation based on the soft symbols, and the third exploits the closed form SNR expressions for linear receivers. A link to system interface model for iterative receivers is developed for evaluating the reference curves for different modulation and coding schemes, and results are validated by comparing the simulated and predicted frame error rates. It is shown that linear and residual noise‐based SNR approximations result in a very good prediction performance whereas the performance of QR decomposition‐based method degrades for higher order modulations and coding schemes. This paper presents link to system interfacing technique for MIMO iterative receivers. A link to system interface model for iterative receivers is developed for evaluating the reference curves for different modulation and coding schemes, and results are validated by comparing the simulated and predicted frame error rates. Three post detection SNR evaluation schemes have been proposed for link to system interfacing all of which give good prediction performance especially at lower order modulation.     

Multi-access interference suppression in canonical space-timecoordinates: A decentralized approach

We propose a decentralized space-time multiuser detection scheme based on the notion of canonical space-time coordinates (CSTCs) for representing the received signal. The CSTC representation provides a natural framework for decentralized multi-access interference (MAI) suppression in lower dimensional subspaces that results in complexity reduction relative to existing chip rate filtering schemes. The framework is based on a partitioning of the signal space into active and inactive CSTCs. The active CSTCs contain the signal of the desired user, facilitate maximal diversity exploitation and minimal complexity interference suppression. The inactive CSTCs only contain MAI and can be included progressively to attain a desired level of MAI suppression at the cost of increased complexity. We develop CSTC-based linear coherent multiuser detectors using the linearly constrained minimum variance (LCMV) criterion. We characterize the set of inactive coordinates and analyze the performance of the LCMV receiver as a function of the number of inactive CSTCs. Channel estimation and detector sensitivity to channel estimation errors are discussed. We demonstrate that the low-complexity adaptive receivers designed via the CSTC framework are more robust to channel estimation errors than existing chip-domain filtering schemes     

Space-time multiuser detection in multipath CDMA channels

The problem of multiuser detection in multipath CDMA channels with receiver antenna array is considered. The optimal space-time multiuser receiver structure is first derived, followed by linear space-time multiuser detection methods based on iterative interference cancellation. Blind adaptive space-time multiuser detection techniques are then proposed, which require prior knowledge of only the spreading waveform and the timing of the desired user's signal. Single-user-based space-time processing methods are also considered and are compared with the multiuser approach. It is seen that the proposed multiuser space-time processing techniques offer substantial performance gains over the single-user-based methods, especially in a near-far situation     

Linear receivers for the multiple-input multiple-output multiple-access channel

We consider a multiuser multiple-input multiple-output (MIMO) communication system using code-division multiple access (CDMA) and multiuser detection to discriminate the different users. Our focus is on the CDMA uplink of a frequency-nonselective Rayleigh fading channel. We study two types of receivers: joint receivers, which address simultaneously both spatial and multiple-access interference; and separate receivers, addressing the two types of interference individually. This approach allows assessing the benefits of adding MIMO processing capabilities to existing multiuser single-input single-output systems. For both receiver types, we analyze solutions based on linear (matched filter, decorrelator, minimum mean-square error) and maximum-likelihood receivers. For all the receivers considered, we provide closed-form expressions (as expectations of given functions) of the resulting pairwise error probabilities. Performance results are obtained in terms of frame-error rate versus E/sub b//N/sub 0/, following two different approaches. An analytic approach using large-system asymptotic methods, whereby the system parameters (number of users and antennas, spreading gain) are assumed to grow to infinity with finite limiting ratios. A computer-simulation approach is used to illustrate the differences between asymptotic and simulation results.     

On channel parameter estimation in a space-time bit-interleaved-coded modulation system for multipath DS-CDMA uplink with receive diversity

In this paper, we consider iterative space-time multiuser detection and channel parameter estimation in a bit-interleaved coded modulation scheme for asynchronous direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective, slowly fading channels. Accurate estimation of the channel parameters is critical as it has great impact on the overall BER performance. We present an iterative space-time multiuser (STMU) turbo detection and estimation scheme, based on space alternating generalized expectation maximization (SAGE) algorithm. This algorithm operates on the coded symbols by exchanging soft information between the detector and the estimator. We show through computer simulations that the proposed low complexity STMU receiver considerably outperforms conventional estimation schemes and achieves excellent performance, both in terms of BER and estimation error variance. Finally, we will consider different mapping strategies and investigate their impact on the performance and complexity of the estimator.     

Linear space-time multiuser detection for multipath CDMA channels

We consider the problem of detecting synchronous code division multiple access (CDMA) signals in multipath channels that result in multiple access interference (MAI). It is well known that such challenging conditions may create severe near-far situations in which the standard techniques of combined power control and temporal single-user RAKE receivers provide poor performance. To address the shortcomings of the RAKE receiver, multiple antenna receivers combining space-time processing with multiuser detection have been proposed in the literature. Specifically, a space-time detector based on minimizing the mean-squared output between the data stream and the linear combiner output has shown great potential in achieving good near-far performance with much less complexity than the optimum space-time multiuser detector. Moreover, this space-time minimum mean-squared error (ST-MMSE) multiuser detector has the additional advantage of being well suited for adaptive implementation. We propose novel trained and blind adaptive algorithms based on stochastic gradient techniques, which are shown to approximate the ST-MMSE solution without requiring knowledge of the channel. We show that these linear space-time detectors can potentially provide significant capacity enhancements (up to one order of magnitude) over the conventional temporal single-user RAKE receiver     

Iterative nonlinear detection for SFBC SC–FDMA uplink MIMO transmission systems

Single‐carrier frequency division multiple access (SC‐FDMA) systems with space frequency block coding (SFBC) transmissions achieve both spatial and frequency diversity gains in wireless communications. However, SFBC SC‐FDMA schemes using linear detectors suffer from severe performance deterioration because of noise enhancement propagation and additive noise presence in the detected output. Both issues are similar to inter‐symbol‐interference (ISI). Traditionally, SC‐FDMA system decision feedback equalizer (DFE) is often used to eliminate ISI caused by multipath propagation. This article proposes frequency domain turbo equalization based on nonlinear multiuser detection for uplink SFBC SC‐FDMA transmission systems. The presented iterative receiver performs equalization with soft decisions feedback for ISI mitigation. Its coefficients are derived using minimum mean squared error criteria. The receiver configuration study is Alamouti's SFBC with two transmit and two receive antennas. New receiver approach is compared with the recently proposed suboptimal linear detector for SFBC SC‐FDMA systems. Simulation results confirm that the performance of the proposed iterative detection outperforms conventional detection techniques. After a few iterations, bit‐error‐rate performance of the proposed receiver design is closely to the matched filter bound. Copyright © 2016 John Wiley & Sons, Ltd.     

Iterative interference cancellation and channel estimation in multibeam satellite systems

This paper deals with the use of non‐linear multiuser detection techniques to mitigate co‐channel interference on the reverse link of multibeam satellite systems. These techniques allow more capacity efficient frequency reuse strategies than classical ones, as they make possible to cope with lower C/I. The considered system takes as a starting point the DVB‐RCS standard, with the use of convolutional coding, and the use of the Ka‐band. We propose different iterative interference cancellation schemes, which operate at the beamformer outputs, and which use information from decoders. The proposed receivers assume an initial single‐user synchronization step: frame synchronization and timing recovery, and then perform channel estimation: beamformer coefficients; signal carrier phases and signal amplitudes. In a first step, these receivers are evaluated by simulation in terms of bit error rate and of channel estimation error on two interference configurations. For one of these receivers, sensitivity to imperfect timing recovery and to low‐frequency offsets from user terminals is evaluated. In a second step, since the receiver performances highly depend on the interference configuration, we propose an approach to evaluate performances on a multibeam coverage (by taking into account the variability of interference configurations on the coverage). This method is used to compare different receivers on an example based on a coverage designed on a digital focal array feed reflector antenna. Copyright © 2007 John Wiley & Sons, Ltd.