top of page
Search

Radio Network Temporary Identifiers (RNTIs) in 5G NR

  • Writer: Venkateshu Kamarthi
    Venkateshu Kamarthi
  • Aug 1, 2025
  • 7 min read

Introduction

In 5G NR, Radio Network Temporary Identifiers (RNTIs) are pivotal for distinguishing UEs (User Equipments), groups of UEs, or specific types of messages and procedures at the physical (PHY) and MAC layers. RNTIs are used by the gNB (5G base station) and UEs to exchange, scramble, or decode all critical messages—controlling everything from initial access to fine-grained scheduling and power control. Each RNTI serves specific functions and scenarios in the RAN architecture, ensuring robust, secure, and flexible resource management.

 

Think of RNTI masking like a key that locks/unlocks the control messages:

  • The CRC is the lock.

  • The RNTI is the key.

  • Only the UE with the right “key” (RNTI) can “unlock” and verify the control message by properly unmasking the CRC checksum.

 

Core principles of RNTI:

  • 16-bit Identifier: (Except some control-plane RNTIs) Each RNTI is a 16-bit value that scrambles the CRC of certain DL/UL control or data messages.

  • Context-Specific: RNTIs may be dedicated (per-UE) or common (shared) depending on context and procedure.

  • Dynamic Lifecycle: Most RNTIs are valid only for specific protocol phases, events, or procedures.


Comprehensive List of 5G NR RNTIs: Usage, Functionality & Scenarios

Below are the most significant RNTIs in 5G NR, grouped by function, with typical scenarios of use.

 

1. C-RNTI (Cell RNTI)

  • Purpose: Uniquely identifies a connected (RRC_CONNECTED) UE within a cell. Used for DL/UL scheduling, HARQ, and dedicated RRC procedures.

  • Assignment: Allocated post-successful random access or during handover.

  • Scenarios:

    • gNB uses C-RNTI to address PDCCH grants for specific UEs.

    • UE uses C-RNTI for uplink transmission identification.

  • How Generated?:

    • The gNB selects a unique value per active UE in the cell.

    • Provided to UE in contention resolution or as part of RRC Connection Setup/Resume or via SIB for non-contention based random access.

  • Lifecycle: Until UE leaves the cell or suspends its context.

2. Temporary C-RNTI (TC-RNTI)

  • Purpose: Temporarily identifies a UE during random access and handover before a permanent C-RNTI is assigned.

  • Assignment: Delivered in Random Access Response (RAR); promoted to C-RNTI upon successful contention resolution.

  • How Generated?:

    • During random access, the gNB includes a Temporary C-RNTI in the RAR (Random Access Response).

    • UE stores the value received as TC-RNTI for further message exchanges during contention resolution.

  • Scenarios:

    • Used in random access procedures and handover initiation.

3. RA-RNTI (Random Access RNTI)

  • Purpose: Identifies UEs awaiting a Random Access Response during the PRACH procedure.

  • Assignment: Calculated based on PRACH preamble time/frequency indices. Not unique—may address multiple UEs running RA simultaneously.

  • Scenarios:

    • Used for the scrambling of PDCCH during RAR phase.

    • UE monitors for RA-RNTI-masked messages right after sending PRACH preamble.

  • How Generated?:


    The RA-RNTI is tied to the time-frequency resources of the PRACH occasion for which the UE sends its preamble.

    • Formula:


      RA-RNTI = 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id

    • s_id: index of the first OFDM symbol of the PRACH occasion,0 ≤ s_id < 14

    • t_id:index of the first slot of the PRACH occasion in a system frame,   0 ≤ t_id < 80 

    • f_id:index of the PRACH occasion in the frequency domain,0 ≤ f_id < 8

    • ul_carrier_id:UL carrier used for Random Access Preamble transmission

      • 0 for NUL carrier

      • 1 for SUL carrier

  • Both UE (using PRACH configuration) and gNB (scheduling) can derive the same value.

 

4. P-RNTI (Paging RNTI)

  • Purpose: Common to all UEs in cell; used to broadcast paging or system information change notifications.

  • How Generated?:

    • Value is always 65534 (0xFFFE)

  • Usage:

    • Used by all UEs to detect paging DCI/messages. gNB uses it to page any idle/inactive UE.

  • How Generated?:

    • Value is always 65535 (0xFFFF)

  • Scenarios:

    • Any UE in idle/inactive mode listens for P-RNTI to detect incoming paging.

5. SI-RNTI (System Information RNTI)

  • Purpose: Used by gNB to broadcast System Information (SI) to all UEs in a cell via the SI messages.

  • How Generated?:

    • Value is always 65535 (0xFFFF)

  • Scenarios:

    • UEs monitor SI-RNTI for decoding MIB/SIBs during cell search and re-selection.

 

6. MCS-C-RNTI (Modulation and Coding Scheme RNTI)

  • Purpose: When gNB needs to dynamically (differently) schedule multiple UEs for updated MCS settings using a special DCI format.

·       Values are either picked from valid 16-bit ranges defined by 3GPP, assigned for a specific group or UE, and communicated to targeted UEs via higher layer signaling (RRC).

  • Scenarios:

    • Used for dynamic link adaptation.

7. CS-RNTI (Configured Scheduling RNTI)

  • Purpose: Allows the gNB to schedule resources in a pre-configured, semi-dynamic fashion (e.g., for URLLC/IoT devices).

  • Values are either picked from valid 16-bit ranges defined by 3GPP, assigned for a specific group or UE, and communicated to targeted UEs via higher layer signaling (RRC).

  • Scenarios:

    • Used for low-latency scheduled transmissions in time-sensitive IoT or URLLC scenarios.

8. TPC-RNTIs (Transmit Power Control RNTIs)

  • Purpose: Handle uplink transmit power control commands for sets of UEs.

·       Values are either picked from valid 16-bit ranges defined by 3GPP, assigned for a specific group or UE, and communicated to targeted UEs via higher layer signaling (RRC).

  • Types & Scenarios:

    • TPC-PUCCH-RNTI: Power control for Uplink Control Channel.

    • TPC-PUSCH-RNTI: Power control for Uplink Shared Channel.

    • TPC-SRS-RNTI: Power control adjustments for SRS transmissions.

  • Typical Use: Used for uplink power regulation commands to groups of UEs by the gNB.

9. INT-RNTI (Interruption RNTI)

  • Purpose: Signals certain types of downlink interruptions or resource preemption.

  • Scenarios:

    • Used by the network to preempt ongoing downlink transmissions to prioritize more urgent traffic.

10. SFI-RNTI (Slot Format Indicator RNTI)

  • Purpose: Indicates slot format changes (e.g., switch between UL, DL, flexible) to all UEs in TDD-configured NR cells.

  • Scenarios:

    • Employed in dynamic TDD slot adaptation.

11. SP-CSI-RNTI (Semi-Persistent CSI RNTI)

  • Purpose: Used for activation and resource allocation of semi-persistent Channel State Information (CSI) reporting in the UL.

  • Scenarios:

    • Relevant for massive MIMO or advanced scheduling where periodic UL channel reporting is essential.

12. SL-RNTI (Sidelink RNTI family)

  • Purpose: Identify and manage sidelink transmissions for V2X (vehicle-to-everything) and device-to-device communications.

  • Types:

    • SL-RNTI: Dynamically scheduled SL.

    • SLCS-RNTI: Configured SL.

    • Semi-Persistent Scheduling V2X-RNTI: For V2X persistent scheduling.

 

13. I-RNTI (Inactive RNTI)

  • Purpose: RRC-level RNTI to identify a suspended context in RRC_INACTIVE state; used for faster session resumption and efficient idle-inactive cycling.


Typical Lifecycle of RNTI Usage

  1. Cell Search/Initial Access:

    • UE decodes SI-RNTI or P-RNTI for system information and paging.

  2. Random Access:

    • UE derives RA-RNTI to listen for gNB's random access response.

    • Receives Temporary C-RNTI for initial transmission.

  3. Connection Establishment:

    • gNB assigns a permanent C-RNTI upon successful random access and contention resolution.

  4. Scheduling:

    • gNB uses C-RNTI for dedicated messages and grants.

    • May use MCS-C-RNTI or CS-RNTI for advanced scheduling.

  5. Power Control:

    • TPC-RNTIs are periodically used for uplink power adjustments.

  6. Mobility/Handover:

    • TC-RNTI and (re)assignment of C-RNTI for seamless transition.

  7. Advanced and Group Messaging:

    • SP-CSI-RNTI, SFI-RNTI, INT-RNTI, and various SL-RNTIs are used for massive MIMO, TDD flexibility, multicast, or D2D/V2X.

 

Table: Quick Reference – Selected RNTIs, Usage, and Scenarios

RNTI Type

Usage Scenario

Typical Physical Channel

Purpose/Benefit

C-RNTI

Connected UE, individual grant

PDCCH, PUSCH, PUCCH

Dedicated addressing, scheduling

Temporary C-RNTI

Initial RA, handover, msg3

PUSCH, PDCCH

Temporary ID before full setup

RA-RNTI

Random Access

PDCCH (RAR phase)

Addressing UEs doing RA procedure

P-RNTI

Paging

PDCCH, PDSCH

Notify idle/inactive UE for new messages

SI-RNTI

System Info Broadcast

PDSCH

System Info for all UEs

MCS-C-RNTI

Dynamic MCS Scheduling

PDCCH

Group/unicast dynamic scheduling

CS-RNTI

Configured/Low-latency sched.

PDCCH

Semi-persistent, URLLC/IoT

TPC-[x]-RNTI

Uplink Power Control

PDCCH

Command many UEs to adjust transmit power

INT-RNTI

Resource preemption

PDCCH

Preempt ongoing DL transmission

SFI-RNTI

Slot Format context

PDCCH, system signals

Indicate slot (UL/DL/Flexible) change

SP-CSI-RNTI

Semi-persistent CSI activation

PUSCH, PUCCH

Advanced CSI/massive MIMO

SL-RNTI family

Sidelink/V2X comms

SL-SCH, SCCH/STCH

Group D2D or V2X communication orchestration

I-RNTI

RRC Inactive context

RRC signalling

Fast resume for mobility/energy savings

Additional Technical Considerations

  • RNTI Masking: RNTIs mask (scramble) PDCCH DCI CRCs, so UEs only decode messages relevant to their current procedural state, saving power and resources. The CRC bits are XORed (masked) with the 16-bit RNTI value relevant for that message (e.g., C-RNTI, RA-RNTI, P-RNTI).

Masking the CRC with an RNTI enables each UE to filter out only the relevant control information intended specifically for it or its group.

  • Group vs Dedicated RNTIs: Some (like TPC-RNTIs) apply to groups of UEs; others (like C-RNTI) are unique per UE.

  • Dynamic Value Assignment: Temporary and special-purpose RNTIs (e.g., TC-RNTI, RA-RNTI) are managed by protocol timers and event triggers, automatically retired when obsolete.

 

DCI Formats and RNTIs used

A UE identifies the payload size and RNTI used for CRC scrambling for a DCI format. There are various DCI formats and RNTIs, as summarized in Table below.


LTE RNTI Types

LTE (as per 3GPP 36.321/36.212) includes these main RNTIs:

  • C-RNTI (Cell RNTI): Uniquely identifies a connected UE within a cell.

  • Temporary C-RNTI (T-CRNTI): For use during random access or before contention resolution.

  • RA-RNTI (Random Access RNTI): Identifies UEs during the random access (RA) procedure.

  • P-RNTI (Paging RNTI): Used for paging messages to groups of UEs.

  • SI-RNTI (System Information RNTI): Used for broadcast system information.

  • SPS-C-RNTI (Semi-Persistent Scheduling C-RNTI): For UEs configured for SPS (primarily VoLTE).

  • TPC-PUSCH-RNTI / TPC-PUCCH-RNTI: Group RNTIs for uplink transmit power control for PUSCH/PUCCH.

  • M-RNTI (MBMS RNTI): Used for MBMS (Multimedia Broadcast Multicast Service).


Building on LTE, 5G NR (as defined in 3GPP TS 38.321) keeps many legacy RNTIs but introduces new ones to handle advanced 5G requirements such as URLLC, flexible slot structures, massive MIMO, NR sidelink, and sophisticated scheduling.


RNTI Type

LTE Present

5G NR Present

5G NR Specific Purpose

C-RNTI

Yes

Yes

Dedicated addressing

T-CRNTI

Yes

Yes

Temporary, Random Access

RA-RNTI

Yes

Yes

Random Access Response

P-RNTI

Yes

Yes

Paging messages

SI-RNTI

Yes

Yes

System Information Broadcast

SPS/CS-RNTI

Yes

Yes, expanded

Configured/Semi-persistent scheduling

TPC-RNTI

Yes

Yes, expanded

Per-channel (PUSCH/PUCCH/SRS in NR) power control RNTIs

M-RNTI

Optional

No

MBMS (not carried forward in NR as is)

MCS-C-RNTI

No

Yes

Dynamic group MCS scheduling

INT-RNTI

No

Yes

Interruption/preemption signaling

SFI-RNTI

No

Yes

Slot format indication for TDD

SP-CSI-RNTI

No

Yes

Semi-persistent CSI reporting scheduling

SL-RNTI Family

Limited

Yes

NR V2X, sidelink, group communications

I-RNTI

No

Yes

RRC INACTIVE fast context restoration

PO-RNTI

No

Yes

Paging Occasion alignment

Conclusion

RNTIs are the "control keys" of the 5G NR air interface, making fine-grained scheduling, efficient group/broadcast control, robust random access, and seamless handover possible. They power everything from low-level physical transmissions to advanced features like NR sidelink, URLLC, and massive MIMO.

 

References:

 

​

 

bottom of page