# 5G-NR Physical Layer-Based Solutions to Support High Mobility in 6G Non-Terrestrial Networks
## 4 Proposed 5G-NR-Based Solutions for High Mobility
Channel equalization is one of the most challenging issues in wireless communication because signal propagation is affected by various factors, including path loss, phase shifts, delay spread, fading, and Doppler spread. The latter is particularly problematic in mobile communication systems because it can alter the frequency of the transmitted signal significantly on the receiver side. This paper analyzes the impact of the channel estimation method on high mobility. The pilot-aided perfect channel estimation and DM-RS-based practical channel estimation [21] are used in this paper. The transmission of pilot tones within the OFDM symbols with previously determined amplitudes and phases at the receiver side is one of the techniques used in OFDM systems for channel estimation. The data and the pilot subcarriers will have identical amplitude and phase variations while transmitting the composite OFDM signal on the channel. On the other hand, it is possible to estimate the channel characteristics in the frequency domain at these pilot frequencies because of prior knowledge about pilot levels prior to the advent of the channel effect. Understanding some aspects of the channel transfer function makes it easier to estimate the remainder of the transfer function at the desired data sub-carrier frequencies, which greatly reduces the channel effect and improves reception quality. The 3 GPP TS 38.211 [19] specifies the mapping types A (slot-wise) and B (non-slot-wise) for PDSCH symbol allocation. The first DM-RS symbol for mapping type A can be found at position 2 or 3 in a slot, which is denoted as \(I_{0}\). In contrast, the duration of OFDM symbols is denoted as \(I_{d}\) in Figure 4, which is defined as the number of symbols between the first and last OFDM symbol of the slot. Fast decoding is possible because of the front-loaded DM-RS, allotted in the first position of the slot [18]. This paper suggests using front-loaded DM-RS-based channel estimation with a denser DM-RS symbol pattern to support high mobility in NR-NTN evolution. A significant performance impact will be experienced if only a front-loaded DM-RS is allocated, especially in high-mobility applications where the channel is sensitive to time-domain oscillations [18]. As a result, it is crucial to allocate more DM-RS symbols after the front-loaded symbols. As a result, the use of more DM-RS symbols is suggested to produce a denser DM-RS pattern, as shown in Figure 4. As a result, increasing time-domain DM-RS density can also result in an increase in DM-RS overhead and degradation in spectral efficiency [18]. Figure 4: 5G-NR sub-frame with DM-RS symbol configuration to support high mobility. The main objective of DM-RS is to estimate the channel coefficient for coherent detection. The design of DM-RS is closely connected to the channel characteristics. The channel fluctuates more rapidly in the frequency domain. In that situation, the DM-RS density within the frequency domain must be raised, resulting in a low channel coherence bandwidth [18]. This front-loaded DM-RS architecture is particularly useful for minimizing decoding delay when there is high mobility and the channel coherence time is longer than the period of the front-loaded DM-RS. On the other hand, if only the front-loaded DM-RS is assigned, link performance may degrade as UE speed increases because of the shorter channel coherence time. Figure 4 shows the DM-RS configuration used in practical channel estimation. The additional three DM-RS symbols are used to develop a denser pilot pattern to support high mobility.