RNTIs in LTE are thoroughly
discussed in the post ‘RNTIs in LTE’. The RNTIs in NR are
similar to those of LTE. The current post will discuss about ‘RNTIs in 5G NR’
in detail.
What is an RNTI?
An RNTI stands for ‘Radio Network Temporary Identifier’
and is used to differentiate/identify a connected mode UE
in the cell, a specific radio channel, a group of UEs in case of paging, a
group of UEs for which power control is issued by the gNB, system information transmitted for all the UEs by the gNB etc…
There are a several
RNTI types in 5G NR such as P-RNTI, SI-RNTI, RA-RNTI, Temporary C-RNTI,
C-RNTI, MCS-C-RNTI, CS-RNTI, TPC-PUCCH-RNTI,
TPC-PUSCH-RNTI,
TPC-SRS-RNTI, INT-RNTI, SFI-RNTI, I-RNTI,
and SP-CSI-RNTI.
Except
for I-RNTI, the length of an RNTI is 2 bytes (16-bits) and is used to scramble
CRC bits before CRC bits are attached to the corresponding transmission
(uplink/downlink). I-RNTI is an RRC level RNTI and is used to identify the suspended UE context of an UE in RRC_Inactive
state.
Each RNTI’s usage (except I-RNTI), its
associated transport/logical channel and value range are given in the below table and are thoroughly
discussed subsequently.
Below is the summary of RNTIs (except I-RNTI) and their corresponding
DCI formats.
P-RNTI (Paging-RNTI)
P-RNTI
is used by the UEs for the reception of paging in the downlink.
It
is also used to inform the UEs about SI Modifications and/or ETWS/CMAS
notifications using ‘Short Message’ transmitted with P-RNTI over DCI. Based on
the si-SchedulingInfo configuration, the UEs after receiving ‘Short
Message’, shall acquire SIB6 for ‘ETWS primary notification’ or SIB7 for ‘ETWS
secondary notification’ or SIB8 for CMAS notification.
Both
Paging messages and Short Messages are addressed to P-RNTI on
PDCCH, but while paging is sent on PCCH logical channel, the SI
Modification/ETWS/CMAS is sent over PDCCH directly.
P-RNTI
is a common RNTI meaning that, it is not allocated to any UE explicitly. It’s
value is fixed/standardized to be 65534 (0xFFFE).
PDCCH
DCI format 1_0 is used for paging and notification for SI modifications/ETWS/CMAS
purposes.
Paging
message is carried by PCCH logical channel which is mapped to PCH transport
channel. The PCH transport channel is mapped to PDSCH physical channel. The gNB
scrambles PDCCH’s CRC with P-RNTI for transmission of PDSCH that carries paging
information.
SI-RNTI (System Information-RNTI)
SI-RNTI
is used for identification
of Broadcast and System Information in the downlink.
It
is a common RNTI meaning that, it is not allocated to any UE explicitly. SI-RNTI’s value is
fixed/standardized to be 65535 (0xFFFF).
PDCCH DCI format 1_0 is
used for SI purpose.
Broadcast
of System Information uses BCCH logical channel which is then mapped to DL-SCH
transport channel which intern mapped to PDSCH physical channel.
The
UE shall receive the PDCCH scrambled
with SI-RNTI in the PDCCH monitoring occasion(s) for SI
message acquisition, from the start of the SI-window and continue until
the end of the SI-window or until the SI message was received.
If
the SI message was not received by the end of the SI-window, repeat
reception at the next SI-window occasion for the concerned SI message in
the current modification period.
RA-RNTI (Random
Access-RNTI)
RA-RNTI is used to identify the Random Access Response (RAR) in the
downlink.
As part of RA procedure, the gNB’s MAC generates RAR(s) as a response to
the Random Access Preamble transmitted by the UE(s). 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 configured by ra-ResponseWindow.
RA_RNTI
calculation (given below) is a bit different as compared to LTE.
RA-RNTI= 1 +
s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id
-
s_id is the index of the first OFDM symbol of the
specified PRACH (0 £ s_id < 14)
-
t_id
is the index of the first slot of the specified PRACH in a system frame (0 £ t_id <
80)
-
f_id
is the index of the specified PRACH in the frequency domain (0 £ f_id <
8)
-
ul_carrier_id
is the UL carrier used for Msg1 transmission (0 for NUL and 1 for SUL carrier)
RA_RNTI unambiguously identifies which time-frequency
resource was utilized by the UE to transmit the RA preamble.
There is a chance that more than one UE is transmitting same RA preamble
(Msg1), and all those UEs would be monitoring for RAR on PDCCH DCI scrambled by
same RA-RNTI i.e., multiple UEs might decode PDCCH scrambled by the same
RA-RNTI.
The RAR is transmitted on DL-SCH transport channel which intern is
mapped to PDSCH. The gNB scrambles PDCCH’s CRC with RA-RNTI for transmission of
PDSCH that carries RAR(s).
TC-RNTI (Temporary
C-RNTI)
As part of Random Access procedure, the gNB’s MAC generates Random
Access Response (RAR) as a response to the RA Preamble transmitted by the UE.
The MAC RAR contains TC-RNTI which shall be used by the UE for the rest of the
RA procedure.
The UE shall use TC-RNTI received in RAR for scrambling of Msg3 (PUSCH
corresponding to RAR grant) and it’s retransmissions in the uplink. If the gNB
fails to decode Msg3, it would send a grant (DCI format 0_0) to the UE
which is addressed to TC-RNTI.
In downlink, (if applicable) UE Contention Resolution (UE Contention
Resolution Identity MAC Control Element) is sent on PDSCH for which PDCCH (DCI
format 1_0) will be scrambled by TC-RNTI.
The TC-RNTI is promoted to C-RNTI for a UE which wins the contention and
does not already have a C-RNTI. It is dropped by others (for which contention
is not successful).
The UE shall discard the TC-RNTI value received in RAR during
non-contention based random access procedure.
A UE which successfully completes RA procedure and already have a
C-RNTI, resumes using its C-RNTI and discards TC-RNTI received in RAR.
TC-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
C-RNTI (Cell-RNTI)
C-RNTI is a unique identification used for identifying RRC Connection
and scheduling.
C-RNTI is dedicated to a particular UE. The gNB uses C-RNTI to allocate
a UE with uplink grants, downlink assignments, PDCCH order etc. C-RNTI is also
used by the gNB to differentiate uplink transmissions (e.g. PUSCH, PUCCH) of a
UE from others.
The gNB assigns different C-RNTI values to different UEs. When Carrier
Aggregation is configured, the same C-RNTI applies to all serving cells.
In MR-DC, two C-RNTIs are independently allocated to the UE: one for
MCG, and one for SCG.
After successful completion of contention based RA procedure, the UE
promotes Temporary C-RNTI to C-RNTI (if the UE doesn’t have a C-RNTI already)
During handover within NR or from other RAT to NR, C-RNTI (as newUE-Identity)
is explicitly provided by the network in the reconfiguration message.
During contention-free RA procedure for ‘Beam Failure Recovery’, the UE
monitors for RAR using C-RNTI instead of RA-RNTI.
The gNB can dynamically allocate resources to UEs via the C-RNTI on
PDCCH(s). DCI Format 0_0 and 0_1 are used for uplink grants and DCI Format 1_0
and 1_1 are used for downlink assignments on PDCCH.
C-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
MCS-C-RNTI
MCS-C-RNTI is an unique UE identification used for indicating an alternative
MCS table for PDSCH and PUSCH.
For scheduling data packets with higher reliability (e.g. URLCC), 64QAM
MCS tables containing entries with lower spectral efficiency are introduced for
both downlink and uplink. The MCS tables can be configured semi-statically or
dynamically. The dynamic signaling of MCS table is supported by configuring the
UE with MCS-C-RNTI, where the scrambling of DCI CRC by MCS-C-RNTI indicates the
64QAM MCS tables with entries of lower spectral efficiency.
For uplink, if the UE is configured with MCS-C-RNTI and the PUSCH is
scheduled by a PDCCH with CRC scrambled by MCS-C-RNTI, the UE shall use IMCS
and Table 5.1.3.1-3 from TS 38.214 to determine the modulation order
(Qm) and Target code rate (R) used in the physical
uplink shared channel.
For downlink, if the UE is configured with MCS-C-RNTI and the PDSCH is
scheduled by a PDCCH with CRC scrambled by MCS-C-RNTI, the UE shall use IMCS
and Table 5.1.3.1-3 from TS 38.214 to determine the modulation order
(Qm) and Target code rate (R) used in the physical
downlink shared channel.
The gNB can configure the UE with MCS-C-RNTI via RRC configuration through
IE PhysicalCellGroupConfig.
MCS-C-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
CS-RNTI (Configured
Scheduling-RNTI)
CS-RNTI is an unique UE identification used for Semi-Persistent
Scheduling (SPS) in the downlink and configured grant in the uplink. The
handling of CS-RNTI is similar to LTE’s SPS C-RNTI.
In the downlink, Activation and deactivation of SPS is done via
PDCCH addressed to CS-RNTI. Retransmissions for the original transmission using
SPS downlink assignment can also be scheduled using PDCCH addressed to CS-RNTI.
In the uplink, two types of configured grants are defined. For ConfiguredGrantType1,
CS-RNTI is only used for re-transmissions as the activation and
deactivation is done by the RRC itself. For ConfiguredGrantType2, CS-RNTI
used for activation, deactivation and re-transmission.
The gNB can configure the UE with CS-RNTI via RRC configuration through
IE PhysicalCellGroupConfig.
CS-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
SP-CSI-RNTI (Semi-Persistent
CSI-RNTI)
SP-CSI-RNTI is used to activate/deactivate semi-persistent CSI reporting
on PUSCH.
For semi-persistent reporting on PUSCH, a set of trigger states are
higher layer configured by CSI-SemiPersistentOnPUSCH-TriggerStateList, where
the CSI request field in DCI format 0_1 scrambled with SP-CSI-RNTI
activates one of the trigger states.
The gNB can configure the UE with SP-CSI-RNTI via RRC configuration through
IE PhysicalCellGroupConfig.
SP-CSI-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
SFI-RNTI (Slot
Format Indication-RNTI)
SFI-RNTI is used for the notification of slot format information over
PDCCH and is generally assigned to a group of UEs.
DCI format 2_0 is used to dynamically change the slot format.
DCI format 2_0 whose CRC scrambled by SFI-RNTI carries Slot
format indicator 1, Slot format indicator 2, ..., Slot format indicator N. The
size of DCI format 2_0 is configurable by RRC layer up to 128 bits. positionInDCI
indicates the (starting) position (bit) of the slotFormatCombinationId
(SFI-Index) for this serving cell (servingCellId) within the DCI payload.
The gNB can configure the UE with SFI-RNTI via RRC configuration within
the IE SlotFormatIndicator which also carries information about total
length of the DCI payload scrambled with SFI-RNTI (dci-PayloadSize).
SFI-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
INT-RNTI (Interruption-RNTI)
INT-RNTI is used for the identification of preemption in the downlink.
The gNB may preempt an ongoing PDSCH transmission to one UE with a
latency-critical transmission to another UE. The gNB can configure UEs to
monitor interrupted transmission indications using INT-RNTI on a PDCCH. If a UE
receives the interrupted transmission indication, the UE may assume that no
useful information to that UE was carried by the resource elements included in
the indication, even if some of those resource elements were already scheduled
to this UE.
DCI format 2_1 is used for notifying the PRB(s) and OFDM
symbol(s) where UE may assume no transmission is intended for the UE.
Normally INT-RNTI is assigned to a group of UEs. The gNB can configure
INT-RNTI and other parameters related to preemption in the IE DownlinkPreemption
within PDCCH-Config IE.
INT-RNTI is of 16-bit in length and its value can range from 1 to 65519
(0x0001 to 0xFFEF).
TPC-PUCCH-RNTI and
TPC–PUSCH-RNTI
TPC-PUSCH-RNTI and TPC-PUCCH-RNTI are used for PUSCH and PUCCH power
control purposes respectively.
DCI format 2_2 is used for the transmission of TPC commands for PUCCH
and PUSCH.
Normally TPC RNTI is assigned to a group of UEs. DCI format 2_2
with CRC scrambled by TPC-PUSCH-RNTI or TPC-PUCCH-RNTI carries TPC commands for
N UEs by means of block number 1, block number 2,.., block number N. For
a specific UE, the parameter tpc-PUSCH or tpc-PUCCH configured by
gNB determines the index to the block number.
The gNB can configure the UE with TPC-PUSCH-RNTI or TPC-PUCCH-RNTI using
IE PhysicalCellGroupConfig via RRC configuration through IE PhysicalCellGroupConfig.
TPC-PUSCH-RNTI/TPC-PUCCH-RNTI is of 16-bit in length and its value can
range from 1 to 65519 (0x0001 to 0xFFEF).
TPC–SRS-RNTI
TPC-SRS-RNTI is used for the identification to control power (TPC
commands over DCI) for Sounding Reference Signal (SRS). This is useful for
UE(s) which doesn’t have a link between SRS and PUSCH power control, either
because of independent power control is desirable or the UE was configured with
a serving cell without PUSCH and PUCCH.
DCI format 2_3 is used for the transmission of a group of TPC commands
for SRS transmissions by one or more UEs. Along with a TPC command, an SRS
request may also be transmitted within DCI.
DCI format 2_3 with CRC scrambled by TPC-SRS-RNTI carries TPC
commands for N UEs by means of block number 1, block number 2,.., block
number N. For a specific UE, the parameter startingBitOfFormat2-3
provided by RRC configuration within SRS-TPC-CommandConfig indicates the
starting bit position of a block within the group DCI with SRS request fields
(optional) and TPC commands.
DCI format 2_3 is applicable for uplink carrier(s) of serving cells
where a UE is not configured for PUSCH and PUCCH transmissions or for uplink
carrier(s) of a serving cell where srs-PowerControlAdjustmentStates indicates
a separate power control adjustment state between SRS transmissions and PUSCH
transmissions.
The gNB can configure the UE with TPC-SRS-RNTI using IE PhysicalCellGroupConfig
via RRC configuration through IE PhysicalCellGroupConfig.
TPC-SRS-RNTI is of 16-bit in length and its value can range from 1 to
65519 (0x0001 to 0xFFEF).
I-RNTI
(Inactive-RNTI)
I-RNTI is used to identify the suspended UE context of a UE in RRC_Inactive
state.
The network assigns I-RNTI to the UE when moving from RRC_CONNECTED to
RRC_Inactive in RRCRelease message within SuspendConfig. Two
types of I-RNTIs namely fullI-RNTI and shortI-RNTI are defined.
The network informs the UE in SIB1 which I-RNTI to be used while resuming the
connection (in the RRCResumeRequest message).
If the field useFullResumeID is present in SIB1, the UE shall use
fullI-RNTI, otherwise, the UE shall use shortI-RNTI.
While resuming the connection, if fullI-RNTI is used, the UE
sends RRCResumeRequest1 message, else if shortI-RNTI is used, the
UE sends RRCResumeRequest message.
The I-RNTI is also used for paging purposes for UEs in RRC_Inactive
state. The network uses only fullI-RNTI as part of PagingUE-Identity
within the Paging message.
The fullI-RNTI is a bit string of length 40 bits while the shortI-RNTI
is a bit string of length 24 bits.
Reference: 3GPP 38.321, 38.212, 38.213, 38.214, 38.300
and 38.331
