Green Communication via Power-Optimized HARQ Protocols

Recently, the efficient use of energy has become an essential research topic for green communication. This paper studies the effect of optimal power controllers on the performance of delay-sensitive communication setups that utilize hybrid automatic repeat request (HARQ). The results are obtained for repetition time diversity (RTD) and incremental redundancy (INR) HARQ protocols. In all cases, the optimal power allocation, minimizing the outage-limited average transmission power, is obtained under both continuous and bursting communication models. Also, we investigate the system throughput in different conditions. The results indicate that the power efficiency is substantially increased if adaptive power allocation is utilized. For example, assume a Rayleigh fading channel with a maximum of two (re)transmission rounds with rates {1, [1/2]} nats-per-channel-use and an outage probability constraint 10^-3. Then, compared with uniform power allocation, optimal power allocation in RTD reduces the average power by 9 and 11 dB in the bursting and continuous communication models, respectively. In INR, these values are obtained to be 8 and 9 dB, respectively.