Combining techniques for the reception of signals from satellite constellations with path diversity at the User Terminals

The large-scale deployment of non‑terrestrial networks (NTNs) based on non‑geostationary orbit (NGSO) satellite constellations is transforming the design paradigm of satellite communication systems. The availability of multiple simultaneously visible satellites at the user terminal (UT) enables the exploitation of spatial diversity through cooperative transmission. However, practical realization of such techniques is severely constrained by symbol‑level time misalignment caused by differential propagation delays, satellite motion, and heterogeneous onboard clocks. These impairments fundamentally challenge the applicability of classical transmit‑diversity techniques, such as space–time block coding (STBC), which rely on strict synchronization assumptions that are rarely satisfied in realistic satellite environments.

This doctoral thesis investigates the feasibility of combining signals from distributed satellite transmitters at the user terminal under realistic timing, synchronization, and hardware constraints. The research focuses on UT‑assisted diversity combining strategies that do not rely on global channel state information at the gateway and that remain robust to symbol‑time misalignment. A comprehensive modeling framework is first developed to characterize waveform misalignment resulting from differential orbital dynamics and clock mismatch in low Earth orbit (LEO) constellations. Based on this model, a novel sample time misalignment estimation and distributed compensation architecture is proposed, combining coarse frame‑level synchronization with fine symbol‑time tracking using delay‑locked loop and Kalman‑filter‑based techniques for the estimation, and a distributed control loop between UT and satellite for the compensation.

Building on this synchronization framework, the thesis introduces a low‑complexity UT receiver architecture capable of combining STBC‑encoded signals under imperfect symbol‑time alignment. The proposed approach explicitly incorporates time‑misalignment information into the diversity‑combining process, enabling effective mitigation of inter‑symbol interference induced by asynchronous waveform arrival. An analytical formulation of the combining scheme is presented, along with an efficient numerical implementation that exploits block‑circulant matrix structures and Fourier‑domain processing to significantly reduce computational complexity.

The performance of the proposed algorithms is thoroughly evaluated through numerical simulations using DVB‑S2X waveforms under realistic satellite impairments, including Doppler shifts, power imbalance, and imperfect channel estimation. To validate practical feasibility, a real‑time hardware test-bed based on National Instruments (NI) universal software radio peripheral (USRP) platforms and field programmable gate array (FPGA) accelerated processing is developed. Experimental results demonstrate close agreement with theoretical and simulation outcomes, confirming that the proposed UT‑based diversity combining achieves stable signal‑to‑interference‑plus‑noise ratio (SINR) gains and robust operation even under symbol‑time misalignment.
Overall, this thesis advances the state of the art in cooperative satellite communications by demonstrating that practical transmit diversity can be achieved at the UT without stringent synchronization requirements at the gateway. The proposed techniques contribute to scalable, resilient, and hardware‑validated solutions suitable for future non‑terrestrial networks.