Exploiting Transmission and Caching Diversity in Cache-Enabled User-Centric Network: Analysis and Optimization

User-centric network (UCN) organizes a dynamic base station group (BSG) for each user equipment (UE), consisting of potential base stations (BSs) with caching ability. However, the performance gain brought by the increased BS diversity of potential BSs has not yet been analyzed. Using stochastic geometry, the analytical expression of average outage probability is derived for the cache-enabled UCN, where a practical scenario is considered wherein inter-cache data transfer cannot be performed between BSs and the serving BS must have the requested content. Specifically, the gain of BS diversity within the BSG is characterized, where the UE benefits from transmission resource diversity of the BSG by always connecting to the BS with the best coverage performance among those with the required content, and the increased caching content diversity is utilized by searching the content within the BSG. Based on the analysis, to minimize the lower bound of outage probability, closed-form optimal caching distribution is obtained which is more uniform than the request distribution. Outage probability of the cache-enabled UCN is decreased by 42.1% compared to the case where the UE connects to the nearest BS with the required content. Moreover, the optimal caching distribution decreases outage probability by 28.5% compared to the policy of caching the most popular contents.