Constellation Rotation for DVB Multiple Access Channels With Raptor Coding

In this paper we investigate a technique for increasing the capacity of a Digital Video Broadcasting-Return Channel via Satellite (DVB-RCS) system. In our previous work, we proposed a scheme consisting in adding a non-orthogonal interfering source to a DVB-RCS channel carrying data from an existing source. The inclusion of the new source does not cause bandwidth increase. The secondary source uses Raptor code. The destination will use successive decoding. It decodes the interfering source first, and after its removal, decodes the main source. In this paper, we generalize our work to allow possibility of decoding either the secondary source data (as in our previous work) or the main source data first. We investigate the performance and delay for each decoding scheme. Since the channels are non-orthogonal, it is possible that for some power allocation scenarios constellation points get erased. To address this problem, we use constellation rotation. The constellation map of the secondary source is rotated to increase the average distance between the points in the constellation resulting from the superposition of the main and interfering sources' constellations. Finally, we determine the optimum constellation rotation angle for the interfering source analytically and confirm it with simulations.