project: ieee p802.15 working group for wireless personal area networks (wpans)

Joe Decuir, MCCI

6 October 2004

Joe Decuir, MCCISlide 1

doc.: IEEE 802.15-04a/0573r0

Submission

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: [Two Way Time Transfer based ranging]Date Submitted: [October 6, 2004]Source: [Joe Decuir] Company [MCCI.]Address [18814 SE 42nd St, Issaquah, WA, USA]Voice:[(425)603-1188], FAX: [(425)603-0279], E-Mail:[[email protected]]

Re: [TG4a Ranging]

Abstract: [An application of Time-of-Flight measurements to ranging]

Purpose: [Contribute to ranging in IEEE 802.15 TG4a.]

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.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Table of Contents• Introduction to the concept• Sorting functions between layers• Two Way Time Transfer variations• TWTT requirements• Example MB-UWB PHY implementation• Example MB-UWB MAC implementation• Range calculations• Error analysis and compensation

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Introduction to time-based ranging

• The concept is simple in principle:– Measure the radio signal flight time– multiply by c (speed of light)

• The trick is to accurately measure flight time, given:– channel impairments: noise, multipath, etc– circuit and logic delays– manufacturing tolerances: crystal differences

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Sorting functions into layers• Times of flight are short: 33ns/10m

– basic timing is likely to be in the PHY

• Conducting measurements requires some fast logic, responding quickly to frames.– the protocol is likely to be in the MAC

• Calculations are more complex but not time critical– Location awareness is above the MAC

• see Roberts [3] page 3 of 9

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Where are the time references?• If a network of devices has synchronized

clocks, then a signal can be sent at a known time and detected at a measured time [1].– synchronizing clocks precisely enough is hard

• If pairs of devices have similar clocks with minimal frequency error, then a pair of signals can be exchanged, and average time-of-flight measured.– focus of this paper

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Two Way Time Transfer (TWTT)

• Initiating device measures time– from sending the first signal, to– receiving the second signal

• Responding device either:– responds in a fixed and known delay time [2] or [3]– measures its own response delay time and reports that

to the initiator [4] & [5]

• Initiator subtracts the two delays, yielding two times-of-flight– the calculation is easy: multiply by c/2

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Two-Way Time Transfer Model [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 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.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

TWTT in PAN environment• Original TWTT was long range

– response delays were negligible

– free space = no multipath

• In PAN environment– Device response delays may exceed flight times

– The message frames themselves are much longer than the flight times (10s of usec vs 10s of nsec)

– Multipath signal propagation is common

– Clock frequencies limit resolution

– Clock frequency differences limit accuracy.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Example TWTT UWB Implementation

• Choose an easy-to-detect signal feature– e.g. feature of standard PHY preamble

• PHY: Add a fast timer and capture latch

• MAC: Add a simple cooperative measurement transaction

• Describe simple and complex upper layer calculations

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

PHY Ranging Resources [5]TX PHY RX PHY

ModDSP

DemodDSP

timer latch timer latch

counter counter

TX PHY captures the counter when the reference signal is sent into the modulator DSP. RX PHY captures the counter when the referencesignal is detected by the demodulator DSP.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

PHY Calibration Constants

• RTD = Ranging Transmit Delay: As per the previous slide, there will be a delay between the time the reference signal is fed into the modulator and the time that signal appears at the antenna.

• RRD = Ranging Receive Delay: There will also be a delay between the time the reference signal arrives at the antenna and the time that signal is detected in the demodulator.

• Each MAC needs these constants to correct time measurements.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Ranging Transaction Overview• Initiator (DEV1) MAC reserves time• 6 frame ranging exchange transaction:

– RRQ & ACK: DEV1 ranging request– RM1 & RM2: measurement frames– RM2 = DEV2’s ACK to DEV1’s RM1– RMR & ACK: DEV2 ranging measurement report

back to DEV1

• DEV1 collects 4 timer values per pair• Initiator upper layers do calculations

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Example RM1/RM2 Timing: MB-UWB

Initiator, Dev1

Responder, Dev2

preamble

preamble

flight times

RM1

RM1 preamble

preamble

SIFS

RM2

RM2

T1c

R1c

R2c

T2c

The preamble and the SIFS are both 10 usec.Actual flight times would be <33ns for <10m.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Time value capture & correction

• DEV1 captures the RM1 transmit time T1 – T1c = T1 + RTD(dev1)

• DEV2 captures the RM1 receive time R1– R1c = R1 – RRD(dev2)

• DEV2 captures the RM2 transmit time T2 – T2c = T2 + RTD(dev2)

• DEV1 captures the RM2 receive time R2– R2c = R2 – RRD(dev1)

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Single measurement exampleDev 1, Initiator

Dev2, Responder

RRQ

RM2+RMR

RM1

ACK

ACK

123us

This example shows only one TWTT measurement.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Four measurement exampleDev 1, Initiator

Dev2, Responder

RRQ

RM2+RMR

RM1

ACK

ACK

264us

This example shows four TWTT measurements:123 + 3 x 46.8 + 10 flight times (<.3us) ~ 264 us

RM2

RM1 RM1 RM1

RM2 RM2

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Example Range Calculation• Suppose the Timer clock is 528 MHz

• The complete exchange is R2c – T1c.– Both measurements from the same timer.

• The delay through Dev2 is T2c – R1c.– Both measurements from the same timer.

• The difference is two flight times = 2Ft.

• 2Ft = (R2c – T1c) – (T2c – R1c)

• Range = Ft x c (speed of light)

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Primary Error Sources• Signal bandwidth limits spatial resolution of

the timing signal [3].

• Multipath delayed signals make the range look longer than it is.

• Timer resolution limits spatial resolution:c/528MHz = 56.8cm; c/4224 MHz = 7.1cm.

• Clock frequency differences generate errors– see next slide for example

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

Example Frequency Offset Errors

• Given 4224 MHz nominal clocks

• Given Clock tolerance of +/- 20ppm

• Aggregate tolerance is +/- 40ppm

• 23.7 usec is approximately 100,000 clock periods at 4224 MHz.

• The max distance error due to clock frequency error could be 4 clock cycles– 4c/4224MHz = 28.4 cm.

Joe Decuir, MCCI

6 October 2004


doc.: IEEE 802.15-04a/0573r0

Submission

REFERENCES

[1] 15-04-0418[2] 15-03-0541[3] 15-04-0300[4] 15-04-0050[5] 15-04-0493

project: ieee p802.15 working group for wireless personal area networks (wpans)

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