Uplink performance evaluation of multicode DS/CDMA systems in the presence of nonlinear distortions

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 can 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.