Optimal Pilot Placement for Semi-Blind Channel Tracking of Packetized Transmission over Time-Varying Channels

SUMMARY The problem of optimal placement of pilot symbols is considered for single carrier packet-based transmission over time varying channels. Both flat and frequency-selective fading channels are considered, and the time variation of the channel is modeled by Gauss-Markov process. The semi-blind linear minimum mean-square error (LMMSE) channel estimation is used. Two different performance criteria, namely the maximum mean square error (MSE) of the channel tap state over a packet and the cumulative channel MSE over a packet, are used to compare different placement schemes. The pilot symbols are assumed to be placed in clusters of length (2L +1 ) whereL is the channel order, and only one non-zero training symbols is placed at the center of each cluster. It is shown that, at high SNR, either performance metric is minimized by distributing the pilot clusters throughout the packet periodically. It is shown that at low SNR, the placement is in fact not optimal. Finally, the performance under the periodic placement is compared with that obtained with superimposed pilots.