Physical layer security in cache-enabled heterogeneous cellular networks

In this chapter, we discuss physical layer security in a cache-enabled heterogeneous cellular network comprised of a macro base station and multiple small base stations (SBSs). We jointly devise caching placement and file delivery for achieving secure and energy-efficient transmissions against randomly distributed eavesdroppers. Specifically, we propose a novel hybrid "most popular content" and "largest content diversity" caching placement policy to distribute files of different popularities. Depending on the availability and placement of the requested file, we employ three cooperative transmission schemes, namely distributed beamforming, frequency-domain orthogonal transmission, and best SBS relaying. We derive analytical expressions for the connection outage probability and secrecy outage probability for each transmission scheme. Afterwards, we design the optimal transmission rates and caching allocation successively for maximizing the overall secrecy throughput and secrecy energy efficiency, respectively. We eventually provide numerical results to verify the proposed theoretical analysis and to demonstrate the superiority of the proposed hybrid caching policy.