Spectral efficiency of direct-sequence spread-spectrum multiaccess with random spreading

Information theoretic aspects of code division multiple access (CDMA) random direct-sequence spread-spectrum (DSSS) are investigated. The CDMA-DSSS channel with randomly and independently chosen spreading sequences accurately models the situation where pseudo-noise sequences span many symbol periods. We analyze the spectral efficiency (total capacity per chip) as a function of the number of users, spreading gain and signal-to-noise ratio, and we quantify the loss in efficiency relative to an optimally chosen set of signature sequences and to an optimal multiaccess system without spreading. Particular attention is given to the limiting spectral efficiency as the number of users grows without bound. White Gaussian background noise and equal-power synchronous users are assumed. The analysis comprises the following receivers: a) optimal joint processing, b) single-user matched filtering; c) decorrelation and d) minimum mean square error linear processing.