Echo-MIMO: a Two-Way Channel Training Method for Matched Cooperative Beamforming

A two-way multi-input multi-output (MIMO) channel sounding method for enabling both a transmitter and a receiver to have complete knowledge of the channel between them is presented. The key idea is that a receiver reflects, or "echoes", the received signal back to the transmitter, from which it estimates its own outgoing channel fading coefficients for all transmit/receive pairs of antenna elements. The weighted non-linear least squares problem to jointly estimate the outgoing and incoming channels together is solved for the case of frequency-flat fading. The channel estimates are used to obtain a matched pair of maximum ratio transmission/maximum ratio combining (MRT/MRC) weight vectors for cooperative beamforming. Simulation results are given that show the echo MIMO with MRT/MRC approach can outperform a quantized MRT/MRC codebook method. For fully correlated channels, the echo MIMO MRT/MRC transmission scheme approaches the ideal beamformer which uses perfect channel estimates