# 5G-NR Physical Layer-Based Solutions to Support High Mobility in 6G Non-Terrestrial Networks
## 3 Overview of Non-Terrestrial Networks
### Channel Models for NR-NTN  
The third-generation partnership project (3 GPP) released a technical report on channel models for frequencies ranging from 0.5 to 100 GHz [9] that would be employed in 5G communication systems to support the development of 6G technology services. The objective of this paper is to contribute to the design and analysis of physical layer performance for non-terrestrial networks using single- and multi-path channel models under high UE mobility. In 3 GPP technical reports, two channel models, such as clustered delay line (CDL) and tapped delay line (TDL), have been proposed to be utilized in link-level simulations [1; 8; 9; 10]. The CDL is a channel model used when the received signal consists of several distinct delayed clusters. Each cluster consists of many nodes in the network with the same delay but varying angles of departure (AOD) and angles of arrival (AOA). CDL channel models expand TDL profiles and are developed for 3D channels. There are five different CDL profiles available. There are three CDL models applicable to NLOS: CDL-A, CDL-B, and CDL-C. CDL-D and CDL-E are CDL models for the LOS environments [9]. The generation of the azimuth angles of departure and arrival is the first stage in creating the channel coefficients for the CDL profiles. Therefore, the arrival angles for the \(m\)th ray of the \(n\)th cluster can be determined as [8; 9]:  
\[\varnothing_{n,m},AoA=\varnothing_{n},AoA+C_{ASA}\;\alpha_{m} \tag{1}\]  
The azimuth angle of arrival (\(AoA\)) is defined by \(\varnothing_{n},AoA\), and the azimuth spread of arrival angle is represented by \(C_{ASA}\). The ray offset angle within a cluster is defined by \(\alpha_{m}\). Similarly, the departure angles for the \(m\)th ray of the \(n\)th cluster can be determined as [8; 9]:  
\[\varnothing_{n,m},AoD=\varnothing_{n},AoD+C_{ASD}\;\alpha_{m} \tag{2}\]  
The azimuth angle of departure (AoD) is represented by \(\varnothing_{n},AoD\), and the cluster-wise Root Mean Square (RMS) azimuth spread of the departure angle is defined by \(C_{ASD}\). The TDL channel model offers five different profiles depending on the environment scenario. Present profiles for non-line-of-sight (NLOS) scenarios are TDL-A, TDL-B, and TDL-C, whereas TDL-D and TDL-E are the corresponding LOS profiles [9]. The channel impulse response of the TDL channel model is specified as:  
\[H(t,\tau)=\sum_{k=1}^{m}a_{k}(t)\delta(\tau-\tau_{k}) \tag{3}\]The amplitude at the \(\tau_{k}\) delay for the \(k\)th tap is denoted as \(a_{k}\)(t). Except for the TDL-C profile, which has 24 taps, all designed NLOS profiles contain 23 taps. The TDL-D and TDL-E are LOS profiles containing 13 and 14 taps, respectively, with a Ricean fading distribution for the first tap. The parameters for the normalized latency of each profile and the relative power for each tap can be found in [8; 10]. Doppler spectra for each tap are characterized by a classical (Jakes) spectrum shape and a maximum Doppler shift \(f_{D}\). \[f_{D}=|\overline{v}|/\lambda_{0} \tag{4}\]  
The Doppler spectrum for those taps also contains an amplitude peak, resulting in a fading distribution with the required K-factor at the Doppler shift \(f_{s}=0.7f_{D}\)[10]. In order to select the propagation characteristics for a high-mobility scenario, we have utilized the tapped delay line (TDL) channel model developed by 3 GPP in [1; 9] for our link-level simulation. Since our aim is to identify the impact of the different DMRS symbol patterns and the MIMO configurations on the high UE mobility, we have utilized the TDL channel model. In [8; 9; 21; 22] TDL-A for NLOS and TDL-E for LOS models are recommended for the evolution of DMRS of the 3 GPP 5G-NR high-speed train scenario, where the Rician K-factor is 13.3 dB. In these profiles, the TDL channel offers the delay spread as a parameter to scale the path delays, which is a scaling delay spread of 10 ns. Using a shorter delay spread for LOS applications and longer delay spreads for NLOS paths is recommended by [9].