e DM-RS occasions within a slot. However, unlike LTE, it is not possible to interpolate channel estimations between slots, or in general different trans- mission occasions, since different slots may be transmitted to different devices and/or in dif- ferent beam directions [14]. New Radio Access Physical Layer Aspects (Part 2) 419 Type 1 single-symbol DM-RS Type 1 double-symbol DM-RS Type 2 single-symbol DM-RS Type 2 double-symbol DM-RS structure structure structure structure Figure 4.3 Various NR PDSCH DM-RS Type A time-frequency patterns [6]. In LoS-dominant channel conditions, the delay spread is expected to be shorter (or equiva- lently the channel coherence bandwidth becomes larger); thus one can consider reducing frequency-domain density of the DM-RS without significant degradation of channel estima- tion precision. By doing so, the overhead due to the DM-RS can be reduced. One example of such low-density DM-RS patterns in frequency-domain is shown in Fig. 4.3. For MIMO trans- mission up to two frequency-domain orthogonal DM-RS ports are supported. The DM-RS is UE-specific, can be beamformed, is confined in the UE scheduled resources, and is transmit- ted only when necessary, both in the downlink and uplink and is used to estimate the commu- nication channel prior to coherent demodulation. To support multi-layer MIMO transmission, multiple orthogonal DM-RS ports can be scheduled, one for each layer. Orthogonality is achieved by means of FDM (comb structure), TDM, and/or CDM (with cyclic shift of the base sequence or orthogonal cover codes) methods. The basic DM-RS pattern is front loaded, as the DM-RS design considers the early decoding requirement to support low-latency appli- cations. For low-speed scenarios, DM-RS uses low density in the time domain. However, for high-speed scenarios, the time density of DM-RS is increased to track fast changes in the 420 Chapter 4 radio channel. The NR defines two time-domain DM-RS structures which differ in the loca- tion of the first DM-RS symbol [14]: Mapping Type A, 