New power allocation methods for AF relay assisted OFDM cognitive radio networks with outdated CSI

In this paper, we investigate optimal and suboptimal power allocation schemes for amplify-and-forward (AF) relay assisted OFDM-based cognitive radio (CR) transmission. Due to channel feedback delay, the available channel state information (CSI) between the secondary and primary users is considered as an outdated but correlated version of the actual instantaneous CSI.We develop an optimal power allocation scheme to maximize the instantaneous capacity of the CR transmission assuming that the primary users are subject to average interference constraints, and the CR transmitters are subject to maximum transmit power constraint. A suboptimal scheme with reduced complexity has also been investigated and compared with the uniform loading algorithm. Numerical results confirm that the proposed optimal power loading scheme achieves on average, 60% capacity improvement than the uniform loading algorithm. Also the proposed suboptimal scheme outperforms the uniform loading approach and can be used as a less complex alternative for optimal power allocation with about 10% (on average) capacity degradation.