Preamble-Based Channel Estimation for Amplify-and-Forward OFDM Relay Networks

In this paper, we address the preamble-based channel estimation problem for orthogonal frequency-division multiplexing (OFDM) systems using multiple amplify-and-forward (AF) relays. We first establish a linear minimum mean-square-error (LMMSE) channel estimator which directly estimates the overall channels from the source to the destination. Then, based on the assumption that the channel correlations between the relays are unknown at the source terminal, a suboptimal training sequence and precoding matrices in closed form are designed to approach the optimal estimation performance at low-complexity.