  In this paper, we aim to minimize the average file transmission delay via
bandwidth allocation and cache placement in two-tier heterogeneous networks
with limited storage capacity, which consists of cache capacity and buffer
capacity. For average delay minimization problem with fixed bandwidth
allocation, although this problem is nonconvex, the optimal solution is
obtained in closed form by comparing all locally optimal solutions calculated
from solving the Karush-Kuhn-Tucker conditions. To jointly optimize bandwidth
allocation and cache placement, the optimal bandwidth allocation is first
derived and then substituted into the original problem. The structure of the
optimal caching strategy is presented, which shows that it is optimal to cache
the files with high popularity instead of the files with big size. Based on
this optimal structure, we propose an iterative algorithm with low complexity
to obtain a suboptimal solution, where the closed-from expression is obtained
in each step. Numerical results show the superiority of our solution compared
to the conventional cache strategy without considering cache and buffer
tradeoff in terms of delay.
