  Edge caching can effectively reduce backhaul burden at core network and
increase quality-ofservice at wireless edge nodes. However, the beneficial role
of edge caching cannot be fully realized when the offloading link is in deep
fade. Fortunately, the impairments induced by wireless propagation environments
could be renovated by a reconfigurable intelligent surface (RIS). In this
paper, a new RIS-aided edge caching system is proposed, where a network cost
minimization problem is formulated to optimize content placement at cache
units, active beamforming at base station and passive phase shifting at RIS.
After decoupling the content placement subproblem with hybrid beamforming
design, we propose an alternating optimization algorithm to tackle the active
beamforming and passive phase shifting. For active beamforming, we transform
the problem into a semidefinite programming (SDP) and prove that the optimal
solution of SDP is always rank-1. For passive phase shifting, we introduce
block coordinate descent method to alternately optimize the auxiliary variables
and the RIS phase shifts. Further, a conjugate gradient algorithm based on
manifold optimization is proposed to deal with the non-convex unit-modulus
constraints. Numerical results show that our RIS-aided edge caching design can
effectively decrease the network cost in terms of backhaul capacity and power
consumption.
