A Low Feedback Scheme for WMAN MIMO Beamforming

Two techniques, i.e. differential and complementary feedbacks, are derived to reduce the feedback overhead for MIMO transmit beamforming. The unitary beamforming matrix is the cumulative product of the fed back unitary matrixes for differential feedback. The fed back matrix depicts the incremental change of the beamforming matrix and it is quantized by small vector codebooks. The overhead can be further reduced by complementary feedback for channel with N transmit and M receive antennas, where M is N or N-1. One unit vector is fed back instead of one unitary matrix. The vector corresponds to the complementary subspace of the beamforming subspace for M=N-1 or corresponds to the strongest or weakest singular mode for M=N. Simulation with 3GPP LTE settings demonstrates that both techniques can reduce the feedback overhead by more than 30% with little performance degradation