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3GPP Release 16 features for URLLC in 5G
Release 16 where the physical layer work was completed in December 2019 continued to develop further the physical layer design for URLLC to deal with the unsolved problems in Release 15 as well as support Industrial Internet of Things with more stringent requirements (higher reliability of 106, lower latency of 0.5 to 1 ms) in some URLLC use cases of Release 16: factory automation, transport industry including the remote driving use case and electrical power distribution.
Physical downlink control channel (PDCCH) enhancements
As presented in Section 1.3.1.1, sub-slot-based transmission is one of the features in Release 15 of URLLC. In DL transmission, this feature requires the UE to monitor DL data including PDCCH and PDSCH in subslot level. The location of PDSCH is indicated by PDCCH so the UE needs to decode PDCCH before decoding PDSCH. However, the UE does not know the exact location of PDCCH so it carries out blind decoding in a search space. Each possible location of PDCCH in the search space is called PDCCH candidate. However, in Release 15, the number of PDCCH candidates that the UE can monitor in a slot is limited as shown in Table 1.2. Moreover, the resource for PDCCH in a slot is also limited as shown by the number of control channel elements (CCEs) in Table 1.2. A CCE consists of 6 resource element groups. A resource element group equals to one resource block during one OFDM symbol that contains 12 resource elements. The number of CCEs that a PDCCH has is dened as the aggregation level (AL) (for example, 1 CCE is AL 1, 2 CCEs are AL 2). The transmission might be in sub-slot level while PDCCH monitoring capability is only dened in slot level. This limit degrades the ability of the UE to operate in sub-slot-based transmission when not all PDCCHs can be transmitted from the gNB and monitored by the UE. For example, if the gNB transmits PDCCH in a sub-slot of 2 OFDM symbols with SCS of 60 kHz, the UE has 7 occasions to monitor PDCCH in a slot of 14 symbols. Therefore, the UE, on average, only can monitor 3 PDCCH candidates and 7 non-overlapping CCEs per sub-slot based on Table 1.2. When AL 8 (8 CCEs) is needed to guarantee PDCCH reliability, there is not enough CCEs for that PDCCH to be transmitted and monitored in a sub-slot. Moreover, with 3 PDCCH candidates per sub-slot, if the UE monitors 2 PDCCH candidates with AL 2 and 1 PDCCH candidate with AL 4, it is not capable of monitoring another PDCCH candidate with AL 8 so this PDCCH AL 8 is dropped or PDCCH with a lower AL is used that decreases reliability.
Multiple PUCCHs for hybrid automatic repeat request-acknowledgement (HARQ- ACK) within a slot
DL transmission in sub-slot level that is featured in Release 15 requires an improvement in feedback transmission. The UE is expected to transmit feedback on sub-slot level as DL data because a fast Negative acknowledgment (NACK) feedback on sub-slot level reduces the reception time of feedback at the gNB and guarantees a retransmission in latency budget of URLLC. However, in Release 15, a UE is able to transmit only one PUCCH with HARQ-ACK information in a slot. If the UE nishes decoding process of a packet after the PUCCH resource for HARQ feedback in a slot, it must wait until the next slot to transmit feedback that delays feedback transmission and a retransmission if necessary. Moreover, if HARQ-ACK for URLLC PDSCH occurs in the same slot as HARQ-ACK for other eMBB/URLLC PDSCHs, all the HARQ-ACK information will be multiplexed together and transmitted over the PUCCH resource indicated in the latest DL assignment. The multiplexing degrades the reliability of HARQ feedback.
In Release 16, therefore, sub-slot-based HARQ-ACK feedback procedure is supported where PUCCH resources are congured per sub-slot of 2 or 7 symbols so multiple PUCCHs for HARQ-ACK can be transmitted within a slot. Any sub-slot PUCCH resource is not across sub-slot boundaries and no more than one transmitted PUCCH carrying HARQ-ACK starts in a sub-slot. In this way, HARQ-ACK feedback is also transmitted in sub-slot level to match with DL transmission in sub-slot level.
UCI intra-UE multiplexing
In Release 15, the number of PUCCHs transmitted by a UE in a slot is limited to 2. Therefore, when the UE has multiple overlapping PUCCHs in a slot or overlapping PUCCHs and PUSCHs in a slot, the UE multiplexes dierent UCI types in one PUCCH/PUSCH. However, in URLLC transmission, low latency requires urgent schedules that cause an overlap of URLLC UCI with PUCCH/PUSCH of a dierent type services with lower priority where the multiplexing causes a degradation of the URLLC transmission. Moreover, if the ending symbol of the multiplexing PUCCH/PUSCH is later than the ending symbol of URLLC UCI, it causes an additional delay to URLLC transmission. For these reasons, the behavior of the UEs must be specied to guarantee URLLC service.
In Release 16, the behaviors of the UE are standardized following UCI prioritization based on two-level priority so that if there is an overlap between two low priority (LP) and high priority (HP) UL transmissions, the LP UL transmission such as eMBB PUSCH/PUCCH is cancelled instead of being multiplexed with the HP UL transmission such as URLLC PUSCH/PUCCH. In the non-overlapping cancelled symbols of the LP UL transmission, the UE is not scheduled to transmit. In case the UE encounters the intra-collision of more than two UL PUSCH/PUCCH transmissions, the UE resolves collision between UL transmissions with same priority by UCI multiplexing then resolves collision between UL transmission with dierent priorities by UCI prioritization.
3GPP Release 17 features for URLLC in 5G
In Release 16, PUCCH repetitions are done in slot level where there is only one PUCCH repetition per slot and PUCCH repetitions cannot cross slot boundary. The reliability of PUCCH can be enhanced by allowing PUCCH repetitions in sub-slot level as PUSCH repetition Type B in Release 16. There are more PUCCH repetitions allowed in URLLC latency constraint and a long PUCCH can also be segmented to small PUCCH repetitions to cross slot boundary.
In Release 17, if the feedback for a DL SPS transmission is pointed to a DL symbol instead of an UL symbol in time division duplex (TDD) conguration, the SPS feedback is deferred to the next available PUCCH based on semi-static conguration of slot format. There is a limit on the maximum deferral of the SPS feedback that is congured by the gNB based on data requirements.
URLLC enhancements in unlicensed spectrum
In Release 15 and 16, URLLC is specied to operate only in licensed spectrum. However, due to new use cases in the industrial scenario, unlicensed spectrum becomes a complement to URLLC operation in licensed spectrum. One important use case is the industrial automation in controlled environments with restricted access. The features of transmission in unlicensed spectrum have been specied since Release 13. However, the features of unlicensed spectrum do not take into account the features of URLLC specied in Release 15 and 16. This incompatibility requires the work in the ongoing Release 17 to harmonize the features of unlicensed spectrum and URLLC so that URLLC can operate in unlicensed spectrum and still attains the latency and reliability requirements.
In unlicensed spectrum, a transmitter is required to do Listen before talk (LBT) through the channel access mechanisms to access to the channel and transmit data in the duration of channel occupancy time (COT). One of the channel access mechanisms is frame based equipment (FBE) where the transmitter is allowed to do LBT in the xed moments. The periodicity between two consecutive LBT moments is a xed frame period (FFP) from 1ms to 10ms. In Release 16, only the gNB is allowed to initiate a COT by doing LBT in the xed moments. After obtaining the channel, the gNB might share the COT to the UE so that it can transmit the UL transmission. This may cause long latency in UL transmission due to two reasons.
First, if LBT fails, the gNB must wait from 1ms to 10ms to do LBT in the next moment. In that interval, the UE also cannot start its UL transmission because no COT is initiated by the gNB. Second, if the gNB has no DL data to transmit, it does not initiate a COT. If the UE has UL data at that time, it also cannot transmit because of the absence of the gNB-initiated COT. Therefore, to reduce latency and support URLLC in unlicensed spectrum, in Release 17, the UE is allowed to initiate its own COT to transmit UL data. The UE is able to determine whether a scheduled UL transmission is transmitted according to the shared gNBinitiated COT or UE-initiated COT based on a predetermined rule. If the transmission is conned within a gNB FFP before the idle period of that gNB FFP and the UE has already determined that gNB initiated that gNB FFP, the UE assumes that the congured UL transmission corresponds to gNB-initiated COT. Otherwise, the UE assumes that the congured UL transmission corresponds to a UE-initiated COT. The FFP parameters of the UE-initiated COT such as the period of FFP, oset of FFP’s starting point (having a symbol granularity) can be provided to the UE by dedicated RRC.
In unlicensed spectrum, when a UE transmits data in CG resources, it can be congured to transmit UCI containing redundancy version, HARQ identity and new data indicator in parallel with data in PUSCH.
The gNB also can transmit ACK feedback to the UE when it decodes correctly the packet. Both the uses of CG UCI and CG ACK are enabled or disabled for unlicensed using one RRC parameter i.e. cg
Table of contents :
Abstract
Acknowledgements
Abbreviations
1 Introduction
1.1 5G New Radio overview
1.2 URLLC requirements
1.3 Physical layer design for URLLC in 3GPP Releases 15, 16, and
1.3.1 3GPP Release 15 – the foundation for URLLC in 5G
1.3.2 3GPP Release 16 features for URLLC in 5G
1.3.3 3GPP Release 17 features for URLLC in 5G
1.4 Thesis outline and contributions
1.5 Thesis perspective from practical (3GPP works) and more fundamental aspects
2 Ensuring Latency and Reliability of the UL Congured Grant transmissions
2.1 Problem formulation
2.2 Related works
2.3 Optimal reserved resources to ensure K repetitions
2.3.1 Reserved resources
2.3.2 System model
2.3.3 Collision probability in reserved resources
2.3.4 Group access to the reserved resources
2.3.5 Optimal reserved resources with a successive interference cancellation (SIC) receiver at the gNB
2.4 Explicit HARQ feedback structure to reduce packet loss in the less-than-K-repetition situation
2.4.1 Operation of the explicit HARQ feedback structure
2.4.2 Design of the explicit HARQ feedback
2.5 Additional SR to reduce packet loss in the less-than-K-repetition situation
2.6 Numerical results and performance evaluation
2.6.1 Optimal reserved resources
2.6.2 Explicit feedback structure and additional SR in less-than-K-repetition transmission
2.7 Conclusion
3 UL eMBB and URLLC multiplexing
3.1 Problem of multiplexing URLLC and eMBB in the CG resources
3.2 Related works
3.3 Strategy to multiplex the eMBB and URLLC UEs in the CG resources
3.3.1 The overlap indication and the explicit HARQ ACK feedback
3.3.2 The overlap indication and the additional SR
3.3.3 Conguration and Signalling for the Overlap Indication
3.3.4 Design of the Explicit HARQ Feedback
3.4 Numerical results and performance evaluation
3.5 Conclusion
4 Feedback Enhancements for Downlink Semi-Persistent Scheduling Transmissions in Ultra- Reliable Low-Latency Communication
4.1 Feedback cancellation in DL SPS transmission in TDD conguration
4.2 Related works
4.3 Enhancements for HARQ feedback in DL SPS transmission in TDD
4.3.1 Dynamically indication of K1 value for each DL SPS transmission
4.3.2 ACK-only feedback structure
4.4 Numerical results
4.5 Conclusion
5 Load based channel access enhancements in unlicensed spectrum for NR URLLC trans- missions
5.1 Load based channel access mechanism
5.2 Related works
5.3 Analysis of Type 1 channel access procedures in unlicensed spectrum
5.3.1 System model
5.3.2 Probabilities of the states in Markov chain for Type 1 channel access procedures
5.3.3 Transmitter’s average channel access time in Type 1 channel access procedures
5.3.4 DL and UL transmissions’ latency in unlicensed spectrum
5.4 Conditions and enhancements in using Type 1 channel access procedures
5.4.1 Numerical results of the impact of channel access on URLLC transmission .
5.4.2 New proposed tables of channel access priority class for URLLC DL and UL transmission
5.5 Conclusion
6 Frame based channel access enhancements in unlicensed spectrum for NR URLLC trans- missions
6.1 Frame based channel access mechanism
6.2 Related works
6.3 Analysis of FBE in unlicensed spectrum
6.3.1 System model
6.3.2 Probabilities of the states and channel access in Markov chain for FBE channel access
6.3.3 Relation between the probability of no data and the probability of sensing a busy channel
6.3.4 URLLC operation with FBE in unlicensed spectrum
6.4 Multiple congurations of FFP in FBE for URLLC in unlicensed spectrum
6.4.1 Multiple congurations of FFP
6.4.2 The Markov chain of FBE channel access with multiple congurations of FFP
6.5 FFP arrangement based on the transmitter’s priority
6.6 Numerical and simulation results
6.7 Conclusion
7 Dynamic switching between load based and frame based channel access mechanisms in unlicensed spectrum
7.1 Markov chain model for the coexistence of the devices using LBE and FBE in unlicensed spectrum
7.1.1 System model
7.1.2 LBE’s model
7.1.3 FBE’s model
7.1.4 Coexistence of LBE and FBE’s model
7.2 Dynamic switch between LBE and FBE at the UE in unlicensed spectrum
7.2.1 Switch from FBE to LBE
7.2.2 Switch from LBE to FBE
7.3 Numerical results
7.4 Conclusion
8 Enhancements of PUSCH repetitions for URLLC in unlicensed spectrum
8.1 Gap in the middle of PUSCH repetitions
8.1.1 Gap due to UL/DL directions
8.1.2 Gap due to orphan symbols
8.2 Related works
8.3 Enhancements of PUSCH repetitions in licensed and unlicensed spectrum
8.3.1 Handling gap due to UL/DL directions
8.3.2 Handling orphan symbols
8.4 Simulation results
8.4.1 Performance of the scheme to handle UL/DL directions
8.4.2 Performance of the scheme to handle orphan symbols
8.5 Conclusion
9 Conclusions
9.1 Concluding remarks
9.2 Future perspectives
References
