Space-Frequency Precoding with Space-Tap Correlation Information at the Transmitter

In closed loop methods for obtaining exact channel state information at the transmitter (CSI-Tx), the overhead associated with the feedback can be excessive for fast mobiles. Channel-statistics-based CSI-Tx requires a much smaller overhead, and is, therefore, attractive for use with fast mobiles. We study ways to exploit correlation-based CSI-Tx in a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. We focus on a channel environment in which spatial and tap correlation are present. We propose a channel model for the case such that spatial and tap correlation can be separated, and show that this case channel correlation decreases the ergodic capacity of a MIMO-OFDM system when no CSI-Tx is available. However, this decrease can be mitigated when correlation based CSI-Tx is exploited. We introduce an optimal precoding approach to maximize capacity with spatial- and tap-correlation-based CSI-Tx. We also propose a statistical waterfilling scheme, which leads to almost optimal capacity performance without requiring computationally intensive numerical optimization. Based on these approaches, the impact of spatial and tap correlation is investigated.