Multi‐group linear turbo equalization with intercell interference cancellation for MC‐CDMA cellular systems

In this paper, we investigate multi‐group linear turbo equalization using single antenna interference cancellation (SAIC) techniques to mitigate the intercell interference for multi‐carrier code division multiple access (MC‐CDMA) cellular systems. It is important for the mobile station to mitigate the intercell interference as the performance of the users close to cell edge is mainly degraded by the intercell interference. The complexity of the proposed iterative detector and receiver is low as the one‐tap minimum mean square error (MMSE) equalizer is employed for mitigating the intracell interference, while a simple group interference canceller is used for suppressing the intercell interference. Simulation results show that the proposed iterative detector and receiver can mitigate the intercell interference effectively through iterations for both uncoded and coded signals. Copyright © 2009 John Wiley & Sons, Ltd.     

Partial zero-forcing adaptive MMSE receiver for DS-CDMA uplink in multicell environments

Adaptive multi-user detection techniques for interference suppression in direct-sequence code division multiple access (DS-CDMA) systems have gained much attention since they do not require any information on interfering users. In the uplink of DS-CDMA systems, however, the base station receiver typically knows the spreading waveforms of the users within its cell but does not know those of the users in other cells. We propose a partial zero-forcing adaptive minimum mean squared error (MMSE) receiver for the DS-CDMA uplink utilizing the spreading waveforms known at the base station as well as training data. The proposed receiver first removes the intracell interference using a linear filter based on the knowledge of the spreading waveforms of the interfering users within the cell. Then the intercell interference remaining in the output of the linear filter is mitigated by adaptive MMSE detection. To speed up the convergence of the adaptive filter weights without loss of the steady-state performance, we develop a modified least mean square (LMS) algorithm based on the canonical representation of the filter weights. It is shown through analysis and simulation results that the proposed receiver improves the convergence speed and the steady-state performance.     

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.      

Minimum probability of error-based methods for adaptive multiuser detection in multipath DS-CDMA channels

Direct-sequence code-division multiple-access (DS-CDMA) is a popular multiple-access technology for wireless communications. However, its performance is limited by multiple-access interference and multipath distortion. Multiuser detection and space-time processing are two signal processing techniques employed to improve the performance of DS-CDMA. Two minimum probability of error-based space-time multiuser detection algorithms are proposed in this paper. The first algorithm, minimum joint probability of error (MJPOE), aims to minimize the joint probability of error for all users. The second algorithm, minimum conditional probability of error (MCPOE), minimizes the probability of error of each user conditioned on the transmitted bit vector, for each user individually. In both the algorithms, the optimal filter weights are computed adaptively using a gradient descent approach. The MJPOE algorithm is blind and offers a bit-error-rate (BER) performance better than the nonadaptive minimum mean squared error (MMSE) algorithm, at the cost of higher computational complexity. An approach for reducing the computational overheads of MJPOE using Gram-Schmidt orthogonalization is suggested. The BER performance of the MCPOE algorithm is slightly inferior to MMSE, however, it has a computational complexity linear in the number of users. Both blind and training-based implementations for MCPOE are proposed. Both MJPOE and MCPOE have a convergence rate much faster than earlier known adaptive implementations of the MMSE detector, viz. least mean square and recursive least squares. Simulation results are presented for synchronous single path channels as well as asynchronous multipath channels, with multiple antennas employed at the receiver.     

Design of Turbo-MUD Receivers for Overloaded CDMA Systems by Density Evolution Technique

This paper deals with a turbo multiuser detector suitable for applications in overloaded coded DS-CDMA systems. The turbo-MUD receiver is based on the use of a linear MMSE detector in the first iteration and a parallel interference cancellation scheme in the successive ones. The inputs of the interference cancellator are both the detector outputs and the soft information from a bank of turbo decoders. The performance of the proposed receiver has been derived by means of computer simulations and applications of the density evolution theory: in particular, this technique permits to properly evaluate the number of MMSE iterations, simplifying the overall receiver design.     

An Adaptive MMOE-PIC Detector for Asynchronous DS-CDMA Communications

We propose a modified linear parallel interference cancelation (PIC) structure using the adaptive minimum mean output-energy (MMOE) algorithm for direct-sequence code-division multiple-access (DS-CDMA) systems. The complexity of the proposed receiver structure is shown to be linear in the number of users and hence, lower complexity than the centralized minimum mean-squared error (MMSE) multiuser detector. It is demonstrated that the proposed receiver structure can significantly reduce the long training period required by the standard adaptive MMOE receiver in near-far environments. Both numerical and theoretical results show that the proposed receiver performs close to the optimum MMSE receiver whereas the conventional adaptive MMOE detector suffers from high BER’s due to the imperfect filter coefficients. Also our results show a three fold increase in the number of users when the MMOE-PIC is used relative to the conventional MMOE receiver. Furthermore, the transient behavior of the proposed MMOE-PIC receiver due to abrupt changes in the interference level is examined. It is shown that the proposed adaptive receiver offers much faster self recovery, with less signal-to-interference ratio (SIR) degradation, than the standard MMOE in sever near-far scenarios.     

A Study of the Technique of Linear Multiuser

The technique of linear multiuser detection in DS-CDMA systems is studied in this paper. The purpose is to find a receiver structure with good performance and moderate complexity, so that the receiver can efficiently suppress multiple-access interference(MAI) and multipath interference and has good near-far resistant ability, which may improve the system's capability while reducing the requirement for power control. The main work of the dissertation can be summarized as follows: the performance of MMSE multiuser detector in synchronous/asynchronous DS-CDMA systems over different channels is analyzed in chapter 2 of the dissertation. Using matrix method, we analyze the relation between performance measurement and spreading code correlation matrix, Signal-Interference-Ratio(SIR) and near-far factor, and prove that the performance of MMSE detector is better than that of the decorrelating detector. For fading channel, we analyze the performance of MMSE detector in DS-SS system firstly. Results show that the detector can efficiently suppress multipath interference. Extending to synchronous/asynchronous DS-CDMA systems over fading channels, we propose a simple linear detector structure that accomplishes despreading, detection and combining. Thus, the receiver is easy for implementation. Based on the proposed notion of combined spreading codes, we prove that the synchronous/asynchronous CDMA system is equivalent to the synchronous CDMA system over AWGN channel with double users. Therefore, the MMSE detector can efficiently suppress MAI and multipath interference in steady state, and has good near-far resistant ability. In chapter 3, we study the adaptive algorithm based on MMSE criterion. Firstly, the approach to the blind algorithm based on subspace is analyzed. We improve the algorithm in the part of channel estimation, which decreases the computational complexity while guaranteeing the performance. Meanwhile, we point out that CMOE-RLS algorithm is essentially an algorithm based on subspace approach. Also, it is shown from simulation that PASTd subspace tracking algorithm is not applicable for multiuser detection. Secondly, we propose an adaptive algorithm based on pilot channel, called PCA/PCRA. The algorithm does not require channel estimation, and has a rapid convergence rate. The steady state performance can be achieved by increasing the transmitting power in pilot channel. Computational complexity is only O(N2). Therefore, PCA/PCRA is suitable for the engineering application. The cost is that a pilot channel is needed for each user in the system. In chapter 4, constant algorithms for multiuser detection are studied. Firstly, we analyze the capture performance of CMA, and point out there exist many local stationary points. Initializations to guarantee CMA converges to the desired point are discussed. Results show that the convergence of CMA is decided by constant, step-size, spreading code correlation matrix and near-far factor. Secondly, we propose the constrained constant algorithm (C-CMA) for multiuser detection. It is shown that when the constant is greater than the triple power of the desired user, C-CMA globally converges to the desired point. Simulations illustrate that C-CMA has a rapid convergence rate and the steady state performance is good. However, great step-size can also reult in dispersion for the algorithm. Since C-CMA is a variable step-size CMOE-LMS algorithm, we propose a variable step-size constraint algorithm (VSCA). VSCA has the advantages of both CMOE-LMS and C-CMA such as robust, rapid convergence rate and good steady state performance. Thus, VSCA is suitable for engineering application. But the improper selection of step-size coefficients may degrade performance seriously. The computational complexity of the above constant algorithms is only O(N). In Section 5, the cyclostationarity of spreading signals is analyzed in the first part. We prove that spreading signals are ergodic cyclostationary signals with a cyclic period that is equal to the period of spreading code. Methods for processing cyclostationary signals are then given. It is shown that this method can mitigate the interference from a stationary noise for channel estimation. But the computational complexity for cyclostationary correlation is high, which prevents its application in implementation. In the second part, we discuss the application of oversampling technique in spreading communication systems. Although the oversampling can improve the performance of the linear multiuser detector, the improvement is trivial. On the contrary, oversampling increases the computational complexity of the weight vector greatly, which prevents its application in implementation. Additionally, we prove that FSE plus despreading or despreading pus FSE is equivalent to the linear detector with different lengths of delay line. However, the two kinds of structure have lower computational complexity.     

An Iterative Multiuser Detector for Overloaded LDPC Coded CDMA Systems

Maximum number of users in a Code Division Multiple Access (CDMA) system, disregarding the type of used signature sequences, is equal to the processing gain; but in overloaded CDMA systems, it is tried to use some special methods of applying signature sequences so that the number of users exceeds the processing gain of the system. This growth in capacity is gain at the cost of decrease in performance of the conventional systems; and usually it is tried to use channel coding methods or multi-user detectors to compensate this decrease. Because of advantages of using coding methods joined with multiuser detectors in achieving better performance and also because of some benefits of using (Low Density Parity Check) LDPC method in comparison with similar capacity achieving coding methods, in this article, an iterative multi-user detector for an overloaded LDPC Coded CDMA system is proposed. This receiver consists of a combination of matched filters in the first stage and a linear (Minimum Mean Square Error) MMSE detector and an Interference Cancellation (IC) scheme in the successive stages. In the suggested method, a bank of LDPC decoders gives the soft information to the IC blocks, which help for the better interference cancellation. Comparing the performance of the proposed system with that of Turbo coded system shows that the proposed system, in addition to advantages of using LDPC codes instead of Turbo codes, has better bit error rate performance.     

Randomly spread CDMA: asymptotics via statistical physics

This paper studies randomly spread code-division multiple access (CDMA) and multiuser detection in the large-system limit using the replica method developed in statistical physics. Arbitrary input distributions and flat fading are considered. A generic multiuser detector in the form of the posterior mean estimator is applied before single-user decoding. The generic detector can be particularized to the matched filter, decorrelator, linear minimum mean-square error (MMSE) detector, the jointly or the individually optimal detector, and others. It is found that the detection output for each user, although in general asymptotically non-Gaussian conditioned on the transmitted symbol, converges as the number of users go to infinity to a deterministic function of a "hidden" Gaussian statistic independent of the interferers. Thus, the multiuser channel can be decoupled: Each user experiences an equivalent single-user Gaussian channel, whose signal-to-noise ratio (SNR) suffers a degradation due to the multiple-access interference (MAI). The uncoded error performance (e.g., symbol error rate) and the mutual information can then be fully characterized using the degradation factor, also known as the multiuser efficiency, which can be obtained by solving a pair of coupled fixed-point equations identified in this paper. Based on a general linear vector channel model, the results are also applicable to multiple-input multiple-output (MIMO) channels such as in multiantenna systems.     

Asynchronous block-based minimum mean square error (B-MMSE) CDMA multi-user detection

This paper introduces a novel asynchronous CDMA multi-user detector, the block-based MMSE (B-MMSE) multi-user detector, in which the data stream is segmented into blocks by inserting zero bits and detection takes place block-by-block without compromising the MMSE detection efficiency. The BER performance of the B-MMSE detector is studied and the results are compared with those of the decorrelating detector. It is shown that the B-MMSE detector offers promising detection efficiency at a much lower implementation complexity, which is linear in the product of number of users and block size, than that of the traditional MMSE detector. © 1998 John Wiley & Sons, Ltd.     

Multi-user detection for DS-CDMA communications

Direct-sequence code-division multiple access (DS-CDMA) is a popular wireless technology. In DS-CDMA communications, all of the users' signals overlap in time and frequency and cause mutual interference. The conventional DS-CDMA detector follows a single-user detection strategy in which each user is detected separately without regard for the other users. A better strategy is multi-user detection, where information about multiple users is used to improve detection of each individual user. This article describes a number of important multiuser DS-CDMA detectors that have been proposed     

On the average near-far resistance for MMSE detection of directsequence CDMA signals with random spreading

The performance of a near-far-resistant, finite-complexity, minimum mean squared error (MMSE) linear detector for demodulating direct sequence (DS) code-division multiple access (CDMA) signals is studied, assuming that the users are assigned random signature sequences. We obtain tight upper and lower bounds on the expected near-far resistance of the MMSE detector, averaged over signature sequences and delays, as a function of the processing gain and the number of users. Since the MMSE detector is optimally near-far-resistant, these bounds apply to any multiuser detector that uses the same observation interval and sampling rate. The lower bound on near-far resistance implies that, even without power control, linear multiuser detection provides near-far-resistant performance for a number of users that grows linearly with the processing gain     

Joint Iterative Multiuser and Narrowband Interference Suppression in Coded Asynchronous DS-CDMA Channels

Multiuser interference suppression in coded direct sequence code division multiple access (DS-CDMA) uplink channels is significantly impacted by the application of the turbo processing concept. This paradigm essentially involves the iterative exchange of soft information between a multiuser demodulator and a bank of single-user decoders, to their mutual benefit. The present work proposes a joint iterative minimum mean square error (MMSE) multiuser and narrowband interference suppressor for coded asynchronous DS-CDMA channels. Since the parameters of the narrowband interference are unknown a priori, the first iteration in this scheme is effectively just MMSE multiuser interference suppression. The outputs of all users' soft decoders (available at the end of each iteration) are fed back to subtract their (estimated) cumulative contribution from the received signal vector. The residue comprises the narrowband interference embedded in wideband noise, so that the former can be piecewise interpolated over chip intervals of appropriate duration, and then subtracted from the received signal, to provide a narrowband-interference-free input signal for the next iteration. The soft-decoded feedback estimates of the interferers' signals are used in each iteration also to perform soft MMSE multiuser interference suppression, in conjunction with subtractive interference cancellation. This scheme performs well at both low and high received signal powers, and displays the successive cancellation property across iterations, exhibiting good near-far resistance. Introducing multisensor arrays at the receiver relaxes the limitations imposed by the inherent suboptimality of MMSE multiuser demodulation, insofar as it enhances interuser separation, now in the spatial sense. Simulations indicate that the performance of the proposed technique surpasses that of all existing suboptimal algorithms in this context.     

Multistage linear receivers for DS-CDMA systems

A new family of multistage low-complexity linear receivers for direct sequence code division multiple access (DS-CDMA) communications is introduced. The objective of the proposed design is to mitigate the effect of multiple access interference (MAI), the most significant limiting factor of user capacity in the conventional DS-CDMA channel. The receivers presented here employ joint detection of multiple users and therefore require knowledge of all the signature codes and their timing. In addition, for a multipath environment, reliable estimates of the received powers and phases are assumed available for maximal ratio RAKE combining. Each stage of the underlying design recreates the overall modulation, noiseless channel, and demodulation process. The outputs of these stages are then linearly combined. The combining weights can be chosen to implement different linear detectors, including the decorrelating and minimum mean square error (MMSE) detectors. In this paper, we focus on implementing the MMSE detector. Simulation results illustrate that significant performance gains can be achieved in both synchronous and asynchronous systems.This work was presented in part at IEEE Communication Theory Workshop, April 23–26, 1995, and at IEEE MILCOM '95, November 5–8, 1995.This work was submitted in partial fulfillment of Ph.D. requirements at The City University of New York.     

An Iterative Receiver for Uplink of Coded MIMO DS-CDMA System Employing Layered Space-Time Transmission

We consider a coded multiple-input multiple-output (MIMO) DS-CDMA system using layered space-time transmission in multipath wireless channels, where space-time signals from multiple antennas of multiple users propagate through rich scattering multipath fading. We propose a receiver employing iterative joint detection and decoding with a reduced-complexity detector using linear minimum mean squared error filtering with a priori information and parallel soft-input soft-output (SISO) decoders. Computer simulation results show that the proposed receiver for multi-user MIMO transmission provides high-spectral efficiency and performance approaching to single-user bound. Furthermore, the reduced-complexity receiver outperforms an iterative soft decision-directed maximal ratio combining (DD-MRC) receiver, RAKE receiver as well as a conventional non-iterative receiver.     

A flexible receiver for CDMA multiuser communications

Code division multiple access (CDMA) capacity is limited by interference amongst users. The effect of this interference on receiver outputs depends on the users' signatures and the actual detector used in the receiver. A matched filter receiver is particularly sensitive to interference, whereas an optimum multiuser receiver is less sensitive but infeasible due to its exponential complexity. We propose a receiver structure that trades detection performance for reduced complexity. It can interpolate between the performances and complexities of these two receivers. Our detector uses a tree structure, and some of its special cases are the decision feedback detector, the decorrelating detector, and the optimal linear detector. We show that at equal complexity levels, a particular implementation of our detector outperforms these detectors. We also show that our approach can be used with a minimum-mean-square-error design criterion and coded CDMA transmission     

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.     

Adaptive space-time feedforward/feedback detection for high datarate CDMA in frequency-selective fading

We investigate linear and nonlinear space-time minimum mean-square-error (MMSE) multiuser detectors for high data rate wireless code-division multiple-access (CDMA) networks. The centralized reverse-link detectors comprise a space-time feedforward filter and a multiuser feedback filter which processes the previously detected symbols of all in-sector users. The feedforward filter processes chip-rate samples from a bank of chip-matched filters which operate on the baseband outputs from an array of antennas. We present an adaptive multiuser recursive least squares (RLS) algorithm which determines the MMSE adjusted filter coefficients with less complexity than individual adaptation for each user. We calculate the outage probabilities and isolate the effects of antenna, diversity, and interference suppression gains for linear and nonlinear filtering and for CDMA systems with varying levels of system control (e.g., timing control, code assignment, cell layout). For eight users transmitting uncoded 2-Mb/s quadrature phase-shift keying with a spreading gain of eight chips per symbol over a fading channel with a multipath delay spread of 1.25 μs, the performance of a three-antenna feedforward/feedback detector was within 1 dB (in signal-to-noise ratio per antenna) of ideal detection in the absence of interference. By training for 10% of a 5-ms frame, RLS adaptation enabled the same detector to suffer less than a 0.5-dB penalty due to the combined effects of imperfect coefficients and error propagation. The advantage of nonlinear feedforward/feedback detection over linear feedforward detection was shown to be significantly larger for a CDMA system with enhanced system control     

Iterative (turbo) soft interference cancellation and decoding forcoded CDMA

The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive soft-output decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached     

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.