Spectrum Leasing in Cognitive Radio Networks: A Survey

Spectrum scarcity and underutilization in wireless networks, arising due to traditional static spectrum allocation, have been of great concern since the past decade. The suboptimal use of spectrum motivates new wireless concepts where unlicensed users opportunistically access licensed spectrum using cognitive radio technology. A cognitive radio can change its operating parameter intelligently in real time to account for dynamic changes in a wireless environment. Licensed user (primary user) and cognitive radio users (secondary users) may coexist in a spectrum band, provided that secondary users operate without interfering with primary user’s transmission. Secondary users have the ability to sense a primary user’s activity in a band employing various signal processing techniques. In addition to hierarchal coexistence with primary users, secondary networks compete for a limited number of resources with other cognitive radio networks in the system, i.e. self-coexistence. Both coexistence and self-coexistence in cognitive radio networks force a secondary user to learn and adapt to its environment quickly. The complexity of dynamic spectrum access due to fluctuating spectrum availability, heterogeneous secondary users, diverse quality of service requirements, and increasing network density indicates a need to look beyond traditional approaches. Parallels have been drawn between dynamic spectrum access and biology inspired paradigms describing social and asocial interactions between agents. Proposed schemes using decision theory model complex secondary-secondary dynamics. Foraging insect societies have inspired schemes where self-organizing cognitive radio networks achieve spectrum access fairness. In this survey, although both spectrum sensing and spectrum sharing are critical aspects of dynamic spectrum access, we focus primarily on dynamic spectrum sharing paradigms drawn from decision theory and the study of foraging societies. A general overview of cognitive network architecture, cognitive radio duty cycle, and spectrum sensing is given to emphasize the underlying limitations and challenges in cognitive radio networks.