Multiuser Code-division Multiple-access System Research Articles

Multi-user Detection in DS-CDMA System Using Biogeography Based Optimization

To attain large information rates, code division multiple access (CDMA) is a well-built aspirant for the downlink of cell phone communications. However, while transmitting any signal over fading channel, the performance of CDMA system is highly affected. For that reason, Multiuser detection (MUD) and channel estimation play a significant function for overcoming the interference and characterizing the channel. In this paper, biogeography base optimization (BBO) algorithm introduced for multiuser detection CDMA system. The objective of BBO algorithm is used to minimize the error rate of the user transmitting signal. According to immigration rate and emigration rate, the optimal solution of the detection problem is decided. Therefore, the user detection complexity and the interference of user transmitting signal are resolved. Proposed multiuser detection algorithm is appropriate for run time user identification process; since the detection complication and the required time are concentrated. The proposed approach is implemented in MATLAB working platform and the performance measures are examined. The error rate of proposed algorithm is compared with harmony search algorithm, enhanced harmony search algorithm, and without multiuser detection procedure.

Decoder-Assisted Timing Synchronization in Multiuser CDMA Systems

The problem of synchronizing weak user signals in multiuser code-division multiple access systems is addressed. We integrate timing synchronization into an iterative multiuser detection (MUD) framework for the improved synchronization performance of weak users. First, the effect of correctly detected users on the remaining undetected users is suppressed via soft parallel interference cancellation. Then, the time offset (TO) of each undetected user is estimated. To speed up the MUD, interference cancellation is performed after only a few decoder iterations of the detected users, and the TO estimator is designed taking into account the residual multiple access interference. A low-complexity non-data-aided TO estimator that estimates the TO of each undetected user separately is proposed. The proposed technique significantly improves the synchronization performance of weak users under high near-far conditions compared to the full interference case. Assuming Gaussian distributed log-likelihood ratios at the decoder outputs, we directly relate the missed detection probability of a desired user to the root-mean-squared error of the detected user's data sequences. The extrinsic information transfer chart of the decoder is used to analyze the acquisition performance of the desired user with the number of decoder iterations of the detected users. An upper bound for the missed detection probability of the proposed estimator is derived.

Receiver Design for Uplink Multiuser Code Division Multiple Access Communication System Based on Neural Network

In this paper, we consider the receiver design problem for the uplink multiuser code division multiple access (CDMA) communication system based on the neural network technique. The uplink multiuser CDMA communication system model is described in the form of space---time domain through antenna array and multipath fading expression. Novel suitable neural network technique is proposed as an effective signal processing method for the receiver of such an uplink multiuser CDMA system. By the appropriate choice of the channel state information for the neural network parameters, the neural network can collectively resolve the effects of both the inter-symbol interference due to the multipath fading channel and the multiple access interference in the receiver of the uplink multiuser CDMA communication system. The dynamics of the proposed neural network receiver for the uplink multiuser CDMA communication system is also studied.

A neural network approach to MMSE receivers in a DS-CDMA multipath fading environment

This letter deals with an advanced minimum mean-squared error receiver for applications to uplink transmissions in a multiuser code-division multiple-access system. The receiver is implemented by means of a suitable neural network in order to enhance the receiver convergence speed in the case of fast fading. Performance comparisons with classical approaches highlights a better behavior for the proposed scheme.

Multi-user MIMO CDMA systems using complete complementary sequences

The authors propose a novel use of complete complementary (CC) sequences for increased spectral efficiency in a multiple-input–multiple-output (MIMO) code division multiple access (CDMA) system. The new method overcomes many of the problems and limitations persistent in single-input–single-output (SISO) and proves that under the proposed MIMO CC-CDMA system, the capacity for the number of supported users increases linearly by the number of transmitters. The paper also shows that the MIMO CC-CDMA system demonstrates a superior performance over those using traditional Walsh spreading sequences (Walsh-CDMA). Results include the bit error rate comparison for CC-CDMA frequency selective fading channels and that of the Walsh-CDMA under flat fading channels.

Equalization with oversampling in multiuser CDMA systems

Some of the major challenges in the design of new-generation wireless mobile systems are the suppression of multiuser interference (MUI) and inter-symbol interference (ISI) within a single user created by the multipath propagation. Both of these problems were addressed successfully in a recent design of A Mutually Orthogonal Usercode-Receiver (AMOUR) for asynchronous or quasisynchronous code division multiple access (CDMA) systems. AMOUR converts a multiuser CDMA system into parallel single-user systems regardless of the multipath and guarantees ISI mitigation, irrespective of the channel null locations. However, the noise amplification at the receiver can be significant in some multipath channels. In this paper, we propose to oversample the received signal as a way of improving the performance of AMOUR systems. We design Fractionally Spaced AMOUR (FSAMOUR) receivers with integral and rational amounts of oversampling and compare their performance with the conventional method. An important point that is often overlooked in the design of zero-forcing channel equalizers is that sometimes, they are not unique. This becomes especially significant in multiuser applications where, as we will show, the nonuniqueness is practically guaranteed. We exploit this flexibility in the design of AMOUR and FSAMOUR receivers and achieve noticeable improvements in performance.

Performance Evaluation of a Novel CDMA Detection Technique: The Two-State Approach

The use of code division multiple access (CDMA) makes third-generation wireless systems interference limited rather than noise limited. The research for new methods to reduce interference and increase efficiency led us to formulate a signaling method where fast impulsive silence states are mapped on zero-energy symbols. The theoretical formulation of the optimum receiver is reported and the asymptotic multiuser efficiency (AME) as well as an upper bound of the probability of error have been derived and applied to the conventional receiver and the decorrelating detector. Moreover, computer simulations have been performed to show the advantages of the proposed two-state scheme over the traditional single-state receiver in a multiuser CDMA system operating in a multipath fading channel.

A full-rank regularization technique for MMSE detection in multiuser CDMA systems

In multiuser code-division multiple-access (CDMA) environments, the ill-conditioning of the covariance matrix of the received signal may degrade the performance of minimum mean-squared error (MMSE) detectors, especially when few samples are available for the covariance matrix estimation. In order to mitigate this performance degradation, we propose a full-rank regularized MMSE detector based on the covariance matrix tapering (CMT) technique. Simulation results show the effectiveness of the proposed technique at high signal-to-noise ratio (SNR).

A full-rank regularization technique for MMSE detection in multiuser CDMA systems

In multiuser code-division multiple-access (CDMA) environments, the ill-conditioning of the covariance matrix of the received signal may degrade the performance of minimum mean-squared error (MMSE) detectors, especially when few samples are available for the covariance matrix estimation. In order to mitigate this performance degradation, we propose a full-rank regularized MMSE detector based on the covariance matrix tapering (CMT) technique. Simulation results show the effectiveness of the proposed technique at high signal-to-noise ratio (SNR).

AMOUR-generalized multicarrier transceivers for blind CDMA regardless of multipath

Suppression of multiuser interference (MUI) and mitigation of multipath effects constitute major challenges in the design of third-generation wireless mobile systems. Most wide-band and multicarrier uplink code-division multiple-access (CDMA) schemes suppress MUI statistically in the presence of unknown multipath. For fading resistance, they all rely on transmit- or receive-diversity and multichannel equalization based on bandwidth-consuming training sequences or self-recovering techniques at the receiver end. Either way, they impose restrictive and difficult to check conditions on the finite-impulse response channel nulls. Relying on block-symbol spreading, we design a mutually-orthogonal usercode-receiver (AMOUR) system for quasi-synchronous blind CDMA that eliminates MUI deterministically and mitigates fading regardless of the unknown multipath and the adopted signal constellation. AMOUR converts a multiuser CDMA system into parallel single-user systems regardless of multipath and guarantees identifiability of users' symbols without restrictive conditions on channel nulls in both blind and nonblind setups. An alternative AMOUR design called Vandermonde-Lagrange AMOUR is derived to add flexibility in the code assignment procedure. Analytic evaluation and preliminary simulations reveal the generality, flexibility, and superior performance of AMOUR over competing alternatives.

Near optimum tree-search detection schemes for bit-synchronous multiuser CDMA systems over Gaussian and two-path Rayleigh-fading channels

Simple reduced tree-search detection schemes of the breadth-first type are applied to suboptimal joint multiuser detection in bit-synchronous code-division multiple access (CDMA) systems over both Gaussian and two-path Rayleigh-fading channels. It is pointed out that in contrast to the case of the optimal multiuser detector, the choice of the receiver filter severely influences the performance of suboptimal multiuser detectors. Simulation results supported by analysis show that breadth-first tree-search algorithms using a decorrelating noise whitening receiver filter perform better than similar receivers, which solely use a matched filter (MF) for virtually all nonsingular spreading code sets studied. Most of the code sets are randomly generated. The M- and T-algorithm detectors based on decorrelating noise whitening filter (WF) outputs can achieve near optimum performance at a very low complexity compared to the optimal detector, although the proposed detectors are more complex than some known suboptimum detectors. Furthermore, the use of combining techniques is considered for a two-path Rayleigh-fading channel, and a semi-synchronous CDMA structure is proposed. It is shown that if maximum ratio combining (MRC) is employed, the decorrelating noise WF still exists. The corresponding suboptimal combining detector with a decorrelating noise WF outperforms a similar noncombining detector.

A simple way to compute the minimum distance in multiuser CDMA systems

A simple method to compute the minimum distance D/sub min/ in multiuser code division multiple access (CDMA) systems based on the Cholesky decomposition of the positive-definite symmetric correlation matrix is proposed. Although finding D/sub min/ is known to be NP-hard, this decomposition allows the computation of the minimum distance and the asymptotic efficiency of optimum multiuser detection to be performed very efficiently in almost all cases of practical interest. Numerical results for synchronous CDMA with binary random signature waveforms of length 31 are used to illustrate the method.

The design and performance analysis for several new classes of codes for optical synchronous CDMA and for arbitrary-medium time-hopping synchronous CDMA communication systems

New families of spread-spectrum codes are constructed, that are applicable to optical synchronous code-division multiple-access (CDMA) communications as well as to arbitrary-medium time-hopping synchronous CDMA communications. Proposed constructions are based on the mappings from integer sequences into binary sequences. The authors use the concept of number theoretic quadratic congruences and a subset of Reed-Solomon codes similar to the one utilized in the Welch-Costas frequency-hop (FH) patterns. The properties of the codes are as good as or better than the properties of existing codes for synchronous CDMA communications: both the number of code-sequences within a single code family and the number of code families with good properties are significantly increased when compared to the known code designs. Possible applications are presented. To evaluate the performance of the proposed codes, a new class of hit arrays called cyclical hit arrays is recalled, which give insight into the previously unknown properties of the few classes of number theoretic FH patterns. Cyclical hit arrays and the proposed mappings are used to determine the exact probability distribution functions of random variables that represent interference between users of a time-hopping or optical CDMA system. Expressions for the bit error probability in multi-user CDMA systems are derived as a function of the number of simultaneous CDMA system users, the length of signature sequences and the threshold of a matched filter detector. The performance results are compared with the results for some previously known codes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>