Coded Caching for Time-Varying Files Popularities and Asynchronous Delivery

Data communication has seen exponential growth recently, and it currently dominates wireless communication. As a result, proactive caching was developed to minimize peak traffic rates by storing content, in advance, at different nodes in the network. We consider proactive caching for a broadcast wireless network with one central hub such as a satellite (ST) and
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associated mobile units (MUs) such as mobile mini-ground stations or end users. The ST has a library of files, and the MUs demands are assumed to be limited to this library, while the popularity of the library files changes over time. We assume that the MUs demands arrive at different times, and hence, asynchronous file delivery is necessary. We propose a new scheme that minimizes the files delivery sum rate and show that we can use the file delivery messages to proactively and constantly update the MU finite caches. We show that this mechanism reduces the downloaded traffic of the network. The proposed scheme uses index coding to jointly encode the delivery of different demanded files with the cache updates to other MUs to follow the changes in the files popularities. An offline optimization of the delivery sum rate of the scheme is proposed, where it requires knowledge of the files popularities across the whole transmission period. In particular, the problem is formulated as a linear program and the optimal caching is obtained numerically. Moreover closed form solutions to two special cases are derived and a lowerbound to the achievable delivery sum rate is developed. Numerical results show the benefits of the proposed scheme over conventional caching schemes, in terms of reducing the delivery sum rate.