The Performance of Quaternary Amplitude Modulation with Quaternary Spreading in the Presence of Interfering Signals

Multiple-access digital communications is considered with direct-sequence spread-spectrum (DS-SS) quadrature amplitude modulation and quaternary spreading in each transmission. Each transmitted signal undergoes attenuation and a delay at a receiver in the additive white Gaussian noise channel. The receiver uses coherent demodulation with a correlation detector synchronized to the one of the K received signals so that the K - 1 other transmissions act as interference. The average probability of error at the output of the detector is determined using Monte Carlo simulation and compared with an approximation in which the interference component in each detection statistic is approximated by a Gaussian random variable. Closed-form expressions are derived for the rst and second moments of the interference under several circumstances. The moments are used with the \u27Gaussian approximation\u27, and the accuracy of the approximation is investigated for each of the circumstances