 resource TRS period: TRS 10/20/40/80 (OFDM symbols) Figure 4.7 Example TRS structure (four single-port, Density-3 CSI-RS over two consecutive slots [9]). 432 Chapter 4 set or the TRS can be configured with a periodicity of 10, 20, 40, or 80 ms. It must be noted that the exact set of time-frequency resource elements used for the TRS may vary; however, the two CSI-RS within a slot are always separated by four symbols in the time domain. This time-domain separation sets a limit for the maximum frequency error that can be com- pensated. Likewise, the frequency-domain separation of four subcarriers sets a limit for the maximum timing error that can be compensated. There is an alternative TRS structure with the same per-slot structure as the TRS structure shown in Fig. 4.7 with only two CSI-RS within a slot, compared to two consecutive slots for the TRS structure shown in the figure. In LTE, the CRS served the same purpose as the TRS; however, the TRS has relatively lower overhead, has one antenna port, and only present in two slots in every TRS period. As we mentioned earlier, the CSI-RS for tracking or TRS is a special configuration of CSI-RS which is specifically configured for a UE. The TRS is used for fine time and fre- quency tracking as well as path delay spread and Doppler spread estimation. A UE in RRC_CONNECTED mode would receive information on a CSI-RS resource set which is configured specifically for the purpose of time/frequency tracking. In that case, the UE is configured with RRC parameter trs-Info and will assume that TRS is transmitted from the antenna port with the same port index of the configured NZP CSI-RS resources in the CSI-RS resource set. In frequency range 1, the UE may be configured with a CSI-RS resource set consisting of four periodic CSI-RS resources in two consecutive slots with two periodic CSI-RS resources in each slot, whereas in frequency range 2, the UE may be con- figured with a CSI-RS resource set of two periodic CSI-RS resources in one slot or with a CSI-RS resource set of four 