  In this paper, we propose a novel wireless caching scheme to enhance the
physical layer security of video streaming in cellular networks with limited
backhaul capacity. By proactively sharing video data across a subset of base
stations (BSs) through both caching and backhaul loading, secure cooperative
joint transmission of several BSs can be dynamically enabled in accordance with
the cache status, the channel conditions, and the backhaul capacity. Assuming
imperfect channel state information (CSI) at the transmitters, we formulate a
two-stage non-convex mixed-integer robust optimization problem for minimizing
the total transmit power while providing quality of service (QoS) and
guaranteeing communication secrecy during video delivery, where the caching and
the cooperative transmission policy are optimized in an offline video caching
stage and an online video delivery stage, respectively. Although the formulated
optimization problem turns out to be NP-hard, low-complexity polynomial-time
algorithms, whose solutions are globally optimal under certain conditions, are
proposed for cache training and video delivery control. Caching is shown to be
beneficial as it reduces the data sharing overhead imposed on the
capacity-constrained backhaul links, introduces additional secure degrees of
freedom, and enables a power-efficient communication system design. Simulation
results confirm that the proposed caching scheme achieves simultaneously a low
secrecy outage probability and a high power efficiency. Furthermore, due to the
proposed robust optimization, the performance loss caused by imperfect CSI
knowledge can be significantly reduced when the cache capacity becomes large.
