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MICROWAVE LINK DESIGN
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What is Microwave
Communication
A communication system that utilizes
the radio frequency band spanning 2 to
60 GHz. As per IEEE, electromagneticwaves between 30 and 300 GHz are
called millimeter waves (MMW) instead
of microwaves as their wavelengths areabout 1 to 10mm.
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Small capacity systems generally
employ the frequencies less than 3 GHz
while medium and large capacitysystems utilize frequencies ranging
from 3 to 15 GHz. Frequencies > 15
GHz are essentially used for short-haultransmission.
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Advantagesof Microwave Radio
Less affected by natural calamities
Less prone to accidental damage
Links across mountains and rivers are
more economically feasible
Single point installation and maintenance
Single point security
They are quickly deployed
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Line-of-Sight Considerations
Microwave radio communication requires a
clear line-of-sight (LOS) condition
Under normal atmospheric conditions, theradio horizon is around 30 percent beyond
the optical horizon
Radio LOS takes into account the concept of
Fresnel ellipsoids and their clearance criteria
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Fresnel Zone - Areas of constructive anddestructive interference created whenelectromagnetic wave propagation in free space
is reflected (multipath) or diffracted as the waveintersects obstacles. Fresnel zones are specifiedemploying ordinal numbers that correspond to thenumber of half wavelength multiples thatrepresent the difference in radio wavepropagation path from the direct path
The Fresnel Zone must be clear of allobstructions.
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Radius of the first Fresnel zone
R=17.32(x(d-x)/fd)1/2
where d = distance between antennas (in Km)
R= first Fresnel zone radius in meters
f= frequency in GHz
x
y
d=x+yR
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Line-of-Sight Considerations
Typically the first Fresnel zone (N=1) is used todetermine obstruction loss
The direct path between the transmitter and thereceiver needs a clearance above ground of at
least 60% of the radius of the first Fresnel zone toachieve free space propagation conditions
Earth-radius factor k compensates the refractionin the atmosphere
Clearance is described as any criterion to ensuresufficient antenna heights so that, in the worstcase of refraction (for which k is minimum) thereceiver antenna is not placed in the diffractionregion
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Effective Earths Radius = k * True Earths RadiusTrue Earths radius= 6371 Km
k=4/3=1.33, standard atmosphere with normallyrefracted path (this value should be used whenever
local value is not provided)
Variations o f the ray curvature as a
funct ion of k
True Earths curvature
= 6,371 Km
K=1
K=0.5
K=0.33
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Clearance criteria to be satisfied under
normal propagation conditions
- Clearance of 60% or greater at theminimum k suggested for the certain path
- Clearance of 100% or greater at k=4/3
- In case of space diversity, the antenna canhave a 60% clearance at k=4/3 plus
allowance for tree growth, buildings (usually
3 meter)
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Microwave Link Design
Microwave Link Design is a methodical,
systematic and sometimes lengthy
process that includes Loss/attenuation Calculations
Fading and fade margins calculations
Frequency planning and interferencecalculations
Quality and availability calculations
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Microwave Link Design Process
The whole process is iterative and may go through
many redesign phases before the required quality and
availability are achieved
Frequency
Planning
Link Budget
Quality
and
Availability
Calculations
Fading
Predictions
Interference
analysis
Propagation losses
Branching
losses
Other Losses
Rain
attenuation
Diffraction-refraction
losses
Multipath
propagation
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Loss / Attenuation Calculations
The loss/attenuation calculations are
composed of three main contributions
Propagation losses
(Due to Earths atmosphere and terrain)
Branching losses
(comes from the hardware used to deliver
the transmitter/receiver output to/from theantenna)
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Miscellaneous (other) losses
(unpredictable and sporadic in character
like fog, moving objects crossing the path,poor equipment installation and less than
perfect antenna alignment etc)
This contribution is not calculated but is
considered in the planning process as an
additional loss
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Propagation Losses
Free-space loss - when the transmitter andreceiver have a clear, unobstructed line-of-sight
Lfsl=92.45+20log(f)+20log(d) [dB]where f = frequency (GHz)
d = LOS range between antennas (km)
Vegetation attenuation (provision should be
taken for 5 years of vegetation growth)L=0.2f0.3R0.6(dB)f=frequency (MHz)R=depth of vegetation in meters (forR
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Obstacle Lossalso called Diffraction Loss or Diffraction
Attenuation. One method of calculation is based on knife
edge approximation.
Having an obstacle free 60% of the Fresnel zone gives 0
dB loss
0 dB
20dB16dB6dB0 dB
First Fresnel Zone
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Gas absorption
Primarily due to the water vapor and
oxygen in the atmosphere in the radio relayregion.The absorption peaks are locatedaround 23GHz for water molecules and 50to 70 GHz for oxygen molecules.The
specific attenuation (dB/Km)is stronglydependent on frequency, temperature andthe absolute or relative humidity of theatmosphere.
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Gas attenuation versus frequency
T=30o
RH=50%
Frequency (GHz)
0 25 50
0.4
T=40oC
RH=80%
1.0
23GHzTotal specific
gas attenuation
(dB/Km)
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Attenuation due to precipitation
Rain attenuation is the main contributor in thefrequency range used by commercial radiolinks
Rain attenuation increases exponentially withrain intensity
The percentage of time for which a given rainintensity is attained or exceeded is availablefor 15 different rain zones covering the entireearths surface
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The specific attenuation of rain is dependenton many parameters such as the form and sizeof distribution of the raindrops, polarization,
rain intensity and frequency Horizontal polarization gives more rain
attenuation than vertical polarization
Rain attenuation increases with frequency andbecomes a major contributor in the frequencybands above 10 GHz
The contribution due to rain attenuation is notincluded in the link budget and is used only inthe calculation of rain fading
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Ground Reflection
Reflection on the Earths surface may give
rise to multipath propagation
The direct ray at the receiver may interferedwith by the ground-reflected ray and the
reflection loss can be significant
Since the refraction properties of the
atmosphere are constantly changing the
reflection loss varies.
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The loss due to reflection on the ground isdependent on the total reflection coefficient ofthe ground and the phase shift
The highest value of signal strength isobtained for a phase angle of 0o and thelowest value is for a phase angle of 180o
The reflection coefficient is dependent on thefrequency, grazing angle (angle between theray beam and the horizontal plane),polarization and ground properties
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The grazing angle of radio-relay paths is verysmall usually less than 1o
It is
recommended to avoid ground reflectionby shielding the path against the indirect ray
The contribution resulting from reflection lossis not automatically included in the link
budget.When reflection cannot be avoided,the fade margin may be adjusted by includingthis contribution as additional loss in the linkbudget
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Signal strength versus reflection
coefficient
+10
0
-20
0.2 0.6 1.0
Amax
Amin
SignalStrength
(dB)
Total reflection coefficient
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Link Budget
The link budget is a calculation involving
the gain and loss factors associated with
the antennas, transmitters, transmissionlines and propagation environment, to
determine the maximum distance at which
a transmitter and receiver can successfullyoperate
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Receiver sensitivity threshold is the signal
level at which the radio runs continuous
errors at a specified bit rate
System gain depends on the modulation
used (2PSK, 4PSK, 8PSK, 16QAM,
32QAM, 64QAM,128QAM,256QAM) and
on the design of the radio
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The gains from the antenna at each end are
added to the system gain (larger antennas
provide a higher gain). The free space loss of the radio signal is
subtracted. The longer the link the higher the
loss. These calculations give the fade margin.
In most cases since the same duplex radio
setup is applied to both stations the calculation
of the received signal level is independent of
direction
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Receive Signal Level (RSL)RSL = Po Lctx + Gatx Lcrx + Gatx FSL
Link feasibility formulaRSL Rx (receiver sensitivity threshold)
Po = output power of the transmitter (dBm)
Lctx, Lcrx = Loss (cable,connectors, branching unit)between transmitter/receiver and antenna(dB)
Gatx = gain of transmitter/receiver antenna (dBi)
FSL = free space loss (dB)
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The fade margin is calculated with
respect to the receiver threshold level
for a given bit-error rate (BER).Theradio can handle anything that affects
the radio signal within the fade margin
but if it is exceeded, then the link couldgo down and therefore become
unavailable
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The threshold level for BER=10-6 for
microwave equipment used to be about
3dB higher than for BER=10-3.Consequently the fade margin was 3 dB
larger for BER=10-6 than BER=10-3. In
new generation microwave radios withpower forward error correction schemes
this difference is 0.5 to 1.5 dB
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Radio path link budget
Transmitter 1
Receiver 1
Splitter Splitter
Transmitter 2
Receiver 2
Output
Power (Tx)
Branching
Losses
waveguide
Propagation
Lo
sses
AntennaGain
BranchingLosses
Received
Power (Rx)
Receiver threshold Value
Fade Margin
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Fading and Fade margins
Fading is defined as the variation of the
strength of a received radio carrier signal
due to atmospheric changes and/or
ground and water reflections in thepropagation path.Four fading types are
considered while planning links.They are
all dependent on path length and areestimated as the probability of exceeding
a given (calculated) fade margin
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Multipath fading
- Flat fading
- Frequency-selective fading
Rain fading
Refraction-diffraction fading (k-type
fading)
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Multipath Fading is the dominant fading
mechanism for frequencies lower than
10GHz. A reflected wave causes a multipath,i.e.when a reflected wave reaches the
receiver as the direct wave that travels in a
straight line from the transmitter
If the two signals reach in phase then thesignal amplifies. This is called upfade
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Upfademax=10 log d 0.03d (dB)
d is path length in Km
If the two waves reach the receiver out ofphase they weaken the overall signal.Alocation where a signal is canceled out bymultipath is called null or downfade
As a thumb rule, multipath fading, for radiolinks having bandwidths less than 40MHz andpath lengths less than 30Km is described asflat instead of frequency selective
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Flat fading
A fade where all frequencies in the channel areequally affected.There is barely noticeablevariation of the amplitude of the signal across the
channel bandwidth If necessary flat fade margin of a link can be
improved by using larger antennas, a higher-power microwave transmitter, lowerloss feedline and splitting a longer path into two shorter
hops On water paths at frequencies above 3 GHz, it is
advantageous to choose vertical polarization
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Frequency-selective fading
There are amplitude and group delaydistortions across the channel bandwidth
It affects medium and high capacity radiolinks (>32 Mbps)
The sensitivity of digital radio equipment to
frequency-selective fading can be describedby the signature curve of the equipment
This curve can be used to calculate theDispersive Fade Margin (DFM)
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DFM = 17.6 10log[2(f)e-B/3.8/158.4] dB
f = signature width of the equipment
B = notch depth of the equipment
Modern digital radios are very robust andimmune to spectrum- distorting fade activity.
Only a major error in path engineering (wrongantenna or misalignment) over the high-clearance path could cause dispersive fadingproblems
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Rain Fading
Rain attenuates the signal caused by the
scattering and absorption of
electromagnetic waves by rain drops
It is significant for long paths (>10Km)
It starts increasing at about 10GHz and for
frequencies above 15 GHz, rain fading is thedominant fading mechanism
Rain outage increases dramatically with
frequency and then with path length
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Microwave path lengths must be reduced inareas where rain outages are severe
The available rainfall data is usually in theform of a statistical description of the amount
of rain that falls at a given measurement pointover a period of time.The total annual rainfallin an area has little relation to the rainattenuation for the area
Hence a margin is included to compensate for
the effects of rain at a given level ofavailability. Increased fade margin (margins ashigh as 45 to 60dB) is of some help in rainfallattenuation fading.
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Reducing the Effects of Rain Multipath fading is at its minimum during periods of
heavy rainfall with well aligned dishes, so entire path
fade margin is available to combat the rain attenuation
(wet-radome loss effects are minimum with shrouded
antennas)
When permitted, crossband diversity is very effective Route diversity with paths separated by more than
about 8 Km can be used successfullyRadios with
Automatic Transmitter Power Control have been used
in some highly vulnerable links
Vertical polarization is far less susceptible to rainfallattenuation (40 to 60%) than are horizontal polarisation
frequencies.
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Refraction Diffraction Fading Also known as k-type fading
For low k values, the Earths surface becomes curved and
terrain irregularities, man-made structures and other objects
may intercept the Fresnel Zone.
For high k values, the Earths surface gets close to a plane
surface and better LOS(lower antenna height) is obtained
The probability of refraction-diffraction fading is therefore
indirectly connected to obstruction attenuation for a given
value of Earthradius factor
Since the Earth-radius factor is not constant, the probability
of refraction-diffraction fading is calculated based on
cumulative distributions of the Earth-radius factor
B i R d ti
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Basic Recommendations
Use higher frequency bands for shorter hops andlower frequency bands for longer hops
Avoid lower frequency bands in urban areas
Use star and hub configurations for smaller
networks and ring configuration for larger networks In areas with heavy precipitation , if possible, use
frequency bands below 10 GHz.
Use protected systems (1+1) for all importantand/or high-capacity links
Leave enough spare capacity for future expansionof the system
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Space diversity is a very expensive way ofimproving the performance of the microwave link
and it should be used carefully and as a last resort The activities of microwave path planning and
frequency planning preferably should be performedin parallel with line of sight activities and other
network design activities for best efficiency. Use updated maps that are not more than a year
old. The terrain itself can change drastically in avery short time period.Make sure everyone on the
project is using the same maps, datums andcoordinate systems.
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Perform detailed path surveys on ALL microwave
hops.Maps are used only for initial planning, as a
first approximation.
Below 10 GHz , multipath outage increases
rapidly with path length.It also increases with
frequency , climatic factors and average annualtemperature.Multipath effect can be reduced with
higher fade margin. If the path has excessive path
outage the performance can be improved by
using one of the diversity methods.
Diffi lt A f Mi Li k
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Difficult Areas for Microwave Links
In areas with lots of rain, use the lowestfrequency band allowed for the project.
Microwave hops over or in the vicinity of the
large water surfaces and flat land areas cancause severe multipath fading.Reflections may
be avoided by selecting sites that are shielded
from the reflected rays.
Hot and humid coastal areas
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Thank you