Distributed cohesive radio systems for spaceborne applications

During the last decade, the use of small satellites has revolutionized the field of space exploration and communications. This has opened up new possibilities, such as the feasibility of grouping them into Cohesive Distributed Satellite Systems (CDSSs). A CDSS is a multi-satellite configuration that appears as a single solid entity from an external perspective, which includes data reception, processing, and transmission operations. This entails improving several DSS applications such as Earth observation, geolocation, navigation, imaging, and communications. The synchronization of CDSSs involves precisely aligning time, frequency, and phase among multiple satellites, which is a significant challenge due to the inherent characteristics of space-based environments. For instance, the spacecraft mobility, the round-trip delay, and the resource constraints make the synchronization of DSSs more challenging than its equivalent in wireless terrestrial networks. However, it is simultaneously an unavoidable challenge for future space communications. This requirement does not only apply to small satellites DSS but also to avoid interference in crowded orbits and enable the federated satellites’ system paradigm.
This thesis aims to identify the technical synchronization requirements and design the synchronization and coordination techniques to perform cohesive transmission in a DSS. First, we studied the state-of-the-art synchronization techniques and analyzed their feasibility for DSS. Additionally, we summarized other methods related to the synchronization of DSS, such as inter-satellite ranging and positioning. Then, we considered a first approximation to the problem, assuming accurate time synchronization and relative positioning among the satellites in a DSS. This problem is equivalent to synchronizing the local oscillators’ phase in a precoding-enabled multi-beam satellite system.
One of the most significant synchronization impairments for implementing CDSSs is the phase noise of the LOs in different spacecraft. In this regard, the two-state phase noise model was implemented and integrated into the channel emulator of the MIMO end-to-end satellite emulator, which allowed us to validate the results included in this thesis. Next, we analyzed the impact of the phase errors and uncertainties in operating a precoded forward link satellite communication system. We formally demonstrated that the uplink phase variations affect precoding performance even when all the LOs share a single frequency reference.
Additionally, we identified the individual contributions of each system element to the overall synchronization uncertainties in practical precoding implementations. Besides, for linear and non-linear precoding, we formally demonstrated that the UTs can track slow time variations in the channel if they equally affect all the beams. The compensation loop to mitigate these impairments was designed, implemented, and integrated into the GW of the MIMO end-to-end satellite emulator. The solution is a closed-loop algorithm that uses the periodical channel phase measurements sent to the GW by the UTs as part of traditional precoding implementations. The proportional-integral controller included in the GW calculates the compensation phase required to align all the beams to the phase of the designated reference beam. Besides, we compared different approaches to combine the channel phase estimations obtained from the UTs using the amplitude of the estimated channel and the UT’s thermal noise. The compensation loop and the combining estimations hardware implementations were used in real-time experiments to assess the feasibility of the precoding technique for GEO satellite systems.