5G-NR GNSS Independent Time and Frequency Synchronization in NTN Scenarios

Abstract

In recent years, we observed a growing interest to re-use the terrestrial 5G-NR waveform over satellite or non-terrestrial networks (NTNs). The 5G-NR radio waveform is based on multi-carrier orthogonal frequency division multiplex (OFDM) modulation while existing satellite waveforms like DVB-S2X and DVB-RCS are based on square root raised cosine (SRRC) pulse-shaped single carrier transmissions. Next to some clear advantages of OFDM for multi-channel transmissions, there are some drawbacks. A critical disadvantage is the higher sensitive of the OFDM waveform to time and frequency misalignment of the terminal. Terminal misalignment in time or frequency will result in non-orthogonality of the different subcarriers and cause intersymbol interference (ISI) and intercarrier interference (ICI). 5G-NR thus requires tighter time and frequency synchronization than existing satellite waveforms. The 5G-NR standardization body (3GPP) has put forward the use of global navigation satellite systems (GNSS) and satellite orbit information to pre-compensate the mobility effects. The aim of the present solution is to avoid the GNSS information and satellite orbit data dependency for reliable communication. This paper will first review the critical synchronization accuracy needs and the Doppler requirements for 5G-NR NTNs. Next, we explore different time and frequency tracking options highlighting their shortcomings, and finally, we propose a novel solution based on higher order closed loop tracking.