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

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ Proposed classes of Ranging service] Date Submitted: [16 May, 2005] Source: [Vern Brethour] Company [Time Domain Corp.]

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

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Proposed classes of Ranging service] Date Submitted: [16 May, 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: [3 Classes of ranging service as well as three algorithmic protocols are proposed for 802.15.4a.] Purpose: [To serve as “targets” in the selection of signaling parameters and protocols for 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. 3 Classes of ranging service for 802.15.4a Proposed to serve as “targets” for determining aspects of signaling. Brethour, Time Domain

  3. Why is this an issue? • The ranging guidance we have in the PAR is not very specific: for example, the PAR is silent about ranging in the face of SOP as well as time-on-the air targets. • The PAR talks about asset tracking in paragraph 18a and the implication is that we intend to support positioning in addition to just ranging. • The PAR makes a casual inferred equality between positioning accuracy delivered by the application and the range accuracy delivered by the PHY. Brethour, Time Domain

  4. Review some of the factors in ranging accuracy • During April and May the Ranging Edit team has had weekly calls to discuss these factors. • Only a condensed overview is given here. Brethour, Time Domain

  5. The GDOP issue was introduced in an e-mail to the reflector on April 7, 2005 • GDOP stands for Geometric Dilution of Precision. • The assumption is that Range is not enough, but that the application is looking for position. • 04-085r5 presents an idealized overview of the techniques for turning ranges into position. • The Ranging committee final report does not include GDOP. Brethour, Time Domain

  6. What is GDOP about? • It’s NOT about band plans, chipping rates, pulse sizes, header lengths or any other factor that is under our control. • GDOP is about the relative positioning of the deployed nodes. • We usually have no control whatever over the position of the deployed nodes. Brethour, Time Domain

  7. Positioning from TOA 3 anchors with known positions (at least) are required to retrieve a 2D-position from 3 TOAs Anchor 2 (xA2,yA2) Anchor 1 (xA1,yA1) Mobile (xm,ym) Anchor 3 (xA3,yA3) Estimated Position Measurements Specific Positioning Algorithms TOA Ranging from 04-0581r5 Brethour, Time Domain

  8. Good GDOP positioning situation Tracked node is here Reference nodes are here Brethour, Time Domain

  9. Bad GDOP positioning situation Tracked node is here Reference nodes are here Brethour, Time Domain

  10. We have no control over the GDOP situation. • It’s something our customer will do for (or to) himself. • We must make some allowance for it when setting targets for ranging accuracy when we ultimately want positioning accuracy. • A historically used GDOP allowance is “4” • GDOP allowance of 4 means that to achieve a positioning accuracy of 1 meter we need to achieve a ranging accuracy of 25 cm. Brethour, Time Domain

  11. Ranging accuracy is a function of the length of the channel sounding symbol. • The use of the channel sounding header was presented and discussed as 0222r0. • Channel sounding does not HAVE to be part of the header. • If it is part of the header, more flexibility is allowed with the acquisition search. Brethour, Time Domain

  12. 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. The question today is: WHEN exactly do we click these stopwatches? This picture is from 04-0581r5 Brethour, Time Domain

  13. Artists’ concept of a UWB message: Data (to include the time stamp of when the delimiter was at the antenna of the transmitter. Channel sounding Acquisition The “Delimiter” is a unique signal event that defines the end of the acquisition header. Brethour, Time Domain

  14. If acquisition picks up the receive waveform here Do we call that the receive time? We better not: The receive time is here. Brethour, Time Domain

  15. Finding the leading edge to high accuracy was discussed in 0246r1 • In a effort to achieve good ranging in an SOP environment, the bandwidth used for ranging has been limited in the discussions to 500 MHz to allow use of FDM for isolation. • How do we do accurate ranging with only 500 MHz of bandwidth? Brethour, Time Domain

  16. What do we do with the base band envelope? Let’s think about the leading edge problem in free space: Base band envelope (500 MHz) mixed to DC. About 5 ns for 500 MHz Brethour, Time Domain

  17. Consider finding the leading edge in free space: only one arriving pulse envelope. Base band envelope (500 MHz) mixed to DC. Sample times (1 GHz) Actual Samples Correct answer for position of leading edge Brethour, Time Domain

  18. How do we find the green arrow? 500 MHz base band envelope mixed to DC. and sampled at 1 GHz Correct answer for position of leading edge (The elusive green arrow) One popular algorithm simply finds the first non-zero (in practice, above some threshold) value and calls that sample position the location of the leading edge. In this example, that algorithm would say the leading edge is here. Brethour, Time Domain

  19. Alternative algorithm: Find the green arrow! Do some math & calculate this position. Correct answer for position of leading edge Another algorithm uses the first two non-zero (in practice, above some threshold) values and does trig computations knowing that they are samples of a known length cosine to calculate the location of the leading edge. Brethour, Time Domain

  20. That looks like it will need high S/N to work. Do some math & calculate this position. Correct answer for position of leading edge High S/N is achieved with high processing gain. Brethour, Time Domain

  21. The need for high processing gain was the driver in the discussion in 246r1. • One of the most important decisions we will make is picking the length of the packet header. How much time for the header? Data (to include the time stamp of when the delimiter was at the antenna of the transmitter. Channel sounding Acquisition A delimiter signaling event separates the header from the rest of the packet. Brethour, Time Domain

  22. A spreadsheet was presented as 245r1 which makes a calculation of header lengths. We make decisions and trade off numbers in this part of the spreadsheet While we keep our eye on these two answers: These are the projected preamble lengths needed to satisfy the conditions Brethour, Time Domain

  23. 246r1 is an attempt at honesty and realism • (Except that a path loss exponent of 3 is used.) • Otherwise, the assumptions are realistic. Brethour, Time Domain

  24. The Predicted header lengths are a few milliseconds. • The target range was 50 meters. (That was chosen because 100 meters didn’t seem possible.) • A fairly complex reference receiver was used. That was because accurate positioning with a simple receiver didn’t seen possible. Brethour, Time Domain

  25. The Proposed three classes roughly defined: • Accurate Ranging • Fast Ranging • Cost-effective Ranging Brethour, Time Domain

  26. Accurate Ranging • One way ranging accuracy of 25 cm or better at 50 meters in 4 ms in 90% of channels. • Against White Gaussian Noise • All nodes stationary during measurement • No other 15.4a transmitters operating (on any channel) during the measurement • (Targeting a 2 way ranging accuracy 25 cm.) • (Targeting a positioning accuracy of 1 meter with a GDOP degradation factor of 4.) Brethour, Time Domain

  27. Fast Ranging • One way message accuracy of 25 cm or better at 20 meters in 500 us in 90% of channels. • Against White Gaussian Noise • All nodes stationary during measurement • No other 15.4a transmitters operating (on any channel) during the measurement • (Targeting a 2 way ranging accuracy 25 cm.) • (Targeting a positioning accuracy of 1 meter with a GDOP degradation factor of 4.) Brethour, Time Domain

  28. Cost-effective Ranging • One way message accuracy of 1m or better at 20 meters in 4 ms in 90% of channels. • Against White Gaussian Noise • All nodes stationary during measurement • No other 15.4a transmitters operating (on any channel) during the measurement • This service is intended for ranging – not precision positioning. Brethour, Time Domain

  29. Comments • The allowance factor {GDOP degradation = 4} simply represents an experience based guess, actual results will vary wildly depending upon node placement upon deployment. • What is actually measured in two way ranging is the round trip range which will be the sum of two one way ranges, each with a statistical error having a maximum of 25 cm. We will divide that sum by 2 to get the one way error which now should have an error of less than 25 cm (hoping to get lucky with the statistics). • The hope is that there will be additional luck left over to cover crystal drift during message turn-around time, and round-off errors and a host of little things. • The “Fast Ranging” service is intended to deal with the global position update rate in a constellation of many nodes in relative motion. Brethour, Time Domain

  30. Additionally, ranging protocols were presented in 0234r0 • First; let’s review the vocabulary established in 15-04-0581-05-004a (The Ranging Subcommittee Report) • TOA Ranging • TDOA Ranging mode 1 (SOI is Rx) • TDOA Ranging mode 2 (SOI is Tx) Brethour, Time Domain

  31. What about TDOA ? Two modes from 04-0581r5 to consider. • Mode 1 – The station of interest (SOI) receives multiple reference packets and calculates the TDOA • LORAN-C type operation and the processing burden is on the receiver to run the hyperbolic location algorithms • Mode 2 – The station of interest transmits a reference packet which is received by multiple fixed nodes • The fixed nodes must forward the TDOA information to a central node which then runs the hyperbolic location algorithms reference node reference node SOI SOI Key: Sync Pulse Location Pulse Position Report Key: Sync Pulse Location Pulse TDOA backhaul controller controller Mode 1 - Passive Mode 2 - Active Brethour, Time Domain

  32. Review: How many one way transmissions to keep track of n SOIs ? • With TOA it’s {n squared plus n}. • With TDOA Mode 2 it’s {n}. • With TDOA Mode 1 it’s {independent of n}. Brethour, Time Domain

  33. Let’s compare the SOIs • With Mode 2, the SOI can be very simple. A non-coherent receiver might be adequate. There is on need for an on-board solver. • With Mode 1, the SOI must be as capable as an anchor device. Brethour, Time Domain

  34. Recommendations: • We should develop the 15.4a ranging standard to support BOTH TOA and TDOA mode 2 ranging protocols. • In the interest of harvesting “low hanging fruit” we should also support TDOA mode 1. Brethour, Time Domain

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