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.