# How Much Can D2D Communication Reduce Content Delivery Latency in Fog Networks with Edge Caching?

## TL;DR

This paper investigates how enabling D2D communication in Fog-RANs with edge caching can significantly reduce content delivery latency, using an information-theoretic framework and optimal caching and transmission strategies.

## Contribution

It introduces joint caching, transmission, and D2D policies that are near-optimal in minimizing delivery latency in fog networks.

## Key findings

- D2D communication can substantially lower delivery latency.
- Proposed policies are within a factor of two of optimal.
- Certain scenarios achieve the absolute minimum latency.

## Abstract

A Fog-Radio Access Network (F-RAN) is studied in which cache-enabled Edge Nodes (ENs) with dedicated fronthaul connections to the cloud aim at delivering contents to mobile users. Using an information-theoretic approach, this work tackles the problem of quantifying the potential latency reduction that can be obtained by enabling Device-to-Device (D2D) communication over out-of-band broadcast links. Following prior work, the Normalized Delivery Time (NDT) --- a metric that captures the high signal-to-noise ratio worst-case latency --- is adopted as the performance criterion of interest. Joint edge caching, downlink transmission, and D2D communication policies based on compress-and-forward are proposed that are shown to be information-theoretically optimal to within a constant multiplicative factor of two for all values of the problem parameters, and to achieve the minimum NDT for a number of special cases. The analysis provides insights on the role of D2D cooperation in improving the delivery latency.