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Error Recovery Scheme for Scheduled Ack

Error Recovery Scheme for Scheduled Ack. Date: 2010-0 5 - 19. Authors:. 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

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Error Recovery Scheme for Scheduled Ack

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  1. Error Recovery Scheme for Scheduled Ack Date: 2010-05-19 Authors:

  2. 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

  3. 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

  4. Downlink MU-MIMO functions • General Downlink MU-MIMO functions • Sounding • Group control schemes • NAV distribution • Beamformed data transmissions • Response phase  Scope of this contribution

  5. DL MU MIMO using scheduled acks without error • 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 By channel access rules MU-MIMO data transmission AP SIFS STA1 BA SIFS STA2 BA SIFS STA3 BA

  6. DL MU MIMO using scheduled acks with an error: Invalid 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 By channel access rules MU-MIMO data transmission AP SIFS STA1 BA SIFS STA2 BA SIFS STA3 BA

  7. DL MU MIMO using scheduled acks with an error: Missing 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 By medium access rules MU-MIMO data for STA1 MU-MIMO data for STA2 MU-MIMO data for STA3 AP SIFS STA1 BA SIFS STA2 SIFS STA3 BA

  8. Recovery mechanism for Invalid BlockAck • 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 By medium access rules MU-MIMO data transmission Backoff procedure AP Group BAR … SIFS STA1 BA SIFS SIFS STA2 BA BA SIFS SIFS STA3 BA BA

  9. Recovery mechanism for Missing BlockAck • 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 By medium access rules MU-MIMO data for STA1 MU-MIMO data for STA2 MU-MIMO data for STA3 PIFS AP Group BAR SIFS STA1 BA SIFS STA2 SIFS BA STA3 SIFS BA

  10. Example of group BlockAckReq frame • 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 Group ID receiving STAs’ information Figure. Group Control Information configuration (example)

  11. 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

  12. 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

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