FemtoCaching: Wireless Video Content Delivery through Distributed Caching Helpers

TL;DR

This paper proposes a distributed caching system with helpers to improve wireless video delivery, analyzing optimal file placement strategies and demonstrating near-optimal algorithms for minimizing download times.

Contribution

It introduces a novel caching framework with both uncoded and Fountain-encoded files, providing algorithms with provable approximation guarantees for optimal file placement.

Findings

Greedy algorithm achieves within a factor of 2 of optimal for uncoded caching.

Efficient algorithm with ratio $1-(1-1/d)^d$ for special uncoded case.

Convex formulation for coded cache assignment enabling linear programming solutions.

Abstract

Video on-demand streaming from Internet-based servers is becoming one of the most important services offered by wireless networks today. In order to improve the area spectral efficiency of video transmission in cellular systems, small cells heterogeneous architectures (e.g., femtocells, WiFi off-loading) are being proposed, such that video traffic to nomadic users can be handled by short-range links to the nearest small cell access points (referred to as "helpers"). As the helper deployment density increases, the backhaul capacity becomes the system bottleneck. In order to alleviate such bottleneck we propose a system where helpers with low-rate backhaul but high storage capacity cache popular video files. Files not available from helpers are transmitted by the cellular base station. We analyze the optimum way of assigning files to the helpers, in order to minimize the expected downloading time for files. We distinguish between the uncoded case (where only complete files are stored) and the coded case, where segments of Fountain-encoded versions of the video files are stored at helpers. We show that the uncoded optimum file assignment is NP-hard, and develop a greedy strategy that is provably within a factor 2 of the optimum. Further, for a special case we provide an efficient algorithm achieving a provably better approximation ratio of $1-(1-1/d)^d$, where $d$ is the maximum number of helpers a user can be connected to. We also show that the coded optimum cache assignment problem is convex that can be further reduced to a linear program. We present numerical results comparing the proposed schemes.