  This paper studies physical-layer security for a cache-enabled heterogeneous
cellular network comprised of a macro base station and multiple small base
stations (SBSs). We investigate a joint design on caching placement and file
delivery for realizing secure and energy-efficient transmissions against
randomly distributed eavesdroppers. 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, respectively. 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 to achieve a maximal overall secrecy throughput and secrecy energy
efficiency, respectively. Numerical results verify the theoretical analyses and
demonstrate the superiority of the proposed hybrid caching policy.
