Coding across multicodes and time in CDMA systems and turbo equalizations

Jeongsoon Park, Purdue University

Abstract

This thesis considers three topics of interest in wireless communications: channel coding in multicode CDMA systems, sparse channel turbo equalization and reduced-delay turbo equalization. First, optimization of channel coding in multicode CDMA systems is considered. When combining a multicode CDMA system with convolutional coding, two methods have been considered in the literature. In one method, coding is across time in each multicode channel while in the other the coding is across both multicodes and time. In this thesis, a performance/complexity analysis of decoding metrics and trellis structures for the two schemes is carried out. It is shown that the latter scheme can exploit the multicode diversity inherent in convolutionally coded direct sequence code division multiple access (DS-CDMA) systems which employ minimum mean squared error (MMSE) multiuser detectors. In particular, when the MMSE detector provides sufficiently different signal-to-interference ratios (SIR'S) for the multicode channels, coding across multicodes and time can obtain significant performance gain over coding across time, with nearly the same decoding complexity. Second, turbo equalization for sparse channels is investigated. The turbo equalization scheme provides nearly optimal performance but when it is employed in sparse multipath channel environments, the equalizer complexity can prohibit its use. In this thesis, we develop a parallel trellis n-tap soft-input soft-output maximum a posteriori (MAP) equalizer which can consider n dominant taps of sparse channels with reasonable complexity. It is shown that turbo equalization employing the n-tap MAP equalizer can nearly achieve the asymptotic performance of the optimal maximum likelihood (ML) detector for the combined channel encoder and intersymbol interference (ISI) channel assuming perfect ISI cancellation. We also propose a tap allocation scheme for a time-varying minimum mean squared error (MMSE) equalizer which provides a good performance/complexity trade-off. Third, turbo equalization for delay constrained channels is examined. In turbo equalization the delay due to interleaver can limit its use in delay sensitive applications. We propose a new scheme which can partially exploit the turbo principle when the interleaver is inadequate to yield the conventional independence assumptions. Here continuous decoding is performed by a staged equalization and decoding with greatly reduced decoding delay. It is shown that the proposed scheme can provide asymptotically optimal performance for some channels which can be characterized by a simple criterion.

Degree

Ph.D.

Advisors

Gelfand, Purdue University.

Subject Area

Electrical engineering

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