A genetic algorithm-assisted semi-adaptive MMSE multi-user detection for MC-CDMA mobile communication systems

In this work, a novel Minimum-Mean Squared-Error (MMSE) multi-user detector is proposed for MC-CDMA transmission systems working over mobile radio channels characterized by time-varying multipath fading. The proposed MUD algorithm is based on a Genetic Algorithm (GA)-assisted per-carrier MMSE criterion. The GA block works in two successive steps: a training-aided step aimed at computing the optimal receiver weights using a very short training sequence, and a decision-directed step aimed at dynamically updating the weights vector during a channel coherence period. Numerical results evidenced BER performances almost coincident with ones yielded by ideal MMSE-MUD based on the perfect knowledge of channel impulse response. The proposed GA-assisted MMSE-MUD clearly outperforms state-of-the-art adaptive MMSE receivers based on deterministic gradient algorithms, especially for high number of transmitting users

Novel multiuser detection and multi-rate schemes for multi-carrier CDMA

A large variety of services is [sic] expected for wireless systems, in particular, high data rate services, such as wireless Internet access. Users with different data rates and quality of service (QoS) requirements must be accommodated. A suitable multiple access scheme is key to enabling wireless systems to support both the high data rate and the integrated multiple data rate transmissions with satisfactory performance and flexibility. A multi-carrier code division multiple access (MC-CDMA) scheme is a promising candidate for emerging broadband wireless systems. MC-CDMA is a hybrid of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). The most salient feature of MC-CDMA is that the rate of transmission is not limited by the wireless channel\u27s frequency-selective fading effects caused by multipath propagation. In MC-CDMA, each chip of the desired user\u27s spreading code, multiplied by the current data bit, is modulated onto a separate subcarrier. Therefore, each subcarrier has a narrow bandwidth and undergoes frequency-flat fading. Two important issues for an MC-CDMA wireless system, multiuser detection and multi-rate access, are discussed in this dissertation. Several advanced receiver structures capable of suppressing multiuser interference in an uplink MC-CDMA system, operating in a frequency-selective fading channel, are studied in this dissertation. One receiver is based on a so-called multishot structure, in which the interference introduced by the asynchronous reception of different users is successfully suppressed by a receiver based on the minimum mean-square error (MMSE) criterion with a built-in de-biasing feature. Like many other multiuser schemes, this receiver is very sensitive to a delay estimation error. A blind adaptive two-stage decorrelating receiver based on the bootstrap algorithm is developed to combat severe performance degradation due to a delay estimation error. It is observed that in the presence of a delay estimation error the blind adaptive bootstrap receiver is more near-far resistant than the MMSE receiver. Furthermore, a differential bootstrap receiver is proposed to extend the limited operating range of the two-stage bootstrap receiver which suffers from a phase ambiguity problem. Another receiver is based on a partial sampling (PS) demodulation structure, which further reduces the sensitivity to unknown user delays in an uplink scenario. Using this partial sampling structure, it is no longer necessary to synchronize the receiver with the desired user. Following the partial sampling demodulator, a minimum mean-square error combining (MMSEC) detector is applied. The partial sampling MMSEC (PS-MMSEC) receiver is shown to have strong interference suppression and timing acquisition capabilities. The complexity of this receiver can be reduced significantly, with negligible performance loss, by choosing a suitable partial sampling rate and using a structure called reduced complexity PS-MMSEC (RPS-MMSEC). The adaptive implementation of these receivers yields a superior rate of convergence and symbol error rate performance in comparison to a conventional MMSEC receiver with known timing. All the above receiver structures are for a single-rate MC-CDMA. Three novel multi-rate access schemes for multi-rate MC-CDMA, fixed spreading length (FSL), coded FSL (CFSL) and variable spreading length (VSL), have been developed. These multi-rate access schemes enable users to transmit information at different data rates in one MC-CDMA system. Hence, voice, data, image and video can be transmitted seamlessly through a wireless infrastructure. The bit error rate performance of these schemes is investigated for both low-rate and high-rate users

Lattice-structure based adaptive MMSE detectors for DS-CDMA systems.

Thesis (M.Sc.Eng.)-University of Natal, Durban, 2001.There has been significant interest in the research community on detectors for DS-CDMA systems. The conventional detector, which detects users ' data bits, by using a filter matched to the users' spreading codes, has two major drawbacks. These drawbacks are (1) its capacity is limited by multiple access interference (MAl) and (2) it suffers from the near-far problem. The remedy to these problems is to use a multiuser detector, which exploits knowledge of users ' transmission and channel parameters to mitigate MAl. Such detectors are called multi user detectors (MUD). A number of these detectors have been proposed in the literature. The first such detector is the optimal detector proposed by Verdu. Following Verdu's work a number of suboptimal detector were proposed. These detectors offer better computational complexity at the expense of the bit error rate performance. Examples of these detectors are the decorrelating detector, the minimum mean squared error detector (MMSE), the successive interference cancellation and parallel interference cancellation. In this thesis, we consider the adaptive DS-CDMA MMSE detector, where lattice-based filter algorithms are employed to suppress MAl. Most of the work in the literature has considered the implementation of this detector using the Least Mean Square (LMS) algorithm. The disadvantage of using the LMS algorithm to implement the MMSE detector is that the LMS algorithm converges very slowly. The main aims of this thesis are as follows. A review of the literature on MUD is presented. A lattice based MUD is then proposed and its performance evaluated using both simulation and analytical methods. The results obtained are compared with those of the LMSMMSE detector. From the results obtained the adaptive Lattice-MMSE detector is shown to offer good performance tradeoff between convergence results and BER results

Wavelet-based multi-carrier code division multiple access systems

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Power and performance trade-off in DS-CDMA receivers based on adaptive LMS-MMSE multi-user detector.

Thesis (M.Sc.Eng.)-University of Natal, Durban, 2003.Third generation cellular communication systems based on CDMA techniques have shown great scope for improvement in system capacity. Over the last decade, there has been significant interest in DS-CDMA detectors. The conventional detector, the optimal detector and a number of sub-optimal multi-user detectors (MUD) have been extensively analyzed in the literature. Recently, the reduction of power consumption in DS-CDMA systems has also become another important consideration in both system design and in implementation. In order to support wireless multimedia services, all CDMA-based systems for third generation systems have a large bandwidth and a high data rate, therefore the power consumed by the digital signal processor (DSP) is high. This thesis focuses on power consumption in the adaptive Minimum Mean Square Error (MMSE) detector which is based on the Least Mean Square (LMS) algorithm. This thesis presents a literature survey on MUD and adaptive filter algorithms. A system model of the quantized LMS-MMSE MUD is proposed and its performance is analyzed. The quantization effects in the finite precision LMS-MMSE adaptive MUD including the steady-state weight covariance, mean square error (MSE) and bit error rate (BER) versus wordlength of data and coefficient are investigated when both the data and filter coefficients are quantized. The effects of wordlength size on power consumption are investigated and the tradeoff between the power consumption and performance degradation and the optimal allocation of bits to data and to LMS coefficients under power constraint is presented

Contribution à la mise en oeuvre de récepteurs et de techniques d'estimation de canal pour les systèmes mobiles de DS-CDMA multi-porteuse

Ce mémoire traite du développement de récepteurs et de techniques déstimation de canal pour les systèmes mobiles sans fil de type DS-CDMA multi-porteuse. Deux problèmes principaux doivent être pris en compte dans ce cas. Premièrement, l'Interférence d'Accès Multiple (IAM) causée par d'autres utilisateurs. Deuxièmement, les propriétés des canaux de propagation dans les systèmes radio mobiles. Ainsi, dans la première partie du manuscrit, nous proposons deux structures adaptatives (dites détection séparée et détection jointe) pour la mise en oeuvre de récepteurs minimisant lérreur quadratique moyenne (MMSE), fondés sur un Algorithme de Projection Affine (APA). Ces récepteurs permettent de supprimer les IAM, notamment lorsque le canal d'évanouissement est invariant dans le temps. Cependant, comme ces récepteurs nécessitent les séquences d'apprentissage de chaque utilisateur actif, nous développons ensuite deux récepteurs adaptatifs dits aveugles, fondés sur un algorithme de type projection affine. Dans ce cas, seule la séquence d'étalement de l'utilisateur désiré est nécessaire. Quand les séquences d'étalement de tous les utilisateurs sont disponibles, un récepteur reposant sur le décorrélateur est aussi proposé et permet d'éliminer les IAM, sans qu'une période pour l'adaptation soit nécessaire. Dans la seconde partie, comme la mise en oeuvre de récepteurs exige léstimation du canal, nous proposons plusieurs algorithmes pour léstimation des canaux d'évanouissement de Rayleigh, variables dans le temps et produits dans les systèmes multi-porteuses. A cette fin, les canaux sont approximés par des processus autorégressifs (AR) d'ordre supérieur à deux. Le premier algorithme repose sur deux filtres de Kalman interactifs pour léstimation conjointe du canal et de ses paramètres AR. Puis, pour nous affranchir des hypothèses de gaussianité nécessaires à la mise en oeuvre d'un filtre optimal de Kalman, nous étudions la pertinence d'une structure fondée sur deux filtres H1 interactifs. Enfin, léstimation de canal peut ^etre vue telle un problème déstimation fondée sur un modèle à erreur- sur-les-variables (EIV). Les paramètres AR du canal et les variances de processus générateur et du bruit d'observation dans la représentation de léspace d'état du système sont dans ce cas estimés conjointement à partir du noyau des matrices d'autocorrélation appropriées.This dissertation deals with the development of receivers and channel estimation techniques for multi-carrier DS- CDMA mobile wireless systems. Two major problems should be taken into account in that case. Firstly, the Multiple Access Interference (MAI) caused by other users. Secondly, the multi-path fading of mobile wireless channels. In the first part of the dissertation, we propose two adaptive structures (called separate and joint detection) to design Minimum Mean Square Error (MMSE) receivers, based on the Affine Projection Algorithm (APA). These receivers are able to suppress the MAI, particularly when the fading channel is time-invariant. However, as they require a training sequence for every active user, we then propose two blind adaptive multiuser receiver structures based on a blind APA-like multiuser detector. In that case, only the knowledge of the spreading code of the desired user is required. When the spreading codes of all users are available, a decorrelating detector based receiver is proposed and is able to completely eliminate the MAI without any training. In the second part, as receiver design usually requires the estimation of the channel, we propose several training-based algorithms for the estimation of time-varying Rayleigh fading channels in multi-carrier systems. For this purpose, the fading channels are approximated by autoregressive (AR) processes whose order is higher than two. The first algorithm makes it possible to jointly estimate the channel and its AR parameters based on two-cross-coupled Kalman filters. Nevertheless, this filtering is based on restrictive Gaussian assumptions. To relax them, we investigate the relevance of a structure based on two-cross-coupled H1 filters. This method consists in minimizing the influence of the disturbances such as the additive noise on the estimation error. Finally, we propose to view the channel estimation as an Errors-In-Variables (EIV) issue. In that case, the channel AR parameters and the variances of both the driving process and the measurement noise in the state-space representation of the system are estimated from the null space of suitable correlation matrices