Blind Multiuser Receiver Research Articles

Precoder Based Collaborative Blind Multiuser Detection of CDMA Signals

A new collaborative blind multiuser detection scheme based on precoding and signal subspace estimation is proposed for binary phase-shift keying/direct-sequence code-division multiple-access signals. In this scheme, precoding the phase of the data enables separation and detection of a desired user's signal from the received signal with the prior knowledge of only the precoding sequence, signature waveform and timing of the user of interest. In this approach, instead of assigning unique precoding sequence to each user, same precoding sequence is assigned to a group of users which enables group precoding and deprecoding operations. The proposed collaborative blind multiuser receiver consists of two stages: group deprecoding stage to minimize multiple-access interference due to multiple groups in the system and blind subspace multiuser detection stage to detect desired user data from the respective group. The simulation results confirm that the performance gain afforded by the proposed scheme can be substantial when compared to the conventional linear subspace blind detectors.

Blind Multiuser Detection for Long Code Multipath DS-CDMA Systems with Bayesian MC Techniques

This paper presents an efficient Bayesian blind multiuser receiver for long code multipath CDMA systems. The proposed receiver employs the adaptive sampling method for the Bayesian inference procedure to estimate the data symbols and multipath parameters. Compared to the other reported Bayesian Monte Carlo receivers for long code multipath CDMA systems, the proposed one achieves a faster convergence and a lower computational complexity to attain comparable performance. Simulation results are presented to demonstrate the effectiveness of the proposed Bayesian blind multiuser receiver.

Blind multiuser receiver for DS-CDMA wireless system

We consider blind signal detection in an asynchronous code-division multiple-access (CDMA) system employing short spreading sequences in the presence of unknown multipath fading. This approach is capable of countering the presence of multiple-access interference (MAI) in CDMA fading channels. The proposed blind multiuser detector is based on an independent component analysis (ICA) to mitigate both MAI and noise. This algorithm has been utilised in blind source separation (BSS) of unknown sources from their mixtures. It can also be used for estimating the basis vectors of BSS. The aim is to include an ICA algorithm within a wireless receiver in order to reduce the level of interference in wideband systems. This blind multiuser detector requires no training sequence compared with the conventional multiuser detection receiver. The proposed ICA blind multiuser detector is made robust with respect to knowledge of signature waveforms and the timing of the user of interest. Several experiments are performed in order to verify the validity of the proposed ICA algorithm.

Robust blind multiuser detection based on the worst-case performance optimization of the MMSE receiver

The conventional minimum mean-square error (MMSE) multiuser receiver requires that each particular user transmits a sequence of training symbols known to the receiver and the receiver estimates the user signatures using this knowledge. However, in the presence of multiaccess interference (MAI) during the training period and/or in scenarios with short training sequence length, the signature estimates can be erroneous, and the performance of the MMSE multiuser receiver can degrade substantially. In this paper, we propose a new blind multiuser receiver that is robust against the effects of erroneously presumed desired user signature and short data length. Our approach is based on the explicit modeling of possible mismatches in the mean-square error cost function and worst-case performance optimization. We show that this approach leads to a multiuser receiver which uses the data covariance matrix with an adaptive diagonal loading . The proposed method has simple implementation with a computational complexity comparable with that of the fixed diagonal loading-based multiuser receiver. Simulation results show performance improvements achieved by our approach relative to existing techniques.

Blind Multiuser Receiver for Space-Time Coded CDMA Signals in Frequency-Selective Channels

Blind detection of a desired user's signal in a direct sequence code-division multiple-access (DS-CDMA) system using multiple transmit and receive antennas and space-time (ST) block coding is considered. We consider Alamouti's ST coding scheme for two transmit antennas designed for flat-fading channels but allow the channel to be frequency selective. A (spreading) code-constrained inverse filter criterion (IFC)-based linear blind detector to detect a desired user's signal for uncoded CDMA signals exploiting only a single transmit and single receive antenna was recently presented. The IFC method exploits the higher order statistics of the data. In ST-coded multiantenna CDMA systems, a user signal may be treated as the superposition of several virtual user signals derived from the same parent user. In particular, for Alamouti's scheme, one has two virtual users for each user if the information symbols are real valued. The previously proposed code-constrained IFC-based detector is modified to detect a given virtual-user signal. The original user signal can then be recovered from the associated detected virtual user signals obtained by running two code-constrained IFC-based detectors in parallel. We also propose a novel approach combining the code-constrained IFC and a penalty function to simultaneously extract the two virtual user signals associated with a given user. Global minima of the proposed cost functions are analyzed. An illustrative simulation example is presented.

A Low-Complexity Blind Multiuser Receiver for Long-Code CDMA

Receivers for long-code systems are for computational reasons usually based on simple matched-filter techniques, and hence suffer from multiaccess interference. Decorrelating RAKE and MMSE receivers do not have this problem but have not been widely studied due to the apparent complexity of the inversion of a large code matrix. Tong, van der Veen Dewilde, and Sung (IEEE Tr. Signal Proc., 2003) derived a blind decorrelating RAKE receiver (DRR) and channel estimation algorithm for long-code CDMA systems, and showed how it can be efficiently implemented. In this paper, we continue on that work. We propose both single-user and multiuser blind source-channel estimation algorithms by making use of an iterative estimation scheme initialized by the DRR. Simulation results show significant improvement, even in heavily loaded systems. Moreover, with an implementation based on time-varying system theory, the proposed algorithm can be implemented efficiently at a cost similar to the RAKE.

A Two-Stage CMA-Based Receiver for Blind Joint Equalization and Multiuser Detection in High Data-Rate DS-CDMA Systems

The paper deals with the problem of blind mitigation of intersymbol interference (ISI) as well as multiple-access interference (MAI) in asynchronous high data-rate direct-sequence code-division multiple-access systems. A blind adaptive multiuser receiver based on the constant-modulus algorithm (CMA) is proposed, which demodulates each desired user by exploiting only the knowledge of its spreading code, without requiring estimation of the users's channels and timings. In order to overcome the CMA interference capture problem, which arises in a multiuser scenario, a two-stage adaptive receiver is adopted: In the first stage, partial MAI and ISI suppression is blindly achieved by exploiting the desired user signature structure properties; in the second stage, the residual MAI and the ISI are removed by using the CMA, and the information symbols of the desired user are reliably recovered. Theoretical analysis and simulation results show that the first stage is an effective blind adaptive strategy which allows the CMA detector in the second stage to lock on the desired-user symbol, at a particular delay. The proposed blind receiver achieves a significant performance gain in comparison with existing blind methods.

Blind Multiuser Receivers for Chaos Based Communications

Recently, there has been a good deal of interest in the use of chaotic signals for direct sequence code division multiple access (DS-CDMA) communication systems. The capacity of DS-CDMA systems is interference-limited, and can therefore be increased by techniques that suppress interference. This letter is devoted to the evaluation of the impact of blind multiuser detection techniques on chaos based DS-CDMA systems. Blind receivers can suppress multiple access interference and do not require knowledge of the code sequences and propagation channels of the interference. We demonstrate that, for chaotic sequence-based communications, blind multiuser receivers significantly improve the BER with respect to single-user receivers, and that their use is practically essential with a high number of users.

Blind multiuser detection via interference identification

Previous results on blind multiuser detection apply in situations where the signal parameters of the users of interest are known, and those of the interferers; are unknown. In this paper, we consider the new paradigm of an N-user system, in which K users are active, and the problem is to detect G users of interest out of those K active users when the signal parameters (codes, amplitudes) of the G users of interest are known, as are the codes of all N users. What is not known at the receiver, however, is K - G, the number of active interferers, and the identity of these interferers. A solution to such a problem could be to ignore the knowledge of the remaining N - G codes, and apply known blind multiuser detectors based on stochastic approximation or subspace tracking techniques. However, it is shown here that the additional knowledge of those codes can be used to obtain an interference-identification-based blind multiuser receiver that has much faster convergence properties. We illustrate the underlying principle in the context of blind group detection in synchronous direct-sequence/code-division multiple-access (DS/CDMA) systems operating in channels that exhibit frequency-selective fading.

Capon multiuser receiver for CDMA systems with space-time coding

We present in this paper a linear blind multiuser receiver, referred to as the Capon receiver, for code-division multiple-access (CDMA) systems utilizing multiple transmit antennas and space-time (ST) block coding. The Capon receiver is designed by exploiting signal structures imposed by both spreading and ST coding. We highlight the unique ST coding induced structure, which is shown to be critical in establishing several analytical results, including self-interference (i.e., spatially mixed signals of the same user) cancellation, receiver output signal-to-interference-and-noise ratio (SINR), and blind channel estimation of the Capon receiver. To resolve the scalar ambiguity intrinsic to all blind schemes, we propose a semi-blind implementation of the Capon receiver, which capitalizes on periodically inserted pilots and the interference suppression ability of the Capon filters, for (slowly) time-varying channels. Numerical examples are presented to compare the Capon receiver with several other training-assisted and (semi-)blind receivers and to illustrate the performance gain of ST-coded CDMA systems over those without ST coding.

Blind turbo multiuser detection for long-code multipath CDMA

We consider the problem of demodulating and decoding multiuser information symbols in an uplink asynchronous coded code-division multiple-access (CDMA) system employing long (aperiodic) spreading sequences, in the presence of unknown multipath channels, out-cell multiple-access interference (OMAI), and narrow-band interference (NBI). A blind turbo multiuser receiver, consisting of a novel blind Bayesian multiuser detector and a bank of MAP decoders, is developed for such a system. The effect of OMAI and NBI is modeled as colored Gaussian noise with some unknown covariance matrix. The main contribution of this paper is to develop blind Bayesian multiuser detectors for long-code multipath CDMA systems under both white and colored Gaussian noise. Such detectors are based on the Bayesian inference of all unknown quantities. The Gibbs sampler, a Markov chain Monte Carlo procedure, is then used to calculate the Bayesian estimates of the unknowns. The blind Bayesian multiuser detector computes the a posteriori probabilities of the channel coded symbols, which are differentially encoded before being sent to the channel. Being soft-input soft-output in nature, the proposed blind Bayesian multiuser detectors and the MAP decoders can iteratively exchange the extrinsic information to successively refine the performance, leading to the so-called blind turbo multiuser receiver.

Blind adaptive algorithms for minimum variance CDMA receivers

Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. We develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver.

Differentially Coherent Blind Adaptive Receiver for Rayleigh Fading CDMA Channels

Interest in code division multiple access (CDMA) has increased dramatically in both the industry and academia. This is due to the potential increase in user capacity and interference, multipath fading, and jam-resistant capabilities. Different approaches of multiuser detection schemes have shown improvement over the conventional receiver. However, multiuser detectors require system parameters and adaptive multiuser receivers require training sequences. These parameters are difficult to obtain in multipath fading channels with a degradation in performance. A blind adaptive multiuser receiver for flat fading and frequency-selective fading environments is studied in this paper. This receiver alleviates the problem of multiuser detectors and adaptive multiuser receivers by using a blind adaptive equalization method. The structure of the receiver is similar to that of the adaptive MMSE receiver which consists of an adaptive equalizer and an adaptive mechanism that uses a standard least mean-squared (LMS) algorithm, but uses a matched filter to generate a reference signal. The receiver is shown to be effective in different fading channels with perfect power control and a near-far effect. Furthermore, the simulation results show that the blind MMSE receiver outperforms the conventional receiver and the RAKE receiver.

Blind Multiuser Detection with Array Observations

Cochannel interference is usually a major limitation to the performance of mobile wireless systems. Examples of different forms of cochannel interference include multi-access interference in CDMA systems and cochannel interference resulting from frequency reuse in TDMA systems. In order to mitigate the interference from other users we present a blind multiuser receiver which utilizes array observations and performs both spatial and temporal processing of the received signal. The presented technique is completely blind in the sense that no signature sequences, channel state or spatial location needs to be known a priori, nor use of a training sequences, channel state or spatial location needs to be known a priori, nor use of a training sequence is required for the adaptation. The diversity introduced by the array observations can be efficiently combined with the use of CDMA signature sequences. After initial convergence, a reliable estimate of the combined temporal and spatial signature for each user is provided that can be employed by a multiuser receiver of lower complexity.