  Caching at small base stations (SBSs) has demonstrated significant benefits
in alleviating the backhaul requirement in heterogeneous cellular networks
(HetNets). While many existing works focus on what contents to cache at each
SBS, an equally important problem is what contents to deliver so as to satisfy
dynamic user demands given the cache status. In this paper, we study optimal
content delivery in cache-enabled HetNets by taking into account the inherent
multicast capability of wireless medium. We consider stochastic content
multicast scheduling to jointly minimize the average network delay and power
costs under a multiple access constraint. We establish a content-centric
request queue model and formulate this stochastic optimization problem as an
infinite horizon average cost Markov decision process (MDP). By using
\emph{relative value iteration} and special properties of the request queue
dynamics, we characterize some properties of the value function of the MDP.
Based on these properties, we show that the optimal multicast scheduling policy
is of threshold type. Then, we propose a structure-aware optimal algorithm to
obtain the optimal policy. We also propose a low-complexity suboptimal policy,
which possesses similar structural properties to the optimal policy, and
develop a low-complexity algorithm to obtain this policy.
