Coding Across Multicodes and Time in CDMA Systems Employing MMSE Multiuser Detector

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 paper, 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 (SIRs) for the multicode channels, coding across multicodes and time can obtain significant performance gain over coding across time, with nearly the same decoding complexity.