MMSE interference suppression for rapidly faded CDMA systems: A new formulation for differentially modulated signals

While MMSE interference suppression for DS-CDMA systems offers large potential gains relative to the conventional matched filter receiver, standard MMSE adaptation for coherent demodulation is unable to track a typical fading channel. However, with the use of differentially coherent demodulation, the task of tracking the fading gain of the desired user can be avoided altogether. In this paper, we propose a new optimization problem, involving a quadratic cost function and a quadratic constraint, for linear interference suppression in differentially modulated systems. The solution to the optimization problem turns out to be the MMSE correlator, which is known to be near-far resistant. Our formulation therefore provides a systematic framework for obtaining new recursive and block adaptive implementations of the MMSE correlator which are robust to time variations due to fading.