Physical-Layer Secure Transmissions in Cache-Enabled Cooperative Small Cell Networks

This paper explores physical-layer security in a small cell network with cooperative cache-enabled small base stations (SBSs) in the presence of randomly distributed eavesdroppers. We put forward a hybrid caching placement strategy where a proportion of the cache space in each SBS is assigned to store the most popular files (MPFs), while the remaining is used to cache the disjoint subfiles (DSFs) of less popular files in different SBSs as a means to improve secrecy and content diversity. We then propose two coordinated multi-point techniques, namely, joint transmission and orthogonal transmission, to deliver the MPFs and DSFs, respectively. We jointly design the optimal transmission rate and caching assignment proportion to maximize the secure content delivery probability, and provide various insights into the optimal results. Numerical results are also presented to verify the theoretical findings and to demonstrate the superiority of our caching and transmission strategies.