Carrier frequency

Description

Carrier frequency in 3GPP terminology refers to the center of the Absolute Radio Frequency Channel Number (ARFCN) under test. The ARFCN is a unique identifier for a specific radio frequency channel in a cellular network, and its center frequency is the nominal frequency around which the carrier is transmitted or received. This definition is fundamental to the physical layer of cellular systems, as it determines the exact frequency resource used for communication. The carrier frequency is used in testing and measurement scenarios to verify the performance of user equipment (UE) and base stations (BS) under specific frequency conditions.

The concept of carrier frequency is closely tied to the frequency bands defined by 3GPP for different radio access technologies (RATs), including GSM, UTRA (UMTS), E-UTRA (LTE), and NR (5G). Each band has a set of ARFCNs, and each ARFCN corresponds to a specific carrier frequency. For example, in LTE, the carrier frequency is derived from the E-UTRA Absolute Radio Frequency Channel Number (EARFCN) using a formula that depends on the band. In 5G NR, the NR-ARFCN is used, and the relationship between ARFCN and frequency is defined in 3GPP TS 38.104. The carrier frequency is critical for ensuring that transmitters and receivers operate on the correct frequency and meet regulatory requirements.

In practice, the carrier frequency is used in conformance testing to evaluate parameters such as frequency error, modulation quality, and adjacent channel leakage ratio (ACLR). Test equipment generates signals at the specified carrier frequency, and the device under test (DUT) is configured to operate on that frequency. The definition 'centre of the ARFCN under test' emphasizes that the carrier frequency is the nominal center of the channel, not the edge frequencies. This is important because the channel bandwidth extends around the carrier frequency, and the center is used as the reference point for measurements.

The carrier frequency also plays a role in network planning and deployment. Operators must carefully choose carrier frequencies to avoid interference and optimize coverage and capacity. In multi-carrier and carrier aggregation scenarios, multiple carrier frequencies are used simultaneously, and their relative positions affect the overall performance. The 3GPP specifications provide detailed tables and formulas to map ARFCNs to carrier frequencies for each band, ensuring global interoperability.

In summary, carrier frequency is a basic but essential concept in cellular communications. It defines the center frequency of a radio channel and is used throughout the lifecycle of a network, from design and testing to deployment and optimization. Its definition in 3GPP specifications ensures a common understanding and consistent implementation across different vendors and operators.

Purpose & Motivation

The definition of carrier frequency exists to provide a precise and unambiguous reference for the center frequency of a radio channel in cellular networks. It solves the problem of identifying and measuring the exact frequency at which a carrier operates, which is crucial for testing, certification, and interoperability. Without a standardized definition, different stakeholders might interpret the frequency of a carrier differently, leading to inconsistencies in measurements and potential interference.

Historically, as cellular technologies evolved from GSM to UMTS, LTE, and 5G NR, the need for a common frequency reference became more critical. Each technology introduced its own ARFCN scheme, but the concept of carrier frequency as the center of the ARFCN remained consistent. This allows test equipment and measurement procedures to be defined in a technology-agnostic way, while still accommodating the specific ARFCN mappings of each RAT.

The carrier frequency definition also supports regulatory compliance. Regulatory bodies assign spectrum in terms of frequency blocks, and the carrier frequency must fall within the allocated block. By defining the carrier frequency as the center of the ARFCN, 3GPP ensures that the transmitted signal is centered within the assigned channel, minimizing out-of-band emissions and facilitating spectrum sharing. This is particularly important in shared and unlicensed spectrum scenarios, where precise frequency control is required to avoid interference with other users.