Viability of location determination in an 802.11 WLAN Sept 2005 doc.: IEEE 802.11-05/0952r0 Sept 2005 Viability of location determination in an 802.11 WLAN Date: 2005-09-20 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>. Darwin Engwer, Nortel Darwin Engwer, Nortel

Sept 2005 doc.: IEEE 802.11-05/0952r0 Sept 2005 Abstract During discussions in 802.11 TGv, questions were raised as to the viability of various accurate ranging measurement techniques which could in turn be used for determining the location of a mobile STA wrt a given set of APs. This submission surveys and summarizes a few of the available ranging measurement techniques. Darwin Engwer, Nortel Darwin Engwer, Nortel

Gamut of Location Accuracy Measurement Approaches Sept 2005 Gamut of Location Accuracy Measurement Approaches Note that the actual implementation of any approach will vary depending on the PHY type. * Location determination to just the current AP may be adequate for some applications. Darwin Engwer, Nortel

Locating an MU between three APs Sept 2005 Locating an MU between three APs BSS BSS BSS BSS MU AP#1 AP#2 BSS AP#3 BSS BSS Darwin Engwer, Nortel

Time-of-flight (TOF) Method Sept 2005 Time-of-flight (TOF) Method Seeks to measure the distance between the MU and n APs by directly measuring the over-the-air time-of-flight for a frame exchange sequence between a pair of devices. During discussions in TGv, questions were raised as to the viability of performing accurate TOF ranging measurements. let’s examine … Darwin Engwer, Nortel

Basis for Time-of-Flight (TOF) Approach Sept 2005 RTS DATA AP Tx CTS’ ACK’ AP Rx AP Gate RTS’ DATA’ MU Rx CTS ACK MU Tx tPD tPD tMUP tCTS tGATE tGATE = (2 * tPD) + tMUP + tCTS Note: This timing diagram is not to scale. Darwin Engwer, Nortel

Example TOF Method tCTS is a (known) constant Sept 2005 Example TOF Method tCTS is a (known) constant tPD is the over-the-air propagation delay time, which is directly proportional to the separation distance between the two devices (speed of light, approx. 3 ns/m) tMUP is typically 5 orders of magnitude greater than tCTS but, observe that the MU processing time (tMUP) is quantized by the MAC state machine’s synchronous clock - e.g. 44 MHz for the 802.11b PHY therefore, by taking a number of measurements across multiple frame exchange sequences and applying a bit of statistics one can factor out tMUP , leaving 2*tPD, which is the desired ranging measurement. Darwin Engwer, Nortel

TOF Statistical Method Sept 2005 TOF Statistical Method Requires only the addition of a hires timer (ns accuracy) at the end of the link where it is desired to do the ranging measurement. hence existing MU implementations can be used in conjunction with a new “ranging measurement capable” AP (or vice versa) Darwin Engwer, Nortel

Sept 2005 TOF “TSD” Method Stuart Golden’s TOF TSD method [2] is, I believe, based on including the value of tMUP in the reply frame. Using that approach, a single frame exchange sequence can yield the desired ranging measurement. Requires: the addition of a hires timer (ns accuracy) at the end of the link where it is desired to do the ranging measurement remote end modification to include the processing delay time in the response frame Darwin Engwer, Nortel

Viability of the TOF Method Sept 2005 Viability of the TOF Method let’s look at some actual test data ... Darwin Engwer, Nortel

Test Data for TOF Statistical Method Sept 2005 Test Data for TOF Statistical Method Scope trace showing AP Tx and AP Rx (w/ RTS, CTS, Data, ACK) and AP_Gate ([1] Figure 5.6, page 51) Round trip time measurements ([1] Figure 5.8, page 56) Number of measurements vs. resolution ([1] Figure 5.10, page 59) Darwin Engwer, Nortel

TOF measurements: AP Tx, AP Rx and AP Gate Sept 2005 TOF measurements: AP Tx, AP Rx and AP Gate Darwin Engwer, Nortel

TOF: Round trip time measurements Sept 2005 TOF: Round trip time measurements Darwin Engwer, Nortel

TOF: Number of measurements vs. resolution Sept 2005 TOF: Number of measurements vs. resolution Darwin Engwer, Nortel

Time-of-flight (TOF) Summary Sept 2005 Time-of-flight (TOF) Summary Using time-of-flight as an accurate ranging mechanism for the basis of location determination is viable. QED. Darwin Engwer, Nortel

Sept 2005 Summary This submission does not advocate a particular approach to location determination in a WLAN. This submission provides an overview of the ranging measurement field and in the process demonstrates that location determination is a viable area for consideration for future amendments to the 802.11 standard. Darwin Engwer, Nortel

Sept 2005 References [1] Morrison, J.D., "IEEE 802.11 Wireless local area network security through location authentication", September 2002, http://cisr.nps.navy.mil/downloads/theses/02thesis_morrison.pdf, or, Google: “Darwin Engwer” Morrison [2] Golden, Stuart, "Enabling Localization in WLAN by Time-Stamp Differences (TSD)", IEEE 802.11 submission 11-05-0891-00, September 2005 Darwin Engwer, Nortel

Additional comments on ref [2] Sept 2005 Additional comments on ref [2] using superfluous probe requests/ probe response frame exchange sequences could place an undue burden or load on the network - just for the MUs to be able to determine their current location other frame exchange sequences can be used that do not add additional load to the network Darwin Engwer, Nortel