This paper investigates the behavior of multi-code DS/CDMA systems in the presence of nonlinear distortions caused by high power amplifiers. A technique that uses a small number of parallel transmitted sequences is proposed for combating the nonlinearity. This technique can be modified to have a constant envelope for achieving further performance improvement. It is seen from performance comparisons that the proposed scheme is very effective for combating the nonlinearity.

This paper investigates the behavior of multicode direct sequence code division multiple access (DS/CDMA) systems in the presence of nonlinear distortions caused by high power amplifiers. The performance of the multicode system can be degraded due to a large variation in the envelope of the multicode signal. A technique that uses a small number of parallel transmitted sequences is proposed for combating the nonlinearity. This technique ran be modified to have a constant envelope for achieving further performance improvement. Comprehensive comparisons are made for different systems, based on both theoretical analysis and computer simulation. Our study shows that the technique employing a few parallel transmitted sequences, in conjunction with quasi-ML (maximum likelihood) detection, and the use of the modified scheme, are very effective for combating the nonlinearity.

A general model for M-ary direct sequence-spread spectrum (DS/SS) communications is presented to provide multiple services in direct sequence-spread spectrum multiple access (DS/SSMA). The main idea is to allow each user to use parallel orthogonal spreading sequences, depending upon its needs. More specifically, each service level is associated with certain combinations of the sequences. We use a code, which we refer to as a "mapping code", to form a multidimensional signal constellation. Thus, the data rate can be changed by changing the mapping code. Further, techniques of sequence sharing (reuse) and sequence arrangement (allocation), suitable for packet switching, are proposed; this allows support of multimedia services, while resulting in a reduction of sequence consumption. We evaluate error rates in the presence of multiple access interference (MAI) and additive white Gaussian noise (AWGN). Numerical results are given to show the effects of the system configuration, the data rates, and other system performance measures.