Transcript Microwave Radio Communication

Microwave Radio Communication
 Electromagnetic waves with Frequency range from
approximately 300MHz to 300 GHz.
 High frequency > Short wavelengths > “Microwave”
 Wavelengths of microwaves that are used for
communication fall between 1 cm to 60 cm.
 For full-duplex microwave communication the
frequency band is divided into two halves.
 Lower half is called low band.
 Upper half is called high band.
 If Tx operates in low band Rx has to operate in high
band and vice versa.
 Microwave frequency band:
Frequency
Band
Range
(GHz)
Application
L Band
1-2
Military, Mobile, Satellite
S Band
2-4
Television, Mobile, Satellite
C Band
4-8
Military, Satellite
X Band
8-12
Military, Satellite
Ku Band
12-18
Cable TV, Satellite
K Band
18-27
Satellite
Ka Band
27-40
Military , Satellite
Millimeter
40-100
Satellite
Submillimeter
100-300 Not used
 Modulation and multiplexing
Modulation
Multiplexing
Analog
Microwave radio
relay systems
Frequency
Modulation
Frequency
Division
Multiplexing
Digital
Microwave radio
relay systems
Phase Shift
Keying or
Quadrature
Amplitude
Modulation
Pulse Coded
Modulation Time
Division
Multiplexing
 Based on the distance Microwave systems can be
classified as:
 Short haul Intrastate or feeder service applications
 Long haul  Interstate or backbone route applications
 Microwave radio link:
Side view:
Top view:
 Information originates and terminates at the terminal
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station.
Repeaters relay or conveys the information to the
next downlink microwave station.
Microwave stations must be geographically placed in
such a way that natural or man-made barriers should
not interfere with transmissions between stations.
Microwave stations are placed about 15 to 30 miles
apart .
Microwave radio systems has a capacity to carry
thousands of information channels without the need
of any physical facilities such as coaxial cables or
optical fibers.
 Advantages:
1. Right-of –way acquisition between stations is not needed
2. Each station requires a purchase or lease of only a small
area of land.
3. Requires relatively small antennas (shorter wavelength)
4. Because of high operating frequencies it can carry large
quantity of information
5. Propagation is easy around physical obstacles such as
water and high mountains.
6. Fewer repeaters are necessary for amplification
7. Underground facilities are minimized
8. Minimum delay times are introduced
9. Minimal crosstalk exists between voice channels
10. Increased reliability and less maintenance
 Disadvantages:
1. More difficult to analyze and design circuits at
microwave frequencies.
2. Measuring techniques are more difficult to perfect and
implement at microwave frequencies.
3. It is difficult to implement conventional circuit
components(resisters, capacitors, inductors etc) at
microwave frequencies.
4. Transient time more critical at microwave frequencies.
5. It is often necessary to use specialized components for
microwave frequencies.
6. Microwave frequencies propagate in straight line,
which limits their use to line-of-sight applications.
Frequency modulated microwave radio system:
 Provide flexible, reliable and economical point-topoint communication.
 It can simultaneously carry thousands of voice and
data channels.
 FM microwave radio transmitter:
 Baseband signal is a composite signal consists of,
 FDM voice band channels
 TDM voice band channels
 Broadcast quality composite video
 Wideband data
 Pre-emphasis network provides an artificial boost in
amplitude to the higher baseband frequencies. This
will provide a uniform signal to noise ratio.
 FM modulator provides modulation of the IF
carrier(60-70Mhz) (that eventually becomes the
microwave carrier).
 The IF and its associated sidebands are up-converted
to the microwave region by the mixer, microwave
oscillator, and bandpass filter.
 Mixing, rather than multiplying, is used to translate
the IF frequencies to RF frequencies.
 Microwave generators consists of crystal oscillators
and frequency multipliers.
FM microwave radio receiver:
 Channel separation network provides the isolation and
filtering necessary separate individual microwave
channels and direct them to their respective receivers.
 The bandpass filter, mixer and microwave oscillator
down-convert the RF microwave frequency to IF
frequency .
 FM demodulator is a non-coherent FM detector (PLL
demodulator)
 De-emphasis restores the baseband signal to its
original amplitude-versus-frequency characteristics.
FM Microwave radio repeaters:
 The permissible distance between FM microwave Tx
and its associated microwave receiver depends on
 Tx output power
 Receiver noise threshold
 Terrain
 Atmospheric conditions
 System capacity
 Performance expectations
 Typical distance is 15 miles to 40 miles.
 A microwave repeater is a receiver and a transmitter
placed back to back or in tandem with the system.
 A repeater station receives a signal, amplifies and
reshapes it and then transmits the signal to the next
repeater or terminal station down line from it.
 There are 3 types of repeaters: IF, Baseband and RF.
 IF repeater: (Heterodyne repeater)
Baseband intelligence is unmodified here.
 Baseband repeater:
 Here the baseband signal, which is FDM voice band
channels is further demodulated to master group,
super group, group or even channel level. This allows
the baseband signal to be reconfigured to meet the
routing needs of the overall communication network.
 Modified baseband repeater:
 Here there is no reconfiguration of baseband signals.
The operation is same as IF repeaters. The only
difference is that the amplifier and filters acts only on
the baseband frequencies rather than IF frequencies.
So the design of filters and amplifiers becomes simple
and less expensive.
 RF repeater:
 Here the RF frequency is up-converted or down-
converted by mixing with a local oscillator frequency
(shift oscillator). The RF signal is simply converted and
then re-amplified and transmitted to the next down
line repeater or terminal station. Here reconfiguration
and reshaping are not possible.
 LINE-OF-SIGHT PATH CHARACTERISTICS:
 The free-space path is the line-of-sight path directly
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between the transmit and receive antennas (this is also
called the direct wave).
The ground-reflected wave is the portion of the transmit
signal that is reflected off Earth’s surface and captured by
the receive antenna.
The surface wave consists of the electric and magnetic
fields associated with the currents induced in Earth's
surface.
The magnitude of the surface wave depends on the
characteristics of Earth's surface and the electromagnetic
polarization of the wave. The sum of these three paths
(taking into account their amplitude and phase) is called
the ground wave.
The sky wave is the portion of the transmit signal that is
returned (reflected) back to Earth's surface by the ionized
layers of Earth's atmosphere.