SFN

System Frame Number

Physical Layer →
Introduced in R99 Also in: User Equipment

SFN is a counter that numbers the radio frames in a cell, providing timing synchronization for essential radio procedures like system information scheduling and paging.

Category
Physical Layer
Introduced
R99
Where
Radio Access Network › NG-RAN (5G)
Also touches
1 segments
Specifications
40 specs
SFN Description Purpose Related Classification Detected Changes Specifications

Description

The System Frame Number (SFN) is a fundamental timing parameter in cellular networks, serving as a modulo counter that uniquely identifies each radio frame within a cell's transmission timeline. In LTE, the SFN cycles from 0 to 1023, corresponding to a period of 10.24 seconds (1024 frames * 10 ms/frame). In 5G NR, two ranges are defined: the 10-bit SFN (0-1023) for fundamental timing and the 12-bit Hyper-SFN (H-SFN, 0-4095) for extended timing procedures, especially for IoT and reduced capability devices. The SFN is broadcast within the Master Information Block (MIB) on the Physical Broadcast Channel (PBCH). In LTE, the 8 most significant bits of the SFN are carried in the MIB, while the 2 least significant bits are derived from the PBCH decoding timing. In NR, the PBCH payload carries part of the SFN, and the full value is obtained by combining this with information from the PBCH's Demodulation Reference Signals (DM-RS) and the radio frame timing. The SFN is crucial for time-synchronized network operations. It determines the scheduling of System Information Blocks (SIBs), which are transmitted in specific radio frames and subframes according to formulas based on SFN. It governs paging occasions, where UEs wake up to check for pages only in frames where SFN mod T = T_Offset, with T being the paging cycle. For measurements, UEs use SFN to time-stamp measurement reports (e.g., for handover) and to synchronize discontinuous reception (DRX) cycles. In positioning protocols like LTE Positioning Protocol (LPP) and NR Positioning Protocol (NRPP), SFN is used as a common time reference for Observed Time Difference of Arrival (OTDOA) measurements. Essentially, the SFN provides a cell-specific 'clock' that aligns all UE and network activities within the cell's radio resource grid.

Purpose & Motivation

The SFN was introduced from the earliest 3GPP releases (R99) to provide a standardized, cell-level time reference, addressing the need for deterministic scheduling and synchronization in digital cellular systems. Prior analog systems lacked such a unified, broadcast timing counter, making coordinated channel access and power-saving mechanisms difficult. The SFN solves several critical problems: it enables efficient sleep modes (DRX/paging) by allowing UEs to predict exactly when to wake up based on a known cycle, drastically saving battery life. It allows for the periodic and predictable broadcasting of system information, ensuring all UEs can acquire vital network parameters without continuous monitoring. It provides a common timebase for handover measurements and reporting, ensuring the network can accurately compare measurements from different UEs or different times. Furthermore, it supports advanced features like Multimedia Broadcast Multicast Service (MBMS) where synchronized transmission from multiple cells (MBSFN) requires precise frame alignment. The evolution to include H-SFN in later releases (for LTE-M, NB-IoT, and NR) was motivated by the need for even longer timing cycles for ultra-low-power IoT devices, enabling extended DRX cycles beyond 10.24 seconds and more efficient scheduling for small, infrequent data transmissions.

Classification

Part ofH-SFN
Specific typesBFNH-SFN
Related approachesPBCH

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (44 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Rel-15 11 changes

In Release 15, the SFN function was enhanced to support an SFN offset for OTDOA positioning. Furthermore, the release introduced UE capability for NR-DC (New Radio - Dual Connectivity) requiring SFN synchronization between the PCell and PSCell.

  • Introduction of increased number of E-UTRAN data bearers TS 36.331CR3446
  • Support maximum 8 SS/PBCH blocks for unpaired spectrum beyond 2.4GHz TS 38.213CR0006
  • Introduction of UE capability for NR-DC with SFN synchronization between PCell and PSCell TS 38.331CR1265
  • Number of PDCCH/EPDCCH/SPDCCH received parallel TS 36.302CR1198
  • Correction on measurement triggering based on number of cells TS 36.331CR3657
  • SFN offset for OTDOA TS 36.355CR0229

+ 5 more changes

Rel-16 10 changes

In Release 16, there were no specific changes to the System Frame Number (SFN) function itself, as the approved Change Requests for this release focused on other areas such as clarifying the number of pathloss estimates for positioning SRS, correcting HARQ-ACK codebook generation, and extending the number of cells for search space switching. The provided grounding context defines a radio frame as a 10 ms numbered time interval but does not indicate any modifications to SFN handling or procedures. Therefore, Release 16 updates were centered on adjacent radio resource management and physical layer procedures rather than on the SFN mechanism.

  • Add new general abbreviations MCC Note: CR cover sheet wrongly shows CR number as "1118". TS 21.905CR0118
  • CR on determination of the number of RS for RLM TS 38.213CR0121
  • Correction of Type-3 HARQ-ACK codebook generation for a PDSCH with one transport block for a configuration with a maximum number of two TBs TS 38.213CR0187
  • CR on number of PUCCHs with HARQ-ACK in a slot TS 38.213CR0198
  • CR on the number of pathloss estimates maintained by the UE for SRS for positioning. TS 38.213CR0218
  • CR on Number of PUCCH resource sets per PUCCH-config TS 38.213CR0225

+ 4 more changes

Rel-17 11 changes

In Release 17, specific corrections were made to the System Frame Number (SFN) function regarding the validation of activation and release DCI formats, particularly focusing on the presence and value ranges of fields within DCI formats 0_2 and 1_2. These changes ensured proper handling of the redundancy version and HARQ process number fields. Additionally, the release included corrections related to the unified TCI state framework impacting procedures like Beam Failure Recovery.

  • Remove the maximum number of MIMO layers restrictions for SUL TS 38.331CR2465
  • Addition of extended number range for NS value TS 36.331CR4917
  • Correction to support up to 32 HARQ process numbers for FR2-2 TS 38.212CR0126
  • CR on number of HARQ-ACK codebooks configurable for multicast TS 38.212CR0129
  • Correction on BD/CCE decoding with release-specific number of serving cell(s) for NR operation in FR2-2 TS 38.213CR0382
  • Correction of number of configured DL-CCs for BD/CCE budget for FR2-2 TS 38.213CR0425

+ 5 more changes

Rel-18 11 changes

In Release 18, specific SFN-related updates were focused on correcting and updating test conditions for Carrier Aggregation (CA) in High-Speed Train (HST) scenarios, as indicated by the CR title "Update condition for CA HST-SFN test cases." This involved refining the procedures and parameters used to ensure accurate synchronization and measurement reporting under high-speed mobility conditions where the System Frame Number is critical for timing alignment. No other fundamental changes to the SFN definition or its core radio frame structure were introduced in this release.

  • Correction on the maximum number of SSB rsources for L1 measurement without gaps in LTM TS 38.331CR5302
  • Correction to startPreambleForThisPartition and numberOfPreamblesPerSSB-ForThisPartition in RA-report TS 38.331CR5456
  • Update condition for CA HST-SFN test cases TS 38.522CR0528
  • Introduction of network signalling of maximum number of UL segments [Max-RRC-SegUL] TS 36.331CR5084
  • Corrections on network signalling of maximum number of UL segments [Max-RRC-SegUL] TS 36.331CR5089
  • Corrections on uplink power control in unified TCI framework TS 38.331CR4559

+ 5 more changes

Rel-19 1 change

In Release 19, the specific enhancement for the System Frame Number (SFN) function is not detailed in the provided grounding context or the listed Change Request titles. The only cited technical change from Release 19 is the "Introduction of 32 HARQ process numbers," which is a separate enhancement related to hybrid automatic repeat request processes and not directly to the SFN function itself. Therefore, based solely on the provided materials, no new SFN-specific features or modifications are described for Release 19.

  • Introduction of 32 HARQ process numbers in Rel-19 [TN32HARQ] TS 38.212CR0222

Explore further

Broader topics and technologies where SFN plays a role.

Topics
Positioning & LocationMEC (Edge Computing)LTE / LTE-Advanced5G NR (New Radio)Lawful InterceptSMS & Messaging
Technologies
LTE5G

Defining Specifications

3GPP specifications that define or reference SFN, with the latest known release. Sourced from the 3GPP document catalog — see methodology.

SpecificationTitleRelease
TR 21.905 vj00 3GPP Technical Terms and Definitions Rel-19
TS 25.123 vj00 Radio Resource Management for TDD Rel-19
TS 25.133 vj00 UTRAN RRM Requirements for FDD Rel-19
TS 25.171 vj00 A-GPS Minimum Performance Requirements for UTRA FDD UE Rel-19
TS 25.172 vj00 A-GANSS UE Minimum Performance Requirements (FDD) Rel-19
TS 25.173 vj00 A-GANSS Performance Requirements (TDD) Rel-19
TS 25.211 vj00 UTRA FDD Layer 1: Transport & Physical Channels Rel-19
TS 25.212 vj00 UTRA FDD Layer 1 Multiplexing & Channel Coding Rel-19
TS 25.214 vj00 UTRA FDD Physical Layer Procedures Rel-19
TS 25.221 vj00 UTRA TDD Physical Layer Specification Rel-19
TS 25.222 vj00 UTRA TDD Multiplexing & Channel Coding Rel-19
TS 25.223 vj00 UTRA Physical Layer TDD Spreading & Modulation Rel-19
TS 25.224 vj00 UTRA TDD Physical Layer Procedures Rel-19
TS 25.225 vj00 UTRA TDD Physical Layer Measurements Rel-19
TS 25.402 vj00 UTRAN Synchronisation Mechanisms Rel-19
TS 25.423 vj00 UTRAN RNSAP Specification Rel-19
TS 25.800 vc10 UMTS Heterogeneous Networks Study Rel-12
TR 25.912 vj00 Evolved UTRA and UTRAN Technical Report Rel-19
TR 25.931 vj00 UTRAN Signalling Procedures Examples Rel-19
TS 26.802 vj20 Multicast Enhancements for 5G Media Streaming Rel-19
TS 36.133 vj20 E-UTRA RRM Requirements Rel-19
TS 36.171 vj10 A-GNSS Minimum Performance Requirements for UE Rel-19
TS 36.300 vj00 E-UTRAN Radio Interface Protocol Architecture Overview Rel-19
TS 36.302 vj00 E-UTRA Physical Layer Services Rel-19
TS 36.331 vj00 LTE RRC Protocol Specification Rel-19
TS 36.355 vj00 LTE Positioning Protocol (LPP) Rel-19
TS 36.401 vj00 E-UTRAN Overall Architecture Description Rel-19
TS 36.855 vd00 E-UTRA Positioning Enhancements Study Rel-13
TS 36.878 vd00 LTE Performance Enhancements for High Speed Scenarios Rel-13
TS 37.355 vj20 LTE Positioning Protocol (LPP) Rel-19
TS 37.571 vj00 UE Conformance for Positioning Rel-19
TS 38.133 vj20 5G UE Radio Requirements for RRC_IDLE Mobility Rel-19
TS 38.171 vj10 5G A-GNSS UE Positioning Requirements Rel-19
TS 38.212 vj10 NR Multiplexing and Channel Coding Rel-19
TS 38.213 vj10 NR Physical Layer Control Procedures Rel-19
TS 38.331 vj00 NR Radio Resource Control (RRC) Protocol Specification Rel-19
TS 38.401 vj10 NG-RAN Architecture Specification Rel-19
TS 38.522 vj11 UE Conformance Test Applicability Statement Rel-19
TS 38.523 vj20 5G NR UE Conformance Testing: Idle/Inactive Rel-19
TR 38.913 vj00 Next Gen Access Tech Scenarios & Requirements Rel-19