  Delivering 360 degree video streaming for virtual and augmented reality
presents many technical challenges especially in bandwidth starved wireless
environments. Recently, a so-called two-tier approach has been proposed which
delivers a basic-tier chunk and select enhancement-tier chunks to improve user
experience while reducing network resources consumption. The video chunks are
to be transmitted via unicast or multicast over an ultra-dense small cell
infrastructure with enough bandwidth where small cells store video chunks in
local caches. In this setup, user-cell association algorithms play a central
role to efficiently deliver video since users may only download video chunks
from the cell they are associated with. Motivated by this, we jointly formulate
the problem of user-cell association and video chunk multicasting/unicasting as
a mixed integer linear programming, prove its NP-hardness, and study the
optimal solution via the Branch-and-Bound method. We then propose two
polynomial-time, approximation algorithms and show via extensive simulations
that they are near-optimal in practice and improve user experience by 30%
compared to baseline user-cell association schemes.
