1. Submission Title: [Ranging Values]
November, 2005
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Ranging Values]
Date Submitted: [17 September, 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: [Making the case for augmenting timestamp information with figures of merit. ]
Purpose: [To start a discussion of range figures of merit for 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
Brethour, Time Domain

2. Ranging values in Positioning Applications
November, 2005
Ranging values in Positioning Applications
Specifically addressing “Figure of Merit” values and timestamps in the a specification.
Brethour, Time Domain

3. Review of ranging vs. positioning
November, 2005
Review of ranging vs. positioning
Remember that for a position solution, we are looking for the intersection of spheres. For this presentation, let’s do it in 2-D and think about the intersection of circles.
Brethour, Time Domain

4. The solver problem The solver is not part of our standard.
November, 2005
The solver problem
The solver is not part of our standard.
We will probably discuss the solver in an informative annex, but we certainly will not specify how an application does that job.
Brethour, Time Domain

5. The solver will often have more ranges than the minimum needed for a relative position solution.
November, 2005
We are supposed to decide where all these circles intersect.
Brethour, Time Domain

6. Let’s look at our example:
November, 2005
If trustworthy ranges are blue and untrustworthy ranges are red…..
We will call the positioning solution here
Brethour, Time Domain

7. Let’s look again:
November, 2005
If trustworthy ranges are blue and untrustworthy ranges are red…..
We will call the positioning solution here
Brethour, Time Domain

8. November, 2005
Our Experience:
Experience says that the solver is aided greatly by figures of merit being attached to individual range computations.
The important information is to have a hint about which ranges to believe.
Brethour, Time Domain

9. The solver needs to know which situation it’s dealing with:
November, 2005
The solver needs to know which situation it’s dealing with:
Case 1
Case 2
Brethour, Time Domain

10. November, 2005
Each range gets a figure of merit which is the composite of two channel soundings.
Maybe the solver does some fancy weighting thing, maybe it just throws out the unreliable measurements.
What the solver then does with the information is beyond our standard.
Brethour, Time Domain

11. Where does the figure of merit come from?
November, 2005
Where does the figure of merit come from?
The actual line of sight energy is arriving here
We must make a determination about how much we trust our judgment, given the specific channel sounding.
Brethour, Time Domain

12. November, 2005
Recommending a statistical approach to constructing the Figure to a of Merit.
The actual line of sight energy is arriving here
The characterized leading edge could be here
The question we seek to answer with the FOM: What % of the time in the characterized leading edge within 500 ps of the true leading edge?
Brethour, Time Domain

13. November, 2005
Recommending a 5 bit FOM
The top two bits would call out the confidence interval
The bottom three bits would call out the confidence level
Brethour, Time Domain

14. The top 2 bits call out the interval.
November, 2005
The top 2 bits call out the interval. 00 01 10 11
100ps
500ps
1 ns
3ns
For example: a FOM field of 01 means that the characterizing receiver is confident that the value it is using as the leading edge is within 500 ps of the true leading edge.
Brethour, Time Domain

15. The bottom 3 bits call out the confidence level.
November, 2005
The bottom 3 bits call out the confidence level.
000
001
010
011
100
101
110
111
No FOM
20%
55%
75%
85%
92%
97%
99%
For example: a FOM of 101 means that the characterizing receiver is 92% confident that the value it is using as the leading edge is within the confidence interval of the true leading edge.
Brethour, Time Domain

16. A final example of the FOM
November, 2005
A final example of the FOM
A FOM of “10010” would say that the PHY is 55% confident that the time of signal arrival called out in the timestamp is within 1ns of the true time of arrival.
Brethour, Time Domain

17. What about the sizes of the bit fields in the reports?.
November, 2005
What about the sizes of the bit fields in the reports?.
Brethour, Time Domain

18. November, 2005
This is Zafer’s 595r0 slide 4.
The issue is: How much resolution should we have when we represent times in these timestamp reports?
Brethour, Time Domain

19. The fundamental tension:
November, 2005
The fundamental tension:
We could choose to have our least significant bit of a timestamp field represent a super small (sub fempto second, even) time quantity. That would give our standard the theoretical capability of resolving ranges down to sub millimeters.
Why not?
It’s because we have to move those (likely to be meaningless) bits over the air in our timestamp reports.
Brethour, Time Domain

20. What happens if we are stingy with the bits?
November, 2005
What happens if we are stingy with the bits?
If we go too far the other way, there are few bits over the air in the timestamp reports but now the least significant bit might represent (for example) a tenth of a microsecond.
That’s great for conserving bits…….. why not?
Now the ultimate ranging measurement granularity would be (for this example) 30 meters.
Brethour, Time Domain

21. Recommendation for the smallest bit:
November, 2005
Recommendation for the smallest bit:
We can have the smallest bit represent one chip time divided by 128.
In the case of a chipping rate of 494 MHz this comes to ps.
In ps, rf energy in free space travels half a centimeter.
What if the chipping rate is 507 MHz?
We still let the smallest bit represent one chip time divided by ( = 15.4 ps)
The correction will be handled at some higher level.
Brethour, Time Domain

22. Recommendation for the time report fields:
November, 2005
Recommendation for the time report fields:
We can represent our time reports with 32 bits.
If the LSB of a 32 bit field represents 15 ps, then if all 32 bits are ones, we are describing a time interval of 128 ms.
Brethour, Time Domain

23. Recommendation for the crystal offset report fields:
November, 2005
Recommendation for the crystal offset report fields:
With a pair of 50 ppm tollerance crystals both “out” by the maximum amount, we could need as many as 19 bits to represent the maximum possible crystal drift count.
The recommendation is to place the 5 bit FOM directly above the 19 bit crystal drift number to completely fill up a 24 bit field.
Brethour, Time Domain