Section 4 - 5G NR RACH

Question 1

What are the steps for the random access procedures?

Answer 1

First the device transmits a preamble referred to as PRACH. Next the network transmits a random-access response (RAR), indicating reception of the preamble and providing time-alignment command adjusting the transmission timing of the device on the timing of the received preamble. Then msg 3 and msg4 are exchanged to resolve potential collisions due to simultaneous transmissions of the same preamble from multiple devices within the cell. If the process is successful, msg 4 puts the device in connected state.

Question 2

What are some triggers for RA procedures besides registration?

Answer 2

Handover, when synchronization is needed with the new cell. If uplink synchronization is lost due to long time without transmitting. To request uplink scheduling if no dedicated scheduling-request resource has been configured for the device.

Question 3

How does the UE decide the timing and power for preamble?

Answer 3

The timing and power are decided based on the timing and measurements of the SSB

Question 4

What type of signal is transmitted in the preamble?

Answer 4

A DFTS-OFDM based on a Zadoff-Chu sequence

Question 5

What are the preamble formats for long preambles?

Answer 5

There are 4 formats from 0 to 3. Their numerology is 1.25kHz SCS except the format 3 with 5kHz. Their repetition is 1, 2, 4 and 1 respectively. The CP Length is 100, 680, 15 and 100 us respectively. The preamble length is 800, 1600, 3200, 800 respectively not including CP. Long preambles have a length of 839.

Question 6

What are the formats for shot preambles?

Answer 6

There are 9 formats: A1, A2, A3, B1, B2, B3, B4, C0, C2. They have varying numbers of repetitions and length. A and B are identical except for CP

Question 7

What are the similarities in RACH between LTE and NR?

Answer 7

The RACH procedures consists of 4 messages, the preamble is based on Zadoff-Chu sequence. The two types of RACH remain: contention-based and contention-free

Question 8

What are the difference in RACH between LTE and NR?

Answer 8

In NR the RACH configuration is defined in SIB1 (SIB2 in LTE). There is beam corresponding. LTE only has long format. There are new RACH triggers: beam failure recovery, SCG cell add and change failure, request for other SI, transition from RRC_INACTIVE

Question 9

How are the RACH resources selected?

Answer 9

The gNB transmits SSBs in a beam-sweeping manner, containing RMSI. PRACH resources are determined via association between SSB index and resources of that index, specified in RMSI. UE decodees RMSI and extranct the RACH resources associated with the receiver SSB. The resources are linked to the SSB beam.

Question 10

How are RACH parameters passed to the UE?

Answer 10

In NSA they are passed via a UE-dedicated LTE RRC Reconfiguration message. In SA via the RMSI

Question 11

What are the parameters that define the RACH procedure?

Answer 11

PRACH configuration (power ramp-up step size, resources), SSB and RACH occassion assiciation. The RSRP threshold for SSB selection, CSI-RS dedicated RACH threshold, SUL threshold. CORESET for RAR scheduling. PDCCH related information for decoding RAR. Waveforms for MSG-3 and SCS (use of DFTS-OFDM or CP-OFDM for MSG-3 and SCS to be used for MSG-2,3 and 4). Preambles for SI Request and beam failure recover request

Question 12

What are differences in the RA between FR1 and FR2?

Answer 12

In FR1, there can be multiple RO and the UE can use any of them. In FR2 typically there is only one RO but multiple beams

Question 13

How many preambles are supported?

Answer 13

Up to 64 preambles

Question 14

How are the preambles mapped from SSB into RO?

Answer 14

For Contention Free (CFRA), the assiciation with SSB can be reconfigured through RRC signaling. For CBRA, gNB configures in RMSI the number of preambles per SSB per RACH occasion, and the number of SSB per RO.

Question 15

How is the preamble structured?

Answer 15

CP, preamble (which can be repeated) and guard time. Different preambles can be generated from different sequences generated from different root indices and different cyclic shifts of the same root index.

Question 16

Can all frequencies use long preamble?

Answer 16

No, it is only possible to use long preamble on FR1.

Question 17

What are the use cases of long preamble?

Answer 17

Large cells or high speed (SCS of 1.25 or 5kHz)

Question 18

What are the use cases of short preambles?

Answer 18

Small cells or indoor (SCS of 15, 30 for FR1; 60 or 120kHz for FR2)

Question 19

What bandwidth does the long preamble occupy?

Answer 19

1.08 MHz formats 0-3 and 4.32 format 4

Question 20

What is the procedure for the transmission of MSG-1?

Answer 20

The UE receives SSB/PBCH and measures RSRP on each received beam. Selects the SSB with higher RSRP and reads SI. Use SMSI to determine PRACH configuration (RO, preamble format, root sequence index). Calculate RA-RNTI (random access-radio network temporary identifier, each RA-RNTI is associated with a preamble). Calculate Tx power. Finnaly transmit MSG-1.

Question 21

What parameter determines the RACH timing configuration?

Answer 21

The RACH timing is derived from SIB1 parameter prach-ConfigurationIndex. For each index 3GPP provides: system frame number, subframe number, starting symbol, number of PRACH slot in a subframe, number of RACH occasions in a RACH slot. The values differ for FR1 vs FR2 and TDD vs FDD

Question 22

What are the starting symbols for the preamble in sub-6?

Answer 22

For short formats the 0 or 2. For long formats 0, 1 and 3, symbol 0. For format 2, 0 or 6. For 15kHz SCS there can only be one RACH slot in a subframe. 1 or 2 for 30kHz SCS.

Question 23

How many RO can there be in a slot for mmWave?

Answer 23

For sequence format A1/B1 6, for A2/B2 3, for A3/B3 2. For C0 and C2 4 and 2 respectively

Question 24

How is the RACH configuration in frequency determined?

Answer 24

There are the RACH-ConfigGeneric parameters, including msg1-FDM which determines the number of PRACH transmission occassions FDMed in one time instance, ms1-FrequencyStart which sets the offset of lowest PRACH transmission occasion in frequency domain with respective to PRB0 in UL BWP, and the PRACH format. Then there is the RACH-ConfigCommon Parameters with the ms1 SCS

Question 25

What happens if no MSG-2 is received after sending MSG1?

Answer 25

Once the MSG-1 is transmitted we wait to receive MSG-2. If no MSG-2 is received, the retransmission of MSG1 is given by
- Power Ramp-up
- UE determines the initial preamble transmit power based on
- Pathloss estimate, based on the received power of the selected SSB
- Setting of preambleReceivedTargetPower parameter by the gNB
- If Random Acces Response (MSG-2) is not received, UE re-transmits the preamble with higher Tx power as configured by powerRampingStep
- UL Beam Switching during MSG-1 Transmission
- Detection of a new best DL SSB beam during RACH can trigger change of UL beam

Question 26

How are the timing alignment and SCS for msg-1 determined?

Answer 26

For FDD Preamble is transmitted with zero TA (timing advance). Preamble is transmitted with non-zero TA for TDD. The TA depends on the RACH trigger. Initial RRC setup for SI request and for call setup (default TA defined in the spec is used); Beam recovery - TA provided via RRC messaging; Handover - TA provided in handover command.
For the SCS
- CBRA and PDCCH order: Provided in RMSI
- CFRA: Provided in RRC recfg for PScell adding and change

Question 27

What contains MSG-2?

Answer 27

RAR contains:
- A for MSG-3
- UL Grant for MSG-3
- Temp C-RNTI
RAR is carried in NR-PDSCH scheduled by NR-PDCCH in CORESET, configured in RACH configuration, for initial access. For a given frequency band, RAR is confined within NR UE minimum DL BW. RAR window size is configured in SI.

Question 28

What are the characteristics of MSG-3?

Answer 28

MSG-3 is scheduled by the uplink grant in RAR.
- UE adjusts power setting for MSG-3 via the TPC cmd in MSG-2 and Tx power of the latest PRACH preamble
- UE then transmits MSG-3 using the grant received in MSG-2
Waveform for RACH message 3 can be DFT-S-OFDM or CP-OFDM
- gNB signals the waveform for RACH message 3
SCS for MSG-3 transmission
- RACH configuration (RMSI or HO command) provides the SCS of the MSG-3

Question 29

How is the time alignement for MSG-3 configured?

Answer 29

TA received in RAR is used to adjust msg-3 transmission.
- Time advance/alignment compensates for over the air radio transmission RTT and enables sync in time domain for all UL signals
Granularity of timing advance in RAR depends on SCS of MSG-3. Given Ts = 1/(6430.72) us, for each SCS from 15 to 120kHz the TA granularity is {16, 8, 4, 2}64*Ts respectively.
The TA command for RAR is 12 bits

Question 30

What are the characteristics of MSG-4

Answer 30

The SCS for MSG-4 same as MSG-3. RMSI can indicate PDCCH configuration giving search space configuration for RACH procedure (before RRC connection setup is complete)
- One search space in one slot for MSG-2, MSG-3 and MSG-4
- Starting symbol of MSG-2, MSG-3 and MSG-4 search space is the same in every slot