FC

Carrier Center Frequency

Physical Layer →
Introduced in R99 Also in: User Equipment

FC is the exact center frequency of a radio carrier, derived from a global frequency raster, which anchors channel bandwidth allocation and defines the carrier's spectral position for a given numerology.

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

Description

The Carrier Center Frequency (FC) is the absolute radio frequency that defines the midpoint of a carrier's allocated bandwidth. It is not arbitrarily chosen but is derived from a precise formula based on a channel raster defined in 3GPP specifications. The process begins with an RF reference frequency (F_REF), which is a point on a global frequency raster (e.g., a 100 kHz, 15 kHz, or 5 kHz grid depending on the frequency range and technology). This F_REF is mapped to the carrier's center frequency FC according to a specific offset defined by the subcarrier spacing (numerology) and the channel bandwidth.

Technically, for LTE and NR, FC is calculated as F_REF + ΔF. ΔF is an offset that ensures the carrier's resource blocks (RBs) are aligned symmetrically around FC. The exact calculation depends on the numerology (μ), which defines the subcarrier spacing (SCS = 15 * 2^μ kHz). The channel bandwidth, defined by a certain number of RBs (N_RB), is then placed around this FC. This structured mapping guarantees that different carriers, even with different numerologies, can be placed on the frequency raster without overlapping in an undefined manner and facilitates carrier aggregation.

FC serves as the anchor for all physical layer functions. The baseband generates orthogonal frequency-division multiplexing (OFDM) symbols where subcarriers are indexed relative to FC. The RF transmitter upconverts the baseband signal to this center frequency for radiation. Conversely, the receiver uses FC as the local oscillator frequency for downconversion. Synchronization signals (PSS/SSS) and physical broadcast channel (PBCH) are transmitted relative to FC, allowing the User Equipment (UE) to detect and lock onto the cell. Furthermore, RF performance requirements, such as transmitter unwanted emissions and receiver sensitivity, are specified in relation to FC.

Purpose & Motivation

The Carrier Center Frequency exists to provide a standardized, unambiguous, and globally consistent method for identifying the spectral location of a radio carrier. This solves the critical problem of spectrum management and equipment interoperability. Without a standardized definition, network operators and device manufacturers could implement carriers on slightly different frequencies, leading to interference and failed connections.

Historically, as cellular systems evolved from narrowband FDMA to wideband OFDMA, the need for a precise and flexible frequency raster became paramount. In GSM, carrier spacing was a fixed 200 kHz. With 3G UMTS, a fixed 5 MHz channel was used. LTE and NR introduced scalable bandwidths and multiple numerologies, making a rigid raster insufficient. The purpose of the FC definition, with its dependency on numerology and channel bandwidth, is to enable this flexibility while maintaining a predictable and conflict-free frequency plan. It allows for the efficient packing of carriers of different bandwidths within a licensed band, supports carrier aggregation by defining the exact spacing between component carriers, and ensures that UEs can search for and measure cells accurately across a wide frequency range. It is the cornerstone of predictable RF behavior and spectrum coexistence.

Classification

Part ofARFCN

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (40 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Rel-15 5 changes

In Release 15, the "FC" (Filter Criteria) function itself was not newly introduced, as it is a core IMS concept from earlier releases. The provided Change Requests and grounding context do not describe any new introduction or modification to the FC function's definition or its role in the S-CSCF for triggering application services based on service point triggers. The listed CRs pertain to physical layer radio aspects like carrier frequency offsets, band definitions, and performance requirements for NR and NB-IoT.

  • CR to TS 36.104: Implementation of UL PRB to DL PRB center offset for TDD NB-IoT TS 36.104CR4816
  • CR to TS 36.141: Implementation of UL PRB to DL PRB center offset for TDD NB-IoT TS 36.141CR1189
  • CR to TS 37.145-2: mirror of operating band and frequency range declaration from NR, Rel-15 TS 37.145CR0133
  • CR to TS 38.104 on Combined updates from RAN4 #88bis and #89 (including 7.5 kHz carrier shift in UL for remaining bands) TS 38.104CR0016
  • 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 11 changes

In Release 16, specific advancements for the "FC" (Carrier Center Frequency) function included the introduction of formal channel spacing definitions between E-UTRA and NR carriers, and new provisions for spectrum sharing in specific bands like band 48. It also introduced clarified frequency offset parameters for TDD NB-IoT standalone operation between anchor and non-anchor carriers.

  • Introduction of LTE/NR spectrum sharing in band 48/n48 frequency range TS 38.104CR0220
  • CR to 36.104: frequency offset between anchor and non-anchor carrier for TDD NB-IoT standalone operation TS 36.104CR4833
  • CR to 36.141: frequency offset between anchor and non-anchor carrier for TDD NB-IoT standalone operation TS 36.141CR1194
  • Introduction of channel spacing between E-UTRA and NR carriers TS 37.104CR0888
  • Introduction of channel spacing between E-UTRA and NR carriers TS 37.141CR0907
  • CR to TS 37.141: Clarification on manufacturer's declaration of the number of supported NB-IoT carriers TS 37.141CR0946

+ 5 more changes

Rel-17 3 changes

In Release 17, the "FC" (Carrier Center Frequency) function was enhanced to address specific testing and coexistence requirements. This included modifications to support multiple NB-IoT standalone carriers and adjustments to spurious emissions limits for Active Antenna System base stations to ensure coexistence with other systems. Additionally, corrections were made regarding frequency offset symbols for the spectrum emission mask applicable to non-transmitted channels.

  • CR to 37.141 - TC22 generation misalignment when supporting multiple NB-IoT standalone carriers TS 37.141CR1023
  • CR to 37.145-2 to modify AAS BS OTA Spurious emissions limits for co-existence with systems operating in other frequency bands in R17 TS 37.145CR0302
  • CR to TS 38.104: Corrections on frequency offset symbols for spectrum emission mask for non-transmitted channels TS 38.104CR0307
Rel-18 20 changes

In Release 18, the "FC" (Carrier Center Frequency) function saw updates primarily focused on aligning test frequency range requirements for IoT NTN (Non-Terrestrial Network) and regular LTE UEs across various transmitter and receiver test cases, including output power dynamics and reference sensitivity. Specific clarifications were introduced for UE operation in the 1610-1626.5 MHz range and for variable TX-RX frequency separation in LTE/NB-IoT/eMTC NTN contexts. Additionally, corrections and definitions were made to the NR-ARFCN table for band n263 and for GSCN/ARFCN values, alongside operational clarifications for multiple carrier operation in bands n100 and n101.

  • (LTE_NBIOT_eMTC_NTN_req-Core) CR to TS 36.102 on variable TX-RX frequency separation TS 36.102CR0059
  • (IoT_NTN_FDD_LS_band-Core) Clarification for the PSD limits for a UE operating in 1610-1626.5 MHz frequency range TS 36.102CR0063
  • Splitting the IoT NTN frequency error test case TS 36.521CR0014
  • Update of frequency error test cases TS 36.521CR0025
  • Include Notes to further specify test frequency selection criteria of Annex K.1.1 and K.1.2 in the cases when UE supports only one band or supports two bands TS 36.521CR0063
  • Modify transmit power related test cases test frequency rang requirements to be aligned with regular LTE UE requirements TS 36.521CR0074

+ 14 more changes

Rel-19 1 change

In Release 19, the specification for the "FC" (Filter Criteria) function was not updated. The provided Change Requests and grounding context do not describe any modifications to the FC function itself for this release. The listed CR pertains to IoT NTN frequency error testing, which is unrelated to the SIP-based Filter Criteria used for IMS application triggering.

  • Update of Doppler and delay in IoT NTN frequency error test cases TS 36.521CR0137

Explore further

Broader topics and technologies where FC plays a role.

Topics
Spectrum & CoexistenceTesting & ConformanceLTE / LTE-Advanced5G NR (New Radio)Lawful InterceptUMTS / WCDMA
Technologies
LTE

Defining Specifications

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

SpecificationTitleRelease
TS 23.218 vj00 IMS Call Model Specification Rel-19
TS 36.101 vj30 LTE UE Radio Transmission & Reception Requirements Rel-19
TS 36.102 vj10 E-UTRA UE Satellite Access RF Requirements Rel-19
TS 36.104 vj10 Base Station (BS) radio transmission and reception Rel-19
TS 36.141 vj00 E-UTRA BS Conformance Testing Rel-19
TS 36.181 vj30 E-UTRA RF Test Methods for Satellite Access Node Rel-19
TS 36.521 vj00 E-UTRA UE Conformance ICS Proforma Rel-19
TS 36.790 vf00 LAA/eLAA for CBRS 3.5GHz Band in US Rel-15
TS 37.104 vj10 MSR Base Station RF Characteristics Rel-19
TS 37.141 vj10 RF Test Methods for Multi-Standard Radio Base Stations Rel-19
TS 37.145 vj10 AAS Base Station Conducted Conformance Testing Rel-19
TS 37.802 va10 MSR BS RF Requirements for Non-Contiguous Spectrum Rel-10
TS 37.812 vb30 Multi-band Multi-standard Radio BS Requirements Rel-11
TR 37.900 vj00 Multi-Standard Radio (MSR) Base Station Requirements Rel-19
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
TS 38.176 vj20 IAB Conformance Testing Specification Rel-19
TS 38.521 vj20 NR Physical Layer UE Conformance Testing 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