Low Rate UWB for TG4a

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Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Ranging Accuracy of SDS-TWR Subject to Unequal
Reply Times Date Submitted July
2005 Source Shahriar Emami and Celestino A.
Corral Company Freescale Semiconductor,
Inc Address Freescale Semiconductor, Inc. 6100
Broken Sound Pkwy., N.W. Suite 1 Boca Raton, FL 
33487-2790 Voice(561) 544-4064 Re
802.154a Abstract This document addresses
the performance of SDS-TWR technique Purpose To
help evaluate ways to manage crystal
drift 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.
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Symmetric Double Sided-Two Way Ranging (SDS
-TWR)Proposed in document IEEE-15-05-0334-00-004
Device B
Device A
unknown propagation delay
reply time
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Sensitivity of 4 message TWR

• In 4 message two way ranging (SDS-TWR) it is
  required that treplyA treplyB
• Is ranging error sensitive to variation of
  treplyA in reference to treplyB? And to what
  extent?
• Document IEEE-15-05-0002-01-004a considers small
  variations in reply times (up to 2).
• We will compute the range and ranging error due
  to larger variations of reply time at various
  ranges.

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Calculated Ranges
E Range 20 40 60 80 100
10 m 10.30 10.60 10.90 11.20 11.50
20 m 20.30 20.60 20.90 21.20 21.50
30 m 30.30 30.60 30.90 31.20 31.50
40 m 40.30 40.60 40.90 41.20 41.50
50 m 50.30 50.60 50.90 51.20 51.50
10 PPM crystals are deployed. Excess reply
time of B in reference to A or 100 x
(treplyB-treplyA)/treplyA .
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Actual and Calculated Ranges
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Ranging Error
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Actual and Calculated Ranges
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Ranging Error
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Summary

• For a given tolerable percentage error, shorter
  ranges determine the maximum tolerable reply time
  variation.
• Ranging error at 10m and 50m are 3 and 16,
  respectively (10 PPM) subject to 100 variation
  in reply time.
• Ranging error at 10m and 50m are 11 and 60,
  respectively (40 PPM) subject to 100 variation
  in reply time.
• To limit the ranging error to 6, reply time
  variation should be kept under 40 and 10 for 10
  ppm and 40 PPM crystals, respectively.
• If low PPM crystals are used, the ranging error
  is not significant particularly at larger
  distances and especially when reply time
  variations are held under 40.
• On the other hand, the ranging errors for high
  PPM crystal devices are unacceptable if the reply
  time variations exceeds teens. Other methods
  would have to be deployed to compensate for or
  correct crystal drifts.