Iterative semiblind multiuser receivers for a space-time block-coded MC-CDMA uplink system

Iterative multiuser detection and space-time coding are two promising techniques to improve the capacity and performance of coded multiuser systems in wireless channels. In this paper, we present iterative multiuser detection schemes for a space-time block-coded multicarrier code-division multiple-access system with multiple transmit and receive antennas. We consider a more general case of an uplink system in the presence of both intra- and intercell interferences. We propose two types of iterative semiblind space-time receivers for such an uplink environment. The first is based on the minimum mean-square error criterion and the second is a hybrid scheme based on a combination of parallel interference cancellation and linear multiuser detection. These iterative receivers are derived, using a subspace approach, which utilizes known users' information for the computation of log-likelihood ratios (LLRs) while blindly suppressing the unknown interference. The LLRs are refined successively during the iterative process by using the extrinsic information available through decoding of all known users. A turbo code is used for channel coding. Simulation results in a frequency-selective Rayleigh-fading environment are presented to verify the performance of the proposed schemes.     

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.     

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.     

Cross-Layer Design for MRT Systems with Channel Estimation Error and Feedback Delay

Multiple-input multiple-output (MIMO) beamforming is a joint spatial diversity scheme applied in frequency selective channels to enhance system performance. In uplink transmission, the impact of correlation of path gains and mutual coupling at transmitters cannot be ignored because of the size constraint on mobile terminals. In this paper, an iterative MIMO beamforming algorithm with maximal ratio transmit-maximal ratio combining (MRT-MRC) strategy is presented to improve the output signal-to-noise and interference ratio in the presence of spatial correlation and mutual coupling between transmit elements. To reduce the complexity of systems, we also propose an equal gain transmit-maximal ratio combining (EGT-MRC) scheme. In contrast to MRT-MRC scheme, this algorithm can provide better system performance. The results of numerical simulations verify the advantages of this scheme.     

Blind multiuser detection in uplink CDMA with multipath fading: a sequential EM approach

We consider joint channel estimation and data detection in uplink asynchronous code-division multiple-access systems employing aperiodic (long) spreading sequences in the presence of unknown multipath fading. Since maximum-likelihood (ML) sequence estimation is too complex to perform, multiuser receivers are proposed based on the sequential expectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. Moreover, the snapshot estimates of each path are tracked by linear minimum mean-squared error filters. The user data are detected by a ML sequence detector, given the channel estimates. The proposed receivers that use the exact expressions have a computational complexity O(2/sup K/) per bit, where K is the number of users. Using the EM algorithm, we derive low-complexity approximations which have a computational complexity of O(K/sup 2/) per bit. Simulation results demonstrate that the proposed receivers offer substantial performance gains over conventional pilot-symbol-assisted techniques and achieve a performance close to the known channel bounds. Furthermore, the proposed receivers even outperform the single-user RAKE receiver with Nyquist pilot-insertion rate in a single-user environment.     

Iterative Interference Cancellation and Channel Estimation for Mobile SC-FDE Systems

This letter presents a combination of iterative channel equalization and estimation algorithms for mobile users in an interference-prone single carrier (SC) space division multiple access (SDMA) uplink system. It is shown that the soft decision based iterative block decision feedback equalization (SD-IBDFE) with parallel interference cancellation (PIC) and a least squares (LS) adaptation algorithm give significant improvement in estimating the SDMA users' channels in an iterative manner. Also, iterative channel estimation over multiple frames to estimate the desired user's channels provides additional gain. Simulation results show that the combination of these low complexity algorithms gives reasonably good results in interference-dominated doubly selective channels.     

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     

Space-time spreading and block coding for correlated fading channels in the presence of interference

We consider point-to-point wireless links with multiple antennas in the presence of interference, and exploit channel's spatial correlation and the temporal covariance of the interference to design multiantenna transmitters. We develop a space-time spreading scheme that maximizes average signal-to-interference-and-noise ratio, and an optimally power-loaded space-time beamforming (STBF) scheme which improves error-probability performance. In order to increase transmission rates, we combine orthogonal space-time block coding with STBF, optimize power loading across beams, and develop low-complexity receivers. Optimal training for least-squares error channel estimation, and STBF for minimum mean-square error channel estimation, are also studied. Our analytical and simulated results corroborate that STBF with optimal power loading can considerably reduce error probability and channel-estimation errors.     

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.     

Signal Processing by Generalized Receiver in?DS-CDMA Wireless Communication Systems with?Frequency-Selective Channels

The generalized receiver (GR) based on a generalized approach to signal processing (GASP) in noise is investigated in a direct-sequence code-division multiple access (DS-CDMA) wireless communication system with frequency-selective channels. We consider four avenues: linear equalization with finite impulse response (FIR) beamforming filters; channel estimation and spatially correlation; optimal combining; and partial cancellation. We investigate the GR with simple linear equalization and FIR beamforming filters. Numerical results and simulation show that the GR with FIR beamforming filters surpasses in performance the optimum infinite impulse response beamforming filters with conventional receivers, and can closely approach the performance of GR with infinite impulse response beamforming filters. Channel estimation errors are taken into consideration so that DS-CDMA wireless communication system performance will not be degraded under practical channel estimation. GR takes an estimation error of a maximum likelihood (ML) multiple-input multiple-output (MIMO) channel estimation and GR spatially correlation into account in computation of minimum mean square error (MMSE) and log-likelihood ratio (LLR) of each coded bit. The symbol error rate (SER) performance of DS-CDMA employing GR with a quadrature sub-branch hybrid selection/maximal-ratio combining (HS/MRC) scheme for 1-D modulations in Rayleigh fading is obtained and compared with that of conventional HS/MRC receivers. Procedure of selecting a partial cancelation factor (PCF) for the first stage of a hard-decision partial parallel interference cancellation (PPIC) of the GR employed in DS-CDMA wireless communication system is proposed. A range of optimal PCFs is derived based on the Price’s theorem. Computer simulation results show superiority in bit error rate (BER) performance that is very close to that potentially achieved and surpasses the BER performance of the real PCF for DS-CDMA systems discussed in literature.     

Symbol rate processing for the downlink of DS-CDMA systems

Many services of third-generation (3G) mobile radio systems will have higher data rates in the downlink than in the uplink. We propose to utilize adaptive antennas at the base stations because spatial interference suppression is able to reduce the near-far effect in the downlink of single-user detection direct-sequence (DS) code division multiple access (CDMA) systems. Besides the channel parameters in terms of directions of arrival, delays, and medium-term average path attenuations which are estimated in the uplink, we also take into account the correlation properties of the spreading and scrambling codes. In DS-CDMA the users are distinguished by different spreading codes, which change over time due to scrambling. In Brunner et al. (see Proc. EPMCC, p.375-80, 1999), the beamforming vectors at the base station were computed slotwise, whereas in this paper, we favor a symbol-rate beamforming at the base station. The new approach optimizes the actual values of the decision variables of the RAKE demodulators at the receivers. The superiority of the symbol rate beamforming algorithm compared to slotwise beamforming is shown by bit error rate (BER) simulations     

Analysis of a direct-sequence spread-spectrum cellular radio system

The uplink and downlink performance of a digital cellular radio system that uses direct sequence code division multiple access is evaluated. Approximate expressions are derived for the area averaged bit error probability while accounting for the effects of path loss, log-normal shadowing, multipath-fading, multiple-access interference, and background noise. Three differentially coherent receivers are considered: a multipath rejection receiver, a RAKE receiver with predetection selective diversity combining, and a RAKE receiver with postdetection equal gain combining. The RAKE receivers are shown to improve the performance significantly, except when the channel consists of a single faded path. Error correction coding is also shown to substantially improve the performance, except for slowly fading channels     

波束空时分组编码的ICA盲检测方案

理论研究已经证明,结合波束形成和空时分组编码的混合系统与传统的单纯使用波束发射或空时编码的方案相比具有很大的性能提高;传统的译码方案是借助接收端的信道估计来实现的,它需要知道准确的信道状态信息(CSI)。但如果信道估计不易实现,则系统性能将受很大影响。独立分量分析(ICA)作为一种经典的盲信号分离技术可以在不进行信道估计的情况下对发射信号实现有效检测。本文针对接收端的信号结构提出了一种基于ICA的正交检测方案;并通过仿真将新方案与传统方案进行了性能比较。仿真结果表明,新方案具有较好的系统适应性和误码率特性。     

Robust channel estimation and ISI cancellation for OFDM systems with suppressed features

A feature-suppressed orthogonal frequency-division multiplexing (OFDM) system and the corresponding channel estimation and intersymbol interference (ISI) mitigation techniques are investigated in this paper. Cyclic prefix (CP) and pilot tones, which are commonly used in civilian OFDM systems for ISI mitigation and channel estimation, create distinctive waveform features that can be easily used for synchronization and channel estimation purposes by intercepting receivers. As a result, CP and pilot tones are eliminated in the proposed feature suppressed OFDM system to reduce the interception probability. Instead, a set of specially designed OFDM symbols, driven by different pseudorandom sequences, are employed as preambles to avoid unique spectral signature. These preambles are inserted into the OFDM data symbol stream periodically and in a round-robin manner. In addition, a random frequency offset is introduced to each preamble to further mask the multicarrier signature. New challenges arising from these feature suppression efforts are studied, including robust channel estimation and demodulation techniques in the presence of frequency offset and severe interference. Based on our interference analysis, an iterative ISI and intercarrier interference (ICI) estimation-cancellation-based technique is proposed for both channel estimation and OFDM data demodulation. Our channel estimator performs joint frequency offset and channel impulse response estimation based on the maximum-likelihood (ML) principle. To reduce its complexity, we employ a number of techniques, which include approximation of the ML metrics, as well as fast Fourier transform pruning. The performances and feasibility of the proposed feature suppressed OFDM system and the channel estimator are analyzed and verified through numerical simulations.     

波束空时分组码系统中一种基于ICA的正交检测方案

结合波束形成和空时分组编码的混合系统与传统的单波束发射以及单空时编码发射分集方案相比可以大大提高链路性能;传统的译码方案通常以接收端能够获取精确的信道状态信息为前提;但是对于某些特定的通信环境,这种前提条件通常很难满足.独立分量分析(ICA)技术可以在不进行信道估计的情况下对发射信号实现有效检测.本文针对接收端的信号结构提出了一种基于ICA的正交检测方案;并通过仿真将新方案与传统方案进行了性能比较,仿真结果表明,新方案具有较好的系统适应性和误码率特性.     

A Turbo Detection Structure for Uplink Coded MC-CDMA System With Distortion Cancellation Capability

In this paper, a modified turbo detector is proposed, which, by combining a standard turbo detector with an iterative reconstruction method, cancels multiaccess interference (MAI) and clipping distortion in uplink coded multicarrier code-division multiple-access (MC-CDMA) systems. For clipping noise cancellation, signal reconstruction methods use nondistorted samples to reconstruct distorted samples; however, in MC-CDMA systems, the MAI causes these methods to not work properly. On the other hand, because of clipping, MAI-cancellation methods do not work properly. In this paper, we propose a joint iterative MAI and clipping noise-cancellation method, which is composed of a standard turbo detection structure and a clipping error estimator in each iteration. Furthermore, we investigate the convergence of the proposed method. Simulation results show that the proposed method improves the performance of coded MC-CDMA systems in the uplink in the presence of clipping distortion.      

Space-Time Adaptive MMSE Multiuser Decision Feedback Detectors With Multiple-Feedback Interference Cancellation for CDMA Systems

In this paper, we propose a novel space-time minimum mean square error (MMSE) decision feedback (DF) detection scheme for direct-sequence code-division multiple access (DS-CDMA) systems with multiple receive antennas, which employs multiple-parallel-feedback (MPF) branches for interference cancellation. The proposed space-time receiver is then further combined with cascaded DF stages to mitigate the deleterious effects of error propagation for uncoded schemes. To adjust the parameters of the receiver, we also present modified adaptive stochastic gradient (SG) and recursive least squares (RLS) algorithms that automatically switch to the best-available interference cancellation feedback branch and jointly estimate the feedforward and feedback filters. The performance of the system with beamforming and diversity configurations is also considered. Simulation results for an uplink scenario with uncoded systems show that the proposed space-time MPF-DF detector outperforms existing schemes such as linear, parallel DF (P-DF), and successive DF (S-DF) receivers in terms of bit error rate (BER) and achieves a substantial capacity increase in terms of the number of users, compared with the existing schemes. We also derive the expressions for MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback and expressions of signal-to-interference-plus-noise ratio (SINR) for the beamforming and diversity configurations with linear receivers.     

Downlink beamforming for DS-CDMA mobile radio with multimediaservices

Downlink beamforming is a promising technique for direct-sequence code-division multiple-access (DS-CDMA) systems with multimedia services to effectively reduce strong interference induced by high data rate users. A new downlink beamforming technique is proposed that converts the downlink beamforming problem into a virtual uplink one and takes into account the data rate information of all users. Since the main complexity of this method is due to the existence of multidelay paths, two simplified algorithms are suggested using an equivalent one-path channel vector to replace multipath channel vectors. Computer simulation results are given to evaluate the downlink capacity of DS-CDMA systems using a base station antenna array and the new algorithms proposed     

SAGE based iterative receiver for joint synchronization and channel estimation in uplink MIMO-OFDMA systems

A new Turbo iterative receiver structure is proposed for the uplink multiple-input multiple-output orthogonal frequency division multiple access （MIMO-OFDMA） systems. The space-alternating generalized expectation-maximization （SAGE） algorithm is naturally embedded in the framework of iterative receiver to perform synchronization and detection usin~ the Turbo detector outputs. In each iteration, the expectation step intends to remove the multiple access interference （MAI） caused by other asynchronous users, and the maximization step is utilized to estimate the required parameters （i.e., timing offset, carrier frequency offset, channel state information, etc.） sequentially for each user. Simulation results show that the proposed algorithm can approach the performance of ideal receiver closely, while the processing complexity is rather lower than the conventional detectors.     

Detectors and asymptotic analysis for bandwidth-efficient space-time multiple-access systems

In this paper, a narrowband multiple-channel transmission scheme with multiple transmit antennas is proposed and analyzed. The channelization is based on space-time signature matrices, which do not expand bandwidth, unlike conventional schemes such as code-division or time-division multiplexing. The channels can be used by multiple independent users in an uplink or downlink scenario (multiple access or broadcast channels, respectively), or by one user in a multiplexing scenario. The data transmitted on each channel is convolutionally encoded, interleaved, and then space-time block encoded before space-time channelization. Each channel has a unique interleaver and space-time signature, but the convolutional encoder and space-time block code encoder can be identical across channels. It is shown that asymptotic single-user-like performance can be achieved with optimal detection and decoding in a Rayleigh fading channel. Practical receiver algorithms are developed based on the iterative (turbo) detection technique. The simulation results demonstrate that these suboptimal receivers achieve single-user performance at moderate signal-to-noise ratios, and moderate user loads. In the single-user multiplexing case, a significant performance gain over single-channel transmission with the same data rate is obtained.