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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 1

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

Submission Title: [Two considerations on TWR]Date Submitted: [July 1, 2005]Source: [Yihong Qi, Huan-Bang Li, Ryuji Kohno, Company: National Institute of

Information and Communications Technology ]Contact: Yihong Qi Voice:+81 46 847 5092, E-Mail: yhqi@nict.go.jp]Abstract: [TWR with crystal offset management can effectively improve the

ranging accuracy. A separate mode is necessary for precision with mobility]Purpose: [To propose a separate mode for ranging and communications]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.

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 2

Two Considerations on TWR

Yihong Qi, Huan-Bang Li, Ryuji Kohno

National Institute of Information and Communications Technology (NICT)

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 3

Outline

• In TWR with crystal offset tracking (Vern, 05-336r0), knowing an offset difference only is sufficient to reduce the error due to crystal offset.

• Regarding communication and ranging, a separate mode is necessary for precision ranging with mobility.

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 4

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 offset0t

Message 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 from 04-0581r07.

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 5

Let’s be specific. Here is the error due to crystal offsets.

round trip time seen at A

Since the turn-round time (e.g., 2ms) is greatly larger than the propagation delay (e.g., 150ns), the round trip time and turn-round time is almost equal.

BBRBTAATAR TTTTd 121221

turn-round time seen at B

crystal offset at Acrystal offset at B

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 6

TWR with offset tracking (Vern, 05-336r0)

A B

A B

A measures B’s oscillator drift here

B embeds his measurement of A’s oscillator drift as a number in the data. (along with turn-around time)

Initiation message

Response message

TG4a

July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 7

TWR with offset tracking

• Crystal offset between A and B is tracked.

• How about the error due to crystal offset now?

,)(12122

1

121221

AABBRBTAATAR

BBRBTAATAR

TTTTTTTTd

)(12 ABBRBT TT

before:

now:

,221

121221

ApAp

ABRBTATAR

ttTTTTd

since is known

It works!

)( AB

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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 8

Communication and Ranging

• A parallel mode (current)With a common header, communication and ranging are done in parallel.

• A separate ranging mode( to be discussed)When precision ranging with mobility is taken into account, a separate ranging mode is necessary.

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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 9

Ranging time due to mobility vs. header length

• e=ranging error, v=node speed, a=maximum percentage of error increase due to the mobility, e.g., a=10%, e=25cm, total error (1+a)e=27.5cm

• E.g., v=10m/s, a=10%, e=25cm T<2.5ms

• Header length for ranging is also on the order of millisecond, 1~ 4ms.

,veaT

• Ranging time due to mobility can be on the order of millisecond

No time for communication data!

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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 10

What are our options for ranging with high mobility?

• Using shorter preamble lower ranging accuracy

• A separate ranging mode • Using time-saving ranging methods

– TWR with crystal offset tracking– TDOA

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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 11

Regarding a separate ranging mode

• A separate ranging mode for long distance ranging (50m) when a node speed is larger than certain threshold, e.g., 3m/s.

• Otherwise, a parallel ranging and communication mode

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July, 2005 Doc: IEEE 15-05-0375-01-004a

Qi, Li, Kohno (NICT) Slide 12

Advantages of a separate ranging mode

• Flexibility: data packets and data updating period are designed separately to meet different requirements of ranging and communications. E.g.,

• Communication may use the channel information obtained in the ranging mode.

header length Data updating period

communications short long

ranging long short