FBW

Fractional Bandwidth

Physical Layer →
Introduced in Rel-15

FBW is a dimensionless ratio of a signal's bandwidth to its center frequency, used in 5G NR to define channel bandwidths and assess RF performance, particularly at high frequencies.

Category
Physical Layer
Introduced
Rel-15
Where
Radio Access Network › NG-RAN (5G)
Specifications
8 specs
FBW Description Purpose Related Classification Detected Changes Specifications

Description

Fractional Bandwidth (FBW) is a fundamental concept in radio frequency engineering, defined as the ratio of the instantaneous bandwidth (BW) of a signal or a system's operating bandwidth to its center frequency (Fc), typically expressed as a percentage: FBW = (BW / Fc) * 100%. It is a dimensionless figure of merit that indicates how "wideband" a system is relative to its operating frequency. In the context of 3GPP, particularly from Release 15 onwards for 5G New Radio (NR), FBW is critically important for specifying radio requirements and performance metrics, especially as networks utilize higher frequency ranges (e.g., FR2 above 24.25 GHz) where channel bandwidths can be hundreds of MHz or several GHz.

The application of FBW within 3GPP specifications governs the definition of channel bandwidths for different frequency ranges. For lower frequencies (FR1), absolute bandwidth definitions often suffice. However, for mmWave bands in FR2, the same absolute bandwidth represents a much larger fractional bandwidth, which has significant implications for RF component design, such as power amplifiers and filters, whose performance (e.g., gain flatness, efficiency) is often limited by FBW. The specifications (e.g., 38.104) use FBW to delineate requirements for base station and user equipment radio frequency characteristics.

From a system operation perspective, FBW influences waveform design and numerology selection. A high FBW can introduce challenges like increased phase noise and wider variation in antenna gain across the band. 3GPP accounts for this by defining different sets of requirements for narrowband and wideband devices based on their FBW. Measurement procedures for transmitter and receiver tests, such as unwanted emissions or sensitivity, are also adapted based on the FBW of the configured channel. This ensures that performance is evaluated under realistic conditions that reflect the relative bandwidth of the operational signal.

Purpose & Motivation

Fractional Bandwidth exists as a concept to provide a normalized measure of bandwidth that is independent of the absolute frequency of operation. This is essential for comparing the technical challenges and performance of systems operating at vastly different carrier frequencies. A 100 MHz bandwidth is considered wideband at 2 GHz (FBW=5%) but is relatively narrowband at 28 GHz (FBW≈0.36%). The problems it addresses are related to the scalable and consistent design of radio equipment across 5G's diverse spectrum portfolio.

The motivation for its formal adoption in 3GPP 5G standards stems from the introduction of mmWave spectrum. Previous cellular generations (2G, 3G, 4G) primarily operated in sub-6 GHz bands where FBW was generally low, and specifications could rely on absolute bandwidth values. 5G's expansion into FR2 (24.25 GHz to 52.6 GHz and beyond) meant that channel bandwidths up to 400 MHz (and potentially 1 GHz) became standardized. At these frequencies, such bandwidths constitute a significant FBW, directly impacting RF front-end linearity, filter design, and test methodology. Defining requirements based on FBW ensures that radio performance specifications remain physically meaningful and achievable across the entire frequency range, enabling efficient and cost-effective hardware development for both base stations and devices.

Classification

Part ofNR

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (32 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 Fractional Bandwidth (FBW) function was newly introduced to address operating bands with a fractional bandwidth larger than 6%. This required new declarations for the supported frequency ranges representing these FBWs (D11.32) and separate rated beam EIRP levels at the lower and higher frequency ranges of the fractional bandwidth (D11.33 and D11.34).

  • CR to TS 37.145-2: Intermodulation product bandwidth in Tx IMD test (Rel-15) TS 37.145CR0077
  • 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 8 changes

In Release 16, the Fractional Bandwidth (FBW) function was newly introduced for NR operating bands where the FBW exceeds 6%, requiring the declaration of supported fractional bandwidths. This involved adding specific, wider channel bandwidths for various bands, including 30MHz for n41 and n50, 40MHz for n38, and wider bandwidths for bands n7, n28, n66, n77, and n78. Consequently, the specification now defines procedures for declaring rated beam EIRP at both the lower and higher frequency ranges of the fractional bandwidth.

  • Addition channel bandwidth of 30MHz for n50 in TS 38.104 TS 38.104CR0031
  • CR to introduce 30MHz bandwidth of n41 to TS 38.104 TS 38.104CR0035
  • CR for TS 38.104: adding wider channel bandwidths in Band n7 TS 38.104CR0037
  • CR for TS 38.104: adding wider channel bandwidths in Band n77/n78 TS 38.104CR0105
  • CR for TS 38.104: Addition channel bandwidth of 40MHz for n38 TS 38.104CR0106
  • Introducing new channel bandwidth for band n28 TS 38.104CR0131

+ 2 more changes

Rel-17 6 changes

In Release 17, the Fractional Bandwidth (FBW) function was updated to introduce support for new NR channel bandwidths of 35MHz and 45MHz, as detailed in updates to TS 38.104. This expansion required new declarations for supported NR BS channel bandwidths and the definition of rated beam EIRP at the lower and higher frequency ranges of the fractional bandwidth. The changes also involved updates to tables defining BS channel bandwidths per operating band and corrections to the definition of Aggregated Channel Bandwidth.

  • CR for TS 37.145-2: introduction of channel bandwidths 35MHz and 45MHz TS 37.145CR0314
  • CR for TS 38.104: introduction of channel bandwidths 35MHz and 45MHz NOTE The CR is not implemented TS 38.104CR0341
  • CR for TS 38.104: introduction of channel bandwidths 35MHz and 45MHz NOTE The CR is a resubmission of CR R4-2114918 of RP-211909 TS 38.104CR0363
  • CR to TS 38.104 on BS channel bandwidth per operating band TS 38.104CR0310
  • CR to TS 38.104 on tables for BS channel bandwidths TS 38.104CR0415
  • CR to TS38.104: Correction on the Aggregated Channel Bandwidth TS 38.104CR0294
Rel-18 7 changes

In Release 18, the FBW (Fractional Bandwidth) function was extended to introduce and fully specify a new 3 MHz channel bandwidth for NR operation. This included defining its applicable SS raster entries and enabling support for in-band NB-IoT operation within this 3 MHz bandwidth. Furthermore, corrections were made to clarify the supported channel bandwidths for specific bands like n109 and for network-controlled repeater requirements.

  • Big CR to TS 38.104 on introduction of 3 MHz channel bandwidth TS 38.104CR0500
  • CR to TS 38.104 on clarification of applicable SS raster entries for 3 MHz channel bandwidth TS 38.104CR0528
  • CR to TS 38.104 on support of NB-IoT operation in NR in-band for 3 MHz channel bandwidth TS 38.104CR0542
  • (NR_FDD_ULn28_DLn75_n76) CR for TS 38.104: Correct channel bandwidths and SCS supported by band n109 TS 38.104CR0622
  • CR to 38.106: NCR-MT channel bandwidth TS 38.106CR0061
  • (NR_netcon_repeater-Core)CR for TS 38.106, Correction on requirement for 5MHz channel bandwidth for NCR MT TS 38.106CR0086

+ 1 more changes

Rel-19 9 changes

In Release 19, the key new development for the Fractional Bandwidth (FBW) function was the formal introduction and specification of a 7 MHz channel bandwidth for NR operation. This addition required updates across multiple core specifications (TS 38.104, TS 38.141-2, TS 38.176-2, TS 38.115-1, and TS 37.145-2) to define the new bandwidth's characteristics, performance requirements, and restrictions, particularly for channel bandwidths below 10 MHz. The release also included specific corrections and clarifications for other narrow bandwidths, such as 3 MHz, especially concerning reference sensitivity levels for base stations supporting in-band NB-IoT operation.

  • CR to TS 37.145-2: 7MHz channel bandwidth introduction TS 37.145CR0398
  • CR to 38.104 on adding new UL channel bandwidth to band n48 TS 38.104CR0698
  • CR to TS 38.141-2: 7MHz channel bandwidth introduction TS 38.141CR0666
  • CR to TS 38.176-2: restriction of 7MHz channel bandwidth introduction TS 38.176CR0087
  • CR to TS38.104 on transmission bandwidth configuration for LP-WUS TS 38.104CR0745
  • CR to TS 38.115-1 on 7MHz channel bandwidth TS 38.115CR0066

+ 3 more changes

Explore further

Broader topics and technologies where FBW plays a role.

Topics
Spectrum & CoexistenceTesting & ConformanceLTE / LTE-Advanced5G NR (New Radio)Lawful InterceptRadio Access Network
Technologies
LTE5G

Defining Specifications

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

SpecificationTitleRelease
TS 37.145 vj10 AAS Base Station Conducted Conformance Testing Rel-19
TS 38.104 vj20 NR Base Station RF Requirements Rel-19
TS 38.106 vj20 NR Repeater Radio Transmission and Reception Rel-19
TS 38.115 vj20 NR Repeater RF Conformance Testing Part 1 Rel-19
TS 38.141 vj20 NR Base Station RF Conformance Testing Part 1 Rel-19
TS 38.174 vj10 NR Integrated Access and Backhaul Radio Spec Rel-19
TS 38.176 vj20 IAB Conformance Testing Specification Rel-19
TR 38.877 vi10 Technical Report Rel-18