Optimum energy efficient communications for hybrid ARQ systems

The optimum energy efficient communication of a hybrid automatic repeat request (HARQ) system with Chase combining is studied in this paper. The proposed scheme minimizes the energy required to successfully deliver a bit from a source to its destination by identifying the optimum energy distribution, i.e., the sequence of transmission energy that should be employed during the retransmissions of a packet. The optimum energy distribution is calculated with a new backward sequential method, where the energy of the retransmission round k is expressed as a closed-form expression of the energy of the retransmission rounds i > k. The optimum design is performed by considering a wide range of practical system parameters, such as circuit power and coding in the physical layer, and frame length and protocol overhead in the media access control layer. Both analytical and simulation results demonstrate that the proposed scheme can achieve significant energy savings compared to conventional HARQ systems that employ equal energy in all retransmissions.