  Multi-antenna relaying has emerged as a promising technology to enhance the
system performance in cellular networks. However, when precoding techniques are
utilized to obtain multi-antenna gains, the system generally requires channel
state information (CSI) at the transmitters. We consider a linear precoding
scheme in a MIMO relaying broadcast channel with quantized CSI feedback from
both two-hop links. With this scheme, each remote user feeds back its quantized
CSI to the relay, and the relay sends back the quantized precoding information
to the base station (BS). An upper bound on the rate loss due to quantized
channel knowledge is first characterized. Then, in order to maintain the rate
loss within a predetermined gap for growing SNRs, a strategy of scaling
quantization quality of both two-hop links 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 only the bit number of feedback from the relay to the
BS needs to grow with the increasing transmit power at the BS. Numerical
results are provided to verify the proposed strategy for feedback quality
control.
