Content Placement in Cache-Enabled Sub-6 GHz and Millimeter-Wave Multi-antenna Dense Small Cell Networks

TL;DR

This paper analyzes cache placement strategies in multi-antenna dense small cell networks operating at sub-6 GHz and millimeter-wave frequencies, proposing algorithms to optimize content delivery and balance cache diversity with popularity.

Contribution

It introduces a novel successful content delivery probability model for multi-antenna systems and proposes a constrained cross-entropy algorithm and a two-stair heuristic for content placement.

Findings

The two-stair algorithm outperforms simple popularity-based caching.

The constrained cross-entropy method achieves near-optimal content placement.

Tradeoff identified between caching capacity and density in mmWave systems.

Abstract

This paper studies the performance of cache-enabled dense small cell networks consisting of multi-antenna sub-6 GHz and millimeter-wave base stations. Different from the existing works which only consider a single antenna at each base station, the optimal content placement is unknown when the base stations have multiple antennas. We first derive the successful content delivery probability by accounting for the key channel features at sub-6 GHz and mmWave frequencies. The maximization of the successful content delivery probability is a challenging problem. To tackle it, we first propose a constrained cross-entropy algorithm which achieves the near-optimal solution with moderate complexity. We then develop another simple yet effective heuristic probabilistic content placement scheme, termed two-stair algorithm, which strikes a balance between caching the most popular contents and achieving content diversity. Numerical results demonstrate the superior performance of the constrained cross-entropy method and that the two-stair algorithm yields significantly better performance than only caching the most popular contents. The comparisons between the sub-6 GHz and mmWave systems reveal an interesting tradeoff between caching capacity and density for the mmWave system to achieve similar performance as the sub-6 GHz system.