1. Viability of location determination in an 802.11 WLAN
Sept 2005
doc.: IEEE /0952r0
Sept 2005
Viability of location determination in an WLAN
Date:
Authors:
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Darwin Engwer, Nortel
Darwin Engwer, Nortel

2. Sept 2005
doc.: IEEE /0952r0
Sept 2005
Abstract
During discussions in 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

3. 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

4. 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

5. 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

6. 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

7. 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 b 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

8. 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

9. 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

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

11. 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

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

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

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

15. 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

16. 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 standard.
Darwin Engwer, Nortel

17. Sept 2005
References
[1] Morrison, J.D., "IEEE Wireless local area network security through location authentication", September 2002, or, Google: “Darwin Engwer” Morrison
[2] Golden, Stuart, "Enabling Localization in WLAN by Time-Stamp Differences (TSD)", IEEE submission , September 2005
Darwin Engwer, Nortel

18. 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