PHY-Caching in 5G Wireless Networks: Design and Analysis

In this article, we propose a PHY-caching scheme for 5G wireless networks to achieve spectral efficiency gain over conventional caching schemes, which do not induce physical layer cooperation. By properly caching some popular contents at the BSs, the proposed PHY-caching can opportunistically transform the topology of the RAN from an unfavorable topology (e.g., relay or interference topology) into a more favorable MIMO broadcast topology and enjoy spectral efficiency gain. Specifically, we first introduce a generic cache model and cache-assisted PHY transmission, and show that PHY-caching can significantly enhance the spectral efficiency of the wireless network by inducing dynamic side information at the BSs. Then we discuss the design challenges and solutions of PHY-caching. We introduce maximum-distance-separable-coded caching and online cache content placement design as a potential solution. As a case study, we analyze the performance trade-off between PHY-caching at the BS and CN-caching at the gateway under capacity-limited CN-fronthaul. We show that even though PHY-caching covers fewer users than CN-caching and results in a lower cache hit rate, it is still efficient to do PHY-caching at the BS.