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A Tutorial on 5G NR V2X Communications

Mario H. Castañeda Garcia, Alejandro Molina-Galan, Mate Boban, Javier Gozalvez, Baldomero Coll-Perales, Taylan Şahin, Apostolos Kousaridas

arXiv:2102.04538v1eess.SPcs.ITcs.NI

TL;DR

The paper addresses the need to understand the first 5G NR-based V2X standard and its support for advanced connected and automated driving use cases. It presents an in-depth tutorial centered on sidelink, covering the physical layer, resource allocation, QoS, V2N interfaces, coexistence, use cases, architecture, and evaluation. The resulting tutorial organizes Release 16 capabilities and identifies possible enhancements, including Release 17 work.

  • Problem

    Release 16 introduced the first 5G NR-based V2X standard, whose sidelink capabilities support enhanced V2X use cases related to connected and automated driving.

  • Method

    The paper provides an in-depth tutorial of Release 16 NR V2X, focusing on sidelink and covering its physical layer, resource allocation, QoS, V2N support, coexistence, architecture, use cases, and evaluation methodology.

  • Results

    The tutorial presents the major Release 16 NR V2X standard developments and their advanced functionalities, with sidelink as the main focus.

  • Takeaways & Limitations

    The paper provides a reference basis for understanding NR V2X and for future evaluation and enhancement of its performance and capabilities.

Abstract

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The Third Generation Partnership Project (3GPP) has recently published its Release 16 that includes the first Vehicle to-Everything (V2X) standard based on the 5G New Radio (NR) air interface. 5G NR V2X introduces advanced functionalities on top of the 5G NR air interface to support connected and automated driving use cases with stringent requirements. This paper presents an in-depth tutorial of the 3GPP Release 16 5G NR V2X standard for V2X communications, with a particular focus on the sidelink, since it is the most significant part of 5G NR V2X. The main part of the paper is an in-depth treatment of the key aspects of 5G NR V2X: the physical layer, the resource allocation, the quality of service management, the enhancements introduced to the Uu interface and the mobility management for V2N (Vehicle to Network) communications, as well as the co-existence mechanisms between 5G NR V2X and LTE V2X. We also review the use cases, the system architecture, and describe the evaluation methodology and simulation assumptions for 5G NR V2X. Finally, we provide an outlook on possible 5G NR V2X enhancements, including those identified within Release 17.

I. INTRODUCTION

Release 16 introduced the first 5G NR-based V2X standard, complementing LTE V2X with sidelink capabilities aimed at enhanced connected and automated driving use cases. The paper provides a tutorial covering the standard’s architecture, procedures, resource allocation, QoS, coexistence, evaluation, and future enhancements.

  • Motivation and standard evolution: Release 16 introduced 5G NR V2X, including sidelink communications, as the first V2X standard based on the 5G NR air interface.NR V2X sidelink was designed to complement rather than replace LTE V2X sidelink.
  • Scope and contribution: The paper gives a comprehensive tutorial of Release 16 NR V2X, focusing primarily on sidelink communications and the major developments needed to understand its operation.It also reviews LTE V2X to clarify the differences and novelties introduced by NR V2X.
  • Technical coverage: The tutorial covers NR V2X use cases and requirements, system architecture, sidelink physical-layer channels and procedures, resource allocation, QoS management, and V2N support through Uu and mobility enhancements.The physical-layer treatment includes HARQ feedback, synchronization, and power control, while resource allocation covers modes 1 and 2 and congestion control.
  • Coexistence: The paper explains coexistence mechanisms that allow 5G NR V2X and LTE V2X to operate together, including time-division solutions that prevent in-device interference.The coexistence discussion is part of the standard’s effort to extend LTE V2X capabilities with NR V2X functionality.
  • Evaluation and outlook: The paper concludes with 3GPP evaluation methodology and simulation assumptions, followed by possible NR V2X enhancements identified for Release 17.These future directions are presented alongside the tutorial as a basis for evaluating and extending NR V2X capabilities.
  • LTE V2X background: For LTE V2X mode 4, vehicles autonomously select sub-channels through sensing-based semi-persistent scheduling and announce reservations using the resource reservation interval.The paper also describes CBR and CR as congestion metrics because sensing-based scheduling becomes prone to packet collisions as load increases.

A. 3GPP use case groups

The paper organizes V2X use cases by automation and service domain, then relates them to 3GPP requirements, 5GAA service-level requirements, and the supporting 5G system architecture.

  • 3GPP use case groups: 3GPP distinguishes use cases by SAE automation levels from 0 to 5, with higher automation typically requiring stricter NR V2X QoS.The four 3GPP groups include vehicles platooning, advanced driving, remote driving, and extended sensors.
  • 5GAA use case groups: 5GAA groups use cases into safety, vehicle operations management, convenience, autonomous driving, platooning, traffic efficiency and environmental friendliness, and society and community.Examples range from emergency braking and software updates to platooning, GLOSA, and vulnerable road user protection.
  • 3GPP requirements: 3GPP requirements specify payload, transmission rate, maximum end-to-end latency, reliability, data rate, and required communication range for V2X services.Reliability is the probability of correct reception within the specified latency, while communication range is defined under the targeted payload, latency, reliability, and data-rate conditions.
  • 5GAA requirements: 5GAA complements network-centric requirements with service-level requirements covering automotive concerns such as service reliability, interoperability, positioning, and vehicle density.These scenarios differ by road configuration, actors, service flows, and other system-level conditions.
  • 5G system architecture: The 5G system supports V2X over PC5 for sidelink and over Uu for uplink and downlink, with Rel. 16 Uu V2X limited to unicast communication.Rel. 17 includes ongoing study of multicast and broadcast enhancements for 5G, while LTE-Uu can broadcast downlink V2X messages through MBMS.
  • 5G system architecture: In roaming with local breakout, the visited network controls the session-management and user-plane functions, a deployment option identified as critical for reducing latency.PC5 supports roaming and inter-PLMN operation, requiring consistent PC5 parameters among UEs in a geographical area.

A. Physical Layer Structures for NR V2X sidelink

NR V2X sidelink uses scalable OFDM numerology, configured bandwidth parts and resource pools to organize transmissions across time and frequency. Its physical channels support control, data, synchronization, feedback, and flexible unicast, groupcast, and broadcast operation.

  • Numerology: NR V2X supports 15, 30, 60, and 120 kHz subcarrier spacings, with FR1 using up to 60 kHz and FR2 supporting 60 and 120 kHz.The numerology combines subcarrier spacing with either normal or extended cyclic prefix; extended CP is supported only at 60 kHz.
  • Numerology: Larger subcarrier spacing shortens slot duration, while numerology selection depends on carrier frequency, channel conditions, latency requirements, and hardware features.NR V2X uses 2^μ slots per 1 ms subframe, giving a slot duration of 2^-μ ms.
  • Sidelink bandwidth parts: A sidelink bandwidth part is a contiguous carrier region containing all sidelink channels, reference signals, and synchronization signals under one common numerology.Only one SL BWP is configured for all UEs in a carrier, unlike the multiple BWP configurations possible for NR Uu.
  • Resource pools: A resource pool is a configured subset of sidelink slots and common resource blocks shared by UEs, divided into contiguous sub-channels that form the smallest data-transmission unit.The sub-channel size can be configured as 10, 12, 15, 20, 25, 50, 75, or 100 PRBs.
  • Physical channels and signals: NR V2X sidelink uses PSCCH for control, PSSCH for data and additional control, PSBCH for synchronization information, and PSFCH for reception feedback.The associated DMRS supports decoding, while S-PSS and S-SSS provide synchronization within an S-SSB.
  • Physical channels and signals: Two-stage SCI supports unicast, groupcast, and broadcast while allowing sensing UEs to decode only the fixed-size first-stage SCI for resource sensing.This design reduces SCI-decoding complexity because the second-stage SCI carries variable, transmission-specific information for intended receivers.
  • Physical channels and signals: NR V2X adds HARQ feedback for unicast and groupcast transmissions to increase sidelink reliability, with groupcast feedback configured through two options.Option 1 limits feedback to receivers within a specified distance, whereas option 2 requires all receivers to respond.

5) Sidelink Synchronization Signal Block (S-SSB): PSBCH, S-PSS and S-SSS

The S-SSB carries sidelink synchronization and broadcast information through PSBCH, S-PSS, and S-SSS. Its fixed periodicity, configurable repetition, sequence identifiers, and synchronization payload support timing, frequency, coverage, and sidelink configuration acquisition.

  • S-SSB structure: An S-SSB consists of PSBCH, S-PSS, and S-SSS, occupies one slot, and carries sidelink synchronization information from a SyncRef UE.The S-PSS and S-SSS form the SLSS used for time and frequency synchronization, while PSBCH carries synchronization-related system information.
  • Sidelink synchronization signals: The SLSS identifies the SyncRef UE through an SLSS ID formed from 2 S-PSS and 336 S-SSS candidate sequences, yielding 672 unique identifiers.A receiving UE determines the SyncRef UE’s SLSS ID after detecting the transmitted S-PSS and S-SSS.
  • Sidelink synchronization signals: S-PSS and S-SSS use BPSK, occupy 127 subcarriers, and repeat across two consecutive symbols to improve S-SSB coverage.They occupy part of the 132-subcarrier S-SSB bandwidth rather than the full bandwidth.
  • PSBCH: PSBCH provides system-wide and synchronization information, including coverage status, TDD configuration, DFN, and slot index.Its payload is 56 bits plus a 24-bit CRC and uses Polar coding based on the Rel. 15 NR PBCH design.
  • S-SSB periodicity: S-SSBs recur every 160 ms, with the number and placement of transmissions within each period pre-configured according to subcarrier spacing and frequency range.Larger subcarrier spacings support more S-SSBs per period to provide sufficient sidelink coverage.
  • S-SSB transmission: S-SSBs can use different beams, enabling beam sweeping, but sidelink S-SSB transmission is not part of an initial-access procedure as in NR Uu.The S-SSB structure therefore supports synchronization coverage without the NR Uu beam-selection signaling procedure.

C. Physical Layer Procedures for NR V2X sidelink

NR V2X sidelink adds feedback, retransmission, synchronization, and power-control procedures to support unicast and groupcast communications. HARQ feedback is carried on PSFCH resources selected by configured timing, frequency, and code rules.

  • Overview: Rel. 16 NR V2X introduces unicast and groupcast sidelink communications, together with sidelink HARQ feedback, power control, and CSI acquisition for unicast.These mechanisms extend the sidelink functionality beyond earlier NR and LTE V2X procedures.
  • HARQ feedback: HARQ improves transport-block reliability by combining forward error correction and error detection with retransmissions, while feedback can avoid unnecessary blind retransmissions.Blind retransmissions may otherwise waste sidelink resources and increase channel load.
  • HARQ feedback: NR V2X supports ACK/NACK feedback for unicast and two groupcast options: shared NACK-only feedback or separate ACK/NACK feedback for each receiver.Option 2 provides higher reliability because the transmitter can identify missing feedback from specific receivers, but it requires more resources.
  • PSFCH resources: Each PSFCH transmission maps HARQ feedback to one PSFCH symbol, one PRB, and one cyclic shift, with code-division multiplexing supporting multiple receivers and ACK/NACK distinction.The PSFCH symbol is the first PSFCH slot after a configured minimum of K slots following the PSSCH transmission.
  • PSFCH resources: The PSFCH frequency resource is assigned from PRB sets associated with sub-channels and PSSCH slots, allowing feedback resources to be shared systematically across transmissions.For a transmission spanning multiple sub-channels, multiple corresponding PRB sets may be available.

VI. RESOURCE ALLOCATION FOR 5G NR V2X SIDELINK

Release 16 defines two NR V2X sidelink resource-selection modes that parallel LTE V2X modes 3 and 4 while extending sidelink beyond broadcast to broadcast, groupcast, and unicast.

  • NR V2X Rel. 16 defines modes 1 and 2 for selecting sidelink sub-channels over the NR V2X PC5 interface.
  • Modes 1 and 2 correspond to LTE V2X modes 3 and 4, respectively.
  • Unlike LTE V2X, which supports only broadcast sidelink, NR V2X supports broadcast, groupcast, and unicast sidelink communications.

A. Mode 1

Mode 1 uses network-managed sidelink resources through the Uu interface, with dynamic grants or configured grants, while supporting configurable MCS and HARQ feedback mechanisms.

  • Under mode 1, the gNB or eNB assigns and manages sidelink resources through the NR or LTE Uu interface, so UEs require network coverage.
  • Mode 1 can use separate resource pools for modes 1 and 2 or a shared pool, with sharing improving efficiency but potentially causing collisions.
  • Grant scheduling: Dynamic grants require a UE to request resources for each transport block, whereas configured grants pre-allocate resources for transmitting several transport blocks.
  • Grant scheduling: Configured grant type 1 is immediately usable until released, while type 2 requires gNB activation and remains usable until deactivation.
  • Transmission control: Mode 1 supports three MCS tables and allows the gNB to configure one or more MCSs or tables from which the UE selects.
  • Transmission control: For groupcast and unicast, HARQ feedback can reach the gNB through a UE feedback report on the PUCCH, assisting sidelink resource allocation.

B. Mode 2

Mode 2 enables out-of-coverage UEs to select sidelink resources autonomously through dynamic or semi-persistent scheduling, using sensing, candidate filtering, re-evaluation, and pre-emption.

  • Mode 2 lets UEs autonomously select one or more sidelink sub-channels from a resource pool and operate without network coverage.
  • Scheduling schemes: Dynamic scheduling selects resources for one transport block, whereas semi-persistent scheduling selects resources for consecutive transport blocks separated by an RRI.
  • Candidate selection: The selection window spans [n+T1, n+T2], where n is the resource-selection trigger and T1 is the UE processing time.
  • Candidate selection: A two-step algorithm first excludes unavailable candidates using sensing information and then selects N candidate resources for the initial transmission and possible retransmissions.
  • Enhancements: Re-evaluation allows a UE to repeat candidate exclusion after resource selection, improving flexibility under variable traffic and previously undetected interference.
  • Enhancements: Pre-emption makes a low-priority UE release a reserved resource when it estimates that higher-priority traffic will use it, subject to any configured priority threshold.

C. Congestion Control

Release 16 defines sidelink congestion metrics and configurable countermeasures for mode 2, while leaving the specific congestion-control algorithm unspecified.

  • Rel. 16 supports congestion control for NR V2X sidelink mode 2 but does not specify a particular algorithm.
  • The defined congestion metrics are sidelink Channel Busy Ratio and sidelink Channel occupancy Ratio.
  • Metrics: NR V2X estimates sidelink CBR over 100 slots or 100·2^µ slots according to resource-pool configuration and SCS factor µ.
  • Countermeasures: A transmitting UE uses measured sidelink CBR and CR with a configured lookup table containing up to 16 CBR ranges and corresponding CR limits.
  • Countermeasures: The CR limit should depend on the transport-block priority and the absolute speed of the transmitting UE, and it increases as the CBR range decreases.

VII. QOS FRAMEWORK FOR 5G NR V2X

NR V2X adopts a network-configured 5G QoS framework because per-packet PPPP and PPPR management cannot capture all stringent eV2X requirements. Unlike LTE V2X sidelink, QoS is organized around QoS Flows and QoS Profiles rather than UE-specific implementation choices.

  • LTE V2X sidelink leaves mapping PPPP and PPPR values to Sidelink Radio Bearers and logical channels to UE implementation, so prioritization is not unified.
  • NR V2X supports eV2X requirements involving priority, transmission rate, latency, reliability, data rate, and communication range.
  • PPPP- and PPPR-based per-packet QoS management alone is insufficient for all advanced eV2X service requirements.
  • NR V2X sidelink uses a network-configured 5G QoS model based on QoS Flows and QoS Profiles, independent of UE implementation.

A. QoS management for NR V2X sidelink

NR V2X sidelink applies the 5G QoS model through QoS Flows, radio-bearer configuration, and signaling between the network and peer UEs. Configuration procedures differ for unicast versus groupcast and broadcast communication.

  • A 5G QoS Flow is the finest forwarding-treatment granularity, with traffic in one flow receiving the same scheduling, queueing, shaping, and RLC treatment.
  • SL unicast: For unicast, the network provisions PC5 QoS rules and profiles, then UEs configure the SLRB and map PC5 QoS Flows through PC5-RRC signaling.
  • SLRB configuration specifies Layer 2 and PHY parameters needed to meet QoS requirements.
  • SL groupcast and broadcast: Groupcast and broadcast SLRB configuration uses three Rel. 16 procedures, while transmissions remain managed per packet using advanced QoS profiles.

B. PC5 QoS Profiles for V2X sidelink

PC5 QoS Profiles define the parameters and characteristics used to classify and control NR V2X sidelink traffic. These profiles support different cast types and encode identifiers, bitrate limits, range, and reliability-related behavior.

  • PC5 QoS Rules classify packets into PC5 QoS Flows using a PFI, precedence value, and packet filters.
  • PC5 QoS Profiles contain QoS Parameters and QoS Characteristics, with mappings maintained for unicast, groupcast, and broadcast traffic.
  • PQI identifies standardized PC5 QoS characteristics associated with V2X services.
  • GFBR and MFBR control guaranteed and maximum flow bitrates, while PC5 LINK-AMBR limits aggregate non-GBR bitrate on a PC5 unicast link.
  • For groupcast, the Range parameter specifies the minimum transmitter-receiver distance over which the remaining PC5 QoS parameters must be guaranteed.

C. PC5-RRC

Rel. 16 extends PC5-RRC beyond synchronization support to configure unicast sidelink capabilities, radio bearers, measurements, and link-failure handling. Over Uu, it also supports adaptable QoS profiles and analytics-based notifications for service continuity.

  • C. PC5-RRC: NR V2X PC5-RRC supports capability exchange, SLRB configuration, and SL measurement configuration for unicast communication.
  • C. PC5-RRC: Capability Enquiry and SL RRC Reconfiguration procedures exchange peer capabilities, bearer settings, and measurement configurations over the sidelink signaling radio bearer.
  • C. PC5-RRC: A PC5-RRC connection is a logical connection between a source-destination L2-ID pair and is released with the corresponding unicast link.
  • C. PC5-RRC: PC5-RRC detects sidelink radio link failure when RLC reaches the maximum retransmission count, causing immediate connection release.
  • D. Alternative QoS Profiles and Service Requirements for V2X communication over Uu: Alternative QoS profiles let V2X applications continue operating with lower QoS when their initial profile is unavailable.
  • E. QoS Sustainability Analytics for V2X communication over Uu: QoS Sustainability Analytics predicts expected QoS changes so V2X applications can proactively adapt or safely stop services when QoS degrades.

A. Uu enhancements

Release 16 enhances the Uu interface and mobility support for demanding V2N services, while adding mechanisms to reduce handover interruption and coordinate NR V2X with LTE V2X.

  • Uu interface enhancements: Multiple active UL configured grants match semipersistent uplink resources to varying V2N traffic patterns and QoS requirements.UE assistance information reports traffic characteristics such as periodicity, latency requirements, and maximum transport-block size to help the gNB select suitable grants.
  • Mobility enhancements: Rel. 16 introduces DAPS and CHO to address legacy handover interruption and command-reception limitations for stringent eV2X applications.DAPS maintains source and target links during handover, while CHO preconfigures execution conditions based on measurements such as RSRP, RSRQ, and RS-SINR.
  • Mobility enhancements: DAPS allows simultaneous source- and target-cell connectivity, but overlapping uplink transmissions prioritize the target cell and may discard the source-cell transmission.The UE can continue source-cell HARQ retransmissions until the target cell explicitly indicates that source-cell exchange should stop.
  • NR-LTE coexistence: Rel. 16 proposes TDM and FDM solutions for NR V2X and LTE V2X coexistence, applicable across network-controlled and autonomous resource-allocation modes.TDM permits only one RAT to transmit at a time, whereas FDM permits simultaneous transmission with shared power and sufficient frequency separation.
  • NR-LTE coexistence: Short-term TDM leaves conflict-resolution decisions to UE implementation, and frequent conflicts under high load can degrade QoS.Coordination information must be exchanged before a UE-specific deadline bounded by 4 ms; otherwise, the UE determines how to resolve the conflict.

B. Support for Cross-RAT control

Release 16 standardizes cross-RAT resource control and defines detailed V2V sidelink channel models for evaluating NR V2X across urban and highway conditions.

  • Cross-RAT control: Cross-RAT control lets eNBs and gNBs cooperate to manage NR V2X and LTE V2X sidelink resources under cellular coverage.An eNB can manage LTE and NR PC5 resources through LTE Uu, while a gNB can manage both through NR Uu.
  • Cross-RAT control: An eNB can allocate NR V2X mode 1 resources only with configured-grant type 1 scheduling, not dynamic scheduling or configured-grant type 2.This restriction limits one form of cross-RAT resource allocation.
  • V2V sidelink channel models: Rel. 16 defines LOS, NLOS, and NLOSv V2V states according to street geometry and whether buildings or vehicles block the line-of-sight path.NLOSv occurs when vehicles block the LOS path between vehicles traveling on the same street.
  • V2V sidelink channel models: The channel models introduce pathloss and shadow-fading treatments for urban-grid and highway V2V sidelink links, including additional vehicle-blockage loss for NLOSv.LOS and NLOSv use the same pathloss equation, with extra blockage loss added for NLOSv; fast-fading parameters are state-specific.
  • V2V sidelink channel models: Rel. 16 adds dual mobility modeling that accounts for relative transmitter-receiver motion and speed variation among delayed scattered paths.The model uses Doppler components for the LOS path and scattered paths, with scatterer speeds sampled between negative and positive maximum vehicle speed.

B. System level simulations

The Rel. 16 evaluation methodology specifies urban-grid and highway environments, vehicle and traffic configurations, antenna assumptions, bandwidths, and performance metrics for NR V2X system-level studies.

  • Evaluation scenarios: NR V2X evaluation considers urban-grid and highway environments, with road layouts represented by dedicated urban and highway configurations.The urban-grid requirement uses at least a 3x3 road grid, while the highway scenario uses three lanes per direction with wrap-around.
  • System parameters: The evaluation parameters include aggregated system bandwidth, receiver noise figures, carrier-frequency settings, and distinct vehicle types.Assumed bandwidth reaches 100 MHz for sidelink below 6 GHz and 1 GHz above 6 GHz; vehicle types include passenger vehicles and trucks or buses.
  • Vehicle dropping: Vehicles are independently dropped in lanes using an exponential distribution while maintaining a minimum 2-meter bumper distance and fixed lane-assigned speeds.The methodology also supports clustered vehicle dropping for platooning use cases.
  • Traffic models: The methodology defines three periodic and two aperiodic traffic models to represent diverse use-case requirements while limiting simulation complexity.The specified vehicle-dropping options include all vehicles at 60 km/h and intersection movement probabilities of 0.5 straight, 0.25 left, and 0.25 right.
  • Performance metrics: 3GPP system-level evaluations use three defined performance metrics for NR V2X studies.These metrics are summarized in the paper’s evaluation methodology and associated performance-metric table.

C. Link level simulations

The paper specifies link-level simulation parameters and discusses prospective NR V2X enhancements involving FR2, beamforming, synchronization coverage, positioning, and beam-based power control.

  • Simulation assumptions: Link-level simulations use carrier frequency, channel model, packet size, coding, modulation, waveform, SCS, CP length, synchronization error, diversity, receiver algorithm, and timing parameters.Suggested values or ranges are provided in the referenced 3GPP methodology, alongside CDL channel models for computationally efficient wireless-channel simulation.
  • Beamforming in sidelink: Rel. 16 NR V2X primarily targets FR1 and provides no specific FR2 optimization or beam management beyond SL PT-RS.FR2 support can reuse the FR1 design, but the Rel. 17 NR sidelink enhancement work item does not include FR2 optimizations.
  • Beamforming in sidelink: Beamforming could support high-rate, long-distance use cases by compensating higher-frequency pathloss and enabling spatial reuse through reduced interference.The cited use cases require 50-700 Mbps over 200 meters or beyond.
  • Synchronization: SyncRef UEs can transmit synchronization information in multiple directions using repeated or different beams, potentially expanding synchronization coverage outside network coverage.The S-SSB structure supports beam sweeping across configured S-SSBs.
  • Power control: Future beam-based sidelink power control would select transmit power using sidelink and/or downlink pathloss measured for the transmission beam.This would extend beam-aware power-control principles used for NR PUSCH and PUCCH to PSCCH and PSSCH.
  • Positioning: Sidelink positioning can derive relative vehicle positions from time and angular measurements, complementing network-based positioning, sensors, or GNSS when those methods are limited.Network-based positioning requires coverage and can incur additional latency, signaling, and accumulated absolute-position errors.

C. Enhancements to resource allocation

The paper reviews Rel. 17 and future resource-allocation enhancements for NR V2X, including power saving, inter-UE coordination, relaying, and multicast/broadcast support, while emphasizing coexistence with Rel. 16.

  • Mode 2 enhancements: Rel. 17 NR sidelink enhancement targets Rel. 16 mode 2 with power saving and improved reliability and latency, while requiring co-channel coexistence in the same resource pool.These enhancements are specified as additions that must coexist with Rel. 16 NR V2X.
  • Power saving: A studied power-saving mode 2 variant addresses smartphones and other power-limited pedestrian UEs, whose long sensing intervals can severely affect battery consumption.Rel. 16 mode 1 and mode 2 were designed primarily for vehicles or RSUs without strong power limitations.
  • Inter-UE coordination: Rel. 17 adopts inter-UE coordination type B, allowing an assisting UE to provide resource information that helps reduce hidden-terminal collisions.UE B can detect reservations from UEs A and C and provide that status for UE A’s resource selection.
  • Future resource allocation: Mode 2(d) would let a scheduling UE manage sidelink resources for group members through an intermediary role between the gNB and member UEs.The analyzed enhancement was not adopted in Rel. 16 or Rel. 17 and requires at least the scheduling UE to operate in network coverage.
  • Studied alternatives: Other analyzed options include standalone reservation, TFRPs tailored to QoS requirements, and hybrid mode 2 first transmissions with mode 1 retransmissions for reliability.These mechanisms were studied as alternatives or proposals during standardization, with TFRPs ultimately not adopted in Rel. 16.
  • UE relaying: Rel. 17 UE relaying focuses on single-hop NR sidelink-based relays while retaining forward compatibility for future multi-hop support and addressing discovery, selection, and service continuity.The scenarios can include vehicles, smartphones, and wearables, with both in-coverage and partial or out-of-coverage operation considered.
  • Multicast and broadcast: Uu multicast and broadcast services could deliver common information such as sensor data, software updates, or HD maps to UE groups, with multicast feedback supporting reliability.The related work includes group scheduling, simultaneous unicast operation, mobility, and dynamic multicast-unicast changes with service continuity.
  • Conclusion: The tutorial presents Rel. 16 NR V2X as the first 5G NR-based V2X and NR sidelink standard, while identifying further improvements for future evaluation and enhancement.Its evaluation methodology and system- and link-level simulation assumptions are offered as a basis for assessing later capabilities.
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