Energy-efficient power allocation for non-regenerative OFDM relay links

We consider a two-hop non-regenerative (amplify-and-forward) relay link with one source, one destination and one relay, in which orthogonal frequency division multiplexing (OFDM) is used. The power allocation (PA) to maximize the energy efficiency for the whole link is investigated under quality of service (Qos) requirement and separate power constraints. An approximation method with low complexity is first proposed to convert the PA problem into a quasi-concave problem, which can obtain a tight lower bound in high SNR region. Furthermore, a more accurate approximation method with higher complexity is raised, which can approach the actual performance both in the low SNR and high SNR regions. We also introduce subcarrier pairing scheme to further enhance the energy efficiency for the link. Simulation results are shown to compare the performance of the two proposed methods.