The synergistic gains of coded caching and delayed feedback

Abstract—In this paper, we consider the K-user cache-aided wireless MISO broadcast channel (BC) with random fading and delayed CSIT, and identify the optimal cache-aided degrees-offreedom (DoF) performance within a factor of 4. The achieved performance is due to a scheme that combines basic codedcaching with MAT-type schemes, and which efficiently exploits the prospective-hindsight similarities between these two methods. This delivers a powerful synergy between coded caching and delayed feedback, in the sense that the total synergistic DoFgain can be much larger than the sum of the individual gains from delayed CSIT and from coded caching. The derived performance interestingly reveals — for the first time — substantial DoF gains from coded caching, even when the (normalized) cache size (fraction of the library stored at each receiving device) is very small. Specifically, a microscopic  e??G can come within a factor of G from the interference-free optimal. For example, storing at each device only a thousandth of what is deemed as ‘popular’ content (   10??3), we approach the interference-free optimal within a factor of ln(103)  7 (per user DoF of 1=7), for any number of users. This result carries an additional practical ramification as it reveals how to use coded caching to essentially buffer CSI, thus partially ameliorating the urden of having to acquire real-time CSIT.