Combining techniques for DS-CDMA RAKE receiver

Tapped delay line RAKE receivers are commonly used to achieve path diversity gain in DS-CDMA systems: Here, a bank of correlators is used to separate the multipath components and a combiner coherently recombines the multipath energies. In this paper, the optimal combining techniques for DS-CDMA RAKE receivers (under different criteria) are introduced. Specifically, based on the maximum likelihood criteria, optimal combining techniques are derived and discussed for the following scenarios: (1) channel-fade-information is not available at the receiver; (2) a statistical characterization of the channel fades is available at the receiver, but the actual, instantaneous fade information is not; and (3) the instantaneous channel fade information is available at the receiver. This paper shows that maximum ratio combining (MRC), typically considered the optimal combining scheme when fade parameters are perfectly tracked, is sub-optimal. We present the optimal alternative to traditional MRC.