Asymmetric Resource Allocation in OFDMA-Based Relay Systems

In the traditional OFDMA (orthogonal frequency division multiple access) cooperative systems, the bits received by the relay on one dedicated subcarrier are retransmitted to the destination over paired subcarrier. However, this is not optimal in terms of system performance. This paper relaxes the constraint and designs asymmetric resource allocation. This increases the degree of freedom for transmission. The joint optimization problem of relay selection, subcarrier assignment and power control with the objective of maximizing the transmission rate is a mixed discrete programming problem which is hard to tackle efficiently and near optimally. We develop the method based by Lagrangian dual relexation to solve this problem efficiently. The key in the algorithm is the so-called asymptotic strong duality for OFDMA systems. The asymptotic strong duality suggests that it may be possible to devise polynomial time approximation scheme. The computation complexity is linear with the number of subcarriers. Simulation results show the proposed scheme achieves a higher system capacity than existing symmetric resource allocation schemes, and near-optimal to the globally optimal solution.