# 5G-NR Physical Layer-Based Solutions to Support High Mobility in 6G Non-Terrestrial Networks
## 1 Introduction  
In 6G networks, 5G technology is expected to support NTN with enhanced terrestrial coverage and provide upgraded services [1]. Air-to-ground networks, high-altitude platforms (HAPs), and satellite communication networks are all part of the NTN. NTN communications may become critical for 6G wireless to ensure enormous flexibility and the convergence of terrestrial and satellite networks [2]. Air-to-ground networks deploy ground stations that serve the same functions as base stations (BSs) in terrestrial mobile networks to ensure service consistency in order to provide airplanes with in-flight connections [3, 4] and to improve sweep coverage [5, 6]. Ground station antennas in an air-to-ground network are pointed upward, and ground station inter-site distances are substantially greater than in terrestrial mobile networks. Because of its inherent compatibility with air-to-ground networks, 3 GPP NTN research has thus far focused primarily on satellite communications networks [7]. The third-generation partnership project (3 GPP) is now working to standardize 5G new radio (NR) [8, 9]. New features and solutions, including service-based network architecture, two-layer (central unit and distributed unit) radio access network architecture, and air interface with flexible frame structure, have been employed in 3 GPP NR technical reports (TR) and specifications (TS) [1, 10, 11]. The work item for 3 GPP Rel-17 was established to define and assess solutionsin the field of NTN for NR, with a priority on satellite access [11]. Furthermore, the Rel-18 and Rel-19 study items have been recognized, indicating ongoing research. The physical downlink shared channel used by the 5G new radio to transmit user data is known as PDSCH. The reference signals for 5G-NR, namely the demodulation reference signal (DM-RS) and phase tracking reference signal (PT-RS), are related to PDSCH. According to TS 38.211 [8], these signals are generated inside the PDSCH allocation. In coherent PDSCH demodulation, the channel estimation is carried out by DM-RS. The PT-RS is implemented in 3 GPP 5G NR to address common phase error (CPE). The phase noise produced by local oscillators causes significant degradation in system performance. The inter-carrier interference (ICI) and CPE interference are both caused by it. With CPE, each subcarrier's rotation of a received symbol is identical. Subcarriers lose orthogonality because of ICI. The main function of PT-RS is to evaluate and reduce the impact of CPE on system performance. We contribute to the research on NR-based solutions to support transonic speed in the 6G-NTN through this paper. This paper is motivated by the interest that industry and academics have shown in NTN networks over the last decade. It serves as a key identifier and emphasizes the value added for 5G networks. The growing interest in NR-NTN has led to the publication of numerous surveys and tutorials. Nevertheless, the details of 3 GPP standardization are not addressed. Instead, the paper briefly discusses the role of NTNs, application scenarios, and networking challenges. The References provide in-depth coverage of the architectural and key technical concerns for 5G systems, including satellites; however, the discussion there is mostly centered on radio access network (RAN) issues. It is especially important to mention the work conducted by the 3 GPP Service, System Aspects (SA), and Core and Terminals (CT) working groups on the NTN support for high mobility. It is important to verify which set of 5G-NR physical layer specifications will support the high UE mobility communication in NTN must be determined. Our main contributions are as follows:  
* To better comprehend the function of NTN within the 5G new radio (NR) system, this paper introduces the NTN wireless system, highlights its key features based on 3 GPP technical reports, and discusses the current state of the art on NTN.