This dissertation provides a comprehensive analysis of the coding-spreading tradeoff problem in finite-sized synchronous DS-CDMA systems. In contrast to the large system which has a large number of users, the finite-sized system refers to a system with a small number of users. Much work has been performed in the past on the analysis of the spectral efficiency of synchronous DS-CDMA systems and the associated coding-spreading tradeoff problem. However, most of the analysis is based on the large-system assumptions. In this dissertation, we focused on finite-sized systems with the help of numerical methods and Monte-Carlo simulations.Binary-input achievable information rates for finite-sized synchronous DS-CDMA systems with different detection/decoding schemes on AWGN channel are numerically calculated for various coding/spreading apportionments. We use these results to determine the existence and value of an optimal code rate for a number of different multiuser receivers, where optimality is in the sense of minimizing the SNR required for reliable multiuser communication. Our results are consistent with the well-known fact that all coding (no spreading) is optimal for the maximum a posteriori receiver.Simulations of the LDPC-coded synchronous DS-CDMA systems with iterative multiuser detection/decoding and MMSE multiuser detection/single-user decoding are also presented to show that the binary-input capacities can be closely approached with practical schemes. The coding-spreading tradeoff is examined using these LDPC code simulation results, where agreement with the information-theoretic results is demonstrated.We extend our work to the DS-CDMA systems on two idealized Rayleigh flat-fading channels: the chip-level flat-fading (CLFF) and the (code) symbol-level flat-fading (SLFF). These models represent ideal fast fading and slow fading channels, respectively. Both information-theoretic results and LDPC code simulation results are presented to show the effects of channel fading on system performance and the coding-spreading tradeoff. It is shown that fast fading can be beneficial to system performance under the condition of perfect channel state information at receiver, but slow fading is very harmful. Slow fading also increases the importance of coding greatly, compared to the AWGN and fast fading.Finally, we present some comparisons with large-system results on AWGN and CLFF channels, which show both consistencies and discrepancies.

In direct-sequence code division multiple access (DS-CDMA) systems, the heuristic optimization algorithms for multiuser detection include genetic algorithms (GA) and simulated annealing (SA) algorithm. In this thesis, we use particle swarm optimization (PSO) algorithms to solve the optimization problem of multiuser detection (MUD). PSO algorithm has several advantages, such as fast convergence, low computational complexity, and good performance in searching optimum solution. In order to enhance the performance and reduce the number of parameters, we propose two modified PSO algorithms, inertia weighting controlled PSO (W-PSO) and reduced-parameter PSO (R-PSO). From simulation results, the performance of our proposed algorithms can achieve that of optimal solution. Furthermore, our proposed algorithms have faster convergence performance and lower complexity when compared with other conventional algorithms.

Techn. University, Diss., 2007--Darmstadt.

Zugl.: Dresden, Techn. University, Diss., 2006.

Techn. Universiẗat, Diss--Dresden, 2004.

Zugl.: Bremen, Univ., Diss., 2009

Techn. Universiẗat, Diss., 2004--Dresden.

No description available.

One of the most important components of any wireless network is the medium access control protocol. This research deals with wireless ATM (WATM) medium access control (MAC) protocol. Conventional studies concerning WATM have focused mainly on variations of the time-division-multiple-access (TDMA) method for the wireless aspect of WATM networks. However, there are many advantages that the direct-sequence code-division-multiple-access (DS-CDMA) air-interface method has, such as inherent robustness against multipath fading, better resilience against security infringement attempts, and greater overall capacity compared to the TDMA method as proven in the cellular telephone industry. The main reason behind the relatively broader support for the TDMA method is that the source bit rate is generally higher compared to the DS-CDMA method since the maximum data rate per mobile unit is limited by the processing gain of a traditional DS-CDMA method. In this research, the problem of limited data rate often associated with a DS-CDMA air-interface is alleviated by employing the recently conceived multi-coded DS-CDMA as the primary air-interface, which is known to achieve maximum data rate per mobile unit comparable to applications employing TDMA. The focus of this research is on overcoming periods of significant deterioration of the wireless channel by adaptively employing bit combining. A MAC protocol called Adaptive Parallel Multiple Sub-stream CDMA (APMS-CDMA) is proposed to alternate between normal and ¡°rake-in¡± mode to deal with the often hostile environment of a WATM network. Although the context in which this research effort was conducted was a wireless ATM network environment, the protocol and techniques developed here can be applied to other infrastructure wireless systems using multi-code CDMA as their air-interface. Further, independent of the air-interface technique employed, other wireless systems can benefit from the channel estimation and the traffic management techniques used in this research effort. / Ph. D.

There has been great interest in antenna array processing (diversity, beamforming, null steering, and spatial multiplexing) to enhance the received signal quality and the capacity of wireless communications systems. However, in order to properly exploit the characteristics of different array processing techniques, understanding trade-offs among different techniques and parametric investigation, which offers an insight as to what parameters determine system performance under different situations is necessary. In this study, we present analytical framework which can facilitate the performance analysis of systems with antenna array. Five original contributions to the performance analysis of antenna array processing are presented in this study. First, we present theoretical outage probability of a system equipped with an array which suppresses a few dominant interering signals in TDMA cellular networks when the fading statistics of interfering signals are independent but non-identically distributed. Most of the related previous works assumed either independent and identically distributed fading statistics among cochannel interferences (CCI) or Rayleigh fading when CCI signals are subject to i.n.d. fading statistics. Secondly, the performance of multi-branch predetection equal gain combiner for different modulation techniques in equally correlated Nakagami-m fading is presented through analytical analysis. Specifically, the characteristic function (CHF) and the moment generating function (MGF) of EGC output with correlated inputs are derived and used to evaluate the average symbol error probability (ASEP) and the outage probability performance, respectively. Thirdly, we derived analytical expression which can be used to analyze the performance of different types of diversity techniques in equally correlated Nakagami-m or Rice fading channels. Fourthly, asymptotic analysis on different types of diversity combiners in generalized fading channels is presented in a unifying way. Finally, we investigate and present the impact of transmit diversity at handsets on the reverse link DS/CDMA systems in terms of capacity and coverage over generalized fading channels through analytical approaches. Then, we validate the analytical results with simulation results and investigate practical issues which are hard to capture through analytical analysis using system level simulator we developed. Although we have mainly focused on applying the analytical framework we have derived in this work to the performance analysis of physical layer algorithms such as spatial diversity and adaptive null steering, the framework can be extended to assist the analysis and design of wireless communication systems such as, to name a few, distributed multiple input multiple output (MIMO) system in cooperative wireless networks, multipath routing protocol analysis in wireless fading channels, and antenna selection problems in MIMO system. / Ph. D.