Lowest Carrier

Description

The Lowest Carrier is a term used in 3GPP specifications to identify the carrier with the lowest carrier centre frequency among all carriers transmitted or received in the specified operating band(s). It is a key concept for defining RF requirements in multi-carrier and carrier aggregation scenarios, where multiple carriers are used simultaneously within one or more operating bands. The Lowest Carrier serves as a reference point for various transmitter and receiver requirements, such as out-of-band emissions, spurious emissions, and blocking characteristics.

In the context of 3GPP TS 25.104 (UTRA FDD), TS 36.104 (E-UTRA), and TS 38.104 (NR), the Lowest Carrier is used to specify the frequency range over which certain requirements apply. For example, when a UE or BS transmits on multiple carriers, the out-of-band emissions are defined relative to the lowest and highest carrier frequencies. The Lowest Carrier helps in determining the edge of the aggregated channel bandwidth for the purpose of defining unwanted emissions limits. Similarly, for receiver requirements, the Lowest Carrier is used to define the frequency offsets for blocking and adjacent channel selectivity tests.

The concept of the Lowest Carrier became particularly important with the introduction of carrier aggregation in Rel-10, where multiple component carriers can be aggregated to increase bandwidth. In such scenarios, the aggregated signal spans from the lowest to the highest carrier frequency, and the Lowest Carrier marks the lower boundary of this aggregated signal. This is essential for ensuring that the transmitted signal does not cause excessive interference to other systems and that the receiver can properly filter out unwanted signals.

In addition to carrier aggregation, the Lowest Carrier is also relevant for dual connectivity and multi-band operation, where a device may transmit or receive on multiple carriers in different bands. The term is defined per operating band or per set of operating bands, depending on the context. The exact definition may vary slightly across specifications, but the core concept remains the same: it is the carrier with the minimum carrier centre frequency.

Overall, the Lowest Carrier is a fundamental parameter for RF design and testing in 3GPP networks. It ensures that requirements are applied consistently across different carrier configurations and helps maintain interoperability and spectrum efficiency.

Purpose & Motivation

The Lowest Carrier concept exists to provide a clear and unambiguous reference for RF requirements when multiple carriers are used. Prior to its introduction, RF requirements were typically defined for a single carrier, and with the advent of carrier aggregation, there was a need to define requirements for the aggregated signal. The Lowest Carrier, along with the Highest Carrier, defines the boundaries of the aggregated channel bandwidth, enabling the specification of out-of-band emissions and other parameters.

This concept solves the problem of how to apply RF requirements to a multi-carrier transmission without having to define separate requirements for each carrier combination. By using the Lowest Carrier as a reference, 3GPP can specify requirements that are applicable to any number of carriers, as long as the lowest and highest carrier frequencies are known. This simplifies the specification and testing process, as test equipment can measure the aggregated signal relative to these reference points.

Furthermore, the Lowest Carrier is essential for ensuring coexistence between different systems and operators. By defining the lower boundary of the transmitted signal, it helps in calculating the necessary guard bands and in assessing the potential for interference. This is particularly important in spectrum sharing scenarios and in bands where multiple operators coexist. The concept has evolved with each release to accommodate new band combinations and carrier aggregation scenarios, but its fundamental purpose remains unchanged.