Section: 16.9.10 Sidelink CA
Content: Carrier aggregation (CA) in sidelink is supported for mode 2 and in V2X case only. It applies to both in coverage UEs and out of coverage UEs. Each resource pool (pre)configured for sidelink is associated to a single carrier. A UE using mode 2 resource allocation performs carrier (re)selection and may select one or more carriers used for sidelink. The carrier(s) that can be used for transmitting data are provided by the V2X layer per QoS flow, and LCP ensures that data from a SLRB is transmitted on a carrier for which all mapped QoS flows are allowed to use the carrier. For groupcast and broadcast, when the V2X layer provides multiple carriers in QoS flow to carrier mapping information to the AS, TX profile is used to indicate whether the transmission corresponding to the QoS flow is backward compatible or not. When backward compatibility is needed, the TX UE uses only the legacy carrier without PDCP duplication, or uses PDCP duplication with at least the legacy carrier. For groupcast and broadcast, carrier selection is performed at MAC layer, depending on the CBR of the configured carriers and logical channel priority. Carrier (re)selection may be performed when resource (re)selection is triggered, or when there is no SL grant for a sidelink process on any allowed carrier, and is triggered for each sidelink process. In order to avoid frequent switching across different carriers, the UE may keep using a carrier already selected for transmission, if the measured CBR on this carrier is lower than a (pre)configured threshold. For a UE using mode 2 resource allocation, logical channel prioritization is performed for a sidelink resource on a carrier depending on the CBR measured on the carrier and the logical channel priority, as specified in TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6]. For unicast, SL CA related capability is exchanged between the TX UE and RX UE, and the TX UE delivers the carrier configuration to the RX UE in PC5-RRC. For unicast, carrier selection and logical channel prioritization is performed similar to groupcast and broadcast among the carriers delivered in the carrier configuration. SL CA for unicast is not applied until the carrier configuration signalling is complete. Carrier (re)selection may be performed and a new carrier configuration is sent to the RX UE when the TX UE detects carrier failure on a specific carrier, as specified in TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6]. Sidelink packet duplication is supported for sidelink CA and is performed at PDCP layer. For sidelink packet duplication for transmission, a PDCP PDU is duplicated at the PDCP entity. The duplicated PDCP PDUs of the same PDCP entity are submitted to two different RLC entities and associated to two different sidelink logical channels respectively. The duplicated PDCP PDUs of the same PDCP entity are only allowed to be transmitted on different sidelink carriers. For a SL DRB, sidelink packet duplication is (pre)configured in the bearer configuration. For applicable SL SRBs, whether to use duplication is decided by the TX UE. In unicast, the TX UE sends the duplication configuration to the RX UE in PC5-RRC. There are specified logical channel identities which apply to the sidelink logical channel used for sidelink packet duplication exclusively as specified in TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6].