A study of direct-sequence spread-spectrum (DS/SS) techniques for high capacity mobile communications systems is proposed. This research aims to develop an understanding of how coding and modulation affect the multiple access capability of these systems. In particular, this research will focus on the development of combined coding and modulation techniques for DS/SS systems. Although coded modulation techniques have yielded dramatic performance improvements when applied to communications over narrowband channels, attempts to apply these techniques to spread-spectrum systems have thus far produced modest results Rather than attempt to develop entire new classes of codes, this investigation will examine existing classes of convolutional codes. When the choice of convolutional code is combined with the choice of spreading sequence, a combined coding and modulation technique results. It is conjectured that the resulting codes will perform as well as any possible coded modulation technique for a DS/SS system. The objectives of this research are to understand the fundamental performance limits of coded modulation techniques for DS/SS systems, to identify promising coded modulation techniques, and to analyze the performance of these techniques using a channel model which incorporates the effects of multiple access interference and frequency selective fading. The research proposed here will have a twofold impact. While understanding the relationship between coding and modulation in a DS/SS system is of long term theoretical interest, the development of practical error correction techniques for DS/SS is of immediate practical value to the mobile communications community.
