Private Coded Caching in Hierarchical Satellite Networks

Coded caching was introduced into single-layer satellite network, which brought great improvement on transmission rate. We extend coded caching to the two-layers satellite networks, where Low Earth Orbit (LEO) satellites are moving. Employing time-division method, we propose a baseline scheme by applying the hierarchical coded caching scheme of static scenario in each timeslot. In baseline scheme, geostationary earth orbit satellites help LEO satellites recover certain files first. Cache redundancy between LEO satellites is required in this process. However, these files are cached by other LEO satellites who can serve these files in other timeslots without recovery. Motivated by this observation, we propose a new achievable scheme that utilizes the movement of satellites and eliminates the cache redundancy between LEO satellites. Compared with baseline scheme, it reduces the rate of first layer without increasing the rate of second layer. To further investigate the impact of caches of LEO satellites on transmission, we consider the private delivery problem in our model. Any user could be served by multiple satellites in one cycle, satisfying the Private Information Retrieval (PIR) setting. We propose a private coded caching scheme by concatenating the construction of PIR-queries and coded messages of coded caching. The cost of delivering a message privately is inversely proportional to the number of servers caching this message. Cache redundancy determines the number of satellites caching the same messages. Large cache redundancy between LEO satellites is beneficial to reduce the rate of second layer in private delivery. Numerical results present the superior performance of the proposed schemes.