Recursive and trellis-based feedback reduction for MIMO-OFDM with transmit beamforming

We consider a MIMO-OFDM system with transmit beamforming applied on each OFDM subcarrier, where each beamforming vector is drawn from a codebook with finite size. Depending on the channel realization, the receiver decides the optimal beamforming vector on each subcarrier, and informs the transmitter through a rate-limited feedback link. Exploiting the fact that the channel responses across OFDM subcarriers are correlated, we propose two methods to reduce the amount of needed feedback. One is recursive feedback encoding that selects the optimal beamforming vectors sequentially across the subcarriers, and adopts a smaller-size time-varying codebook per subcarrier depending on prior decisions. The other is trellis-based feedback encoding that selects the optimal beamforming vectors for all subcarriers at once along a trellis structure via the A. The trellis-based feedback encoding outperforms the recursive feedback encoding at the expense of encoding complexity at the receiver. Simulation results demonstrate that our trellis-based approach outperforms an existing interpolation-based alternative, as the latter incurs diversity loss at high SNR.