Study on physical layer enhancements for NR ultra-reliable and low latency case (URLLC)V16.0.0 (2019-03)

Introduction

In Release 15 the basic support for URLLC was introduced with TTI structures for low latency as well as methods for improved reliability. Further use cases with tighter requirements have been identified as important for NR evolution, in addition to the need for enhancing the Release 15 enabled use cases.

The follow key use cases were identified to be considered:

• Release 15 enabled use case improvements
  • Such as AR/VR (Entertainment industry)
• New Release 16 use cases with higher requirements
• Factory automation
• Transport Industry, including the remote driving use case
• Electrical Power Distribution

The objective of this study item is to investigate enhancements to URLLC (Ultra Reliable Low Latency Communications), considering both FR1 and FR2 as well as TDD and FDD, with the already existing solutions for NR as the baseline. The TR reports the results and findings from the study item, mainly on baseline performance achievable with Rel-15 NR URLLC, layer 1 enhancements (including PDCCH enhancements, UCI enhancements, PUSCH enhancements and enhancements to scheduling/HARQ/CSI processing timeline), UL inter UE Tx prioritization/multiplexing and enhanced UL configured grant (grant free) transmissions.

Enhancement features

PDCCH enhancements

For the DCI format(s) scheduling Rel-16 NR URLLC, it is identified to be beneficial to support configurable sizes for some fields and potential reduction of the number of bits for some field(s) compared to Rel-15 DCI, to enable a minimum DCI size target a reduction of 10~16 bits less than the DCI format size of Rel-15 fallback DCI, and to provide the possibility to align with the size of the Rel-15 fallback DCI. The maximum DCI size can be larger than Rel-15 fallback DCI. It is also concluded on the potential list of fields with reduction of the number of bits and the potential list of configurable fields.

Increased PDCCH monitoring capability is identified to be beneficial while it may increase UE complexity. It is concluded to support increased PDCCH monitoring capability on at least the maximum number of non-overlapped CCEs per slot for channel estimation for Rel-16 NR URLLC for at least one SCS subject to some restrictions. Enhancements for PDCCH monitoring capability on the maximum number of monitored PDCCH candidates per slot (with potential restrictions) for Rel-16 NR URLLC can be further considered in work item phase.

It is concluded that PDCCH repetition is not further considered in this study item.

UCI enhancements

In NR Rel-15, only one PUCCH for HARQ-ACK transmission is supported within a slot. Enabling more than one PUCCH for HARQ-ACK transmission within a slot is beneficial as it may enable fast HARQ-ACK feedback to reduce the latency and facilitate separate HARQ-ACK feedback for URLLC and eMBB. It is concluded that more than one PUCCH for HARQ-ACK transmission within a slot should be supported in Rel-16.

Intended for supporting different service types for a UE, it is concluded that at least two HARQ-ACK codebooks can be simultaneously constructed for a Rel-16 UE. Rules for the HARQ-ACK codebooks for supporting different service types should be specified in Rel-16, if at least two HARQ-ACK codebooks are due to be transmitted in resources overlapping in time. When at least two HARQ-ACK codebooks are simultaneously constructed for supporting different service types for a UE, a HARQ-ACK codebook can be identified based on some PHY indications/properties.

Enhanced CSI feedback are studied from several aspects, including DMRS based CSI measurement, A-CSI on PUCCH (e.g. triggering by DL assignment), enhanced CSI reporting mode. The following options have been identified as potential candidates for A-CSI on PUCCH:

• Option 1: A-CSI report on PUCCH triggered by DL-scheduling DCI
• Option 2: A-CSI report on PUCCH based on group-common PDCCHEnhanced CSI feedback is studied. There is no consensus in RAN1 for supporting A-CSI on PUCCH in Rel-16.
• There is no consensus in RAN1 for supporting A-CSI on PUCCH in R16.

PUSCH enhancements

Whether to allow one PUSCH transmission instance to cross the slot boundary when the remaining symbols within one slot is not enough for one PUSCH transmission instance was studied. The conclusion is that one PUSCH transmission instance is not allowed to cross the slot boundary for grant-based PUSCH.

Potential enhancements for grant-based PUSCH and configured grant based PUSCH are studied:

1. Option 1 (Mini-slot level repetition):
   One UL grant scheduling two or more PUSCH repetitions that can be in one slot, or across slot boundary in consecutive available slots
   • For time domain resource determination for mini-slot level repetition, for grant based PUSCH, the time domain resource assignment field in the DCI indicates the resource for the first repetition. The time domain resources for the remaining repetitions are derived based at least on the resources for the first repetition and the UL/DL direction of the symbols. Each repetition occupies contiguous symbols.
   • For frequency hopping for mini-slot level repetition, support at least inter-PUSCH-repetition hopping and inter-slot hopping.
2. Option 2 (Multi-segment transmission):
   One UL grant scheduling two or more PUSCH repetitions in consecutive available slots, with one repetition in each slot with possibly different starting symbols and/or durations
   • For time domain resource determination for multi-segment transmission, for grant based PUSCH, the time domain resource assignment field in the DCI indicates the starting symbol and the transmission duration of all the repetitions.
   • For the transmission within one slot for multi-segment transmission, if there are more than one UL period within a slot, where each UL period is the duration of a set of contiguous symbols within a slot for potential UL transmission as determined by the UE, one repetition is within one UL period and each repetition occupies contiguous symbols. Otherwise, a single PUSCH repetition is transmitted within a slot following Rel-15 behavior.
   • For frequency hopping for multi-segment transmission, support at least inter-slot frequency hopping.
3. Option 3:
   N (N≥2) UL grants scheduling N PUSCH repetitions on consecutive available slots, with one repetition in each slot, and the i-th UL grant can be received before the end of the PUSCH transmission scheduled by the (i-1)th UL grant
4. Option 4:
   One or more actual PUSCH repetitions in one slot, or two or more actual PUSCH repetitions across slot boundary in consecutive available slots, is supported using one UL grant for dynamic PUSCH, and one configured grant configuration for configured grant PUSCH.
   
   
5. Option 5:
   One or more actual PUSCH repetitions in one slot, or two or more actual PUSCH repetitions across slot boundary in consecutive available slots, is supported using one UL grant for dynamic PUSCH, and one configured grant configuration for configured grant PUSCH.
   
   
6. Option 6:
   One or more PUSCH repetitions in one slot, or two or more PUSCH repetitions across slot boundary in consecutive available slots, is supported using one UL grant for dynamic PUSCH, and one configured grant configuration for configured grant PUSCH.

It is identified to be beneficial to support enhancements for both grant-based PUSCH and configured grant based PUSCH in Rel-16, to enable one UL grant scheduling two or more PUSCH repetitions that can be in one slot, or across slot boundary in consecutive available slots. It is concluded to finalize the details regarding how to use "option 1" vs. "option 2" during the work item phase, using option 4, 5 and 6 as a starting point.

Enhancements to scheduling/HARQ/CSI processing timeline

1. Enhancements to scheduling/HARQ processing timeline
   It is concluded that in Rel-16 NR there is no PDSCH and PUSCH processing timing enhancement as compared to Rel-15 NR for SCS = 15 kHz. Please refer to TS 38.824 clause 6.4.2 for conclusisions regarding other SCS.
2. Out-of-order HARQ-ACK and PUSCH scheduling
   In Rel-15, out-of-order HARQ-ACK and PUSCH scheduling are not supported as defined in section 5.1 and section 6.1 in TS 38.214. Out-of-order HARQ-ACK and PUSCH scheduling are studied in this study item and is identified to be beneficial. It is concluded to support out-of-order HARQ-ACK and out-of-order PUSCH scheduling as below.
   • Out-of-order HARQ-ACK
     For a Rel-16 URLLC UE and dynamic downlink scheduling, on the active BWP of a given serving cell, the HARQ-ACK associated with the second PDSCH with HARQ process ID x received after the first PDSCH with HARQ process ID y (x != y) can be sent before the HARQ-ACK of the first PDSCH.
3. Out-of-order PUSCH scheduling
   For a Rel-16 UE, on the active BWP of a given serving cell, the UE can be scheduled with a second PUSCH associated with HARQ process x starting earlier than the ending symbol of the first PUSCH associated with HARQ process y (x != y) with a PDCCH that does not end earlier than the ending symbol of first scheduling PDCCH.

Inter UE Tx prioritization/multiplexing

It is identified to be beneficial to support enhancements for inter UE Tx prioritization/multiplexing, and it is recommended to support UL cancelation scheme and enhanced UL power control scheme in Rel-16.

1. UE UL cancelation mechanisms
   Upon detecting an UL cancelation indication, UE cancels the corresponding UL transmission. The corresponding UL transmission may include an on-going UL transmission, or an UL transmission that has not been started. After cancelation, the UE may resume the transmission afterwards as one option, or may not resume the transmission afterwards as another option.
2. Enhanced UL power control
   It is concluded that the potential enhanced UL power control may include UE determining the power control parameter set (e.g. P0, alpha) based on scheduling DCI indication without using SRI, or based on group-common DCI indication. Increased TPC range compared to Rel-15 may also be considered. Power boosting is not applicable to power limited UEs.

Enhanced UL configured grant (grant free) transmissions

1. Multiple active configured grants
   

   Multiple active configured grants for a BWP of a serving cell is identified to be beneficial. It is concluded that multiple active configured grant configurations for a given BWP of a serving cell should be supported at least for different services/traffic types and/or for enhancing reliability and reducing latency. As to repetitions with multiple configurations for a BWP of a serving cell, it is concluded that transmission of a TB based on the configured grant is associated with a single active configuration for both Type 1 and Type 2 configured grant and when multiple active configurations are configured in a BWP in Rel-16, even if the transmission is repeated.

2. Enhancing reliability and reducing latency
   Mechanisms to ensure a sufficient number of repetitions to meet the latency and reliability requirements were studied, including multiple active configured grant configurations for a BWP of a serving cell and repetition(s) across the boundary of a period P. Multiple configured grant configurations has been considered to be used for enhancing reliability and reducing latency. In addition, using single configured grant configuration to enhance reliability and reduce latency was discussed and may be further considered.
3. PUSCH repetitions within a slot for grant free transmission
   It is understood that the potential enhancements for dynamically scheduled PUSCH and configured grant PUSCH should be studied jointly.
4. Explicit HARQ-ACK for UL configured grant transmission
   gNB's missed detection performance of the PUSCH under configured grant is considered as one aspect for evaluating whether to support explicit HARQ-ACK for UL configured grant transmission. It is concluded that there is no consensus on the necessity of explicit HARQ-ACK for configured grant PUSCH for this study item.

Recommendation

1. For PDCCH enhancement, it is recommended to support the following in Rel-16:
   • DCI format(s) with configurable sizes for some fields and potential reduction of the number of bits for some field(s) compared to Rel-15 DCI, while enabling the minimum DCI size target a reduction of 10~16 bits less than the DCI format size of Rel-15 fallback DCI and the maximum DCI size can be larger than Rel-15 fallback DCI, and provide the possibility to align with the size of the Rel-15 fallback DCI (including possible zero padding if any).
   • Increased PDCCH monitoring capability on at least the maximum number of non-overlapped CCEs per slot for channel estimation for Rel-16 NR URLLC for at least one SCS subject to some restrictions, including at least explicit limitation on the maximum number of BDs/non-overlapping CCEs per monitoring occasion and/or per monitoring span, and the set of applicable SCS(s).
2. For UCI enhancement, it is recommended to support the followings in Rel-16:
   • More than one PUCCH for HARQ-ACK transmission within a slot.
   • At least two HARQ-ACK codebooks can be simultaneously constructed for a Rel-16 UE, intended for supporting different service types for a UE.
3. For PUSCH enhancements, it is recommended to support enhancements for both grant-based PUSCH and configured grant based PUSCH in Rel-16, to enable one UL grant scheduling two or more PUSCH repetitions that can be in one slot, or across slot boundary in consecutive available slots.
4. For enhancements to scheduling/HARQ, it is recommended to support the following in Rel-16:
   • Out-of-order HARQ-ACK:
     HARQ-ACK associated with the second PDSCH with HARQ process ID x received after the first PDSCH with HARQ process ID y (x != y) can be sent before the HARQ-ACK of the first PDSCH on the active BWP of a given serving cell.
   • Out-of-order PUSCH scheduling:
     A UE can be scheduled with a second PUSCH associated with HARQ process x starting earlier than the ending symbol of the first PUSCH associated with HARQ process y (x != y) with a PDCCH that does not end earlier than the ending symbol of first scheduling PDCCH on the active BWP of a given serving cell
5. For inter UE Tx prioritization/multiplexing, it is recommended to support UL cancelation scheme and enhanced UL power control scheme in Rel-16.
6. For enhanced UL configured grant transmission, it is recommended to support multiple active configured grant type 1 and type 2 configurations for a given BWP of a serving cell in Rel-16.