Energy-Efficient Hybrid One- and Two-Way Relay Transmission

Two-way relaying is more spectral efficient than one-way relaying and is expected to consume less transmit power than one-way relaying to achieve the same data rate. However, when circuit power consumption is also taken into account, two-way relaying may not be more energy efficient, particularly when the bits to be transmitted from two source nodes have different numbers. To always offer high energy efficiency (EE), we propose a hybrid relay strategy, which conveys bidirectional messages with both one-way and two-way relaying. To maximize its EE, we jointly optimize the number of bits and transmission time allocated to the one- and two-way relaying stages to minimize the overall energy consumption, including the transmit power and circuit power. To reveal the behavior of the optimized hybrid relay strategy, we analyze the optimal bit allocation for the hybrid relaying under different circuit power consumption and bidirectional data amounts. Analytical and simulation results show that, in a high-traffic region where the transmit power dominates the energy consumption, the hybrid relaying degenerates into two-way relaying. By contrast, in a low-traffic region where the circuit power consumption is dominant, the proposed hybrid relaying offers significant performance gain in the sense of EE over the pure one- and two-way relaying.