model for RACH transmission is character- ized by a physical RACH (PRACH) preamble format that consists of a cyclic prefix, a pre- amble, and a guard time during which no signal is transmitted. 592 Chapter 4 4.2.2 Reference Signals 4.2.2.1 Demodulation Reference Signals Uplink DM-RSs are used for channel estimation and, as shown in Fig. 4.73, they are subject to the same precoding as uplink shared channel. Uplink reference signals are required to have small power variation in the frequency domain to allow similar channel-estimation quality for all frequencies spanned by the reference signals. This requirement is fulfilled for CP-OFDM waveform by using a pseudo-random sequence with good autocorrelation prop- erties. However, for DFT-precoded OFDM waveform, limited power variations as a func- tion of time are also important to achieve signal transmission with a low cubic metric. Furthermore, sufficient number of reference signal sequences of a given length, correspond- ing to a certain reference signal bandwidth, should be available in order to avoid restrictions when scheduling multiple devices in different cells. It is shown that Zadoff-Chu sequences can satisfy these requirements. From a Zadoff-Chu sequence with a given group index and sequence index, additional reference signal sequences can be generated by applying differ- ent linear phase rotations in the frequency domain. The DFT-precoded OFDM waveform in the uplink only supports single-layer transmission and is primarily specified for limited link-budget scenarios. Due to the importance of low cubic metric property and the corresponding high-power-amplifier efficiency, the reference signal structure is somewhat different compared to the CP-OFDM uplink case. In general, transmitting reference signals that are frequency-multiplexed with other uplink transmis- sions from the same device is not suitable for the uplink as it would negatively impact the device power-amplifier efficiency due to increased cubic metric. Instead, certain OFDM Modulated symbols Layer 