V2X in 3GPP Standardization: NR Sidelink in Rel-16 and Beyond

1 INTRODUCTION

• Vehicular networks have expanded beyond road safety and traffic efficiency to include autonomous-driving and infotainment applications.Infotainment applications require high throughput and low latency.Source: §1 · p. 1
• NR sidelink is direct communication between two user equipments without a base station participating in data transmission or reception.The paper describes sidelink as a key enabler of V2X communication.Source: §1 · p. 1
• The paper surveys general and RAN aspects of NR-V2X sidelink specified in Rel-16 and discusses Rel-17 or later work under study or planned.Its scope includes future standardization activities, not only the first NR-V2X release.Source: §1 · p. 1
• Related work includes a review and study of LTE-V2X based on 3GPP Rel-14/15.The paper's chapter plan also covers 3GPP standardization, V2X history and applications, NR sidelink, LTE-V2X comparison, and future releases.Source: §1 · p. 1

2 3GPP STANDARDIZATION ACTIVITIES

• 3GPP standardization activities are organized into three Technical Specification Groups: RAN, SA, and CT.RAN covers access-network functions and interfaces; SA covers system architecture and service capabilities; CT covers terminals, interfaces, and core-network development.Source: §2 · pp. 1–2
• RAN TSGs define access-network functions, requirements, and interfaces across physical, layer 2, and layer 3, and conduct conformance testing.The groups include RAN1 through RAN6.Source: §2 · p. 1
• SA TSGs define and maintain overall system architecture and service capabilities, and support coordination between TSGs..Source: §2 · p. 1
• A Study Item produces a Technical Report; after agreement, it can become a Work Item whose final agreements are documented in a Technical Specification.The listed RAN document series focus on LTE (36.xxx), multiple radio access technologies (37.xxx), and radio technology beyond LTE such as NR (38.xxx).Source: §2 · p. 2

3 HISTORY OF CELLULAR V2X

• LTE's Rel-12 introduced device-to-device communication, a direct UE-to-UE feature that enables V2X communication.LTE itself was defined from Rel-8, beginning in 2008.Source: §3 · p. 2
• LTE-V2X was studied and standardized in Rel-14, with Rel-15 also part of the LTE-V2X release period.Two of four LTE D2D resource-allocation modes were specifically for V2X; broadcast was its only cast type, and all three coverage scenarios were supported.Source: §3 · p. 2
• NR-V2X Rel-16 was the first NR sidelink specification focused on V2X, targeting advanced use cases as well as road safety.Its use cases include platooning, extended sensors, advanced driving, and remote driving.Source: §3 · p. 2
• Rel-16 added unicast and groupcast alongside broadcast and supports sidelink HARQ to improve communication reliability.The release was designed to enhance reliability, latency, capacity, and flexibility.Source: §3 · p. 2

4 GENERAL ASPECTS OF NR-V2X APPLICATION

• NR-V2X use cases have strict QoS requirements, and the paper identifies extended sensors, platooning, remote driving, and advanced driving as key examples.Extended sensors exchange data among vehicles, pedestrians, and other road users to provide each vehicle a holistic environmental view.Source: §4 · p. 2
• Platooning uses low-latency, highly reliable communication for vehicles following at reduced safety distances, with the lead vehicle sending commands and receiving feedback.Advanced driving involves sharing local sensing information to coordinate trajectories and maneuvers.Source: §4 · p. 2
• A use case may combine communication types including vehicle-to-vehicle and vehicle-to-infrastructure links.The passage also lists vehicle-to-pedestrian and vehicle-to-network communication.Source: §4 · p. 3
• V2X communication may take place in-coverage, out-of-coverage, or partial-coverage, depending on whether communicating UEs are within gNB coverage.Partial coverage means at least one communicating UE is in coverage and connected to a gNB.Source: §4 · p. 3

5.1.1 Protocol Stack

• The NR sidelink control and user planes comprise PHY, MAC, RLC, PDCP, SDAP, and RRC functions for transmitting data and signaling.The paper briefly explains services and functionality of these layers.Source: §5.1.1 · p. 3

5.1.2 Cast Types

• NR-V2X supports unicast, broadcast, and groupcast to meet requirements across a wider range of vehicular use cases.HARQ is supported for unicast and groupcast messages.Source: §5.1.2 · p. 3

5.1.3 Frequency ranges

• Rel-16 defines V2X operating bands at 5.9 GHz (n47) and 2.5 GHz (n38), with 10, 20, 30, and 40 MHz channel bandwidths in both.The initial Rel-16 design is based on FR1; FR2 is not the primary focus of the studies.Source: §5.1.3 · p. 3

5.2.1 Radio Resources

• NR sidelink uses NR radio-frame, subframe, and slot structures and supports numerologies that produce shorter slot times for low-latency use cases.The radio resources are defined in time and frequency domains.Source: §5.2.1 · pp. 3–4

5.2.2 Physical Structure

• NR sidelink physical channels carry higher-layer information; PSCCH carries first-stage SCI and is paired with PSSCH, which carries transport blocks and second-stage SCI.PSFCH carries HARQ feedback, while PSBCH carries periodically sent sidelink system information in the synchronization signal block.Source: §5.2.2 · p. 4

5.2.3 Modulation Scheme and Numerology in NR Sidelink

• NR sidelink supports 15, 30, and 60 kHz subcarrier spacing in FR1 and 60 and 120 kHz in FR2, using CP-OFDM and QPSK through 256-QAM.Modulation choice supports reliability or throughput according to application requirements and channel state.Source: §5.2.3 · pp. 4–5, Table 2 · p. 5

5.2.4 Slot Format

• A slot can contain PSSCH, PSCCH, PSFCH, AGC, and guard symbols; guard symbols support switching between sidelink reception and transmission.AGC symbols provide receiver level control, and guards follow PSSCH, PSFCH, or S-SSB.Source: §5.2.4 · p. 5, Figure 3 · p. 5

5.2.5 Radio Resource Allocation

• Mode 2 lets a UE autonomously select resources through sensing, while Mode 1 assigns resources through the gNB for in-coverage UEs.Mode 2 uses recent sensing results, priority-aware exclusion and selection, and can reserve up to three resources; selection is bounded by the transport block's remaining delay budget.Source: §5.2.5 · pp. 5–6

5.2.6 Congestion control

• NR-V2X congestion control uses CBR and CR measurements, and adapts CR and transmission parameters based on traffic priority and CBR.NR-V2X uses 1 or 2 ms calculation intervals for CBR and CR, compared with 4 ms in LTE-V2X.Source: §5.2.6 · p. 6

5.2.7 QoS in Physical Layer

• A 3-bit priority field in first-stage SCI indicates eight physical-layer QoS levels mapped to higher-layer QoS classes.Preemption can be requested when received reservations leave insufficient suitable resources, subject to priority and configuration conditions.Source: §5.2.7 · pp. 6–7

5.2.8 Synchronization

• UE synchronization may come from GNSS, a base station, another synchronized UE, or the UE's internal clock.The first two sources are considered highest quality; a UE uses its internal clock if it cannot synchronize through GNSS, a base station, or a synchronization-reference UE.Source: §5.2.8 · p. 7

5.3.1 Sidelink MAC

• SL MAC handles radio resource selection, uplink-versus-sidelink priority, channel-state reporting, and logical-channel data transfer.It supports ACK/NACK HARQ for unicast and groupcast; NACK-only feedback for groupcast reduces feedback resources when many receivers are present.Source: §5.3.1 · pp. 7–8

5.3.2 Sidelink RLC

• RLC offers Transparent, Unacknowledged, and Acknowledged modes; UM is the only mode for groupcast and broadcast, while AM is only for unicast.RLC services include sequence numbering, ARQ error correction, segmentation, and reassembly.Source: §5.3.2 · p. 8

5.3.3 Sidelink PDCP

• PDCP supports functions including sequencing, ordering, integrity, and duplicate handling, but sidelink does not support duplication.Out-of-order delivery is supported only for unicast.Source: §5.3.3 · p. 8

5.3.4 Sidelink SDAP

• SDAP maps QoS flows to sidelink data radio bearers, with one SDAP entity per destination for unicast, groupcast, and broadcast.Source: §5.3.4 · p. 8

5.3.5 Sidelink RRC

• Sidelink RRC transfers configuration and capability messages, maintains and releases PC5-RRC connections, and detects sidelink radio-link failure.Upon failure, the UE releases connection resources and indicates the event to upper layers; connected UEs also notify the network.Source: §5.3.5 · p. 8

6 COMPARISON OF LTE-V2X AND NR-V2X

• NR-V2X supports unicast and groupcast in addition to broadcast, unlike LTE-V2X, whose only supported cast type was broadcast.The paper relates the additional cast types to applications such as platooning and extended sensors.Source: §6 · p. 8
• NR-V2X adds sidelink HARQ for unicast and groupcast retransmissions based on receiver feedback, a feature LTE-V2X lacks.LTE-V2X supports only broadcast communication in sidelink.Source: §6 · p. 8
• NR-V2X supports 15, 30, 60, and 120 kHz sidelink subcarrier spacing, enabling shorter slots and lower transmission latency than LTE-V2X's 15 kHz support.Source: §6 · p. 8
• NR-V2X adds SDAP to the sidelink user-plane protocol stack to provide flow-based QoS.Source: §6 · p. 8

7 NR-V2X SIDELINK COMMUNICATION IN 3GPP RELEASE 17

• Rel-17 sidelink work addresses Rel-16’s inability to meet all V2X, public-safety, and other commercial requirements and scenarios.Agreed study and work items target wider coverage, increased reliability, reduced latency, and power saving for battery-based UEs.Source: §7 · p. 9

7.1 Sidelink Enhancements

• Rel-17 enhancements target battery-powered UEs and reliability and latency under high load or poor channel conditions.The study considers autonomous resource allocation to reduce power use, inter-UE coordination to improve reliability and latency, and sidelink DRX for extended sleep and aligned wake times.Source: §7.1 · p. 9

7.2 Sidelink Relaying

• Sidelink relaying studies aim to extend sidelink and cell coverage, with UE-to-network and UE-to-UE relay types.UE-to-network relaying supports cell-edge or out-of-coverage users reaching the PDN; UE-to-UE relaying addresses the limits of Rel-16 single-hop sidelink and includes study of relay selection, authorization, QoS, continuity, security, and discovery.Source: §7.2 · p. 9

7.3 Sidelink Positioning

• The positioning study identifies V2X and public-safety use cases, requirements, and deployment scenarios across in-coverage, partial-coverage, and out-of-coverage settings.One cited platooning requirement is relative longitudinal position accuracy below 0.5 m; study results are expected to inform new Rel-18 work.Source: §7.3 · p. 9

• Further sidelink use cases are anticipated in industrial communication, including communication between machines and robots.Source: LEASE 17 · p. 9
• Rel-17 sidelink positioning is a starting point; relative vehicle positioning is expected to become a major Rel-18 work item.The paper describes retrieving distance between vehicles as an important functionality sought by companies.Source: LEASE 17 · p. 9
• QoS-aware resource allocation and power efficiency may be relevant in highly loaded networks to support communication with high QoS requirements.The paper also mentions a study on QoS-aware power-efficient sidelink communication for pedestrian UEs.Source: LEASE 17 · pp. 9–10
• AI/ML-based resource allocation using environmental, road-specific, and radio conditions as inputs is expected to improve sidelink reliability, latency, and throughput.Source: LEASE 17 · p. 10

• Rel-16 NR-V2X adds different subcarrier spacing, sidelink HARQ, unicast and groupcast, and faster reaction to V2X sidelink congestion.Source: §9 · p. 10
• The paper covers general, physical-layer, and higher-layer aspects of Rel-16 NR sidelink communication.Source: §9 · p. 10
• Rel-17 sidelink work aims to provide lower latency, higher reliability, extended coverage, and reduced power consumption for battery-based V2X UEs.Source: §9 · p. 10