Content for TS 38.300 Word version: 18.4.0

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16.15 eXtended Reality Services 16.15.1 General 16.15.2 Awareness 16.15.3 Power Saving 16.15.4 Capacity 16.15.4.1 Physical Layer Enhancements 16.15.4.2 Layer 2 Enhancements 16.15.4.2.1 Assistance Information 16.15.4.2.2 Discard 16.15.5 Non-Homogeneous support of PDU set based handling in NG-RAN 16.16 ECN marking for L4S and congestion information exposure 16.17 Support for TSN enabled Transport Network
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16.15 eXtended Reality Services |R18| p. 216

16.15.1 General p. 216

This clause describes the functionalities for the support of eXtended Reality (XR) services that require high data rate and low latency communications. An overview of XR services is available in TR 38.835, while the service requirements are documented in TS 22.261.

16.15.2 Awareness p. 216

XR-Awareness relies on QoS flows, PDU Sets, Data Bursts and traffic assistance information (see TS 23.501).

The following PDU Set QoS Parameters may be provided by the SMF to the gNB as part of the QoS profile of the QoS flow, and to enable PDU Set based QoS handling at least one of them shall be provided:

PDU Set Delay Budget (PSDB): as defined in TS 23.501, upper bound for the duration between the reception time of the first PDU (at the UPF for DL, at the UE for UL) and the time when all PDUs of a PDU Set have been successfully received (at the UE in DL, at the UPF in UL). When available, supersedes the PDB of the QoS flow.
PDU Set Error Rate (PSER): as defined in TS 23.501, upper bound for a rate of non-congestion related PDU Set losses between RAN and the UE. When available, it supersedes the PER of the QoS flow.

NOTE 1:
In this release, a PDU set is considered as successfully delivered only when all PDUs of a PDU Set are delivered successfully.
PDU Set Integrated Handling Information (PSIHI): indicates whether all PDUs of the PDU Set are needed for the usage of PDU Set by application layer, as defined in TS 23.501.

NOTE 2:
For a given QoS flow, the PDU Set QoS parameters are common for all PDU Sets but can be different for UL and DL.

During the Xn-handover preparation procedure, the source gNB sends the stored PDU Set QoS Parameters as part of the QoS profile to the target NG-RAN node. For NG handover, the AMF provides the PDU Set QoS parameters to the target gNB by means of the NGAP HANDOVER REQUEST message.

In addition, the UPF can identify PDUs that belong to PDU Sets, and may indicate to the gNB the following PDU Set Information in the GTP-U header:

PDU Set Sequence Number;
Indication of End PDU of the PDU Set;
PDU Sequence Number within a PDU Set;
PDU Set Size in bytes;
PDU Set Importance (PSI), which identifies the relative importance of a PDU Set compared to other PDU Sets within the same QoS Flow.

5GC may provide XR traffic assistance information to gNB through NG AP TSC Assistance Information (TSCAI) as specified in clause 5.37.8 of TS 23.501 (for both GBR and non-GBR QoS flows):

UL and/or DL Periodicity;
N6 Jitter Information (i.e. between UPF and Data Network) associated with the DL Periodicity.

This assistance information can be used by the gNB to configure DRX to enable better UE power saving.

In addition, 5GC may provide the following information through NG-U as specified in clause 5.37.5.2 of TS 23.501:

Indication of End of Data Burst in the GTP-U header of the last PDU in downlink.

This information can be used by the gNB to push the UE back to sleep when possible.

In the uplink, the UE needs to be able to identify PDU Sets and Data Bursts dynamically, including PSI. How this is done is left up to UE implementation but when possible for a QoS flow, this is indicated to the gNB via UE Assistance Information.

16.15.3 Power Saving p. 217

Most XR video frame rates (15, 30, 45, 60, 72, 90 and 120 fps) correspond to periodicities that are not an integer (66.66, 33.33, 22.22, 16.66, 13.88, 11.11 and 8.33 ms respectively). The gNB may configure a DRX cycle expressed in rational numbers so that the DRX cycle matches those periodicities, e.g. for the traffic with a frame rate of 60 fps, the network may configure the UE with a DRX cycle of 50/3 ms.

Configured grants may be configured without the need for the UE to wake up to monitor possible grants for UL retransmissions of configured grants, thus increasing the number of power saving opportunities for the UE.

16.15.4 Capacity p. 218

16.15.4.1 Physical Layer Enhancements p. 218

The following enhancements for configured grant-based PUSCH transmission are introduced:

Support of multiple CG PUSCH transmission occasions within a single period of a CG configuration;
Indication of unused CG PUSCH occasion(s) of a CG configuration with Uplink Control Information multiplexed in CG PUSCH transmission of the CG configuration.

16.15.4.2 Layer 2 Enhancements p. 218

16.15.4.2.1 Assistance Information p. 218

In order to enhance the scheduling of uplink resources for XR, the following improvements are introduced:

One additional buffer size table to reduce the quantisation errors in BSR and DSR (defined below) reporting (e.g. for high bit rates):
- Whether, for an LCG, the new table can be used in addition to the regular one is configured by the gNB;
- When the new table is configured for an LCG, it is used whenever the amount of the buffered data of that LCG to be reported is within the range of the new table, otherwise the regular table is used.
Delay Status Report (DSR) of buffered data via a dedicated MAC CE:
- Triggered for an LCH when the remaining time before discard of any buffered PDCP SDU goes below a configured threshold (threshold configured per LCG by the gNB);
- When triggered for an LCH, reports the amount of data buffered with a remaining time before discard below the configured threshold, together with the shortest remaining time of any PDCP SDU buffered that has not been transmitted in any MAC PDU.
Reporting of uplink assistance information (jitter range, burst arrival time, UL data burst periodicity) per QoS flow by the UE via UE Assistance Information. In case target gNB receives the burst arrival time from source gNB during the handover preparation procedure, the target gNB may use it by considering the SFN offset of the source gNB.

16.15.4.2.2 Discard p. 218

When the PSIHI indicates that all PDUs of the PDU Set are needed for a QoS flow, as soon as one PDU of a PDU set is known to be lost, the remaining PDUs of that PDU Set can be considered as no longer needed by the application and may be subject to discard operation at the transmitter to free up radio resources.

In uplink, the UE may be configured with PDU Set based discard operation for a specific DRB. When configured, the UE discards all packets in a PDU set when one PDU belonging to this PDU set is discarded due to discard timer expiry.

The gNB may perform downlink PDU Set discarding based on implementation by taking at least PSDB, PSI, PSIHI parameters into account.

In case of congestion, for downlink, the gNB may perform PDCP SDU discarding based on PSI. For uplink, dedicated downlink signalling is used to request the UE to apply a shorter discard timer to PDCP SDUs belonging to low importance PDU Sets in PDCP.

The transmitting PDCP entity can inform the receiving one of gaps in the sequence of transmitted PDCP SN, resulting from PDCP SDU discard, via a PDCP control PDU.

16.15.5 Non-Homogeneous support of PDU set based handling in NG-RAN p. 219

During a handover from a gNB supporting PDU Set based handling to another gNB, the source gNB signals the PDU Set Information over Xn-U if the target node has signalled the support of PDU Set based handling in the Xn Handover Request Acknowledge message.

During a handover, transition from RRC_INACTIVE to RRC_CONNECTED or RRC re-establishment from a gNB not supporting PDU Set based handling to a gNB supporting PDU Set based handling, the target/new serving gNB may indicate the support of PDU Set based handling to the SMF during the Path Switch Request procedure or Handover Resource Allocation procedure (in case of NG handover), the SMF will act as specified in TS 23.501. If the indication is absent, the SMF infers that PDU Set based handling is not supported by the target/new serving gNB node, then the SMF will act as specified in TS 23.501.

During a handover, transition from RRC_INACTIVE to RRC_CONNECTED or RRC re-establishment from a gNB node not supporting PDU Set based handling to a gNB node supporting PDU Set based handling, the target/new serving gNB node may receive unmarked PDU(s) (i.e. PDU(s) without PDU Set Information Container) forwarded from the source/last serving gNB, node and marked PDU(s) (i.e. PDU(s) with PDU Set Information Container) from UPF, how the target/new serving gNB node handles the marked and unmarked PDUs for the same QoS flow is up to implementation.

16.16 ECN marking for L4S and congestion information exposure |R18| p. 219

In order to support ECN marking for L4S at gNB as specified in TS 23.501, SMF provides ECN marking request per QoS flow level to the gNB as part of PDU Session Resource Management procedure. If the gNB supports ECN marking, it provides the status indication back to the SMF which is used by the SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the ECN marking request to target gNB.

When ECN marking for L4S at gNB is enabled for downlink or uplink, the gNB should set the Congestion Experienced (CE) codepoint in downlink or uplink SDAP SDU(s) as per the recommendations in RFC 9330, RFC 9331 and RFC 9332.

For ECN marking for L4S at UPF, SMF requests the gNB to report congestion information per QoS flow level as part of PDU Session Resource Management procedure. If the gNB supports ECN marking for L4S at UPF, it provides the status indication back to the SMF which is used by SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the ECN marking UPF request to target gNB.

For congestion reporting from gNB to UPF, SMF requests the gNB to report congestion information per QoS flow level as part of PDU Session Resource Management procedure. If the NG-RAN supports congestion information reporting, it provides the status indication back to the SMF which is used by the SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the congestion information request to target gNB.

16.17 Support for TSN enabled Transport Network |R18| p. 219

The NG-RAN may support the TSN enabled Transport Network as specified in TS 23.501.