On the Cramer-Rao Lower Bound for Spatial Correlation Matrices of Doubly   Selective Fading Channels for MIMO OFDM Systems
  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. 
Xiaochuan Zhao (1), Tao Peng (1), Ming Yang (1) and Wenbo Wang (1)   ((1) Beijing University of Posts and Telecommunications...