Energy-delay tradeoff for wireless relay systems using HARQ with incremental redundancy

Using relays, wireless relay systems can provide reliable transmissions when the distance between a source node (SN) and a destination node (DN) is sufficiently long and the transmission power is limited. In this paper, we study the application of the hybrid automatic repeat request (HARQ) protocol with incremental redundancy (HARQ-IR) to wireless relay systems for energy efficient reliable packet transmissions when a delay constraint is not stringent. The energy-delay tradeoff (EDT) is discussed for one-way and two-way relay systems. It is shown that direct transmissions without a relay node (RN) can be more energy efficient under a short delay constraint (or in the high power regime). On the other hand, in the low power regime (or under a long delay constraint), we can show that decoding at an RN plays a crucial role in improving energy efficiency. In particular, if decoding is performed at an RN in both one-way and two-way relay systems, relay-aided transmissions become more energy efficient than direct transmissions by a factor of greater than or equal to 8 (with a path loss exponent of 3 over Rayleigh fading channels) as the transmission power approaches 0.