ACK Protection Schemes for the IEEE 802.11ac MU-MIMO Downlink Month Year doc.: IEEE 802.11-yy/xxxxr0 07/10 ACK Protection Schemes for the IEEE 802.11ac MU-MIMO Downlink Date: 2010-07-12 Authors: Haiguang Wang (I2R) John Doe, Some Company

Month Year doc.: IEEE 802.11-yy/xxxxr0 07/10 Abstract In this contributions, we propose an ACK protection scheme for the IEEE 802.11ac 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

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

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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

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)

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)

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)

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

07/10 Q&A Haiguang Wang (I2R)

07/10 References Robert Stacey, Ali Raissinia, Yong Liu, James Cho, Matt Fischer, “DL MU-MIMO ack protocol”, IEEE 802.11-09/1172r0. Michelle Gong, Robert Stacey, James Cho, “DL MU MIMO Error Handling and Simulation Results”, IEEE 802.11-10/0324r1. Yun Joo Kim, Jae Seung Lee, Jeeyon Choi, Jaewoo Park, Sok-kyu Lee, Min-ho Cheong, “Considerations on MU-MIMO Protection in 11ac”, IEEE 802.11-10/0335r1. Coupechoux, Marceau, Fiat, Lionel, “Method for improving handovers in a WLAN”, US Patent 7,545,776 Haiguang Wang (I2R)