Energy-efficient power allocation for multi-user single-AF-relay underlay cognitive radio networks

In underlay relay-based cognitive radio networks, when the interference imposed on primary users is below some pre-determined threshold, secondary users can use the licensed spectrum although it is occupied by primary users. Hence, energy-efficient power allocation among secondary users and cognitive relay is one of the key techniques to utilize this new network paradigm effectively as power is the key component of the interference imposed on primary users. In this paper, for cognitive radio cooperative communication (CRCC) networks where multiple secondary users share a common cognitive relay with amplify-and-forward (AF) mode, we propose an optimal power allocation strategy with the objective to minimize the sum-power consumption of the system. In addition to the constraints that restrict the interference on the primary users within pre-determined threshold, the formulated problem has constraints to ensure the minimum SNR achieved by secondary users and the cognitive relay has a total power constraint. From the inner understanding of the problem, we simplify the formulation. In order to transform the problem to a search problem of finding equilibrium points of power allocation, a theorem is proposed which provides the condition for sum-system-power minimization state. Consequently, an algorithm with linear time complexity is developed based on that theorem. Extensive simulation has been conducted under different system setups to justify the effectiveness and efficacy of our power allocation strategy comparing with other methods usually adopted to solve this type of problem.