FemtoCaching: Wireless Video Content Delivery through Distributed Caching Helpers

摘要

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