1. DL MU MIMO Error Handling and Simulation Results
Month Year
doc.: IEEE yy/0847r0
DL MU MIMO Error Handling and Simulation Results
Authors:
Date:
John Doe, Some Company

2. Outline Motivation Overview of simulated schemes
DL MU MIMO with scheduled ack
DL MU MIMO with polled ack
Overview of MAC protection
Overview of error handling at the AP
Medium access behavior
Simulation scenarios and parameters
Simulation results
Summary

3. Motivation
Downlink Multi-user MIMO is identified as a key technology to improve the overall network performance
Two DL MU MIMO Ack mechanisms were proposed in /r0
The goals of the simulation are
To study the behavior, including the error recovery behavior, of the two DL MU MIMO ack mechanisms
To study the medium access behavior of the AP when transmitting a DL MU MIMO burst
To evaluate the performance difference of the two DL MU MIMO response mechanisms in different scenarios

4. DL MU MIMO with scheduled Ack
The AP contends for the medium using EDCA
Once a contention is won, the AP transmits a downlink MU MIMO burst to multiple STAs
Each data packet defines an offset value such that each STA knows when to transmit back a BA
Scheduled Ack introduces less overhead but a STA needs to schedule its BA transmission
Data (STA1)
Data (STA3) BA
Data (STA2)
SIFS
SIFS or RIFS
pad

5. DL SDMA scheduled ack schemes do not have an error recovery mechanism
If a STA does not correctly receive an A-MPDU, it does not transmit a BA
There will be a gap in the scheduled response sequence
When a BA is transmitted at certain data rates, there is not enough time for the AP to transmit even a NULL data frame to fill the gap
Other STAs that have not correctly set the NAV may try to contend for the medium during the gap
Data (STA1)
Data (STA2)
Data (STA3) BA
SIFS
SIFS
SIFS BA

6. DL MU MIMO with Polled ACKs
The AP contends for the medium using EDCA
Once a contention is won, the AP transmits a downlink MU MIMO burst to multiple STAs
One STA will transmit a BA immediately after receiving the data packet
The AP sends BAR frames to poll the remaining STAs for BAs
Polled Ack introduces more overhead yet STAs don’t need to schedule BA transmissions
Data (STA1)
Data (STA3) BA
Data (STA2)
BAR
SIFS
pad

7. Error recovery for the polled ack scheme
If the AP senses the medium as idle PIFS after transmitting a BAR frame, it transmits a BAR frame to poll the next STA to which it has transmitted a packet in the DL SDMA burst
PIFS
Data (STA1)
BAR
BAR
BAR
Data (STA2)
Data (STA3) BA
SIFS BA

8. Overview of MAC protection
With MAC protection:
One RTS/CTS exchange at the beginning of a TXOP
One RTS sent to a random STA, the STA replies with a CTS
Without MAC protection
RTS
Data (STA1)
BAR
Data (STA2)
Data (STA3) BA
CTS BA
SIFS
SIFS or RIFS BA
SIFS
SIFS or RIFS

9. Medium access behavior at the AP
As long as one BA is received by the AP, the AP treats the DL SDMA transmission as a success
CW = CWmin
If no BA is received from any STA, the AP treats the DL SDMA transmission as a failure
CW = (CW+1)*2-1
Backoff
(CWmin)
Data (STA1)
Data (STA1)
Data (STA2)
Data (STA2)
Data (STA3) BA BA BA BA
SIFS
RIFS
RIFS BA

10. Simulation scenario and Traffic pattern
One AP and multiple STAs in one BSS
Fully loaded network:
Bi-directional video conferencing traffic (UDP)
Study the network capacity in terms of the network saturation throughput

11. Simulation parameters
One AP (4 antennas), three STAs (each with 2 antennas)
TXOP limit: 3 ms
20MHz: 52 data subcarriers, 4 pilot tones
SIFS=16 us, RIFS=2us, aSlotTime=9 us
Data packet size: 1500 bytes
CWmin=7, CWmax=63 for AC_VI; CWmin=15, CWmax=1023 for AC_BE
Data rates:
With BF only:
802.11a, 16QAM, r=1/2 for control rate (BAR/BA), n MCS15 (64QAM, r=5/6, nSS=2) for data rate
With DL SDMA:
802.11a, 16QAM, r=1/2 for control rate (BAR/BA), n MCS7 (64QAM, r=5/6, nSS=1) for downlink data rate
802.11a, 16QAM, r=1/2 for control rate (BAR/BA), n MCS15 (64QAM, r=5/6, nSS=2) for uplink data rate
Assumption: each STA needs 2 antennas to receive one spatial stream in DL SDMA (MMSE precoding and MMSE receiver for resolvable LTFs)
Training with implicit feedback
Comparison:
DL SDMA with scheduled ACK (SIFS)
DL SDMA with scheduled ACK (RIFS)
DL SDMA with polled ACK

12. Without MAC protection (AC_VI)
Throughput achieved by polled ack scheme is 7-9% higher than scheduled ack schemes

13. Without MAC protection (AC_BE)

14. With MAC protection (AC_VI and AC_BE)

15. Simulation scenario (a hidden-node case)
Fully loaded network
Bi-directional UDP traffic
The AP can transmit simultaneously to STA1, 2, and 3
STA4 cannot decode DL SDMA packets that are intended for STA1, 2, and 3
STA2 has high PER
This can be viewed as STA2 experiences frequent collisions from hidden nodes in another BSS
STA2 is always the second one to respond a BA
STA4
AP1
STA1
STA2
STA3

16. In this scenario, a gap may occur in the response sequence for the scheduled ack scheme
STA4 cannot decode the DL SDMA packets and thus does not set the NAV
STA4 and STA1 are hidden nodes (STA4 cannot receive BA from STA1)
STA4 waits for EIFS after receiving the DL SDMA burst
If scheduled ack is used, because STA4 may transmit during the gap in response sequence, the remaining BA will collide with STA4’s transmission
EIFS = 94us
Data (STA1)
BAR
Data (STA2)
Data (STA3) BA
SIFS BA

17. Polled ack scheme performs better than scheduled ack schemes in this scenario

18. Discussion of the hidden-node scenario
Polled ack scheme performs better than scheduled ack schemes
STA4 cannot set NAV based on DL MU MIMO packets
After receiving a BAR frame transmitted from the AP, STA4 can set the NAV correctly
Because STA4 won’t transmit during the gap in the response sequence, the AP can successfully receive the remaining BA (no unnecessary retransmission needed)
PIFS
EIFS = 94us
Data (STA1)
Data (STA1)
BAR
BAR
BAR
Data (STA2)
Data (STA2)
Data (STA3)
Data (STA3) BA BA
SIFS BA
SIFS BA

19. Summary Three DL MU MIMO response schemes Error recovery
Polled response scheme implements an error recovery mechanism
Scheduled response schemes do not have an error recovery mechanism
MAC protection
One RTS sent from the AP to a randomly selected STA
AP’s medium access behavior
The AP initiates exponential backoff only when no BA response is received (i.e. treated as a traditional transmission failure)
The AP initiates success backoff when at least one BA response is received (i.e. treated as a traditional transmission success)

20. Conclusion
With MAC protection, the three response mechanisms have similar performance
Transmitting RTS/CTS for every TXOP lowers MAC efficiency
The overhead of RTS/CTS becomes more significant if the data rate is higher or the TXOP size is smaller
Polled ack mechanism is more robust
In general, polled ack performs better than scheduled ack schemes when there is no MAC protection
Even for some STAs that are not able to decode DL MU MIMO packets, the BAR frame can still set the NAV at those STAs
The polled ack mechanism is preferred over scheduled ack mechanisms
Lower complexity
More robust (Consistent performance is required to support QoS traffic)

21. Straw Poll #1
Do you prefer the polled ack scheme over the scheduled ack scheme?
Yes No

22. Straw Poll #2
Do you support the AP medium access behavior as described below?
The AP shall consider the DL MU MIMO transmission as a failure when no BA response to the DL MU-MIMO TX  is received
2) The AP may consider the DL MU MIMO transmission as a success when at least one BA response to the DL MU-MIMO TX is received
Yes No