Pilot-based estimation of time-varying multipath channels

1 Reliable coherent communication over mobile wireless channels requires accurate estimation of time-varying multipath channel parameters. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) multipath channels: i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission, and ii) impact of channel estimation errors on the coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of lters is derived. A key channel characteristic controlling system performance is the normalized coherence time that approximately corresponds to the number of symbols over which the channel parameters remain strongly correlated. It is shown that the estimator performance is characterized by an eeective signal-to-noise ratio (SNR) | the product of the pilot SNR and the normalized coherence time. Two suboptimal linear estimators are also proposed that are easy to implement and deliver near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits Doppler diversity in addition to mul-tipath diversity. It is shown that the overall receiver performance is controlled by two competing eeects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite eeects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can signiicantly mitigate the error probability oors that plague conventional CDMA receivers under fast fading due to errors in channel estimation.