  We consider the use of multiple co-located satellites to improve the spectral
efficiency of broadcast transmissions. In particular, we assume that two
satellites transmit on overlapping geographical coverage areas, with
overlapping frequencies. We first describe the theoretical framework based on
network information theory and, in particular, on the theory for multiple
access channels. The application to different scenarios will be then
considered, including the bandlimited additive white Gaussian noise channel
with average power constraint and different models for the nonlinear satellite
channel. The comparison with the adoption of frequency division multiplexing
and with the Alamouti space-time block coding is also provided. The main
conclusion is that a strategy based on overlapped signals is the most
convenient in the case of no power unbalance, although it requires the adoption
of a multiuser detection strategy at the receiver.
