In this paper, we investigate the problem of joint resource allocation in two-way OFDM relay systems under the QoS (Quality of Services) constraints of sum rate and symbol error rate. Assuming perfect knowledge of the instantaneous channel gains for the whole system, we propose a joint resource allocation scheme aiming at maximizing the energy efficiency of the system. We first select the best relay node from all the potential relay nodes and then employ subcarrier pairing, and finally we apply joint power allocation and bit loading. Simulation results show that the proposed scheme can significantly improve the energy efficiency of the system. And we also find that bit loading has more effect on the performance of the system than subcarrier pairing does.
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