New Radio Adjacent Channel Leakage Ratio

Description

The New Radio Adjacent Channel Leakage Ratio (NRACLR) is a critical radio frequency (RF) conformance test parameter defined in the 3GPP specifications for 5G New Radio (NR) user equipment (UE) and base stations (gNBs). It is a measure of a transmitter's ability to confine its transmitted power within its assigned channel bandwidth, thereby minimizing unwanted emissions into adjacent frequency channels. This parameter is essential for maintaining spectral efficiency and ensuring coexistence between different network operators or between different carriers within the same operator's spectrum. The measurement is performed by applying a specific measurement filter, defined by the channel bandwidth and subcarrier spacing, to the transmitted signal. The power is measured first within the assigned channel's bandwidth and then within the bandwidth of an adjacent channel, with the ratio expressed in decibels (dB). A lower ACLR value indicates better transmitter performance, meaning less power is leaking into the neighboring channel.

From an architectural perspective, NRACLR testing is integrated into the RF conformance testing suites for both base stations and UEs, as detailed in specifications such as 38.141 and 38.521. The test setup involves a signal generator to create the NR test model and a spectrum analyzer or specialized test equipment to perform the filtered power measurements. The requirements vary based on factors like the operating frequency band (FR1 or FR2), the channel bandwidth, the modulation scheme, and the power class of the device. For base stations, the ACLR requirements ensure that a gNB's transmission does not cause harmful interference to UEs connected to a neighboring cell on an adjacent frequency. For UEs, it ensures that a device transmitting on one carrier does not desensitize the receiver of a nearby device or base station operating on an adjacent carrier.

The role of NRACLR in the network is foundational to spectrum management and network planning. It directly impacts the ability to deploy carriers with narrow guard bands, enabling higher overall spectrum utilization. By strictly defining and testing for ACLR, 3GPP ensures that equipment from different vendors can interoperate in dense, multi-carrier environments without causing mutual interference. This is particularly crucial for spectrum-sharing scenarios, such as dynamic spectrum sharing (DSS) or operations in shared and unlicensed bands, where transmitters from different technologies or operators are in close spectral proximity. Compliance with NRACLR specifications is therefore a mandatory part of the type approval and certification process for all NR radio equipment, guaranteeing a baseline level of spectral cleanliness for the entire ecosystem.

Purpose & Motivation

The purpose of defining the NRACLR metric is to establish a standardized, quantifiable limit for out-of-channel emissions from 5G NR transmitters. Without such a requirement, transmitters could emit excessive power outside their allocated bandwidth, causing interference to receivers tuned to adjacent channels. This interference, known as adjacent channel interference (ACI), can degrade the signal-to-interference-plus-noise ratio (SINR) for victim receivers, leading to reduced data rates, increased block error rates, and ultimately, a degradation of overall network capacity and user experience. The problem is exacerbated in modern cellular networks which employ wide channel bandwidths (e.g., up to 100 MHz in FR1 and 400 MHz in FR2) and dense carrier aggregation, leaving minimal guard bands between channels.

Historically, similar metrics like ACLR existed for UMTS and LTE, but the transition to 5G NR introduced new challenges that necessitated a re-specification. 5G NR supports a much wider range of carrier frequencies (from sub-1 GHz to millimeter wave), new waveform parameters (like flexible numerology with variable subcarrier spacing), and more complex power amplifier architectures for massive MIMO. The previous LTE ACLR definitions and test models were insufficient to characterize the leakage behavior of these new NR signals accurately. Therefore, 3GPP developed NR-specific ACLR requirements (NRACLR) in Release 15 to provide a consistent and rigorous framework for evaluating transmitter performance across the entire 5G NR design space. This ensures that the advanced spectral efficiency promises of 5G can be realized in practice without being undermined by inter-carrier interference.

Key Features

• Defines transmitter unwanted emission limits into adjacent channels.
• Measurement uses a specific NR measurement filter aligned with the channel bandwidth and subcarrier spacing.
• Requirements are specified separately for User Equipment (UE) and Base Stations (gNB).
• Values depend on frequency range (FR1 or FR2), bandwidth, and modulation.
• Critical for enabling narrow guard bands and efficient spectrum utilization.
• A key parameter for conformance testing and equipment certification.

Evolution Across Releases

Defining Specifications