Convolutional and turbo coded multicarrier direct sequence CDMA, and applications of turbo codes to hybrid ARQ communication systems.

摘要

This dissertation presents a multicarrier asynchronous Direct Sequence (DS) Code Division Multiple Access (CDMA) system wherein the output of a convolutional encoder modulates multiple bandlimited DS-CDMA waveforms which are transmitted in parallel at different carrier frequencies. The receiver detects and combines signals for the desired user and feeds a soft decision Viterbi decoder. The performance of this system is compared to that of a conventional single carrier DS-CDMA system with a RAKE receiver, assuming a slowly varying, frequency selective, Rayleigh fading channel and assuming the presence of additive white Gaussian noise (AWGN), multiple access interference, and narrowband interference. Results will demonstrate similar performance at roughly equal receiver complexity in the absence of narrowband interference, and superior performance of the multicarrier system in the presence of narrowband interference. These results are further extended and evaluated using turbo codes. In this case, the convolutional encoder is replaced by a turbo encoder, and the soft decision Viterbi decoder is replaced by an iterative turbo decoder. The performance and complexity of these implementation alternatives are then discussed.; The second portion of this dissertation introduces a hybrid forward error correction (FEC) and automatic repeat-request (ARQ) system that employs Rate Compatible Punctured Turbo (RCPT) codes to achieve enhanced throughput performance over a nonstationary Gaussian channel. The proposed RCPT-ARQ system combines the performance of turbo codes with the frugal use of incremental redundancy inherent in the rate compatible punctured convolutional (RCPC) codes of Hagenauer. Moreover, we introduce the notion of puncturing the systematic code symbols of a turbo code to maximize throughput at signal-to-noise ratios (SNR) of interest. The resulting system provides both an efficient family of achievable code rates at middle to high SNR, and powerful low rate error correction capability at low SNR. These results are then extended to consider performance in a multipath fading environment.