Energy-Efficient Relay Selection Scheme for Physical Layer Security in Cognitive Radio Networks

Security is a critical issue in cognitive radio (CR) relay networks. Most previous work concentrates on maximizing secrecy capacity (SC) as a criterion to guarantee the security requirements in CR relay networks. However, under the requirement of "green" radio communication, the energy consumption is largely ignored. This paper proposes a relay selection scheme which jointly considers the best relay selection and dynamic power allocation in order to maximize SC and to minimize energy consumption. Moreover, we consider finite-state Markov channels and residual relay energy in the relay selection and power allocation process. Specifically, the formulation of the proposed relay selection and power allocation scheme is based on the restless bandit problem, which is solved by the primal-dual index heuristic algorithm. Additionally, the obtained optimal relay selection policy has an indexability property that dramatically reduces the computational complexity. Numerical results are presented to show that our proposed scheme has the maximum SC and minimum energy consumption compared to the existing ones.