  Due to the large bandwidth, low latency and computationally intensive
features of virtual reality (VR) video applications, the current
resource-constrained wireless and edge networks cannot meet the requirements of
on-demand VR delivery. In this letter, we propose a joint horizontal and
vertical collaboration architecture in mobile edge computing (MEC)-enabled
small-cell networks for VR delivery. In the proposed architecture, multiple MEC
servers can jointly provide VR head-mounted devices (HMDs) with edge caching
and viewpoint computation services, while the computation tasks can also be
performed at HMDs or on the cloud. Power allocation at base stations (BSs) is
considered in coordination with horizontal collaboration (HC) and vertical
collaboration (VC) of MEC servers to obtain lower end-to-end latency of VR
delivery. A joint caching, power allocation and task offloading problem is then
formulated, and a discrete branch-reduce-and-bound (DBRB) algorithm inspired by
monotone optimization is proposed to effectively solve the problem. Simulation
results demonstrate the advantage of the proposed architecture and algorithm in
terms of existing ones.
