Chapter 24: Microwave Radio Communications and System Gain – Review Notes

(Last Updated On: December 8, 2017)

Chapter 24: Microwave Radio Communications and System Gain

This is the summary notes of the important terms and concepts in Chapter 24 of the book "Electronic Communications System" by Wayne Tomasi. The notes are properly synchronized and concise for much better understanding of the book. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.


CHAPTER 24

MICROWAVE RADIO
COMMUNICATIONS AND SYSTEM GAIN


Items

Definitions

Terms

1

Electromagnetic waves with frequencies that range from
approximately 500 MHz to 300 GHz or more.

Microwaves

2

The wavelengths for microwave frequencies, which is than
infrared energy.

1 cm and 60 cm slightly longer

3

The name given to microwave signals, because of their inherently
high frequencies, have short wavelengths.

“Microwave” waves

4

Each frequency is divided in half with the lower half
identified as the low band and the upper half as narrow band.

Full-Duplex (Two-way)

5

Communications system used to carry information for relatively
short distances such as between cities with the same state.

Short Haul

6

Microwave systems that are used to carry information for
relatively long distances, such as interstate and backbone route
applications.

Long Haul

7

It propagate signals through Earth’s atmosphere between
transmitters and receivers often located on top of tower spaced about 15
miles to 30 miles apart.
Advantages of Microwave Radio:
·        
Radio
systems do not require a right-of way acquisition between stations.
·        
Each
station requires the purchase or lease of only a small area of land.
·        
Because
of their high operating frequencies, microwave radio systems can carry large
quantities of information.
·        
High
frequencies mean short wavelengths, which require relatively small antennas.
·        
Radio
signals are more easily propagated around physical obstacles such as water
and high mountains
·        
Fewer
repeaters are necessary for amplification.
·        
Distances
between switching centers are less.
·        
Underground
facilities are minimized.
·        
Minimum
delays are introduced.
·        
Minimal
crosstalk exists between voice channels.
·        
Ø
Increased reliability and less maintenance are important factors.
Disadvantages of Microwave Radio:
·        
It
is more difficult to analyze and design circuits at microwave frequencies.
·        
Measuring
techniques are more difficult to perfect and implement at microwave
frequencies.
·        
It
is difficult to implement conventional circuit components at microwave
frequencies.
·        
Transient
time is more critical at microwave frequencies.
·        
It
is often necessary to use specialized components for microwave frequencies.
·        
Microwave
frequencies propagate in a straight line, which limits their use to line-of-sight
applications.

Microwave Radios

8

Propagates signals outside the Earth’s atmosphere and are
capable of carrying signals much farther while utilizing fewer transmitters
and receivers.

Satellite Systems

9

It is used in microwave radio systems rather than amplitude
modulation because AM signals are more sensitive to amplitude nonlinearities
inherent in wideband microwave amplifiers.

Frequency Modulation

10

Major factor when designing FM Radio systems. It is caused by
repeater amplitude nonlinearity in AM, while in FM, it is caused by
transmission gain and delay distortion.

Intermodulation Noise

11

The composite signal that modulates the FM carrier and may
comprise one or more of the following:
·        
Frequency-division
multiplexed voice band channels
·        
Time-division-multiplexed
voice-band channels
·        
Broadcast-quality
composite video or picture phone
·        
Wideband
data

Baseband

12

It provides an artificial boost in amplitude to the higher
baseband frequencies.

Preemphasis Network

13

Frequency modulation index used in the FM deviator. Typically,
modulation indices are kept between 0.5 and 1.

Low-Index

14

FM signal that is produces at the output of the deviator with
a low-index frequency modulation.

Narrowband FM

15

A receiver and a transmitter placed back to back or in tandem
with the system.

Microwave Repeaters

16

It receives a signal, amplifies and reshapes it, and then
retransmits the signal to the next repeater or terminal station down line
from it.
Types of Microwave repeaters:
·        
IF
·        
Baseband

·        
RF


Repeater Station

17

The received RF carrier is down-converted to an IF frequency,
amplified, reshaped, up-converted to an RF frequency, and then retransmitted.

IF Repeater

18

Generally less than 9 MHz.

Baseband Frequencies

19

The range id 60 MHz to 80MHz.

IF frequencies

20

Another name for a Local oscillator, is considerably lower in
frequency than either the received or the transmitted radio frequencies.

Shift Oscillator

21

Transmission used by microwave systems wherein a direct signal
path must exist between the transmit and receive antennas.

Line-of Site Transmission

22

A temporary reduction in signal strength which last in
milliseconds for several hours or even days.

Radio Fade

23

It suggests that there is more than one transmission path or
method of transmission available a transmitter and a receiver.

Diversity

24

It is simply modulating two different RF carrier frequencies
with the same IF intelligence, then transmitting both RF signals to a given
destination.

Frequency Diversity

25

The output of a transmitter is fed to two or more antennas
that are physically separated by an appreciable number of wavelengths.

Space Diversity

26

A single RF carrier is propagated with two different
electromagnetic polarizations. It is generally used in conjunction with space
diversity.

Polarization Diversity

27

It is more than one receiver for a single radio-frequency
channel. With frequency diversity, it is necessary to also use receiver
diversity because each transmitted frequency requires its own receiver

Receiver Diversity

28

Another form of Hybrid diversity and undoubtly provides the
most reliable transmission but most expensive. It combines frequency, space
polarization and receiver diversity into one system.

Quad Diversity

29

A specialized form of diversity that consist of a standard
frequency diversity path where the two transmitter/ receiver pairs at one end
of the path are separated from each other and connected to different antennas
that are vertically separated as in space diversity.

Hybrid Diversity

30

Alternate facilities temporarily made to avoid a service
interruption during periods of deep fades or equipment failures.
Types of protection switching arrangements:
·        
hot
standby
·        
diversity


Protection Switching Arrangement

31

Each working radio channel has a dedicated backup or spare
channel.

Hot Standby Protection

32

A single backup channel is made available to as many as 11
working channels.

Diversity Protection

33

In hot standby protection, it splits the signal power and
directs it to the working and the spare (standby) microwave channels
simultaneously.

Head-End Bridge

34

It has two working channels, one spare channel, and an
auxiliary channel.

Diversity Protection

35

A low-capacity low-power microwave radio that is designed to
be used for a maintenance channel only.

Auxilliary Channel

36

It is where the number of repeater stations between protection
switches depends.

Reliability Objectives of the Systems

37

Points in the system where baseband signals either originate
or terminate.
Four major sections:
·        
baseband

·        
wireline
entrance link (WLEL)
·        
FM-IF

·        
RF
sections


Terminal Stations

38

Points in a system where baseband signals may be reconfigured
or where RF carriers are simply “repeated” or amplified.

Repeater Stations

39

Stands for WireLine Entrance Link, it serves as the interface
between the multiplex terminal equipment and the FM_IF equipment.

WLEL

40

A balanced modulator that, when used in conjunction with a
microwave generator, power amplifier, and band-pass filter, up-converts the
IF carrier to an RF carrier and amplifies the RF to the desired output power.

Transmod

41

t must be capable of amplifying very high frequencies and
passing very wide bandwidth signals for microwave radios.
devices used in microwave amplifiers:
·        
Klystron
Tubes
·        
Traveling-wave
tubes (TWTs)
·        
IMPATT
(Impact avalanche and transit time)

Power Amplifiers

42

It provides the RF carrier input to the up-converter. It is
called as microwave generator rather than an oscillator because it is
difficult to construct a stable circuit that will oscillate in the gigahertz
range.

Microwave Generator

43

It operates in the range 5 MHz to 25 MHz, used to provide a
base frequency that is multiplied up to the desired RF carrier frequency.

Crystal-controlled
Oscillator

44

A unidirectional device often made from ferrite material. It
used in conjunction with a channel-combining network to prevent the output of
one transmitter from interfering with the output of another transmitter.

Isolator

45

Stands for Automatic Gain Control, is a circuit in an IF
amplifier.

AGC

46

It occurs only when three stations are placed in a
geographical straight line in the system.

Multi-hop Interference

47

It prevents the power that “leaks” out the back and sides of a
transmit antenna from interfering with the signal entering the input of a
nearby receive antenna.

High/Low-Frequency
Scheme

48

The signal entering the input of a nearby receive antenna.

Ring around

49

It means that these channels are propagated with vertical
polarization.

V Channels

50

The line-of-sight directly between the transmit and receive
antenna. Also called as the Direct Wave.

Free-Space Path

51

It consists of the electric and magnetic fields associated
with the currents induced in earth’s surface.

Surface Wave

52

The portion of the transmit signal that is reflected off
Earth’s surface and captured by the receive antenna.

Ground-Reflected Wave

53

The portion of the transmit signal that is returned back to
Earth’s surface by the ionized layers of earth’s atmosphere.

Sky Wave

54

The loss incurred by an electromagnetic wave as it propagates
in a straight line through a vacuum with no absorption or reflection of
energy from nearby objects.

Free-Space Path Loss

55

A phenomenon wherein electromagnetic energy is spread out as
it propagates away from the source resulting in lower relative power
densities.

Spreading Loss

56

The reduction in signal strength at the input to a receiver.

Fading

57

The difference between the nominal output power of a
transmitter and the minimum input power to a receiver necessary to achieve
satisfactory performance.

System Gain

58

Sometimes called as Link Margin, is essentially a “fudge
Factor” included in system gain equations that considers the non ideal and
less predictable characteristics of radio wave propagation and terrain
sensitivity.

Fade Margin

59

He described ways of calculating outage time due to fading on
a non diversity path as a function of terrain, climate, path length, and fade
margin, in April 1969.

W.T. Barnett

60

From Bell Laboratories, he derived formulas for calculating
the effective improvement achievable by vertical space diversity as a
function of the spacing distance, path length, and frequency in June 1970.

Arvids Vignant

61

The ratio of the wideband “carrier” to the wideband noise
power.

Carrier-to-Noise Ratio (C/N)

62

Also called Receiver Sensitivity, is the minimum wide band
carrier power at the input to a receiver that will provide a usable baseband
output.

Receiver Threshold

63

The carrier-to-noise ratio before the FM demodulator.

Pre-detection Signal-to-Noise Ratio

64

The carrier-to-noise ratio after the FM demodulator.

Post detection Signal-to-Noise Ratio

65

A ratio of input signal-to-noise ratio to output signal to
noise ratio.

Noise Factor (F)

66

The noise factor stated in dB and is a parameter commonly used
to indicate the quality of a receiver.

Noise Figure

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Chapter 24: Microwave Radio Communications and System Gain – Review Notes
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