As wireless video is the fastest-growing form of data traffic, methods for spectrally efficient ondemand wireless video streaming are essential to both service providers and users. A key property of video on-demand is the asynchronous content reuse, such that a few popular files account for a large part of the traffic, but are viewed by users at different times. Caching of content on wireless devices in conjunction with device-to-device (D2D) communications allows to exploit this property, and provide a network throughput that is significantly in excess of both the conventional approach of unicasting from cellular base stations and the traditional D2D networks for "regular" data traffic. This paper presents in a tutorial and concise form some recent results on the throughput scaling laws of wireless networks with caching and asynchronous content reuse, contrasting the D2D approach with a few other competing approaches including conventional unicasting, harmonic broadcasting and a novel coded multicasting approach based on combinatorial cache design and network coded transmission from the cellular base station only. Somehow surprisingly, the D2D scheme with spatial reuse and simple decentralized random caching achieves the same near-optimal throughput scaling law as coded multicasting. Both schemes achieve an unbounded throughput gain (in terms of scaling law) with respect to conventional unicasting and harmonic broadcasting, in the relevant regime where the number of video files in the library is smaller than the total size of the distributed cache capacity in the network. In order to better understand the relative merits of these competing approaches, we consider a holistic D2D system design that incorporates traditional microwave (2 GHz) as well as millimeter-wave (mm-wave) D2D links; the direct connections to the base station can be used to provide those rare video requests that cannot be found in local caches. We provide extensive simulation results under a variety of system settings, and compare our scheme with conventional unicasting, harmonic broadcasting and coded multicasting. We show that, also in realistic conditions and non-asymptotic regimes, in realistic conditions and non-asymptotic regimes the proposed D2D approach offers very significant throughput gains with respect to the base station-only schemes. The authors are with the
First, the aggregate caching capacity grows with the number of devices in the network, and thus the benefit of device caching improves as the network size increases REF .
Wireless Device-to-Device Caching Networks: Basic Principles and System Performance
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