UWB Propagation Phenomena

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission
Title UWB Propagation Phenomena Date
Submitted 4 July, 2002 r3 16 July,
2002 Source Kai Siwiak Company Time
Domain Corporation Address 7057 Old Madison
Pike, Huntsville, AL 35806 Voice
256-990-9062 E-Mail kai.siwiak_at_timedomain
.com Re Response to the Call for
Contributions on UWB Channel Models (IEEE
P802.15-02/208r1-SG3a). Abstract This
contribution exposes a behavior of UWB signals in
multipath which is relevant to propagation laws
in multipath and hence for models intended for
evaluating UWB physical layer submissions for a
high-rate extension to IEEE 802.15.3. Purpose A
connection is shown between measured multipath
delay spread and the propagation law. this leads
to a theory for the connection and to a
generalized propagation law model that offers a
better understanding of UWB multipath channels
model which can be used to compare different UWB
PHYs. 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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UWB Propagation Phenomena

• Kai Siwiak
• Time Domain Corporation
• 16 July, 2002 IEEE 802.15-02/301r3

3
Overview

• High resolution UWB propagation measurements
  reveal a close connection between propagation law
  and multipath
• A theory based on measurements suggests to a
  generalized multi-slope multipath propagation law
  model
• Result has implications on receiving signals,
  RAKE gain and understanding of the UWB channel
  and channel models

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UWB Propagation Measurements

• A set of UWB propagation measurements 1 were
  carried out using a UWB pulse transmitter and a
  UWB scanning receiver
• The data were processed using the CLEAN algorithm
  2 to extract
• Strongest pulse power density vs. distance
• Total power density vs. distance
• RMS delay spread vs. distance

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Strongest pulse vs. Distance
The indoor UWB measurements 1 reveal that the
strongest pulse power density propagates
approximately as 30 log(d), distance d is meters
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Indoor Channel Impulse Response
Channel pulses (left) are processed with CLEAN
algorithm to obtain CIRs (right). The total
power density represented by the CIR can then be
reported
Source Yano 1
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Total Power Density vs. Distance
The same measurements 1 reveal that the total
power density propagates approximately as 20
log(d), distance d is meters, and with smaller
variance
8
RMS Delay Spread vs. Distance
The same measurements 1 further reveal that the
rms delay spread varies approximately as t0d,
distance d is in meters, and here t03 nano
seconds
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Initial Assumptions

• A simple Gedankenexperiment leads us to
  conclude that in a lossless 3-d environment of
  copolarized scatterers, total power propagates
  with inverse square of distance
• P(d) Ptx/4pd2
• We further assert that the multipath power delay
  profile is exponentially distributed 3
• S e-t/trms/trms

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Dissipative Losses

• There are additional dissipative losses of the
  form e-2ad which will increase the rate of signal
  attenuation beyond free space loss
• Dissipative lass basis for a very simple UWB
  propagation law 5 which modeled successive wall
  reflections as homogeneous attenuation
• Here, however a nepers/m, is actual dissipative
  I2R loss constant for total power propagation

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Model Parameters

• Rays or pulses arrive at uniform rate 1/t0
• For this measurements set, trmst0d
• The fractional power density associated with
  strongest ray P1 is

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Propagation Law

• Pulses propagate as
• P1(d) Ptx/4pd21- exp(-t0/trms)e-2ad
• where trmst0d
• The value of t0/t0 establishes the transition
  between free space and higher order power law
• When t0/t0d is small (increasing d) we use the
  approximation
• ex 1 x

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Deriving Higher Order Power Law

• Substituting
• P1(d) Ptx/4pd2t0/t0de-2ad
• ... and we have the theoretical basis for inverse
  3rd power propagation law in multipath scattering
  for this data set
• P1(d) Ptx/d3t0/4pt0e-2ad

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Relation to Outdoor Propagation

• Outdoors, the delay spread increases
  approximately with the square root of distance
  4
• Applying the same analysis, power density (in the
  absence of dissipative losses) gives a plausible
  inverse 2.5 power propagation law for this
  mechanism alone
• Poutside(d) CPtx/d 2.5

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Model Constants

• The theory can be generalized to non-uniform
  arrival rates for rays and other data sets
• Values of trms(d), and hence t0 3 ns and t03
  ns here, can be derived directly from UWB pulse
  propagation measurements
• Dissipation factor 20alog(e) dB/m is found from
  the propagation of total power density, here
  alt0.006 nepers/m (lt0.05 dB/m)

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Implications to RAKE Gain

• The maximum possible RAKE gain based on this
  measurement set is the ratio of total power
  density to single pulse power density
• Gmax,RAKE 10 log(d) indoors
• Gmax,RAKE 5 log(d) outside
• Propagation measurements are dependent on how the
  power is collected with respect to multipath in
  the receiver

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Summary

• Generalized propagation law in multipath is
• P1(d) Ptx/4pd21- exp(-t0/trms(d))e-2ad
• Which with this indoor data set for d gtgtt0/t0
  reduces to
• P1(d) Ptx/d3t0/4pt0e-2ad
• Maximum possible indoor RAKE gain here is
• Gmax,RAKE 10 log(d)
• The propagation law is general also applies to
  narrow band signal propagation

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Resulting Multi-Slope Model

• We can postulate a propagation law based on the
  theory
• P1(d) Ptx/4pd 21- exp(-(d1/d )g-2)e-2ad
• d1 is breakpoint distance, m, from free space to
  higher order law
• g is slope of higher order law
• a is dissipative loss constant 20alog(e) dB/m

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Conclusions

• A theoretical basis for propagation law in
  scattering was derived, general model proposed
• Propagation model can be applied to other data
  sets
• Propagation law, attenuation losses, multipath
  delay spread, and RAKE gain are closely connected
• Has implications on UWB channel models channel
  model must couple propagation law, dissipative
  losses and multipath correctly

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References
1 S. M. Yano Investigating the Ultra-wideband
Indoor Wireless Channel, Proc. IEEE VTC2002
Spring Conf., May 7-9, 2002, Birmingham, AL, Vol.
3, pp. 1200-1204 2 J.A. Högbom, Aperture
Synthesis with a Non-Regular Distribution of
Interferometer Baselines, Astron. and Astrophys.
Suppl. Ser, Vol. 15, 1974 3 William C. Jakes,
Microwave Mobile Communications, 1974, IEEE Press
Reprint 4 Private e-mail communication with
Henry L. Bertoni, 6 June 2002 5 K. Siwiak, A.
Petroff , A Path Link Model for UWB Pulse
Transmissions, Conference Proceedings of the
IEEE VTC-2001, Rhodes, Greece, May 6-9, May 2001