6 October 2004 Joe Decuir, MCCISlide 1 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Project: IEEE P 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 42 nd St, Issaquah, WA, USA] Voice:[(425) ], FAX: [(425) ], Re: [TG4a Ranging] Abstract:[An application of Time-of-Flight measurements to ranging] Purpose:[Contribute to ranging in IEEE TG4a.] Notice:This document has been prepared to assist the IEEE P 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 P

6 October 2004 Joe Decuir, MCCISlide 2 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 3 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 4 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 5 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 6 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 7 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Two-Way Time Transfer Model [4] Device ADevice B Two equations in two unknowns yield: * US Naval Observatory, Telstar Satellite, circa Unmatched detect-delays in the two devices may require one-time offset calibration. Unknown propagation delay Unknown clock offset Message 1 Message 2 Multiple measurements of t p and t o yield finer precision & accuracy, and allow frequency offset correction.

6 October 2004 Joe Decuir, MCCISlide 8 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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.

6 October 2004 Joe Decuir, MCCISlide 9 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 10 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI PHY Ranging Resources [5] TX PHYRX PHY Mod DSP Demod DSP timer latch counter 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.

6 October 2004 Joe Decuir, MCCISlide 11 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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.

6 October 2004 Joe Decuir, MCCISlide 12 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Ranging Transaction Overview Initiator (DEV1) MAC reserves time 6 frame ranging exchange transaction: –RRQ & ACK: DEV1 ranging request –RM1 & RM2: measurement frames –RM2 = DEV2s ACK to DEV1s RM1 –RMR & ACK: DEV2 ranging measurement report back to DEV1 DEV1 collects 4 timer values per pair Initiator upper layers do calculations

6 October 2004 Joe Decuir, MCCISlide 13 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Example RM1/RM2 Timing: MB-UWB Initiator, Dev1 Responder, Dev2 preamble flight times RM1 preamble SIFS RM2 T1c R1c R2c T2c The preamble and the SIFS are both 10 usec. Actual flight times would be <33ns for <10m.

6 October 2004 Joe Decuir, MCCISlide 14 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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)

6 October 2004 Joe Decuir, MCCISlide 15 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Single measurement example Dev 1, Initiator Dev2, Responder RRQ RM2+RMR RM1 ACK 123us This example shows only one TWTT measurement.

6 October 2004 Joe Decuir, MCCISlide 16 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI Four measurement example Dev 1, Initiator Dev2, Responder RRQ RM2+RMR RM1 ACK 264us This example shows four TWTT measurements: x flight times (<.3us) ~ 264 us RM2 RM1 RM2

6 October 2004 Joe Decuir, MCCISlide 17 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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)

6 October 2004 Joe Decuir, MCCISlide 18 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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

6 October 2004 Joe Decuir, MCCISlide 19 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI 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.

6 October 2004 Joe Decuir, MCCISlide 20 doc.: IEEE a/0573r0 Submission Joe Decuir, MCCI REFERENCES [1] [2] [3] [4] [5]