Random access: random access response (RAR, MSG2)V15.11.0

Note: Random access response (RAR, or MSG2) is indicated using DCI format 1_0 and RA-RNTI.


Random Access RNTI (RA-RNTI) is used during Random Access procedure, gNB's MAC generates Random Access Response (MSG2, RAR) as a response to the Random Access Preamble (MSG1) transmitted by the UE. RAR is transmitted on DL-SCH transport channel (PDSCH). gNB scrambles PDCCH CRC with RA-RNTI for transmission of PDSCH that carries RAR(s). RA-RNTI can be addressed to multiple UEs, i.e., multiple UEs might decode PDCCH scrambled by the same.

The RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted, is computed as:

Toolbox: RA-RNTI calculatorRA-RNTI = 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id
VariableMeaningRangeExample
s_idindex of the first OFDM symbol of the PRACH occasion0 ≤ s_id < 14
t_idindex of the first slot of the PRACH occasion in a system frame0 ≤ t_id < 80
f_idindex of the PRACH occasion in the frequency domain0 ≤ f_id < 8
ul_carrier_idUL carrier used for Random Access Preamble transmission
0 for NUL carrier
1 for SUL carrier
RA-RNTIUsed to scramble PDCCH CRC for RAR1 ≤ RARNTI ≤ 179205789

In response to a PRACH transmission, a UE attempts to detect a DCI format 1_0 with CRC scrambled by a corresponding RA-RNTI during a window controlled by higher layers:

  • The window starts at the first symbol of the earliest CORESET the UE is configured to receive PDCCH for Type1-PDCCH CSS set, that is at least one symbol after the last symbol of the PRACH occasion corresponding to the PRACH transmission, where the symbol duration corresponds to the SCS for Type1-PDCCH CSS set.
  • The length of the window in number of slots, based on the SCS for Type1-PDCCH CSS set, is provided by ra-ResponseWindow. Possible values of ra-ResponseWindow is {1, 2, 4, 8, 10, 20, 40, 80} slot(s), and the duration is less than or equal to 10 ms.

If the UE detects the DCI format 1_0 with CRC scrambled by the corresponding RA-RNTI and a transport block in a corresponding PDSCH within the window:

  • UE passes the transport block to higher layers.
  • higher layers parse the transport block for a random access preamble identity (RAPID) associated with the PRACH transmission.
    • If the higher layers identify the RAPID in RAR message(s) of the transport block, the higher layers indicate an uplink grant to the physical layer. This is referred to as random access response (RAR) UL grant in the physical layer.

The higher layers can indicate to the physical layer to transmit a PRACH, if

  • UE does not detect the DCI format 1_0 with CRC scrambled by the corresponding RA-RNTI within the window, or
  • UE does not correctly receive the transport block in the corresponding PDSCH within the window, or
  • higher layers do not identify the RAPID associated with the PRACH transmission from the UE.

If requested by higher layers, the UE is expected to transmit a PRACH no later than NT,1 + 0.75 msec after the last symbol of the window, or the last symbol of the PDSCH reception, where

  • NT,1 is a time duration of N1 symbols corresponding to a PDSCH processing time for UE processing capability 1
  • μ corresponds to the smallest SCS configuration among the SCS configurations for the PDCCH carrying the DCI format 1_0, the corresponding PDSCH when additional PDSCH DM-RS is configured, and the corresponding PRACH.
  • For μ = 0, the UE assumes N1,0 = 14. For a PRACH transmission using 1.25 kHz or 5 kHz SCS, the UE determines N1 assuming SCS configuration μ = 0.
  • N1 = {14, 13, 20, 24} for μ = {0, 1, 2, 3}
msg2 timeRAR monitoring window and retransmission of PRACH

Quasi co-location properties:

  • If the UE detects a DCI format 1_0 with CRC scrambled by the corresponding RA-RNTI and receives a transport block in a corresponding PDSCH, the UE may assume same DM-RS antenna port quasi co-location properties as for a SS/PBCH block or a CSI-RS resource the UE used for PRACH association, regardless of whether or not the UE is provided TCI-State for the CORESET where the UE receives the PDCCH with the DCI format 1_0.
  • If the UE attempts to detect the DCI format 1_0 with CRC scrambled by the corresponding RA-RNTI in response to a PRACH transmission initiated by a PDCCH order that triggers a contention free random access procedure for the SpCell, the UE may assume that the PDCCH that includes the DCI format 1_0 and the PDCCH order have same DM-RS antenna port quasi co-location properties.
  • If the UE attempts to detect the DCI format 1_0 with CRC scrambled by the corresponding RA-RNTI in response to a PRACH transmission initiated by a PDCCH order that triggers a contention-free random access procedure for a secondary cell, the UE may assume the DM-RS antenna port quasi co-location properties of the CORESET associated with the Type1-PDCCH CSS set for receiving the PDCCH that includes the DCI format 1_0.

RAR UL grant

A RAR UL grant schedules a PUSCH transmission (MSG3) from the UE. The contents of the RAR UL grant, starting with the MSB and ending with the LSB, are given in Table 8.2-1.

Table 8.2-1: Random Access Response Grant Content field size
RAR grant fieldNum of bitsMeaning
frequency hopping flag10: PUSCH without frequency hopping; 1: with hopping
PUSCH frequency resource allocation14
PUSCH time resource allocation4
MCS4from the first 16 indexes of the PUSCH MCS index table defined in TS 38.214.
TPC command for PUSCH3δmsg2,b,f,c, interpreted according to Table 8.2-2.
CSI request1reserved
Table 8.2-2: TPC Command δmsg2, b,f,c for PUSCH
TPC command01234567
Value [dB]-6-4-202468

Unless the UE is configured a SCS, the UE receives subsequent PDSCH using same SCS as for the PDSCH reception providing the RAR message.

If the UE does not detect the DCI format with CRC scrambled by the corresponding RA-RNTI or the UE does not correctly receive a corresponding transport block within the window, the UE procedure is as described in TS 38.321.