Fallback band combination

Description

A fallback band combination is a concept in 3GPP radio access network specifications that defines a subset of a more complex band combination. It was introduced in Release 13 (Rel-13) and has evolved through Rel-19. The definition states: 'A Uu band combination that would result from another Uu band combination (parent band combination) by releasing at least one SCell or uplink configuration of SCell, or SCG, or SUL. A PC5 band combination that would result from another PC5 band combination (parent band combination) by releasing at least one sidelink carrier. An intra-band non-contiguous band combination is not considered to be a fallback band combination of an intra-band contiguous band combination. A fallback band combination supports the same channel bandwidth(s) for each carrier as its parent band combination(s).'

In carrier aggregation (CA) and dual connectivity (DC), a UE can support multiple component carriers (CCs) across different bands. A parent band combination represents a full set of carriers that the UE can aggregate. A fallback band combination is a simplified version where one or more carriers or configurations are removed. For example, if a UE supports CA with three downlink carriers and one uplink carrier, a fallback band combination might be CA with two downlink carriers and one uplink carrier. The UE is required to support all fallback band combinations of a parent combination it supports, ensuring backward compatibility and flexibility in network scheduling.

The concept is crucial for network operators because it allows them to configure the UE with a subset of carriers based on available resources or load conditions. If the network cannot allocate all carriers of the parent combination, it can fall back to a simpler combination that the UE is guaranteed to support. This is particularly important in heterogeneous networks where different cells may have different capabilities. The fallback band combination ensures that the UE can still operate with reduced throughput rather than failing to connect.

The definition distinguishes between Uu (radio interface between UE and gNB/eNB) and PC5 (sidelink interface for V2X) band combinations. For Uu, releasing an SCell, uplink configuration, SCG (Secondary Cell Group in DC), or SUL (Supplementary Uplink) results in a fallback combination. For PC5, releasing a sidelink carrier results in a fallback combination. The rule that an intra-band non-contiguous combination is not a fallback of an intra-band contiguous combination is important because contiguous and non-contiguous aggregations have different RF requirements and UE capabilities. A UE supporting contiguous aggregation may not support non-contiguous, so they are treated as separate.

The requirement that a fallback band combination supports the same channel bandwidth(s) for each carrier as its parent ensures that the UE's RF capabilities are consistent. For example, if the parent combination supports 20 MHz per carrier, the fallback must also support 20 MHz per carrier for the remaining carriers. This simplifies UE implementation and testing.

Fallback band combinations are specified in 3GPP TS 36.306 (E-UTRA UE radio access capabilities) and TS 38.306 (NR UE radio access capabilities). These specifications define the signaling of UE capabilities, including which band combinations and fallback combinations the UE supports. The network uses this information to configure the UE appropriately. The concept has evolved over releases, with Rel-14 clarifying bandwidth support, Rel-16 extending to SCG, SUL, and PC5, and later releases maintaining the definition.

Purpose & Motivation

The fallback band combination concept was introduced to address the complexity of UE capability signaling in carrier aggregation and dual connectivity. Before its introduction, if a UE supported a complex band combination, it was unclear whether it also supported simpler subsets. This led to interoperability issues and inefficient use of network resources. The fallback band combination provides a standardized way to define subsets, ensuring that a UE supporting a parent combination automatically supports its fallback combinations.

The motivation was to simplify network configuration and scheduling. When a network wants to configure a UE with a subset of carriers, it can rely on the fallback principle to know that the UE will support it. This reduces signaling overhead and avoids the need for the UE to explicitly signal every possible combination. It also enables dynamic adaptation to changing radio conditions or network load.

Historically, as CA and DC became more complex with the introduction of new features like SUL, SCG, and sidelink, the fallback concept was extended to cover these new dimensions. The Rel-16 update was significant because it explicitly included SCG, SUL, and PC5, reflecting the growing importance of dual connectivity and V2X. The fallback band combination solves the problem of capability signaling explosion and ensures that UEs can operate in reduced configurations when necessary.