16.9 Sidelink

38.3003GPPNRNR and NG-RAN Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

16.9.1 General

In this clause, an overview of NR sidelink communication and how NG-RAN supports NR sidelink communication and V2X sidelink communication is given. V2X sidelink communication is specified in TS 36.300 [2].

The NG-RAN architecture supports the PC5 interface as illustrated in Figure 16.9.1-1. Sidelink transmission and reception over the PC5 interface are supported when the UE is inside NG-RAN coverage, irrespective of which RRC state the UE is in, and when the UE is outside NG-RAN coverage.

Figure 16.9.1-1: NG-RAN Architecture supporting the PC5 interface

Support of V2X services via the PC5 interface can be provided by NR sidelink communication and/or V2X sidelink communication. NR sidelink communication may be used to support other services than V2X services.

NR sidelink communication can support one of three types of transmission modes for a pair of a Source Layer-2 ID and a Destination Layer-2 ID in the AS:

– Unicast transmission, characterized by:

– Support of one PC5-RRC connection between peer UEs for the pair;

– Transmission and reception of control information and user traffic between peer UEs in sidelink;

– Support of sidelink HARQ feedback;

– Support of sidelink transmit power control;

– Support of RLC AM;

– Detection of radio link failure for the PC5-RRC connection.

– Groupcast transmission, characterized by:

– Transmission and reception of user traffic among UEs belonging to a group in sidelink;

– Support of sidelink HARQ feedback.

– Broadcast transmission, characterized by:

– Transmission and reception of user traffic among UEs in sidelink.

16.9.2 Radio Protocol Architecture for NR sidelink communication

16.9.2.1 Overview

The AS protocol stack for the control plane for SCCH for RRC in the PC5 interface consists of RRC, PDCP, RLC and MAC sublayers, and the physical layer. The protocol stack of control plane for SCCH for RRC is shown in Figure 16.9.2.1-1.

Figure 16.9.2.1-1: Control plane protocol stack for SCCH for RRC.

For support of PC5-S protocol specified in TS 23.287 [40], PC5-S is located on top of PDCP, RLC and MAC sublayers, and the physical layer in the control plane protocol stack for SCCH for PC5-S, as shown in Figure 16.9.2.1-2.

Figure 16.9.2.1-2: Control plane protocol stack for SCCH for PC5-S.

The AS protocol stack for SBCCH in the PC5 interface consists of RRC, RLC, MAC sublayers, and the physical layer as shown below in Figure 16.9.2.1-3.

Figure 16.9.2.1-3: Control plane protocol stack for SBCCH.

The AS protocol stack for user plane in the PC5 interface consists of SDAP, PDCP, RLC and MAC sublayers, and the physical layer. The protocol stack of user plane is shown in Figure 16.9.2.1-4.

Figure 16.9.2.1-4: User plane protocol stack for STCH.

Sidelink Radio bearers (SLRB) are categorized into two groups: sidelink data radio bearers (SL DRB) for user plane data and sidelink signalling radio bearers (SL SRB) for control plane data. Separate SL SRBs using different SCCHs are configured for PC5-RRC and PC5-S signalling respectively.

16.9.2.2 MAC

The MAC sublayer provides the following services and functions over the PC5 interface in addition to the services and functions specified in clause 6.2.1:

– Radio resource selection;

– Packet filtering;

– Priority handling between uplink and sidelink transmissions for a given UE;

– Sidelink CSI reporting.

With LCP restrictions in MAC, only sidelink logical channels belonging to the same destination can be multiplexed into a MAC PDU for every unicast, groupcast and broadcast transmission which is associated to the destination. NG-RAN can also control whether a sidelink logical channel can utilise the resources allocated to a configured sidelink grant Type 1 (see clause 16.9.3.2). For transmissions to RX UE(s) using SL DRX operation, LCP ensures that a TX UE transmits data in the active time of the RX UE(s).

For packet filtering, a SL-SCH MAC header including portions of both Source Layer-2 ID and a Destination Layer-2 ID is added to each MAC PDU as specified in clause 8.4. LCID included within a MAC subheader uniquely identifies a logical channel within the scope of the Source Layer-2 ID and Destination Layer-2 ID combination.

The following logical channels are used in sidelink:

– Sidelink Control Channel (SCCH): a sidelink channel for transmitting control information (i.e. PC5-RRC and PC5-S messages) from one UE to other UE(s);

– Sidelink Traffic Channel (STCH): a sidelink channel for transmitting user information from one UE to other UE(s);

– Sidelink Broadcast Control Channel (SBCCH): a sidelink channel for broadcasting sidelink system information from one UE to other UE(s).

The following connections between logical channels and transport channels exist:

– SCCH can be mapped to SL-SCH;

– STCH can be mapped to SL-SCH;

– SBCCH can be mapped to SL-BCH.

16.9.2.3 RLC

The services and functions of the RLC sublayer as specified in clause 6.3.2 are supported for sidelink. TM is used for SBCCH. Both UM and AM are used in unicast transmission while only UM is used in groupcast or broadcast transmission. For UM, only unidirectional transmission is supported for groupcast and broadcast.

16.9.2.4 PDCP

The services and functions of the PDCP sublayer as specified in clause 6.4.1 are supported for sidelink with some restrictions:

– Out-of-order delivery is supported only for unicast transmission;

– Duplication is not supported over the PC5 interface.

16.9.2.5 SDAP

The SDAP sublayer provides the following service and function over the PC5 interface:

– Mapping between a QoS flow and a sidelink data radio bearer.

There is one SDAP entity per destination for one of unicast, groupcast and broadcast which is associated to the destination. Reflective QoS is not supported over the PC5 interface.

16.9.2.6 RRC

The RRC sublayer provides the following services and functions over the PC5 interface:

– Transfer of a PC5-RRC message between peer UEs;

– Maintenance and release of a PC5-RRC connection between two UEs;

– Detection of sidelink radio link failure for a PC5-RRC connection.

A PC5-RRC connection is a logical connection between two UEs for a pair of Source and Destination Layer-2 IDs which is considered to be established after a corresponding PC5 unicast link is established as specified in TS 23.287 [40]. There is one-to-one correspondence between the PC5-RRC connection and the PC5 unicast link. A UE may have multiple PC5-RRC connections with one or more UEs for different pairs of Source and Destination Layer-2 IDs.

Separate PC5-RRC procedures and messages are used for a UE to transfer UE capability and sidelink configuration to the peer UE, as specified in TS 38.331 [3]. Both peer UEs can exchange their own UE capability and sidelink configuration using separate bi-directional procedures in both sidelink directions.

If it is not interested in sidelink transmission, if sidelink RLF on the PC5-RRC connection is declared, or if the Layer-2 link release procedure is completed as specified in TS 23.287 [40], UE releases the PC5-RRC connection.

16.9.3 Radio Resource Allocation

16.9.3.1 General

For NR sidelink communication, the UE can operate in two modes as specified in 5.7.2 for resource allocation in sidelink:

– Scheduled resource allocation, characterized by:

– The UE needs to be RRC_CONNECTED in order to transmit data;

– NG-RAN schedules transmission resources.

– UE autonomous resource selection, characterized by:

– The UE can transmit data when inside NG-RAN coverage, irrespective of which RRC state the UE is in, and when outside NG-RAN coverage;

– The UE autonomously selects transmission resources from resource pool(s).

– For NR sidelink communication, the UE performs sidelink transmissions only on a single carrier.

16.9.3.2 Scheduled Resource Allocation

NG-RAN can dynamically allocate resources to the UE via the SL-RNTI on PDCCH(s) for NR sidelink communication.

In addition, NG-RAN can allocate sidelink resources to a UE with two types of configured sidelink grants:

– With type 1, RRC directly provides the configured sidelink grant only for NR sidelink communication;

– With type 2, RRC defines the periodicity of the configured sidelink grant while PDCCH can either signal and activate the configured sidelink grant, or deactivate it. The PDCCH is addressed to SL-CS-RNTI for NR sidelink communication.

Besides, NG-RAN can also semi-persistently allocate sidelink resources to the UE via the SL Semi-Persistent Scheduling V-RNTI on PDCCH(s) for V2X sidelink communication.

For the UE performing NR sidelink communication, there can be more than one configured sidelink grant activated at a time on the carrier configured for sidelink transmission.

When beam failure or physical layer problem occurs on MCG, the UE can continue using the configured sidelink grant Type 1 until initiation of the RRC connection re-establishment procedure as specified in TS 38.331 [12]. During handover, the UE can be provided with configured sidelink grants via handover command, regardless of the type. If provided, the UE activates the configured sidelink grant Type 1 upon reception of the handover command or execution of CHO.

The UE can send sidelink buffer status report to support scheduler operation in NG-RAN. For NR sidelink communication, the sidelink buffer status reports refer to the data that is buffered in for a group of logical channels (LCG) per destination in the UE. Eight LCGs are used for reporting of the sidelink buffer status reports. Two formats, which are SL BSR and truncated SL BSR, are used.

16.9.3.3 UE Autonomous Resource Selection

The UE autonomously selects sidelink resource(s) from resource pool(s) provided by broadcast system information or dedicated signalling while inside NG-RAN coverage or by pre-configuration while outside NG-RAN coverage.

For NR sidelink communication, the resource pool(s) can be provided for a given validity area where the UE does not need to acquire a new pool of resources while moving within the validity area, at least when this pool is provided by SIB. The NR SIB area scope mechanism as specified in TS 38.331 [12] is reused to enable validity area for SL resource pool configured via broadcasted system information.

The UE is allowed to temporarily use UE autonomous resource selection with random selection for sidelink transmission based on configuration of the exceptional transmission resource pool as specified in TS 38.331 [12].

16.9.4 Uu Control

16.9.4.1 General

When a UE is inside NG-RAN coverage, NR sidelink communication and/or V2X sidelink communication can be configured and controlled by NG-RAN via dedicated signalling or system information:

– The UE should support and be authorized to perform NR sidelink communication and/or V2X sidelink communication in NG-RAN;

– If configured, the UE performs V2X sidelink communication as specified in TS 36.300 [2] unless otherwise specified, with the restriction that the dynamic scheduling for V2X sidelink communication (i.e. based on SL-V-RNTI) is not supported;

– NG-RAN can provide the UE with intra-carrier sidelink configuration, inter-carrier sidelink configuration and anchor carrier(s) which provide sidelink configuration via a Uu carrier for NR sidelink communication and/or V2X sidelink communication;

– When the UE cannot simultaneously perform both NR sidelink transmission and NR uplink transmission in time domain, prioritization between both transmissions is done based on their priorities and thresholds configured by the NG-RAN or pre-configured. When the UE cannot simultaneously perform both V2X sidelink transmission and NR uplink transmission in time domain, prioritization between both transmissions is done based on the priorities (i.e. PPPP) of V2X sidelink communication and a threshold configured by the NG-RAN or pre-configured.

When a UE is outside NG-RAN coverage, SL DRB configuration(s) are preconfigured to the UE for NR sidelink communication. If UE changes the RRC state but has not received the SL DRB configuration(s) for the new RRC state, UE continues using the configuration obtained in the previous RRC state to perform sidelink data transmissions and receptions until the configuration for the new RRC state is received.

16.9.4.2 Control of connected UEs

The UE in RRC_CONNECTED performs NR sidelink communication and/or V2X sidelink communication, as configured by the upper layers. The UE sends Sidelink UE Information to NG-RAN in order to request or release sidelink resources and report QoS information for each destination.

NG-RAN provides RRCReconfiguration to the UE in order to provide the UE with dedicated sidelink configuration. The RRCReconfiguration may include SL DRB configuration(s) for NR sidelink communication as well as mode 1 resource configuration and/or mode 2 resource configuration. If UE has received SL DRB configuration via system information, UE should continue using the configuration to perform sidelink data transmissions and receptions until a new configuration is received via the RRCReconfiguration.

NG-RAN may also configure measurement and reporting of CBR for NR sidelink communication and V2X sidelink communication, and reporting of location information for V2X sidelink communication to the UE via RRCReconfiguration.

During handover, the UE performs sidelink transmission and reception based on configuration of the exceptional transmission resource pool or configured sidelink grant Type 1 (for NR sidelink communication only) and reception resource pool of the target cell as provided in the handover command.

16.9.4.3 Control of idle/inactive UEs

The UE in RRC_IDLE or RRC_INACTIVE performs NR sidelink communication and/or V2X sidelink communication, as configured by the upper layers. NG-RAN may provide common sidelink configuration to the UE in RRC_IDLE or RRC_INACTIVE via system information for NR sidelink communication and/or V2X sidelink communication. UE receives resource pool configuration and SL DRB configuration via SIB12 for NR sidelink communication as specified in TS 38.331 [12], and/or resource pool configuration via SIB13 and SIB14 for V2X sidelink communication as specified in TS 38.331 [12].

When the UE performs cell reselection, the UE interested in V2X service(s) considers at least whether NR sidelink communication and/or V2X sidelink communication are supported by the cell. The UE may consider the following carrier frequency as the highest priority frequency, except for the carrier only providing the anchor carrier:

– the frequency providing both NR sidelink communication configuration and V2X sidelink communication configuration, if configured to perform both NR sidelink communication and V2X sidelink communication;

– the frequency providing NR sidelink communication configuration, if configured to perform only NR sidelink communication.

– the frequency providing V2X sidelink communication configuration, if configured to perform only V2X sidelink communication.

16.9.5 Sidelink Discovery

The UE may perform NR sidelink discovery while in-coverage or out-of-coverage for non-relay operation.

The Relay discovery mechanism described in clause 16.12.3 (except the U2N Relay specific threshold based discovery message transmission) is also applied to sidelink discovery.

16.9.6 SL DRX

16.9.6.1 General

Sidelink supports SL DRX for unicast, groupcast, and broadcast. Similar parameters as defined in clause 11 for Uu (on-duration, inactivity-timer, retransmission-timer, cycle) are defined for SL to determine the SL active time for SL DRX. During the SL active time, the UE performs SCI monitoring for data reception (i.e., PSCCH and 2^nd stage SCI on PSSCH). The UE may skip monitoring of SCI for data reception during SL DRX inactive time.

The actual parameters supported for each cast type (unicast, groupcast, broadcast) are specified in the following clauses.

The SL active time of the RX UE includes the time in which any of its applicable SL on-duration timer(s), SL inactivity-timer(s) or SL retransmission timer(s) (for any of unicast, groupcast, or broadcast) are running. In addition, the slots associated with announced periodic transmissions by the TX UE and the time in which a UE is expecting CSI report following a CSI request (for unicast) are considered as SL active time of the RX UE.

The TX UE maintains a set of timers corresponding to the SL DRX timers in the RX UE(s) for each pair of source/destination L2 ID for unicast or destination L2 ID for groupcast/broadcast. When data is available for transmission to one or more RX UE(s) configured with SL DRX, the TX UE selects resources taking into account the active time of the RX UE(s) determined by the timers maintained at the TX UE.

The UE can determine from SIB12 whether the gNB supports SL DRX or not.

A default SL DRX configuration for groupcast/broadcast can be used for discovery message in sidelink discovery in clause 16.9.5 and for relay discovery messages in clause 16.12.3.

16.9.6.2 Unicast

For unicast, SL DRX is configured per pair of source L2 ID and destination L2 ID.

The UE maintains a set of SL DRX timers for each direction per pair of source L2 ID and destination L2 ID. The SL DRX configuration for a pair of source/destination L2 IDs for a direction may be negotiated between the UEs in the AS layer. For SL DRX configuration of each direction, where one UE is the TX UE and the other is the RX UE:

– RX UE may send assistance information, which includes its desired SL on-duration timer, SL DRX start offset, and SL DRX cycle, to the TX UE and the mode 2 TX UE may use it to determine the SL DRX configuration for the RX UE;

– Regardless of whether assistance information is provided or not, the TX UE in RRC_IDLE/RRC_INACTIVE/OOC, or in RRC_CONNECTED and using mode 2 resource allocation, determines the SL DRX Configuration for the RX UE. For a TX UE in RRC_CONNECTED and using mode 1 resource allocation, the SL DRX configuration for the RX UE is determined by the serving gNB of the TX UE;

– TX UE sends the SL DRX configuration to be used by the RX UE to the RX UE;

– The RX UE may accept or reject the SL DRX configuration.

A default SL DRX configuration for groupcast/broadcast can be used for DCR messages.

When the TX UE is in RRC_CONNECTED, the TX UE may report the received assistance information to its serving gNB and sends the SL DRX configuration to the RX UE upon receiving the SL DRX configuration in dedicated RRC signalling from the gNB. When the RX UE is in RRC_CONNECTED, the RX UE can report the received SL DRX configuration to its serving gNB, e.g. for alignment of the Uu and SL DRX configurations.

SL on-duration timer, SL inactivity-timer, SL HARQ RTT timer, and SL HARQ retransmission timer are supported in unicast. SL HARQ RTT timer and SL HARQ retransmission timer are maintained per SL process at the RX UE. In addition to (pre)configured values for each of these timers, SL HARQ RTT timer value can be derived from the retransmission resource timing when SCI indicates more than one transmission resource.

SL DRX MAC CE is introduced for SL DRX operation in unicast only.

16.9.6.3 Groupcast/Broadcast

For groupcast/broadcast, SL DRX is configured commonly among multiple UEs based on QoS profile and Destination L2 ID. Multiple SL DRX configurations can be supported for each of groupcast/broadcast.

SL on-duration timer, SL inactivity-timer, SL HARQ RTT and SL retransmission timers are supported for groupcast. Only SL on-duration timer is supported for broadcast. SL DRX cycle, SL on-duration, and SL inactivity timer (only for groupcast) are configured per QoS profile. The starting offset and slot offset of the SL DRX cycle is determined based on the destination L2 ID. The SL HARQ RTT timer (only for groupcast) and SL HARQ retransmission timer (only for groupcast) are not configured per QoS profile or per destination L2 ID. For groupcast, the RX UE maintains a SL inactivity timer for each destination L2 ID, and selects the largest SL inactivity timer value if multiple SL inactivity timer values associated with different QoS profiles are configured for that L2 ID. For groupcast and broadcast, the RX UE maintains a single SL DRX cycle (selected as the smallest SL DRX cycle of any QoS profile of that L2 ID) and single SL on-duration (selected as the largest SL on-duration of any QoS profile of that L2 ID) for each destination L2 ID when multiple QoS profiles are configured for that L2 ID.

For groupcast, SL HARQ RTT timer and SL retransmission timer are maintained per SL process at the RX UE. SL HARQ RTT timer can be set to different values to support both HARQ enabled and HARQ disabled transmissions.

A default SL DRX configuration, common between groupcast and broadcast, can be used for a QoS profile which is not mapped onto any non-default SL DRX configuration(s).

In-coverage TX and RX UEs in RRC_IDLE/RRC_INACTIVE obtain their SL DRX configuration from SIB. UEs (TX or RX) in RRC_CONNECTED can obtain the SL DRX configuration from SIB, or from dedicated RRC signalling during handover. For the out of coverage case, the SL DRX configuration is obtained from pre-configuration.

For groupcast, the TX UE restarts its timer corresponding to the SL inactivity timer for the destination L2 ID (used for determining the allowable transmission time) upon reception of new data with the same destination L2 ID.

TX profile is introduced to ensure compatibility for groupcast and broadcast transmissions between UEs supporting/not-supporting SL DRX functionality. A TX profile is provided by upper layers to AS layer and identifies one or more sidelink feature group(s). Multiple TX profiles with the support of SL DRX and without the support of SL DRX can be associated to a destination L2 ID. A TX UE only assumes SL DRX for the destination L2 IDs when all the associated TX profiles correspond to support of SL DRX. A Tx UE assumes no SL DRX for the destination L2 ID if there is no associated TX profile. An RX UE determines that SL DRX is used if all destination L2 IDs of interest are assumed to support SL DRX. For groupcast, the UE reports each destination L2 ID and associated SL DRX on/off indication to the gNB.

16.9.6.4 Alignment between Uu DRX and SL DRX

Alignment of Uu DRX and SL DRX for a UE in RRC_CONNECTED is supported for unicast, groupcast, and broadcast. Alignment of Uu DRX and SL DRX at the same UE is supported. In addition, for mode 1 scheduling, the alignment of Uu DRX of the TX UE and SL DRX of the RX UE is supported.

Alignment may comprise of either full overlap or partial overlap in time between Uu DRX and SL DRX. For SL RX UEs in RRC_CONNECTED, alignment is achieved by the gNB.

16.9.7 Power Savings Resource Allocation

The SL UE in Mode 2 can support partial sensing-based resource allocation and random resource selection as power saving resource allocation methods. A SL mode 2 TX resource pool can be (pre)configured to allow full sensing only, partial sensing only, random selection only, or any combination(s) thereof. A UE decides which resource allocation scheme(s) can be used in the AS based on its capability (for a UE in RRC_IDLE/RRC_INACTIVE/OOC) and the allowed resource schemes in the resource pool configuration.

Random resource selection is applicable to both periodic and aperiodic traffic.

A UE capable for partial sensing can perform periodic-based partial sensing and/or contiguous partial sensing for resource (re)selection. Periodic-based partial sensing can only be performed if periodic resource reservation is configured in the resource pool. In periodic-based partial sensing, the UE monitors slots in periodic sensing occasion(s) for a given resource reservation periodicity. Contiguous partial sensing is performed by a UE capable of partial sensing when resource (re)selection is triggered by a UE in a TX pool configured with partial sensing. In contiguous partial sensing, the UE monitors slots in a contiguous sensing window which occur prior to the selected transmission resource.

16.9.8 Inter-UE Coordination (IUC)

The SL UE can support inter-UE coordination (IUC) in Mode 2, whereby a UE-A sends information about resources to UE-B, which UE-B then uses for resource (re)selection. The following schemes of inter-UE coordination are supported:

– IUC scheme 1, where the IUC information sent from a UE-A to a UE-B is the preferred or non-preferred resources for UE-B’s transmission, and

– IUC scheme 2, where the IUC information sent from a UE-A to a UE-B is the presence of expected/potential resource conflict on the resources indicated by UE-B’s SCI.

In scheme 1, the transmission of IUC information from UE-A can be triggered by an explicit request from UE-B, or by a condition at UE-A. UE-A determines the set of resources reserved by other UEs or slots where UE-A, when it is the intended receiver of UE-B, does not expect to perform SL reception from UE-B due to half-duplex operation. UE-A uses these resources as the set of non-preferred resources, or excludes these resources to determine a set of preferred resources and sends the preferred/non-preferred resources to UE-B. UE-B’s resources for resource (re)selection can be based on both UE-B’s sensing results (if available) and the IUC information received from UE-A, or it can be based only on IUC information received from UE-A. For scheme 1, MAC CE and second-stage SCI or MAC CE only can be used to send IUC information. For transmission of the explicit request and reporting for IUC information in unicast manner is supported.

In scheme 2, UE-A determines the expected/potential resource conflict within the resources indicated by UE-B’s SCI as either resources reserved by other UEs and identified by UE-A as fully/partially overlapping with the resources indicated by UE-B’s SCI, or as slots where UE-A is the intended receiver of UE-B and does not expect to perform SL reception on those slots due to half-duplex operation. UE-B uses the conflicting resources to determine the resources to be reselected and exclude the conflicting resources from the reselected resources. For scheme 2, PSFCH is used to send IUC information.