On the average near-far resistance for MMSE detection of direct sequence CDMA signals with random spreading

The performance of a near–far-resistant, finite-complexity, minimum mean squared error (MMSE) linear detector for demodulating direct sequence (DS) code-division multiple access (CDMA) signals is studied, assuming that the users are assigned random signature sequences. We obtain tight upper and lower bounds on the expected near–far resistance of the MMSE detector, averaged over signature sequences and delays, as a function of the processing gain and the number of users. Since the MMSE detector is optimally near–far-resistant, these bounds apply to any multiuser detector that uses the same observation interval and sampling rate. The lower bound on near–far resistance implies that, even without power control, linear multiuser detection provides near–farresistant performance for a number of users that grows linearly with the processing gain.