# 5G-NR Physical Layer-Based Solutions to Support High Mobility in 6G Non-Terrestrial Networks
## 4 Proposed 5G-NR-Based Solutions for High Mobility  
This paper proposes 5G-NR physical layer components for the downlink communication to meet high-mobility requirements in NTN. The flexible numerology is one of the essential features of the 5G-NR physical layer. Figure 2 shows the NR frame structure specified in 3 GPP TS 38.211 [19] for flexible numerology. The minimum subcarrier spacing is determined by the carrier frequency, phase noise, and Doppler effect [25]. A smaller subcarrier spacing would generate either high error vector magnitude (EVM) owing to phase noise or very high requirements on the local oscillator [32]. In high-mobility scenarios, excessively narrow subcarrier spacing can degrade the performance. The required cyclic prefix (CP) overhead and projected delay spread establish an upper limit for subcarrier spacing. Therefore, choosing a very high subcarrier spacing may result in high CP overhead. The channel bandwidth is determined by the OFDM modulator's maximum fast Fourier transform (FFT) size and subcarrier spacing [32]. Based on these factors, the subcarrier spacing should be robust to phase noise and the Doppler effect because of the high mobility and provide the appropriate channel bandwidth. In  
Figure 2: 5G-NR frame structure with flexible numerology. this paper, we propose adaptive numerology to support UE mobility of 500 km/h for NTN based on the optimization of sub-carrier spacing. The PDSCH is used to downlink user data, system information, paging, and some higher-layer information regarding user equipment to all connected devices within the covered area [8]. It is the primary data transmission channel in 5G-NR allocated for the users on an active and adaptable basis. The processing chain for link-level simulation is shown in Figure 3. In this processing chain, we configure the PDSCH transmitter and receiver in which the basic communication steps are followed by generating the data, PDSCH encoding, MIMO precoding, modulation, propagation channels such as the TDL channel model and noise addition, demodulation, pilot-aided perfect and DM-RS-based practical channel estimation and compensation, and LDPC decoding to obtain the BER, maximum achievable throughput, and spectral efficiency. The modulation schemes, such as QPSK and different orders of QAM (e.g., 16, 64, and 256), are used in NR-PDSCH. For NR data channels, a low-density parity-check (LDPC) coding scheme is used. The TDL channel model is utilized for the link-level performance analysis of the simulation. The single-path (1P) Rayleigh and Rician fading components are analyzed from the TDL channel model, designed by 3 GPP, where one tap is considered a single path [10]. Table 2 lists the simulation parameters for link-level performance analysis. This paper proposes solutions for high mobility based on the optimization of numerologies and the impact of various modulation and coding schemes (MCS), 3 GPP NR-NTN channel models, and MIMO/beamforming schemes with link-level simulation under pilot-aided-based perfect and DM-RS-based practical channel estimation at stationary UE and high mobility of 500 km/h. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Carrier Frequency & 5.9 GHz \\ \hline System Bandwidth & 100 MHz \\ \hline Sub-Carrier Spacing (SCS) & 15, 30, 60, 120 kHz \\ \hline Cyclic Prefix & Normal \\ \hline Modulation Scheme & QPSK,16,64,256-QAM \\ \hline Propagation Channel Model & 1-P Rayleigh, 1-P Rician, TDL-A, and TDL-E \\ \hline Coding Rate & 490/1024 LDPC \\ \hline Transmitting Antenna (Tx) & 1/2 \\ \hline Receiving Antenna (Rx) & 1/2/4/8 \\ \hline \multirow{2}{*}{Channel Estimation} & Perfect and Practical \\  & (DM-RS + PTRS) \\ \hline UE Mobility & Stationary UE and 500 km/h \\ \hline Beamforming Technique & Precoding \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Link-level simulation parameters. Figure 3: Link-level simulation model of the 5G-NR physical layer to support high mobility.