Content for  TS 38.300  Word version:  18.4.0

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.

For support of PC5-S protocol specified in TS 23.287, 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.

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.

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 PC5-U is shown in Figure 16.9.2.1-4.

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.

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) and NR sidelink discovery 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.

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.

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.

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.

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;
• Measurement configuration and reporting related to sidelink.

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. 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. 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, UE releases the PC5-RRC connection.

For NR sidelink communication, the UE can operate in two modes as specified in clause 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.

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. 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.

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 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.

NR sidelink supports co-channel coexistence with LTE sidelink, allowing NR sidelink transmissions to share co-channel resources with LTE sidelink transmissions. Dynamic co-channel coexistence is used, in which a resource pool for NR sidelink transmissions is overlapping with a resource pool for LTE sidelink transmissions. Sidelink relay is not supported in a resource pool that uses dynamic co-channel coexistence. Dynamic co-channel coexistence is further specified in TS 38.321, TS 38.213 and TS 38.214.

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 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.

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 and reception resource pool of the target cell as provided in the handover command.

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, and/or resource pool configuration via SIB13 and SIB14 for V2X sidelink communication as specified in TS 38.331. 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.

When the UE performs cell reselection, the UE interested in ProSe service(s) considers at least whether NR sidelink communication/discovery is supported by the cell. The UE may consider the carrier frequency providing NR sidelink communication/discovery configuration as the highest priority frequency, except for the carrier only providing the anchor carrier.

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 2nd 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 HARQ 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 time for the unicast link establishment procedure and the time for the PC5 RRC reconfiguration with initial SL DRX configuration procedure as specified in clause 5.28.2 of TS 38.321 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). The UE can determine from SIB12 whether the gNB supports SL DRX or not.

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, SL DRX slot offset, and SL DRX cycle, to the TX UE and the TX UE using mode 2 resource allocation 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.

When the TX UE is in RRC_CONNECTED and using mode 1 resource allocation, the TX UE may report the received assistance information or the received SL DRX configuration reject information to its serving gNB supporting SL DRX 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 and using mode 1 resource allocation, the RX UE can report the received SL DRX configuration to its serving gNB supporting SL DRX, 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 HARQ RTT timer can be set to different values to support both HARQ enabled and HARQ disabled transmissions. SL DRX MAC CE is introduced for SL DRX operation in unicast only.

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 HARQ 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 HARQ 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). The default SL DRX configuration for groupcast and broadcast can also be used for discovery message in sidelink discovery in clause 16.9.5 and relay discovery messages in clause 16.12.3, for ProSe Direct Link Establishment Request message as specified in TS 24.554, and for Direct Link Establishment Request message as specified in TS 24.587. 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 communication 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. For a given destination L2 ID, all TX and RX UEs should be configured with the same set of TX profile(s). A UE only assumes SL DRX for the given destination L2 ID when all the associated TX profiles correspond to support of SL DRX. A UE assumes no SL DRX for the given 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, when the UE is in RRC_CONNECTED and using mode 1 resource allocation, the UE reports each destination L2 ID and associated SL DRX on/off indication to the gNB supporting 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 resource allocation, 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.

Sidelink supports operation in shared spectrum for both mode 1 and mode 2 for ProSe use case only. Sidelink positioning in shared spectrum is not supported. Type1 and Type2 (2A/2B/2C) channel access procedures specified in TS 37.213 are used for NR sidelink operation in a shared channel.

The SL-CAPC of SL radio bearers and SL MAC CEs are either fixed or (pre)configurable as follows:

• Fixed to the highest priority for all SL-SRBs and SL MAC CEs
• (Pre)configurable per DRB for all SL-DRBs

When choosing the SL-CAPC for a SL-DRB the network takes into account the PQIs of all QoS flows mapped to that SL DRB. Table 16.9.9-1 below shows which SL-CAPC should be used for which standardized PQI(s), i.e., which SL-CAPC to use for a given QoS flow. For a UE in RRC_IDLE/RRC_INACTIVE or OOC, the UE uses the SL-CAPC configured in SIB/Pre-configuration when the QoS flow can be mapped to a non-default SLRB, or when the default SLRB is used for the QoS flow and the SL-CAPC of the default SLRB is configured. When the default SLRB is used for the QoS flow and the SL-CAPC of the default SLRB is not configured:

• if the QoS flow is associated with standardized PQI, the UE derives SL-CAPC for the flow directly from the Table below
• if the QoS flow is associated with non-standardized PQI, the UE may select the SL-CAPC of the standardized PQI having the closest PDB.

The UE then selects the lowest SL-CAPC priority level (highest SL-CAPC value) among the QoS flows to determine the SL-CAPC for the default SLRB.

When performing Type 1 LBT for the transmission of a sidelink TB, the SL UE shall select the SL-CAPC as follows:

• If only SL MAC CE(s) are included in the SL TB, the highest priority SL-CAPC is used; or
• If SCCH SDU(s) are included in the SL TB, the highest priority SL-CAPC is used; or
• The lowest priority SL-CAPC of the SL logical channel(s) with MAC SDU(s) multiplexed in the TB is used otherwise.

The highest priority SL-CAPC is used for SBCCH SDU transmissions and PSFCH transmissions.

SL-specific consistent LBT failure detection and recovery procedure is supported for SL-U. When the UE detects SL-specific consistent LBT failure, it performs the actions as specified in TS 38.321. SL-specific consistent LBT failure detection is per RB-set. A SL UE in RRC_CONNECTED can indicate SL-specific consistent LBT failure to the gNB using a SL MAC CE that indicates the RB set(s) where SL-specific consistent LBT failure was detected. A SL UE using mode 2 resource allocation triggers resource reselection and/or resource pool reselection upon SL-specific consistent LBT failure. In such case, resources in failed RB set(s) are excluded from resource (re)selection until consistent LBT failure on the RB set(s) is cancelled based on conditions in TS 38.321. The UE triggers SL RLF for all PC5-RRC connections when the UE has triggered SL-specific consistent LBT failure in all RB sets.

UE to UE COT sharing is supported in NR sidelink operation for SL-U. When performing SL-SSB transmission(s), a responding UE can utilize a COT shared by a COT initiating UE when the responding UE intends to transmit SL-SSB in the shared COT. When performing PSFCH transmissions, a responding UE can utilize a COT shared by a COT initiating UE when at least one of the responding UE's PSFCH transmissions is intended for the COT initiating UE. When performing PSSCH/PSCCH transmissions, a responding UE can utilize a COT shared by a COT initiating UE when the responding UE's transmission(s) is intended for the COT initiating UE. In unicast, the destination/source ID of the responding UE's transmission should match the source/destination ID of the initiator UE's transmission for the same unicast link. In groupcast/broadcast, destination ID of the responding UE's transmission should match the initiator's destination ID. In addition, a COT initiating UE may transmit an additional pair of source/destination ID in either unicast, groupcast, or broadcast. In this case, the responding UE can utilize a COT shared by a COT initiating UE if the destination/source (for unicast) or destination (for groupcast/broadcast) of the responding UE transmission match what is carried in the additional ID(s). In all cases, a UE's transmissions in a shared COT should have SL-CAPC value that is equal to or smaller than the SL-CAPC value indicated in the COT sharing information. In order to satisfy the COT sharing requirements, a responding UE may perform an enhanced LCP procedure as specified in 38.321 [6]. When receiving multiple COT sharing indications from different COT initiators, it is up to UE implementation which shared COT is used.

A UE using mode 2 resource allocation may support resource selection enhancements to avoid LBT blocking and increase COT resource utilization. If transmission in slot(s) before a reserved resource is able to share its initiated COT to the reservation, the UE may be (pre)configured to prioritize/select resource(s) in the slot(s) for transmission. To avoid blocking, a UE may avoid selection of a number of consecutive resources (up to UE implementation) before a reserved resource. Furthermore, a UE may avoid selection of a number of consecutive resources (up to UE implementation) after a reserved resource. A UE using mode 2 resource allocation supports resource selection for multiple consecutive slot transmission (MCSt). A UE autonomously determines whether to use MCSt, and the number of consecutive slots in an MCSt up to the maximum COT duration for a specific SL-CAPC as defined in TS 37.213. MCSt can be used for transmission of a single TB or multiple TBs. For each TB transmitted in an MCSt, the UE triggers resource (re)selection only when LBT failure is detected on the resources for the initial transmission and all retransmissions of the TB.