Cache Placement in Two-Tier HetNets with Limited Storage Capacity: Cache or Buffer?

TL;DR

This paper develops an optimal cache placement and bandwidth allocation strategy in two-tier HetNets with limited storage, minimizing average delay by caching popular files and proposing a low-complexity iterative solution.

Contribution

It derives a closed-form optimal solution for delay minimization and reveals that caching popular files is optimal, introducing an efficient iterative algorithm for joint optimization.

Findings

Optimal cache placement favors popular files over larger ones.

The proposed iterative algorithm achieves lower delay than conventional strategies.

Numerical results confirm the effectiveness of the joint optimization approach.

Abstract

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.