Diversity Combining Scheme for Time-Varying STBC NGSO Multi-Satellite Systems

Current trends in non-geostationary orbit (NGSO) satellite communications pose the challenge of attaining high spectral efficiency with low-complexity user terminals (UTs). In this context, this letter investigates the downlink/forward transmission of linear space-time block coded (STBC) signals through multiple geographically distributed NGSO satellites. As such, by combining the signals through multiple satellites, STBC provides a diversity advantage, which helps in combating the adverse effects of fading and interference at the UT. The main challenge of implementing such a scheme is the lack of time synchronization and the interference effect through a multi-satellite system toward the UT. To address this issue, we propose a digital receiver structure with multiple branches specifically designed to lock onto each satellite of the multi-satellite system, thus providing a solution for simple UT without requiring additional complexity in terms of radio-frequency components. Herein, we consider the zero-forcing (ZF) based diversity combining scheme while deriving the signal-to-interference-plus-noise ratio (SINR) expression at UT. Our results demonstrate the SINR improvement brought by the ZF combiner.