  The analytic expression of CRLB and the maximum likelihood estimator for
spatial correlation matrices in time-varying multipath fading channels for MIMO
OFDM systems are reported in this paper. The analytical and numerical results
reveal that the amount of samples and the order of frequency selectivity have
dominant impact on the CRLB. Moreover, the number of pilot tones, SNR as well
as the normalized maximum Doppler spread together influence the effective order
of frequency selectivity.
