Energy efficiency of hybrid-ARQ systems under QoS constraints

In this paper, energy efficiency of hybrid automatic repeat request (HARQ) schemes under QoS constraints is studied in the low power regime by characterizing the minimum energy per bit and wideband slope. The energy efficiency is investigated when either an outage constraint is imposed and (the transmission rate is selected accordingly) or the transmission rate is optimized to maximize the throughput. In both cases, it is also assumed that there is a limitation on the number of retransmissions due to deadline constraints. Under these assumptions, closed-form expressions are obtained for the minimum energy per bit and wideband slope for HARQ with chase combining (CC). Through numerical results, the performances of HARQ-CC and HARQ with incremental redundancy (IR) are compared. Moreover, the impact of deadline constraints, outage probability, QoS constraints on the energy efficiency is analyzed.