September 2005

Stuart Golden, Intel Corp.

Slide 1

doc.: IEEE 802.11-05/0891r0

Submission

Enabling Localization in WLAN by Time-Stamp Differences (TSD)

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>.

Date: 2005-09-15

Name Company Address Phone email Stuart Golden Intel Corp. 2111 NE 25th Ave.,

Hillsboro, OR 97124

(503) 712-8088

stuart.golden@intel.com

Authors:

September 2005

Stuart Golden, Intel Corp.

Slide 2

doc.: IEEE 802.11-05/0891r0

Submission

Abstract

TGv has identified location as a possible requirement. This document proposes additional requirements that can be added to TGv to enable significant performance improvements over signal strength methods. The proposed requirements introduce the concept of a Time-Stamp Difference (TSD).

September 2005

Stuart Golden, Intel Corp.

Slide 3

doc.: IEEE 802.11-05/0891r0

Submission

Localization Motivation

• Motivation: GPS does not work in certain areas– Indoors – Roofs/Walls attenuate signal below sensitivity threshold– Metropolitan Areas (Urban Canyon) – Tall buildings obstruct view to

satellite

• Vision: Any client could determine Lat, Long, Alt on a WLAN -- “Like GPS”

• No service fees -- location does not require association – Like GPS• Infrastructure enables but does not calculate each client’s location

– Very scalable – Like GPS

• Privacy driven – Client computes location and distributes as appropriate • Maps and other information may be available after association

• Video– Usage Models on Localization in the Enterprise (5 minute Video here)

September 2005

Stuart Golden, Intel Corp.

Slide 4

doc.: IEEE 802.11-05/0891r0

Submission

WLAN Localization Options

• Signal Strength (SS)– Distance Measurement (node-to-node)– Measures distance by power loss of the transmitted signal

• Time Difference of Arrival (TDOA)– TDOA – 1 Receiver

• Many Transmitters and One Receiver (GPS style)• Not WLAN Friendly

– TDOA – 1 Transmitter• Many Receivers and One Transmitter• Possible with WLAN – not preferred

• Time of Arrival (TOA)– Distance Measure (node-to-node)– Measures distance by time delay of the transmitted signal

September 2005

Stuart Golden, Intel Corp.

Slide 5

doc.: IEEE 802.11-05/0891r0

Submission

Signal Strength

• RCPI – Received Channel Power Indicator (802.11k)– 0.5 dB resolution steps

– +/- 5 dB accuracy requirement

• Classical r2 path loss equation: dB=10log10(K/ r2)

• Environment (shadowing, …) produce significant gain errors

• Signal strength errors imply significant range error

• Variations of several dB or more are common in indoor environments [1] [2].

dB error % range error

0.5 6%

1 12%

5 78%

September 2005

Stuart Golden, Intel Corp.

Slide 6

doc.: IEEE 802.11-05/0891r0

Submission

Time-Stamp Difference (TSD) Definition

• TS Concept:– Capture packets into data buffer. Matched filtering of data buffer

is performed with known waveform (preamble). – Times at which matched filter peaks occur are Time-Stamps (TS)

on station’s own clock. – Different station’s clocks are not synchronized – no origin.

• TS Definition:– Station’s time of when a packet was transmitted or received

• TSD Definition:– Difference of station’s TS of when acknowledgement arrives

minus station’s TS of when message arrives -- using station’s own clock

September 2005

Stuart Golden, Intel Corp.

Slide 7

doc.: IEEE 802.11-05/0891r0

Submission

Distance Measurement (TOA) Compared with Radar using Time-Stamp

Differences (TSD)

MessageAcknowledgement

Client’s TSD is time between ack rec’d

and msg sent

Access Point’s TSD is additional delay introduced by

sending/receiving different packets

September 2005

Stuart Golden, Intel Corp.

Slide 8

doc.: IEEE 802.11-05/0891r0

Submission

Client-AP Location Probe

• Scales Easily– No intensive AP computing

– No network server required

• Time-Stamp Difference (TSD) sent back in Probe Response’s Information Element (IE)

• Information passed pre-association

Directed Probe RequestACK

Probe Response (IE: TSD ~ 4 bytes)

September 2005

Stuart Golden, Intel Corp.

Slide 9

doc.: IEEE 802.11-05/0891r0

Submission

Implementing TDOA using Time-Stamp Differences (TSD)

Server uses multiple TSDs to compute

location Participating

Access Point’s TSD

Listening Access Point’s TSD

Here client does not need to compute TSD, but additional APs need to listen in.

MessageAcknowledgement

September 2005

Stuart Golden, Intel Corp.

Slide 10

doc.: IEEE 802.11-05/0891r0

Submission

TDOA versus TOA

– TDOA • Requires all receivers to be listening for the single transmission• Access Points are the logical listeners but are set to particular frequencies far away. • 802.11a has more channels making AP separation more dramatic.• All receivers “still” need to be synchronized in some sense.

Ch 1 Ch 6 Ch 11

Ch 11 Ch 1Ch 6

Ch 6Ch 1 Ch 11

Ch 6

September 2005

Stuart Golden, Intel Corp.

Slide 11

doc.: IEEE 802.11-05/0891r0

Submission

TSD Validation

• Localization is difficult to validate– Requires a network

– Air interface to multiple nodes – introduces significant entropy

• TOA Ranging– Validation in a controlled environment

– Connect 2 stations via cable• Cable has measured delay (e.g. Network Analyzer)

• TSD are collected from both stations

• Calculated delay from TSD compared with Measured Cable Delay

– Experiment repeatable with different length cables

September 2005

Stuart Golden, Intel Corp.

Slide 12

doc.: IEEE 802.11-05/0891r0

Submission

Typical Performance of TOA LocalizationMean Square Error = 0.9 meters

• TOA Localization Test-bed created in San Diego

Sigma1 m

Typical Signal Strength PerformanceSigma = 7 m

Typical Precision Location PerformanceSigma = 1 m

Circles show a 95% confidence interval

September 2005

Stuart Golden, Intel Corp.

Slide 13

doc.: IEEE 802.11-05/0891r0

Submission

Summary

• Motivation Summary– Accurate localization is needed in WLAN

• SS alone (802.11k) does not provide sufficient accuracy

– TSDs enable TOA and TDOA approaches

– TSDs can potentially enable new applications that require synchronization

• Implementation Summary– Shown how TSD is used in TOA & TDOA

– Simple measurement can be added to WLAN

– Illustrated example TOA performance (sub-meter)

September 2005

Stuart Golden, Intel Corp.

Slide 14

doc.: IEEE 802.11-05/0891r0

Submission

Proposed Text for TGv Objectives

Add the following text to the objectives document

• TGv should provide measurements to support higher accuracy localization over signal strength using TOA and TDOA methods.

• Provide Time-Stamp Differences (TSD) to enable both TOA and TDOA approaches

September 2005

Stuart Golden, Intel Corp.

Slide 15

doc.: IEEE 802.11-05/0891r0

Submission

Poll for TGv Objectives

• Should we modify the TGv Objectives to support higher accuracy localization using TOA and TDOA methods?

• YES _______

• NO _______

• ABSTAIN _______

September 2005

Stuart Golden, Intel Corp.

Slide 16

doc.: IEEE 802.11-05/0891r0

Submission

References

[1] Pahlavan, K., Krishnamurthy, P., Beneat, J., “Wideband Radio Propagation Modeling for Indoor Geolocation Applications,” IEEE Communications Magazine, April 1998, vol. 36, no. 4, pp. 60-65.

[2] Kaemarungsi, K., Krishnamurthy, P., “Properties of Indoor Received Signal Strength for WLAN Location Fingerprinting,” MobiQuitous ’04.