16.3 Random access procedure
36.2133GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Physical layer proceduresRelease 17TS
Prior to initiation of the non-synchronized physical random access procedure, Layer 1 shall receive the following information from the higher layers:
– Narrowband Random access channel parameters (NPRACH configuration)
16.3.1 Physical non-synchronized random access procedure
From the physical layer perspective, the L1 random access procedure encompasses the transmission of narrowband random access preamble and narrowband 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 one subcarrier per set of consecutive symbols reserved for narrowband random access preamble transmissions.
The following steps are required for the L1 random access procedure:
– Layer 1 procedure is triggered upon request of a narrowband preamble transmission by higher layers.
– A target narrowband preamble received power (NARROWBAND_PREAMBLE_RECEIVED_TARGET_POWER), a corresponding RA-RNTI and a NPRACH resource are indicated by higher layers as part of the request.
– If enhanced random access power control is not applied, for the lowest configured repetition level; and if enhanced random access power control is applied then for all configured repetition levels, a narrowband preamble transmission power PNPRACH is determined as
PNPRACH = min{
, NARROWBAND_PREAMBLE_RECEIVED_TARGET_POWER + 
}_[dBm], where 
is the configured UE transmit power for narrowband IoT transmission defined in [6] for subframe i of serving cell 
and 
is the downlink path loss estimate calculated in the UE for serving cell 
. If enhanced random access power control is not applied, for a repetition level other than the lowest configured repetition level, PNPRACH is set to 
.
– The narrowband preamble is transmitted with transmission power PNPRACH commencing on the indicated NPRACH resource. The narrowband preamble is transmitted for the number of NPRACH repetitions for the associated NPRACH repetition level as indicated by higher layers.
- Detection of a NPDCCH 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 16.3.3
16.3.2 Timing
For the L1 random access procedure, UE’s uplink transmission timing after a random access preamble transmission is as follows.
a) If a NPDCCH with associated RA-RNTI is detected and the corresponding DL-SCH transport block 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 according to Clause 16.3.3.
b) If a random access response is received and the corresponding DL-SCH transport block ending in subframe n 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 the NB-IoT UL slot starting 12 milliseconds after the end of subframe n.
c) If no NPDCCH 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 the NB-IoT UL slot starting 12 milliseconds after the end of subframe n.
d) If an NPDCCH scheduling random access response 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 the NB-IoT UL slot starting 12 milliseconds after the end of subframe n.
In case a random access procedure is initiated by a "PDCCH order" ending in subframe n, the UE shall, if requested by higher layers, start transmission of random access preamble at the end of the first subframe , 
, where a NPRACH resource is available.
The "PDCCH order" in DCI format N1 indicates to the UE,
– allocated subcarrier for NPRACH, 
where 
is the subcarrier indication field in the corresponding DCI, 
is reserved for preamble format 0/1, 
is reserved for preamble format 2 if nprach-ParametersListFmt2 is configured and the UE indicates the nprach-Format2 capability and Preamble format indicator is set to 1.
– a repetition number (
) for NPRACH determined by the repetition number field (
) in the corresponding DCI according to Table 16.3.2-1 where R1, R2 (if any) and R3 (if any) are given by the higher layer parameter numRepetitionsPerPreambleAttempt for each NPRACH resource, respectively. R1 < R2 <R3.
Table 16.3.2-1: Number of repetitions (
) for NPRACH following a "PDCCH order"
|
|
|
|
0 |
R1 |
|
1 |
R2 |
|
2 |
R3 |
|
3 |
Reserved |
The UE shall transmit random access preamble on the NB-IoT carrier indicated by "Carrier indication of NPRACH" field, if the field is present in the "PDCCH order". If the value of "Carrier indication of NPRACH" is non-zero, it indicates a NPRACH carrier derived from SystemInformationBlockType22-NB [11] for which the index in the list is equal to the carrier indication. If the value of "Carrier indication of NPRACH" is zero, the uplink carrier used for NPRACH is derived from SystemInformationBlockType2-NB [11].
If nprach-ParametersListFmt2 is configured and the UE indicates the nprach-Format2 capability, the UE shall transmit the preamble format indicated by "Preamble format indicator" field, otherwise the UE shall transmit preamble format 0/1.
16.3.3 Narrowband 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 as the Narrowband Random Access Response Grant in the physical layer.
Nr-bit =15, and the content of these 15 bits starting with the MSB and ending with the LSB are as follows:
– Uplink subcarrier spacing 
is ‘0’=3.75 kHz or ‘1’=15 kHz – 1 bit
– Subcarrier indication field 
as determined in Clause 16.5.1.1 – 6 bits
– Scheduling delay field (
) as determined in Clause 16.5.1 with k0 = 12 for IDelay = 0 , where NB-IoT DL subframe n is the last subframe in which the NPDSCH associated with the Narrowband Random Access Response Grant is transmitted – 2 bits
– Msg3 repetition number 
as determined in Clause 16.5.1.1 – 3 bits
– MCS index indicating TBS, modulation, and number of RUs for Msg3 – 3 bits
The redundancy version for the first transmission of Msg3 is 0.
If the UE is not using higher layer parameter edt-Parameters, or the UE is using higher layer parameter edt-parameters and 
,
– the TBS, modulation, and number of RUs for Msg3 are determined according to Table 16.3.3-1
otherwise,
– if the UE is configured with higher layer parameter edt-SmallTBS-Enabled set to ‘false’,
– the TBS is given by higher layer parameter edt-TBS
– otherwise,
- the UE selects a TBS from the allowed TBS values according to Table 16.3.3-2
- the repetition number for Msg3 is the smallest integer multiple of L value that is equal to or larger than
where 
is the selected TBS for Msg3, and 
is given by higher layer parameter edt-TBS
– if 
and 
and 
, then 
is used in clause 16.5.1.2, otherwise 
is used
– the number of RUs for Msg3 are determined according to Table 16.3.3-3
- π/4 QPSK modulation is used for
and for 
with 
; QPSK modulation is used for
with 
Table 16.3.3-1: MCS index for Msg3 NPUSCH
|
MCS Index |
Modulation
|
Modulation
|
Number of RUs
|
TBS |
|
‘000’ |
pi/2 BPSK |
QPSK |
4 |
88 bits |
|
‘001’ |
pi/4 QPSK |
QPSK |
3 |
88 bits |
|
‘010’ |
pi/4 QPSK |
QPSK |
1 |
88 bits |
|
‘011’ |
reserved |
reserved |
reserved |
reserved |
|
‘100’ |
reserved |
reserved |
reserved |
reserved |
|
‘101’ |
reserved |
reserved |
reserved |
reserved |
|
‘110’ |
reserved |
reserved |
reserved |
reserved |
|
‘111’ |
reserved |
reserved |
reserved |
reserved |
or 
and
and
Table 16.3.3-2: EDT TBS for Msg3 NPUSCH with edt-SmallTBS-Enabled set to ‘true’
|
edt-TBS |
edt-SmallTBS-Subset |
Allowable TBS values |
|
408 |
not configured |
328, 408 |
|
504 |
not configured |
328, 408, 504 |
|
504 |
enabled |
408, 504 |
|
584 |
not configured |
328, 408, 504, 584 |
|
584 |
enabled |
408, 584 |
|
680 |
not configured |
328, 456, 584, 680 |
|
680 |
enabled |
456, 680 |
|
808 |
not configured |
328, 504, 680, 808 |
|
808 |
enabled |
504, 808 |
|
936 |
not configured |
328, 504, 712, 936 |
|
936 |
enabled |
504, 936 |
|
1000 |
not configured |
328, 536, 776, 1000 |
|
1000 |
enabled |
536, 1000 |
Table 16.3.3-3: MCS index for Msg3 NPUSCH and EDT
|
MCS Index |
Number of RUs
|
||
|
edt-TBS = 328, 408, 504, or 584 |
edt-TBS = 680 |
edt-TBS = 808, 936, or 1000 |
|
|
‘011’ |
3 |
3 |
4 |
|
‘100’ |
4 |
4 |
5 |
|
‘101’ |
5 |
5 |
6 |
|
‘110’ |
6 |
8 |
8 |
|
‘111’ |
8 |
10 |
10 |
