Limited feedback design for MIMO-relay assisted cellular networks with beamforming

We consider a MIMO relay downlink system using beamforming techniques with quantized channel state information (CSI) feedback from two hop channels. With this scheme, each remote user feeds back its quantized CSI to the relay, and the relay sends the quantized beamforming vectors to the base station (BS). An upper bound on the rate loss due to feedback quantization is first characterized. Then, a strategy of scaling the quantization accuracy of both hop links in order to maintain the rate loss within a predetermined gap for growing SNRs is proposed. It is revealed that the numbers of feedback bits of both links should scale linearly with the transmit power at the relay, while the number of feedback bits from the relay to the BS needs to grow with the transmit power at the BS.