Space-time Multiuser Detection Research Articles

Two approaches to multiuser detection over fading channels

In this paper, two different receiver structures to multiuser detection that are appropriate for the code-division multiple-access systems with antenna arrays in fading channels are investigated and compared. We analyze and compare the performance of the two different multiuser detection structures for uplink or downlink channels. The number of elements of receiving antenna array may be limited in the downlink channel due to the small size of receivers. We assume a synchronous system, but it can be easily extended to an asynchronous system. The first approach is based on the distributed decorrelator where the signal decorrelation is performed by each receiving antenna element independently and decorrelated outputs are combined according to the maximum ratio. The second approach is the central decorrelator where the signal decorrelation is performed once collectively on the outputs from all elements of receiving antenna array. Both decorrelators provide the same performance in the additive white Gaussian noise channels. The distributed decorrelator provides the better performance in flat fading channels. We employ the decorrelator to demonstrate our results. The results discussed in the present paper can be extended to other configurations such as the blind adaptive space-time multiuser detection.

Sphere Decoding for Space-time Multiuser Detection in STBC-DS-CDMA System

In this paper, a new uplink DS-CDMA system based on Alamouti STBC is proposed in order to restrain the multiple access interference (MAI) in DS-CDMA system. In the new system, data is modulated by BPSK and encoded by sing Alamouti space time block codes before transmitting. And we apply sphere decoding algorithm in the space-time multiuser detection process. The equivalent MIMO structure of received signals is also analyzed. Simulation result shows superiority of combining the new uplink system with space-time multiuser detection using sphere decoding.

MMSE Space-Time Multi-User Detection in OFDM Systems Based on a Variety of Modulation

MIMO-OFDM is considered as candidates for future 3G/B3G broadband wireless services. High quality mobile services have to eliminate multiple-access interference (MAI) that affects multi-user receiver performance in MIMO-OFDM system. In this paper, we analyze the effect of MAI on MIMO-OFDM system and adapt a successive interference cancellation (SIC) multi-user detection algorithm which based on minimum mean square error (MMSE) algorithm for MIMO-OFDM system. This algorithm by optimizing the sorting structure enables the receiver algorithm has better performance. The simulation results show that the algorithm can obtain rate is better than the conventional linear MMSE algorithm performance in the error, and constellation mapping point spacing also affect the detection performance. As the spacing is reducing, the bit error probability will rise.

Layered Space-Time Multi-User Detection Based on Kalman Filter

This paper represents the theories of mathematical model of DS-CDMA system, principle of Kalman filter and layered space-time coding. Then, for the purpose of overcoming the disadvantages of traditional multi-user detection algorithm, the paper proposes the layered space-time multi-user detection algorithm based on kalman filter. Simulation results show the effectiveness of the proposed algorithm, such as the convergence speed is quickened, bit error rate is reduced greatly and channel tracking capability is improved significantly.

Integrated Cooperative Opportunistic Packet Forwarding and Distributed Error Control in MIMO Ad Hoc Networks

In this paper, we address the problem of packet forwarding in multi-hop wireless networks of nodes equipped with multiple antennas. Through the use of MIMO (multiple-input multiple-output) technology, the physical layer is able to superimpose multiple concurrent information flows, thereby increasing the communications parallelism in the network. We propose a cooperative cross-layer scheme that integrates distributed incremental redundancy hybrid automatic retransmission request (HARQ) error control with routing in order to increase the efficiency of the transmissions and decrease the interference load in the network. In the proposed scheme, next hop relays are opportunistically selected during the HARQ process in order to avoid overloaded receivers and guarantee the geographical advancement of the packets. Thus, cooperation is used to both strengthen weak links and select the path to the destination. Numerical results are presented for a specific instantiation where a layered space-time multiuser detection (LASTMUD) transmitter-receiver architecture with spread-spectrum signals is implemented at the physical layer and packets are encoded with a linear erasure code (LEC).

Soft information assisted space-time multiuser detection for highly loaded CDMA

This letter presents an effective space-time multiuser detector (MUD) with the assistance of soft information in multipath code division multiple access (CDMA) channels. The space-time MUD considered is a simple, separable spatial-temporal filter which consists of a single spatial filter and a single temporal filter. Based on the soft-decision outputs determined in the previous iteration, the soft information is then exchanged in the alternating updates of either the spatial filters or the temporal filters. Furnished simulations show that the proposed scheme can offer substantial performance improvement compared with previous works, especially in highly loaded scenarios.

Time- and Frequency-Domain-Spread Generalized Multicarrier DS-CDMA Using Subspace-Based Blind and Group-Blind Space–Time Multiuser Detection

Subspace-based blind and group-blind space-time (ST) multiuser detection (MUD) is invoked for a smart-antenna-aided generalized multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) system communicating over a dispersive ST Rayleigh fading channel, where the channel estimates are attained by using subspace-based blind techniques. Smart antennas are employed to provide increased degrees of freedom and, hence, to improve both the attainable single-user performance and the multiuser capacity. Furthermore, two adaptive subspace-tracking algorithms, i.e., the projection approximation subspace-tracking deflation (PASTd) algorithm and the noise-averaged Hermitian-Jacobi fast subspace tracking (NAHJ-FST) algorithm, are employed for the sake of reducing the computational complexity imposed by eigenvalue decomposition. When employing the NAHJ-FST algorithm, the group-blind MUD employing an antenna array having M = 2 elements exposed to uncorrelated fading achieved a 4.2-dB signal-to-noise-ratio (SNR) gain at a bit error rate (BER) of 10-4 compared with that employing a single antenna, which is an explicit benefit of its increased computational complexity.

Multistage MMSE PIC Space–Time Receiver With Non-Mutually Exclusive Grouping

The arrival of new data services for wireless mobile communications requires an efficient use of the available bandwidth. Interference-limited cellular systems based on code-division multiple access (CDMA) can benefit from multiuser detection (MUD) and beamforming with antenna array to reduce multiple-access interference. Group-based techniques have been proposed to reduce the complexity of space-time MUD and have been shown to provide a performance-complexity tradeoff between matched filtering and full MUD. In this paper, the intergroup interference, which is a limiting factor in group-based systems, is reduced using multistage parallel interference cancellation after group-based minimum mean square error (MMSE) linear filtering. In addition, the extra resources that are available at the receiver are exploited by sharing users among groups. The proposed receiver is shown to converge, as the number of stages increases, to the full space-time MMSE linear MUD filter. The results show that the new approach provides bit error rate (BER) performance close to the full MUD receiver at a fraction of the complexity.

Group-Based Space-Time Multiuser Detection with User Sharing

To reduce the complexity of space-time multiuser detection, it has been proposed recently to exploit the spatial dimension by forming groups of users and apply the detection individually to each group. In this work we propose a new space-time receiver structure based on the group-optimal MMSE linear detector along with a new grouping algorithm that respects practical hardware limitations. Furthermore, an extension of the proposed structure which allows non-mutually exclusive grouping is presented. The simulation results show that the proposed reduced-complexity receiver structure provides a bit error rate (BER) performance close to the full linear MMSE multiuser detector.

A Blind Space-Time Constrained Minimum Output Energy Detector for DS-CDMA Communication System

In this paper, a blind space-time (ST) multiuser detector, called ST constrained minimum output energy (ST-CMOE) detector, is proposed. It can be viewed as an extension of the CMOE method ( Proc. Globecom, p. 379, 1994), and therefore it is immune to near-far effect. It processes the antenna array output jointly in spatial and temporal domain. Simulation results show that the proposed algorithm can outperform the space-CMOE detector proposed by Wang and Poor ( IEEE Trans. Signal Process. vol. 47, p. 2356, 1999) which process the received signal in spatial and temporal domain separately. It can also outperform the ST subspace detector proposed by Chkeif (IEEE Trans. Commun. vol. 48, p. 729, 2000), which, although processes the received signal jointly in spatial and temporal domains, suffers from the mismatch problem in the multipath environment. The simulation results also show that the proposed ST-CMOE detector can achieve the same performance as the training-based ST minimum-mean-square-error detector. In order to reduce the computational load and adapt to dynamic multiple-access channels, an adaptive algorithm is presented. The computational load of this adaptive detector is comparable to Hu's method (IEEE Trans. Signal Process . vol. 52, p. 1862, 2004), while it does not require any initial training or channel estimation in the multipath environment. Simulation results show that the proposed adaptive ST-CMOE detector can work well in a dynamic multiple-access channel, where interferences may enter or leave the channel at any time

User cooperation in an asynchronous cellular uplink

We present in this paper, multiuser decode-and-forward (DF) sharing scheme and a practical low-complexity adaptive pseudo-linear receiver design for cooperative diversity in an asynchronous cellular uplink. The DF schemes provide near maximum-likelihood (ML) performance, including full diversity and significant throughput improvements over no-sharing and existing schemes, under linear receiver design. The proposed linear receiver jointly separates users while suppressing the multiple-access interference (i.e., employs multiuser detection) and performs space-time decoding for diversity exploitation. We provide bit-error-probability and throughput simulations for the practical, noisy inter-user channels. The simulation results demonstrate that the three-user sharing scheme fully exploits available diversity with inter-user demodulation errors as high as 0.1% using linear joint space-time decoding and multiuser detection.

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.

Blind detection for multi-rate DS-CDMA system with antenna array at the base station

In this paper, we propose the blind space-time high rate multi-user detector for synchronous uplink multi-rate Direct Sequence Code Division Multiple Access (DS-CDMA) systems with antenna array at the base station. By employing antenna array at the base stations, the spatial dimension is used efficiently to suppress co-channel interference and increase the capacity for multi-rate CDMA system. After low rate physical users in the system are modeled as corresponding high rate virtual users, we construct the space-time signature vectors of virtual users. And subspace projection algorithm is employed to estimate space-time signature vectors blindly. Then a soft-decision high rate multiuser detector is proposed based on the estimated signature vectors, which avoids estimating the ambiguous complex factors which are necessary in traditional blind detector. Numerical simulation results evaluate the performance in terms of Bit Error Rate (BER) for the proposed scheme. Simultaneously, it demonstrates that the system capability increases two times when using two-element antenna array.

Performance analysis of space-time MMSE multiuser detection for coded DS-CDMA systems in multipath fading channels

This paper investigates the use of convolutional coding in space-time minimum mean-square-error (MMSE) multiuser-based receivers over asynchronous multipath Rayleigh fading channels. We focus on the performance gain attained through error control coding when used with binary-phase-shift-keyed modulation (BPSK) and multiuser access based on direct sequence-code-division multiple access (DS-CDMA). In our analysis, we derive an approximation for the uncoded probability of bit-error in multipath fading channels. This bit-error rate (BER) approximation is shown to be very accurate when compared to the exact performance. For a convolutionally coded system, we obtain a closed form expression for the bit-error rate upper bound. This error bound is noted to be tight as the number of quantization levels increased beyond eight. Using our theoretical results, we obtain an estimate for the achieved user-capacity that accrues due to error control coding. It is found that using convolutional coding with 3-bit soft-decision decoding, a user-capacity gain as much as 300% can easily be achieved when complete fading state information plus ideal channel interleaving are assumed.

Turbo Interference Mitigation in Layered Space-Time MIMO DS-CDMA Uplink with TTCM(Turbo Trellis Coded Modulation)

In this paper we propose the use of a turbo receiver for the uplink of a MIMO CDMA system employing layered space-time transmission. The proposed receiver consists of a low complexity layered space-time multi-user detector using minimum mean squared error (MMSE) filtering with a-priori information and a bank of MAP SISO decoders. Using the soft estimates from a bank of MAP decoders we obtain soft values of the interfering symbols. The SISO multiuser detector subtracts the vector of the interfering symbols from the incoming signal. The resulting vector is then filtered by an adaptive MMSE filter to reduce the residual MAI. This process iterates by exchanging extrinsic informations between the bank of MAP decoders and the SISO multiuser detector. Turbo Trellis Coded Modulation (TTCM) is used as Forward Error Correcting (FEC) code due to its high bandwidth efficiency. Our computer simulations show that the proposed structure outperforms a classical iterative receiver based on parallel interference cancellation (PIC) as well as a non-iterative MMSE receiver. Furthermore, in a multiuser context, the proposed receiver offers an error performance similar to that of single-user case at high SNR.

A novel blind space-time multiuser detector for DS-CDMA system

In this letter, a novel blind space-time multiuser detector, called space-time constrained minimum output energy (ST-CMOE) detector, is proposed. It can achieve the same performance as the training based space-time minimum mean square error (ST-MMSE) detector. An adaptive version of the proposed ST-CMOE detector is also presented. Simulation shows that, the adaptive ST-CMOE detector can work well in a dynamic multiple-access channel.

On Channel Parameter Estimation in a Space–Time Bit-Interleaved-Coded Modulation System for Multipath DS-CDMA Uplink With Receive Diversity

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

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.

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.

Downlink capacity of interference-limited MIMO systems with joint detection

The capacity of downlink cellular multiple-input multiple-output (MIMO) systems, where co-channel interference is the dominant channel impairment, is investigated in this paper, mainly from a signal-processing perspective. Turbo space-time multiuser detection (ST MUD) is employed for intracell communications and is shown to closely approach the ultimate capacity limits in Gaussian ambient noise for an isolated cell. Then, it is combined with various multiuser detection methods for combating intercell interference. Among various multiuser detection techniques examined, linear minimum-mean-square-error (MMSE) MUD and successive interference cancellation are shown to be feasible and effective. Based on these two multiuser detection schemes, one of which may outperform the other for different settings, an adaptive detection scheme is developed, which together with a Turbo ST MUD structure offers substantial performance gain over the well-known V-BLAST techniques with coding in this interference-limited cellular environment. The obtained multiuser capacity is excellent in the high to medium signal-to-interference ratio scenario. Nonetheless, numerical results also indicate that a further increase in system complexity, using base-station cooperation, could lead to further significant increases of the system capacity. The asymptotic multicell MIMO capacity with linear MMSE MUD preprocessing is also derived, and this analysis agrees well with the simulation results.