quisition of the DL transmission beams from gNB during the beam management procedure. Unlike LTE, CSI-RS in NR can be also used for time-frequency tracking with periodic (with periodicity of 10, 20, 40, or 80 ms) and aperiodic CSI-RS configurations (3GPP TS Figure 3.5.12 Basic units for CSI-RS. 3 5G System Overview 1 2 3 4 5 6 7 8 9 10 11 12 13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Figure 3.5.13 CSI-RS for time-frequency tracking. 38.214). In this use case, CSI-RS can be only transmitted with a single antenna port over two OFDM symbols within a slot using the same set of subcarriers. Depending on the FR, one or two slots (see Figure 3.5.13), can be used for CSI-RS transmission to provide a better trade-off between overhead and accuracy for synchronization as well as for channel delay spread and Doppler spread estimation. 3.5.7.2 DM-RS The demodulation of the physical channels in NR is performed based on the channel esti- mated from DM-RS (3GPP TS 38.211). DM-RS are precoded in the same way as a physical channel and confined within the scheduled resource elements of the corresponding chan- nel. Unlike LTE-A, for the physical downlink shared channels (PDSCH or PUSCH), NR offers more flexible DM-RS structure supporting a wide range of use cases. In particular, two DM-RS types denoted Type I and Type II can be used to provide trade-off between frequency domain density and DM-RS overhead. In particular, DM-RS Type I supports two code divi- sion multiplexing (CDM) groups, while DM-RS Type II supports three CDM groups per OFDM symbol as shown in Figure 3.5.14. For both DM-RS types, up to two DM-RS ports can be multiplexed in one CDM group by using orthogonal cover codes in the frequency 0123456 0123456 Figure 3.5.14 DM-RS Type I and Type II. 3.5 NR Physical Layer domain, thus providing a total DM-RS multiplexing capacity of four and six DM-RS ports per OFDM symbol, respectively. For scenarios with massive multi-user MIMO (MU-MIMO) or single-user MIMO (SU-MIMO) transmission with a large number of MIMO layers, the number of DM-