Secrecy-Based Energy-Efficient Data Offloading via Dual Connectivity Over Unlicensed Spectrums

Offloading cellular mobile users' (MUs') data traffic to small-cell networks is a cost-effective approach to relieve congestion in macrocell cellular networks. However, as many small-cell networks operate in the unlicensed bands, the data offloading might suffer from a security issue, i.e., some eavesdropper could overhear the offloaded data over unlicensed spectrums. This motivates us to investigate a secrecy-based energy-efficient uplink data offloading scheme. Specifically, we consider the recent paradigm of traffic offloading via dual connectivity, which enables an MU to simultaneously deliver traffic to a macro base station (mBS) over the licensed channel and a small-cell access point (sAP) over the unlicensed channel. We formulate an MU's joint optimization of traffic scheduling and power allocation problem, with the objective of minimizing the total power consumption while meeting both the MU's traffic demand and secrecy requirement. Despite the non-convex nature of the joint optimization problem, we propose an efficient algorithm to compute the optimal offloading solution. By evaluating the impact of the MU's secrecy requirement and the eavesdropper's channel condition, we quantify the conditions under which the optimal offloading solution corresponds to the full-offloading and zero-offloading, respectively. Numerical results validate the optimal performance of our proposed algorithm, and show that the optimal offloading can significantly reduce the total power consumption compared with some fixed offloading schemes. Based on the optimal offloading solution for each MU, we further analyze the scenario of multiple MUs and sAPs, and investigate how to optimally exploit the sAPs' total offloading capacity to serve the MUs while accounting for the MUs' corresponding power consumptions for offloading data. To this end, we formulate a total network-benefit maximization problem that accounts for the reward for serving the MUs successfully, the mBS's bandwidth usage, and the MUs' power consumptions. Numerical results show that the optimal solution can improve the total network benefit compared with some heuristic sAP-selection scheme.