Error Recovery Scheme for Scheduled Ack Month Year doc.: IEEE 802.11-yy/xxxxr0 Error Recovery Scheme for Scheduled Ack Date: 2010-05-19 Authors:

Month Year doc.: IEEE 802.11-yy/xxxxr0 Background Two ACK mechanisms for DL MU MIMO are being discussed in TGac Scheduled Ack protocol Polled Ack protocol Pros and Cons for polled ack mechanism More robust when there is no MAC protection and has lower complexity More overhead by sending multiple BAR/BA exchanges yet STAs don’t need to schedule BA transmissions Pros and Cons for scheduled ack Slightly more efficient than polled ack Still there is no solution to fill the gap caused by missing BA  Error recovery mechanism is necessary

Outline Possible DL MU MIMO scenarios using scheduled ack without error recovery mechanism Scheduled ack without error Scheduled ack with an error Introduction of scheduled ack protocol with error recovery Error recovery for invalid BlockAck Error recovery for missing BlockAck Conclusions

Downlink MU-MIMO functions Month Year doc.: IEEE 802.11-yy/xxxxr0 Downlink MU-MIMO functions General Downlink MU-MIMO functions Sounding Group control schemes NAV distribution Beamformed data transmissions Response phase  Scope of this contribution

DL MU MIMO using scheduled acks without error Month Year doc.: IEEE 802.11-yy/xxxxr0 DL MU MIMO using scheduled acks without error By channel access rules MU-MIMO data transmission AP STA1 SIFS BA STA2 SIFS BA STA3 SIFS BA By channel access rules, MU MIMO data transmission follows sounding procedure, group control scheme and NAV distribution procedure After MU-MIMO data transmission, each STA sends its BlockAck frame using SIFS time in sequence

DL MU MIMO using scheduled acks with an error: Invalid BA Month Year doc.: IEEE 802.11-yy/xxxxr0 DL MU MIMO using scheduled acks with an error: Invalid BA By channel access rules MU-MIMO data transmission AP STA1 SIFS BA STA2 SIFS BA STA3 SIFS BA When the AP does not correctly receive an BlockAck frame from STA2, the arrived frame is invalid but the channel is not idle According to the scheduled ack mechanism, the AP has to wait for the arrival of the last BA frame in multiple BA sequence

DL MU MIMO using scheduled acks with an error: Missing BA Month Year doc.: IEEE 802.11-yy/xxxxr0 DL MU MIMO using scheduled acks with an error: Missing BA By medium access rules MU-MIMO data for STA1 MU-MIMO data for STA2 MU-MIMO data for STA3 AP STA1 SIFS BA STA2 SIFS STA3 SIFS BA When STA2 does not correctly receive the beamformed data from AP, it does not send BA and the external channel is idle The probability of collision for the medium is increased during the gap because the AP also waits for the arrival of the last BA frame

Recovery mechanism for Invalid BlockAck Month Year doc.: IEEE 802.11-yy/xxxxr0 Recovery mechanism for Invalid BlockAck By medium access rules MU-MIMO data transmission AP Backoff procedure Group BAR … STA1 SIFS BA STA2 SIFS SIFS BA BA STA3 SIFS SIFS BA BA After the last BA frame in the response sequence is arrived, the AP sends a Group BAR frame for recovery at a rate that is less or equal to the rate of the previous BA frame arrived Group BAR solicits BlockAck from STAs whose BlockAck was not correctly received

Recovery mechanism for Missing BlockAck Month Year doc.: IEEE 802.11-yy/xxxxr0 Recovery mechanism for Missing BlockAck By medium access rules MU-MIMO data for STA1 MU-MIMO data for STA2 MU-MIMO data for STA3 PIFS AP Group BAR STA1 SIFS BA STA2 SIFS SIFS BA STA3 SIFS BA If an AP senses a missing BlockAck frame in response at predefined time, the AP sends Group BAR frame for recovery at PIFS after the previous BlockAck frame Group BAR frame occupies the medium during gap owing to missing BA and the probability of collision is reduced Group BAR may reschedule the BlockAck response sequence

Example of group BlockAckReq frame Month Year doc.: IEEE 802.11-yy/xxxxr0 Example of group BlockAckReq frame Group ID receiving STAs’ information Figure. Group Control Information configuration (example) Group BlockAckReq is a BAR frame containing group control information Group control information contains the Group ID and the receiving STAs’ information Using this information, Group BAR can reschedule the BA response sequence on the wireless circumstance

Conclusion In general, polled ack is more robust than scheduled ack schemes on error prone channels Scheduled Ack mechanism introduces less overhead but a STA needs to schedule its BA transmission and does not have an error recovery mechanism Proposed an error recovery scheme to overcome disadvantages of scheduled ack  It is necessary to consider using two ack protocols for DL MU-MIMO transmission

Month Year doc.: IEEE 802.11-yy/xxxxr0 References [1] Robert Stacey, et. al., DL MU-MIMO ack protocol, IEEE 802.11-09/1172r0, Nov. 16, 2009 [2] Michelle Gong, et. al., DL MU MIMO Error Handling and Simulation Results, IEEE 802.11-10/0324r1, Mar. 15, 2010