Short Training Sequence Compatibility with Legacy 802.11g Devices January 2005 doc.: IEEE 802.11-05/1590r0 January 2005 Short Training Sequence Compatibility with Legacy 802.11g Devices Date: 2004-12-30 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>. Christopher Hansen, Broadcom Christopher Hansen, Broadcom

Background 802.11a/b/g are now heavily deployed January 2005 Background 802.11a/b/g are now heavily deployed 802.11n will operate in a legacy environment Backward compatibility is essential Need a method for legacy stations to know when .11n frames are on the air PHY level protection Multiple antenna preamble that can be received and decoded by legacy stations Christopher Hansen, Broadcom

Multiple Antenna Transmission to Legacy Stations January 2005 Multiple Antenna Transmission to Legacy Stations 802.11n stations will have multiple TX paths Need a method for communicating with legacy stations Transmit on 1 antenna Less efficient Transmit on multiple antennas Cyclic Delay Diversity (CDD)? What is the best delay? Christopher Hansen, Broadcom

Single Antenna Inefficiency January 2005 Single Antenna Inefficiency Power reduction (3 to 6 dB) Range reduction for legacy protection 802.11n MIMO TX Legacy RX Christopher Hansen, Broadcom

Multiple Antenna Benefits January 2005 Multiple Antenna Benefits Legacy STA sees signal from all TX paths More TX power available Nothing turned off No large signal transients on TX paths Greater chance that signal will get through The Challenge? What multiple antenna signal will work with legacy stations? Christopher Hansen, Broadcom

Cyclic Shift of ST Sequence Across Antennas January 2005 Cyclic Shift of ST Sequence Across Antennas STRN1 STRN2 STRNN Tx 1 Tx 2 Tx N STRN1 is the legacy .11a/.11g short training sequence STRN2 through STRNN are cyclic shifted versions of STRN1 Christopher Hansen, Broadcom

Choice of Cyclic Shifts January 2005 Choice of Cyclic Shifts Long cyclic shifts are preferable Short cyclic shifts (50 nS or less) produce correlated signals across TX array Produces variations in power level across tones and directions Leads to “hidden” nodes Long cyclic shifts are less prone to this problem Which cyclic shift is best? Compute power estimation error Verify with real legacy devices Christopher Hansen, Broadcom

Effect of Cyclic Shift on Power Estimation Accuracy January 2005 Effect of Cyclic Shift on Power Estimation Accuracy 400 ns shift minimizes power estimation error Christopher Hansen, Broadcom

Legacy Performance Cyclic shift works with legacy 11g devices January 2005 Legacy Performance Cyclic shift works with legacy 11g devices Legacy devices are able to acknowledge packets with rotated preambles, even at high rates Can infer that legacy devices are able to carry out preamble processing Tested products from multiple 11g chipset vendors Atheros Texas Instruments Intersil Broadcom Christopher Hansen, Broadcom

Test Setup Block Diagram January 2005 Test Setup Block Diagram B A Legacy DUT 2:1 Splitter 2:1 Splitter Attenuator AWG -50 dBm MAC addr = AAA SSID = ‘test’ Driver running MAC addr = BBB SSID = ‘test’ 50dB pad 2:1 Splitter C AWG = Arbitrary Waveform Generator (Agilent E8267C) DUT = Device Under Test Sniffer No driver running MAC addr = AAA (same as packets from AWG) SSID = ‘test’ Christopher Hansen, Broadcom

Test Procedure 6 Mbps test frames created in MATLAB. January 2005 Test Procedure 6 Mbps test frames created in MATLAB. DUT’s destination address/BSSID (unicast), data frame, 5 data bytes long Control: Legacy SISO signal. (i.e. 1 TX antenna) 500 frames created, each convolved with randomly generated channel and saved for playback with the Aribrtrary Waveform Generator (AWG) For each candidate 802.11n STSs: 2 TX antenna case (MISO case) TX 1 antenna has legacy STS. 500 frames created, each convolved with randomly generated channel h1(t) TX2 antenna has cyclically shifted STS. 500 frames created, each convolved with randomly generated channel h2(t) (Note: h1(t) and h2(t) are independent) Signals are combined and saved for playback with the AWG Christopher Hansen, Broadcom

Test Procedure Received level carefully calibrated at legacy DUT. January 2005 Test Procedure Received level carefully calibrated at legacy DUT. DUT will send ACK frames when it is able to receive and correctly decode a frame Count the percentage of ACK’d frames to ascertain performance Since DUT is running a driver, a small number of frames may be lost because of collisions with beacons, etc. Each legacy device will have a different sensitivity point X axis shows relative sensitivity of MISO to legacy, single TX antenna performance Christopher Hansen, Broadcom

Results Performance of different cyclic shifts is presented January 2005 Results Performance of different cyclic shifts is presented 25 and 50 ns rms delay spread channels tested Results reported as a ratio of MISO ACKs (.11n TX) to SISO ACKs (legacy TX) Christopher Hansen, Broadcom

Results (Vendor #1 25nsec rms) January 2005 Results (Vendor #1 25nsec rms) Christopher Hansen, Broadcom

Results (Vendor #1 50 nsec rms) January 2005 Results (Vendor #1 50 nsec rms) Christopher Hansen, Broadcom

Results Vendor #2 (25 nsec rms) January 2005 Results Vendor #2 (25 nsec rms) Christopher Hansen, Broadcom

Results Vendor #2 (50 nsec rms) January 2005 Results Vendor #2 (50 nsec rms) Christopher Hansen, Broadcom

Results Vendor #3 (25 nsec rms) January 2005 Results Vendor #3 (25 nsec rms) Christopher Hansen, Broadcom

Results Vendor #3 (50 nsec rms) January 2005 Results Vendor #3 (50 nsec rms) Christopher Hansen, Broadcom

Results Vendor #4 (25 nsec rms) January 2005 Results Vendor #4 (25 nsec rms) Christopher Hansen, Broadcom

Results Vendor #4 (50 nsec rms) January 2005 Results Vendor #4 (50 nsec rms) Christopher Hansen, Broadcom

January 2005 Conclusions Analytical results show that 400 nsec shift provides best power estimation accuracy Laboratory results show that 400 nsec shift provides the best legacy protection Backward compatible MIMO preambles based on a 400 nsec cyclic shift on the 2nd antenna (2 TX case) are appropriate for 802.11n Christopher Hansen, Broadcom