Blind Constant Modulus Multiuser Detection in Multipath Fading Channel via Contracting Regularized BFGS

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

A blind adaptive step-size time-domain receiver for multi-carrier code-division multiple access (MC-CDMA) systems is presented. Adjustment rules for the receiver tap-weight as well as its step-size are based upon the stochastic approximation of the constant modulus (CM) criterion. The ability of the proposed receiver to detect the desired user in multipath fading channels at full load is assessed. Sensitivity to the various initial values of the step-size and the adaptation rates of the algorithm is also investigated.

We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance.

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each processing window. The asymptotic convergence of the mean of the iterative estimate to the actual channel is also shown. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. Finally, the proposed iterative channel estimation technique is extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimate with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.

The authors investigate the problem of blind multi-user detection (MUD) for uplink of asynchronous direct-sequence code-division multiple access (DS-CDMA) systems with hostile jamming. To cope with the lack of prior knowledge of spread signals, hostile jamming and channel state information, the authors propose a scheme based on blind source separation (BSS) to solve this problem. Dual receive antennas are used in the proposed scheme. By exploiting the structure of the two received signals, a BSS model with dependent sources is formed, and then blind hostile jamming cancellation and MUD are obtained by solving this BSS problem. The proposed scheme does not require to know any information about the hostile jamming, and can work well under multiple kinds of hostile jamming. Simulation results further demonstrate that if such proposed approach is used to cancel hostile jamming and make MUD, then the bit-error-rate performance can be substantially improved as compared with earlier detectors.

We develop a novel multistage group-blind multiuser detection technique for reverse link in multi-cell direct sequence (DS) code-division multiple access (CDMA) systems. This approach fully utilizes all known users' information, performs interference cancellation (IC) to suppress the interferers within the cell, and exploits blind adaptive decision-directed (DD) MMSE detector to suppress the inter-cell interference. This process can be made iterative and in the multistage process, the tentative decision bits from DD-MMSE detectors can be fed back to the IC part to make interference regeneration more accurate. A simplified RLS (SRLS) algorithm is derived particularly for DD-MMSE detectors and steady-state SIR performance of the SRLS algorithm is analyzed. It is shown that the proposed detector with SRLS algorithm is sufficient to perform accurate multiuser detection and has significant performance gain over that of the pure blind detectors and the non-blind detectors for the given scenario. Moreover, compared with the conventional RLS algorithm, the SRLS algorithm has lower computational complexity and the convergence speed is faster. We also develop two group-blind channel estimation methods to estimate the effective signature waveforms in multipath fading channel. It is seen that the group-blind channel estimation methods using updated received signal correlation matrix can effectively combat the mismatch problem faced by pure blind detectors.

Due to the open access nature of wireless communication environment, physical layer security has brought increasingly high attention in recent years, attempting to protect the confidential communication away from eavesdroppers. In contrast to the current secrecy designs, which generally exploit the spatial Degree-of-Freedom (DoF) (e.g. MIMO and cooperative relay approaches) or the temporal DoF (e.g. waveform-based schemes), we propose in this paper a novel binary signature design for the secure transmission of downlink code-division multiple-access (CDMA) systems by exploring the DoF in the code domain. In particular, we exploit the characteristics of CDMA communications and the diversity of multipath Rayleigh fading channels. We propose to elaborately design the spreading code to minimize the output signal-to-interference-plus-noise ratio (SINR) of the eavesdropper while ensuring the legitimate users' quality of service (QoS)/SINR. The proposed algorithms can provide a near optimal performance compared with the exhausting searching approach, but with a much lower computational complexity. Simulation studies confirm our analytical performance predictions and demonstrate the benefits of the proposed secure signature design algorithms.

Distributed multiple-input multiple-output (MIMO) system is a new architecture for next generation wireless communication systems with distributed multiple antennas. In this paper, an uplink model for multiuser distributed MIMO systems with wideband code-division multiple-access (CDMA) technology is presented. We focus on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. The spectral efficiency of the uplink system with single-user detectors at the base stations under multipath fading channels is investigated carefully. Two cases with different assumptions of interference knowledge are studied in detail. The spreading sequences of the CDMA systems are assumed to employ loosely synchronized (LS) spreading sequences and random spreading sequences respectively. LS spreading sequences exhibit a so-called interference free window (IFW), where both the auto-correlation and cross-correlation of the codes become zero. Therefore LS sequences have the promise of mitigating the effects of inter symbol interference (ISI) and multiple access interference (MAI) in multipath fading channels. It is shown by numerical results that the uplink spectral efficiency of the distributed MIMO-CDMA system using LS sequences can significantly outperform that of the system using random spreading sequences. Copyright © 2005 John Wiley & Sons, Ltd.

This paper presents an iterative, yet effective minimum mean square error (MMSE) space-time multiuser detector (MUD) with the assistance of soft interference to enhance the performance in the multipath code division multiple access (CDMA) systems. The proposed space-time MUD is of rank one and comprises of a spatial filter and a temporal filter. To offset the performance loss due to such a constraint, the new MUD alternatively updates the spatial filter or the temporal filter with the soft interference cancellation in between so as to refine the received signal. Computer simulations show that the new MUD can drastically enhance the performance compared to previous works with modest computations, especially in highly loaded scenarios.

In this contribution, we consider the performance of multiple-input multiple-output (MIMO) code division multiple access (CDMA) system for digital image transmission over correlated Rayleigh fading channel. MIMO system employs multiple antennas both at the transmitter and receiver. In our work, we consider turbo coded system to investigate the performance of MIMO-CDMA system. We realize turbo code using parallel concatenated convolutional encoder and second convolutional encoder is preceded by random interleaver. Further the output of two convolutional encoder are punctured to get rate of ½. At the receiver, we employ iterative decoding algorithm to reduce the effect of multiple access interference (MAI) and Inter-Symbol-Interference (ISI) and median filters to reduce the effect of salt and pepper noise.. In particular, we evaluate the performance of coded MIMO-CDMA system using Zero forcing (ZF) - multi-user detection (MUD) and minimum mean square error (MMSE)-MUD detection technique to mitigate the effects of multi-stream interference (MSI). It is shown through the simulation study that our considered coded MIMO-CDMA system with MMSE algorithm achieves better picture quality with less signal-to-noise ratio (SNR) as compared to ZF MUD.

Group-blind multiuser detectors for uplink code-division multiple-access (CDMA) were recently developed by Wang and Host-Madsen. These detectors make use of the spreading sequences of known users to construct a group constraint to suppress the intracell interference. However, such techniques demand the estimation of the multipath channels and the delays of the known users. In this paper, several improved blind linear detectors are developed for CDMA in fading multipath channels. The proposed detectors utilize the correlation information between consecutively received signals to generate the corresponding group constraint. It is shown that by incorporating this group constraint, the proposed detectors can provide different performance gains in both uplink and downlink environments. Compared with the previously reported group-blind detectors, our new methods only need to estimate the multipath channel of the desired user and do not require the channel estimation of other users. Simulation results demonstrate that the proposed detectors outperform the conventional blind linear multiuser detectors.

In recent years, the joint decoding of combined source and channel coding have shown the capability of delivering performances exceeding that of separately decoded schemes. In this paper, a well-known joint source/channel coding scheme is investigated for possible use in future 4th Generation (4G) direct sequence spread spectrum multiple access (DS/SSMA) code division multiple access (CDMA) communication systems. Due to the availability of potentially large sets of sequences with good correlation characteristics, the DS/SSMA CDMA system considered in this paper employs complex spreading sequences (CSS). A multiuser CSS-based DS/SSMA CDMA communication system was implemented as test platform for the joint source/channel coding scheme. Several families of CSSs, including Zadoff-Chu (ZC), double sideband constant envelope linearly interpolated root-of-unity filtered general chirp-like (DSB CE-LI-RU Filtered GCL), analytical bandlimited complex (ABC) and quadriphase (QPH) sequences are investigated on this platform. Simulated bit error rate (BER) performances are presented for uncoded, separate source and channel coded and joint source/channel coded systems under realistic multipath fading channel conditions.

A parallel combinatory (PC)-hybrid pulse interval modulation-optical code division multiple access (hPIM-OCDMA) system is proposed. In the PC-hPIM-OCDMA system, a user chooses e among K code sequences assigned to the user and sends e2b-ary PIM symbols for a given frame, where b is the number of bits per PIM symbol. The PC-hPIM-OCDMA system can offer higher bit rate than hPIM-OCDMA systems, and then it can reduce the effect of MAI, without reducing the bit rate. The bit error rate (BER) of PC-hPIM-OCDMA is derived, by considering that the dominant interference is MAI, which is caused by cross-correlation of strict optical orthogonal codes (SOOCs). Numerical results show that the PC-hPIM-OCDMA system can offer lower BER and accommodate more users than hPIM-OCDMA systems.

A blind two-stage multiple-input multiple-output (MIMO) receiver is proposed for code-division multiple access (CDMA) systems utilizing space-time block coding (STBC) over a multipath fading channel. Specifically, in the first stage, a signal-blocked (SB) interference-blocked (IB) detector is first constructed for collection of the desired signals and suppression of multiple access interference (MAI). In the second-stage, a decision-directed scheme is developed to alleviate desired signal cancellation. Computer simulations demonstrate that the proposed blind receiver can achieve a reliable output signal-to-interference-plus-noise ratio (SINR) performance approximating that of the optimal minimum mean squared error (MMSE) receiver and can exhibit the robustness against the finite data sample effect.

Blind adaptive multiuser detection based on the constant modulus algorithm (CMA) has received a lot of attention. In the blind detectors proposed previously, the CMA is applied to the output of the chip match filters. In this paper, we consider the use of the CMA at the output of a bank of filters matched to the signature waveforms of each user. For simplicity, a synchronous code division multiple access (CDMA) system is adopted. The convergence performance of the blind detector is also analyzed and the results indicate that the CMA does not have the stable undesired equilibrium, which has been proven to exist for the practical Goddard algorithm with the finite tap equalizer when used in intersymbol interference (ISI) channels. However, the permutation ambiguity and the phase ambiguity occur. To avoid the permutation and phase ambiguities, some constraints are introduced into the CMA and their validity is also proven by analysis and simulations. When the CMA for each user converges to the desired solution, the blind detector is approaching to the decorrelating detector.