y, if the channel varies faster in time-domain (i.e., shorter channel coherence time), denser DM-RS allocation across time is required. After determining frequency/time-domain DM-RS densities, the DM-RS loca- tions in the time-frequency resource grid should be considered. Assuming stationary channel conditions, uniform DM-RS allocation in both frequency and time-domain is preferred for minimizing interpolation error and reducing implementation complexity. Since no user data is transmitted by DM-RS per se, allocating DM-RS with a proper density is required to maximize the throughput. In NR, a front-loaded DM-RS structure is used as a baseline to achieve low-latency decod- ing (see Fig. 4.2). In the time-frequency resource grid, the front-loaded DM-RS can be located just after the control region, followed by data region. As soon as channel is 418 Chapter 4 Single-symbol Double-symbol Slot duration Slot duration Transmission duration Transmission duration Figure 4.2 Comparison of DM-RS Type A and Type B mappings [6]. estimated based on the front-loaded DM-RS, the receiver can coherently demodulate data in the data region. The front-loaded DM-RS structure is particularly advantageous in decoding-latency reduction for low-mobility scenarios where channel coherence time is lon- ger than the duration of the front-loaded DM-RS. However, allocating only the front-loaded DM-RS can degrade the link performance at higher UE speeds (i.e., channel coherence time becomes shorter). Although the channel information in the data region can be obtained by interpolation, the channel information accuracy diminishes with higher mobility. Therefore, we consider the front-loaded DM-RS patterns with 2 X and 4 X time-domain densities as shown in Fig. 4.3 [70]. To support high-speed scenarios, it is possible to configure up to three additional DM-RS occasions in a slot. The channel estimation in the receiver side can use these additional reference signals for more accurate channel estimation, for example, to perform interpolation between th