TGn Sync MAC Response to Reasons and Cures Relating to Link Adaptation Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 TGn Sync MAC Response to Reasons and Cures Relating to Link Adaptation Date: 2005-05-16 Authors: Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <stuart.kerry@philips.com> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <patcom@ieee.org>. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Authors (continued): May 2005 Month Year doc.: IEEE 802.11-yy/xxxxr0 Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Comments: MCS Feedback Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Comments: MCS Feedback Summary of Comments #3, 12, 20, 36, 47, 85 Immediate (SIFS) MCS Feedback is complex, adds too much overhead and shows no benefit MCS Feedback must be optional since HCCA simulations showed no performance benefit Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Responses #3, 12, 20, 36, 47, 85 We have previously shown in document 05/1630r1 MCS Feedback provides significant performance advantage over so-called “open loop” schemes that rely on ACK/NAK feedback to adjust MCS. Open loop must back-off on data rate to meet very low latency and PER requirements of media distribution applications. Closed loop MCS feedback Is able to maximize throughput under latency and PER constraints. SIFS feedback is NOT necessary. Can be provided with very low overhead. The following analysis is further evidence of the value and cost/benefit of MCS Feedback. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Channel model B Spatial spreading SNR = 20 dB 2x2 Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Channel model B Spatial spreading SNR = 20 dB 2x2 Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

WLAN Channel Variations Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 WLAN Channel Variations 2x2. Channel model B. Spatial spreading only SNR = 20 dB Significant channel SNR variations Per stream SNR (averaged across frequency bins) for SS varies between 5dB and 20dB. Fading cycle is a few tens of ms. To maintain a target PHY PER of 2% Either, track channel variations with MCS Feedback latency of 10 ms Or, with open loop or long MCS Feedback latency need to back-off from average SNR by 10 dB. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

MCS Feedback Latency and Back-off Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 MCS Feedback Latency and Back-off Study the tradeoff between MCS Feedback latency and PHY Data Rate back-off to maintain target PER. Link simulations Consider fixed back-off values of 1 dB, 2 dB and 3 dB Higher back-off is required to achieve target PER but results in loss of data rate. Limit the size of the fixed back-off at the PHY, since PHY does not know the delay with which the MCS Feedback will be delivered and used by the MAC. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

PHY Rate versus SNR Mean PHY Rate (Mbps) Backoff SNR = 10 dB Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 PHY Rate versus SNR Mean PHY Rate (Mbps) Backoff SNR = 10 dB SNR = 20 dB SNR = 30 dB 1 dB 18 68 140 2 dB 13 63 130 3 dB 9 56 125 Channel Model B, Spatial Spreading, 2x2. Backoff must be increased to meet target PER. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

PER versus MFB Latency for 3 dB Backoff Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 PER versus MFB Latency for 3 dB Backoff Channel Model B, Spatial Spreading, 2x2. At higher SNRs, fixed back-off = 3 dB is insufficient to maintain target PER Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Observations from Link Simulations Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Observations from Link Simulations However, even a fixed 3 dB backoff is insufficient to achieve a target 2% PER if the MCS feedback latency exceeds 10 ms. Require even larger backoff to bring down the PER when the MCS feedback latency is larger than 10-20 ms. Back-off value of 1-2 dB allows the PER target to be met if the latency is less than 10 ms. Low latency MCS feedback to maximize data rate while achieving target PER. This case is studied next using system level simulations with an “outer loop” that adjusts backoff to achieve a target PER. Large MCS feedback latency results in large backoff. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

System level simulation setup Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 System level simulation setup PHY Parameters 2x2 Link Distance: 32 m. SNR = 20 dB Channel model B with no impairments Spatial Spreading System simulation parameters One AP-STA FTP flow; 150 Mbps offered; 1500 B packets; Delay req: 300ms Outer loop targets 2% PER Delayed Block Ack MCS Feedback (MFB) latency varied between 2 to 200 ms. Report MAC goodput (MSDU correct and within delay requirement) Mean PHY mean rate PHY PER Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Impact of MCS Feedback Delay Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Impact of MCS Feedback Delay Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Observations from System Simulation Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Observations from System Simulation With MFB every 20 ms MAC rate is ~15% lower compared to MFB every 2 ms With MFB every 100 ms MAC rate is less than half the rate achieved with MFB every 2 ms Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Complexity of MCS Feedback Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Complexity of MCS Feedback Generation of MCS Feedback is very low complexity. The estimate of post-detection SNR is a by-product of the spatial equalizer that the receiver implements to decode the received frame. Mapping the SNR to MCS feedback introduces little additional implementation complexity. So, immediate MCS feedback comes at virtually no additional cost. MCS feedback latency of 10-20 ms would be sufficient to adapt to SNR variations in typical WLAN channels. Performance results show that immediate MCS feedback (within SIFS) is not necessary. Immediate MCS Feedback is not mandated. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Conclusions on MCS Feedback Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Conclusions on MCS Feedback We find significant performance advantage of low latency MCS Feedback compared to “open loop” MCS feedback latency of 10-20 ms is sufficient to adapt to SNR variations in typical WLAN channels. Immediate MCS Feedback is not mandated. Performance results show that immediate MCS feedback (within SIFS) is not necessary. Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Beamforming and Calibration Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Beamforming and Calibration MAC Related Reasons: 14, 82, 83, 86, 93, 95 Beamforming must be truly optional Calibration related procedures should be optional and part of capability negotiation. Possibly mandatory for AP. Beamforming is not effective for TGn Sync MAC. (Incompatibility of beamforming with MRA) Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company

Response: Beamforming and Calibration Month Year doc.: IEEE 802.11-yy/xxxxr0 May 2005 Response: Beamforming and Calibration A-PPDU and A-MPDU have similar properties regarding beamforming (from MAC point of view) Cannot change beamforming training during the packet. PPDU Bursting is effective with beamforming Each PPDU may contain an aggregate (A-MPDU) Each PPDU directed to a single RA may use beamforming Support for Calibration is optional at the STA We have added a new HT Capability bit “Calibration Supported” Sanjiv Nanda, Qualcomm Inc., et al John Doe, Some Company