Subspace Interference Alignment for multibeam satellite communications systems

In this work, we examine the feasibility of achieving Subspace Interference Alignment (SIA) in a multibeam satellite interference uplink channel. The channel is assumed to be Line-Of-Sight where link is a single physical path with complex gain and a large fixed delay (e.g. GEO satellite). SIA achieves interference-free degree of freedom in cellular networks owing to the rich scattering present in the terrestrial channel. In this paper, we show that it is also possible to achieve interference-free degree of freedom in the multibeam satellite system. We propose the geometrical arrangement constraints for both, the beam and the satellite, for full per-station interference-free transmission using finite dimensions. The theoretical potential is shown to be very large compared to the current schemes, which are based on orthogonalization and reuse. However, our results show the necessity of large bandwidth scaling with increase in number of stations. Moreover, we analyze the required scaling in a realistic scenario, which turns out to be unfeasible. Consequently, SIA is theoretically advantageous for a LOS satellite multibeam but practical schemes exploiting alignment in other domains should be developed.