Random access: PUSCH scheduled by RAR UL grant (MSG3)V15.11.0

The transmission of MSG3 is as scheduled by MSG2 with the RAR UL grant, which includes scheduling informations for frequency hopping, frequency and time domain reqsource allocation, MCS, and transmit power control.


Frequency domain resource allocation

An active UL bandwidth part (BWP) for a PUSCH transmission scheduled by a RAR UL grant is indicated by higher layers. For determining the frequency domain resource allocation for the PUSCH transmission within the active UL BWP:

  • if the active UL BWP and the initial UL BWP have same SCS and same CP length and the active UL BWP includes all RBs of the initial UL BWP, or the active UL BWP is the initial UL BWP: the initial UL BWP is used,
  • else:
    • the RB numbering starts from the first RB of the active UL BWP
    • the maximum number of RBs for frequency domain resource allocation equals the number of RBs in the initial UL BWP
  • Note: in both cases, despite the start RB may be different, the maximum number of RBs for frequency domain resource allocation always equals the number of RBs in the initial UL BWP.

The frequency domain resource allocation is by uplink resource allocation type 1 (TS 38.214). For an initial UL BWP size of RBs, a UE processes the frequency domain resource assignment field as follows:

  • if : truncate the frequency domain resource assignment field to its least significant bits.
    • Why 180? , thus, 14 bits resource allocation can indicate up to 180 RBs.
  • else: expand the frequency domain resource assignment field by inserting bit(s) of zero after the NUL,hop bits, where NUL,hop is provided in Table 8.3-1.
  • The trancated or expanded frequency resource assignment field is then interpreted as the frequency resource assignment field in DCI format 0_0.

For a PUSCH transmission with frequency hopping scheduled by RAR UL grant or for a Msg3 PUSCH retransmission, the frequency offset for the second hop is given in Table 8.3-1.

Table 8.3-1: Frequency offset for second hop of PUSCH transmission with frequency hopping scheduled by RAR UL grant or of Msg3 PUSCH retransmission
Number of PRBs in initial UL BWPValue of NUL,hop Hopping BitsFrequency offsete for 2nd hop
0
1
00
01
10
11Reserved
RAR UL grant, freq domainTruncation/expansion of original frequency resource assignment field

Time domain resource allocation

With reference to slots for a PUSCH transmission scheduled by a RAR UL grant, if a UE receives a PDSCH with a RAR message ending in slot n for a corresponding PRACH transmission from the UE, the UE transmits the PUSCH in slot n + k2 + Δ, where k2 and Δ are (TS 38.214):

Time resource allocation is a set of 4 bits which represents a index to a PUSCH time domain resource allocation configuration table of up to 16 entries:

Table 6.1.2.1.1-2: Default PUSCH time domain resource allocation A for normal CP
Row indexPUSCH mapping typeK2SL
1Type Aj014
2012
3010
4Type B210
5410
648
746
8Type Aj+1014
9012
10010
11j+2014
12012
13010
14Type Bj86
15Type Aj+3014
16010

Table 6.1.2.1.1-4 and 5: Definition of value j and Δ
μjΔ
012
113
224
336

The UE may assume a minimum time between the last symbol of a PDSCH reception conveying a RAR message with a RAR UL grant and the first symbol of a corresponding PUSCH transmission scheduled by the RAR UL grant is equal to NT,1 + NT,2 + 0.5 msec, where:

  • NT,1 is a time duration of N1 symbols corresponding to a PDSCH processing time for UE processing capability 1 when additional PDSCH DM-RS is configured
    • for μ = 0, N1,0 = 14
  • NT,2 is a time duration of N2 symbols corresponding to a PUSCH preparation time for UE processing capability 1
  • N1 and N2 correspond to the smaller of the SCS configurations for the PDSCH and the PUSCH
MSG3 timing: minimum time between RAR and PUSCH
μN1 [symb]NT,1 [ms]N2 [symb]NT,2 [ms]Minimum time [ms]
0141.00100.712.21
1130.46120.431.39
2200.36230.411.27
3240.21360.321.04

Toolbox: MSG3 transmit slot calculatornMSG3 = nRAR + k2 + Δ

Note: default allocation table A is assumed, i.e., pusch-TimeDomainAllocationList not provided

μtime resource allocation
from RAR UL grant
jK2ΔK2 + ΔN1N2minimum time
NT,1+NT,2+0.5
1134 slots13121.39 ms

time msg2 to msg3Time relation: from MSG2 to MSG3

Other configurations

A SCS for the PUSCH transmission is provided by subcarrierSpacing in BWP-UplinkCommon.

A UE transmits PRACH and the PUSCH on a same uplink carrier of a same serving cell.

A UE transmits a transport block in a PUSCH scheduled by a RAR UL grant in a corresponding RAR message using redundancy version number 0.

If a TC-RNTI is provided by higher layers, the scrambling initialization of the PUSCH corresponding to the RAR UL grant is by TC-RNTI. Otherwise, the scrambling initialization of the PUSCH corresponding to the RAR UL grant is by C-RNTI.

Msg3 PUSCH retransmissions, if any, of the transport block, are scheduled by a DCI format 0_0 with CRC scrambled by a TC-RNTI provided in the corresponding RAR message. The UE always transmits the PUSCH scheduled by a RAR UL grant without repetitions.

A UE determines whether or not to apply transform precoding based on msg3-transformPrecoder in RACH-ConfigCommon (as described in clause 6.1.3 of TS 38.214).