options as the PDSCH (QPSK, 16-QAM, 64-QAM, and 256-QAM), but Pi/2-BPSK is also included as an optional modulation. The use of Pi/2-BPSK was studied initially 6 5G Physical Layer Short PUCCH Long PUCCH PDCCH GP PUSCH PUCCH PDCCH GP PUSCH PUCCH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 (14 OFDM symbols) (14 OFDM symbols) Figure 6.6 5G short and long PUCCH. with LTE development during Release 8 studies [1], but was not included at the time in the specifications as LTE was able to reach comparable coverage with QPSK already. Physical Random Access Channel (PRACH), for enabling the random access proce- dure. The corresponding transport channel is the Random Access Channel (RACH). Besides the physical channels, the following physical signals are defined in Release 15: Demodulation References Signals (DM-RSs), which are used to aid channel esti- mation. Note that as in LTE, there is no longer the concept of Common Reference Signals (CRS). DM-RS is needed naturally for both uplink and downlink directions. As described later, the amount of DM-RS can vary depending on the environment, that is, on whether there are high-velocity UEs or not. Phase-tracking Reference Signals (PT-RSs) are used with high frequency bands (if configured by the network). PT-RS is repeated periodically in the frequency domain while continuing a sequence of known data. It allows reduction of the common phase error on the receiver side (phase noise compensation). PT-RS is also available in both downlink and uplink directions. Channel State Information Reference Signals (CSI-RSs) (in the downlink) are also known as signals which allow a UE to make a CSI measurement report. The approach is similar to the LTE with known CSI-RS location in the time and frequency domains, making feedback necessary for frequency domain scheduling in the gNB. PSSs, together with the SSSs, form the physical cell ID. There are three different sequences for the PSS and 336 different sequences for the SSS, thus together they create a space of 1008 differe