# Wireless Caching: Making Radio Access Networks More than Bit-Pipelines
## 2 Proactive Service: Gains, Costs, and Needs
### Caching Gains: A Time-Domain Perspective  
Revisiting wireless caching gains is critical in the optimization of cache-empowered RANs. Those gains reported in the existing literature can be mainly cast into three main categories. * Caching enables physical layer multicasting [12]. In theory, caching is capable of serving infinitely many users with a common request, thereby making RANs scalable. Classic on-demand transmission can seldom benefit from the multicasting gain because users seldom ask for a common message simultaneously. Aligning common requests in the time domain may, however, cause severe delay and damage Quality-of-Service (QoS). Proactive caching brings a solution to attain multicasting gain without inducing delay in data services. Even when users have different requests, judiciously designed coded caching strategies [13; 14] allow RANs to enjoy the multicasting gain. * Caching extends the tolerable transmission time, thereby bringing spectral efficiency (SE) or energy efficiency (EE) gains. Lazy scheduling [15], opportunistic scheduling [16; 17], opportunistic spectrum access (OSA) [18], and energy harvesting (EH) [19] may increase the SE and EE. However, their applications are usually prohibited or limited due to their random transmission delay. Caching enables content transmission before user requests and hence substantially prolongs the delay tolerance. * Caching enables low-complexity interference mitigation or alignment [10]. It is well known that a user can cancel a signal's interference based on prior knowledge about the message that the signal bears. Classic successive interference cancellation (SIC) decodes the interference first by treating the desired signal as noise. However, SIC can suffer from high complexity and error propagation. By contrast, caching provides reliable prior knowledge on the interfering signal, which significantly reduces the complexity of interference cancellation. We present a unified time-domain framework to trace the common root of caching gains revealed by a large body of research. In many wireless techniques, there exists a fundamental tradeoff between transmission time and radio resource efficiency, as summarized in Table 1. In practice, however, trading the transmission time for energy efficiency (EE) \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Transmission Techniques** & **Application Scenarios** & **How Is SE or EE Gain Attained?** & **Why Is Delay Increased?** \\ \hline Lazy Scheduling & Additive White Gaussian Noise Channels & Due to the convexity of Shannon capacity, EE is a decreasing function of the transmission power/rate. & Low data rate \\ \hline Opportunistic Scheduling & Fading Channels & EE/SE is increased by time domain water-filling, or simply accessing good channels only. & Channel states remaining poor \\ \hline Opportunistic Spectrum Access & Secondary Users & SE is increased by sensing and accessing idle timeslots or spectrum holes. & Spectrum remaining busy \\ \hline Energy Harvesting & Renewable Energy Powered BSs/UEs & The renewable energy harvested from solar panels, wind turbines, or even the RF environment helps to save grid power. & No or little energy harvested \\ \hline Physical-Layer Multicasting & Users with Common Interests & Multiple users located in the same cell are served by broadcasting a common signal to them. & Waiting for common requests \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tradeoff Between the Transmission Time and SE/EE. or spectral efficiency (SE) is mostly prohibited due to the reactive service mode. When content items are transmitted upon user requests, an increased transmission time results in a large delay, leading to poor QoS. Caching is a straightforward solution prolonging the transmission time by allowing RANs to push files to the network edge or a user's device before they are requested. In the reactive mode, a user's request time is usually regarded as the time origin of scheduling. Hence content placement is launched on the negative time-axis.