Frequency Offset Out-of-Band boundary

Description

The Frequency Offset Out-of-Band boundary (FOOB) is a technical parameter defined in 3GPP specifications for New Radio (NR), particularly in conformance test specifications like TS 38.521-1. It is not a physical component but a conceptual frequency offset value that demarcates two distinct regions for radio frequency (RF) emissions from a User Equipment (UE) transmitter: the Out-of-Band (OOB) domain and the Spurious emission domain. This boundary is defined relative to the edge of the channel bandwidth allocated to the UE's transmission.

Operationally, emissions measured at frequencies within the OOB domain (closer to the transmitted carrier) are subject to Out-of-Band emission limits. These emissions are primarily due to modulation, power amplifier non-linearity, and noise, and their limits are designed to protect adjacent channels in the same or different operating bands. The measurement bandwidth for OOB emissions is typically aligned with the resource block bandwidth. Beyond the FOOB frequency offset, emissions fall into the Spurious domain. Spurious emissions are unwanted emissions at discrete frequencies, often harmonics or intermodulation products, which can cause interference to systems operating in completely different frequency bands. Spurious emission limits are generally stricter and are measured with a fixed reference bandwidth.

The exact value of the FOOB is not a single number but is defined by a formula based on the NR channel bandwidth and subcarrier spacing. For example, it is often calculated as the channel bandwidth divided by two, plus a specific offset. This calculation ensures the boundary scales appropriately with different NR configurations. During UE RF conformance testing, standardized test setups measure transmitter emissions across a wide spectrum. The FOOB is the pivotal point that determines which set of limit tables (OOB or Spurious) from the core specification TS 38.101-1 must be applied to the measured power level at a given frequency offset, ensuring the device complies with regulatory and coexistence requirements.

Purpose & Motivation

The FOOB was defined to create a clear and standardized technical demarcation for emission control in 5G NR devices. As radio systems became more complex with wide bandwidths and carrier aggregation, the spectrum of unwanted emissions from a transmitter is vast. Regulators and standards bodies needed a consistent rule to categorize these emissions for testing and compliance purposes. The problem was to distinguish between emissions that affect nearby channels (OOB) and those that could affect far-off, unrelated services (Spurious).

Historically, different standards or regions might have used varying definitions, complicating global device certification. The introduction of FOOB in 3GPP Release 17 provided a unified, formula-based definition intrinsic to the NR technology parameters. This solved the ambiguity in applying the correct test limits, especially for new wideband NR carriers where the traditional fixed-frequency offsets from older technologies like LTE were inadequate.

Its creation was motivated by the need for precise and scalable test requirements for the 5G NR ecosystem. It addresses the limitation of a one-size-fits-all boundary by tying the FOOB directly to the channel bandwidth, ensuring that the transition point between emission domains is always logically related to the signal's own occupied bandwidth. This allows for effective spectrum management, protects a wide range of other radio services from interference, and provides manufacturers with clear targets for transmitter design and filter implementation.