Methods of Hybrid Cognitive Radio Network: A Survey

In advanced radio communication systems, most of the radio spectrum remains underutilized. To fully utilize the radio spectrum, an efficient allocation of the scarce and expensive radio resources is most important and challenging. As a solution of this problem, cognitive radio network (CRN) can make better use of the radio spectrum by allowing the secondary users (SU) to opportunistically access and share the licensed spectrum using dynamic spectrum access (DSA) technology. There are two main sharing techniques in CRN based on the access technology: 1) over-lay; 2) under-lay. In over-lay mode, secondary user can access only vacant spectrum in the absence of primary user (PU) but in underlay mode, the SUs can coexist with the PUs in the same channel under the SINR (signal to interference plus noise ratio) constraints. To further improve the spectrum scarcity, both over-lay and under-lay approaches gets combined under SINR constraints to make a hybrid cognitive radio. In this paper, we provide a survey for different methods of hybrid cognitive radio in which under-lay and over-lay modes are merged using different techniques to efficiently utilize the scarce radio spectrum.