On the Cramer-Rao Lower Bound for Frequency Correlation Matrices of   Doubly Selective Fading Channels for OFDM Systems
  The analytic expression of CRLB and the maximum likelihood estimator for the sample frequency correlation matrices in doubly selective fading channels for OFDM systems are reported in this paper. According to the analytical and numerical results, the amount of samples affects the average mean square error dominantly while the SNR and the Doppler spread do negligibly. 
Xiaochuan Zhao (1), Ming Yang (1), Tao Peng (1) and Wenbo Wang (1)   ((1) Beijing University of Posts and Telecommunications...