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 considerations on TWR] Date Submitted: [July 1, 2005] Source: [Yihong Qi, Huan-Bang Li, Ryuji Kohno, Company: National Institute of Information and Communications Technology ] Contact: Yihong Qi Voice:+81 46 847 5092, E-Mail: yhqi@nict.go.jp] Abstract: [TWR with crystal offset management can effectively improve the ranging accuracy. A separate mode is necessary for precision with mobility] Purpose: [To propose a separate mode for ranging and communications] 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.

2. Two Considerations on TWR Yihong Qi, Huan-Bang Li, Ryuji Kohno National Institute of Information and Communications Technology (NICT)

3. Outline • In TWR with crystal offset tracking (Vern, 05-336r0), knowing an offset difference only is sufficient to reduce the error due to crystal offset. • Regarding communication and ranging, a separate mode is necessary for precision ranging with mobility.

4. 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. Figure 1 Two-Way Time Transfer Model from 04-0581r07.

5. Let’s be specific. Here is the error due to crystal offsets. crystal offset at A crystal offset at B round trip time seen at A turn-round time seen at B Since the turn-round time (e.g., 2ms) is greatly larger than the propagation delay (e.g., 150ns), the round trip time and turn-round time is almost equal.

6. Initiation message A B 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) TWR with offset tracking (Vern, 05-336r0)

7. TWR with offset tracking • Crystal offset between A and B is tracked. • How about the error due to crystal offset now? before: now: It works! since is known

8. Communication and Ranging • A parallel mode (current) With a common header, communication and ranging are done in parallel. • A separate ranging mode( to be discussed) When precision ranging with mobility is taken into account, a separate ranging mode is necessary.

9. Ranging time due to mobility vs. header length • e=ranging error, v=node speed, a=maximum percentage of error increase due to the mobility, e.g., a=10%, e=25cm, total error (1+a)e=27.5cm • E.g., v=10m/s, a=10%, e=25cm  T<2.5ms • Header length for ranging is also on the order of millisecond, 1~ 4ms. • Ranging time due to mobility can be on the order of millisecond  No time for communication data!

10. What are our options for ranging with high mobility? • Using shorter preamble  lower ranging accuracy • A separate ranging mode • Using time-saving ranging methods • TWR with crystal offset tracking • TDOA

11. Regarding a separate ranging mode • A separate ranging mode for long distance ranging (50m) when a node speed is larger than certain threshold, e.g., 3m/s. • Otherwise, a parallel ranging and communication mode

12. Advantages of a separate ranging mode • Flexibility: data packets and data updating period are designed separately to meet different requirements of ranging and communications. E.g., • Communication may use the channel information obtained in the ranging mode.