Energy-efficient Resource Allocation for OFDM Transmission with Opportunistic DF Relaying

This paper addresses an energy-efficiency (EE) maximized resource allocation (RA) problem for cooperative orthogonal frequency division modulation (OFDM) transmission, when the system's total power consumption (including the circuit power and transmission power) is limited below a peak value. The improved DF relaying protocol proposed in [1] is used with optimized subcarrier pairing. The EE is evaluated by the number of message bits delivered for per Joule energy consumed by the whole system. A polynomial-complexity algorithm is designed to find the optimum RA (i.e., power allocation, subcarrier pairing and transmission-mode selection) for maximizing the EE based on the Dinkelbach method as well as the dual method to solve a subproblem. The effectiveness of the algorithm is illustrated by numerical results. It is shown that the improved DF protocol with optimized subcarrier pairing leads to a higher maximum EE than the noncooperative transmission protocol which always uses every subcarrier for the direct-mode transmission. The optimum EE improves when the relay moves towards the middle region between the source and the destination. The optimum RA produced by the proposed algorithm leads to a significantly higher EE but a smaller SE than the SE-maximized RA, especially when the peak power allowed is high.