IDENTIFICATION OF SUITABLE CARRIER FREQUENCY FOR MOBILE TERRESTRIAL
COMMUNICATION
SYSTEMS WITH LOW ANTENNA HEIGHTS
By Robert F. Graham, Jr.,
205/97 1-6725, fax: 205/97
Sponsored
by Keith
Science Applications
International
Corporation,
1-6428, email: robert.f.graham-2
@cpmx.saic.com
Anderson,
Joint
Project
Office,
Unmanned
Ground
6725
Vehicle
Odyssey
Systems,
Drive,
Huntsville,
AL,
35806,ph:
AMCOM,
Redstone
Arsenal,
AL.
Figure 1. A Ground to Ground
Earth with No Obstructions
Propagation
Path over Plane
Figure 2. A Ground to Ground
Earth with an Obstruction
Propagation
Path over Plane
ABSTRACT
Terrestrial
communication
systems require
reliable
wireless links for mobile to base connectivi~
beyond the line of
sight where both ends of the link have antennas in close
proximity
to the ground.
Frequency sele:cti;lewill
partially
determine
the
performance
of
a”
terrestrial
communications
system in an intense multipath
environment.
This paper is part of an experimental
e~ort to examine many
aspects of RF propagation
and radio hardware
issues and
focuses on the efiects of rolling terrain, trees, and buildings on
radio performance
over a wide range of unmodulated
carrier
frequencies.
Experimental
data were collected
using the
Unmanned Ground Vehicle Technology
Test Bed (UGV TTB)
and other transmitter/receiver
test f~tures.
Experimental
data
were analyzed to determine
the sensitivities
of propagation
mechanisms to carrier jiequency.
The experimental/analytical
results along with bandwidth requirements and legal constraints
indicate suitable ranges of carrier frequencies for terrestrial
communication
systems.
INTRODUCTION
The reason for determining a suitable carrier for carrier
frequency
for terrestrial
communications
is to minimize
transmitter power requirements at a given desired range. Three
considerations
drive the selection of carrier frequency:
good
accommodation
of
required
propagation
performance,
bandwidth,
and available
channel
space.
Similar
to
performance, cost, and schedule, the above considerations form a
box around the selection process for carrier frequency.
This paper gives first priority
to good propagation
performance and investigates the sensitivity of RF propagation
over a terrestrial channel to carrier frequency.
Accommodation
of required bandwidth
and available channel space refine the
selection of suitable carrier frequencies, especially with respect
to signals with video bandwidths.
PECULIARITIES
OF TERRESTRIAL
LINKS
A terrestrial link of modest range, for example a few
kilometers,
would suggest little technical challenge if the link
were not from one point on the ground to another point on the
ground.
A 4 km link at 300 MHz would require an effective
radiated power (ERP) of about 50 microwatt
in space but might
require as much as 500 W with antennas close to the ground.
Figure 1 and 2 depict typical ground to ground configurations.
Figure 1 shows a typical ground to ground propagation
path with no obstructions.
The obstructionless path is attenuated
far below the free space level by the coincidence of the direct ray
and the ground reflected ray at the receive antenna. The severe
attenuation is caused by the ground reflected ray being inverted
by the ground reflection, thus tending to cancel the direct ray.
Figure 2 adds an obstruction to the propagation
path.
An obstruction such as a tree is translucent to RF and propagates
an attenuated ray through the tree. Also, the portion of the direct
wavefront and ground reflected wavefront (represented as rays in
Figure 2) that is not blocked by the tree is diffracted around and
over the tree and then passed to the receive antenna as another
attenuated ray.
Therefore there are three attenuating
mechanisms to
These
consider for ground to ground propagation
paths.
mechanisms are reflection, absorption, and blockage.
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
For most signal paths, except for paths through very
wooded areas, absorption
is considerably
less important than
reflection or blockage.
Compared to blockage, absorption can
be viewed as a gain, because some RF is being transmitted
through the absorbing medium in addition to the blocked signal
that is diffracted over and around the absorbing medium.
That
is, blockage constitutes a worse case than does absorption.
For
example, if a forest was replaced with a mound of earth
occupying
the same volume, the signal 10SS would be made
greater, because earth absorbs more completely than trees and is
opaque to RF. One scenario where absorption is particularly
a
problem is where both ends of a link are located inside or near
the edge of an absorbing medium, because the ends of the link
come in close proximity
to the absorbing
medium
where
maximum
blockage occurs.
In the case of jungle, only an
extremely powertid
transmitter could support a link at 4 km.
The jungle
case is usually rendered irrelevant
for mounted
radios by the lack of mobility of a mobile unit inside a jungle.
profile.
In Figure 4, propagation data was collected with a 400
MHz CW signal along a nearly straight line path for 2 km on a
more hilly (30 meter height difference)
terrain profile.
The
following
information
is common to both Figures 4 and 5. The
receiver was in a stationary vehicle at the start of the path for
both runs.
A marker indicating
the top of a each hill was
recorded in the data by the mobile operator turning off the
transmitter.
The marker also roughly indicates the boundary
between line of sight and non-line of sight.
The ERP at the
transmitter was +44 dBm (10 watts with nominal stub antenna
gain). The height of both antennas above their respective points
on the ground was 2 meters. Also, a terrain profile is shown
near the bottom of each plot. The height difference is 10 meters
for Figure 4 and 30 meters for Figure 5.
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COMPARISON
MODELS
OF
EXPERIMENTAL
DATA
AND
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Figures 4 and 5 show both raw data and calculated
data on the same plot.
The raw data and calculated data are
explained in separate sections below.
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DATA:
Four dozen propagation
data files were collected
along several roads on and near Redstone Arsenal in Alabama.
The distances along these roads was from 1.5 to 5 km. All
roads were straight, wide, and paved except for Overlook Road,
which turned into a narrow dirt road after 1 km and wound its
way over the hill at the end of the road. All roads where lined
with a combination
of trees, buildings,
and clearings.
Ten
frequencies were used ranging from 30 MHz to 2320 MHz.
Refer to Figure 3 for a block diagram of the equipment setup for
the data collection experiments.
In all experiments, the receiver
was located in the stationary vehicle and the transmitter was
located in the moving vehicle.
Stub antennas were cut for
specific frequencies.
In the experiments represented by Figures
4 and 5, a synthesized source with a 10 watt amplifier was used.
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DISTANCE,
METERS
Figure 4. Prediction of Rideout Road at 161 MHz with an ERP
or +44 dBm and 2 Meter Antenna Heights
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Figure 5. Prediction of Overlook Road at 400 MHz
ERP of +44 dBm and 2 Meter Antenna Heights
with
an
There are two things to notice about the levels of the
measured propagation data for these two data files.
First, the
mean signal strength within the line of sight is the same for both
r?equencies. Second, several places on the plots show the raw
data below the level of all predictions.
RECEIVER
I
Figure 3.
Equipment
Setup for Data Collection
In Figure 4, propagation data was collected with a 161
MHz continuous wave (CW) signal along a straight line path for
4 km on a relatively
flat (10 meter height difference) terrain
The mean signal strength at 1000
dBm for both the 161 MHz and 400 MHz
of the raw data at the end of the 400 MHz
than predicted by any model. The distance
of the 4 km requirement, but the transmitter
hill. This is an example of a situation that
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
meters is about -70
data runs. The level
data run was lower
at this point is half
is behind a 30 meter
has more blockage,
reflection,
or absorption
than anticipated,
because complete
cancellation of the signal is always possible and zero signal is a
signal strength of minus infinity decibels.
a
o
s
s,
.,,
A
second
prediction
was generated
from
the
Okumura-Hata
model.
This model has been used extensively
for cellular network design and offers more accuracy at higher
frequencies and longer distances. At 161 MHz, the OkumuraHata model is accurate within about 20 dB. At 400 MHz, the
Okumura-Hata
model is accurate within
about 15 dB.
The
Okumura-Hata model is not a suitable model for short distances
of 4 km or less, because it does not increase the effect of
blockage with distance, and thus overestimates
the effect of
blockage for non-line of sight cases at short distances.
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DISTANCE,
METERS
Figure6.
Comparison of Highest
Frequencies at Rldeout Road
and Lowest Measured
Figure 6 compares experimental
data along Rideout
Road (terrain protlle near bottom of plot) for the highest and
Note three
lowest frequencies
used in the experiments.
contrasting features: mean level, deviation from mean level, and
sensitivity to terrain variations.
First, the mean level in the line
of sight region (Line of sight is everywhere along the path
except the end of the run.) is about 20 dB higher for the 34.2
MHz signal than it is for the 2320 MHz signal. At 1000 meters,
the curve for 2320 MHz has a signal strength of-70 dBm, which
is in agreement with the data curves for 161 MHz and 400 MHz
in Figures 4 and 5 respectively.
The 20 dB advantage for 34.2
MHz is caused by a ground surface wave that was observed for
all experimental runs below 80 MHz. The ground surface wave
uses the soil of the ground as a conduit for RF energy. This
conduit is very lossy at high frequencies.
Second, the deviation
from mean level or fhst fading noise on the two curves is greater
for the higher frequency.
Trees md buildings
have higher
cross-sectional
areas at higher frequencies,
and thus higher
frequencies contribute more scattered field to the signal level at
the receiver.
Third, the curve for the 2320 MHz signal shows
more sensitivity to terrain variation than does the curve for the
34.2 MHz signal.
The higher frequency is more sensitive to
terrain
variation
because the shadows caused by higher
frequencies are more crisply defined than the shadows caused
by lower frequencies due to the shorter wavelength of higher
frequencies.
MODELING:
Several models were used to predict the propagation
loss. The models are called Free Space, Okumura-Hata,
Plane
Earth, and Site Specific listed in top to bottom order as they
appear in Figures 4 and 5.
The Free Space model, which accounts for only the
spreading of the wavefront with distance, would ordinarily
be
accurate for space and airborne links.
For the two data runs
shown, the Free Space predictions
are optimistic
by 30 to 50
decibels (dB).
The Free Space model for 161 MHz is more
optimistic than for 400 MHz because free space loss increases
with frequency, whereas th~ measured performance is frequency
independent within the line of sight.
A third model is the Plane Earth model.
The Plane
Earth model adds the effect of a specular ground reflection to
the free space loss. The Plane Earth model gives the correct
answer within about 10 dB. Note that the Plane Earth model is
relatively
accurate even though
the terrain
profile
is not
perfectly flat. The accuracy of the Plane Earth model indicates
that the ground reflection
causes most of the loss associated
with ground to ground data links. The Plane Earth model
predicts the LOS frequency independence and a path loss that is
inversely proportional
to the fourth power of the range as
outlined in Burlington
[1] and Bertoni et al [9]. Essentially, the
ground modifies the antenna pattern such that the antenna beam
lifts off the ground, which is caused by the 180 degree phase
shift on the electric field of the ground reflected ray.
A fourth model is the Site Specific model, which was
developed in conjunction
with this paper.
The Site Specific
model is derived from the Plane Earth model by adding terrain
dependent blockage
loss to the Plane Earth model.
The
blockage point for each mobile position along the course was
found and the effect of blockage was computed using a table of
a Cornu spiral representation
of the received field.
The Site
to within
about
6 dB.
Specific
prediction
is accurate
Accounting
for blockage allows the prediction to be somewhat
more accurate than the Plane Earth model and allows the
prediction to follow the undulations of the terrain.
SELECTING
CARRIER
GENERIC
MODEL
FREQUENCY
BASED
ON
A
Models predict mentioned
above predict the mean
level of the raw data only. Inspection of the raw data in Figures
4 through 6 shows that the signal strength at any instant in time
can be considerably below the mean level.
The correct mean
level model will be above the raw data 50~o of the time.
A
generic model used to support the design of a radio link, should
allow the raw data to be above the model predicted level 99°/0
or
more/less of the time, depending on the error tolerance of the
system containing the radio link. Thus, a fade margin should be
added to the link budget to ensure the reliability
of the link at
moderately
low signal strengths.
The fade margin ensures
protection of the system against errors due to thermal noise, but
does not ensure protection of the system against errors due to
distortion of the propagated RF in the channel.
A generic model can deal with blockage by adding a
shadow margin to the link budget.
For rolling hill terrain, a
suitable method of adding shadow margin is outlined in the
second section below.
FADE
MARGIN:
A fade margin
is added to the link budget by
statistically characterizing fast fading. The fade margin given at
the 1’%. level ensures that signal strength is above the fade
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
margin 99% of the time.
If the fade margin in dB is roughly
doubled, the signal strength can be ensured to be above the
doubled fade margin 99.9% of the time. The fast fading is truly
transitory provided that the mobile vehicle is moving.
The
effect of fast fading for a moving vehicle is to introduce burst
errors into the received data.
The Rayleigh distribution
is the theoretical limit for
the case of a group of scattered rays having no dominant ray in
the group.
Statistical cumulative
distributions
of experimental
data roughly agree with the distributions
in Bullington’s
“Radio
Propagation Fundamentals”
[1] as shown in Figure 7. Note that
fade margins must increase with frequency.
10[
apply for urban terrain with
terrain.
COMPARISON
tall buildings
or extremely
rugged
OF FREQUENCIES:
A generic non-site specific prediction
can be used to
help determine optimum carrier frequency.
A plot of generic
data was made over a 100 to 4000 meter range.
The curves
were generated using the plane earth model
minus two
frequency dependent margins, fade margin and shadow margin.
Both margins are taken from Bullington
[1 and 5]. The shadow
margin is for a terrain variation of 200 meters and is ramped in
from zero to full value over the 4 km range. The ramping of the
shadow margin corresponds to the increasing
probability
of
shadowing with distance.
The Okumura-Hata
model was not
used, because it does not increase the effect of blockage with
distance, and thus overestimates the effect of blockage at short
distances.
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Non-site Specific Prediction
Figure 8.
dBm and 2 Meter Antenna Heights
SIGNAL
LEVEL
IN DB
BELOW
MEAN
LEVEL
Figure 7. Fade Level Chart from “Radio Propagation
Fundamentals”
by Kenneth Bullington
[1]
SHADOW
MARGIN:
Shadow margin is an allowance
for a reasonable
“worst” case amount of shadow loss due to signal blockage.
The amount of the shadow loss is the difference
in signal
strength in dB between an unobstructed path and an obstructed
path. The shadow loss is directly proportional
to frequency.
If
the frequency is raised one order of magnitude, then the shadow
loss increases 10 dB. The direct proportionality
of shadow loss
to frequency was stated by Bertoni, et al [9] and verified in the
calculation of the blockage loss in the site-specific model.
A 200 meter high terrain variation gives a reasonable
“worst” case for a 4 km link, because there is probably only one
200 meter hill over a 4 km range. Choosing a lesser height is
too benign to represent a “worst” case, and choosing a greater
height creates a scenario where there M probably less than one
hill of this height.
If there is less than one hill over a range,
then the scenario is carried out on one side of that hill or just
barely over the crest of that hill, which is also too benign to
represent a “worst”
case. This type of reasoning would not
for an ERP of +44
The curves in Figure 8 assume an ERP of +44 dBm
and antenna heights of 2 meters. Note that the curve for 1 GHz
ends up at -130 dBm at 4 km. If the power at the transmitter is
raised 10 dB and the antenna height at one end is raised to 20
meters for a 20 dB increase in height gain, the signal strength at
4 km for 1 GHz becomes -100 dBm. A sensitivity of-100 dBm
would support low resolution real time video or high resolution
non-real time video. High resolution
real time video has a
threshold at about -90 dBm. The message of Figure 8 is that
improves
as carrier
frequency
propagation
performance
However,
when bandwidth
accommodation
and
decreases.
available
channel
space are considered,
higher
carrier
frequencies may become necessary.
The curve for 10 MHz has a 30 dB gain due to the
presence of a surface wave for frequencies below 80 MHz. 10
MHz has an ambient noise floor that is at least 10 dB higher
than the noise floor at 100 MHz.
Thus, the advantage of 10
MHz over 100 MHz is about 20 dB. 20 dB is still a tremendous
advantage,
corresponding
to a 100 to
1 power
ratio.
Unfortunately,
there are no available video channels below 80
MHz;
so that the ground
surface wave cannot be taken
advantage of without
accepting a narrow band assignment
requiring extreme compression of the video data rate and large
antennas.
There is a 20 dB spread in performance between 100
MHz and 1 GHz due mainly to fade margin.
This region
commonly supports video bandwidth channels and has available
channel space especially at the high end.
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
10 GHz appears to be out of contention as a possible
carrier frequency
except for the fact that high frequencies
support small high gain antennas.
If a commitment
could be
made to support the pointing
of directional
antennas at both
ends of the link, 10 GHz might be able to perform as well as 1
GHz, because of the gain of directional antennas may over come
fade margin and additional shadow loss. The higher bands may
still be viable if directional
antennas are used and subsystems
are bought or developed to point the directional amtennas.
1.
Ground communications
propagation losses are as much as
70 dB higher than space communications
propagation
losses.
2.
Within the line of sight, mean signal strength is frequency
independent and antenna height dependent.
3.
Fast fading
iiequency.
4.
Available
frequencies between 100 MHz and 1 GHz are
usable terrestrial
video links without
requirements
for
directional
antennas
or data compression
to reduce
bandwidth but will require elevated antennas and 100 watts
or more of transmitter power for low antenna heights.
5.
and
shadow
[1]
K. Burlington,
“Radio Propagation
Fundamentals,”
The Bell System Technical Journal., Vol. XXXVI,
No. 3, pp.
593-626, May 1957.
H. T. Head,
“The Influence of Tress on Television
[2]
Field Strengths at Ultra-High
Frequencies,” Proceedings of the
IRE, pp. 1016-1020, June 1960.
T. Tamir,
[3]
“On Radio
Propagation
in
Environments,”
IEEE Trans. Ant. and Prop., Vol. AP-15,
pp. 806-817, November 1967.
SUMMARY
losses
REFERENCES
losses
increase
with
Frequencies below 100 MHz require video data rates in the
tens of KHz and compression factors
of about 100 or
better for real time video. Frequencies below 100 MHz are
appropriate for audio and low data rate communications
where available channels can be found.
6.
Frequencies above 1 GHz require directional
antennas of
10 dB gain or better and tracking subsystems for pointing
the antennas.
7.
The provision
of sufficient
link budget for a radio link
ensu;es only a specified reliability
against errors due to
RF propagation
through
an intense
thermal
noise.
multipath
channel causes signal distortion
at very high
wide
bandwidth
strengths,
particularly
for
signal
waveforms.
Forest
No. 6,
A. G. Longley,
R. K, Reasoner,
[4]
“Comparison
of
Propagation Measurements with Predicted Values in the 20 to
10,000 MHz Range,” ESSA Technical
Report ERL 148-ITS
97., SAMSO-IR-70-51
Vol. 1, pp. 1-102, January 1970.
K. Burlington,
“Radio Propagation
for Vehicular
[5]
Communications,”
IEEE Trans. Veh. Technol., Vol. VT-26, pp.
295-308, November 1977.
[6]
Office
1978.
“Radio Propagation in Urban Areas,”
A. G. Longley,
of Telecommunications
Report 78-144,
pp. 1-49, April
R. K. Tewari, S. Swarup, M.N. Roy,
“Radio Wave
[7]
Propagation Through Rain Forest of India,” IEEE Trans. Ant.
and Prop., Vol. 368, No. 4, pp. 433-449, April 1990.
A. Tavakoli, K. Sarabandi, F. T. Ulaby,
“Horizontal
[8]
Propagation
Through
Periodic
Vegetation
Canopies,”
IEEE
Trans. Ant. and Prop., Vol. 39, No. 7, pp. 1014-1023,
July
1991.
H. L. Bertoni, W. Honcharenko,
L. R. Maciel, H. H.
“UHF Propagation
Prediction
for Wireless Personal
Co&munications,”
Proceedings of the IEEE, Vol. 82, No. 9, pp.
1333-1359, September 1994
J;:
J. T. Hviid, J. B. Andersen,
J. Toftgard,
J. Bojer,
[10]
“Terrain-Based
Propagation Model for Rural Area- An Integral
Equation Approach,”
IEEE Trans. Ant. and Prop., Vol. 43, No.
1, pp. 41-46, January 1995.
“LEO Satellite Channels for Packet
D. A. Jiraud,
[11]
Communications:’
Applied Microwave and Wireless, pp. 42-55,
Summer 1995.
ACKNOWLEDGEMENTS
I would like to thank Tom Barr and Warren Harper,
both senior engineers at SAIC, for their support on this paper. I
would also like to thank the UGV Joint Project Office for their
funding and guidance, and, in particular, Keith Anderson for his
support of the experiments.
0-7803-4902-4/98/$10.00 (c) 1998 IEEE