Optimal power allocation for secondary users in CR networks

Cognitive Radio (CR) increases spectrum utilization by allowing unlicensed secondary users to concurrently operate with the licensed primary users. The side-lobes of an OFDM-based secondary user (SU) causes unwanted interference to the sub-carriers of OFDM-based primary user (PU). The objective is to assign power to the sub-carriers of SU in such a way that capacity of SU is maximized while satisfying the total power budget constraint and interference (introduced to the PU from SU) constraint. Previously, this power allocation to the SU´s sub-carriers (i.e. convex optimization algorithm) was solved using KKT conditions & iterative algorithms. These algorithms maximize capacity asymptotically when the iterations are very large. In this paper, closed-form solution is presented that calculate the power allocation to the SU´s sub-carriers. Simulation results are included which shows that complexity of the closed-form solution is small when compared to the complexity of the iterative algorithms.