Transcript 15-04-0573-00-004a-two-way-time-transfer-based-ranging.ppt

6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 1
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Table of Contents
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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
Submission
Slide 2
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 3
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 4
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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 timeof-flight measured.
– focus of this paper
Joe Decuir, MCCI
Submission
Slide 5
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 6
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Two-Way Time Transfer Model [4]
Unknown propagation delay
tp
Unknown clock offset t 0
Message 1
Message 2
Device A
T2 AR  T2 BT  to  t p
Device B
T1BR  T1 AT  to  t p
Two equations in two unknowns yield:
tp 
1
2
 T2 AR  T1 AT   T2 BT  T1BR  
to 
1
2
 T2 BT  T1BR   T2 AR  T1AT  
Multiple measurements of tp
and to yield finer precision &
accuracy, and allow frequency
offset correction.
* US Naval Observatory, Telstar Satellite, circa 1962
http://www.boulder.nist.gov/timefreq/time/twoway.htm
Unmatched detect-delays in the two devices may require one-time
offset calibration.
Joe Decuir, MCCI
Submission
Slide 7
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 8
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 9
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
PHY Ranging Resources [5]
TX PHY
RX PHY
Mod
DSP
Demod
DSP
counter
timer latch
counter
timer latch
TX PHY captures the counter when the reference
signal is sent into the modulator DSP.
RX PHY captures the counter when the reference
signal is detected by the demodulator DSP.
Joe Decuir, MCCI
Submission
Slide 10
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 11
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Ranging Transaction Overview
• Initiator (DEV1) MAC reserves time
• 6 frame ranging exchange transaction:
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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
Submission
Slide 12
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Example RM1/RM2 Timing:
MB-UWB
Initiator, Dev1
R2c
T1c
preamble RM1
preamble RM2
flight times
preamble RM1
R1c
Responder, Dev2
SIFS
preamble RM2
T2c
The preamble and the SIFS are both 10 usec.
Actual flight times would be <33ns for <10m.
Joe Decuir, MCCI
Submission
Slide 13
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 14
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Single measurement example
Dev 1, Initiator
RRQ
RM1
ACK
123us
ACK
RM2+RMR
Dev2, Responder
This example shows only one TWTT measurement.
Joe Decuir, MCCI
Submission
Slide 15
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Four measurement example
Dev 1, Initiator
RRQ
RM1
RM1
RM1
RM1
ACK
264us
ACK
RM2
RM2
RM2
RM2+RMR
Dev2, Responder
This example shows four TWTT measurements:
123 + 3 x 46.8 + 10 flight times (<.3us) ~ 264 us
Joe Decuir, MCCI
Submission
Slide 16
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 17
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
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
Submission
Slide 18
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
Example Frequency Offset Errors
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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
Submission
Slide 19
Joe Decuir, MCCI
6 October 2004
doc.: IEEE 802.15-04a/0573r0
REFERENCES
[1] 15-04-0418
[2] 15-03-0541
[3] 15-04-0300
[4] 15-04-0050
[5] 15-04-0493
Joe Decuir, MCCI
Submission
Slide 20
Joe Decuir, MCCI