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

1. 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:[joe@mcci.com] 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

2. 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

3. 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

4. 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

5. 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. 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

7. Unknown propagation delay Unknown clock offset Message 1 Message 2 Two-Way Time Transfer Model [4] Device A Device B Two equations in two unknowns yield: Multiple measurements of tpand 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

8. 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

9. 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

10. PHY Ranging Resources [5] TX PHY RX PHY Mod DSP Demod DSP counter counter timer latch 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

11. 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

12. 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

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

14. 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

15. Single measurement example Dev 1, Initiator RRQ ACK RM1 123us ACK RM2+RMR Dev2, Responder This example shows only one TWTT measurement. Joe Decuir, MCCI

16. 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

17. 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

18. 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

19. 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

20. REFERENCES [1] 15-04-0418 [2] 15-03-0541 [3] 15-04-0300 [4] 15-04-0050 [5] 15-04-0493 Joe Decuir, MCCI