eriodic, semi-persistent, or aperiodic (DCI triggered). When used for time frequency track- ing, the CSI-RS can be periodic or aperiodic. In this case, a single port is configured, and the signal is transmitted in the form of bursts over two or four symbols that are spread over one or two slots [71]. A configured CSI-RS resource may correspond to up to 32 different antenna ports. In NR, a CSI-RS is always configured on a per-device basis. It must be noted that the UE-specific configuration of CSI-RS does not necessarily mean that a transmitted CSI-RS can only be used by a single device, rather the same set of CSI-RS resources can be separately config- ured for multiple devices, which means that a single CSI-RS can be shared among multiple devices. As illustrated in Fig. 4.5, a single-port CSI-RS occupies a single resource element within a resource block in the frequency domain and one slot in the time domain. While the CSI-RS can be configured to occur anywhere within the resource block, in practice there are some restrictions on CSI-RS resource assignment to avoid collisions with other down- link physical channels and signals. The transmission of a configured CSI-RS is expected not to collide with a CORESET configured for the device; the DM-RS associated with PDSCH transmissions scheduled for the device; and the SS blocks transmissions. A multiport CSI-RS can be viewed as multiple orthogonal per antenna-port CSI-RS sharing the set of resource elements assigned for transmission of the configured multiport CSI-RS. In general, the resource sharing is achieved through a combination of code-domain New Radio Access Physical Layer Aspects (Part 2) 427 (i.e.,different per antenna-port CSI-RS are transmitted on the same set of resource elements with separation achieved by spreading the CSI-RS with different orthogonal codes), frequency-domain (i.e., different per antenna-port CSI-RS are transmitted on different sub- carriers over an OFDM symbol), and time-domain (i.e., different per antenna-port CSI-RS are transmitted on