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Doc.: IEEE 802.15-05-0346-00-004a Submission June, 2005 Brethour, Time DomainSlide 1 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks.

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Presentation on theme: "Doc.: IEEE 802.15-05-0346-00-004a Submission June, 2005 Brethour, Time DomainSlide 1 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks."— Presentation transcript:

1 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 1 Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Sources of error When Ranging.] Date Submitted: [12 June, 2005] Source: [Vern Brethour] Company [Time Domain Corp.] Address [7057 Old Madison Pike; Suite 250; Huntsville, Alabama 35806; USA] Voice:[(256) ], FAX: [(256) ], Re: [ a.] Abstract:[A discussion of the sources of error in ranging measurements for each of the currently proposed ranging protocols in 15.4a.] Purpose:[To promote discussion in a.] Notice:This document has been prepared to assist the IEEE P 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 P

2 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 2 Sources of error and a comparison of two ranging protocols.

3 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 3 When doing UWB ranging, the error in the final range estimate is the composite of many sources. In r0, r1, r0, and r0 we have discussed the crystal offset component of the ranging error. This is a good thing to discuss, crystal offsets are very important.

4 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 4 On June 6, two different ways to manage crystal offsets were presented. 334r0: SDS-TWR 336r0: Offset management through tracking

5 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 5 We have two different ways to address the same problem. How do we choose? As explained in , all 15.4a UWB systems must have tracking capability, so the need for tracking equipment is not unique to the 2 message protocol. That protocol simply uses information that is present in the tracking offset measurement.

6 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 6 There is a natural preference for more efficient use of air time, so: Why would we want to support the 4 message protocol? Easy answer! We would be eager to support the 4 message protocol if it yielded more accurate ranging results.

7 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 7 To compare accuracy of the ranging protocols we must look at where the ranging errors come from. The protocols are affected by errors in leading edge estimation in generally similar ways, but there are other differences.

8 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 8 The protocols are affected by different error sources: The 4 message protocol is sensitive to un- equal message turn-around times. (The 2 message protocol doesn’t care about this at all.) The 2 message protocol is sensitive to errors in the estimation of the tracking offset. (The 4 message protocol doesn’t care about this at all.)

9 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 9 What’s going on with the turn around time? This is a well known issue. And it was pointed out appropriately in the original presentation. This requirement has also come up again during the discussions on the ranging conference calls.

10 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 10 If the turn-around times are not equal, the crystal drift during the different size turn around times cannot be cancelled out. Where does the turn around time come from? If it’s going to be the same for all implementations, it will have to come from our spec! What’s going on with the turn around time?

11 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 11 What’s going on during the turn-around time? The receiver is parsing the just arrived message to discover that it’s supposed to respond. The receiver is computing message checksums to decide if it trusts the just arrived message. The receiver is computing timestamps and formatting the response message to send back.

12 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 12 Is it natural for all 15.4a implementations to have similar turn-around times? Not really: High performance implementations will handle the turn-around activities in hardware and can do it in effectively ZERO time. Lower performance implementations might want help from a processor parsing messages, computing checksums, and formatting responses.

13 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 13 The High Performance implementation does the turn-around in ZERO time? Yeah, it’s a trick. The way we do it is to parse the message and the checksum in hardware (which is very fast) and then immediately start transmitting the outgoing preamble. While the outgoing acquisition preamble is going out on the air, we do the timestamp computations and format the payload part of the message.

14 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 14 What will happen if we legislate the turnaround time for all implementations during the drafting of the spec? Either the high performance camp must give up performance, or the low cost camp must rely less on their processor. It raises the potential for unhappy human designers.

15 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 15 So what happens if we rely on tracking offsets ? As discussed in , some kind of tracking offset measurement will be available in all of the receivers. But if we use tracking offsets to correct for crystal differences AND we have tracking errors (it happens!) then the tracking errors get into our range measurement.

16 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 16 So which protocol is most accurate? IF we constrain the 4 message turn-around times to be equal, And If we use reasonable estimates for the sizes of the other error sources. And If we are careful to line up the errors in the most unfavorable way in both cases, Then: the resulting range errors are essentially equal.

17 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 17 “line up the errors in the most unfavorable way in both cases…” Huh? What’s that all about? There are so many small error sources, in both protocols, that if they are not lined up (by a human with malice) to all pull the same way, the errors can cancel out and both protocols give spectacular results. That’s not real! Nature has malice too, it’s just less systematic.

18 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 18 How is a malicious human supposed to keep track of all the errors to enforce pessimism? It’s hard to do manually. A spreadsheet called r0 accompanies this presentation. The spreadsheet can be used to adjust the signs of the errors (in a trial and error kind of way) to keep things always heading in the bad direction.

19 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 19 Okay, we have a spreadsheet: What’s the bottom line? Playing with the spreadsheet is encouraged. A typical set-up situation has a 10 ns prop time being measured by the 4 message protocol as 9.85 ns and the same situation is measured by the 2 message protocol as 9.827ns

20 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 20 “playing with the spreadsheet in encouraged…” How do we do that? This is r0 The cover sheet is not interesting, go to the “compute ranges sheet” We play around with numbers in this box While we watch the answers in the green cells

21 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 21 Conclusions The 4 message protocol is not more accurate than the 2 message with tracking protocol. Either protocol handles crystal offsets sufficiently that leading edge detection errors become dominant. The 4 message protocol does require that the message turn-around time be the same in all implementations.

22 doc.: IEEE a Submission June, 2005 Brethour, Time DomainSlide 22 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.


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