# 5G-NR Physical Layer-Based Solutions to Support High Mobility in 6G Non-Terrestrial Networks
## 2 Related Works  
The NR-NTN research is not limited to 3 GPP reports. [PERSON] et al. presented a novel network architecture for an integrated nanosatellite-5G system operating in the millimeter wave (mmWave) domain [12], whereas [13] identified the most promising satellite networking configurations and discussed some design trade-offs in this domain. The authors in [14; 15; 16] describe the physical layer communication research beyond 5G because of the numerous extra features provided to NR, PHY abstraction of NR-based systems is a difficult task. The variable number of resource blocks (RBs) and OFDM symbols that can be allocated to each user in NR results in a significant increase in the number of supported transport block sizes (TBSs) [15]. The introduction of LDPC coding with varying lifting sizes and two different base graph types complicates the code block segmentation procedure at PHY [16]. Furthermore, NR supports modulation orders up to 256-QAM and a wide range of MCS table combinations, which is discussed in 6G evolution [17]. Moreover, 6G researchers are working on constructing non-terrestrial networks (NTN) to enable ubiquitous and high-capacity global connections [18]. Unlike previous wireless generation networks, which have traditionally been designed to provide connectivity for a quasi-bi dimensional space, 6G envisions a three-dimensional (3D) heterogeneous architecture in which terrestrial infrastructures are supplemented by non-terrestrial stations such as unmanned aerial vehicles (UAVs), high-altitude platforms (HAPs), and satellites [19]. These features can provide on-demand, cost-effective coverage in densely populated and unserved areas and ensure backhauling, support for high-speed mobility, and high-throughput hybrid multiple services. In particular, the significance of NTN has been acknowledged in standard activities [20]. [PERSON] et al. [2] reviewed NTN research activities, described the features and enabling technologies of NTNs in the 6G landscape, and focused on the problems in the field that remain open for research. For example, they examined how different networking configurations affect the performance of an NTN scenario in which aerial/space vehicles use millimeter wave (mmWave) frequencies to offer access connectivity to on-the-ground mobile terminals. The proposed DM-RS design sets by [PERSON] et al. [21] are intended to provide accurate channel estimates in a very high-mobility environment of up to 500 km/h while limiting the DM-RS overhead to an acceptable degree in the terrestrial networks. Extensive link-level simulations from [22] show that even with significant Doppler shift/spread, the proposed DM-RS design sets can achieve satisfactory block error rate (BLER) and spectrum efficiency performance. DM-RS patterns applicable to the 3 GPP 5G-NR high-speed train (HST) scenario with MIMO schemes and TDL channel model are also discussed in [21; 22]. [PERSON] et al. discussed initial access, mobility management, and linear cell designs in [23], in which they discuss the evaluation parameters for the 3 GPP-adopted HST scenario and the proposed physical layer technologies for HST applications, including numerology and reference signal designs. For high-speed rail broadband mobile communications, the authors in [24] propose massive MIMO and beamforming-based methods that support high mobility. The analysis of flexible numerologies of 5G-NR to support integrated-satellite-terrestrial networks with low earth orbit (LEO) satellites are described by authors in [25]. Furthermore, there are multiple case studies of NTN deployments in other countries, as well as efforts by international organizations and NGOs for NR-NTN evolution in 6G [7]. On the other hand, despite recent research, some questions remain unanswered for proper network architecture [26].