RNTIs in LTE

What is RNTI?

RNTI stands for Radio Network Temporary Identifier. RNTIs are 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 eNB, system information transmitted for all the UEs by the eNB etc…

There are a several RNTI types in LTE such as SI-RNTI, P-RNTI, C-RNTI, Temporary C-RNTI, SPS-CRNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI, RA-RNTI, and M-RNTI. Each RNTI’s usage, its value range etc…are discussed in detail below

SI-RNTI (System Information RNTI)

• SI-RNTI is used for broadcast of system information.
• It is a common RNTI meaning that, it is not allocated to any UE explicitly.
• SI-RNTI is of 16-bit in length and its value is fixed to 65535 (0xFFFF). A single SI-RNTI is used to address SystemInformationBlockType1 as well as all SI messages
• 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 UEs should know the scheduling information for PDSCH which is carrying System Information. The required scheduling information is contained in DCI (Downlink Control Information) whose CRC is scrambled by SI-RNTI
• DCI Formats which carries scheduling information for System Information are DCI Format 1A and DCI Format 1C in common search space
• The UE starts decoding PDCCH scrambled with SI-RNTI at the start of SI-Window (for the concerned SI message) until the end of the SI-window, or until the SI message was received excluding the following subframes. 

          - subframe #5 in radio frames for which SFN mod 2 = 0
          - any MBSFN subframes
          - any uplink subframes in TDD

• If the SI message was not received by the end of the SI-window, the UE repeats reception at the next SI-window occasion for the concerned SI message

P-RNTI (Paging RNTI)

• P-RNTI is used by the UEs for the reception of paging.
• It is a common RNTI meaning that it is not allocated to any UE explicitly.
• P-RNTI is of 16-bit in length and its value is fixed to 65534 (0xFFFE).
• 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 eNB scrambles PDCCH’s CRC with P-RNTI for transmission of PDSCH that carries paging information
• DCI Formats which carries scheduling information for paging are DCI Format 1A and DCI Format 1C in common search space

RA-RNTI (Random Access RNTI)

• As part of Random Access procedure, the eNB’s MAC generates Random Access Response (RAR) as a response to the Random Access Preamble transmitted by the UE. RAR is transmitted on DL-SCH transport channel which intern is mapped to PDSCH. The eNB scrambles PDCCH’s 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 RA-RNTI.
• RA-RNTI unambiguously identifies which time-frequency resource was utilized by the UE to transmit the Random Access preamble (explained below)
• RA-RNTI is of 16-bit in length and its value is derived from the below equation where t_id is the index of the first subframe of the specified PRACH (0≤ t_id <10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0 ≤ f_id< 6).

                                              RA-RNTI= 1 + t_id+10*f_id

• For FDD there is at most one PRACH resource per subframe ⇨ f_id = 0 ⇨ RA-RNTI range is 1 to 10 whereas for TDD, the RA-RNTI ranges from 1 to 60
• The values corresponding to the RA-RNTI values of a cell’s PRACH configuration are not used in the cell for any other RNTI (C-RNTI, Semi-Persistent Scheduling C-RNTI, Temporary C-RNTI, TPC-PUCCH-RNTI or TPC-PUSCH-RNTI)
• DCI Formats which carries scheduling information for RAR are DCI Format 1A and DCI Format 1C in common search space

Temporary C-RNTI

• As part of Random Access procedure, the eNB’s MAC generates Random Access Response (RAR) as a response to the Random Access Preamble transmitted by the UE. The MAC RAR contains Temporary C-RNTI
• Temporary C-RNTI is of 16-bit in length and its value can range from 1 to 65523 (0x0001 to 0xFFF3).
• During contention based random access procedure, the UE stores received Temporary C-RNTI (received in RAR) and uses it during random access procedure. The UE shall discard the Temporary C-RNTI value received in RAR during non-contention based random access procedure
• The UE shall use Temporary C-RNTI for scrambling of msg3 (PUSCH corresponding to RAR grant) and it’s retransmissions
• During contention based RA procedure, the UE monitors PDCCH scrambled with Temporary C-RNTI for DCI0 in common search space (This DCI0 monitoring is for possible retransmission of msg3)
• In downlink, UE Contention Resolution (UE Contention Resolution Identity MAC Control Element) is on PDSCH for which PDCCH will be scrambled by Temporary C-RNTI. This is applicable only if UE has included CCCH SDU (RRC Connection Request or RRC Connection Re-establishment) in Msg3
• DCI Format which carriers scheduling information for Contention Resolution in the downlink can be either DCI Format 1A in Common and UE specific search space or DCI Format 1 in UE specific search space
• The Temporary C-RNTI is promoted to C-RNTI for a UE which detects RA success and does not already have a C-RNTI. It is dropped by others (for which contention is not successful)
• A UE which detects RA success and already has a C-RNTI, resumes using its C-RNTI and discards Temporary C-RNTI

C-RNTI (Cell RNTI)

• C-RNTI is a unique identification used for identifying RRC Connection and scheduling which is dedicated to a particular UE.
• The eNB assigns different C-RNTI values to different UEs. When Carrier Aggregation is configured, same C-RNTI applies to all serving cells.
• The eNB uses C-RNTI to allocate a UE with uplink grants, downlink assignments, PDCCH orders etc.
• The eNB also uses C-RNTI to differentiate uplink transmissions (e.g. PUSCH, PUCCH) of a UE from others.
• C-RNTI is of 16-bit in length and its value can range from 1 to 65523 (0x0001 to 0xFFF3).
• After connection establishment or re-establishment the Temporary C-RNTI (as explained above) is promoted to C-RNTI.
•  During Handovers within E-UTRA or from other RAT to E-UTRA, C-RNTI is explicitly provided by the eNB in MobilityControlInfo container with IE newUE-Identity
• DCI Formats 0 and 1A in Common and UE specific search space and DCI Formats 1, 1B, 1D, 2, 2A, 2B, 2C, and 4 in UE specific search space will be transmitted on PDCCH with CRC scrambled by the C-RNTI
• The scrambling initialization of PDSCH corresponding to the PDCCHs with DCI Formats 1, 1A, 1B, 1D, 2, 2A, 2B, and 2C is by C-RNTI
• The scrambling initialization of the PUSCH corresponding to the PDCCHs with DCI Format 0/4 and the PUSCH retransmission for the same transport block is by C-RNTI

SPS C-RNTI (Semi-Persistent Scheduling C-RNTI)

• SPS C-RNTI identifies semi-persistent grants/assignments. 
• SPS C-RNTI is a dedicated RNTI and is configured by RRC. eNB configures the UE with a SPS C-RNTI as part of sps-Config as discussed in detail here
• The CRC parity bits obtained for the PDCCH payload are scrambled with the SPS C-RNTI for SPS activation, release, re-activation and retransmission
• SPS C-RNTI is of 16-bit in length and its value can range from 1 to 65523 (0x0001 to 0xFFF3).
• For SPS activation/re-activation/retransmission in downlink, DCI Format 1A (common and UE specific search space) and DCI Formats 1/2/2A/2B/2C (UE specific search space) with PDCCH’s CRC is scrambled by SPS C-RNTI are used. The scrambling initialization of PDSCH corresponding to these PDCCHs and PDSCH without a corresponding PDCCH is by SPS C-RNTI
• For SPS release in downlink, DCI Format 1A (common and UE specific search space) with PDCCH’s CRC is scrambled with SPS C-RNTI is used
• For SPS activation/re-activation/retransmission/release in uplink, DCI Format 0 (common and UE specific search space) with PDCCH’s CRC is scrambled by SPS C-RNTI is used. The scrambling initialization of PUSCH corresponding to these PDCCHs and PUSCH retransmission for the same transport block is by SPS C-RNTI. The scrambling initialization of initial transmission of PUSCH without a corresponding PDCCH and the PUSCH retransmission for the same transport block is by SPS C-RNTI.
• UE monitors SPS C-RNTI only on PCell

TPC RNTI (Transmit Power Control RNTI)

• TPC RNTI is used for uplink power control purpose. There are two types, TPC-PUSCH-RNTI and TPC-PUCCH-RNTI
• Normally TPC RNTI is assigned to a group of UEs. eNB may configure the UE with TPC-PUSCH-RNTI and TPC-PUCCH-RNTI via RRC signalling
• TPC-PUSCH-RNTI/TPC-PUCCH-RNTI is of 16-bit in length and its value can range from 1 to 65523 (0x0001 to 0xFFF3).
• DCI format 3/3A (common search space) whose CRC is scrambled with TPC-PUCCH-RNTI is used for the transmission of TPC commands for PUCCH
• DCI format 3/3A (common search space) whose CRC is scrambled with TPC-PUSCH-RNTI is used for the transmission of TPC commands for PUSCH
• DCI format 3 is used for the transmission of TPC commands for PUCCH and PUSCH with 2-bit power adjustments. DCI format 3A is used for the transmission of TPC commands for PUCCH and PUSCH with single bit power adjustments.
• The notation 3/3A implies that the UE shall receive either DCI format 3 or DCI format 3A depending on the configuration

M-RNTI (MBMS RNTI)

• Indication of an MBMS specific RNTI, the M-RNTI, on PDCCH is used to inform UEs in RRC_IDLE and UEs in RRC_CONNECTED about an MCCH information change
• The DCI format 1C (common search space) with M-RNTI is used for notification and includes an 8-bit bitmap to indicate the one or more MBSFN Area(s) in which the MCCH change(s)
• M-RNTI is of 16-bit in length and its value is fixed to 65533 (0xFFFD)

Reference 3GPP TS 36.300, 36.331, 36.211, 36.212, 36.213, 36.321