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Relaying is supposed to be a low energy consumption technique since the long distance transmission is divided into several short distance transmissions. When the power consumptions (PCs) other than that consumed by transmitting information bits is taken into account, however, relaying may not be energy efficient. In this article, we study the energy efficiencies (EEs) of one-way relay transmission (OWRT) and two-way relay transmission (TWRT) by comparing with direct transmission (DT). We consider a system where two source nodes transmit to each other with the assistance of a half-duplex amplify-and-forward relay node. We first find the maximum EEs of DT, OWRT, and TWRT by optimizing the transmission time and the transmit powers at each node. Then we compare the maximum EEs of the three strategies, and analyze the impact of circuit PCs and data amount. Analytical and simulation results show that relaying is not always more energy efficient than DT. Moreover, TWRT is not always more energy efficient than OWRT, despite that it is more spectral efficient. The EE of TWRT is higher than those of DT and OWRT in symmetric systems where the circuit PCs at each node are identical and the numbers of bits to be transmitted in two directions are equal. In asymmetric systems, however, OWRT may provide higher EE than TWRT when the numbers of bits in two directions differ significantly.

Energy efficiency analysis of one-way and two-way relay systems