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Recent accomplishments in link-level and radio technologies have significantly improved the performance of wireless links. Wireless mobile ad hoc networks, however, typically only take limited advantage of these enhancements. In this research, the medium access control protocol and ad hoc routing protocol are extended to take advantage of radios offering multi-user interference cancellation and direct-sequence spread-spectrum functionality, by encouraging multiple simultaneous connections and adaptively changing communication parameters on a per-packet basis. Through its environment characterization techniques, the adaptive direct sequence spread spectrum MAC protocol for non-broadcast multiple access networks (ADIM-NB) improves several aspects of the wireless mobile ad hoc network performance, including throughput, delay, stability, and power consumption, through its use of spread-spectrum multiple access and four different adaptive algorithms. The four adaptive algorithms change processing gain, forward error correction coding rate, transmit power, and number of simultaneous connections. In addition, the ad hoc routing protocol is extended with the clustering algorithm for mobile ad hoc network (CAMEN). With ADIM-NB in mind, CAMEN discourages the use of broadcast messages, supplements ADIM-NB's functionality at the network level, and improves the network scalability by aggregating nodes into clusters. Both protocols are intended to lead to more powerful and flexible communication capabilities for wireless nodes. Simulation models have been developed and simulated to verify the performance improvements of both protocols at the network-level as well as provide a means to perform trade-off analysis. Results indicate that the network capacity is increased between 50% in a moderately loaded network to 100% in a heavily loaded network over a non-adaptive MAC protocol. The delay also improve significantly in most scenarios of interest. / Ph. D.

The focus of this dissertation is the study of advanced processing techniques for multiuser interference cancellation in direct sequence code division multiple access communications. Emphasis is placed on the development of efficient techniques that are practical to implement. The work begins with a study of several sub-optimal multiuser detection techniques under a variety of conditions. Multistage parallel interference cancellation is identified as a practical and robust approach for mitigating multiple access interference. In order to reduce the effect of biased decision statistics inherent to parallel cancellation, a low-complexity modification to parallel interference cancellation that significantly improves performance is derived. Based on this approach, two real-time DSP implementations are devised, one fully coherent and one non-coherent. Multi-symbol differential detection is then explored as an alternative for improving the performance of the non-coherent approach. Additionally, dual-antenna diversity techniques are also investigated as a means for improving performance in multipath environments. / Ph. D.

Parallel interference cancellation for DS-CDMA has been shown to suffer from biased amplitude estimates if a matched-filter estimator is used. The bias magnitude is proportional to the number of interfering users. For heavy system loads, the bias has been shown to adversely effect the accuracy of the interference cancellation process, thereby impairing BER after cancellation. Empirical simulation work has demonstrated that weighting down interference estimates can improve BER performance. This thesis substantiates these BER improvements by modelling and analyzing a soft interference cancellation technique which mitigates the effects of the bias by minimizing BER after cancellation in a bit-synchronous parallel interference cancellation CDMA receiver. We analyze system decision metrics with down-scaled interference estimates and determine both the mean and variance of the biased decision statistics. From these two metric moments, system BER is evaluated, and the optimal interference scaling function which minimizes BER is derived. We demonstrate BER performance enhancements by simulating this soft interference cancellation technique in systems under perfect power control and in the near-far situation. We further discuss the applicability of the results to asynchronous systems. / Master of Science

In Code Division Multiple Access (CDMA) systems forward error correction (FEC) is an important factor in improving system performance. Because of the multiple access interference observed in asynchronous CDMA, FEC is required to improve system capacity. Several alternative coding techniques are available, including "trellis codes" specifically designed for CDMA, very low rate convolutional codes and conventional convolutional codes. An analytic approach is developed to compare the performance of different coding techniques for CDMA on the same basis by extending several improved analysis techniques which were developed for uncoded CDMA. Analytic results are presented for additive white Gaussian noise (AWGN) channels and frequency non-selective Rician Fading (FNRF) channels. The performance of coded CDMA systems in frequency selective Rayleigh fading channels is also analyzed using simulation techniques. Both analytic and simulation results show that by using trellis codes or very low rate convolutional codes significant performance improvement is achieved over conventional convolutional codes. Trellis codes outperform very low rate convolutional codes in AWGN and FNRF channels while both codes show similar performance in frequency selective Rayleigh fading channels. / Master of Science

The developing field of smart structures and skins provides an application to which fiber optic sensors bring unique capabilities and benefits. The primary cost in a network of fiber sensors is in the sources, receivers and associated hardware and can be prohibitive for a large number of sensors. Multiplexing of sensors based on spread spectrum techniques offers an efficient and cost effective solution to this limitation. The system hardware developed in this research work is capable of the real time monitoring of a four sensor network. Experimental results with embedded and attached Extrinsic Fabry Perot Interferometers (EFPI) are presented. The system can be used to multiplex any type of sensor which translates the measurand into intensity variations of the light. A measure of the system efficiency is obtained using crosstalk measurements. A suppression of 40 dB has been obtained between the desired sensor signal and the interference. The effect on system performance has been observed by varying system parameters such as code length and separation between codes. Highly sensitive embedded interferometric sensors have been used in a multimeasurand environment to measure temperature and strain. A solution to the inherent 'near-far' problem in an optical COMA system has been proposed and results for the same are presented. A novel WDM/CDM hybrid (Wavelength Division Multiplexing/Code Dhtision Multiplexing) scheme has been proposed to increase the light intensity at the detector thereby increasing the number of sensors in the system. Methods to optimize and upgrade the system are discussed. / Master of Science

Lately, spread spectrum systems are being increasingly used for commercial wireless communications because of their ability to reject various types of interference. This ability allows them to be used in multiple access systems. Direct sequence and frequency hopping systems have been the primary spread spectrum techniques used in practice. One technique which has not received much attention until recently is time hopping. In time hopping, a symbol is transmitted at a random position within the symbol period using a pulse width which is much smaller than the symbol period. Ultra-wideband (UWB) technology is a radar technology which shows promise for an relatively simple implementation of a time hopping system. This thesis looks at the error probability performance of a UWB time hopping multiple access system. Previous work has led to an estimate of the performance using a Gaussian approximation similar to that used for direct sequence systems. Through the use of a fast simulation technique, it will be shown that in certain situations, the Gaussian approximation fails to accurately predict the performance. A numerical analysis which uses characteristic functions is developed and shown to correctly predict the system’s performance under a wide range of situations. This numerical analysis also contributes to the understanding of the system. / Master of Science

京都大学 / 新制・課程博士 / 博士(農学) / 甲第24246号 / 農博第2525号 / 新制||農||1094(附属図書館) / 学位論文||R4||N5417(農学部図書室) / 京都大学大学院農学研究科地域環境科学専攻 / (主査)教授 近藤 直, 教授 飯田 訓久, 准教授 小川 雄一 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

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The capacity of a direct-sequence spread-spectrum code division multiple access (DSSS-CDMA) system is limited by multiple access interference (MAJ) and the near-far problem. There are two approaches to mitigating these problems: multiuser detection and single-user detection techniques. Multiuser detection techniques cancel the interference and enhance system capacity, but have large computational requirements and require the knowledge of MAI parameters. Single-user detection techniques require only the knowledge of the desired user’s spreading code and timing, and have a complexity comparable to the conventional receiver. This thesis reviews a wide range of DSSS-CDMaA single-user detectors found in the literature. The receivers are explained with a common approach using an adaptive antenna array perspective and noting that single-user detectors exploit spectral redundancy, while adaptive arrays exploit spatial redundancy. Commonly used trained adaptation algorithms for single-user detection are first presented, and are followed by newly proposed blind adaptation algorithms. These new blind algorithms are Griffiths’ algorithm, and the linearly constrained constant modulus algorithm (LCCMA ). Through simulation, a blindly-adapted single-user detector is shown to greatly outperform the conventional receiver in terms of bit-error-rate (BER) performance, and to perform almost as well as in the case of trained adaptation. The receivers are shown to be near-far resistant, and are computationally attractive for a mobile receiver. Both receivers have good convergence properties and don’t suffer from catastrophic failure. / Master of Science