Information delivery through broadcasting in satellite communication networks

Satellite broadcast is an important candidate for large-scale multimedia information distribution due to the inherent wide-range multicasting capability of satellites and the asymmetry of satellite communications (high bandwidth downlink, limited bandwidth uplink) that matches nicely the information flow asymmetry in multimedia applications. We consider a data broadcasting model that is encountered in most asymmetric satellite communication environments. The problem of scheduling the data broadcast such that average response time experienced by the users is low is considered. In a push-based system, where the users cannot place requests directly to the server and the broadcast schedule should be determined based solely on the access probabilities, we formulate a deterministic dynamic optimization problem, the solution of which provides the optimal broadcast schedule. Properties of the optimal solution are obtained and then we propose a suboptimal dynamic policy which achieves average response time close to the lower bound. In a pull-based system where the users may place requests about information items directly to the server, the scheduling can be based on the number of pending requests for each item. Suboptimal policies with good performance are obtained in this case as well. If a user has local memory, it can alleviate its access latency by selectively prefetching the items from the broadcast and storing them in the memory. A good memory management strategy can substantially reduce the user's access latency. An optimal memory management policy is identified, that minimizes the expected aggregate latency. Memory update strategies with limited look-ahead are presented as implementable approximations of the optimal policy as well. We also consider the problem of joint broadcast scheduling and user's cache management and propos a joint optimization scheme which can achieve the performance up to 40% better than the existing non-joint approach.