Throughput-Delay Performance of Half-Duplex Hybrid-ARQ Relay Channels

The throughput-delay performance of a half-duplex relay channel with hybrid-automatic retransmission request (HARQ) is analyzed in this paper. The protocol uses a form of incremental redundancy HARQ transmission with assistance from the relay via space-time coding if the relay decodes the message before the destination. The packet delay constraint is represented by L, the maximum number of HARQ rounds. An outage is declared if the packet is unsuccessful after L HARQ rounds. A delay-limited throughput is defined in this paper to explicitly account for finite delay constraints and associated non-zero packet outage probabilities. For small outage probabilities, this delay-limited throughput is greater than the conventional long-term average throughput. An exact expression is obtained for the outage probability to compare the throughput-delay performance of this half-duplex relay protocol with direct transmission. The delay-limited throughput of the relay channel is significantly larger than that of direct transmission for a wide range of signal-to-noise ratios (SNRs), target outage probabilities, delay constraints and relay positions.