doc.: ieee 802.11-05/0161r0 submission march 2005 stuart golden, intel corp.slide 1 usage of...

March 2005

Stuart Golden, Intel Corp.

Slide 1

doc.: IEEE 802.11-05/0161r0

Submission

Usage of Timestamps in WLAN for Localization and other Applications

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 <[email protected]> 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 <[email protected]>.

Date: 2005-05-16

Name Company Address Phone email Stuart Golden Intel Corp. 13280 Evening

Creek Dr., San Diego CA 92128

(858) 391-4329

[email protected]

Authors:

March 2005


Slide 2

doc.: IEEE 802.11-05/0161r0

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)

March 2005


Slide 3

doc.: IEEE 802.11-05/0161r0

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

• Angle of Arrival (AOA)– Phased array antenna can give directional information

• TOA & AOA– Combining TOA & AOA can potentially provide “single” fixed node localization

March 2005


Slide 4

doc.: IEEE 802.11-05/0161r0

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

March 2005


Slide 5

doc.: IEEE 802.11-05/0161r0

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%

March 2005


Slide 6

doc.: IEEE 802.11-05/0161r0

Submission

Motivation Summary

• Accurate localization is needed in WLAN– SS alone (802.11k) does not provide sufficient accuracy

• Timestamps enable TOA and TDOA approaches

• Timestamps can potentially enable new applications that require synchronization

• Now, let’s discuss obtaining and using timestamps …

March 2005


Slide 7

doc.: IEEE 802.11-05/0161r0

Submission

TOA: Range Measurement via Timestamps

• Square represents long preamble of Probe Request

• Triangle represents long preamble of ACK– Definitions:

= Time of arrival between Client and AP (desired) t0 = Time at which ACK is sent on Client’s clock = Mis-synchronization between the two clocks

– Measurements:• Client’s Time Difference = t0 +

• AP’s Time Difference = t0 -

– Estimates from measurements• t0 = ½*(Client’s Time Diff + AP’s Time Diff)

= ½*(Client’s Time Diff - AP’s Time Diff)

• Range = speed of light *

Probe

Ack

Client Receive

APReceive

0t

Probe

Ack

00t

0t

0t

March 2005


Slide 8

doc.: IEEE 802.11-05/0161r0

Submission

Client-AP Location Probe

• Scales Easily– No intensive AP computing

– No network server required

• Timestamps (TS) sent back in Probe Response’s Information Element (IE)

• Information passed pre-association

Directed Probe RequestACK

Probe Response (IE: 2 timestamps ~ 10 bytes)

March 2005


Slide 9

doc.: IEEE 802.11-05/0161r0

Submission

Timestamp (TS) Measurement

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

with known waveform (preamble). Times at which peaks occur are TS.

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

– Station’s clock is not synchronized to any other station clock – no origin

– Station’s clock is periodic – 0 to Max Value and repeat

– Station’s clock has some Resolution increment

• Example: 5 Bytes per TS– .01 ns Resolution

– 11 second Max Value

March 2005


Slide 10

doc.: IEEE 802.11-05/0161r0

Submission

Calculation of TS• TS has two components:

– 1) sample counter – sample number for Matched Filter peak– 2) sub-sample component – resolution to sub-sample accuracy

• Interpolate Matched Filter output for sub-sample accuracy • Matched Filter (once per packet – e.g. part of equalizer)

– Time Domain Implementation• Use FIR Filter at output of A/D

– Frequency Domain Implementation• Perform inverse FFT at the output of the FFT multiplied by Expected Spectrum

• Transmit Packet TS– Method 1: Radio may have deterministic delay from specified trigger– Method 2: Measure Transmit TS

• Turn on receiver while transmitting• May have to turn off LNA during this mode so not to saturate receiver

RF Front End

LNA

DAC

ADC

TX Baseband

RX Baseband

Up Conversion

Down Conversion LNA

PA

Simplified Direct Conversion Radio

PA

Transmit/ReceiveSwitch

Transmitter Chain

Receiver Chain

AntennaSystem

March 2005


Slide 11

doc.: IEEE 802.11-05/0161r0

Submission

TS 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)

• TS are collected from both stations

• Calculated delay from TS compared with Measured Cable Delay

– Experiment repeatable with different length cables

March 2005


Slide 12

doc.: IEEE 802.11-05/0161r0

Submission

Typical Performance of TOA LocalizationMean Square Error = 0.9 meters

• TOA Localization Test-bed created in San Diego

March 2005


Slide 13

doc.: IEEE 802.11-05/0161r0

Submission

Synchronization or Measuring Mis-Synchronization

• Option 1: Measurement Only– Measure TS for each Tx or Rx packet

– System still is completely unsynchronized

– Mis-synchronization is measured

• Option 2: Station Synchronization– Provides ability to synchronize stations

– Measure TS for each Rx packet

– Capability to have a station send a packet at a specified TS• Like TDMA where each station uses CSMA before transmitting at

start of slot

March 2005


Slide 14

doc.: IEEE 802.11-05/0161r0

Submission

Summary

• Motivation Summary– Accurate localization is needed in WLAN

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

– Timestamps enable TOA and TDOA approaches

– Timestamps can potentially enable new applications that require synchronization

• Implementation Summary– Shown how TS are used in TOA

– Simple measurement can be added to WLAN

– Illustrated example TOA performance (sub-meter)

March 2005


Slide 15

doc.: IEEE 802.11-05/0161r0

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.

March 2005


Slide 16

doc.: IEEE 802.11-05/0161r0

Submission

Feedback for Next Steps

• Interest in Localization and/or Synchronization Study Group?

• YES _______

• NO _______

• ABSTAIN _______

doc.: ieee 802.11-05/0161r0 submission march 2005 stuart golden, intel corp.slide 1 usage of...

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