Broadcasting over Two-Hop Relay Fading Channels

We consider a two-hop relay channel, with stringent delay constraints, which require transmission without channel state information (CSI), where only decoders possess perfect CSI estimation. Various relaying protocols and single-user broadcasting strategies are studied. For a decode-and-forward (DF) relay several broadcasting strategies are considered. Then, for an amplify-and-forward (AF) relay, a maximal broadcasting achievable rate is analytically derived. A quantize-and-forward (QF) relay is also considered. When coupled with a single-level code at the source it uses codebooks matched to the received signal power and performs optimal minimal mean square error (MMSE) quantization. This scheme is simplified by a hybrid amplify-quantize-forward (AQF) relay, which scales the input, and performs optimal MMSE quantization with a single codebook. It is shown that the latter is optimal in means of throughput on the relay-destination link, while maintaining a lower coding complexity than the QF setting. A further extension of the AQF allows the relay to perform successive refinement quantization coupled with a matched multi-level code. Numerical results show that for high SNRs the broadcast approach over AF relay may achieve higher throughput gains than the other relaying protocols that were numerically tractable