Power allocation in DF relay assisted OFDM cognitive radio networks with outdated CSI

In this paper, we investigate power allocation in decode-and-forward (DF) relay assisted OFDM cognitive radio (CR) networks. The objective is to maximize the instantaneous capacity of the CR system. 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delay, the available channel state information (CSI) between the secondary and primary users is considered as an outdated but correlated version of the actual instantaneous CSI. An optimal power allocation scheme is presented assuming that the primary users are subject to average interference constraints and the CR transmitters are subject to maximum transmit power constraint. In addition, a suboptimal scheme with reduced complexity is also proposed. Numerical results confirm that the proposed suboptimal power allocation scheme can be used as a less complex alternative for optimal power allocation with some degradation in capacity.