(The study on a hybrid cognitive radio system having overlay and underlay modes

Abstract

This paper proposes a hybrid cognitive radio (CR) system where underlay and overlay CR approaches are combined. By controlling switching rate from an overlay CR mode to an underlay CR mode in a probabilistic sense, the stability of secondary user`s queue is maintained and its throughput is maximized while the target departure rate of a primary (licensed) user is retained. Since a primary user in a hybrid system is likely to suffer from additional interference compared to a single-mode overlay CR system, we find the optimal transmit power of a secondary user in an overlay mode for given switching rate according to three different interference constraints Then, the range of the switching rate in which secondary user`s queue is stable is identified and the optimal switching rate for throughput maximization is derived. Our analysis and numerical results show that the proposed hybrid CR system attains higher throughput than a single-mode CR system.