On the Outage Exponent of Fading Relay Channels with Partial Channel State Information

The problem of optimal dimension and power allocation to maximize the asymptotic outage exponent over a decode- and-forward fading relay channel with quantized channel state feedback (CSF) is studied. Three different scenarios are considered: relay-source, destination-relay, and destination-source feedback. It is found that even with just one bit of CSF from the relay to control the source power is sufficient to achieve the multiantenna upperbound in a range of multiplexing gain, with fixed-length codes, i.e., with coding schemes significantly simpler than "dynamic decode-and-forward." CSF from destination to control the relay power slightly outperforms dynamic decode- and-forward at high multiplexing gains. CSF from destination when the source-relay channel gain is unknown to the destination, achieves a diversity exponent equal to K times the diversity exponent of dynamic decode and forward, for any multiplexing gain. This linear growth of diversity with the number of feedback levels stands in sharp contrast to the exponential growth for multiantenna channels (fully cooperative transmitters).