In recent years, heterogeneous radio access technologies have experienced rapid development and gradually achieved effective coordination and integration, resulting in heterogeneous networks (HetNets). In this paper, we consider the downlink secure transmission of HetNets where the information transmission from base stations (BSs) to legitimate users is subject to the interception of eavesdroppers. In particular, we stress the problem of joint user association and power allocation of the BSs. To achieve data transmission in a secure and energy efficient manner, we introduce the concept of secrecy energy efficiency which is defined as the ratio of the secrecy transmission rate and power consumption of the BSs and formulate the problem of joint user association and power allocation as an optimization problem which maximizes the joint secrecy energy efficiency of all the BSs under the power constraint of the BSs and the minimum data rate constraint of user equipment (UE). By equivalently transforming the optimization problem into two subproblems, that is, power allocation subproblem and user association subproblem of the BSs, and applying iterative method and Kuhn-Munkres (K-M) algorithm to solve the two subproblems, respectively, the optimal user association and power allocation strategies can be obtained. Numerical results demonstrate that the proposed algorithm outperforms previously proposed algorithms.

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In this paper, a multi-hop cognitive relay network consisting of multiple PU transmission pairs, SU transmission pairs and relay SUs is considered. To achieve the performance enhancement of multi-hop transmission links between SU pairs, an energy efficient constrained shortest path first (CSPF)based joint resource allocation and route selection algorithm is proposed, which consists of two sub-algorithms, i.e., CSPFbased route selection sub-algorithm and energy efficiencybased resource allocation sub-algorithm. More specifically, we first apply CSPF-based route selection sub-algorithm to obtain the shortest transmission route under the transmission constraints. Then, an energy efficiency-based resource allocation problem of the shortest routes is formulated and solved by applying iterative algorithm and Lagrange dual method. Finally, the energy efficiency of the shortest transmission routes is examined and the globally optimal route selection and resource allocation strategy is obtained which offers the maximal energy efficiency of the transmission route. Simulation results demonstrate the effectiveness of the proposed algorithm.