6 Random access procedure
36.2133GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical layer proceduresRelease 17TS
If the UE is configured with a SCG, the UE shall apply the procedures described in this clause for both MCG and SCG
– When the procedures are applied for MCG, the terms ‘secondary cell’, ‘secondary cells’, ‘serving cell’, ‘serving cells’ in this clause refer to secondary cell, secondary cells, serving cell, serving cells belonging to the MCG respectively.
– When the procedures are applied for SCG, the terms ‘secondary cell’, ‘secondary cells’, ‘serving cell’, ‘serving cells’ in this clause refer to secondary cell, secondary cells (not including PSCell), serving cell, serving cells belonging to the SCG respectively. The term ‘primary cell’ in this clause refers to the PSCell of the SCG
For a UE configured with EN-DC/NE-DC and serving cell frame structure type 1, if the UE is configured with tdm-PatternConfig/tdm-PatternConfigNE-DC for the serving cell, the UE is not expected to transmit any uplink physical channel or signal in the serving cell on subframes other than offset-UL subframes, where the offset-UL subframes are determined by applying an offset value given by harq-Offset-r15 to the subframes denoted as uplink in the UL/DL configuration tdm-PatternConfig/tdm-PatternConfigNE-DC.
Prior to initiation of the non-synchronized physical random access procedure, Layer 1 shall receive the following information from the higher layers:
– Random access channel parameters (PRACH configuration and frequency position)
– Parameters for determining the root sequences and their cyclic shifts in the preamble sequence set for the primary cell (index to logical root sequence table, cyclic shift (), and set type (unrestricted or restricted set))
6.1 Physical non-synchronized random access procedure
From the physical layer perspective, the L1 random access procedure encompasses the transmission of random access preamble and random access response. The remaining messages are scheduled for transmission by the higher layer on the shared data channel and are not considered part of the L1 random access procedure. A random access channel occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmissions. The eNodeB is not prohibited from scheduling data in the resource blocks reserved for random access channel preamble transmission.
A UE is not expected to be configured with PRACH on a LAA SCell.
The following steps are required for the L1 random access procedure:
– Layer 1 procedure is triggered upon request of a preamble transmission by higher layers.
– A preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), a corresponding RA-RNTI and a PRACH resource are indicated by higher layers as part of the request.
– For a BL/CE UE, a number of PRACH repetitions for preamble transmission attempt is also indicated by higher layers as part of the request. For a non-BL/CE UE or for a BL/CE UE with the PRACH coverage enhancement level 0/1/2, a preamble transmission power PPRACH is determined as
PPRACH = min{, PREAMBLE_RECEIVED_TARGET_POWER + }_[dBm], where is the configured UE transmit power defined in [6] for subframe i of serving cell and is the downlink path loss estimate calculated in the UE for serving cell . For a BL/CE UE, PPRACH is set to for the highest PRACH coverage enhancement level 3.
– A preamble sequence is selected from the preamble sequence set using the preamble index.
– A single preamble is transmitted using the selected preamble sequence with transmission power PPRACH on the indicated PRACH resource. For a BL/CE UE, the single preamble is transmitted for the number of PRACH repetitions for the associated PRACH coverage enhancement level as indicated by higher layers.
– For non-BL/CE UEs, detection of a PDCCH with the indicated RA-RNTI is attempted during a window controlled by higher layers (see [8], Clause 5.1.4). If detected, the corresponding DL-SCH transport block is passed to higher layers. The higher layers parse the transport block and indicate the 20-bit uplink grant to the physical layer, which is processed according to Clause 6.2.
– For BL/CE UEs, detection of a MPDCCH with DCI scrambled by RA-RNTI is attempted during a window controlled by higher layers (see [8], Clause 5.1.4). If detected, the corresponding DL-SCH transport block is passed to higher layers. The higher layers parse the transport block and indicate the Nr-bit uplink grant to the physical layer, which is processed according to Clause 6.2.
6.1.1 Timing
Throughout this clause, for a BL/CE UE, if the UE is configured with the higher layer parameter CellSpecificKoffset,
– where
is the parameter CellSpecificKoffset provided by higher layers, and
is the parameter UESpecificKoffset provided by higher layers, otherwise
otherwise,
– , .
For the L1 random access procedure, a non-BL/CE UE’s uplink transmission timing after a random access preamble transmission is as follows.
a) If a PDCCH with associated RA-RNTI is detected in subframe n, and the corresponding DL-SCH transport block contains a response to the transmitted preamble sequence, the UE shall, according to the information in the response, transmit an UL-SCH transport block in the first subframe . If the UE supports reduced control plane latency and reducedControlPlaneLatency is enabled, 
, otherwise, 
. If the UL delay field in Clause 6.2 is set to zero, is the first available UL subframe for PUSCH transmission, where for TDD serving cell, the first UL subframe for PUSCH transmission is determined based on the UL/DL configuration (i.e., the parameter subframeAssignment) indicated by higher layers. The UE shall postpone the PUSCH transmission to the next available UL subframe after if the field is set to 1.
b) If a random access response is received in subframe n, and the corresponding DL-SCH transport block does not contain a response to the transmitted preamble sequence, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe .
c) If no random access response is received in subframe n, where subframe n is the last subframe of the random access response window, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe , except if the transmitted preamble sequence is on a TDD serving cell not configured for PUSCH/PUCCH transmission.
For the L1 random access procedure, a BL/CE UE’s uplink transmission after a random access preamble transmission is as follows.
a) If a MPDCCH with associated RA-RNTI is detected and the corresponding DL-SCH transport block reception ending in subframe n contains a response to the transmitted preamble sequence, the UE shall, according to the information in the response, transmit an UL-SCH transport block in the first subframe , , if the UL delay field in Clause 6.2 is set to zero where the subframe is the first available UL subframe for PUSCH transmission, where for TDD serving cell, the first UL subframe for PUSCH transmission is determined based on the UL/DL configuration (i.e., the parameter subframeAssignment) indicated by higher layers.
When the number of Msg3 PUSCH repetitions, , as indicated in the random access response, is greater than 1, the subframe is the first available UL subframe in the set of BL/CE UL subframes. The UE shall postpone the PUSCH transmission to the next available UL subframe after , if the UL delay field is set to 1.
When the number of Msg3 PUSCH repetitions,, as indicated in the random access response, is equal to 1, the subframe is the first available UL subframe for PUSCH transmission determined by for FDD and the parameter subframeAssignment for TDD. The UE shall postpone the PUSCH transmission to the next available UL subframe after , if the UL delay field is set to 1.
b) If a random access response is received and its reception ends in subframe n, and the corresponding DL-SCH transport block does not contain a response to the transmitted preamble sequence, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe .
c) If the most recent PRACH coverage enhancement level for the UE is 0 or 1,
– if no random access response is received in subframe n, where subframe n is the last subframe of the random access response window, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe .
If the most recent PRACH coverage enhancement level for the UE is 2 or 3,
– if no MPDCCH scheduling random access response is received in subframe n, where subframe n is the last subframe of the random access response window, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe ;
– if an MPDCCH with associated RA-RNTI is detected and the corresponding DL-SCH transport block reception ending in subframe n cannot be successfully decoded, the UE shall, if requested by higher layers, be ready to transmit a new preamble sequence no later than in subframe .
In case a random access procedure is initiated by a "PDCCH order" in subframe n for non-BL/CE UEs, the UE shall, if requested by higher layers, transmit random access preamble in the first subframe , , where a PRACH resource is available.
In case a random access procedure is initiated by a "PDCCH order" reception ending in subframe n for BL/CE UEs, the UE shall, if requested by higher layers, transmit random access preamble in the first subframe ,
, where a PRACH resource is available.
If a UE is configured with multiple TAGs, and if the UE is configured with the carrier indicator field for a given serving cell, the UE shall use the carrier indicator field value from the detected "PDCCH order" to determine the serving cell for the corresponding random access preamble transmission.
6.2 Random Access Response Grant
The higher layers indicate the Nr-bit UL Grant to the physical layer, as defined in 3GPP TS 36.321 [8].
This is referred to the Random Access Response Grant in the physical layer.
If BL/CE UE then
– If the most recent PRACH coverage enhancement level for the UE is 0 or 1, the contents of the Random Access Response Grant are interpreted according to CEModeA.
– If the most recent PRACH coverage enhancement level for the UE is 2 or 3, the contents of the Random Access Response Grant are interpreted according to CEModeB.
– The content of these Nr bits starting with the MSB and ending with the LSB are given in Table 6-2 for CEmodeA and CEmodeB if the higher layers do not indicate EDT to the physical layer as defined in [8], and in Table 6.2-F if the higher layers indicate EDT.:
– where = and
Table 6-2: Random Access Response Grant Content field size
|
DCI contents |
CEmodeA |
CEmodeB |
|
Msg3 PUSCH narrowband index |
 |
2 |
|
Msg3 PUSCH Resource allocation |
4 |
3 |
|
Number of Repetitions for Msg3 PUSCH |
2 |
3 |
|
MCS |
3 |
0 |
|
TBS |
0 |
2 |
|
TPC |
3 |
0 |
|
CSI request |
1 |
0 |
|
UL delay |
1 |
0 |
|
Msg3/4 MPDCCH narrowband index |
2 |
2 |
|
Zero padding |
4 –
|
0 |
|
Total Nr-bits |
20 |
12 |
Table 6.2-F: Random Access Response Grant Content field size for EDT
|
DCI contents |
CEmodeA |
CEmodeB |
|
Msg3 PUSCH narrowband index |
|
3 |
|
Msg3 PUSCH Resource allocation |
5 |
3 |
|
Number of Repetitions for Msg3 PUSCH |
2 |
3 |
|
TPC |
3 |
0 |
|
CSI request |
1 |
0 |
|
UL delay |
1 |
0 |
|
Msg3/4 MPDCCH narrowband index |
|
3 |
|
Zero padding |
8 – |
0 |
|
Total Nr-bits |
20 |
12 |
– For CEmodeB, the Msg3 PUSCH narrowband index indicates the narrowband to be used for first subframe of Msg3 PUSCH transmission as given in Table 6.2-A if the higher layers do not indicate EDT to the physical layer as defined in [8], Table 6.2-G otherwise.
– given in Table 6.2-A, Table 6.2-B and Table 6.2-G is the narrow band used for first subframe of MPDCCH for Random Access Response and is determined by higher layer parameter mpdcch-NarrowbandsToMonitor-r13 if only one narrowband is configured, otherwise, it is determined by Table 6-2-E.
Table 6.2-A: Msg3 PUSCH Narrowband Value for CEmodeB.
|
Value of ‘Msg3 narrowband index’ |
Msg3 PUSCH Narrowband |
|
’00’ |
 |
|
’01’ |
 |
|
’10’ |
 |
|
’11’ |
 |
Table 6.2-G: Msg3 PUSCH Narrowband Value for CEmodeB and EDT.
|
Value of ‘Msg3 narrowband index’ |
Msg3 PUSCH Narrowband |
|
‘000’ |
|
|
‘001’ |
|
|
‘010’ |
|
|
‘011’ |
|
|
‘100’ |
|
|
‘101’ |
|
|
‘110’ |
|
|
‘111’ |
|
– The Msg3/4 MPDCCH narrowband index indicates the narrowband used for first subframe of MPDCCH configured by Temporary C-RNTI and/or C-RNTI during random access procedure as given in Table 6.2-B if the higher layers do not indicate EDT to the physical layer as defined in [8], value of 
for CEModeA and Table 6.2-H for CEModeB otherwise. The number of downlink narrowbands is given by = .
Table 6.2-B: Msg3/4 MPDCCH Narrowband Value for CEmodeA and CEmodeB.
|
Value of ‘Msg3/4 MPDCCH narrowband index’ |
Msg3/4 MPDCCH Narrowband |
|
’00’ |
 |
|
’01’ |
 |
|
’10’ |
 |
|
’11’ |
 |
Table 6.2-H: Msg3/4 MPDCCH Narrowband Value for CEmodeB and EDT.
|
Value of ‘Msg3/4 MPDCCH narrowband index’ |
Msg3/4 MPDCCH Narrowband |
|
‘000’ |
|
|
‘001’ |
|
|
‘010’ |
|
|
‘011’ |
|
|
‘100’ |
|
|
‘101’ |
|
|
‘110’ |
|
|
‘111’ |
|
– The repetition number field in the random access response grant configured by higher layers indicates the repetition level (
) for the initial transmission of Msg3 PUSCH as given in Table 6.2-C for CEmodeA and Table 6.2-D for CEmodeB, where
– is determined by higher layer parameter pusch-maxNumRepetitionCEmodeA-r13 if it is signaled, otherwise = 8,
– is determined by higher layer parameter pusch-maxNumRepetitionCEmodeB-r13 if it is signaled, otherwise = 512.
If the higher layers indicate EDT to the physical layer as defined in [8] and if the UE is configured with higher layer parameter edt-SmallTBS-Enabled-r15, the repetition number for the initial transmission of Msg3 PUSCH is the smallest integer multiple of M that is equal to or larger than
where 
is the TBS of Msg3 PUSCH as determined in clause 8.6.2, and 
is the value of the higher layer parameter edt-TBS-r15. M=4 if 
> 4, M = 1 otherwise.
Table 6.2-C: Msg3 PUSCH Repetition Level Value for CEmodeA.
|
Value of ‘Repetition number’ |
Msg3 PUSCH Repetition level |
|
’00’ |
 |
|
’01’ |
 |
|
’10’ |
 |
|
’11’ |
 |
Table 6.2-D: Msg3 PUSCH Repetition Level Value for CEmodeB.
|
Value of ‘Repetition number’ |
Msg3 PUSCH Repetition level |
|
‘000’ |
 |
|
‘001’ |
 |
|
‘010’ |
 |
|
‘011’ |
 |
|
‘100’ |
 |
|
‘101’ |
 |
|
‘110’ |
 |
|
‘111’ |
 |
Table 6.2-E: Narrowband () for MPDCCH RAR.
|
Mapped Preamble Index |
 |
|
mod( Preamble Index, 2 )=0 |
First narrowband configured by high layer parameter mpdcch-NarrowbandsToMonitor-r13 |
|
mod( Preamble Index, 2 )=1 |
Second narrowband configured by high layer parameter mpdcch-NarrowbandsToMonitor-r13 |
– The resource allocation field is interpreted as follows:
– For CEmodeA,
– if the higher layers indicate EDT to the physical layer as defined in [8], then
– interpret the resource allocation using UL resource allocation type 0 within the indicated narrowband
– else,
– insert one most significant bit with value set to ‘0’, and interpret the expanded resource allocation using UL resource allocation type 0 within the indicated narrowband.
– For CEmodeB, interpret the resource allocation using UL resource allocation type 2 within the indicated narrowband.
– The truncated modulation and coding scheme field is interpreted such that the modulation and coding scheme corresponding to the Random Access Response grant is determined from MCS indices 0 through 7 for CEmodeA in Table 8.6.1-1
The truncated TBS field is interpreted such that the TBS value corresponding to the Random Access Response grant is determined from TBS indices 0 through 3 for CEmodeB in Table 7.1.7.2.1-1
else,
– Nr=20, and the content of these 20 bits starting with the MSB and ending with the LSB are as follows:
– Hopping flag – 1 bit
– Fixed size resource block assignment – 10 bits
– Truncated modulation and coding scheme – 4 bits
If a UE is performing non-contention based random access procedure and is configured with higher layer parameter pusch-EnhancementsConfig, then
– Repetition number of Msg3 – 3 bits
else
– TPC command for scheduled PUSCH – 3 bits
– UL delay – 1 bit
– CSI request – 1 bit
– The UE shall use the single-antenna port uplink transmission scheme for the PUSCH transmission corresponding to the Random Access Response Grant and the PUSCH retransmission for the same transport block.
– The UE shall perform PUSCH frequency hopping if the single bit frequency hopping (FH) field in a corresponding Random Access Response Grant is set as 1 and the uplink resource block assignment is type 0, otherwise no PUSCH frequency hopping is performed. When the hopping flag is set, the UE shall perform PUSCH hopping as indicated via the fixed size resource block assignment detailed below.
– The fixed size resource block assignment field is interpreted as follows:
– if
– Truncate the fixed size resource block assignment to its b least significant bits, where , and interpret the truncated resource block assignment according to the rules for a regular DCI format 0
– else
– Insert b most significant bits with value set to ‘0’ after the NUL_hop hopping bits in the fixed size resource block assignment, where the number of hopping bits NUL_hop is zero when the hopping flag bit is not set to 1, and is defined in Table 8.4-1 when the hopping flag bit is set to 1, and , and interpret the expanded resource block assignment according to the rules for a regular DCI format 0
– end if
– The truncated modulation and coding scheme field is interpreted such that the modulation and coding scheme corresponding to the Random Access Response grant is determined from MCS indices 0 through 15 in Table 8.6.1-1.
– The TPC command shall be used for setting the power of the PUSCH, and is interpreted according to Table 6.2-1.
end if
Table 6.2-1: TPC Command for Scheduled PUSCH
|
TPC Command |
Value (in dB) |
|
0 |
-6 |
|
1 |
-4 |
|
2 |
-2 |
|
3 |
0 |
|
4 |
2 |
|
5 |
4 |
|
6 |
6 |
|
7 |
8 |
In non-contention based random access procedure, the CSI request field is interpreted to determine whether an aperiodic CQI, PMI, RI, and CRI report is included in the corresponding PUSCH transmission according to Clause 7.2.1. In contention based random access procedure, the CSI request field is reserved.
The UL delay applies for TDD, FDD and FDD-TDD and this field can be set to 0 or 1 to indicate whether the delay of PUSCH is introduced as shown in Clause 6.1.1. A BL/CE UE interpreting the contents of the random access response according to CEModeB shall follow the description of UL delay field set to 0.