Terrestrial and Space Microwave Radio Communication

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Terrestrial and Space Microwave Radio
Communication
by Miroslav Kasal
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Microwave radio relay is a technology for
transmitting digital and analog signals, such as
long-distance telephone calls, multimedia data
transfer and the relay of television programs to
transmitters between two locations on a line of
sight radio path.
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In microwave radio relay, radio waves are
transmitted between the two locations with
directional antennas, forming a fixed radio
connection between the two points. Long
daisy-chained series of such links form
transcontinental telephone, multimedia data
and/or television communication systems. Another
way is an application of space transponder placed
into a satellite for long distance communication.
From technology point of view both ways are
similar with specific differences.
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Because a line of sight radio link is made, the
radio frequencies used occupy only a narrow path
between stations (with the exception of a certain
radius of each station). Antennas used must have
a high directive effect these antennas are
installed in elevated locations such as large
radio towers.
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Typical types of antenna used in radio relay link
installations are parabolic reflectors, shell
antennas and horn radiators, which have a
diameter of up to 4 meters for terrestrial and up
to 70 meters for space communication. Highly
directive antennas permit an economical use of
the available frequency spectrum, despite long
transmission distances.
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Parabolic dish antennas
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Horn antennas
PenziasWilson New Jersey - Bell Laboratories
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Because of the high frequencies used, a
quasi-optical line of sight between the stations
is generally required. Additionally, in order to
form the line of sight connection between the two
stations, the first Fresnel zone must be free
from obstacles so the radio waves can propagate
across a nearly uninterrupted path. Obstacles in
the signal field cause unwanted attenuation, and
are as a result only acceptable in exceptional
cases.
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Obstacles, the curvature of the Earth, the
geography of the area and reception issues
arising from the use of nearby land (such as in
manufacturing and forestry) are important issues
to consider when planning radio links. In the
planning process, it is essential that "path
profiles" are produced, which provide information
about the terrain and Fresnel zones affecting the
transmission path.
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The presence of a water surface, such as a lake
or river, in the mid-path region also must be
taken into consideration as it can result in a
near-perfect reflection (even modulated by wave
or tide motions), creating multipath distortion
as the two received signals ("wanted" and
"unwanted") swing in and out of phase. Multipath
fades are usually deep only in a small spot and a
narrow frequency band, so space and frequency
diversity schemes were usually applied in the
third quarter of the 20th century.
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The effects of atmospheric stratification cause
the radio path to bend downward in a typical
situation so a major distance is possible as the
earth equivalent curvature increases from 6370 km
to about 8500 km (a 4/3 equivalent radius
effect). Rare events of temperature, humidity and
pressure profile versus height, may produce large
deviations and distortion of the propagation and
affect transmission quality.
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High intensity rain and snow must also be
considered as an impairment factor, especially at
frequencies above 10 GHz. All previous factors
make it necessary to compute suitable power
margins, in order to maintain the link operative
for a high percentage of time, like the standard
99.99 or 99.999 used in 'carrier class'
services of most telecommunication operators.
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Satellite Radio Relay The idea of satellite
communication system was firstly formulated by
Arthur C. Clark in 1945.
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Every communication satellite in its simplest
form involves the transmission of information
from an originating ground station to the
satellite (the uplink), followed by a
retransmission of the information from the
satellite back to the ground (the downlink).
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The downlink may either be to a select number of
ground stations or it may be broadcast to
everyone in a large area. Hence the satellite
must have a receiver and a receive antenna, a
transmitter and a transmit antenna, some method
for connecting the uplink to the downlink for
retransmission, and prime electrical power to run
all of the electronics. The exact nature of these
components will differ, depending on the orbit
and the system architecture, but every
communication satellite must have these basic
components.
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Another big difference between the geosynchronous
antenna and the low earth antenna is the
difficulty of meeting the requirement that the
satellite antennas always be "pointed" at the
earth. For the geosynchronous satellite, of
course, it is relatively easy. As seen from the
earth station, the satellite never appears to
move any significant distance. As seen from the
satellite, the earth station never appears to
move. We only need to maintain the orientation of
the satellite.
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For communication satellites four typical orbits
are applied LEO - Low Earth Orbit lt 2000 km ICO
- Intermediate Circular Orbit (MEO) 5000 -
15000 km HEO - High Elliptical Orbit high of
apogee 40000 km, i ? 63 deg GEO -
GEostationary Orbit (GSO) 35786 km, i 0 deg
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Link Analysis It will be assumed that the
frequency of the modulated carrier is between 1
GHz and 30 GHz. In this case only direct wave can
be considered in most application of the high
gain antennas links with high elevation.
Consider the two isotropic antennas at a
distance r. The first one is being fed by the
radiofrequency signal with power P1. Let us
determine what power P2 will be attached by the
second isotropic antenna in point 2.
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In all points on a spherical surface with
radius r
P2
P1

r
1
2
the magnitude of Poynting vector is
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The power P2, which can be measured in a matched
load to the second isotropic antenna will be
where S2 is the effective area of the receiving
antenna. For isotropic antenna the effective area
is
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Then, for a power transfer it can be written
The value
is called the propagation loss of the direct
electromagnetic wave in free space.
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In praxis, logarithmic expression of the direct
electromagnetic wave attenuation is often used
Link budget of a microwave link is then defined
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C - is the power of the received modulated
carrier, N0 - the power spectral density of the
Additive White Gaussian Noise (AWGN) at the
same point of the receiver, T - is the
equivalent noise temperature, k - is the
Boltzmann constant 1,38.10-23 J/K PT - is the
transmitting power, GT, GR - are the gains of
transmitting and receiving antenna relative to
an isotropic antenna.
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The Equivalent Isotropic Radiated Power (EIRP) is
given by
Then, the previous equation can be written as
follows
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While the transmitting side is fully
characterized by EIRP, the receiving side is
fully characterized by ratio GR/T. For this
reason the ratio GR/T is called the figure of
merit or G/T (in dB/K) of the receiving
equipment and expresses receiving system
sensitivity. For practical reason the equation is
used often in logarithmic form
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The total noise power in a channel with noise
bandwidth Bn is given by
and the C/N ratio is given by
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as well as
The ratio Eb/N0 is introduced for the systems
with a two state discrete modulation. Eb is
average energy of the signal corresponding to one
bit. The ratio Eb/N0 is given by
where the fb is data rate.
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After substitution can be written
The Es/N0 ratio is introduced for higher state
discrete modulation. Relationship between these
two ratios is given by
where M is a number of modulation states.
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Parameter Eb/N0 is very important because it
allows to trade off different systems aside from
data rate. There are two basic criteria energy
budget and spectral efficiency fb/Bn. While for
terrestrial communication the spectral efficiency
is preferred, low energy budget is most important
for satellite communication.
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System Noise Temperature Consider the receiving
equipment shown in figure. This consists of an
antenna connected to the receiver.
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The connection is a lossy one and is at
a thermodynamic temperature TF (which is close
to the outdoor temperature T0 290 K). It
introduces an attenuation LF which corresponds to
a gain GF 1/ LF. The equivalent noise
temperature TS will be determined at the point
corresponding to the receiver input as follows
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The TA is the noise temperature of the antenna
and TR is the equivalent noise temperature of the
receiver itself. This noise temperature TS,
which takes account of the noise generated by
antenna and the connection together with the
receiver noise is called the system noise
temperature. When the sky is clear the noise
captured by the antenna consists of the noise
from the sky and the noise due to radiation
from the Earth.
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In the absence of meteorological formations the
antenna noise temperature contains contributions
due to the sky and the surrounding ground.
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For lossless antenna the noise is determined from
expression
where
is the brightness temperature of the sky in the
direction .
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In practice, only that part of the sky in the
direction of the antenna boresight contributes to
the integral as the gain has a high value in that
direction. As an consequence, the noise
contribution of the clear sky TSKY can be
assimilated with the brightness temperature for
the angle of elevation of the antenna.
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Radiation of the ground in the vicinity of the
earth station is captured by the side lobes of
the antenna radiation pattern and partly by the
main lobe when the elevation angle is small. The
sum of such contributions yields the value TG.
The antenna noise temperature is thus given by
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However, the antenna noise temperature increases
during the presence of meteorological formation,
such as clouds and rain, which constitute an
absorbent, and consequently emissive, medium.
Because the meteorological formation behaves as
an attenuator at the input of the system, the
antenna noise temperature is given by
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The ARAIN is the attenuation and Tm the mean
thermodynamic temperature of formation in
question. In conclusion, the antenna noise
temperature TA is function of the frequency, the
elevation angle and the atmospheric conditions.
Consequently, the figure of merit of an earth
station must be specified for particular
conditions of frequency, elevation angle and
atmospheric conditions.
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Conclusion The quality of the microwave link
between a transmitter and receiver can be
characterized by the ratio of the signal power to
the noise power spectral density. This is a
function of the characteristics of the terminal
equipment of the link, the transmitter EIRP and
the receiver figure of merit G/T and the
properties of transmission medium L0.
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In a satellite link between two earth stations,
two links must be considered the uplink and the
downlink. The propagation conditions in the
atmosphere affect the uplink and the downlink
differently. The attenuation L0 increases by rain
for both the uplink and the downlink according to
their frequencies. However for downlink the
system noise temperature increases by rain in
addition.
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Thank you for attention !