Outage Evaluation for STAR-RIS Assisted Satellite-UAV-Terrestrial NOMA Networks With Imperfect CSI

Due to some transmission delays and channel estimation errors, it is always difficult to acquire perfect channel state information (CSI) in practical communications. Therefore, in this study, taking the practical scenario of imperfect CSI into consideration, a simultaneously transmitting and reflecting-reconfigurable intelligent surface (STAR-RIS)-assisted satellite-unmanned aerial vehicle (UAV)-terrestrial non-orthogonal multiple access (NOMA) network is investigated, in which the UAV is assumed to be randomly located in a spherical cap space while the users are randomly located in an inner circular and an outer annular plane on the ground. Specifically, a satellite source first sends NOMA signals to a UAV relay, which then forwards them to both ground users via a STAR-RIS through reflection as well as transmission. In addition, considering that satellite-UAV link is subject to shadowed-Rician while other links are subject to Nakagami-m distribution, the cumulative distribution functions and probability density functions of channel gains in the presence of imperfect CSI are derived. Furthermore, in practical application scenarios, the positions of the UAV and both users may be randomly located in some regions. Therefore, three randomly distributed scenarios are considered: 1) The users are randomly located while the UAV's position is fixed; 2) The UAV is randomly located while both users are in fixed locations; and 3) Both are randomly located. Under those three scenarios, both analytical and asymptotic expressions of outage probability (OP) for two users as well as the system OP are derived using the stochastic geometry approach, and their accuracy is confirmed with Monte-Carlo simulations.