16.1 URLLC

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

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

16.1.1 Overview

The support of Ultra-Reliable and Low Latency Communications (URLLC) services is facilitated by the introduction of the mechanisms described in the following clauses. Please note however that those mechanisms need not be limited to the provision of URLLC services. Furthermore, RRC can associate logical channels with different SR configurations, for instance, to provide more frequent SR opportunities to URLLC services.

16.1.2 LCP Restrictions

With LCP restrictions in MAC, RRC can restrict the mapping of a logical channel to a subset of the configured cells, numerologies, PUSCH transmission durations, configured grant configurations and control whether a logical channel can utilise the resources allocated by a Type 1 Configured Grant (see clause 10.3) or whether a logical channel can utilise dynamic grants indicating a certain physical priority level. With such restrictions, it then becomes possible to reserve, for instance, the numerology with the largest subcarrier spacing and/or shortest PUSCH transmission duration for URLLC services. Furthermore, RRC can associate logical channels with different SR configurations, for instance, to provide more frequent SR opportunities to URLLC services.

16.1.3 Packet Duplication

When duplication is configured for a radio bearer by RRC, at least one secondary RLC entity is added to the radio bearer to handle the duplicated PDCP PDUs as depicted on Figure 16.1.3-1, where the logical channel corresponding to the primary RLC entity is referred to as the primary logical channel, and the logical channel corresponding to the secondary RLC entity(ies), the secondary logical channel(s). All RLC entities have the same RLC mode. Duplication at PDCP therefore consists in submitting the same PDCP PDUs multiple times: once to each activated RLC entity for the radio bearer. With multiple independent transmission paths, packet duplication therefore increases reliability and reduces latency and is especially beneficial for URLLC services.

Figure 16.1.3-1: Packet Duplication

NOTE: PDCP control PDUs are not duplicated and always submitted to the primary RLC entity.

When configuring duplication for a DRB, RRC also sets the state of PDCP duplication (either activated or deactivated) at the time of (re-)configuration. After the configuration, the PDCP duplication state can then be dynamically controlled by means of a MAC control element and in DC, the UE applies the MAC CE commands regardless of their origin (MCG or SCG). When duplication is configured for an SRB the state is always active and cannot be dynamically controlled. When configuring duplication for a DRB with more than one secondary RLC entity, RRC also sets the state of each of them (i.e. either activated or deactivated). Subsequently, a MAC CE can be used to dynamically control whether each of the configured secondary RLC entities for a DRB should be activated or deactivated, i.e. which of the RLC entities shall be used for duplicate transmission. Primary RLC entity cannot be deactivated. When duplication is deactivated for a DRB, all secondary RLC entities associated to this DRB are deactivated. When a secondary RLC entity is deactivated, it is not re-established, the HARQ buffers are not flushed, and the transmitting PDCP entity should indicate to the secondary RLC entity to discard all duplicated PDCP PDUs.

When activating duplication for a DRB, NG-RAN should ensure that at least one serving cell is activated for each logical channel associated with an activated RLC entity of the DRB; and when the deactivation of SCells leaves no serving cells activated for a logical channel of the DRB, NG-RAN should ensure that duplication is also deactivated for the RLC entity associated with the logical channel.

When duplication is activated, the original PDCP PDU and the corresponding duplicate(s) shall not be transmitted on the same carrier. The logical channels of a radio bearer configured with duplication can either belong to the same MAC entity (referred to as CA duplication) or to different ones (referred to as DC duplication). CA duplication can also be configured in either or both of the MAC entities together with DC duplication when duplication over more than two RLC entities is configured for the radio bearer. In CA duplication, logical channel mapping restrictions are used in a MAC entity to ensure that the different logical channels of a radio bearer in the MAC entity are not sent on the same carrier. When CA duplication is configured for an SRB, one of the logical channels associated to the SRB is mapped to SpCell.

When CA duplication is deactivated for a DRB in a MAC entity (i.e. none or only one of RLC entities of the DRB in the MAC entity remains activated), the logical channel mapping restrictions of the logical channels of the DRB are lifted for as long as CA duplication remains deactivated for the DRB in the MAC entity.

When an RLC entity acknowledges the transmission of a PDCP PDU, the PDCP entity shall indicate to the other RLC entity(ies) to discard it. In addition, in case of CA duplication, when an RLC entity restricted to only SCell(s) reaches the maximum number of retransmissions for a PDCP PDU, the UE informs the gNB but does not trigger RLF.

16.1.4 CQI and MCS

For channel state reporting, a CQI table for target block error rate 10^-5 is introduced.

For scheduling data packets with higher reliability, 64QAM MCS tables containing entries with lower spectral efficiency are introduced for both downlink and uplink. The tables are different for CP-OFDM and DFT-s-OFDM. The MCS tables can be configured semi-statically or dynamically. The dynamic signalling of MCS table is supported by configuring UE with MCS-C-RNTI, where the scrambling of DCI CRC by MCS-C-RNTI indicates the 64QAM MCS tables with entries of lower spectral efficiency.

16.1.5 DCI formats

For PDCCH transmission with higher reliability, two DCI formats are introduced for uplink and downlink scheduling respectively.

16.1.6 Higher layer multi-connectivity

The redundant transmission may be applied on the user plane path between the UE and the network for URLLC service as specified in TS 23.501 [3].

16.1.6.1 Redundant user plane paths based on dual connectivity

UE may initiate two redundant PDU Sessions over the 5G network. The 5GS sets up the user plane paths of the two redundant PDU sessions to be disjoint. When PDU session setup or modification is initiated, the RAN can configure dual connectivity in one NG-RAN node or two NG-RAN nodes for the two redundant PDU sessions to ensure the disjoint user plane paths according to the redundancy information received from the 5GC. The RAN shall ensure that the resources of the data radio bearers for the two redundant PDU sessions are isolated. If the RAN cannot satisfy the disjoint user plane requirement, the redundant PDU sessions may be kept or not kept according to the RAN local configuration. The redundancy information is transferred to the target NG-RAN node in case of handover.

16.1.6.2 Redundant data transmission via single UPF and single RAN node

Two NG-U tunnels are setup between single UPF and single NG-RAN node for redundant transmission of the QoS flows when PDU session setup or modification is initiated. The two NG-U tunnels are transferred via disjointed transport layer paths. The 5GC provides the indicator per QoS flow to the NG-RAN for the redundant transmission. For downlink, the NG-RAN node eliminates the duplicated packets per QoS flow. For uplink, the NG-RAN node replicates the packets and transmits them via the two NG-U tunnels. The indicator per QoS flow for redundant transmission is transferred to the target NG-RAN node in case of handover.

16.1.7 URLLC in Unlicensed Controlled Environment

URLLC services can be supported in shared spectrum where LBT failures are assumed to be not frequent. In this case, a channel access procedure for semi-static channel occupancy can be initiated by the gNB or the UE, or the gNB operates in dynamic channel access mode, as described in TS 37.213 [37]. To handle potential LBT failures on configured grants (CG), the CG retransmission timer can be optionally configured to enable autonomous retransmissions, and it may be configured simultaneously with enhanced intra-UE overlapping resource prioritization mechanisms. When the CG retransmission timer is configured, the UE shall select the HARQ process for each CG resource by itself. If the enhanced intra-UE overlapping resource prioritization mechanisms is also configured, the UE may be further configured to select the HARQ process for a CG resource based on logical channel priority.

16.1.8 PUCCH cell switching for TDD cells

To reduce the delay for HARQ-ACK feedback for TDD operation with URLLC services, PUCCH cell switching for TDD cells is supported. The UE can be provided in each PUCCH group with a PUCCH switching SCell (PUCCH sSCell) that can be used for PUCCH transmission in addition to PCell / PSCell / PUCCH SCell. The applicable cell for PUCCH transmission to be either on PCell /PSCell / PUCCH SCell or the PUCCH sSCell at a time is either defined by:

– a higher layer configured semi-static time-domain pattern of the applicable cell for PUCCH transmission; or

– dynamic indication of the cell for PUCCH transmission through a PDCCH scheduling a PUCCH transmission.

The PUCCH cell switching is applicable to all UCI types when using the higher layer configured time-domain pattern, but is only applicable to HARQ feedback for the dynamic indication of the cell for PUCCH transmission through a PDCCH scheduling PUCCH.