Microwave communication
Electronic Communication systems
Fundamental Through Advanced
Wayne Tomasi
Reference Books
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Digital Communication System by Proakis
Satellite Communications by Timothy Pratt
Digital Communications by Grant and Glover
Microwaves
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Microwaves are broadband_ large amount of data can be handled easily
microwaves - electromagnetic waves with a frequency between 500 MHz
1GHz (wavelength 30cm) and 300GHz
Hence have inherently shorter wavelengths in microns therefore microwaves
microwave frequency are further categorized into frequency bands: L (1-2
GHz), S (2-4 GHz), C (4-8 GHz), X (8-12 GHz)
In Full duplex communication the band is divided into 2 halves low band
and high band
Transmitters can operate in the low/high band ; RX in the high/low band
receivers need an unobstructed view of the sender to successfully receive
microwaves
microwaves are ideal when large areas need to be covered and there are no
obstacles in the path
Advantages of microwaves over radio waves
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Due to high operating frequencies, more data can be sent through microwaves
Can carry large quantities of information
High frequency implies short wave length and smaller antenna
smaller antennas produce a more focused beam which is difficult to intercept and
decode
increased bandwidth, higher speeds
Fewer repeaters necessary
Distances between switching centers is comparatively less
Minimum delay times
Minimal cross talk between voice channels
Increased reliability and less maintenance
Disadvantages of microwave communication
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Propagates on LOS
Obstacles are deterrent to its transmission
The cost of implementing the communication infrastructure is high
Microwaves are susceptible to rain, snow, electromagnetic interference
Microwave Engineering Considerations
• Free space &
atmospheric
attenuation
• Reflections
• Diffractions
• Attenuation caused by
Rain
• Skin affect
• Line of Sight (LOS)
• Fading
• Range
• Interference
Microwave Radio Link Planning
The design and construction of a microwave radio link network is based on a number
of factors.
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Distance between microwave radio terminals;
Terrain properties, eg bodies of water, cliffs, forests, snow;
Frequency of operation, often governed by licensing costs, frequency availability,
planned distances and even susceptibility to rain fading;
Interference management to the microwave link receiver. Generally managed by
allocating a clear frequency pair by the Regulator, but for frequency bands 'sold at
auction' or with delegation, eg Defence communications and large carriers, this
becomes the management responsibility of the band licensee/owner;
Fading, dispersion and multipath distortion;
Size of antennas, feedline properties, need for towers and masts, and for high gain
antennas - even the stability (both tilt and torsional properties) of the supporting
mast must be engineered to avoid the antenna beam being mis-directed due to
wind or ice on the structure;
Microwave Radio Link Planning
The design and construction of a microwave radio link network is based on a number
of factors.
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Management of moisture inside external waveguides;
Management of equipment, power and security alarms, remote control switching
and order wire systems.
Council, Local Government, FAA, CASA and community development permissions
governing visual and controlled airspace intrusions;
Cost of equipment and cost benefit analysis including equipment maintenance;
Satellite communication links are also classed as microwave radio links, but given
their minimal exposure to atmospheric conditions, these type of microwave links
can operate at minimal fade margins, ie having minimal contingency in the level of
received signal strengths;
Availability of equipment, spares, maintenance, test equipment and skilled staff;
Sun transits for microwave link receivers facing at the eastern or western horizons.
The issue here is that the "sun noise" will often overwhelm broadband microwave
receivers, generating what is called a 'sun transit outage'. Same deal for satellite
communication links as well.
Microwave applications
• Used for long-haul telecommunications
• Displaced by fiber optic networks
• Still viable for right-of-way bypass and geographic obstruction
avoidance
▪ carrier waves in satellite communications
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cellular communication
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Bluetooth
▪ WiMAX
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wireless local area network
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GPS (Global Positioning System)
Microwave communication concepts
• microwaves are generated by magnetrons through
vibration of electrons
• LoS (Line of Sight) – is a visible straight line between the
sender and the receiver
• LoS propagation – propagation of microwaves in a straight
line free from any obstructions
• Fresnel zone – eliptical area around the LoS between a
sender and receiver; microwaves spread into this area once
are generated by an antenna; this area should be free of
any obstacles:
FM microwave Radio system
• FM is used in microwave radio systems
• Noise tolerance is high
Microwave FM transmitter
Microwave FM transmitter
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Pre emphasis network _amplifying the high frequency signals
Lower baseband signals FM the IF and the higher baseband signals PM the IF
IF selected is 60 MHz and 80 MHz; common is 70 MHz
-0.5< mf <1 Narrow band transmission
BW = 2 times highest baseband signal
Up conversion to higher microwave frequencies
This leads to increase in the frequency deviation, BW modulation index etc
Microwave FM receiver
Microwave repeaters
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Typically distance between 15mi to 40 mi
Depends on various factors
Tx power, Rx power, atmosphere, terrain, noise, system capacity, reliability, etc
Microwave Impairments
• Equipment, antenna, and waveguide failures
• Fading and distortion from multipath reflections
• Absorption from rain, fog, and other atmospheric
conditions
• Interference from other frequencies
Microwave diversity
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Microwaves use LOS
Direct signal must exist between the Tx and Rx antenna
Suffers over time from radio fading (few minutes to hours /days)
Diversity is employed to increase the reliability of the system
Diversity _ more than one path available for transmission of the signal to increase
the reliability
A reliability of 99.99% = 53 minutes of outage time per year
99.9999% = 32 seconds of outage time per year
System selects the best signal depending on the C/N carrier to Noise ratio
Microwave diversity
Diversity Applied in microwave transmission
• Space diversity
• Frequency diversity
• Polarization diversity
• Hybrid diversity
• Quad diversity
FADE MARGIN
• A design allowance that provides for sufficient
system gain or sensitivity to accommodate
expected fading, for the purpose of ensuring
that the required quality of service is
maintained.
Space Diversity
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Space Diversity protects against multi-path fading by automatic switch over to
another antenna place below the primary antenna. This is done at the BER failure
point or signal strength attenuation point to the secondary antenna that is receiving
the transmitted signal at a stronger power rating.
Frequency Diversity
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Frequency Diversity uses separate frequencies (dual transmit and receive systems)
monitors primary for fail over and switches to standby.
Interference usually affects only one range of frequencies.
Not allowed in non-carrier applications because of spectrum scarcity.
Microwave diversity
Polarization diversity:
• Signals with different Polarization (vertical and horizontal) are transmitted
• Generally used in conjunction with space diversity
Hybrid diversity
• Uses a std frequency diversity path where the Tx and Rx pairs are separated by
frequency and vertical separation
Quad Diversity
• Combines all the diversities in one hence expensive to implement
Protection Switching arrangements
• Diversity and Protection switching
arrangement
• Diversity_ provides an alternate route both
can fail
• Protection switching system: provides
protection against fading and equipment
failures_these work on the sytem including
the repeaters etc covers a wide area
Protection Switching arrangements
• Diversity
• Hot Standby switching system: Hot implying
its live _carries identical live information_ if
the main system fails the standby kicks in
immediately
Bit Error Rate (BER)
• The BER is a performance measure of microwave signaling
throughput
• The bit error rate (BER) is the number of bit errors per unit
time.
• The bit error ratio (also BER) is the number of bit errors
divided by the total number of transferred bits during a
studied time interval.
• BER is a unit less performance measure, often expressed as
a%
– 10 or one error per million transmitted bits of information
– Data fail over is at 10; voice traffic can withstand this error rate
System gain and Receiver Threshold
System gain (Gs):
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Must be  sum of all gains and losses incurred during transmission from the Tx to
the Rx
Gs = Pt − C min  losses − gain
Gs  Fm + LP + L f + Lb − APt − APr
Gs=system gain
Pt = transmitted power and
Cmin= minimum power received with given conditions
Gains in the system will be :
G AT , G AR , indB
Loss in the system (dB) will be:
free space path loss, waveguide feeder loss,
total coupling of branching losses, fade margin
Receiver Threshold sensitivity
(C/N)
• C/N : Carrier to Noise ratio is the most important
parameter when evaluating a microwave system
• Receiver threshold is defined as the minimum Carrier
power at the input to a receiver that will provide
usable baseband output
• C/N is a pre-detection of the system performance
Carrier to noise ratiomeasured across the same impedance
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 Vcarrier   Vc 
=CNR=C/N= 
 = 
 Vnoise   Vn 
2
2
 Vc 
 Vc 
CNRdB = C / N = 10 log10   = 20 log10  
 Vn 
 Vn 
Cmin recieved = CNRmin + N
Pt = GS + Cmin
FSPL
FPSL = LdB = 32.4 + 20 log( f MHz ) + 20 log( Dkm )
FPSL = LdB = 92.4 + 20 log( fGHz ) + 20 log( Dkm )
fade margin (dB) = FD = 30 log D + 10 log(6 ABf ) − 10 log(1 − R) − 70
D 
Parabolic dish antenna gain A p = 
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  
generally  =55%
5.4 D 2 f 2
Ap =
C2
A p (dB) = 20 log f MHz + 20 LogDm − 42.2
2
Examples on C/N
Microwave communication system
• BER is related to Noise figure in a nonlinear
way
Microwave communication
applications
• Terrestrial microwave links: Analogue systems,
digital systems, LOS propagation
• Fixed point satellite communication
• VSAT systems