Stay or switch? Analysis and comparison of delays in cognitive radio networks with interweave and underlay spectrum access

Cognitive Networks have been proposed to opportunistically discover and exploit licensed spectrum bands, in which the secondary users' (SU) activity is subordinated to primary users (PU). Depending on the nature of interaction between the SU and PU, there are two frequently encountered types of spectrum access: underlay and interweave. While a lot of research effort has been devoted to each mode, there is no clear consensus about which type of access performs better in different scenarios and for different metrics. To this end, in this paper we approach this question analytically, and provide closed-form expressions that allow one to compare the performance of the two types of access under a common network setup. We focus on the average delay as the key metric, which we analyze using queueing theory. This allows an SU to decide when one type of access technique provides better performance, as a function of the metric of interest and key network parameters. What is more, based on this analysis, we propose a dynamic (hybrid) policy, that can decide at any point to switch from one type of access to the other, offering up to 50% of additional performance improvement, compared to the optimal " static " policy in the scenario at hand. We provide extensive validation results using a wide range of realistic simulation scenarios.