Code division multiple access (CDMA) is one of the most promising candidates for the next generation wireless personal and mobile communication systems. Unfortunately, due to the multipath nature of the wireless channels, the effective signature waveforms among the active mobile users in the receiver end are often non-orthogonal. Therefore, CDMA systems suffer from cochannel interference, which results in the near-far problem and constitutes a major impediment to the overall system performance.
This dissertation addresses the issue of near-far resistance for both optimum and suboptimum detectors under multipath channel scenario. First, the theoretical optimum near-far resistance under multipath is derived. It serves the least upper bound on the near-far resistance of any detector, as well as a benchmark for relative performance of any suboptimum detector. Then the near-far resistance of three typical suboptimum detectors (i.e., subspace method, linear prediction method and constrained optimization method) as well as the general minimum mean square error (MMSE) detector are derived. Factors which may affect the practical near-far resistance are also investigated. Based on those results, two methods (i.e., zero bit insertion method and linear smoothing method) are proposed to enhance the near-far resistance of suboptimum detectors. Computational complexity of the proposed methods is derived and compared. Furthermore, the near-far resistance of CDMA with spatial diversity is studied. Both CDMA with receiver antenna array and space-time coded CDMA (ST-coded CDMA) are investigated. It is shown that near-far resistance is enhanced when spatial diversity is introduced. And the receiver end diversity contributes more to the enhancement of near-far resistance than transmitter end diversity. The rank of the channel matrix is also investigated for CDMA with receiver spatial diversity. It is shown that second order statistics (SOS) blind equalization methods are much safer to use in CDMA with spatial diversity than systems without spatial diversity. Furthermore, the relationship between the average near-far resistance and the rank of the channel matrix is revealed. Finally, the near-far resistance of multicarrier CDMA is investigated. It is shown that the near-far resistance of MC-CDMA with cyclic prefix is better than those of MC-CDMA without cyclic prefix and of the direct sequence CDMA (DS-CDMA), at the expense of lower bandwidth efficiency. Simulations are presented to verify theoretical findings.
|School:||University of Cincinnati|
|Department:||Engineering : Electrical Engineering|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Access, Code, Communication, Division, Far, Near-Far, Resistance|
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