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MICROWAVE TRANSMISSIONPRINCIPLES
By
Tope Okunlola
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IntroductionRadio Frequencies
Wireless telecommunications involve theuse of radio frequencies for
telecommunication purposes
Radio Frequencies are electromagnetic
radiation spanning from roughly 300Khz to
300Ghz
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Microwave Frequencies
They are sub band of the Radio Frequency
spectrum
They roughly range from 2Ghz to 60Ghz
They are used for modern commercial
telecommunications purposes
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Microwave Link
A Microwave Link is a communications
system that uses radio waves in the
microwave frequency range to transmit
information between two fixed locations on
the earth. That is, it is a point to point
wireless communication technology for the
transmission of data, voice and video(Broadband). Microwave links move large
amounts of information at high speed.
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Microwave Users
Wireless ISPs use microwave links to
provide high speed internet access to their
clients
Telephone companies transmit calls
between switching centers on microwave
links
Cellular telephone calls are carried
between cells on microwave link
Broadcasters send programmes from
studio to transmission station on microwave
link
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Microwave Advantages
It has the following advantages:
Fast and Easy Deployment
Cheaper cost of deploymentCheaper cost of Radio Equipment
Flexibility in terms of upgrading,
redeploying, and capacity increase
Resilience to natural calamities/problems
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Microwave Network
This consists of the portions of the network
communicating by Radio (Microwave)
frequenciesCommercial telecommunications Networks
use Radios for both its Access and Transport
Networks
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Access Network
This comprises of the subscriber wireless
terminals and the Base Station Access
Radios
It is also known as the Point to Multipoint
Network
There must be a means of transporting
information from one BS to another as
subscribers may reside on different BSIn a wireless telecommunication system,
access network is always wireless
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Transport Network
This is the network that serves as a pipe to
carry voice, data and Video signals from one
BS to anotherIt is also known as Point to Point
It is also known as the Backbone
In a wireless telecommunication system,
transport network is not always wirelessTransport choices may be mixed in the
Transport network
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Transport Choices
Leased linesMicrowave
Fibre Optics
Satellite
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Microwave Protection Scheme
Space Diversity
Frequency Diversity
Monitored Hot StandbyRoute Diversity
Hybrid Diversity
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Microwave Topology
Linear/Chain
Star
Tree
Ring
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Components of Microwave
Radio Equipment
Transmission cable
Antenna
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Contents
Line of Sight: there are Visual LOS and RF
LOS; Visual LOS occurs when one is able
to see the transmitting antenna with thevisible eyes from the receiving end, and
vise versa. RF LOS occurs like visual LOS
with the exception that the LOS can be
affected by the blockage of the Fresnel
Zone.
Microwave Terms with meaningsMicrowave Terms with meanings
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Microwave Terms with meaningsMicrowave Terms with meanings
Fresnel Zone: This is the portion of the
space (air) that the RF transmitted signals
travel through from the transmitter to thereceiver. It is ellipsoid. The first 60% of the
fresnel zone must be free of obstruction for
a good link.
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Microwave Terms with meaningsMicrowave Terms with meanings Obstructions: This may be Natural, or Man made.
Natural e.g. Mountains, hills. Man Made e.g.
Buildings. They may be referred to as clutter or
terrain when designing many hops.
Hop: A hop is a single link involving two Base
Stations communication each other.
Transmitting Power: This is the amount powerthat is released from the antenna source into the
atmosphere. It does not include Loss of cables and
Gain of antenna. It is measured in mW or dBm.
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Propagation Loss:
i. Reflection: This occurs when part of the
signals transmitted are reflected back bythe earth terrain and clutter. The
wavelength of the reflected signals is
smaller compared to the diameter of the
obstructing surface.
Microwave Terms with meaningsMicrowave Terms with meanings
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Microwave Terms with meaningsMicrowave Terms with meanings
ii. Refraction: This occurs when part of the signals
transmitted passes through another medium of
different density and then bends away from thenormal route.
iii. Scattering: This occurs when part of the
transmitted signals meet some obstacles that are
having diameters smaller compared to theirwavelength. These obstacles may be rough
surfaces, or even fine particles in the air.
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Microwave Terms with meaningsMicrowave Terms with meaningsiv. Diffraction: This occurs when the transmitted
signals meet obstacles on the way and could not
pass through, but passes either sides of the
obstacles. It is effect of the waves turning, or
bending around the obstacles.
v. Absorption: This occurs when part of the
transmitted signals strikes and are absorbed intothe obstacles in a way that it does not pass
through, reflect off, or bend around the object.
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Microwave Terms with meaningsMicrowave Terms with meanings
iv. Path Length: This is the distance between the
Transmitter and the Receiver. In other words, the
distance of a hop.v. Free Space Loss: This is the loss that occurs in the
air/space when the wave is being transmitted from the
transmitting antenna to the receiving antenna. It is the
loss occurred in the path. It can be referred to as Path
Loss. Mathematically,FSL: 92.45 + 20LogF + 20LogD
D= distance in Km, F= Frequency in GHz
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Microwave Terms with meaningsMicrowave Terms with meaningsiv. Loss: This is a decrease in signal strength. Loss can be
cable loss, or FSL. Signal degradation can be caused as a
result of Impedance mismatches in the cables and the
connectors. This is also referred to as attenuation. The unit
is decibel, Db.
v. Gain: This is an increase in signal amplitude. It can be
when an external power source such as RF amplifier is
used to amplify the signal or a high gain antenna is used to
focus the beam width of a signal to increase the signal
amplitude. The unit is decibel, Db.
vi. Antenna: An antenna converts electrical energy from the
cable into waves in case of a transmitting antenna, and
waves into electrical signal in case of a receiving antenna.
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Microwave Terms with meaningsMicrowave Terms with meanings
vii. Antenna Gain: This is proportional to the antenna
physical shape and size. Antenna creates the effect of
amplification by virtue of its physical shape. Antenna
gain is as a result of focusing the signal into a tighter
beam. The focusing of the radiation is measured by wayofbeamwidths, which are either measured in horizontal
or vertical degrees. Antenna are of different types viz:-
Omni directional, Semi directional, Highly directional.
viii. Voltage Wave Standing Ratio: This occurs when there is
mismatched impedance that is, resistance to current flow
measured in Ohms between the devices in the
Microwave system. The effect can be a decrease in the
amplitude of the transmitted signal.
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Microwave Terms with meaningsMicrowave Terms with meaningsx. Radiated Power: This is the amount of power as measured
in the main lobe of the transmitting antenna. This puts into
consideration the losses and the gains encountered during
propagation. It can be referred to as Effective IsotropicRadiated Power. Mathematically, EIRP = Transmit Power
+ Transmit Antenna Gain + Cable Loss of the transmitter.
It is measured in mW or dBm.
xi. Receiver Sensitivity: This is also known as Receiver
Threshold. It is the minimum Power level at the Input of aradio receiver for it to decode a packet of data without an
error, to transfer information without error. A good
example is the Elastic Limit.
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Microwave Terms with meaningsMicrowave Terms with meaningsxii. Fade Margin: This is also known as Link Margin. This is the
allowance given to the link when designing, for it to function
properly putting multipath effects into consideration and any
unforeseen contingencies that may occur even afterimplementation. It can be viewed as link insurance. The
bigger the FM the better the link reliability. Mathematically,
FM = Received Signal Level Receiver Sensitivity.
xiii. Link Reliability: This is the percentage of the time the link is
expected to be up. The higher the FM, the more reliable thelink is. At least 99.995% is required for a short-haul. And
99.9999% for a long-haul. Reliability is also referred to as
Availability.
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Microwave Terms with meaningsMicrowave Terms with meaningsxiv. Signal to Noise Ratio: This is the minimum
power difference to achieve between the wanted
received signal and the noise level/floor. The
noise can be thermal noise, interfering noisefrom other network or within the same network.
For the microwave system to be able to work at
a certain data rate, the system needs a minimum
S/N ratio. If the signal is more powerful than thenoise, signal/noise ratio will be positive; if the
signal is buried in the noise, the signal/noise
ratio will be negative.
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Microwave Terms with meaningsMicrowave Terms with meaningsxv. Multipath Fading: This occurs as a result of the signalstraveling in many/multiple paths or directions between the
transmitter and the receiver. It occurs as a result of the
signals being reflected, refracted, scattered, absorbed, and
diffracted. Some of these signals are lost in the air beforereaching the receiver, thus reducing the RSL of the link.
The effect can be reduced by space diversity.
xvi. Radio Relay: This is the Transmitting Station that is used
to boost signals from one Base Station to the other. It is
eitherActive Station orPassive Station. An ActiveStation consists of the transmission equipments, while the
Passive Station consists of a good reflective surface placed
at certain angles to boost the signals.
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Microwave Terms with meaningsMicrowave Terms with meaningsxvii. Received Signal Level: This is the signal level received
that is measured at the antenna port of the receiving
antenna. It is measured in dBm. The higher the RSL, the
better the link. Mathematically, RSL = transmit power +transmit antenna gain transmit cable losses free space
loss + receive antenna gain receive cable loss.
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Link BudgetLink BudgetThis is the Power Budget of a microwave link. It
takes into account all the gains and losses in the link.
It is computation of the whole transmission chain.The link working condition is: total transmit + total
Propagation + total receive; must be greater than
Zero (0). The remain gives the Margin of the
system.
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Link Budget ContdLink Budget Contd
Mathematically, Link Budget:
Total Transmit (dBm): Transmit Output Power (dBm) cable
loss (dB) + antenna gain (dB).
Propagation (dB): Free Space Loss (dB)
Total Received (dBm): antenna gain (dBi) cable loss (dB)
receiver sensitivity (dBm)
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InterferenceInterferenceIn microwave links, interference occurs whenever a receiver
receives a signal from an unintended transmitter. It can either
be Intrasystem orIntersystem.
Intrasystem Interference occurs when the interference sourceis from within the system itself, like through antennas,
waveguides. Severe forms of intrasystem interference occurs
as a result of bad transmission network design; things like bad
frequency or polarization assignment.Intersystem Interference occurs when the interference source
is from another network.
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ModulationModulationThis is process of putting information on the radio waves by
altering the characteristics on the radio waves being
Transmitted. It can be done by either digital or by analog. In
other words, you modulate to carry your information throughaltering the characteristics since information signals cannot
travel far but microwave can. Waves characteristics to be
altered are:
1. Amplitude2. Frequency
3. Phase
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Modulation contdModulation contd
Few examples of analog modulation scheme are:
AM, FM while few of digital are: QAMS, QPSK.
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Important ConsiderationsImportant Considerations
Perform link analysis and budget for each hop to
determine antenna sizes to use.
Per hop reliability objectives are expected to bemet.
Frequency and Polarization to be well assigned
to reduce interference to the minimum level.
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Planning ToolsPlanning Tools
Software tools are used in the planning and the
design simulation and analysis. Examples of such are:
ATDI ICS telecoms and Pathloss 4.0
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THANKS!THANKS!
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