Department of Electrical and Computer Engineering
Ph.D. Thesis Abstract
Quality of Service for Multi-rate DS/CDMA Systems with Multi-user Detection
This thesis addresses the use of decorrelators for a dual-rate synchronous DS/CDMA system that serves both low bit rate and high bit rate users. Here it is assumed that in an interval of duration T, a low rate user transmits one bit while a high rate user transmits M bits. Applying a standard decorrelator to the interval of duration T yields an M bit processing delay for high rate users and a computational complexity that grows with M. This situation prompts us to propose a decorrelator that generates bit decisions for each high rate user in every subinterval of duration T=M. To decode a low rate user, a soft decoding rule applies maximal ratio combining on M separate decorrelated outputs of each low rate user. It is proven that the bit error rate of the standard decorrelator is less than or equal to that of the soft decoding decorrelator. In addition, we develop a dual-rate decision feedback decorrelator which retains the desirable properties of the soft decoding decorrelator and outperforms the standard decorrelator.
For an asynchronous multi-rate DS/CDMA system, a truncated window decorrelator is proposed. We prove that the performance of any truncated window decorrelator improves monotonically with the observation window length. On the other hand, a longer observation window results in greater computational complexity. Therefore, we decode a user by extending the observation window over sufficient number of its bits. We develop simple upper and lower bounds for the asymptotic eciency of both the truncated window and infinite window decorrelators to characterize practical window sizes. Using a decorrelator in a multi-rate DS/CDMA system, it may be necessary for different users to combat the noise enhancement and the propagation losses to varying degrees depending on individual requirements. In this context, we propose an iterative power control algorithm which is suitable for a class of BER objectives. If the uplink channel gain of the desired user is known, then it is simple to choose the transmitter power needed to meet its target BER objective. In practice, however, the uplink channel gain is often difficult to measure. To avoid this measurement, we employ stochastic approximation methods to develop the power control algorithm.
Ph.D. Dissertation Director: Professor Roy D. Yates
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