Code phase and delay settings that minimize CDMA interference

When code-division multiple access (CDMA) signals are linearly multiplexed (phase-coherently summed) at a transmit terminal, a receiver demodulating information from one component signal experiences multiple access interference (MAI) from all others to the extent that the waveforms are not strictly orthogonal. Under a model for which: (1) the users are homogeneous, i.e., identical in those parameters that govern the amount of MAI they produce at a receiver, and (2) the various codes are random binary sequences, one may develop an expression for the amount of induced MAI as a function of the spread-spectrum chip delays and phases. It is not immediately obvious from this expression, however, what selection of delays and phases produces the most benign distribution of MAI among users. This paper presents an analytic characterization of MAI for linear multiplexing of a set of CDMA codes, culminating in a deterministic design of delays and phases that achieves MAI noise levels substantially smaller than those predicted by worst-case or random assignment results. It is believed that the design achieves the minimum MAI level, although it is not proved.