1. DL MU-MIMO ack protocol
Month Year
doc.: IEEE yy/xxxxr0
DL MU-MIMO ack protocol
Date:
Authors:
John Doe, Some Company

2. Overview Analyze two ack protocol options for DL MU-MIMO:
Polled acks
Scheduled acks
Look at robustness, efficiency and complexity

3. Polled Acks One user responds immediately to implicit BAR
Other users wait to be polled with BAR frame
Simple: No change to existing protocol
Permitted under existing 11n channel access rules

4. Scheduled Acks All users respond to implicit BAR
Each user has a different offset for response to avoid collision
Need to add ‘offset’ field to Data MPDUs
Slightly more efficient than polled acks
Probably requires NAV protection:
Sequence gaps are possible (missing BA)
 Third party STA may access the medium and collide with scheduled ack

5. Efficiency gain of Scheduled Acks over Polled Acks
TXOP duration
2 STA
3 STA
4 STA
1.5 ms
3.5%
6.9%
10.4%
3 ms
1.7%
5.2%
Assuming 52 us overhead for SIFS+BAR (12 Mbps 11a format BAR)

6. Error recovery with polled acks
Normal error recovery rules apply with polled ack
Sequence is fully under AP control

7. NAV Protection for Scheduled Acks
Need NAV to protect against sequence gaps (missing BA)
L-SIG TXOP protection: has EIFS problem with legacy STAs
Prefer RTS/CTS
RTS/CTS adds overhead, but is beneficial with long TXOP and on a busy network
RTS/CTS acts as a collision detect mechanism: in case of collision, entire TXOP (3ms) is not lost

8. Conclusion Polled ack mechanism is supported under existing 11n rules
Should be supported as baseline mechanism
Proposed enhancements should be compared to this technique
Scheduled acks has some efficiency gain over polled ack
But slightly more complex
Protection overhead and benefit applies to both schemes