A hybrid Overlay-Underlay strategy for OFDM-based cognitive radio systems and its maximum achievable capacity

In this paper we propose a novel power allocation scheme for Hybrid Overlay-Underlay spectrum sharing technique in OFDM-Based Cognitive Radio (CR) Systems. In OFDM-Based Cognitive Radio systems, the interference power limits of the primary users (PUs) introduce subcarriers transmit power constraints for the secondary user (SU). The power allocation among the subcarriers of SU should be subjected to both the sum transmit power constraint (P
s
) as well as the tolerable interference (Q) that is imposed to primary user (PU) due to transmit power and the leakage of sidelobes signal. Different from previous works, we deal with a more general case in where the subcarriers sidelobes signals are specially considered in both Idle and Busy bands. A general power allocation algorithm is proposed to obtain the optimal solution (by dual decomposition method) that maximizes the capacity while satisfying both these constraints. We have divided the assigned power into two kinds: P
I
(the allocated power to subcarriers which are active in the Idle band) and P
B
(the power allocated to subcarriers which are active in the Busy band). Numerical results revealed that the current Hybrid Overlay-Underlay approach is more efficient than both overlay and underlay schemes.