A Two-Step Approach to Power Allocation for OFDM Signals Over Two-Way Amplify-and-Forward Relay

A two-way relay channel (TWRC) in which two terminals T-1 and T-2 exchange orthogonal frequency division multiplexing (OFDM) signals with the help of an amplify-and-forward (AF) relay T-3 is considered here, and an efficient technique for allocating powers to parallel tones of OFDM is developed. A sum rate maximization problem is formulated by replacing the individual power constraints of the conventional sum rate maximization problem, which limit the power of each terminal, with the total power constraint limiting the sum of powers of all terminals. The maximization problem with the total power constraint yields a more efficient power allocation policy than the conventional problem with individual power constraints. It is shown that the closed-form solution of the maximization problem under the total power constraint can be obtained for a single-tone system (N = 1). Based on this result, a two-step suboptimal approach is proposed in which the power is optimally assigned to each tone first, and then at each tone the assigned power is distributed to the three terminals. The proposed method is shown to assign 50% of the total power to relay T-3 irrespective of the channels. It is demonstrated that the proposed method is considerably simpler to implement than the conventional dual-decomposition method (DDM), yet the performance of the former is almost identical to that of the latter.