  As wireless video transmission is the fastest-growing form of data traffic,
methods for spectrally efficient video on-demand wireless streaming are
essential to service providers and users alike. A key property of video
on-demand is the asynchronous content reuse, such that a few dominant videos
account for a large part of the traffic, but are viewed by users at different
times. Caching of content on devices in conjunction with 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
the base station and the traditional D2D networks for regular data traffic.
This paper presents in a semi-tutorial 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 competing approach based on
combinatorial cache design and network coded transmission from the base station
(BS) only, referred to as coded multicasting. Interestingly, the spatial reuse
gain of the former and the coded multicasting gain of the latter yield, somehow
surprisingly, the same near-optimal throughput behavior in the relevant regime
where the number of video files in the library is smaller than the number of
streaming users. Based on our recent theoretical results, we propose a holistic
D2D system design that incorporates traditional microwave (2 GHz) as well as
millimeter-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 simulations under a variety of system settings, and
compare our scheme with other existing schemes by the BS. We show that, despite
the similar behavior of the scaling laws, the proposed D2D approach offers very
significant throughput gains with respect to the BS-only schemes.
