Erlangen, Nürnberg, Univ., Diss., 2003. / Erscheinungsjahr an der Haupttitelstelle: 2002. Computerdatei im Fernzugriff.

This work presents the application of the theory and algorithms of Independent Component Analysis (ICA) to blind multiuser symbol estimation in downlink of Direct-Sequence Code Division Multiple Access (DS-CDMA) communication system. The main focus is on blind separation of convolved CDMA mixture and the improvement of the downlink symbol estimation. Term blind implies that the separation is performed based upon the observation only. Since the knowledge of system parameter is available only in the downlink environment, the blind multiuser detection algorithm is an attractive option in the downlink.<p>Firstly, the basic principles of ICA are introduced. The objective function and optimization algorithm of ICA are discussed. A typical ICA method, one of the benchmark methods for ICA, FastICA, is considered in details. Another typical ICA algorithm, InfoMAX, is introduced as well, followed by numerical experiment to evaluate two ICA algorithms.<p>Secondly, FastICA is proposed for blind multiuser symbol estimation as the statistical independence condition of the source signals is always met. The system model of simulation in downlink of DS-CDMA system is discussed and then an ICA based DS-CDMA downlink detector has been implemented with MATLAB. A comparison between the conventional Single User Detection (SUD) receiver and ICA detector has been made and the simulation results are analyzed as well. The results show that ICA detector is capable of blindly solving multiuser symbol estimation problem in downlink of DS-CDMA system.<p>The convergence of ICA algorithm is, then, discussed to obtain more stable simulation results. A joint detector, which combines ICA and SUD and where ICA is considered as an additional element attached to SUD detector, has been implemented. It was demonstrated that the joint detector gives the lowest error probability compared to conventional SUD receiver and pure ICA detector with training sequences.

This work presents the application of the theory and algorithms of Independent Component Analysis (ICA) to blind multiuser symbol estimation in downlink of Direct-Sequence Code Division Multiple Access (DS-CDMA) communication system. The main focus is on blind separation of convolved CDMA mixture and the improvement of the downlink symbol estimation. Term blind implies that the separation is performed based upon the observation only. Since the knowledge of system parameter is available only in the downlink environment, the blind multiuser detection algorithm is an attractive option in the downlink.<p>Firstly, the basic principles of ICA are introduced. The objective function and optimization algorithm of ICA are discussed. A typical ICA method, one of the benchmark methods for ICA, FastICA, is considered in details. Another typical ICA algorithm, InfoMAX, is introduced as well, followed by numerical experiment to evaluate two ICA algorithms.<p>Secondly, FastICA is proposed for blind multiuser symbol estimation as the statistical independence condition of the source signals is always met. The system model of simulation in downlink of DS-CDMA system is discussed and then an ICA based DS-CDMA downlink detector has been implemented with MATLAB. A comparison between the conventional Single User Detection (SUD) receiver and ICA detector has been made and the simulation results are analyzed as well. The results show that ICA detector is capable of blindly solving multiuser symbol estimation problem in downlink of DS-CDMA system.<p>The convergence of ICA algorithm is, then, discussed to obtain more stable simulation results. A joint detector, which combines ICA and SUD and where ICA is considered as an additional element attached to SUD detector, has been implemented. It was demonstrated that the joint detector gives the lowest error probability compared to conventional SUD receiver and pure ICA detector with training sequences.

PN code synchronization is a common and essential task at the receiver in CDMA communication systems. In most literatures, there usually exists assumption of synchronization between the received spreading sequence and a locally generated replica. Few practical and detail synchronization schemes are showed in literatures. Moreover, there are no sufficient knowledge about operation characteristics of loop elements and operation strategies. Hence, in this thesis we propose some schemes about PN code synchronization and then do some analysis to compare performance of those schemes in AWGN channel. Code synchronization at DSSS receivers is performed in two steps. One is code acquisition and the other is code tracking. In acquisition process, we propose so called some ¡§auxiliary decision criterions¡¨ in multiple-dwell detection strategies. Analytical results are validated with computer simulations. By use of auxiliary decision criterions, the acquisition process is outperforms the conventional multiple-dwell and single-dwell detection strategies in AWGN channel. In code tracking process, we use coherent DLL. Loop filter parameters are tuned by observing phase-plane trajectories to acquire stable lock points and convergence property. In addition, we proposed a coherent DLL chip implementation, and accomplish the design flow that includes verification, synthesis, place and route, and simulation.

In the direct sequence-code division multiple access (DS-CDMA) system, which uses direct sequence spread spectrum (DSSS) technique to perform multiple-access, the major limitation of the system capacity is the capability of interference rejection. In this system, multiuser receivers usually divided into two groups, the first group is called the ¡§centralized receiver,¡¨ because it must know the information of total users, including the spreading sequence of each user, channel response, etc. Due to the complexity of computation, this kind of receivers is suitable for the base station. The second group is called the ¡§decentralized receiver,¡¨ because it only needs to know the information of desired user, therefore, it is very suitable for mobile station. The decentralized receiver can be further separate into two kinds: data-aided and non-data-aided receivers. Usually, the non-data-aided receiver is also called the blind receiver; our proposed interference cancellator belongs to this blind one. This thesis mainly discusses the performance of our proposed interference cancellator in different conditions. There is a novel interference detector which can efficiently detect strong interferers in our proposed interference cancellator. When strong interferers exist, the received signal will be passed through the interference-blocking transformer, which exploits the subspace approach to block strong interference. After interference cancelled, conventional de-spreading technique is used to obtain the desired data. In this thesis, besides the complete mathematical analysis of our proposed interference cancellator, computer simulations are also used to observe its performance behavior in different conditions. The simulation results exhibit that this interference cancellator has good performance in different conditions, and due to have the property of low complexity, our proposed interference cancellator is very suitable for the mobile station. Finally, we make a conclusion for this blind interference cancellator, and expect to realize a mature multiuser receiver based on this technique in the future.

In this thesis, an improved genetic algorithm (GA) for multi-user detectors is investigated in synchronous and asynchronous DS-CDMA systems. Although the embedded GA and multistage detector (E-GA/MSD) can also improve the conventional GA performance for multiuser detection, the MSD incurs high computational consumption when the users are growing. To eliminate the MSD operation at each generation, an improved GA with adjustable parent size and enlarged sampling space is developed. The proposed GA with much less complexity can achieve almost the same BER performance as the E-GA/MSD. Simulation results demonstrate that the proposed GA is near to an optimum detector. Additionally, we apply the proposed GA to a spatial diversity reception assisted multiuser detector over uncorrelated flat Rayleigh fading channels in synchronous CDMA systems. As expected, computer simulations also show that the proposed GA can achieve the lower BER performance than conventional GA.

Nürnberg, University, Diss., 2003--Erlangen. / Erscheinungsjahr an der Haupttitelstelle: 2002.

International Telemetering Conference Proceedings / October 21, 2002 / Town & Country Hotel and Conference Center, San Diego, California / This paper presents the development of two distributed terrestrial radiolocation algorithms that use range estimates derived from DS-CDMA waveforms. The first algorithm, which is RLS-based, is derived as the solution of an approximate least-squares positioning problem. This algorithm has the advantage of reduced computational complexity, compared with the EKF-based algorithm that is presented. It is shown via simulations that both positioning algorithms perform well, with the performance of the EKF-based algorithm being superior.

In this thesis, we propose a modified blind adaptive approach to MAI cancellation for synchronous DS-CDMA systems, termed PCMA Approach. It has the same advantage as MOE and CMA of suppressing MAI without the explicit knowledge of interference parameters. We study the effect of arrival and departure of new users to the synchronous blind multiuser receivers, namely MOE and CMA. The perturbation caused by this sudden birth or death of interferers is examined from the viewpoint of the change of the cost function under the noiseless assumption. It is shown that in the vicinity of lock convergence, the changes in the CMA and MOE costs are the same if the perturbation is small enough. But it is also observed that the transient behavior of the CMA detector becomes significantly less robust as the existing number of users approaches the processing gain. To overcome this question of the CMA detector, we employ a projection matrix P that can remove the desired signal from the received signal with other interferers unchanged. Finally its performance has been demonstrated by numerical results in comparison with CMOE, ANMMSE, BAMMSE and Linear MMSE. The proposed receiver can combat MAI, approach the performance of Linear MMSE and have a higher capacity and fast convergence rate for a time-varying user population.

In a multiuser DS-CDMA system with frequency selectivity, each user’s spreading sequence is transmitted through a different channel and the autocorrelation and the cross correlation properties of the received sequences will not be the same as that of the transmitted sequences. The best way of designing spreading sequences for frequency selective channels is to design them at the receiver exploiting the users’ channel characteristics. By doing so, we can show that the designed sequences outperform single user AWGN performance. In existing sequence design algorithms for frequency selective channels, the design is done in the time domain and the connection to frequency domain properties is not established. We approach the design of spreading sequences based on their frequency domain characteristics. Based on the frequency domain characteristics of the spreading sequences with unconstrained amplitudes and phases, we propose a reduced-rank sequence design algorithm that reduces the computational complexity, feedback bandwidth and improves the performance of some existing sequence design algorithms proposed for frequency selective channels. We propose several different approaches to design the spreading sequences with constrained amplitudes and phases for frequency selective channels. First, we use the frequency domain characteristics of the unconstrained spreading sequences to find a set of constrained amplitude sequences for a given set of channels. This is done either by carefully assigning an already existing set of sequences for a given set of users or by mapping unconstrained sequences onto a unit circle. Secondly, we use an information theoretic approach to design the spreading sequences by matching the spectrum of each user’s sequence to the water-filling spectrum of the user’s channel. Finally, the design of inner shaping codes for single-head and multi-head magnetic recoding channels is discussed. The shaping sequences are designed considering them as short spreading codes matched to the recoding channels. The outer channel code is matched to the inner shaping code using the extrinsic information transfer chart analysis. In this dissertation we introduce a new frequency domain approach to design spreading sequences for frequency selective channels. We also extend this proposed technique to design inner shaping codes for partial response channels.