Optimal packet scheduling in tree-structured LEO satellite clusters

Clusters of mobile (LEO) satellites, flying at a nongeostationary orbit, have been recently proposed, designed and made operational for achieving a global coverage of roaming users. An example of these is Teledesic. The satellites of these clusters are equippedwith a radio frequency switch and connected via intersatellite links to form a specific topology. Uncoordinated packet transmission in these systems may result in collisions. Collided packets must be retransmitted, with an obvious degradation in performance, in terms of both bandwidth usage and delay. This performance degradation can be overcome by a proper scheduling of the packets. In this paper, we consider optimal (namely minimum length) packet scheduling in LEO satellite clusters. We present a preprocessing algorithm which, together with the optimal, polynomial time, packet scheduling algorithm for isolated satellites, produces an optimal schedule in tree-connected clusters. The overall algorithm inherit the time complexity of the optimal one for isolated systems, and is O(N4) for a cluster serving N roaming users.