Hybrid Analog/Digital CSI Feedback for Transmit Beamforming Systems in Time-Selective Fading Channels

In the IEEE 802.16 WiMAX standard, fixed codebooks are used for the purpose of transmitter beamforming. Transmission of the codebook indices is accomplished via digital limited feedback uplink. We propose an extension to the feedback scheme that can improve BER performance at the cost of slight increase in feedback rate. Namely, for each feedback period, we deploy an additional analog feedback symbol to encapsulate channel information that is otherwise lost in the codebook quantization process. This analog/digital hybrid feedback scheme essentially converts the fixed codebook into an adaptive one that matches the instantaneous channel condition, allowing us to properly perform beamforming in a time-selective fading channel. As verified in our simulation, the hybrid feedback scheme offers better BER performance compared to the fixed codebook pure digital feedback scheme.