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Two Way Ranging using Tracking Information to Manage Crystal Offsets

This project explores the use of tracking information in two-way ranging to manage crystal offsets for high accuracy measurements. It proposes a method to reduce ranging errors and improve accuracy in wireless personal area networks.

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Two Way Ranging using Tracking Information to Manage Crystal Offsets

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Two Way Ranging using tracking information to manage crystal offsets.] Date Submitted: [5 June, 2005] Source: [Vern Brethour] Company [Time Domain Corp.] Address [7057 Old Madison Pike; Suite 250; Huntsville, Alabama 35806; USA] Voice:[(256) 428-6331], FAX: [(256) 922-0387], E-Mail: [vern.brethour@timedomain.com] Re: [802.15.4a.] Abstract: [Very high accuracy TWR with only 2 messages is supported by using tracking information.] Purpose: [To promote discussion in 802.15.4a.] 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. Brethour, Time Domain

  2. TWR using Tracking information. (A way to manage crystal offsets.) Brethour, Time Domain

  3. Managing crystal offsets is not optional. • 05-0334r0 (Rainer Hach) works out an example with uncorrected +/- 10 ppm crystal offsets leading to a 10 ns (3 meter) ranging error on a 10 meter link with only a 1 ms turn around time. • In real life, we want longer links but our turn around times will be longer than 4 ms. Brethour, Time Domain

  4. Where is the turn around time coming from?It’s here! Homogeneous preamble signal 4 ms 50 meter ranging Data (to include the time stamp of when the delimiter was at the antenna of the transmitter. Homogeneous preamble signal 500 us 20 meter Ranging 75 meter communications Homogeneous preamble signal 50 us 30 meter communications Brethour, Time Domain

  5. Our turn around times will be greater than 4 ms. • We also don’t want 10 ns errors! • Our accuracy target is less than 1 ns. • Unless we manage the crystal offsets algorithmically, even 2ppm crystals will not work! • Managing crystal offsets is not optional. Brethour, Time Domain

  6. So fine, we must manage crystal offsets. 05-0334r1 (Rainer Hach) shows one way of managing crystal offsets. SDS-TWR Brethour, Time Domain

  7. We can also manage crystal offsets by looking at tracking information. Homogeneous preamble signal Data (to include the time stamp of when the delimiter was at the antenna of the transmitter. We acquire in this part of the preamble We do channel sounding in this part of the preamble Brethour, Time Domain

  8. We are tracking during this entire time We acquire in this part of the preamble There is really no choice about tracking, because we need high processing gain during the channel sounding. See 05-0335r0. Brethour, Time Domain

  9. I low pass A2D LNA Mag Rectangle to polar osc Phase Q 90 low pass A2D What does “tracking” really mean? Transmit oscillator information arrives here. Receive oscillator is literally here. By watching this over time, we know the exact difference in the oscillators. Brethour, Time Domain

  10. How do we make that protocol work? Initiation message A B B measures A’s oscillator drift here Response message A B A measures B’s oscillator drift here B embeds his measurement of A’s oscillator drift as a number in the data. (along with turn-around time) Brethour, Time Domain

  11. What does Radio A do? A B • Radio A can just correct the turnaround time with his own green number. (Throw the blue number away!) • Accuracy improvement is possible by averaging the blue number and the green number. A measures B’s oscillator drift here B embeds his measurement of A’s oscillator drift as a number in the data. (along with turn-around time) Brethour, Time Domain

  12. I low pass A2D LNA Mag Rectangle to polar osc Phase Q 90 low pass A2D What kind of accuracy do we get? Transmit oscillator info arrives here. Receive oscillator is literally here. We typically track the carrier to within 30 degrees (1/12 of a cycle). 1/12 of a cycle, even at 3 GHZ, is about 30 ps. 30 ps error in 3 ms is .01 ppm !!! (That’s why it’s okay to throw the blue number away on the previous slide.) Brethour, Time Domain

  13. Why do we like this? • Very high measurement accuracy of the carrier offset. • Nearly effortless, if the tracking system is designed with this in mind. (We have to track anyway, during channel sounding.) • Only two messages on the air. • The crystal offset information ends up with both nodes, but most critically with the initiator. Brethour, Time Domain

  14. Recommendation: • That 15.4a use tracking information to manage the crystal offsets in two way ranging. • That 15.4a keep the TWR message count to 2 messages for a ranging exchange. Brethour, Time Domain

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