GPP TS 38.211). Since the phase noise increases with carrier frequency, the use of PT-RS is more beneficial for high carrier frequencies, such as mm Wave. PT-RS in NR has low density in the frequency domain but high density in the time domain. It is supported for both downlink (associated with PDSCH) and uplink (associated with PUSCH). To facilitate phase tracking at the receiver, the antenna port of PT-RS is always associated with one antenna port of DM-RS. For PUSCH transmission the association with the DM-RS port is indicated by the DCI, while for PDSCH transmission the association is fixed in the NR specification. For the CP-OFDM waveform, PT-RS has uniform structure in both frequency and time. In the frequency domain, PT-RS resource elements can be present every second or every fourth PRB depending on the number of allocated PRBs. In the time domain, PT-RS can be transmitted every OFDM symbol, every second OFDM symbol (see Figure 3.5.15), or every fourth OFDM symbol depending on the modulation and coding scheme (MCS). The PT-RS sequence for CP-OFDM is derived from the DM-RS symbol of the associated DM-RS port. For DFT-s-OFDM, PT-RS is transmitted using multiple PT-RS groups, which are multi- plexed with the PUSCH symbols prior to DFT spreading. The number of PT-RS groups as 3 5G System Overview 012345678910 11 12 13 Figure 3.5.15 PT-RS within PRB for CP-OFDM. DM-RS PT-RS well as the number of PT-RS symbols per group can be adapted based on the number of allocated PRBs for PUSCH transmission. In DFT-s-OFDM, the PT-RS resource elements are modulated by /2 BPSK to guarantee power-efficient transmission with low PAPR. 3.5.7.4 SRS Similar to LTE, NR supports SRS, which are used to assist link adaptation and pre- coding matrix selection for uplink transmission as well as to provide downlink CSI for reciprocity-based precoding in TDD systems (3GPP TS 38.211). SRS for DL CSI acqui- sition supports antenna switching when UE has fewer transmit chains than receiving antennas (3GPP TS 38.214). Compared with LTE, SRS in 