Reliable energy-efficient spectrum management and optimization in cognitive radio networks: how often should we switch?

Spectrum handoff is a promising spectrum management approach that enables the secondary user to continue its transmission by switching to a sensed-as-idle channel when the current channel is re-occupied by the primary user. Periodic sensing is commonly employed to facilitate spectrum handoff by improving sensing accuracy so that the secondary user has a better opportunity to avoid transmission collision via switching to a truly idle channel. However, byproducts of spectrum handoff, such as extra energy cost on channel switch, switch delay, and collisions due to excessive switch, have been largely underestimated. These by-products which have significant impacts on the overall performance lead to the questions: Should we switch the channel? How often should we switch? In this article we first review several different periodic sensing and handoff mechanisms. Then we identify the trade-off between the secondary user's energy efficiency and transmission reliability. The proposed switch-stay model is elaborated and studied to balance the aforementioned trade-off in light of sensing accuracy, the probability of collision, throughput and delay constraints. Upper and lower bounds of the stay probability on the current channel are illustrated from these constraints. In addition, we discuss the implementation of an antenna-selection sensing mechanism in order to relax these bounds for a better energy-efficient spectrum management solution. Performance evaluation and some open issues are also discussed in this article.