Symmetric Double Sided Two-Way Ranging: Crystal Tolerances and Solutions

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Symetric Double Sided -Two Way Ranging Date Submitted: [] Source:Rainer Hach Company: Nanotron Technologies Address:Alt-Moabit 61, 10555 Berlin, Germany Voice: +49 30 399 954 207 E-Mail:r.hach@nanotron.com Re: [] Abstract: Demonstrate the problem of crystal tolerances on Two Way Ranging and suggest a solution Purpose: Material to be presented and discussed 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. Hach, Nanotron

2. What’s the problem ? • TWR as defined in the ranging subcommittee final report (0581r07) is vulnerable to timebase mismatch and thus crystal tolerances. • We propose that the UWB PHY should support Symmetric Double Sided-TwoWay Ranging (SDS-TWR). Hach, Nanotron

3. Unknown propagation delay Unknown clock offset Message 1 Message 2 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. “Straightforward TWR Model” according to 04-0581r07 Offset between A and B is reflected here! What about drift? Figure 1 Two-Way Time Transfer Model Hach, Nanotron

4. Let’s analyze the calculation of the propagation delay round trip time measured with clock A reply time measured with clock B Usually the reply time will be significant larger than the propagation delay. Thus round trip time and reply time will be almost equal! Example: tp=30ns, treply=1ms -> tround=1.000060 ms Hach, Nanotron

5. What happens if there is a drift (difference in clock speed) between clock A and clock B? • Assume +10 ppm for clock A and -10 ppm for clock B: big number measured with clock B big number measured with clock A wrong number! +10 ppm -10 ppm tp=0.5*(1.000060e-3*(1+10e-6)-1e-3*(1-10e-6)) = 40e-9 Problem: Difference between large numbers (e.g. 1ms) with different accuracies (+10ns, -10 ns) has high inaccuracy! Hach, Nanotron

6. Effect of high inaccuracy after subtracting two large numbers measured with different clocks can be avoided by using Symmetric Double Sided-Two Way Ranging (SDS -TWR) Device B Device A unknown propagation delay reply time Hach, Nanotron

7. Let’s analyze the calculation of the propagation delay for SDS -TWR two big numbers measured with the same timebase (clock B) two big numbers measured with the same timebase (clock A) Using the same example numbers as before yields: tp=0.25*(1.000060e-3*(1+10e-6)-1e-3*(1+10e-6)+1.000060e-3*(1-10e-6)-1e-3*(1-10e-6)) -> tp= 30e-9 correct number! Hach, Nanotron

8. How many message are required for SDS-TWR? At least 3! Device B Device A Device B Device A these two messages can be combined into one Hach, Nanotron

9. Summary • “Straightforward TWR” requires very low crystal tolerances << 10 ppm or precise phase tracking • SDS-TWR is an enhancement on “Straightforward TWR” which eliminates the need of phase tracking • SDS-TWR needs a minimum of only 3 messages • SDS-TWR can work with standard tolerances (up to 40 ppm or more) Hach, Nanotron