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Channel estimation errors versus Doppler diversity in fast fading channels

This paper studies the impact of channel estimation errors and Doppler diversity on the performance of coherent CDMA receivers under fast fading. A key channel characteristic controlling performance is the coherence time over which the channel remains strongly correlated. We analyze a time-frequency RAKE receiver structure that exploits joint multipath-Doppler diversity. Modeling of channel estimation errors is based on the performance of pilot-based estimators tailored to the receiver structure. It is shown that the overall performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but inherently higher (Doppler) diversity, with opposite effects for longer coherence times. Our results demonstrate that Doppler diversity gains dominate under certain conditions and can significantly mitigate the error probability floors that plague the performance of conventional CDMA receivers under fast fading.