Performance analysis of quaternary DS-SSMA systems with complex signature sequences and complex processing in Rayleigh fading

Performance of quaternary direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with complex signature sequences and complex modulator and correlator in flat Rayleigh Fading is investigated. Due to the availability of potentially large sets of complex sequences with good correlation characteristics, the interest of using complex spreading sequences in DS-SSMA has increased dramatically. In this paper, complex processing in modulator and correlator is also employed in order to take advantage of the correlation properties of complex signature sequences. The average bit error rate (BER) for quaternary synchronous systems is obtained first, and then the BER for quaternary asynchronous systems is achieved using Gaussian approximation method. The complex spreading sequences investigated include the recently introduced orthogonal unified complex Hadamard transform (UCHT) sequences, as well as some well-known sequences. The numerical examples illustrate that the systems based on UCHT spreading sequences perform generally better than the conventional ones.