Abstract
The fifth generation wireless networks must provide fast and reliable connectivity while coping with the ongoing traffic growth. It is of paramount importance that the required resources, such as energy and bandwidth, do not scale with traffic. While the aggregate network traffic is growing at an unprecedented rate, users tend to request the same popular contents at different time instants. Therefore, caching the most popular contents at the network edge is a promising solution to reduce the traffic and the energy consumption over the backhaul links. In this paper, two scenarios are considered, where caching is performed either at a small base station, or directly at the user terminals, which communicate using \ac{D2D} communications. In both scenarios, joint design of the transmission and caching policies is studied when the user demands are known in advance. This joint design offers two different caching gains, namely, the \textit{pre-downloading} and \textit{local caching gains}. It is shown that the finite cache capacity limits the attainable gains, and creates an inherent tradeoff between the two types of gains. In this context, a continuous time optimization problem is formulated to determine the optimal transmission and caching policies that minimize a generic cost function, such as energy, bandwidth, or throughput. The jointly optimal solution is obtained by demonstrating that caching files at a constant rate is optimal, which allows to reformulate the problem as a finite-dimensional convex program. The numerical results show that the proposed joint transmission and caching policy dramatically reduces the total cost, which is particularised to the total energy consumption at the \ac{MBS}, as well as to the total economical cost for the service provider, when users demand economical incentives for delivering content to other users over the D2D links.
Highlights
Wireless traffic has experienced a tremendous growth in the last years due to the wide spread use of hand-held devices connected to the Internet, e.g., mobile phones, tablets, etc
For the first scenario, where caching is performed at the Small Base Stations (SBSs): (i) we show that, within each time slot, it is optimal to cache data at a constant rate, which permits reformulating the problem as a convex program; (ii) we solve this convex problem by means of dual decomposition and propose a subgradient algorithm to obtain the optimal dual variables; and (iii) we derive the structure of the optimal transmission power at the Macro Base Station (MBS) and the caching policy at the SBS
For the second scenario, where information is cached at the users and shared through D2D communications: (i) we show that, within each time slot, each user should cache data at a constant rate; (ii) we show that each user should transmit the files in the D2D links at a constant rate; and (iii) we reformulate the problem as a convex optimization problem
Summary
Wireless traffic has experienced a tremendous growth in the last years due to the wide spread use of hand-held devices connected to the Internet, e.g., mobile phones, tablets, etc. To cope with this growing traffic requirements, lots of efforts have been devoted towards the definition of the fifth generation of cellular communication systems (5G), which is expected to be operative by 2020. Device-to-Device (D2D) communications, small cell densification, millimeter wave, and massive MIMO are currently investigated as main enabling technologies for its success
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