On the Design of Spreading Sequences for CDMA Systems with Nonlinear OQPSK-type Modulations

The main focus of this paper is to present an analytical method for calculate the correlation between OQPSK (Offset Quadrature Phase-Shift Keying) signals modulated by several spreading sequence families. Our analytical model includes nonlinearities, namely by treating the transmitter amplifier as highly nonlinear. Grossly nonlinear power amplification is highly desirable as it allows power-efficient low-cost receivers, with applications in satellite, underwater or deep space communications. OQPSK-type modulations include as special cases CPM (Continuous Phase Modulation) schemes and can be designed to have low envelope fluctuations or even a constant envelope, essential in many communications systems (e.g., satellite). This model and the derived analytical results for the correlation of sequences in such a nonlinear environment are then applied to a DS-CDMA (Direct Sequence - Code Division Multiple Access) system. We present simulation results for three types of spreading sequences: ML (Maximum-Length), Kasami and TCH (Tomlinson, Cercas, Hughes) sequences. The simulation results are presented to compare the performance of those types of sequences in the linear and nonlinear DS-CDMA systems.