1. ACK Protection Schemes for the IEEE 802.11ac MU-MIMO Downlink
Month Year
doc.: IEEE yy/xxxxr0
07/10
ACK Protection Schemes for the IEEE ac MU-MIMO Downlink
Date:
Authors:
Haiguang Wang (I2R)
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
07/10
Abstract
In this contributions, we propose an ACK protection scheme for the IEEE ac MU-MIMO downlink. It allows AP to choose an ACK protection scheme based on transmission durations. The aim is to balance the overhead caused by alternative protection schemes and reduce the loss of ACK due to collision.
Haiguang Wang (I2R)
John Doe, Some Company

3. Agenda TERMS Background
07/10
Agenda
TERMS
Background
Using Thresholds for the ACK protection scheme selection.
Summary
Haiguang Wang (I2R)

4. Terms 07/10 Term Definition ACK Acknowledgment BA
Block Acknowledgement
BAR
Block Acknowledgement Request BS
Base Station
CSMA/CA
Carrier Sensing Multiple Access / Collision Avoidance
CTS
Clear-To-Send
DCF
Distributed Coordinate Functioning
DIFS
DCF Interframe Space
IEEE
Institute of Electrical and Electronic Engineering
MAC
Media Access Control
MU-MIMO
Multi-User Multi Input Multi Output
OFDMA
Orthogonal Frequency-Division Multiple Access
RIFS
Reduced Interframe Space
RTS
Require-To-Send
SIFS
Short Interframe Space
STA
Station
Haiguang Wang (I2R)

5. Downlink Multi-user MIMO
07/10
Downlink Multi-user MIMO
Downlink Multi-user MIMO is identified as a key technology to improve the overall network throughput.
The base station can transfer data packets to multiple stations simultaneously.
Haiguang Wang (I2R)

6. 07/10
Scheduled ACKs
The receiving stations send back acknowledgement based on certain schedule in order to avoid collision
New fields are required in Data MPDUs
Haiguang Wang (I2R)

7. Known issues with Scheduled ACKs
07/10
Known issues with Scheduled ACKs
Legacy stations cannot extract NAV information
If a STA does not correctly receive an A-MPDU, then it does not transmit a BA. This may lead to ACKs loss due to collision.
There will be a gap in the scheduled response sequence
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
Haiguang Wang (I2R)

8. 07/10
Existing Solutions
A few ACK protection mechanisms were proposed previously. They are:
Polled ACKs
Scheduled ACK with protection
CTS-to-Self
RTS/CTS
Haiguang Wang (I2R)

9. Polled ACKs 07/10 One user responds immediately to implicit BAR
Other users wait to be polled with BAR frame
No change is required to existing protocol
Haiguang Wang (I2R)

10. Using RTS/CTS to Protect the ACKs
07/10
Using RTS/CTS to Protect the ACKs
AP transmits RTS/CTS to protect both the data transmission and ACKs.
Haiguang Wang (I2R)

11. Using CTS-to-Self Protect the Scheduled ACK Transmission
07/10
Using CTS-to-Self Protect the Scheduled ACK Transmission
Using CTS-to-Self to protect the transmission of ACKs.
Less overhead compare to the RTS/CTS scheme.
Haiguang Wang (I2R)

12. Issues with Existing Solutions
07/10
Issues with Existing Solutions
Polled ACK and RTS/CTS protection cause overhead.
Overhead is significant when TXOP is short.
Protection with CTS-to-Self is not good enough sometimes when transmission duration is long.
Haiguang Wang (I2R)

13. 07/10
Current Status
No single solution can handle both the ACK protection and overhead issue well.
No protection: can cause ACKs loss due to collision.
Polled ACK, RTS/CTS protection: cause severe overhead when transmission duration is short.
CTS-to-Self: not robust enough when transmission duration is long.
Haiguang Wang (I2R)

14. Using Thresholds for ACKs Protection Schemes Selection
07/10
Using Thresholds for ACKs Protection Schemes Selection
Haiguang Wang (I2R)

15. 07/10
Our Proposal
Using multiple schemes for protections but selection based on thresholds. The candidate protection schemes in consideration are :
No protection
Using scheduled ACK scheme directly.
CTS-to-Self.
RTS/CTS or polled ACK.
Haiguang Wang (I2R)

16. Single Threshold Scheme
07/10
Single Threshold Scheme
Using a single threshold, H0, to select the protection scheme.
D0 to denote transmission duration with scheduled ACK
Dx to denote transmission duration when using protection scheme x.
We calculate the protection overhead Hx as follows:
Hx = (Dx – D0)/D0
Only if Hx < H0, the AP can use the protection scheme x for ACK protection.
If there are multiple schemes satisfy the condition, then AP can choose one of them for protection.
If there is no scheme satisfy the condition, then the AP just transmit the packets without protection.
Haiguang Wang (I2R)

17. 07/10
Two Thresholds Scheme
Using two thresholds, L1 < L2, for ACKs protection schemes selection.
No protection when transmission duration is shorter than L1.
Using CTS-to-Self to protect the ACKs when transmission duration is longer than L1 but less than L2.
Using RTS/CTS or polled ACK when the transmission duration is longer than L2.
Haiguang Wang (I2R)

18. 07/10
Summary
The proposed scheme provides a better way to protect the ACKs for MU-MIMO downlink.
Haiguang Wang (I2R)

19. 07/10
Q&A
Haiguang Wang (I2R)

20. 07/10
References
Robert Stacey, Ali Raissinia, Yong Liu, James Cho, Matt Fischer, “DL MU-MIMO ack protocol”, IEEE /1172r0.
Michelle Gong, Robert Stacey, James Cho, “DL MU MIMO Error Handling and Simulation Results”, IEEE /0324r1.
Yun Joo Kim, Jae Seung Lee, Jeeyon Choi, Jaewoo Park, Sok-kyu Lee, Min-ho Cheong, “Considerations on MU-MIMO Protection in 11ac”, IEEE /0335r1.
Coupechoux, Marceau, Fiat, Lionel, “Method for improving handovers in a WLAN”, US Patent 7,545,776
Haiguang Wang (I2R)