June 2005

Hach, NanotronSlide 1

IEEE-15-05-0334-00-004a

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: Symetric Double Sided -Two Way RangingDate Submitted: [] Source: Rainer HachCompany: Nanotron TechnologiesAddress: Alt-Moabit 61, 10555 Berlin, Germany Voice: +49 30 399 954 207E-Mail: r.hach@nanotron.com

Re: []

Abstract: Demonstrate the problem of crystal tolerances on Two Way Ranging and suggest a solution

Purpose: Material to be presented and discussed

Notice: This document has been prepared to assist the IEEE P802.15. 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 acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

June 2005

Hach, NanotronSlide 2

IEEE-15-05-0334-00-004a

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What’s the problem ?

• TWR as defined in the ranging subcommittee final report (0581r07) is vulnerable to timebase mismatch and thus crystal tolerances.

• We propose that the UWB PHY should support Symmetric Double Sided-Two

Way Ranging (SDS-TWR).

June 2005

Hach, NanotronSlide 3

IEEE-15-05-0334-00-004a

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“Straightforward TWR Model” according to 04-0581r07

Device A Device B

Two equations in two unknowns yield:

* US Naval Observatory, Telstar Satellite, circa 1962http://www.boulder.nist.gov/timefreq/time/twoway.htmUnmatched detect-delays in the two devices may require one-time offset calibration.

ptUnknown propagation delay

poATBR ttTT 11

Unknown clock offset 0tMessage 1

Message 2

BRBTATARp TTTTt 121221

poBTAR ttTT 22

ATARBRBTo TTTTt 121221

Multiple measurements of tp and

to yield finer precision &

accuracy, and allow frequency offset correction.

Figure 1 Two-Way Time Transfer Model

Offset between A and B is reflected here!What about drift?

June 2005

Hach, NanotronSlide 4

IEEE-15-05-0334-00-004a

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Let’s analyze the calculation of the propagation delay

round trip time measured with clock A

Usually the reply time will be significant larger than the propagation delay.Thus round trip time and reply time will be almost equal!

Example: tp=30ns, treply=1ms -> tround=1.000060 ms

BRBTATARp TTTTt 121221

reply time measured with clock B

June 2005

Hach, NanotronSlide 5

IEEE-15-05-0334-00-004a

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What happens if there is a drift (difference in clock speed) between clock A and clock B?

• Assume +10 ppm for clock A and -10 ppm for clock B:

+10 ppm -10 ppm

tp=0.5*(1.000060e-3*(1+10e-6)-1e-3*(1-10e-6)) = 40e-9

Problem: Difference between large numbers (e.g. 1ms) withdifferent accuracies (+10ns, -10 ns) has high inaccuracy!

BRBTATARp TTTTt 121221

wrongnumber!

big number measured with clock A

big number measured with clock B

June 2005

Hach, NanotronSlide 6

IEEE-15-05-0334-00-004a

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Effect of high inaccuracy after subtracting two large numbers measured with different clocks can be avoided by using

Symmetric Double Sided-Two Way Ranging (SDS -TWR)

Device A Device B

pt unknown propagation delay

preplyB tt

pt

replyBreplyA tt pt

pt

reply time

4

replyBroundBreplyAroundAp

ttttt

roundAt

replyAroundB tt

June 2005

Hach, NanotronSlide 7

IEEE-15-05-0334-00-004a

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Let’s analyze the calculation of the propagation delay for SDS -TWR

Using the same example numbers as before yields:

tp=0.25*(1.000060e-3*(1+10e-6)-1e-3*(1+10e-6)+1.000060e-3*(1-10e-6)-1e-3*(1-10e-6))

-> tp= 30e-9

4

replyBroundBreplyAroundAp

ttttt

correctnumber!

two big numbers measured with the same timebase (clock A)

two big numbers measured with the same timebase (clock B)

June 2005

Hach, NanotronSlide 8

IEEE-15-05-0334-00-004a

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How many message are required for SDS-TWR? At least 3!

Device A Device B Device A Device B

these two messages can be combinedinto one

June 2005

Hach, NanotronSlide 9

IEEE-15-05-0334-00-004a

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Summary• “Straightforward TWR” requires very low

crystal tolerances << 10 ppm or precise phase tracking

• SDS-TWR is an enhancement on “Straightforward TWR” which eliminates the need of phase tracking

• SDS-TWR needs a minimum of only 3 messages

• SDS-TWR can work with standard tolerances (up to 40 ppm or more)