Fallback group

Description

A fallback group is a concept introduced in 3GPP Release 15 (Rel-15) for 5G NR carrier aggregation. It defines a group of carrier aggregation bandwidth classes for which it is mandatory for a UE to be able to fall back to lower order CA bandwidth class configurations. The definition states: 'Group of carrier aggregation bandwidth classes for which it is mandatory for a UE to be able to fallback to lower order CA bandwidth class configuration. It is not mandatory for a UE to be able to fallback to lower order CA bandwidth class configuration that belongs to a different fallback group.'

In carrier aggregation, bandwidth classes (e.g., CA_n1A, CA_n1B, CA_n1C) define the aggregation of component carriers with specific bandwidths. A higher order class (e.g., CA_n1C) aggregates more or wider carriers than a lower order class (e.g., CA_n1A). The fallback group concept groups these classes such that if a UE supports a higher order class, it must also support the lower order classes within the same group. However, it is not required to support lower order classes in other groups. This reduces the testing and implementation burden on UEs while ensuring a minimum level of flexibility.

The grouping is typically based on the band and the aggregation type. For example, all bandwidth classes for band n1 might belong to one fallback group, while those for band n78 belong to another. The exact grouping is defined in 3GPP TS 38.101 (NR UE radio transmission and reception) and TS 38.521 (NR UE conformance specification). These specifications list the fallback groups and the mandatory fallback relationships.

The fallback group concept is important for network operators because it allows them to know which lower order configurations a UE will support based on its support of higher order configurations. When scheduling, the network can fall back to a lower order CA configuration within the same group if needed, without explicit signaling from the UE. This is particularly useful in dynamic spectrum sharing or when resources are limited.

For UE vendors, the fallback group simplifies capability signaling and reduces the number of combinations that must be tested. Instead of supporting all possible lower order combinations across all bands, the UE only needs to support those within the same fallback group. This makes UE design more efficient and reduces time to market.

The concept has been maintained unchanged from Rel-15 through Rel-19, indicating its stability and effectiveness. It is a key part of the NR CA framework, ensuring interoperability and efficient use of spectrum.

Purpose & Motivation

The fallback group concept was introduced to address the complexity of carrier aggregation bandwidth classes in 5G NR. With the wide range of bands and bandwidths, the number of possible CA configurations is enormous. Requiring UEs to support all lower order configurations of every higher order configuration they support would be impractical and costly. The fallback group limits the mandatory fallback to within a defined group, reducing the burden on UE vendors while still providing network flexibility.

Before fallback groups, the fallback requirements were less structured, leading to potential ambiguity and interoperability issues. The fallback group provides a clear, standardized framework that both UE vendors and network operators can rely on. It ensures that a UE supporting a high-order CA configuration can still operate in reduced configurations within the same group, which is essential for scenarios like network congestion or coverage limitations.

The motivation was also to align with the modular approach of 5G NR, where different bands and bandwidths are treated as separate entities. By grouping them, the specifications can be more manageable and scalable as new bands are added. The fallback group concept solves the problem of capability signaling explosion and ensures that UEs can gracefully degrade their CA operation without losing connectivity.