In this paper, the Cramér-Rao lower bound (CRLB) of the sample frequency correlation matrices (SFCM) is derived based on a rigorous model of the doubly selective fading channel for orthogonal frequency division multiplexing (OFDM) systems with pilot-symbol-aided modulation. By assuming a fixed pilot sequence and independent samples, SFCM is complex Wishart distributed. Then, the maximum likelihood estimator (MLE) and the exact expression of CRLB are obtained. From CRLB, the lower bounds of total mean squared error (TMSE) and average mean squared error (AvgMSE) independent of the pilot sequence are deduced, which reveal that the amount of samples is the dominant factor affecting AvgMSE while the signal-to-noise ratio and the maximum Doppler spread have negligible effect. Numerical simulations demonstrate the analytic results.
An optimal multiuser detection algorithm with a computational complexity of O(K log K) is proposed for the class of linear multiple-access systems which have constant cross-correlation values. Here the optimal multiuser detection is implemented by searching for a monotone sequence with maximum likelihood, under the ranking of sufficient statistics. The proposed algorithm is intuitive and concise. It is carried out in just two steps, and at each step only one kind of operation is performed. Also, the proposed algorithm can be extended to more complex systems having more than a single cross-correlation value.
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