COMBINED CODING AND SPREADING IN DIRECT-SEQUENCE CDMA SYSTEMS

This paper presents an analysis of a direct-sequence code-division mul tiple-access communication system which assigns a set of M sequences t o each user. Hence, each user transmits log(2) M bits of information p er sequence. The probability of transmission errors depends on both th e interference due to other users and the error correcting capability of the sets of sequences. To evaluate the performance of the considere d system we assume that the channel is disturbed by additive white Gau ssian noise, Further, optimum coherent demodulation and minimum Euclid ean distance decoding of the sets of sequences are employed. By using the Gaussian approximation for the multiple-access interference, an up per bound on the error probability is obtained. The bound applies to m any sets of sequences with good distance properties in the Euclidean s pace. Further, lower bounds on the cross-correlation between different sets of sequences are derived, which take into account the minium Ham ming distance between different sequences in a set.