Content Prefetching of Mobile Caching Devices in Cooperative D2D Communication Systems

The increase of mobile traffic volume in a wireless network using high carrier frequency can lead to coverage and backhaul bottleneck issues, and these issues can be solved by the use of device-to-device (D2D) caching technology that installs a content cache on a device and uses D2D communication to supply contents to other devices. In this paper, a special device that can supply contents using D2D communication to other devices is called a helper, while a device that can receive contents from a helper is called user equipment (UE). Even though a helper may have a large storage capacity compared to a UE, the cache capacity of a helper is still limited, making it difficult to obtain a sufficient hit ratio using a caching scheme based on the long-term content popularity of a large number of UEs while considering the distant future. In this paper, to solve the problem of the conventional D2D caching system, we suggest instead updating the cache contents based on the short-term content preference of individual user for the near future. In the proposed caching scheme, a UE in an area with insufficient data supply predicts the content that will be consumed in the near future. This information is then passed to a mobile helper moving along a predictable travel path, such as a bus and a self-driving car that is passing through or will pass though an area with sufficient data supply. Then, the mobile helper prefetches the contents and moves to update the cache of the nomadic or fixed helper near the UE. The simulation results show that content prefetching considering each UE and the near future can achieve better performance than content caching considering a large number of UEs and the distant future.