  This paper deals with the problem of precoding in multibeam satellite
systems. In contrast to general multiuser multiple-input-multiple-output (MIMO)
cellular schemes, multibeam satellite architectures suffer from different
challenges. First, satellite communications standards embed more than one user
in each frame in order to increase the channel coding gain. This leads to the
different so-called multigroup multicast model, whose optimization requires
computationally complex operations. Second, when the data traffic is generated
by several Earth stations (gateways), the precoding matrix must be
distributively computed and attain additional payload restrictions. Third,
since the feedback channel is adverse (large delay and quantization errors),
the precoding must be able to deal with such uncertainties. In order to solve
the aforementioned problems, we propose a two-stage precoding design in order
to both limit the multibeam interference and to enhance the intra-beam minimum
user signal power (i.e. the one that dictates the rate allocation per beam). A
robust version of the proposed precoder based on a first perturbation model is
presented. This mechanism behaves well when the channel state information is
corrupted. Furthermore, we propose a per beam user grouping mechanism together
with its robust version in order to increase the precoding gain. Finally, a
method for dealing with the multiple gateway architecture is presented, which
offers high throughputs with a low inter-gateway communication. The conceived
designs are evaluated in a close-to-real beam pattern and the latest broadband
communication standard for satellite communications.
