5G physical channels need to provide flexible communication between the 5G gNodeBs and the User Equipment (UEs). 5G NR has specified the physical channels for 5G networks that can be used either for Downlink or Uplink communication. The main 5G NR physical channels used for Uplink communication are the Physical uplink shared channel (PUSCH), the physical uplink control channel (PUCCH), and the Physical Random Access Channel (PRACH). Uplink signals such as DM-RS, PT-RS, and SRS are also supported. 5G NR supports the simultaneous transmission on PUSCH and PUCCH.
Physical Uplink Shared Channel (PUSCH)
The physical uplink shared channel (PUSCH) is used to carry the user data and optionally the Uplink Control Information (UCI). Simultaneous transmission of PUSCH and PUCCH is not part of release 15 but may be introduced in a later version. Two waveforms are possible CP-OFDM (Cyclic Prefix), intended for MIMO and Discrete Fourier Transform Spread OFDM (DFT-s-OFDM), used with a single layer transmission, with intra-slot frequency hopping supported. Front-loaded DM-RS are located at the first OFDM symbol assigned for PUSCH. Additional ones can be configured for high speed. Discrete Fourier Transform Spread OFDM (DFT-s-OFDM) is commonly known as SC-FDMA and adopted by uplink, with lower PAPR than OFDMA. No changes are made compared to LTE.
Physical Uplink Control Channel (PUCCH)
The Physical Uplink Control Channel (PUCCH) is used to carry the Uplink Control Information (UCI) with the Channel State Information (CSI) reports, the HARQ feedback, and the Scheduling Requests (SR). There are two types of PUCCHs, the long PUCCH (µ/2 BPSK and QPSK modulations) and the short PUCCH (BPSK and QPSK modulations). According to 3GPP 38.211, the PUCCH format is defined by the length of the OFDM symbols and the number of bits. PUCCH format 0 is for short duration PUCCH with small Uplink Control Information (UCI) payloads of up to two bits. PUCCH format 1 is for long PUCCH with small payloads of up to two bits and User Equipment multiplexing capacity of up to 84 UEs without frequency hopping and 36 UEs with frequency hopping in the same PRB. PUCCH format 2 is for Short PUCCH with large payloads of more than two bits and no UE multiplexing. PUCCH format 3 is for Long PUCCH with large UCI payloads and no UE multiplexing capability in the same PRBs. Finally, PUCCH format 4 is for Long PUCCH, moderate UCI payloads, and multiplexing capacity of up to 4 UEs in the same PRBs.
Physical Random Access Channel (PRACH)
The Random Access procedure in 5G NR is similar to LTE, although the number of Preamble Formats is increased to 13. For beam establishment, different SS block time indices are associated with different RACH time/frequency occasions. SIB1 provides the “number of SS-block time indices per RACH time/frequency occasion.” SSB time indices are associated with RACH occasions, first in frequency, then in time within a slot, and last in time between slots. For User Equipment (UE) initial access, the reference signals used for beam management are PSS, SSS, and PBCH DMRS (i.e., SSB) for IDLE mode and CSI-RS (DL) and SRS (UL) for CONNECTED mode.
5G-NR Preamble formats
Two different lengths (L_RA) of PRACH preamble are used depending on the subcarrier spacing of the preamble. When the subcarrier spacing of PRACH preamble is 1.25 or 5 kHz, a long sequence (L_RA = 839) is used. When the subcarrier spacing of PRACH preamble is 15, 30, 60, or 120 kHz, a short sequence (L_RA = 139) is used.
Root sequence index planning – short sequences
Short sequences have only 138 root sequences, each of length 139 bits. This means that cell differentiation must be planned in the frequency domain. 5G cell has a configurable number of unique preambles. In case this parameter is not broadcasted, it defaults to 64. Preambles can be generated by a cyclic shift from root sequences. The number of consumed root sequences per cell depends on cyclic shift size.
32 root sequences per cell mean that we can create 138/32=4 unique cell configurations only, while the cell radius is only 2km. That means that the maximum configuration for preamble is 64 in a cell. For example, if we allocate only 16 RA preambles per cell as is the case with some vendors, with NCS 69, we need only 8 root sequences per cell, and the final number of unique cell configuration is 138/8=17.
Zero Correlation Zone
Zero Correlation Zone defines the maximum relative distance between UEs, upon which transmitted preambles stay orthogonal when they are sent towards the base station. In the worst scenario, one UE is covered by the base station, and another UE is at the cell edge, so this parameter defines the maximum cell radius. For the two preambles to interfere with each other, the following conditions must be met. PRACH must be sent from both UEs at the same time (same PRACH time resource), and PRACH must be sent from both UEs at the same frequency resource, and the relative distance between UEs has to be higher than the maximum radius.
Phantom/Ghost RACH
The Phantom or Ghost RACH is a fake channel request caused by noise. The conditions for Phantom or Ghost RACH are the following: The UE must be near a cell edge, with a similar path loss to two gNBs. Both gNBs are configured with the same RootSequence Index parameter. The network is time-synchronized, so the PRACH occurs at the same moment in both gNBs. The Msg1-FrequencyStart is configured with the same value in both gNBs in the frequency domain. The last condition can be avoided, with Msg1-FrequencyStart planning, especially between adjacent cells with the same RootSequenceIndex configuration. However, configuring PRACH further towards the bandwidth center reduces single user equipment maximum Uplink throughput, as it fragments PUSCH into two pieces. This cannot be scheduled for one UE since non-contiguous frequency allocation is not yet possible from 5G UEs.
PRACH bandwidth for short sequence format
Short preambles always have a bandwidth of 12 Resource Blocks. The subcarrier spacing can be configured by broadcast 3GPP parameter msg1-SubcarrierSpacing = [15, 30 kHz]. PRACH may have up to 8 resources in the frequency domain, 3GPP parameter msg1-FDM = [1, 2, 4, 8]. So to avoid the Ghost RACH effect, PRACH of adjacent cells with identical RSI configuration can be distributed into the frequency domain by allocating different Msg1-FrequencyStart values.
However, the msg1-FDM parameter is maximized to 8 with a typical cell bandwidth of 100MHz. The number of frequency positions varies with the Subcarrier spacing (SCS) of PRACH.
