An overview of medium access control strategies for opportunistic spectrum access in cognitive radio networks

Cognitive radio (CR) is a promising wireless technology that provides efficient spectral usage. Medium Access Control (MAC) has an important role in several cognitive radio functions such as sensing, spectrum mobility, resource allocation and spectrum sharing. We focus on the opportunistic spectrum access (OSA) functionality of the CR network MAC layer by which the secondary users (SUs) access licensed spectrum in space and time with no harmful interference to primary users (PUs), without prior information on spectral usage. To achieve this, the unlicensed users should have the ability to adaptively and dynamically seek and exploit opportunities in licensed spectrum in time, polarization and frequency domains. There have been several OSA MAC schemes proposed for CR networks. This article presents a detailed review of such state-of-the-art schemes. First the differences between the conventional MAC protocols and OSA based MAC protocols are discussed. Existing OSA MAC protocols are classified according to their key attributes and their performances. Finally, future research directions are discussed.