Enhancing UHR Latency Support with Preemption in IEEE 802.11-23
Explore the detailed discussion on low latency support in the UHR group, focusing on preemption as a key feature for improving latency in IEEE 802.11-23. Learn about R-TWT mechanisms, multi-AP coordination, and preemption strategies for deterministic and event-based low latency traffic. Discover how preemption within PPDU and across PPDU can efficiently reduce delays for low latency transmissions in UHR settings.
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February 2023 doc.: IEEE 802.11-23/0018r1 Low Latency Support in UHR Date: 2023-02-05 Authors: Name Kiseon Ryu Liwen Chu Huizhao Wang Rui Cao Hongyuan Zhang Affiliations NXP Semiconductors Address Phone email kiseon.ryu@nxp.com Submission Slide 1 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Introduction Latency reduction is one of the main requirement to be improved in the UHR group as described in the approved motion for UHR SG formation. Various UHR contributions were discussed for latency improvement with the following potential technical direction. Preemption [1], [2] AI/ML [3] M-AP coordination [4], [5] Overhead reduction [6] In this contribution, we discuss potential UHR features for low latency support, and especially preemption . Submission Slide 2 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 R-TWT in UHR R-TWT in 11be is a mechanism to protect and prioritize the low latency traffic by allocating R-TWT SPs and overlapping quiet intervals. In 11be WG LB, we had many technical comments for R-TWT which were not reached the group consensus. Some UHR contributions addressed potential improvement on R-TWT such as TWT Information sharing among neighboring APs and protection of neighbor AP s R-TWT SP [4] Multi-AP coordinated resource reservation and protection for low latency traffic [5] Improvement on R-TWT is quite straightforward to support low latency in UHR with the following technical items. R-TWT SP coordination among OBSSs R-TWT SP protection of neighbor APs R-TWT SP protection of other APs in the same Multiple BSSID set R-TWT SP extension Etc. Submission Slide 3 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Preemption For a deterministic low latency traffic, 11be R-TWT and SCS can be used to support the deterministic low latency traffic. When R-TWT SPs are scheduled, the channel can be occupied by other STAs such as non-R-TWT STAs or OBSS STAs during the R-TWT SP. For an event-based low latency traffic, The issue of non-deterministic low latency traffic was discussed in [2] In UHR, an AP needs to schedule at any time an event-based and/or deterministic low latency traffic by preempting the ongoing transmission or the TXOP. Preemption can be used; In case of long PPDU transmission, a TXOP holder can transmit the low latency frame before finishing the current transmission. Preemption within a PPDU In case of low latency frame buffered at a non-TXOP holder, the low latency frame can be transmitted before finishing the current TXOP. Preemption within a TXOP Submission Slide 4 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Preemption within a PPDU Preemption by a TXOP holder TXOP holder is transmitting non-low latency traffic when the low latency traffic arrives. To reduce delay of low latency traffic, the low latency traffic can be sent by interrupting the current PPDU transmission. Two types of preemption can be considered Aggregation in a PPDU being transmitted Two-dimensional resource-based preemption was introduced [1] High efficiency and high complexity Transmission in a separate PPDU [2] Relatively low efficiency and low complexity Example of A-PPDU for preemption [1] Low latency traffic arrives Example of a separate PPDU for preemption [2] Submission Slide 5 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Preemption within a TXOP During other STA s TXOP, low latency data frame may be buffered at a non-TXOP holder. Low latency data for STA2 CTS BA AP Non-Low latency data (AP) STA1's TXOP RTS STA1 STA2 A non-TXOP holder should be able to transmit a low latency data frame by interrupting the current TXOP. Submission Slide 6 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Preemption within a TXOP Possible Cases Case 1: DL low latency data TX in a UL TXOP TXOP responder (AP) needs to preempt the UL TXOP. Case 2: UL low latency data TX in a DL TXOP During a DL TXOP, an AP needs to transmit a Trigger frame for scheduling UL resource for UL low latency traffic STA1s TXOP APs TXOP LL data for STA2 LL data CTS BA TF- R M- BA Non-LL data RTS AP AP Non-LL data RTS CTS BA STA1 STA1 LL data for AP LL data BA STA2 STA2 Preempted TXOP Preempted TXOP Case 3: UL low latency data TX in a UL TXOP A TXOP holder (non-AP STA) needs to allocate a time period for an AP to schedule a UL resource for UL low latency traffic. Case 4: Low latency data TX in an OBSS TXOP Aggressive Spatial Reuse can be defined for an AP/non-AP STA to transmit a low latency frame during an OBSS TXOP. STA1s TXOP CTS BA OBSS AP TF- R M- BA CTS BA AP RTS Non-LL data OBSS STA Non-LL data RTS STA1 OBSS TXOP LL data LL data for AP LL data LL data AP or STA STA2 Preempted TXOP Submission Slide 7 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Preemption within a TXOP Coexistence With SCS An AP may pre-empt a TXOP to schedule the periodic low latency traffic stream (DL/UL/Direct Link) which has been established between the AP and a non-AP STA using the SCS procedure. With TWT An AP may pre-empt a TXOP during an individual and/or a broadcast TWT SP such as an R-TWT SP if a non-AP STA is a TXOP holder of the TXOP. Submission Slide 8 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Further consideration Max. PPDU length limitation for non-low latency traffic Channel access with various IFSs applicable to low latency and non-low latency traffic Time period reservation within a TXOP for low latency traffic Etc. Submission Slide 9 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 Summary Latency reduction is one of the main requirement in UHR. Improvement on 11be R-TWT in UHR is straightforward. For better support of low latency traffic, a TXOP can be preempted by a (non-)TXOP holder during the DL/UL/OBSS TXOP. Submission Slide 10 Kiseon Ryu, NXP
February 2023 doc.: IEEE 802.11-23/0018r1 References [1] 11-22/1393, Latency Reduction Scheme for UHR [2] 11-22/1880, Latency and Reliability enhancements for UHR [3] 11-22/1519, Requirements of Low Latency in UHR [4] 11-22/1530, Multi AP coordination for next-generation Wi-Fi [5] 11-22/1556, Multi-AP Coordination for Low Latency Traffic Delivery [6] 11-22/1926, Challenges to achieve deterministic low latency in next generation 802.11 Submission Slide 11 Kiseon Ryu, NXP
