  This paper studies secure video streaming in cache-enabled small cell
networks, where some of the cache-enabled small cell base stations (BSs)
helping in video delivery are untrusted. Unfavorably, caching improves the
eavesdropping capability of these untrusted helpers as they may intercept both
the cached and the delivered video files. To address this issue, we propose
joint caching and scalable video coding (SVC) of video files to enable secure
cooperative multiple-input multiple-output (MIMO) transmission and, at the same
time, exploit the cache memory of both the trusted and untrusted BSs for
improving the system performance. Considering imperfect channel state
information (CSI) at the transmitters, we formulate a two-timescale non-convex
mixed-integer robust optimization problem to minimize the total transmit power
required for guaranteeing the quality of service (QoS) and secrecy during video
streaming. We develop an iterative algorithm based on a modified generalized
Benders decomposition (GBD) to solve the problem optimally, where the caching
and the cooperative transmission policies are determined via offline
(long-timescale) and online (short-timescale) optimization, respectively.
Furthermore, inspired by the optimal algorithm, a low-complexity suboptimal
algorithm based on a greedy heuristic is proposed. Simulation results show that
the proposed schemes achieve significant gains in power efficiency and secrecy
performance compared to several baseline schemes.
