Abstract: Energy efficiency (EE) can be enhanced by retransmissions and combining in hybrid automatic repeat request (HARQ) system. However, it is difficult to optimize the transmit power of each retransmission when the accurate retransmission number and future channel state information (CSI) cannot be obtained. This paper proposes a simple energy efficient HARQ scheme for point-to-point wireless communication. In the proposed scheme, the conditional word error rate (WER) of each retransmission is fixed and the transmit power is adapted correspondingly. Three performance metrics are analyzed including average transmission number, throughput and EE. Compared with the conventional equal power HARQ scheme, the proposed scheme can significantly improve the EE and other two metrics under the same constraint of average transmit power or average energy consumption. Furthermore, it is found that, selecting a conditional WER which is slightly smaller than the optimal one is sufficient for practical implementation.

Key words: Energy efficiency, hybrid automatic repeat request (HARQ), conditional word error rate