Chip-Level Channel Estimation for the Downlink of a WCDMA System in Very High Mobility Environment

This article proposes a channel estimation method for the downlink channels of a WCDMA system in a high-speed railroad setting. High mobility may cause conventional symbol-level channel estimation to yield severe errors because in conventional methods channel state has to maintain constant within one to several symbol durations. However, in high mobility environment, this assumption may not hold. Errors are particularly more dangerous when using very high spreading factors. In order to counteract the adverse effect of high mobility on channel estimation, we shorten the observation window to that of an N-chip block so that channel conditions or characteristics remain approximately unchanged. We consider channel estimation prior to dispreading the received signal. In other words, channel estimation is done at the chip level rather than the conventional symbol level. The least squares (LS) criterion is employed to acquire channel characteristics for each block of N pilot chips, and the linear interpolation method is used to determine the channel characteristics for each data chip. The LS-based estimator is selected due to its simplicity since it does not need to know channel or noise statistics. An LS-based estimator at the chip level has the further advantage that it is robust against interpath interference (IPI). The uncoded bit error rate (BER) performance of a RAKE receiver using different channel estimation schemes is evaluated and compared through simulations. The proposed scheme is found to be suitable for a high-speed railroad setting.