Optimal placement of training for time varying ISI channels

We consider the problem of optimal placement of training for time varying channels with intersymbol interference (ISI). We assume that channel has a finite impulse response of length (L+1) and that each tap of the channel fades independently and identically. The time variation of the fading is modeled by a Gauss-Markov process. We constrain the training symbols to be placed in clusters of length (2L+1) and each cluster contains only one non-zero training symbol. We show that at high SNR, the cumulative channel MSE is minimized by placing the training clusters periodically in the packet. A special case (L=0) of this result is the optimal placement of pilot symbols for Rayleigh fading channels with Gauss-Markov variation.