Sub-block Bandwidth

Description

Sub-block bandwidth is defined in 3GPP specifications as the bandwidth of one sub-block. Since a sub-block is a contiguous allocated block of spectrum for transmission and reception by the same UE, the sub-block bandwidth represents the width of that block in frequency. This parameter is crucial for RF performance requirements, as it determines the spectral characteristics of the transmitted signal and the filtering needed to meet emission limits. The sub-block bandwidth is used in specifications such as 25.104 (UTRA), 36.101 (E-UTRA), and 38.104 (NR) to define test cases and compliance criteria for UEs and base stations.

In carrier aggregation scenarios, the sub-block bandwidth is often equal to the channel bandwidth of a component carrier, but it can also be the aggregated bandwidth of multiple contiguous component carriers. For example, in intra-band contiguous CA, the sub-block bandwidth is the total bandwidth of the aggregated carriers, which may be wider than a single carrier. This definition ensures that the UE's RF front-end is tested for the actual contiguous spectrum it will use, rather than individual carriers. The sub-block bandwidth is also used to define the spectrum emission mask and adjacent channel leakage ratio (ACLR) requirements, which specify the maximum allowed emissions outside the sub-block.

The sub-block bandwidth is closely related to the RF bandwidth, which is the maximum bandwidth supported by the UE. The RF bandwidth may contain multiple sub-blocks, each with its own sub-block bandwidth. The network scheduler allocates sub-blocks based on the UE's capabilities and the available spectrum, and the sub-block bandwidth must be within the UE's supported RF bandwidth. In NR, the sub-block bandwidth can be flexible, supporting various numerologies and channel bandwidths up to 400 MHz in millimeter wave bands.

From an implementation perspective, the sub-block bandwidth affects the design of power amplifiers, filters, and duplexers. A wider sub-block bandwidth requires more linear power amplifiers and sharper filters to meet emission requirements. It also impacts the UE's power consumption and thermal design. The 3GPP specifications define sub-block bandwidth for each operating band and CA configuration, providing a clear framework for equipment design and testing.

The sub-block bandwidth concept has remained stable since its introduction in Rel-10, with the definition unchanged across releases. However, its application has expanded with new bands and features, such as supplementary uplink and dynamic spectrum sharing. Overall, sub-block bandwidth is a fundamental parameter for ensuring that cellular devices operate efficiently and without causing harmful interference.

Purpose & Motivation

The sub-block bandwidth parameter was introduced to provide a precise metric for RF compliance testing in the context of carrier aggregation and flexible spectrum allocation. Before its definition, RF requirements were often specified per carrier, which was insufficient for scenarios where multiple carriers are aggregated into a single contiguous block. The sub-block bandwidth allows regulators and operators to define emission limits for the actual spectrum block used by a UE, ensuring coexistence with other systems.

It also solves the problem of testing UEs with different CA configurations. By defining the sub-block bandwidth, test houses can verify that a UE meets emission requirements for the specific contiguous spectrum it supports. This is particularly important for intra-band contiguous CA, where the aggregated bandwidth can be much larger than a single carrier. The sub-block bandwidth provides a common language for describing these configurations.

As networks move towards wider bandwidths and higher frequencies, the sub-block bandwidth remains a key parameter for spectrum management. It enables efficient use of spectrum by allowing operators to deploy wide contiguous blocks while ensuring that emissions are controlled. The concept is expected to evolve with new technologies like 6G, but its fundamental purpose of defining spectrum block width will persist.