Inter-band concurrent operation of NR Uu and NR Sidelink

Description

Inter-band concurrent operation is a radio access network feature introduced in 3GPP Release 19 that allows a User Equipment (UE) to simultaneously transmit and/or receive on an NR Uu interface (the standard cellular link between UE and gNB) and an NR Sidelink interface (the direct UE-to-UE link) using carriers in different frequency bands. This capability is defined in the RF performance specifications TS 38.101 (UE radio transmission and reception) and TS 38.521 (UE conformance testing). The fundamental concept is that the Uu link and the Sidelink link operate on separate frequency carriers that belong to different operating bands, as opposed to intra-band operation where both links would share the same band.

Architecturally, the UE must support two independent radio frequency (RF) chains that can be tuned to different bands concurrently. This requires careful RF design to avoid intermodulation interference, harmonic interference, and blocking between the two carriers. The UE also needs to manage its power allocation across both links, ensuring that the total transmit power does not exceed regulatory limits and that both links maintain sufficient quality. The gNB controls the Uu link, while the Sidelink link is managed by the UE itself (or by a group of UEs) using the PC5 interface. The concurrent operation is coordinated at the UE level, with the network providing configuration and resource allocation for the Sidelink via the Uu link.

In terms of protocol stack, the Uu link uses the full NR protocol stack (PHY, MAC, RLC, PDCP, SDAP) for communication with the gNB, while the Sidelink uses a modified stack (PHY, MAC, RLC, PDCP) for direct communication. The two stacks operate in parallel, sharing the UE's processing resources. The MAC layer must handle multiplexing of data from both links, and the PHY layer must manage the dual-carrier transmission and reception. The UE's capability to support inter-band concurrent operation is signaled to the network during capability transfer, allowing the network to configure the Sidelink appropriately.

Key technical challenges addressed by this feature include the need for efficient spectrum utilization, where the Uu link might operate in a licensed band (e.g., 3.5 GHz) and the Sidelink in an Intelligent Transportation System (ITS) band (e.g., 5.9 GHz). This separation allows the Sidelink to use dedicated spectrum for V2X while the Uu link provides wide-area coverage and network coordination. The feature also supports advanced use cases such as platooning, where vehicles need to exchange data directly with low latency while also communicating with the network for traffic information and remote driving support.

Inter-band concurrent operation is a cornerstone for Release 19's enhanced Sidelink features, enabling new services that require both direct communication and network connectivity. It builds on earlier Sidelink designs (introduced in Release 12 for LTE and enhanced in Release 16/17 for NR) by removing the constraint that both links must operate in the same band, thus providing more flexibility for spectrum deployment and improving overall system performance. The feature is expected to be further refined in Release 20, with potential enhancements in power efficiency and multi-band coordination.

Purpose & Motivation

The primary motivation for inter-band concurrent operation is to enable simultaneous NR Uu and NR Sidelink communication without the limitations of intra-band operation. In earlier releases, when a UE operated both links in the same band, it faced challenges such as self-interference, limited spectrum availability, and reduced flexibility in resource allocation. By allowing the two links to use different bands, the UE can leverage dedicated spectrum for Sidelink (e.g., ITS bands) while maintaining cellular connectivity in licensed bands, thus improving reliability and performance for advanced applications like V2X and public safety.

Historically, Sidelink was designed for direct communication in scenarios where network coverage might be unavailable or unreliable. However, many use cases (e.g., platooning, sensor sharing) require both direct links and network connectivity. Intra-band operation forced both links to share the same frequency resources, leading to inefficient spectrum use and potential interference. Inter-band operation solves this by decoupling the two links, allowing each to operate in its optimal band. This also simplifies RF design in some cases, as the bands can be chosen to minimize interference.

The feature was introduced in Release 19 as part of the broader enhancements for NR Sidelink, which aims to support advanced V2X services with higher throughput, lower latency, and better reliability. It addresses the limitations of previous approaches by providing a standardized way for UEs to signal their inter-band concurrent capabilities and for networks to configure the Sidelink accordingly. This ensures interoperability and consistent performance across different UE implementations and network deployments.