  Advancements in satellite technology have made direct-to-device connectivity
a viable solution for ensuring global access. This method is designed to
provide internet connectivity to remote, rural, or underserved areas where
traditional cellular or broadband networks are lacking or insufficient. This
paper is a survey providing an in-depth review of multi-satellite Multiple
Input Multiple Output (MIMO) systems as a potential solution for addressing the
link budget challenge in direct user-satellite communication. Special attention
is given to works considering multi-satellite MIMO systems, both with and
without satellite collaboration. In this context, collaboration refers to
sharing data between satellites to improve the performance of the system. This
survey begins by explaining several fundamental aspects of satellite
communications (SatComs), which are vital prerequisites before investigating
the multi-satellite MIMO systems. These aspects encompass satellite orbits, the
structure of satellite systems, SatCom links, including the inter-satellite
links (ISL) which facilitate satellite cooperation, satellite frequency bands,
satellite antenna design, and satellite channel models, which should be known
or estimated for effective data transmission to and from multiple satellites.
Furthermore, this survey distinguishes itself by providing more comprehensive
insights in comparison to other surveys. It specifically delves into the
Orthogonal Time Frequency Space (OTFS) within the channel model section. It
goes into detail about ISL noise and channel models, and it extends the ISL
section by thoroughly investigating hybrid FSO/RF ISLs. Furthermore, analytical
comparisons of simulation results from these works are presented to highlight
the advantages of employing multi-satellite MIMO systems.
