5G-Advanced: Bridging to 6G

As the first release of 5G-Advanced, Release-18 has ushered in the 5G-Advanced era. The 3GPP standardization on physical layer design was finished in August 2023, and the ASN.1 will be frozen in June 2024, by which all specifications will be stable and ready for implementation.

Release-19, with the initial package approved in December 2023, has commenced exploration into 6G technology. Targeting completion by the end of 2025, it aims to serve as a bridge to 6G.

Release-18: Initiating 5G-Advanced Era

• RAN Part

For RAN, 26 study items or work items have been completed in R18, with 3 of them enabling intelligence or efficiency, 9 supporting verticals, and 14 enhancing network coverage and capacity.

—eSmall data transmission: As a WI led by ZTE, in Rel-18, mobile terminated small data transmissions are supported with the completion of the MT-SDT WI. For DL, MT-SDT (i.e., DL-triggered small data) offers similar benefits as MO-SDT, such as reducing signaling overhead and UE power consumption by avoiding unnecessary transitions to RRC_CONNECTED and reducing latency by allowing fast transmission of small and infrequent packets, e.g. for positioning.

—Network controlled repeaters: This work item, which is also led by ZTE, specifies the signaling and behavior for side control information (i.e., beam-forming, UL-DL TDD operation, and ON-OFF information), control plane signaling and procedures, and solutions for network-controlled repeater management.

—Extended reality (XR): XR Awareness, power saving enhancements and capacity enhancements have been specified in this work item. For XR awareness, one additional buffer size table, a new MAC CE for delay status report (DSR) of buffered data, and reporting of uplink assistance information (jitter range, burst arrival time, UL data burst periodicity) per QoS flow have been introduced to enhance uplink resource scheduling by NG-RAN. For power saving enhancements, the gNB may configure a DRX cycle expressed in rational numbers to match the periodicities of video frame rates. In addition, configured grants may be set without the need for the UE to monitor possible UL retransmissions, thus increasing power savings for the UE. For capacity enhancements, configured grant-based PUSCH transmissions are enhanced with support of multiple CG PUSCH transmission occasions within a single period of a CG configuration. Moreover, indications of unused CG PUSCH occasion(s) of a CG configuration are provided, with uplink control information multiplexed in CG PUSCH transmissions of the CG configuration.

—Network energy savings: Network energy savings are key to 5G/NR success, aiming to reduce environmental impact (greenhouse gas emissions) and achieve operational cost savings. In the initial study conducted within RAN WGs, various techniques spanning time, frequency, spatial and power domains were investigated based on a network energy consumption model for base stations. Following that, the Rel-18 work item introduced several new features including CSI enhancements for adaptations in spatial and power domains, Cell DTX/DRX mechanism in time domain, SSB-less SCell operation for inter-band CA, mechanism to prevent legacy UEs camping and enhancements on CHO procedure, as well as inter-node beam activation and enhancements on paging.

—NTN/NTN-IoT: Rel-18 NR NTN work item specifies coverage enhancement, NR-NTN deployment in above 10 GHz bands, network-verified UE location, and NTN-TN and NTN-NTN mobility and service continuity enhancements. Rel-18 RAN work item further enhances the IoT-NTN in three major areas: performance (HARQ and GNSS enhancements), measurement and mobility enhancements in both idle and connected modes, and enhancements in discontinuous coverage. It also complements the SA2 study on 5GC enhancements for satellite access Phase 2.

• CN Part

For CN, 28 work items have been completed in R18, including 2 for intelligent network enablement, 11 for enhanced network services, 8 for vertical support, and 7 for enhanced network convergence and coverage.

—Network slicing phase 3 & eNSAC: These two work items are led by ZTE. In the network slicing phase 3, functional enhancements and deployment optimizations for 5GS network slicing are specified. Key features include support for slice service continuity, service area not matching TA boundary, temporary or periodic slice deployment, multiple network slice admission control (NSAC) service area, partial slices in the registration area, and improved network control of UE usage. Based on the GSMA-defined network slice template, the eNSAC work item enhances the existing NSAC procedure to control the number of UEs with at least one PDU session/PDN connection per network slice in the case of EPC interworking.

—XR & media services: To better support XR and other media services in the 5G system, this work item enhances policy control and QoS mechanisms for characteristics like multi-modality flows and PDU set handling. It also supports 5GS information exposure for XR and other media services, and provides solutions to meet performance requirements for round-trip latency, jitter, UE power saving, and the trade-off between QoE and UE power saving.

—System support for AI/ML-based services: This work item aims to provide intelligent transmission support for AI/ML-based services at the application layer. This enables service providers to leverage 5GS as an intelligent platform to assist in their AI/ML operations at the application layer.

Release-19: Exploring 6G Technology

• RAN Part

The initial Rel-19 package has included 16 study items and work items, with 8 being further enhancements of Rel-18 topics and 8 being new topics.

—AI/ML for NG-RAN, air interface and mobility: This study item led by ZTE aims to investigate new AI/ML based use cases, i.e., network slicing and CCO, within existing NG-RAN interfaces and architecture (including non-split and split architectures). This goal is to identify enhancements that can support AI/ML functionality. Moreover, the study involves further discussions on the Rel-18 leftovers, including mobility optimization for NR-DC, split architecture support for Rel-18 use cases, energy saving enhancements such as energy cost prediction, continuous MDT collection targeting the same UE across RRC states, and multi-hop UE trajectory across gNBs. The AL/ML for air interface and mobility in NR will also be specified in Rel-19.

—Channel modelling enhancements for 7-24GHz: This study item also led by ZTE aims to validate through measurements the channel model of TR38.901 within the 7-24 GHz frequency range. It will adapt and extend as necessary the channel model of TR38.901 for this frequency range, addressing key aspects for applicable scenarios such as near-field propagation (ensuring consistency between near-field and far-field) and spatial non-stationarity.

—Ambient IoT: This study targets a further assessment at RAN WG-level of ambient IoT, a new 3GPP IoT technology suitable for deployment in a 3GPP system. Ambient IoT relies on ultra-low complexity devices with ultra-low power consumption, specially designed for very-low-end IoT applications. The study will provide clear differentiation, addressing use cases and scenarios that cannot be fulfilled based on existing 3GPP LPWA IoT technology, such as NB-IoT, even when considering reduced peak Tx power.

• CN Part

The initial Rel-19 package has included 15 study items, comprising 8 further enhancements of Rel-18 topics and 7 new topics.

—UPF enhancement for exposure and SBA phase 2: The study item led by ZTE aims to address R18 leftovers to enhance UPF capabilities for exposure and achieve better integration of UPF into 5GC SBA. The specific objectives include supporting selection of UPF for user plane functionalities, optimizing the procedures related to UPF data collection, e.g., direct/indirect subscription of UPF via control plane from application, and enhancing the interface between AF and 5GC to permit UPF handling of headers (e.g., detection of IP header, http header, etc), uplink and downlink, as well as reporting/notifications.

—Architecture support of ambient power-enabled IoT: This study item aims to study the architecture support of ambient power-enabled IoT devices. The specific objectives include validating the device’s ID, managing identification, subscription, registration and connection of ambient IoT devices, and supporting information transfer for ambient IoT services.

—Energy efficiency and energy saving: This study item aims to study architectural impacts and functional extensions required to facilitate efficient energy use and energy saving. The specific objectives include exposing information related to network energy, and implementing subscription and policy control to support energy efficiency and energy saving as service criteria.

—Core network enhanced support for AI/ML: This study item aims to study possible architectural and functional extensions for cross-domain AI/ML interworking and coordination (e.g., UE, RAN, core, applications) to address the overall AI/ML framework. It will investigate how to support collaborative AI/ML operations, involving 5GC/NWDAF and/or AF for vertical federated learning (VFL), and how to support NWDAF-assisted policy control to prevent signaling storms.

IMT-2030 has defined six usage scenarios for 6G, including three extensions from traditional ones: hyper reliable and low-latency communication, massive communication, and immersive communication, and three new ones: AI and communication, integrated sensing and communication, and ubiquitous connectivity (Fig. 1).

With some study items and work items in Rel-18 and Rel-19, it is evident that some pre-research on 6G has been initiated in 5G-Advanced. For instance, NTN aims to provide ubiquitous connectivity, ambient IoT supports massive communication, AI/ML for NR-RAN explores AI and communication, and XR enhancement focuses on immersive communication. Consequently, it can be asserted that 5G-Advanced is establishing a bridge to 6G, facilitating the sustainable digitization and intelligence of the entire world.