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

25 comments:

  1. Hi Kumar

    You have explained so nicely !!!
    I have a question, the last paragraph of RA-RNTI says that "DCI Formats which carries scheduling information for RAR are DCI Format 1A and DCI Format 1C in common search space". So if RAR carries the Uplink Grant then why not DCI format 0 carries the Scheduling Information for RAR, why DCI 1A or 1C is used ?

    Regards
    Benson

    ReplyDelete
  2. hi swami .

    Good Material
    But kindly correct this
    RA-RNTI= 1 + t_id+10*f_id
    t_id ranges 0,9
    f_id ranges 0,5
    you have something like 0,10 for t_id and 0,6 for f_id which is incorrect

    update your material as below

    where t_id addresses the first subframe of the used PRACH, resulting in a possible range between 0
    and 9, and f_id is the ascending frequency index within this subframe with a range between 0 and 5.

    Thanks
    Beginner in LTE

    ReplyDelete
    Replies
    1. Hi,

      The ranges given in this post are correct. I wrote < 10 for t_id and < 6 for f_id so it is correct

      Delete
  3. Agreed :) ,re-read your material

    ReplyDelete
  4. How can 129 bits fit into DCI format 0 which allows 1 CCE = 72 bits only.
    When exactly DCI format 0 is used and how?
    Isn't the case that DCI format 1A is used always instead as for all 1.4 to 20 MHz minimum bits generated from encoding is >72 bits (more than one CCE) which DCI format 0 doesn't allow.

    ReplyDelete
    Replies
    1. Hi ,
      in simple words:
      1CCE =72 CCE but you have to multiply by modulation also. if its QPSk then 72x2= 144 bits so now 129 bit fit in DCI 0 .... &
      DCI 0 /4 use for uplink assignment .
      DCI 1x/2x use for DownLink assignment.
      DCI 3/3A for TPC

      Thanks,
      Prashant

      Delete
  5. What does blank RNTI Type mean ?

    ReplyDelete
    Replies
    1. what do you mean, Could you please elaborate your question?

      Delete
  6. Hi Kumar,
    Why SPS-CRNTI is needed ? Doesn't UE have to do blind decoding with both CRNTI and SPS-CRNTI ?

    ReplyDelete
    Replies
    1. Yes the UE have to try decoding with both C-RNTI and SPS CRNTI. I am not sure if I have understood your question. Could you please elaborate?

      Delete
    2. Thanks for your response Kumar.

      I am not sure when UE should start using SPS-RNTI ? Immediately after
      receiving SPS config ?

      Lets consider the following scenarios:

      1) After receiving SPS configuration, It may be activated any time and eNB has indicate this to UE in DL using DCI, so this ACTIVATION Command DCI is scrambled with SPS RNTI or CRNTI?

      What I mean that, after receiving SPS config, UE have to try decoding PDCCH with both CRNTI and SPS-CRNTI?

      Since during SPS process, ENB can also perform dynamic scheduling .

      Isn't only C-RNTI was enough ?

      Delete
    3. Yes, the UE has to start decoding PDCCH with both CRNTI and SPS CRNTI as soon as the SPS is configured in the UE. Please refer to the other post on SPS which has a detailed explaination:
      http://howltestuffworks.blogspot.se/2013/10/semi-persistent-scheduling.html

      Delete
    4. Hi Kumar,
      Even I am wondering why SPS-RNTI is needed ? Was It really necessary to provide new RNTI just for SPS allocation. Simple field in DCIs should do it?
      Given that UE have to decode PDCCH with both CRNTI & SPS-CRNTI,UE has to do several blind decodings

      Delete
    5. The UE after calculating CRC parity bits, just has to check if the CRC bits are scrambled by C-RNTI or SPS C-RNTI which doesn't need so much processing as one might be thinking, it is completely different to blind decodings.

      Coming to your point, a "special field in DCI", I am not sure how much it can save from this situation.
      The UE has to check for DCI payload with and without the "special field" you have mentioned. It could be a considerable amount of signalling overhead (on PDCCH) to always carry this special field for a set of UE's configured with SPS.

      Delete
  7. Hello Kumar,
    I have a question regarding RACH Procedure.

    What happens when Multiple UEs send the same 'random value (40 Bits)' in MSG3 ?
    In this case, won't RAR PDU would mach for all UEs and all of them would send ACK ??

    ReplyDelete
    Replies
    1. The random number is a 40-bit number so the chances of generating the same random nummber by 2 UEs is almost zero.

      Delete
    2. I agree. But what if it happens :) I mean how long It can continue ?

      Delete
  8. Thank you for such a good article.
    In DL, PDCCH is scrambled with UE RNTI(CRNTI or SPS rnti) , so that UE can know
    DCI belonged to him.
    Then in UL, how eNB knows that particular PUCCH/PUSCH belongs which UE?

    Also how does eNB knows type of PUCCH format used by UE?

    ReplyDelete
    Replies
    1. For PUSCH, the information bits are scrambled with ue specific scrambling sequence which depends on the rnti. Please refer to 5.3.1 of 36.211. More importantly, the set of RBs on which PUSCH is received should let the eNodeB know who is transmitting it.

      For PUCCH,
      Cyclic shift and orthogonal spreading codes differentiate multiple UEs when PUCCH is transmitted by multiple UEs in the same Resource block pair

      Delete
    2. Hi Kumar,
      How this orthogonal sequence are communicated to UEs? Which message carries this?

      Delete
  9. Thank you for such a good article.

    I have some following questions:
    1. How eNB assigns TC-RNTI in RAR message? Perhaps randomly choose?
    2. When will UEs release TC-RNTI or C-RNTI back to eNB and how to inform eNB
    to know that?

    Thanks

    ReplyDelete
    Replies
    1. The UE never informs the eNodeB about the release of C-RNTI. The C-RNTI is linked to a particular RRC Connection/Context. When the Connection or context is released, the corresponding RNTI is released

      Delete
    2. And how eNB assigns TC-RNTI ? Is it up to eNB or depends on preamble send by UE ?

      Delete
  10. Who generates the C-RNTI?

    Will a UE have the same C-RNTI if it gets latched to another eNodeB or will it be assigned a new C-RNTI by the new eNodeB?

    ReplyDelete
  11. SPS-RNTI is not UE Specific it is common for all , for UE specific resource allocation CNTI is used for , pattern based resource allocation means semi persistence resource allocation the PDCCH DCI CRC is scrambled with SPS-RNTI . SPS-RNTI is used in case of VoLTE where alot of scheduling request are sent in network because of progressive communication model , so avoid uplink signalling load SPS-RNTI scrambled PDDCH which will allocate resource in a semi persistence manner will be configured for a UE .

    ReplyDelete