Caching or No Caching in Dense HetNets?

TL;DR

This paper critically examines the effectiveness of caching in dense heterogenous networks, revealing that densification can diminish caching benefits and increase costs, and proposing a new efficiency metric to optimize caching strategies.

Contribution

It introduces a caching efficiency metric and demonstrates that caching only about 3% of content in small cells is optimal, challenging conventional offloading strategies.

Findings

Densification reduces cache-hit probability and increases network cost.

Caching only about 3% of content maximizes efficiency.

Range expansion has limited impact on caching performance.

Abstract

Caching the content closer to the user equipments (UEs) in heterogenous cellular networks (HetNets) improves user-perceived Quality-of-Service (QoS) while lowering the operators backhaul usage/costs. Nevertheless, under the current networking strategy that promotes aggressive densification, it is unclear whether cache-enabled HetNets preserve the claimed cost-effectiveness and the potential benefits. This is due to 1) the collective cost of caching which may inevitably exceed the expensive cost of backhaul in a dense HetNet, and 2) the excessive interference which affects the signal reception irrespective of content placement. We analyze these significant, yet overlooked, issues, showing that while densification reduces backhaul load and increases spectral efficiency in cache-enabled dense networks, it simultaneously reduces cache-hit probability and increases the network cost. We then introduce a caching efficiency metric, area spectral efficiency per unit spent cost, and find it enough to cache only about 3% of the content library size in the cache of smallcell base stations. Furthermore, we show that range expansion, which is known to be of substantial value in wireless networks, is almost impotent to curb the caching inefficiency. Surprisingly, unlike the conventional wisdom recommending traffic offloading from macro cells to small cells, in cache-enabled HetNets, it is generally more beneficial to exclude offloading altogether or to do the opposite.