ensity is reduced from 1 to 2, that is, from PT-RS for each OFDM symbol 598 Chapter 4 to PT-RS for every other OFDM symbol in the case of 256 QAM [54]. In that case, although signaling overhead is halved for time density 2, that is, more information bits can be transmit- ted in each RB, the effective data transmission rate suffers performance loss due to degraded BLER. In contrast, for 64QAM the degradation due to time density reduction is much less. Therefore, the time density of PT-RS can be a function of modulation order and it should increase with higher modulation order. Since the receiver only needs to track the phase difference using the PT-RS, it does not need to know the amplitude of the PT-RS. Therefore, unlike other reference signals, for example, DM-RS, where the reference signals are formed by a pseudo-random sequence of symbols, PT-RS reference signals can use the exact same symbol. In MU-MIMO, PT-RS can be configured for each user and it is possible that the same subcarrier is used for multi- ple users, thus PT-RS collisions may happen, which would degrade the CPE compensation for two reasons: (1) the interference pattern is not completely random since the same sym- bol is used for PT-RS and (2) the interference level is higher when the power of PT-RS is boosted for more accurate CPE compensation. In such cases, it would be better to avoid PT-RS collision SO that the interference is randomized without power boosting. This can be avoided by introducing an RB-level offset when configuring PT-RS for each user [54]. In NR, both DFT-S-OFDM and CP-OFDM waveforms are supported for the uplink transmis- sions and PT-RS signals are necessary for both waveforms. The PT-RS insertion follows a com- mon framework for both downlink and uplink in case of CP-OFDM waveforms. In the case of DFT-S-OFDM, two types of insertion mechanisms for PT-RSs were studied, that is, pre-DFT and post-DFT insertion. In the former mechanism, PT-RS signals are inserted in the frequency domain before DFT precoding SO that the resulting