Performance Analysis of Cooperative Caching and Transmission Diversity in Cache-Enabled UAV Networks

In existing research of cache-enabled UAV networks, cell association is content-centric regardless of UAV line-of-sight (LOS)/non-line-of-sight (NLOS) channel dynamics, and the fixed content distribution hinders system performance. This paper proposes user-centric cooperation for cache-enabled UAV networks to exploit both contents and transmission diversity, and optimizes the probabilistic caching distribution to minimize system outage probability considering UAV interference topology dynamics. Specifically, user-centric UAV group (UAVG) is formed for each user to increase both aggregated cache size and transmission resources, considering UAVs can't realize inter-nodes cache file transform due to limited wireless backhaul. Users will search the required content within the UAVG and associate to the cache hit UAV that provides the strongest signal-to-interference ratio (SIR), thus the intrinsic gain of transmission diversity can be achieved. If the required content can't be successfully transmitted by UAVG, the macro base stations (MBSs) will help transmit the target content. In addition, based on analytical results, the optimal content distribution (OCD) probability is optimized by Lagrange multiplier, thus the tradeoff between content diversity gain and limited cache size is exploited. Analytical results show that, compared with caching the most popular contents policy, our proposed OCD model can decrease system average outage probability by 44.8%.