Random access procedure in 6G Non-Terrestrial Networks

5G/6G Non-Terrestrial Networks (NTN) extend coverage beyond terrestrial infrastructures and provide connectivity in remote scenarios. In this context, the initial access phase is particularly challenging, as large propagation delays, satellite mobility, and Doppler effects might significantly affect uplink synchronization and the reliability of PRACH detection. In 3GPP Release 17 NR NTN, uplink synchronization relies on open-loop pre-compensation by the UE using broadcast timing and ephemeris information, followed by network-side refinement. The accuracy of this procedure depends on the validity duration of the provided parameters, which determines the level of residual timing and frequency errors. This thesis examines the effect of these residual impairments on PRACH detection performance in single and multi-users NTN scenarios. The analysis focuses on residual timing and frequency offsets after UE-side pre-compensation and compares two detector implementations: the MATLAB-based receiver and the detector provided by the srsRAN Project. The study uses a simulation framework that reproduces PRACH generation, propagation delay, timing pre-compensation, residual frequency offset and noisy channel conditions. The results show that detector performance depends on the adopted PRACH format and the magnitude of the residual synchronization error. In general, reliable detection is still possible in NTN scenarios when timing and frequency pre-compensation remain accurate. The analysis also highlights the role of validity duration as a trade-off between synchronization accuracy and update frequency at the UE side. Overall, the work suggests that existing PRACH formats and detection mechanisms can still be effectively used in NTN systems, provided that timing-related parameters and access procedure windows are carefully adapted to the satellite environment.