1. Error Recovery Scheme for Scheduled Ack
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Error Recovery Scheme for Scheduled Ack
Date:
Authors:

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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
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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
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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

6. DL MU MIMO using scheduled acks with an error: Invalid BA
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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

7. DL MU MIMO using scheduled acks with an error: Missing BA
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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

8. Recovery mechanism for Invalid BlockAck
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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

9. Recovery mechanism for Missing BlockAck
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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

10. Example of group BlockAckReq frame
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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

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

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