Sub-band of an Operating Band

Description

In 3GPP specifications, a sub-band is defined as a part of the uplink and downlink frequency range of an operating band. This concept is fundamental to the physical layer design, particularly for spectrum flexibility and carrier aggregation. An operating band, such as Band n78 (3.3-3.8 GHz), can be divided into multiple sub-bands, each covering a contiguous portion of the band. This division allows network operators to deploy carriers in specific sub-bands, facilitating spectrum sharing, interference management, and coexistence with other systems.

The sub-band concept is closely tied to the definition of operating bands and their associated channel bandwidths. For example, in LTE and NR, the supported channel bandwidths (e.g., 5, 10, 20 MHz) are mapped to specific sub-bands within an operating band. The sub-band structure ensures that transmissions occur within the allocated spectrum and comply with regulatory requirements. It also enables the UE to scan and measure specific sub-bands for cell selection and mobility purposes.

In carrier aggregation (CA), sub-bands play a crucial role in defining component carriers (CCs). Each CC occupies a sub-band, and multiple CCs can be aggregated across different sub-bands to increase bandwidth. The sub-band concept also supports supplementary uplink (SUL) and supplementary downlink (SDL) operations, where additional sub-bands are used to enhance coverage or capacity. Furthermore, sub-bands are used in dynamic spectrum sharing (DSS) between LTE and NR, allowing both technologies to operate within the same operating band but in different sub-bands.

The 3GPP specifications 25.104 (UTRA), 36.141 (E-UTRA), 37.104 (NR) and their counterparts define the sub-band structure for each operating band. These specifications detail the allowed sub-band configurations, including the relationship between sub-bands and synchronization raster, channel raster, and resource blocks. The sub-band concept is also relevant for radio frequency (RF) requirements, such as out-of-band emissions and spurious emissions, which are specified per sub-band to ensure coexistence.

From an implementation perspective, sub-bands influence the design of RF front-ends, filters, and antennas. Devices must support the sub-bands defined for their operating bands, and network planning must consider sub-band allocation to avoid interference. The sub-band concept has evolved with new spectrum bands (e.g., millimeter wave) and features like flexible numerology in NR, where sub-bands can have different subcarrier spacings. Overall, sub-bands provide a granular approach to spectrum management, enabling efficient use of frequency resources in modern cellular networks.

Purpose & Motivation

The sub-band concept was introduced to address the need for flexible and efficient spectrum utilization in cellular networks. As operating bands became wider and more fragmented, dividing them into sub-bands allowed operators to deploy carriers in specific portions, reducing interference and simplifying coordination. It also enabled the coexistence of multiple technologies (e.g., LTE and NR) within the same band by allocating different sub-bands to each.

Before sub-bands, spectrum allocation was often coarse-grained, leading to inefficient use and difficulties in managing interference. The sub-band approach provides a standardized way to define spectrum segments, making it easier for equipment vendors to design products that comply with regulatory and operator requirements. It also facilitates international roaming by ensuring that UEs can operate in the sub-bands of visited networks.

In the context of 5G and beyond, sub-bands are essential for supporting diverse services with different bandwidth and latency requirements. They enable network slicing at the spectrum level, where different sub-bands can be dedicated to specific slices. The concept continues to evolve with new bands and features, ensuring that spectrum remains a manageable and scalable resource.