Optimal spectrum sharing and power allocation for OFDM-based two-way relaying

The problem of optimally allocating power for half-duplex two-way relaying in an OFDM system is considered. Assuming two-way relay is performed using analog network coding, and with total network power constraint, we obtain the optimal power allocation across subcarriers and among a relay and two communicating nodes to maximize the achievable sum rate in the network. We show that the resulting solution is a combination of two (usually opposite) power allocation strategies, i.e., water-filling across subcarriers and SNR-balancing between communicating end nodes. Further analysis also shows the optimal power allocation on the relay itself is a water-filling solution.