Mutual information based resource allocation in the two-way relay channel with OFDM

The combination of Physical-Layer Network Coding (PLNC) and Orthogonal Frequency Division Multiple Access (OFDMA) in Two Way Relay (TWR) systems enables promising designs for future generation wireless systems. So far, existing resource allocation schemes for OFDMA in TWR systems do not exploit the advantage of PLNC, i.e. the relay is not necessarily interested in the original user messages, but their XOR combination. Recently, it was shown that the achievable throughput in a TWR system with PLNC is close to its mutual information. Thus, the mutual information might be well suited to address the power allocation problem of a PLNC system with OFDMA. However, as no closed form expression is known for the mutual information of such a system, the optimal resource allocation cannot be found easily. Thus, an appropriate approximation for the mutual information in the critical MAC phase is proposed within this paper. Afterwards, that approximation is used to optimise the power allocations in an OFDMA system with individual power constraints. Simulation results and a comparison to alternative low effort algorithms confirm the efficiency of this approach.