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
  differential reply times in between 2 and 20 us.
• We will compute the range and ranging error due
  to variations of reply time at various ranges
  when absolute reply time is in 0.1-1 ms range.

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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 and reply time is
assumed to be 1 ms. 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
Ranging error at 10m and 50m are 15 and 3,
respectively (10 PPM) subject to 100 variation
in reply time (1ms reply time).
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Actual and Calculated Ranges
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Ranging Error
Ranging error at 10m and 50m are 60 and 12,
respectively (40 PPM) subject to 100 variation
in reply time (1ms reply time
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What If Reply Time Is Decreased?
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What If Reply Time Is Decreased?
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Summary (1)

• For a given tolerable percentage error, shorter
  ranges determine the maximum tolerable excess
  reply time.
• 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 if reply time high could are
  unacceptable. Other methods would have to be
  deployed to compensate for or correct crystal
  drifts
• When reply time was reduced up to an order of
  magnitude (1ms-0.1 ms), ranging error and
  percentage ranging error also went down by up to
  an order of magnitude.

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Summary (2)
Ranging Error
Acceptable Error
Minimum Range
Maximum Range
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Bottom Line

• For a given set of parameters
• PPM ( )
• Reply Time
• Excess Reply Time( )
• Minimum Desired Range
• Calculate ranging error
• If percent ranging error acceptable, you are
  done.
• Otherwise one needs to reduce PPM, reply time,
  excess reply time or increase minimum range.