  This paper introduces a joint optimization framework for user-centric beam
selection and linear precoding (LP) design in a coordinated multiple-satellite
(CoMSat) system, employing a Digital-Fourier-Transform-based (DFT) beamforming
(BF) technique. Regarding serving users at their target SINRs and minimizing
the total transmit power, the scheme aims to efficiently determine satellites
for users to associate with and activate the best cluster of beams together
with optimizing LP for every satellite-to-user transmission. These technical
objectives are first framed as a complex mixed-integer programming (MIP)
challenge. To tackle this, we reformulate it into a joint cluster association
and LP design problem. Then, by theoretically analyzing the duality
relationship between downlink and uplink transmissions, we develop an efficient
iterative method to identify the optimal solution. Additionally, a simpler
duality approach for rapid beam selection and LP design is presented for
comparison purposes. Simulation results underscore the effectiveness of our
proposed schemes across various settings.
