Downlink Beamforming Strategies for Interference-Aware NGSO Satellite Systems

An important part of designing non-geostationary satellite orbit (NGSO) constellations is to ensure that they can co-exist with the legacy geostationary satellite orbit (GSO) systems in terms of interference levels. According to the current radio regulations defined by the International Telecommunication Union (ITU), NGSO systems shall not cause unacceptable interference to GSO systems. However, the traditional interference avoidance methods considered for the NGSO systems impact the coverage and overall service quality of their users. Embracing the trends of equipping NGSO satellites with phased array antennas, this paper investigates downlink beamforming strategies to improve NGSO users’ service and mitigate the interference caused at GSO systems, ensuring seamless co-channel operation for NGSO and GSO network systems. Our proposed aggregate interference-constrained (AIC) beamforming optimization, limits the co-channel interference at GSO ground stations while minimizing the NGSO satellites’ transmitted power. The AIC beamformer is designed under average channel state information (aCSI) to consider a practical scenario in NGSO satellite communications where acquiring perfect instantaneous channel information is challenging due to satellite mobility and long propagation roundtrip. Furthermore, for a more general scenario, this study considers robust AIC beamforming to account for the uncertainty in GSO ground stations’ location when suppressing interference. Numerical results evidence the benefits of our proposed method ensuring service quality and providing the lowest probability for unaccepted interference levels at GSO ground stations among other benchmarks.