Fractional Bandwidth

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.