Rake reception with improved maximal ratio combining weights for the WCDMA forward link

Abstract

In the conventional RAKE reception in a spread spectrum system, the maximal ratio combining (MRC) weight for each of the RAKE fingers is the complex conjugate of the estimated instantaneous fading gain. This design is known to be able to maximise the instantaneous signal-to-interference ratio (SIR) in the combined signal, provided that the interference in each finger has the same power as and is uncorrelated from the interference in all the other fingers. The equal power is easily fulfilled in prevailing Direct Sequence Code Division Multiple Access (DS-CDMA) systems, but not, as we show in this paper, in the forward link of the upcoming Wideband Code Division Multiple Access (WCDMA) systems. The amount of multipath interference (MPI) in each of the fingers of a forward link RAKE receiver in WCDMA is actually unequal to that in the other fingers. In this paper, we derive an algorithm to determine the optimum set of combining weights for the RAKE receiver in WCDMA. Using these new optimum combining weights, we show that notable improvement in the BER performance can be achieved, compared with the conventional weights, in the multipath fading channel, especially when the fading channel is Rician, as for the microcellular and indoor picocellular systems.