Determining code symbol LLRs on diversity channels with applications to cdma2000

The reverse link of cdma2000 uses a code-multiplexed pilot signal to allow for near coherent demodulation at the base station receiver. Typical RAKE receivers approximate maximal ratio combining by using a "pilot filter" to derive a combiner weight estimate from the noisy pilot signal. It is possible to improve on this design by exploiting the channel information that is also embedded in the encoded data-bearing signal. We show how the pilot and data-bearing signals may be used to determine, implicitly, "combiner weights", and explicitly, code symbol log likelihood ratios (LLRs). Simulation results on the cdma2000 reverse link demonstrate SNR gains on the order of 0.5 dB with the default pilot-to-traffic ratios. Even larger gains may be achieved by optimizing the pilot-to-traffic ratio appropriately.