Section: 16.8 Support for Time Sensitive Communications 16.8.1 General
Content: Time Sensitive Communications (TSC), as defined in TS 23.501[ System architecture for the 5G System (5GS) ]  [3], is a communication service that supports deterministic communication and/or isochronous communication with high reliability and availability. Examples of such services are the ones in the area of Industrial Internet of Things, e.g. related to cyber-physical control applications as described in TS 22.104[ Service requirements for cyber-physical control applications in vertical domains ]  [39]. To support strict synchronization accuracy requirements of TSC applications, the gNB may signal 5G system time reference information to the UE using unicast or broadcast RRC signalling with a granularity of 10 ns. Uncertainty parameter may be included in reference time information to indicate its accuracy. The UE may indicate to the gNB a preference to be provisioned with reference time information using UE Assistance Information procedure. Propagation delay compensation (PDC) mechanisms may be applied based on RTT or TA, and can be performed at the UE or gNB side. When performed at UE side, the PDC mechanisms are controlled via RRC signalling by the gNB. The RTT-based PDC mechanism is achieved by using Rx-Tx time difference measurements of a single pair of configured TRS/PRS and SRS. The following figure describes the signalling procedures of UE-side RTT-based PDC: Figure 16.8-1: Signalling Procedure of UE-side RTT-based PDC 1. The gNB provides measurement configurations to the UE; 2a/b. The gNB transmits TRS or PRS to the UE for measurements, and the UE transmits SRS to the gNB for measurement; 3a/b. Both the UE and the gNB perform Rx-Tx time difference measurements; 4. The gNB provides its Rx-Tx time difference measurement to the UE; 5. The UE performs PDC based on Rx-Tx time difference measurements from itself and the gNB. The following figure describes the signalling procedures of gNB-side RTT-based PDC: Figure 16.8-2: Signalling Procedure of gNB-side RTT-based PDC 1. The gNB provides measurement configurations to the UE; 2a/b. The gNB transmits TRS or PRS to the UE for measurements, and the UE transmits SRS to the gNB for measurement; 3a/b. Both the UE and the gNB perform Rx-Tx time difference measurements; 4. The UE reports its Rx-Tx time difference measurement to the gNB; 5. The gNB performs PDC based on Rx-Tx time difference measurements from itself and the UE. The gNB may also receive TSC Assistance Information (TSCAI), see TS 23.501[ System architecture for the 5G System (5GS) ]  [3], from the Core Network, e.g. during QoS flow establishment, or from another gNB during handover. TSCAI contains additional information about the traffic flow such as burst arrival time, burst periodicity, and survival time. TSCAI knowledge may be leveraged in the gNB's scheduler to more efficiently schedule periodic, deterministic traffic flows either via Configured Grants, Semi-Persistent Scheduling or with dynamic grants, and/or to improve the associated link reliability to meet the survival time requirement (see TS 22.104[ Service requirements for cyber-physical control applications in vertical domains ]  [39]). To support uplink periodic traffics of services with survival time requirement, configured grant resources can be used such that the mapping relation between the service and the configured grant is known to both gNB and UE, thus allowing the gNB to use configured grant retransmission scheduling (addressed by CS-RNTI) to trigger survival time state entry for the corresponding DRB. Upon survival time state entry, all RLC entities configured for the DRB are activated by the UE for duplication to prevent failure of subsequent messages and hence fulfilling the survival time requirement. If CA or DC duplication for the DRB is already activated, the DRB should enter survival time state when any retransmission grant for any of its active LCHs is received.