Highest Carrier

Description

In 3GPP specifications, the term 'Highest Carrier' refers to the carrier that has the highest carrier centre frequency among all carriers transmitted or received in a given operating band or set of operating bands. This term is primarily used in the context of radio frequency (RF) requirements for base stations and user equipment, where it helps define the upper boundary of the frequency range that the equipment must support. The definition is found in 21.905 and is referenced in numerous RF performance specifications such as 25.104, 25.105, 36.104, 37.104, 38.104, and others. It is a fundamental concept for testing and ensuring that equipment operates correctly across the entire band.

The Highest Carrier is not a physical entity but a reference point for specifying requirements. For example, when defining out-of-band emissions or spurious emissions, the highest carrier determines the frequency above which certain limits apply. It is also used in the context of carrier aggregation, where multiple carriers are aggregated; the highest carrier is the one with the maximum centre frequency among the aggregated carriers. This is important for defining the maximum operating frequency and for coexistence studies with other systems. The term is often used in conjunction with 'Lowest Carrier', which is the carrier with the lowest centre frequency, to define the operating band edges.

Technically, the carrier centre frequency is the arithmetic mean of the upper and lower frequencies of the carrier. For a given operating band, the highest carrier is typically at the upper edge of the band, but it can be any carrier within the band depending on the deployment. The specifications define test cases where the highest carrier is used to verify that the equipment meets requirements at the band edge, where performance may be more challenging due to filter roll-off and other RF impairments. For example, in 38.104 for 5G NR, the highest carrier is used to define the maximum output power and unwanted emissions for base stations operating in a specific band.

The concept applies to both base stations (BS) and user equipment (UE). For BS, the highest carrier is relevant for conducted and radiated requirements. For UE, it is used in RF conformance testing to ensure that the device can operate at the upper edge of the band without exceeding emission limits or degrading sensitivity. The specifications 36.141 and 38.141 provide the test procedures for base stations, while 37.145 and 38.174 cover additional aspects. The term is also used in multi-band operation, where the highest carrier among multiple bands may be considered for certain requirements.

In summary, the Highest Carrier is a key parameter in RF engineering for 3GPP systems. It provides a clear reference for defining the upper frequency limit of operation within a band, enabling consistent testing and ensuring interoperability and coexistence. Its definition has remained stable since Rel-11, with only minor editorial changes, reflecting its fundamental nature in RF requirements.

Purpose & Motivation

The term 'Highest Carrier' exists to provide a precise and unambiguous reference for the upper frequency limit of operation within a specified operating band. In RF engineering, many requirements such as emission limits, power levels, and sensitivity are defined relative to the band edges. Without a clear definition of the highest carrier, there could be ambiguity in testing and compliance, leading to inconsistent interpretations. This term solves that by explicitly defining it as the carrier with the highest centre frequency transmitted or received in the band(s). It is particularly important for carrier aggregation and multi-band operation, where multiple carriers are used simultaneously, and the highest carrier determines the worst-case frequency for certain impairments.

Historically, as mobile networks evolved from single-carrier to multi-carrier and multi-band operation, the need for such a term became evident. In early releases, requirements were often defined for a single carrier at the band edge. With the introduction of carrier aggregation in Rel-10, it became necessary to define requirements for the aggregated set, and the highest carrier provided a consistent reference. The definition was refined in Rel-11 to clarify that it applies to the specified operating band(s), plural, to cover multi-band scenarios. This ensures that equipment can be tested against a common understanding, facilitating global roaming and interoperability.

The term also aids in coexistence studies with other systems, such as satellite or radar, where the highest carrier determines the potential interference footprint. By standardizing this term, 3GPP ensures that all stakeholders—operators, vendors, and test houses—use the same reference, reducing the risk of misinterpretation and enabling efficient certification. Overall, the Highest Carrier is a foundational concept in 3GPP RF specifications, underpinning the reliable operation of mobile networks.