Performance of π/4-shift DQPSK direct-sequence spread-spectrum multiple-access with diversity combining over frequency-selective Rician fading channels

The bit-error-rate (BER) performance of a π/4-differential quadrature phase shift keying (DQPSK) direct sequence spread spectrum multiple access (DS/SSMA) modem scheme in a digital cellular mobile communication system is analyzed. The system is modeled as a frequency-selective Rician fading channel corrupted by additive white Gaussian noise (AWGN). The correlations of received signals, assumed to be normally distributed, are first obtained, then adopted to average bit error probability. In order to overcome multipath fading effects, the postdetection equal-gain diversity technique is selected. The received baseband signal is composed of specular signals, faded signals by multipath, interferences by other users, and AWGN. The bit-error-rate is finally calculated by numerical integration with the Gold sequence as spread sequence