SSREF

SS block reference frequency position

Physical Layer →
Introduced in Rel-15 Also in: User Equipment

SSREF is the absolute frequency position of the Synchronization Signal block within a 5G NR carrier, which UEs use to find and synchronize to the cell during initial access.

Category
Physical Layer
Introduced
Rel-15
Where
Radio Access Network › NG-RAN (5G)
Also touches
1 segments
Specifications
5 specs
SSREF Description Purpose Related Classification Detected Changes Specifications

Description

SS block reference frequency position (SSREF) is a fundamental parameter in 5G New Radio (NR) that specifies the absolute radio frequency channel number (e.g., in terms of NR-ARFCN) corresponding to the first subcarrier of the first symbol of the Synchronization Signal / Physical Broadcast Channel (SS/PBCH) block. The SS/PBCH block, often called the SSB, is a burst of signals that includes the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and PBCH. The SSREF provides the essential anchor point in the frequency domain that a User Equipment (UE) must locate during the initial cell search procedure. Without knowing SSREF, a UE would have to blindly search the entire carrier bandwidth for the SSB, which would be highly inefficient and power-consuming.

The architecture involves the gNodeB (gNB) configuring and broadcasting the SSREF as part of the cell's configuration. While the SSREF itself is not directly broadcast in a message; it is derived from other system information. The Master Information Block (MIB), carried on the PBCH within the SSB, provides parameters like the systemFrameNumber and the pdcch-ConfigSIB1. The UE uses these, along with pre-defined mapping rules and its measurement of the SSB's position relative to the carrier, to determine the absolute frequency. Specifications such as 3GPP TS 38.101 (UE radio transmission and reception) and TS 38.104 (Base Station radio transmission and reception) define the allowed frequency ranges and granularity for SSREF placement. The SSB's frequency position can be configured within a flexible raster, distinct from the channel raster used for general carrier selection, allowing for more efficient spectrum utilization.

How it works in practice: During initial access, a UE scans a set of frequencies (the channel raster). When it detects energy, it attempts to find the PSS and SSS by correlating known sequences. Once it finds an SSB, it can decode the MIB from the PBCH. The UE combines the information from the MIB with its knowledge of the SSB's subcarrier spacing and the predefined SSB-to-channel raster offset to calculate the SSREF and thus the absolute frequency of the cell's SSB. This calculated SSREF is then used as a reference for all other frequency-domain measurements and configurations within that cell, such as the location of the control resource set (CORESET) for SIB1 and the initial uplink/downlink bandwidth parts. In FR2 (mmWave), where beamforming is used, the SSREF is consistent across different SSB beams, providing a stable frequency reference for beam management procedures.

Purpose & Motivation

SSREF was introduced in 5G NR to address the increased flexibility and bandwidth of NR carriers compared to LTE. In LTE, the primary and secondary synchronization signals (PSS/SSS) were always located at the center of the carrier bandwidth, providing a fixed frequency reference. However, 5G NR supports much wider bandwidths (up to 400 MHz) and diverse spectrum types (including mmWave), making a fixed central location inefficient or impractical. The purpose of SSREF is to decouple the location of the essential synchronization signals from the center of the carrier, allowing network operators to place the SSB at an optimal frequency position within the band.

This flexibility solves several problems. First, it allows efficient support for narrowband UEs (e.g., for IoT) that may only be capable of receiving a small bandwidth part; the SSB can be placed within a region accessible to such devices. Second, in scenarios with spectrum sharing or fragmented spectrum, the SSB can be positioned to avoid interference or align with clear frequency segments. Third, for mmWave bands with high path loss, it allows the placement of the SSB within a frequency range where the gNB's analog beamforming can be most effectively applied during initial access. The motivation for its creation was to enable the wide range of deployment scenarios envisioned for 5G, from low-band massive IoT to high-band enhanced mobile broadband.

Historically, the fixed synchronization signal position in LTE was a limitation for advanced spectrum usage. SSREF, as part of the NR physical layer design from Rel-15 onwards, provides the necessary parameterization to unlock this flexibility. It addresses the challenge of initial cell search in a heterogeneous frequency landscape, ensuring that UEs can reliably and quickly find and synchronize to cells regardless of where the operator chooses to place the synchronization block within the licensed spectrum, thereby reducing access latency and improving network performance.

Classification

Part ofARFCN
Related approachesPSSSSSPBCHMIB

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (25 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Rel-15 2 changes

In Release 15, the SSREF function was newly introduced to define the absolute radio frequency channel number (ARFCN) for the SS block's reference point. This specification allows the SS block to be positioned at a different frequency than the overall NR carrier, providing essential flexibility for initial cell search and synchronization. The details are specified within the channel arrangement clauses of TS 38.104.

  • CR to TS 38.104 on Combined updates from RAN4 #90 This document combines the proposed changes in the following Draft CRs from RAN4 #90: - R4-1900284, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1900763, "Draft CR to TS 38.104: Update of performance requirement numbers for DFT-s-OFDM based PUSCH" - R4-1900876, "Draft CR to TS 38.104: On RX spurious emissions requirement" - R4-1900968, "Draft CR for 38.104: Performance requirements for NR PUCCH format 1" - R4-1901329, "Draft CR to 38.104: Annex C.6 correction" - R4-1901330, "Draft CR to 38.104: Abbreviations addition" - R4-1901387, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1901474, "Draft CR to TS 38.104: Corrections on transmitter co-existence and co-location requirements" - R4-1901483, "Draft CR to TS 38.104: Corrections on general intermodulation requirement" - R4-1902239, "Draft CR to TS 38.104: Addition of missing EIRP/EIS definitions in terminology in clause 3.1" - R4-1902241, "Draft CR to 38.104; clarification of BS power limits" - R4-1902245, "Draft CR to 38.104: Correction to FR2 OTA Interfering signal mean power units" - R4-1902246, "Draft CR to 38.104; Correction to definition of OTA reference sensitivity" - R4-1902260, "draft CR to TS 38.104 - update emissions scaling" - R4-1902338, "Draft CR: Update on FR1 range extension for TS38.104" - R4-1902389, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1902394, "Draft CR to TS 38.104 – PUSCH requirements with CP-OFDM for FR2" - R4-1902396, "CR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1902444, "Draft CR to TS 38.104: Editorial CR for BS demodulation requirements" - R4-1902561, "Draft CR for updating PRACH performance requirements in TS38.104" - R4-1902571, "Corrections to 38.104 Delay profile calculation" - R4-1902642, "Draft CR to TS 38.104: Correction on multi-band operation related requirements" TS 38.104CR0019
  • CR to TS 38.104 Combined updates from RAN4 #90bis and RAN4#91 From RAN4 #90bis: - R4-1903105, "Draft CR to TS 38.104: Corrections on terminologies and editorial errors" - R4-1903319, "Draft CR to TS 38.104: removal of unused definition: "minimum EIRP level under extreme condition"" - R4-1903320, "Draft CR to TS 38.104: OSDD information correction" - R4-1903457, "Draft CR to TS 38.104: Removal of FFS for FR2 TDD OFF power level requirement in clause 9.5" - R4-1903499, "Draft CR to 38.104: Correction to unwanted emissions mask for bands n7 and n38" - R4-1903836, "Draft CR to TS 38.104: Correction on description on multi-band operation in section 4.8" - R4-1904024, "Draft CR to TS 38.104 Applicability rules for BS demodulation" - R4-1904234, "draftCR: Correlation matrix for 8Rx in TS 38.104" - R4-1904723, "Draft CR to TS 38.104: Update of performance requirements for DFT-s-OFDM based PUSCH" - R4-1904726, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1904729, "Draft CR on PRACH performance requirements in TS38.104" - R4-1904734, "Draft CR on TS 38.104 Performance requirement for PUCCH format 1" - R4-1904735, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1904739, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1904745, "draftCR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1904799, "Draft CR to TS 38.104: FRC update for PUSCH FR1 mapping type B and FR2 DMRS 1+1" - R4-1904816, "Draft CR : Clarification on step 5 and step 6 for delay profiles calculation (38.104)" - R4-1904842, "Draft CR to TS 38.104 BS demodulation CP-OFDM PUSCH FR2 requirements" - R4-1905126, "draft CR to 38.104 for TAE requirements" - R4-1905139, "draft CR to TS 38.104 on EVM measurement (Annex B and C)" - R4-1905140, "Draft CR: editorial correction on FR1 spurious emission requirement in TS38.104" - R4-1905143, "Draft CR for TS 38.104: Addition of NOTE for transmitter intermodulation requirements in certain regions" - R4-1905144, "Draft CR to TS 38.104: FRC reference corrections for the Rx requirements" - R4-1905145, "Draft CR to TS 38.104: Clarification on application of interfering signal offsets for ACS, blocking and intermodulation requirements" - R4-1905148, "Draft CR to TS 38.104: Corrections on out-of-band blocking requirement" From RAN4 #91: - R4-1906002, "Draft CR to 38.104: Subclause 6.7 and 9.8 transmitter intermodulation – correction of interfering signal type" - R4-1906096, "Draft CR to 38.104: Correction of frequency range for OTA spurious emissions" - R4-1906311, "Draft CR to 38.104: Correction on FRC (Annex A)" - R4-1906346, "Removal of n65 in Rel-15 38.104" - R4-1906915, "Draft CR to TS 38.104: Clarification on application of interfering signal offsets for OTA ACS, blocking and intermodulation requirements" - R4-1906918, "Draft CR to TS 38.104: Clarification on type of interfering signal for ACS, in-band blocking and ICS requirements" - R4-1907110, "Draft CR to TS 38.104: correction of the fundamental frequency limit of 2.55GHz for the spurious emissions" - R4-1907246, "Draft CR to TS 38.104: Update of performance requirements for DFT-s-OFDM based PUSCH" - R4-1907249, "Draft CR to TS 38.104: Correction on the terminology in PUSCH FRC tables" - R4-1907252, "Draft CR to TS38.104: Updates of PRACH performance requirements" - R4-1907255, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1907258, "Draft CR on NR UCI on PUSCH performance requirements for TS 38.104" - R4-1907261, "draftCR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1907266, "Draft CR on TS 38.104 Performance requirement for PUCCH format 1" - R4-1907267, "Draft CR on TS 38.104 Performance requirement for multi-slot PUCCH format 1" - R4-1907272, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1907275, "Draft CR to TS 38.104 BS demodulation CP-OFDM PUSCH FR2 requirements" - R4-1907277, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1907629, "Draft CR to 38.104: Term “reference signal” replacing by term “ideal signal” in EVM context" - R4-1907634, "Draft CR to 38.104: corrections to the EVM annex" - R4-1907659, "Draft CR to TS 38.104 on Spurious emission Category B in FR2" - R4-1907661, "Draft CR to 38.104 Definition of contiguous transmission bandwidth" - R4-1907662, "Draft CR to 38.104: BS TAE requirements" - R4-1907664, "Draft CR to 38.104: Clarification of interferer RB frequency for narrowband blocking" - R4-1907672, "Draft CR for TS 38.104: Correction on EVM" - R4-1907689, "Correction to CA carrier spacing" TS 38.104CR0029
Rel-16 7 changes

In Release 16, the SSREF function was updated to support new deployment scenarios, including the introduction of LTE/NR spectrum sharing in the n48 band. These changes were part of broader enhancements to channel arrangements and carrier aggregation configurations detailed in the specification. The updates ensured proper synchronization signal block positioning for coexistence and operation within the shared spectrum.

  • Introduction of LTE/NR spectrum sharing in band 48/n48 frequency range TS 38.104CR0220
  • CR to T 38.104: Implementation of endorsed draft CRs from RAN4#92 (Rel-16) (Mirrors changes in R4-1908440 for Rel-15 TS 38.104) - R4-1907940, "Draft CR to TS 38.104: Correction on the terminology in FRC tables in A.1 and A.2" - R4-1908307, "Draft CR to TS 38.104: Clarification on application of OTA receiver requirements for BS supporting polarization" - R4-1908387, "Draft CR for TS38.104: editorial correction for reference meausrement channel" - R4-1908619, "Draft CR to TS38.104: Correction on interferer frequency offset values for ACS" - R4-1908629, "Draft CR to TS38.104: Corrections on EVM window length (Annex B.5.2, C.5.2)" - R4-1908774, "DraftCR to 38.104: Editorial Corrections to redudant units in clause 10.8.3" - R4-1908805, "Draft CR to 38.104: Limits in FCC title 47 for OTA operating band unwanted emissions (9.7)" - R4-1909270, "Draft CR to TS 38.104: Receiver spurious emissions frequency correction" - R4-1909309, "DraftCR to TS 38.104: text corrections, Rel-15" - R4-1909310, "DraftCR to 38.104: correction of TAB connectors mapping to TAB connector TX min cell group, Rel-15" - R4-1909416, "draft CR 38.104 - correct reference to annex F" - R4-1910066, "Draft CR to TS 38.104: Update of performance requirements for DFT-s-OFDM based PUSCH" - R4-1910069, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1910072, "Draft CR on NR UCI on PUSCH performance requirements for TS 38.104" - R4-1910075, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1910078, "Draft CR to TS38.104: Updates to NR PRACH performance requirements" - R4-1910081, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1910084, "Draft CR to TS 38.104 BS demodulation CP-OFDM PUSCH FR2 requirements" - R4-1910088, "Draft CR for 38.104: Performance requirements for NR PUCCH format 1" - R4-1910089, "Draft CR for 38.104: Performance requirements for NR multi-slot PUCCH" - R4-1910094, "draftCR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1910431, "Corrections to EVM calculations in 38.141-1 annex B" - R4-1910462, "Draft CR to 38.104: Correction on regional requirements (4.5)" - R4-1910493, "Draft CR to TS 38.104 correction to Annex C.7" - R4-1910606, "Draft CR for TS 38.104: Channel spacing for adjacent NR carriers" TS 38.104CR0034
  • CR to TS 38.104: Finalization of interfering RB centre frequency offsets in receiver narrowband blocking requirement TS 38.104CR0083
  • CR to TR 38.104: Correction of table reference of interfering signal for ACS requirement TS 38.104CR0085
  • CR to TS 38.104: Correction on interfering signal frequency offsets for receiver intermodulation requirements TS 38.104CR0087
  • IntraSlot frequency hopping applicability in the one OFDM symbol test case TS 38.104CR0145

+ 1 more changes

Rel-17 4 changes

In Release 17, the SSREF function was updated to include a reference to Frequency Reference Channels (FRCs) within the base station RF requirements specification. This change provided a standardized reference point for SS block measurements. The release also introduced specific in-band blocking requirements for multi-band Base Stations to ensure proper operation.

  • CR to TS38.104 on introduction of upper 700MHz A block TS 38.104CR0365
  • CR to TS 38.104 on reference to FRCs TS 38.104CR0411
  • CR to TS 38.104: Corrections on frequency offset symbols for spectrum emission mask for non-transmitted channels TS 38.104CR0307
  • CR to 38.104: In-band blocking for multi-band Base Stations TS 38.104CR0333
Rel-18 7 changes

In Release 18, the SSREF function was updated to define reserved GSCN and ARFCN-ValueNR entries for NR operating bands, ensuring clear frequency positioning for synchronization signals. This work also included corrections and clarifications for extending NR operation into higher frequency ranges, such as up to 71 GHz. Furthermore, specific updates were made to address band-specific requirements, including for Band n100 and Band n104, relating to blocking and reference sensitivity parameters.

  • CR to TS 38.104 on additional narrowband blocking requirement for Band n100 TS 38.104CR0612
  • CR to TS 38.104 on table references for OTA operating band unwanted emission limits TS 38.104CR0437
  • CR to TS 38.104: NR-ARFCN table reference correction for band n263, Rel-18 TS 38.104CR0460
  • CR to TS 38.104 on corrections of table references for extending current NR operation to 71GHz TS 38.104CR0468
  • CR for TR 38.104, Correction on reference of PREFSENS for in-band blocking and out-of-band blocking for band n104 TS 38.104CR0472
  • CR for TS 38.104: Define the reserved GSCN / ARFCN-ValueNR and NR operating band TS 38.104

+ 1 more changes

Rel-19 5 changes

In Release 19, the SSREF function saw no direct changes or enhancements based on the provided CR titles and grounding context. The listed Change Requests for this release focus on correcting and refining other Base Station (BS) conformance test areas, such as out-of-band blocking, reference sensitivity levels for specific bandwidths, and test case annex references. Therefore, the technical specifications governing the SS block reference frequency position itself were not updated in this particular release cycle.

  • CR to TS 38.141-2 OTA test reduction for out-of-band blocking requirement TS 38.141CR0682
  • CR to TS 38.104 correcction on SBFD BS blocking TS 38.104CR0770
  • (NR_FR1_lessthan_5MHz_BW-Core)CR for TS38.104, Correction on reference sensitivity levels requirement apply to BS that supports NB-IoT operation in NR in-band for 3 MHz channel bandwidth TS 38.104CR0739
  • Correct Annex references in TC 8.2.6 TS 38.141CR0695
  • Correct Annex references in TC 8.2.14 TS 38.141CR0697

Explore further

Broader topics and technologies where SSREF plays a role.

Topics
Spectrum & CoexistenceLTE / LTE-Advanced5G NR (New Radio)Lawful InterceptIoT & MTCManagement & Operations
Technologies
LTE5G

Defining Specifications

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

SpecificationTitleRelease
TS 38.101 vj31 NR User Equipment Radio Transmissions Rel-19
TS 38.104 vj20 NR Base Station RF Requirements Rel-19
TS 38.141 vj20 NR Base Station RF Conformance Testing Part 1 Rel-19
TR 38.785 vh00 UE radio transmission for enhanced NR sidelink Rel-17
TR 38.786 vi20 Technical Report for NR Sidelink Evolution Rel-18