Linear Multiuser Detection Research Articles

Iterative low-complexity multi-user detection for asynchronous multi-sequence signalling based bit-interleaved coded modulation code division multiple access systems

An uplink direct sequence spread spectrum communications systems employing a multi-sequence model over a quasi-static frequency-selective fading channel is considered. In analogy with bit-interleaved coded modulation (BICM) technique, a group of bits at the output of a bit-wise interleaver is mapped uniquely to a complex signalling vector belonging to an orthogonal plane sequence modulation signal space, which is constructed over a set of expanded signature sequences. This transmission system provides not only bandwidth efficiency offered by additional signal planes but also time diversity resulting from the BICM technique. It is observed that at high system traffic load error performance could degrade substantially due to user cross-correlations, multi-access asynchronism as well as channel frequency selectivity. The authors employ a 'turbo principle' receiver, consisting of a soft interference cancellation scheme, soft demappers and maximum a posteriori decoders, to avert this capacity loss by exploiting the serially concatenating structure at the transmitter. After simple mathematical manipulation, a soft space-time linear minimum mean-square error multi-user detector could even be explored on the basis of per signal plane per user. Both analytical performance-bound and computer simulation of the proposed framework in terms of bit-error rate (BER) are revealed. Further, performance comparisons with convolutionally coded and conventional bandwidth-efficient coded direct sequence code division multiple access systems under the same system conditions are illustrated. The authors have also investigated the impact of labelling maps on the BER performance.

Diversity reception in short-code CDMA

Short spreading sequences facilitate efficient multiuser detectors and have hence been incorporated into practical code division multiple access systems such as 3rd Generation Partnership Project UTRA-TDD. However, as the number of users approaches the spreading code length, the asymptotic performance and the numerical stability of linear multiuser detection algorithms degrade. Given some simplifications such as synchronous users, it is shown that receive diversity effectively mitigates these degradations and allows the system to be operated with a high load. A lower bound on the mean asymptotic multiuser efficiency is presented and the numerical stability of linear multiuser detection algorithms for systems with receive diversity is analysed. It is further shown that the performance variability is significantly reduced by receive diversity.

Multiuser detection for cooperative networks and performance analysis

We investigate strategies for user cooperation in the uplink of a synchronous direct-sequence code-division multiple-access (DS/CDMA) network employing nonorthogonal spreading codes and analyze their performance. We consider two repetition-based relay schemes: decode-and-forward (DAF) and amplify-and-forward (AAF). Focusing on the use of linear multiuser detectors, we first present cooperation strategies, i.e., signal processing at both the relay nodes and the base station (BS), under the assumption of perfectly known channel conditions of all links; then, we consider the more practical scenario where relays and BS have only partial information about the system parameters, which requires blind multiuser detection methods. We provide performance analysis of the proposed detection strategies in terms of the (asymptotic) signal-to-(interference plus noise) ratio and the bit error rate, and we show that AAF achieves a full second-order diversity when a minimum mean-square-error detector is employed at both the relay side and the BS. A simple, yet effective, partner selection algorithm is also presented. Finally, a thorough performance assessment is undertaken to study the impact of the multiple-access interference on the proposed cooperative strategies under different scenarios and system assumptions

A robust approach to channel estimation and detection for multi-carrier CDMA

We present a robust approach to channel estimation and multiuser detection (MUD) for multi-carrier CDMA by explicitly taking into account prior channel estimation errors. Specifically, robust channel estimates and MUD receivers are obtained by optimizing the worst-case performance over a properly selected bounded uncertainty set. Although the prior channel estimation error is not bounded, it is beneficial to refine the estimate over a properly chosen bounded uncertainty set. Numerical results show that linear MUD detectors resulting from our robust approach yield improved performance over those that ignore the prior estimation errors

A new symbol-alphabet-aware multi-user detection scheme

In this letter, a new multi-user detection scheme is proposed. It employs a novel nonlinear minimum mean square error (MMSE) estimator that exploits the knowledge of symbol alphabet to estimate the interfering signals. After cancelling the estimated interference, a conventional linear multi-user detector such as the MMSE detector or, simply, the matched-filter receiver (MFR) is then used to detect the desired user signal. Simulation results demonstrate that the proposed detector significantly outperforms the conventional linear MMSE detector

Separate linear receiver interfaces for MIMO multiple-access channels

We address the design of a receiver for the uplink of a direct sequence code division multiple access wireless communication system, endowed with multiple transmit and receive antennas and affected by multiple-access interference (MAI) for the presence of multiple transmitting users. The receiver is assumed to keep separate the tasks of mitigating the spatial and MAI in order to be implementable in two options: a baseline option accommodating the MAI mitigation interface only and an enhanced option accommodating the spatial interference mitigation interface as well. The MAI mitigating interface is based on standard linear multiuser detectors. The spatial interference mitigating interface is derived according to a linear minimum mean-square error criterion, which assumes the MAI mitigating interface to be given.

Blind channel estimation and detection for space–time coded CDMA in ISI channels

In order to accommodate high data-rate applications, there is a significant interest in extending space-time (ST) coding, originally proposed for frequency-nonselective channels assumed known to the receiver, to unknown ISI (inter-symbol interference) channels. In this paper, we consider the problem of blind channel identification and linear multiuser receiver design for ST-coded CDMA (code-division multiple-access) systems operating in frequency-selective fading channels. We investigate the identifiability conditions and present a subspace-based blind channel estimator for such systems. To assess the performance of the proposed and other potential estimators, we derive an unconditional Cramer-Rao bound (CRB) which, similarly to the proposed blind channel estimator, does not assume the knowledge of the transmitted information symbols. We discuss various linear multiuser detection schemes that can be used in conjunction with the proposed channel estimator for symbol demodulation in ST-coded CDMA systems. Numerical examples are presented to illustrate the performance of the proposed channel estimator and linear detectors in multipath Rayleigh fading channels.

Outage-based optimal power control for generalized multiuser fading channels

We address the problem of achieving outage probability constraints on the uplink of a code-division multiple-access (CDMA) system employing power control and linear multiuser detection, where we aim to minimize the total expended power. We propose a generalized framework for solving such problems under modest assumptions on the underlying channel fading distribution. Unlike previous work, which dealt with a Rayleigh fast-fading model, we allow each user to have a different fading distribution. We show how this problem can be formed as an optimization over user transmit powers and linear receivers, and, where the problem is feasible, we provide conceptually simple iterative algorithms that find the minimum power solution while achieving outage specifications with equality. We further generalize a mapping from outage probability specifications to average signal-to-interference-ratio constraints that was previously applicable only to Rayleigh-faded channels. This mapping allows us to develop suboptimal, computationally efficient algorithms to solve the original problem. Numerical results are provided that validate the iterative schemes, showing the closeness of the optimal and mapped solutions, even under circumstances where the map does not guarantee that constraints will be achieved.

Multiuser detection of time-hopping PPM UWB system in the presence of multipath fading

Ultra-wideband (UWB) impulse radio (IR) systems are currently being considered for several applications due to their attractive features that include low-power carrierless and ample multipath diversity. Among the various modulation and multiple-access schemes, time-hopping (TH) pulse position modulation (PPM) is a popular technique in application. Most past works rely on strict power control and perform single-user detection (matched filtering) on the desired signal. This paper aims to apply multiuser detection techniques in binary PPM (BPPM) UWB IR multiple-access systems. Moreover, we consider frequency-selective multipath fading channels to account for the wireless cellular environment. A class of linear multiuser detectors (LMDs) is applied to extract the information bits while eliminating multiuser interference (MUI) in the presence of multipath fading. Simulation results are provided to compare the performance of different LMDs.

Efficient BER evaluation of linear multiuser detectors with imperfect channel estimation for CDMA fading channels

In this paper, we present a unified mathematical framework to analyze the bit-error rate (BER) performance of general linear coherent multiuser receivers with diversity reception and imperfect channel estimation for doubly selective Rician-fading asynchronous code-division multiple-access channels. BERs of linear receivers with channel state information and data-aided channel estimation are analyzed, and both exact and low-complexity approximate BER evaluation formulas are presented. Furthermore, by using a Markov chain steady-state analysis, a tight BER approximation for receivers with decision-directed channel estimation is proposed. Numerical and simulation results verify the accuracy of the proposed BER evaluation methods.

Performance of Linear Reduced-Rank Multistage Receivers for DS-CDMA in Frequency-Selective Fading Channels

The performance of a set of linear reduced-rank multistage filter banks is studied in the context of multiuser detection for direct-sequence (DS) code-division multiple-access (CDMA) systems. The set of filter banks under consideration is comprised of the minimum mean-square error (MMSE), the minimum output energy (MOE), the best linear unbiased estimator (BLUE), and the maximum-likelihood (ML) detector. Based on a common framework for the multistage implementations of the aforementioned filter banks, the signal-to-interference plus noise ratios (SINRs) and bit-error rates (BERs) of these reduced-rank filter banks are studied for multipath Rayleigh-fading channels. A generic BER formula is provided for coherent detection and noncoherent differential detection schemes constructed under this common framework. Analysis shows that all of these performance measures are characterized by a kernel matrix K/sub mmse/ whose trace forms the output SINR of the MMSE filter bank. Through investigating the recursive structure of K/sub mmse/, the output SINRs are proven to be monotonically increasing with the number of stages and upper-bounded by a number equal to the paths of the desired user's channel. The condition for asymptotically achieving this upper bound is also provided, which leads to the notion of effective user capacity of linear reduced-rank multiuser detection as well as serves as a test for the existence of a BER floor for coherent detection. In addition, the channel mismatch due to differential detection is also shown to yield a BER floor for noncoherent detection. Based on this analysis, a simple yet effective rule for choosing the number of stages is provided for both coherent and noncoherent linear multistage multiuser detection.

Optimal Throughput and Energy Efficiency for Wireless Sensor Networks: Multiple Access and Multipacket Reception

We investigate two important aspects in sensor network design—the throughput and the energy efficiency. We consider the uplink reachback problem where the receiver is equipped with multiple antennas and linear multiuser detectors. We first assume Rayleigh flat-fading, and analyze two MAC schemes: round-robin and slotted-ALOHA. We optimize the average number of transmissions per slot and the transmission power for two purposes: maximizing the throughput, or minimizing the effective energy (defined as the average energy consumption per successfully received packet) subject to a throughput constraint. For each MAC scheme with a given linear detector, we derive the maximum asymptotic throughput as the signal-to-noise ratio goes to infinity. It is shown that the minimum effective energy grows rapidly as the throughput constraint approaches the maximum asymptotic throughput. By comparing the optimal performance of different MAC schemes equipped with different detectors, we draw important tradeoffs involved in the sensor network design. Finally, we show that multiuser scheduling greatly enhances system performance in a shadow fading environment.

Throughput Analysis for Fully-Connected Ad Hoc Network Joint with Multiuser Detection

The issue of importance of multiuser detection for CDMA-based mobile ad hoc networks is addressed in this paper. For conventional scheme, each terminal in the network uses matched filter to receive packets, so the performance (e.g., throughput) of the network suffers from multi-access interference (MAI). Different from above scheme, in this paper, each terminal of the ad hoc network is equipped with an adaptive blind linear multiuser detector, so the ability of MAI-resistance is gained. Employing slotted-ALOHA protocol in MAC layer and using fully-connected network model, the throughput of ad hoc network is studied. Theoretic analysis and simulation results show that multiuser detection can remarkably improve throughput performance of ad hoc networks.

Space-time multiuser detection for DS-CDMA communication in MIMO systems

Space-time signal processing based on multiple-input multiple-output(MIMO) systems is an active research field in which interfering signals are cancelled and multiuser detection is achieved using space diversity. In a Rayleigh fading channel, space-time block codes using multiple transmitting antennas can improve system performance and reduce bit-error-rate for multiuser detection. In this paper, several antenna configurations are designed for DS-CDMA communication in MIMO systems. Space-time linear multiuser detection and space-time serial interference cancellation multiuser detection are simulated. Bit-error-rate and computation complexities of the two methods are compared. Conclusions are given in the end.

Semiblind linear parallel interference cancellation multiuser detectors for DS-CDMA systems

Semiblind multiuser detection for the reverse link of a multicell code-division multiple-access (CDMA) system is considered. In such a system, although active users are present in the home cell and neighboring cells, the base station receiver only knows the signature information of the users in its own cell. Three new semiblind linear multiuser detectors are proposed based on parallel interference cancellation approach. Compared with some known semiblind detectors, the proposed detectors share a rather simple implementation structure and involve reduced latency for processing, which are of particular importance in a time-varying CDMA system. Numerical results are presented to compare the detection performance of the proposed detectors with that of some existing semiblind multiuser detectors.

Is multiuser detection beneficial to mixed service UMTS networks?

The main objective of this paper is to investigate linear multiuser detection for the uplink of present mixed service UMTS Terrestrial Radio Access Network (UTRAN) Frequency Division Duplex (FDD) networks in terms of system capacity, system coverage as well as power reduction. For the system performance assessments we apply a two-stage power control multiuser detection algorithm where the users are detected by a multiuser block detection. We evaluate the UTRAN FDD load control parameter noise rise and show corresponding system capacity bounds for different service mixes carried over dedicated channels with data rates up to 64 kbit/s. Note that for typical UTRAN FDD services the signal-to-noise operating points of a multiuser detector are very low (less than 0 dB). These low signal-to-interference-plus-noise ratios (SINR) operating points severely limit multiuser detection performance gains. Compared to single-user-matched filtering the minimum mean square error (MMSE) detector leads to fairly modest system capacity gains up to 60% dependent on the service mix assumed while the decorrelator is even worse. With respect to coverage as well as mobile station power, MMSE detection indicates fair to high improvements.

Applicability of new radio technologies for advanced UMTS networks

Abstract. An expanded effort is under the way to support the evolution of UMTS (Universal Mobile Telecommunication System). Apart from delivering high data rates, future UMTS releases will also require to provide high network performance in terms of system capacity, low radiated power, and high coverage. Well promising performance-enhancing technologies are smart antennas as well as multiuser detection. Although these new radio technologies have recently been subject to intense research, main UMTS network integration aspects with their specific constraints have been neglected in many cases. Especially the interaction with UMTS radio resource control being required to meet Quality of Service (QoS) constraints has to be included to assess the applicability of these technologies for UMTS. In this paper, we study the interaction of beamforming concepts as well as multiuser detection with load and power control. We also work out UMTS specific constraints like signal-to-interference-plus-noise ratio (SINR) operating points, pilot power pollution or channel estimation, all strongly limiting network performance. Results are shown for capacity gains and power reduction for all beamforming concepts of interest as well as linear multiuser detection schemes. The results show that fix as well as user-specific beamforming significantly improves network performance gains in downlink. In uplink multiuser detection indicates fairly modest system capacity gains, while it reduces tremendously mobile station power.

New results for probability of error performance in MMSE multiuser detection for CDMA

The minimum-mean-square-error (MMSE) linear multiuser detector considers multiple access interference (MAI) and background Gaussian noise asymptotically Gaussian for a large number of users in asynchronous code division multiple access (CDMA) system. For this asymptotic condition, the MMSE detector outperforms the decorrelating detector only if the value of normalized cross-correlation (NCC) for any pair of code sequence is less than or equal to numerically derived upper bounded value. The available results in literature have been derived for two-user case. In this paper, we have presented a general formula to calculate upper bound on NCC for arbitrary number of users under near-far situation. We also propose Chernoff bound on error probability of MMSE multiuser detector. Its proof based on divergence theorem and study of leakage coefficients for more than two users imposes the stringent condition on signal-to-noise ratio of desired user.

Impact of Channel Estimation Errors on Multiuser Detection via the Replica Method

For practical wireless DS-CDMA systems, channel estimation is imperfect due to noise and interference. In this paper, the impact of channel estimation errors on multiuser detection (MUD) is analyzed under the framework of the replica method. System performance is obtained in the large system limit for optimal MUD, linear MUD, and turbo MUD, and is validated by numerical results for finite systems.

An adaptive WPE multiuser detector

Weighted polynomial expansion (WPE) detectors can be used to approximate linear multiuser detectors to avoid the need to compute matrix inversion. For finite stage WPE detectors, weight vector optimization is an important process since it determines how effective the linear multiuser detectors can be approximated. In this letter, the optimal weight vector for the finite stage WPE detector is derived based on the mean-squared error (MSE) criteria. In addition, a low-complexity adaptive WPE detector based on the least-squares criteria is proposed to simplify the computation of the optimum weight vector. Simulation results show that the proposed adaptive WPE detector outperforms the existing WPE method.