Hybrid Cache Placement for Physical-Layer Security in Cooperative Networks

Due to the broadcast nature of wireless transmission, traditional wireless two-hop relaying is vulnerable for eavesdroppers to overhear messages. In order to enhance the physical-layer security, this paper studies one cooperative network with caching relays to reduce the transmission links overheard by the eavesdropper. Relays are grouped into different clusters so that the secrecy outage probability of any cluster is better than the traditional two-hop transmission. Based on the secrecy outage probability of clusters and the groups of files, we propose a novel hybrid cache placement, which combines the traditional base station caching strategy, most popular content caching strategy and largest content diversity caching strategy to cache the popular contents. The performance analysis and optimal design results are provided on the two objective functions: the secrecy outage probability and average secrecy capacity. From these two objective functions, some interesting observations are reached: for the function of secrecy outage probability, we can always find an optimal value of most popular files to minimize the secrecy outage probability; and for the function of average secrecy capacity, we can also find an optimal value of most popular files to maximize the average secrecy capacity. The numerical and simulation results are finally demonstrated to verify the proposed studies.