Description
The Cell Radio Network Temporary Identifier (C-RNTI) is a fundamental identifier used in 3GPP radio access networks (RAN), including UMTS, LTE, and NR. It is a 16-bit value in LTE and NR, uniquely assigned by the serving base station (eNB in LTE, gNB in NR) to a User Equipment (UE) upon successful random access and connection establishment within that specific cell. The C-RNTI's primary role is to serve as a temporary address for the UE on the physical and MAC layers, allowing the network to efficiently manage and direct radio resources to that specific user.
Architecturally, the C-RNTI is a key component of the RAN's control plane. It is used to scramble the Cyclic Redundancy Check (CRC) of Downlink Control Information (DCI) messages on the Physical Downlink Control Channel (PDCCH). When a UE monitors the PDCCH, it attempts to decode DCI messages using its assigned C-RNTI as part of the de-scrambling process. A successful decode indicates that the control message (e.g., an uplink grant or downlink assignment) is intended for that specific UE. This mechanism provides secure and efficient addressing without requiring constant transmission of longer, permanent UE identities over the vulnerable air interface.
The C-RNTI is central to dynamic scheduling. For every Transmission Time Interval (TTI), the scheduler in the base station uses the C-RNTI to address grants and assignments to specific UEs. It is used for both uplink (UL-SCH) and downlink (DL-SCH) shared channel transmissions. The identifier is temporary and cell-specific; if a UE hands over to a new cell, it is assigned a new C-RNTI by the target cell. This ensures identifier uniqueness within a cell's coverage area and simplifies resource management. The C-RNTI is released when the UE's RRC connection is released or during handover procedures.
Beyond basic scheduling, the C-RNTI plays a role in other procedures. It is used for contention-based random access, where a UE may be assigned a Temporary C-RNTI initially, which can later be confirmed as its permanent C-RNTI for the connection. In connected mode, it is used for power control commands (TPC-PUCCH-RNTI, TPC-PUSCH-RNTI are derived concepts) and other MAC control elements. Its temporary nature is a critical security and privacy feature, preventing long-term tracking of a UE based on its radio signaling identifier.
Purpose & Motivation
The C-RNTI was created to solve the fundamental problem of efficiently and securely addressing a specific User Equipment within a radio cell for the purpose of resource allocation and control signaling. Prior to concepts like the C-RNTI, networks might have relied on longer, permanent identifiers for scheduling, which would be inefficient for frequent, small control messages and would pose a significant privacy risk due to the ease of tracking a device over the air.
Its introduction, particularly as LTE was designed, was motivated by the need for a highly dynamic, packet-scheduled air interface. Unlike circuit-switched systems, LTE and NR allocate resources on a millisecond basis. Transmitting a full UE identity (like the IMSI or S-TMSI) with every scheduling grant would create enormous overhead. The C-RNTI provides a short, locally significant handle that minimizes control channel overhead while enabling the high-speed, low-latency scheduling required for broadband data services.
The C-RNTI also addresses security and privacy concerns. By being temporary and cell-specific, it mitigates the risk of passive eavesdroppers tracking a user's location and connection patterns over a wide area. A UE is assigned a new C-RNTI in each cell, breaking the linkability of its radio signaling identity across different locations. This design is a core part of 3GPP's subscriber privacy protections. Furthermore, it simplifies RAN implementation by confining identifier management to a single cell or gNB, avoiding the need for global coordination of these temporary addresses.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (80 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, the C-RNTI function was extended to the new NR access technology, as part of the introduction of New Radio. This required defining and clarifying related procedures for PDCCH monitoring, including its use for scrambling PUSCH transmissions scheduled by a Random Access Response uplink grant.
- Introduction of New Radio Access Technology in TS 36.300 TS 36.300CR0998
- Clarifying PDCCH Period Definition TS 36.321CR1300
- Defining PDCCH-Subframes for NB-IoT UE TS 36.321CR1327
- Clarification of PDCCH monitoring when not fully aligned with PDCCH periods TS 36.321CR1459
- CR to 36.331 on alignment of use of fullI-RNTI and I-RNTI in paging and InactiveConfig (Alt.2) TS 36.331CR3810
- PDCCH monitoring for overlapped CORESETs TS 38.213CR0018
+ 24 more changes
In Release 16, specific corrections were made to the C-RNTI function regarding its replacement and the conditions for its use during a 2-step Random Access (RA) procedure. Furthermore, a correction was specified for the prioritization between a Downlink Control Information (DCI) with a Downlink Pre-emption Indicator (DCP) and a Random Access Response (RAR) addressed to the C-RNTI during a Contention-Free Random Access (CFRA) for Beam Failure Recovery (BFR). These updates refined the existing rules for C-RNTI handling without introducing new RNTI types or altering its fundamental definition.
- Addition of PUR RNTI in E-UTRA related UE identities TS 36.300CR1297
- Introducing UE Radio Capability Mapping procedure for EN-DC TS 36.300CR1314
- PDCCH-based HARQ-ACK for a specific HARQ process with multi-TB scheduling TS 36.321CR1517
- Incorrect restriction for RLC UM radio bearers TS 36.331CR4385
- Type-3 CSS monitoring with PS-RNTI on primary cell TS 38.213CR0129
- CR on correction on PDCCH monitoring for DAPS HO TS 38.213CR0132
+ 10 more changes
In Release 17, the C-RNTI function itself was not directly modified, but several related procedures for PDCCH monitoring and reception were corrected and enhanced. These changes included corrections for multi-slot PDCCH monitoring in NR-DC and CA scenarios, as well as clarifications on PDCCH skipping and PDCCH Ordered RACH for an SCell. Furthermore, a new UE capability to limit PDCCH monitoring was introduced.
- Align GUTI allocation to best practices of unpredictable identifier generation TS 33.401CR0702
- Avoid linkage between security functions and UE Radio Access Capabilities TS 33.401CR0708
- CR on PDCCH repetition with SSSG switching TS 38.213CR0332
- Correction on the tables for determining Type0 PDCCH monitoring occasions TS 38.213CR0337
- Correction on multi-slot PDCCH monitoring in NR-DC and CA scenarios with mixed capability types TS 38.213CR0342
- Corrections on PDCCH monitoring enhancement for 52-71GHz spectrum TS 38.213CR0346
+ 13 more changes
In Release 18, there were no fundamental changes to the C-RNTI's core definition or allocation principle, as it remains a unique UE identifier within a cell. The release introduced clarifications and corrections for procedures where the C-RNTI is used, such as in PDCCH-ordered contention-free random access (CFRA) for two timing advance groups (2TA) and in the monitoring of PDCCH for specific UE types like RedCap in HD-FDD mode. These updates ensure the C-RNTI's reliable application in enhanced physical downlink control channel (PDCCH) procedures across new scenarios and configurations.
- Clarification of satellite identifiers TS 36.300CR1430
- Clarification of satellite identifiers TS 36.331CR5152
- CR on PDCCH monitoring for dedicated spectrum less than 5 MHz TS 38.213CR0596
- Clarification of PDCCH ordered CFRA for 2TA TS 38.300CR0868
- Clarification on RACH-ConfigCommon for PDCCH order based CFRA and SI request TS 38.331CR4808
- Correction to applied TCI state for mTRP PDCCH reception TS 38.331CR5200
+ 4 more changes
In Release 19, the C-RNTI function itself was not directly modified, but its associated control channel procedures were enhanced. The release introduced PDCCH repetitions for Type0-PDCCH CSS sets in Terrestrial Networks (TNs) and for common PDCCH in Non-Terrestrial Networks (NTN) for TNs, improving reliability for scheduling messages addressed to identifiers like the C-RNTI. Additionally, corrections and alignment were made for parameters related to intra-slot PDCCH repetition and for receiving the Random Access Response when overlapping with an SSB candidate.
- Introduction of PDCCH repetitions for Type0-PDCCH CSS set in TNs [Common_PDCCH_Rep_TN] TS 38.213CR0748
- Introduction of common PDCCH repetition (Rel-19 NTN) for TN [Common_PDCCH_rep_TN] TS 38.300CR1058
- Correction on PDCCH candidates skipping for receiving RAR when overlapping with candidate SSB TS 38.213CR0755
- Alignment on parameter for intra-slot PDCCH repetition TS 38.213CR0761
- Correction on NCR-RNTI configuration TS 38.331CR5623
Explore further
Broader topics and technologies where C-RNTI plays a role.
Defining Specifications
3GPP specifications that define or reference C-RNTI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 25.331 vj00 | UTRAN RRC Protocol Specification | Rel-19 |
| TS 25.423 vj00 | UTRAN RNSAP Specification | Rel-19 |
| TR 25.931 vj00 | UTRAN Signalling Procedures Examples | Rel-19 |
| TS 32.836 vc00 | NM Centralized Coverage and Capacity Optimization Study | Rel-12 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.843 vf10 | Security Study for ProSe UE-to-Network Relay | Rel-15 |
| TS 36.133 vj20 | E-UTRA RRM Requirements | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.401 vj00 | E-UTRAN Overall Architecture Description | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |