  The strong interference suffered by users can be a severe problem in
cache-enabled networks (CENs) due to the content-centric user association
mechanism. To tackle this issue, multi-antenna technology may be employed for
interference management. In this paper, we consider a user-centric interference
nulling (IN) scheme in two-tier multi-user multi-antenna CEN, with a hybrid
most-popular and random caching policy at macro base stations (MBSs) and small
base stations (SBSs) to provide file diversity. All the interfering SBSs within
the IN range of a user are requested to suppress the interference at this user
using zero-forcing beamforming. Using stochastic geometry analysis techniques,
we derive a tractable expression for the area spectral efficiency (ASE). A
lower bound on the ASE is also obtained, with which we then consider ASE
maximization, by optimizing the caching policy and IN coefficient. To solve the
resultant mixed integer programming problem, we design an alternating
optimization algorithm to minimize the lower bound of the ASE. Our numerical
results demonstrate that the proposed caching policy yields performance that is
close to the optimum, and it outperforms several existing baselines.
