Calculation of error probability of CDMA communication systems using saddlepoint method

An efficient method for evaluating the average error probability of direct sequence spread-spectrum code-division multiple-access (CDMA) communication systems is presented. Various phase-shift keying systems are analyzed. Results of Gaussian approximation, saddlepoint approximation, and numerical contour integration by the saddlepoint method are compared. The variance and the error probability can be computed from the moment generating function of the receiver output. General and simple formulas for evaluating the multiple access interference variance are presented for systems using deterministic or random signature sequences. Both quadriphase-shift keying (QPSK) and offset quadriphase-shift keying (OQPSK) systems yield the same performance when the signal-to-noise ratio criterion is adopted. However, accurate numerical computations show that they do not have the same one. The results show that MSK yields better performance than QPSK and OQPSK.