Due to the spectrum varying nature of cognitive radio networks, secondary users are required to perform spectrum handoffs when the spectrum is occupied by primary users, which will lead to a handoff delay. In this paper, based on the multi-armed bandit framework of medium access in decentralized cognitive radio networks, we investigate blind spectrum selection problem of secondary users whose sensing ability of cognitive radio is limited and the channel statistics are a priori unknown, taking the handoff delay as a fixed handoff cost into consideration. In this scenario, secondary users have to make the choice of either staying foregoing spectrum with low availability or handing off to another spectrum with higher availability. We model the problem and investigate the performance of three representative policies, i.e., PRE , SL(K), kth-UCB1. The simulation results show that, despite the inclusion of the fixed handoff cost, these policies achieve the same asymptotic performance as that without handoff cost. Moreover, through comparison of these policies, we found the kth-UCB1 policy has better overall performance.
Channel sensing order setting is crucial for efficient channel exploration and exploitation in cognitive radio (CR) networks. This paper investigates the sensing order setting problem in multi-channel multi-user CR networks for both distributed scenario and centralized scenario. As the optimal solution is too complicated, two suboptimal greedy search algorithms with much less computational complexities are proposed. The channel availability, channel achievable rate, multi-user diversity and collisions among CR users are considered comprehensively in our proposed methods. For the distributed scenario, a novel potential function is proposed to represent the relative advantage of a channel used by a user among multi channels and multi users, based on which each user can get its own sensing order. For the centralized scenario, a sensing matrix is obtained by a coordinator for all the users. It is shown that, CR users' average throughput increases and collision probability decreases with the number of channels due to increased transmission opportunities. The total network throughput increases with the number of user pairs due to multi-user diversity. Simulation results validate the efficacy of the proposed schemes in elevating CR users' throughput and decreasing the probability of collision, and show the performance improvement of the proposed schemes by comparisons with existing works.
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