# Wireless Caching: Making Radio Access Networks More than Bit-Pipelines
###### Abstract
In particular, caching will make next-generation RANs much more proactive and become more than bit-pipelines. It motivates a proactive infrastructure that integrates wireless transceivers, request predictors, and cache memories. This infrastructure predicts what and when a user requests or schedules, when and how to push, and determines how long to cache by itself. As a result, there is a need to revisit caching, and in this paper we do so from the following three perspectives. * From a time-domain perspective, a unified view of caching gains and costs is presented. Pushing cacheable information objects prolongs their transmission times significantly while serving users within their tolerances. To adopt various wireless techniques that trade the transmission time for spectral and/or energy efficiency gains, user-specific request prediction is particularly beneficial, based on which a beyond-bit-pipeline RAN proactively schedules what, when, and how to push based on its own prediction of users' requests. Some fundamental limits of cache-empowered RANs can be revealed from a cross-layer perspective. * From an incentive perspective, a pricing mechanism is conceived to advocate the application and evolution of caching policies. Since no one is willing to pay for what he or she never reads, cache-empowered RANs abandon the bit-pipeline-oriented pricing that charges a user according to her or his throughput or access time. We envision a hierarchical framework to price caching services, which creates incentives for RANs to foresee future requests, push the right files, and participate required collaborations. * From a transparency perspective, recommendation systems (RSs) make cached files visible to users. A cache-friendly RS tells a user not only what he or she might be interested in, but also what has been done for her or him. If the user accepts the recommendation to read the cached file, he or she enjoys low latency and possibly a reduced service fee because caching saves radio resources. A RAN with joint caching and recommendation will no longer serve as a bit-pipeline only, since it manipulatesusers. Furthermore, preference-aware data compression, also referred to as edge compression, is made practical by learning and understanding individual users. The idea of proactive caching can be also borrowed to offload peak-time tasks of mobile edge computing (MEC). The merging of communication, computation, compression, and recommendation makes RANs more than bit-pipelines. In this article, we begin our discussion with gains, costs, and needs of cache-empowered RANs that determine what, when, and how to transmit and cache without waiting for requests from users. Then, a more precise prediction that foresees future user request in the time domain is reviewed. Based on the request time prediction, joint scheduling of communications links and cache memories is enabled to optimize when and how content items are cached. In particular, we review the energy-, spectral-, and memory-efficient caching methods with focus on their theoretical performance limits. To advocate caching and user cooperation therein, a novel hierarchical pricing architecture is also discussed. Finally, we envision that caching will merge several application layer roles, such as recommender systems, data compression, and MEC, thereby holding the potential of making RANs more than bit-pipelines. Figure 1: A unified framework for understanding caching gains from a time-domain perspective. Caching prolongs the transmission time, which enables various wireless techniques e.g., in Table 1 that trade the transmission time for energy and/or spectral efficiencies.