Channel Edge

Description

Channel edge is a term used in 3GPP specifications to define the frequency boundaries of a radio carrier. Specifically, it refers to the lowest or highest frequency of the carrier, separated by the channel bandwidth. In the context of New Radio (NR), the definition has been refined to account for Integrated Access and Backhaul (IAB) nodes, where the channel edge is separated by either the IAB-MT (Mobile Termination) channel bandwidth or the IAB-DU (Distributed Unit) channel bandwidth. This distinction is important because IAB nodes use the same spectrum for both access and backhaul, and the channel edges may differ depending on the direction of transmission.

The channel edge is a critical parameter for RF testing and network planning. It defines the outer limits of the carrier's spectrum, and any transmission outside these limits is considered out-of-band emission. Regulatory bodies set limits on out-of-band emissions to prevent interference with adjacent carriers. Therefore, the channel edge is used as a reference point for measuring adjacent channel leakage ratio (ACLR), spectrum emission mask (SEM), and other RF requirements. In LTE, the channel edge is defined as the lowest and highest frequency of the E-UTRA carrier, separated by the channel bandwidth. For example, a 20 MHz LTE carrier centered at 2.6 GHz would have channel edges at 2.59 GHz and 2.61 GHz.

In 5G NR, the channel edge concept is more complex due to the use of flexible numerology and wider bandwidths. The channel edge is still defined as the lowest or highest frequency of the NR carrier, but the separation is now specified in terms of the IAB-MT or IAB-DU channel bandwidth. This is because IAB nodes may use different bandwidths for access and backhaul links. The IAB-MT channel bandwidth refers to the bandwidth used by the mobile termination part of the IAB node, which communicates with the parent node. The IAB-DU channel bandwidth refers to the bandwidth used by the distributed unit part, which serves UEs. The channel edge for the overall carrier is determined by the larger of the two, ensuring that both access and backhaul transmissions are contained within the allocated spectrum.

The channel edge is also important for carrier aggregation and dual connectivity. When multiple carriers are aggregated, each component carrier has its own channel edges. The aggregated signal must fit within the overall spectrum allocation, and the channel edges of individual carriers must be aligned to avoid gaps or overlaps. In some cases, the channel edges of different carriers may be different, especially when aggregating carriers of different bandwidths. The network must ensure that the aggregated signal meets all RF requirements, including those related to channel edges.

From a testing perspective, the channel edge is used to define the test points for various RF measurements. For example, the error vector magnitude (EVM) is measured at the channel edges to ensure that the signal quality is acceptable across the entire bandwidth. The channel edge is also used in the definition of the occupied bandwidth, which is the bandwidth containing 99% of the total integrated power of the transmitted spectrum. The occupied bandwidth must be less than the channel bandwidth to ensure that the signal is contained within the allocated spectrum.

In summary, the channel edge is a fundamental concept in RF engineering for cellular networks. It defines the spectral boundaries of a carrier and is used as a reference for transmitter and receiver requirements. Its evolution from a simple definition in LTE to a more nuanced one in NR reflects the increasing complexity of modern radio access networks, particularly with the introduction of IAB.

Purpose & Motivation

The concept of a channel edge exists to provide a clear, standardized definition of the spectral boundaries of a radio carrier. This is essential for ensuring that transmissions from different operators and different technologies do not interfere with each other. By defining the channel edge, 3GPP enables regulators to set emission limits and allows equipment manufacturers to design filters and amplifiers that meet these limits. The channel edge also serves as a reference for testing, ensuring that devices comply with RF requirements across the entire operating band.

Historically, the need for a channel edge definition arose as cellular networks moved from narrowband to wideband technologies. In GSM, the 200 kHz channel bandwidth meant that the channel edges were relatively close together, and out-of-band emissions were easier to manage. With the introduction of WCDMA and LTE, the wider bandwidths made it more challenging to contain emissions within the allocated spectrum. The channel edge became a critical parameter for defining spectrum masks and adjacent channel power ratios. In 5G NR, the introduction of IAB further complicated the picture, as the same spectrum is used for both access and backhaul. The channel edge definition was updated to account for the different bandwidths used by IAB-MT and IAB-DU, ensuring that both links can coexist without causing interference.

The limitations of previous approaches included a lack of flexibility in defining channel edges for multi-RAT and multi-band deployments. For example, in a network with both LTE and NR carriers, the channel edges of each carrier must be carefully coordinated to avoid interference. The updated definition in NR provides a more precise framework for such coordination, especially in IAB scenarios where the backhaul link may use a different bandwidth than the access link. This evolution reflects the ongoing need to balance spectral efficiency with interference management in increasingly complex networks.