  Distributed massive multiple-input multiple output (mMIMO) system for low
earth orbit (LEO) satellite networks is introduced as a promising technique to
provide broadband connectivity. Nevertheless, several challenges persist in
implementing distributed mMIMO systems for LEO satellite networks. These
challenges include providing scalable massive access implementation as the
system complexity increases with network size. Another challenging issue is the
asynchronous arrival of signals at the user terminals due to the different
propagation delays among distributed antennas in space, which destroys the
coherent transmission, and consequently degrades the system performance. In
this paper, we propose a scalable distributed mMIMO system for LEO satellite
networks based on dynamic user-centric clustering. Aiming to obtain scalable
implementation, new algorithms for initial cooperative access, cluster
selection, and cluster handover are provided. In addition, phase shift-aware
precoding is implemented to compensate for the propagation delay phase shifts.
The performance of the proposed user-centric distributed mMIMO is compared with
two baseline configurations: the non-cooperative transmission systems, where
each user connects to only a single satellite, and the full-cooperative
distributed mMIMO systems, where all satellites contribute serving each user.
The numerical results show the potential of the proposed distributed mMIMO
system to enhance system spectral efficiency when compared to noncooperative
transmission systems. Additionally, it demonstrates the ability to minimize the
serving cluster size for each user, thereby reducing the overall system
complexity in comparison to the full-cooperative distributed mMIMO systems.
