Optimal Pilot Design for LMMSE Channel Estimation in MIMO-OFDM Systems

In this paper, we investigate the optimal pilot for multiple-input multiple-output and orthogonal frequency-division multiplexing (MIMO-OFDM) systems. The optimal pilot is obtained by minimizing the mean-square-error (MSE) of the linear minimum MSE (LMMSE) channel estimation, which is related to the channel correlation matrices in space, time, and frequency domains. We obtain that the optimal pilot is the statistical precoded sequence in the transmit eigenmode with proper power allocation. In general, the power allocation has no closed-form solution, so we present some suboptimal schemes to approximate the optimal one. Simulations show that the proposed statistical precoded pilot outperforms the conventional one, and the suboptimal schemes can well approximate the optimal one.