FemtoCaching: Wireless Content Delivery Through Distributed Caching Helpers

Abstract

Video on-demand streaming from Internetbased 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.

Secure Video Streaming in Heterogeneous Small Cell Networks With Untrusted Cache Helpers

“…This robust optimization approach has been commonly adopted for studying PLS in the literature, see [29]- [31] and references therein. Constraints C6 and C7 jointly guarantee a minimum achievable secrecy rate of R Problem P0 is a non-convex mixed-integer nonlinear program (MINLP) 7 due to the binary caching decision vector q and the non-convex constraints C6 and C7. This type of problem is NP-hard [41].…”

“…Then, the cooperative transmission policy {w ρ,t , V t } for time t ∈ T 0 is optimized online by solving the following problem 6 Prediction of the users' future requests based on historical user profiles can further improve the cache placement at the cost of an increased computational complexity. 7 For a non-convex MINLP, even if the binary constraints are relaxed into convex ones, the problem remains non-convex [41].…”

“…Consequently, the backhaul traffic is offloaded by reusing the cached content [5], [6]. Caching as an alternative to small cell backhauling was first investigated in [7], where caching was shown to also substantially reduce the average downloading delay. Besides, caching improves the energy efficiency of wireless backhauling systems as was shown in [8].…”

Network-Coded Content Delivery in Femtocaching-Assisted Cellular Networks

“…This growth is expected to further increase by two orders of magnitude in the next few years [2]. This large demand raises the need to enhance the spectrum efficiency of next-generation cellular networks [3].…”

“…One of the recent suggestions to do so in the 5G macrocell architecture is to bring the content closer to the clients, by deploying a large number of low-cost wireless nodes with large storage capacity, in which the popular files (i.e., the files which are most likely to be requested by the clients) are cached [3]. These nodes are usually referred to as femtocaches (FCs) and the approach is called femtocaching [2]. Since file popularity is slowly varying, this caching occurs through the expensive backhaul to the macrocell base station (MBS) with very low rate or in off-peak times.…”