advanced communication-system
TRANSCRIPT
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24051-ADVANCED COMMUNICATION SYSTEM QUESTION BANK
RANGANATHAN POLYTECHNIC COLLEGE, COIMBATORE.
Prepared by A.Indhumathi, Lecturer/ECE
UNIT-I RADAR AND NAVIGATIONAL AIDS
1. Expand RADAR.
RAdio Detection And Ranging
2. What is RADAR?
RADAR is basically a means of gathering information about distant objects or
targets, by sending electromagnetic waves.
3. What are the types of RADAR?
According to the principle of operation, the RADAR is classified into 2 types:
they are,
Pulsed Radar system
Continuous Radar system.
4. What are the applications of RADAR?
Radars used in defense weapons system & in safety 7 navigation
applications
Used to locate enemy missiles, planes & ships
Used as altimeters to measure height
5. Give the expression for RADAR range equation.
𝑟𝑚𝑎𝑥 = 𝑃𝑡𝑆𝐴0
2
4𝜋𝜆2𝑃𝑚𝑖𝑛
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; 𝐴0 = 𝐴𝑝𝜆2
4𝜋
Therefore, 𝑟𝑚𝑎𝑥 = 𝑃𝑡𝑆𝐴𝑝
2 𝜆2
4𝜋 3𝑃𝑚𝑖𝑛
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6. Give any two factors that influencing maximum range.
By increasing the transmitted power, the range can be increased.
The maximum range is achieved by reducing the minimum detectable signal
power.
7. What are the display methods of radar receiver?
A Scope
Plan Position Indicator
Direct feeding of a computer
8. What are the limitations of PPI?
Distortion of true map position will occur if PPI is on an aircraft & its antenna is
not pointing straight down.
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9. What are the applications of PPI?
PPI display can be used in search radars and especially when conical scanning
is employed.
10. What is Automatic Target detection?
The function of the radar operator viewing the ordinary radar display is to
recognize the presence of target & extract their location. When the function is
performed by electronic decision circuitry without intervention of an operator, the
process is known as automatic target detection.
11. What are the other names of automatic target detector?
The automatic detector is also called as plot extractor & data extractor.
12. What is the advantage of automatic target detector?
It is used to overcome the limitations of an operator due to fatigue, boredom &
overload.
13. What is the use of automatic target detector?
It allows the radar output to be transmitted over telephone lines rather than by
more expensive broadband microwave links.
14. What are the basic aspects of automatic target detection?
The four basic aspects are:
Integration of pulses from target
Detection decision & determination of target location
Range
Azimuth
15. What is direction finding?
Radar direction finding is a useful navigational aid. In this system, the waves
propagate away from the transmitter along a great circle, which makes radio direction
finding. This system operates at low & medium frequency ranges.
16. What is radio range?
A navigation system that establishes a course through space along which
aircraft can navigate from one location to another is termed as a radio range.
17. What is radio compass?
An instrument which is used in ships & aircrafts for finding the directions is
called compass. If this system is operated in radio signal, it is called radio compass.
18. What are the operating frequencies of radio telemetry systems?
The operating frequency is 216 to 220MHz & 2.2 to 2.3 GHz.
19. What are the electronic systems used for landing aircraft?
The types of aircraft landing systems are
Ground Controlled Approach
Instrument Landing System
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20. What is glide-path equipment?
The glide-path equipment or glide path facility provides an equi-signal path
type of guidance in the vertical plane by the equi-signal path of the localizer.
21. What are the two radars employed by GCA?
Airport Surveillance Radar (ASR)
Precision Approach Radar (PAR)
22. What is the use of ASR?
The ASR is used for control the traffic pattern of aircraft in the vicinity of the
landing field. It has a range of approximately 30miles. It is a high resolution short
range set. It is used to conduct the actual landing.
23. What is the advantage of GCA system?
No equipment is required in the aircraft other than an ordinary radio receiver &
the ground installation can be mobile.
24. What is the disadvantage of GCA system?
The disadvantages are that there are number of human links in the chain & that
the chief responsibility for making a successful landing is taken away from the pilot.
25. What is a telephone system?
The telephone is derived from the greek words; tele means Far, and phone
means sound. Telephony thus involves the conversion of sound signals into an audio
frequency analog electrical signal.
26. What are the classifications of telephone system?
One-way communication system (Simplex system)
Two-way communication system (Duplex system)
27. What is simplex system?
In this system, transmission takes place in only one direction at a time, but it
provides communication in either direction alternatively.
28. What is duplex system?
In this system, transmission takes place in both directions. A pair of wires used
for this purpose.
29. What are the requirements of telephone systems?
It must be able to transmit voice signals in both directions.
It must provide the means for signaling from each terminal towards the
other.
30. What is ISDN?
ISDN stands for Integrated Service Digital Network for providing an end to end
connectivity over a digital link. It is basically a circuit switched digital network. In
ISDN line a wide range of digital services such as voice, data, image, video, facsimile
etc. can be integrated.
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31. Expand PAR.
Precision Approach Radar
32. What is scanning?
The scanning process examines each line turn & extracting the value for each
pixel on the line in a sequential manner usually from left to right until entire image is
scanned.
33. What is a facsimile communication system?
A facsimile means an exact reproduction. In facsimiles transmission document
or picture is exactly reproduced at the receiving end.
34. Define index of cooperation.
The IOC is a number derived from the width-height ratio.
35. What is direct recording?
Direct recording means that the image is transferred directly to the copy paper,
no post processing is required to develop the image.
36. What are the four reference points in ISDN model?
U – Connection between ISDN exchange in the carrier‟s office & NT1.
T – Connector on NT1 provided to the customer
S – Interface between the ISDN PBX and the ISDN terminals
R – Connection between the terminal adapter and non-ISDN terminals.
37. Expand CCITT.
Consultative Committee for International Telegraphy and Telephony
38. What is the use of synchronization?
In synchronous system, it is necessary for the receiver scanner and the
transmitter scanner to run at exactly the same speed.
39. What are the features of ISDN?
High speed internet access
High flexibility
Video conferencing
Provides data rate of 3Mbps on a local link & 64 Kbps/128Kbps over a wide
area link
40. Expand GCA.
Ground Controlled Approach
41. Expand IOC.
Index Of Cooperation
42. Expand ILS.
Instrument Landing System
43. Expand PPI.
Plan Position Indicator
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44. Expand ISDN.
Integrated Service Digital Network
45. Expand ASR.
Airport Surveillance Radar
46. What are the uses of fax?
To transmit photographs, documents, weather maps and language texts etc.
47. What is ILS?
Instrument Landing System (ILS) is an electronic system used for Aircraft
Landing System (ALS). It consists of essential elements like Runway Localizer,
Glide-path Equipment & Marker Beacons.
UNIT-II DIGITAL COMMUNICATION
1. What is the use of data system?
The data system is required to transmit rectangular pulses at a rate of ranging from
100 to 500 Kbits per second. The typical uses are:
Computer to computer communication
Programming & data collection
Telemetry & alarm system
2. What are the types of digital communication?
On line system
Offline system
3. Define online system.
In an online system the data is transmitted directly to or from a computer.
4. Define offline system.
In an offline system the data is transmitted to or from an intermediate storage
point such as card, a paper tape punch, magnetic tape or disk.
5. What is real time system & non real time system?
If the system requires rapid response then it is a real time system.
In non real system the data is much delayed.
6. What is simplex, semi-simplex & full-duplex connection?
A one-way channel connection is known as simplex connection. An either way
transmission requires semi-duplex connection & a full-duplex is the one in which the
data can be sent in both directions simultaneously.
7. What is a digital signal?
The digital signal does not provide the continuous representation of the original
signal. Instead, the digital signal represents data as a series of digits such as a number.
This digital representation can be considered as a code which approximates the actual
value.
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8. What are the advantages of digital communication?
Relatively inexpensive digital circuits can be used.
Privacy is preserved by using data encryption
Greater dynamic range
In long distance systems, noise does not accumulate from repeater to
repeater.
9. What are the disadvantages of digital communication?
More bandwidth is required.
Synchronization is required.
10. What is the frequency range of telephone channel?
The frequency range of telephone channel is if 300 to 3400Hz, this referred to
within the industry as a 4 KHz channel.
11. What is the frequency range of HF radio & submarine cables?
The HF radio and submarine cables frequency range is 300-2800Hz.
12. What are the characteristics of digital communication?
Bandwidth requirement
Speed & Noise
Crosstalk & Equalizers
Baud rate &Distortion
Echo compressors
13. Define speed.
The transmission speed of a communication channel is described in „baud rate‟.
Baud rate is the unit of signaling speed.
14. Define Nyquist rate.
The effect of the noise on the data channel can be reduced by increasing the
signal to noise ratio for an ideal channel the nyquist rate twice the bandwidth.
15. Define baud rate.
Baud rate is the unit of signaling speed generally the information may transfer
at a rate equal to or different from the baud rate.
16. Define cross talk.
Any transmission system which conveys more than one signal simultaneously
can experience cross talk.
17. Define distortion.
Signals of different frequencies can be passed by a channel with different values
of amplitude attenuation & at different propagation speeds. This results in distortion.
18. Define equalizers.
Phase distortion & attenuation can be reduced by using equalizer on the
channel.
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19. What are the common codes used by computers?
Baudot code, Binary code, ASCII code, EBCDIC code
20. What is adaptive equalization?
The equalizer automatically varies its delay characteristics, while sampling the
return signal to determine when the channel delay plus equalizer delay reaches proper
tolerances. At that time data transmission commences. The data is thereafter sampled
during transmission to ensure that equalization settings are modified in appropriate
level. This type of equalization is called adaptive equalization.
21. Define echo compressors.
Echo compressors or echo suppressors are used on long distance circuits to
overcome echoes caused by circuit imbalances.
22. Mention the uses of baudot code.
Used in teletypewriter machines incorporating a paper to be punched & reader
mechanism.
23. Define Baudot code.
The baudot code is a 5 bit code which has been used in telegraphy & paper tape
systems. There are totally 32 different code combinations (25=32). This is not
sufficient to represent the alphabets (26), numerals (10) and other characters (+,-,>, <,)
hence we increase the total number of combination by incorporating the shift codes.
24. Mention the drawbacks of baudot code.
It does not provide extra combination of bits to code punctuation & various
codes.
All the five bits are used for coding the data. Hence error correction using parity
technique is not possible.
25. Expand CRC.
Cyclic Redundancy Code
26. Define ASCII code.
ASCII stands for American Standard Code for Information Interchange. It is a
seven bit code. The seven bits are formed, based on a standard binary propagation.
27. Mention the limitations of baudot code.
Only 5bits are available.
The numbers of combinations are limited only upto 25=32 codes.
This code is not sequential &It is not used for error detection & correction.
28. What are the advantages of ASCII code?
Error detection can be achieved by increasing the total numbers of bits to 8.The
parity bit is added as the 8th bit, usually the MSB.
It can be easily used in a computer. A modern computer uses hexadecimal code
for internal computations. Since ASCII is an 8 bit code with parity bit, it can be
easily accommodated in computer as 8 bit data.
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29. What is the use of ASCII code?
It is widely used in modern computers.
30. Define EBCDIC code.
EBCDIC code stands for Extended Binary Coded Decimal Interchange Code. It
is a 8 bit fixed length code. Here all bits are used for representing the information.
31. What are the advantages of EBCDIC code?
It is similar to ASCII code. It can be readily used in computer.
Total number of combinations is higher.
32. What are the disadvantages of EBCDIC code?
Here all the 8 bits are used for data encoding.
There is no provision for parity bit.
Here error correction is not possible.
33. What is a parity check code?
The most widely used approach for detecting errors that arise in storing &
moving words in the use of a parity check bit added to each character code group.
Codes of this type are called parity check codes.
34. What are the two types of parity check codes?
Odd parity & even parity.
35. Define redundant code.
Most error detection system use some form of redundancy to check whether the
received data contains errors. That means, additional data is sent with the basic data.
The redundancy takes the form of transmitting the information twice & comparing the
two sets of data to see that are same.
36. Define CRC.
The cyclic redundancy check is a more reliable error detection scheme. It is a
mathematical technique which is applied to the transmitted data. It is effective in
catching 99.9% or more of transmission errors.
37. What is constant ratio code?
The 2 out of 5 and 4 out of 8 codes depend upon the ratio of 1s to 0s, to indicate
that errors have occurred. So they are called constant ratio codes.
38. Mention the types of error correction methods.
Retransmission
Forward error correcting code
Hamming code.
39. Define error correction codes.
Error correction is an important aspect of data transmission. This process
involved with error correction normally results in an increase in the number of bits per
second which are transmitted, & naturally this increases the cost of transmission.
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40. Define hamming code.
Hamming code is an interesting error detection code. This code adds several
parity check bits to a data word. The hamming code is capable of locating a single
error but it fails if multiple errors occur in a single data block.
41. What is dibit?
Each of the 2-bit elements is called a dibit. Each of the 2bit pairs is converted to
a phase values of in the data set.
42. Define ASK modulation.
In amplitude shift keying (ASK) modulation, the digital signal is switched in
between amplitude levels of digital signal. The carrier signal is on & off accordance
with the digital binary input signal. The carrier signal is transmitted during the high
level input signal & blocked during low level signal. This method is also called as
ON-OFF keying (OOK).
43. Define QPSK modulation.
QPSK is a Quaternary Phase Shift keying or Quadrature PSK which is an angle
modulated constant amplitude digital modulation. It transmits higher bit rate.
44. Expand EBCDIC code.
Extended Binary Coded Decimal Interchange Code
45. What is even parity & odd parity?
Even parity means adding an extra bit to the group of bits to make the group of
bits to make the whole number of 1‟s as even. In case of odd parity, add one more bit on
the end of character, to make the whole word as odd number of 1‟s.
46. Define FSK.
Frequency shift keying is that which shifts carrier frequency to indicate a mark
or a space for a common digital signal. In this, the unmodulated carrier frequency
corresponds to mark condition & space by downward frequency shift.
47. Define PSK.
PSK is a system in which the carrier may be phase shifted by +90° for a mark &
-90° for a space. In this, the unmodulated carrier frequency corresponds to mark
condition & space by downward frequency shift.
UNIT-3 OPTICAL COMMUNICATION
1. What is fiber?
Optical fibers are replaced by wire transmission lines in communication
systems. Light is effectively the same as radio frequency radiation, but its frequency is
high. The information carrying capacity of fiber is very much greater than microwave
radio systems.
2. What is the frequency range of an optical waveguide?
The optical carrier frequency is in the range of 1013
to 1015
Hz, while the radio
frequency is about 106Hz and the microwave frequency range is about 10
10Hz.
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3. What are the materials used in fibers?
The material used in fiber is silica glass or silicon dioxide, which costs lower
than other wire lines.
4. What are the advantages of optical fiber?
Wider bandwidth
Low transmission loss
Dielectric waveguide or Electrical isolation
Immunity to crosstalk & EMI
Signal security,
Small size & weight
5. What are the limitations of optical fiber?
Limited application & Low power
Distance & Modulation
Nuclear radiation & Fragility
6. What is light pipe?
When light enters one end of a glass fiber under the light condition most of the
light will propagate or move down the length of the fiber and exit from the far end. A
small part of light will be lost due to internal absorption but a portion of light will be
contained and guided to the far end such a fiber is called light pipe or light guide.
7. What are the conditions for total internal reflection?
The glass inside the fiber core must have a slightly higher index of refraction
“n1” than the index of refraction “n2” of the material(cladding) surrounding the
fiber core.
The light must absorb the wall with an angle of incidence Ф (between the ray
path & the normal to the fiber wall) that is greater than the critical angle Фc,
which is defined as sin Фc= 𝑛2
𝑛1
8. Define critical angle.
The minimum angle at which the external light rays may enter the air/glass
interface and still propagate down the fiber is called critical angle θc
9. Define numerical aperture.
Numerical aperture (NA) is closely related to acceptance and is the figure of
merit commonly used to measure the magnitude of the acceptance angle.
10. Define Snell’s law.
Snell‟s law states that the incidence angle θ0 is related to the refraction angle by
θ1 by the relationship.
sin 𝜃0 = (𝑛1
𝑛0) sin 𝜃1
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11. What is meridonial ray?
The ray has an angle of incidence Ф at the interface which is greater than the
critical angle and is reflected at the same angle to the normal. The light ray is known
as meridonial ray as it passes through the axis of the fiber core.
12. Define acceptance angle.
Acceptance angle 𝜃𝑎 is the maximum angle in which external light rays may
strike the air/glass interface and still propagate down the fiber. Acceptance angle is the
maximum angle.
13. Define core & cladding.
The cylindrical shaped central part of the fiber is known as the core of the fiber.
The core is surrounded by a solid dielectric cladding. The cladding is not necessary for
the light to propagate along the fiber.
14. Define skew rays.
When a light input of the fiber is non-uniform, skew rays will therefore tend to
have a smoothing effect on the distribution of the light as it is transmitted, giving a
more uniform output. The amount of smoothing is dependent on the number of
reflections encountered by the skew rays.
15. What is the purpose of cladding?
The cladding also reduces scattering loss & adds mechanical strength.
16. Define step index fiber.
Step index fiber: In step index fibers the index of refraction of fiber of the fiber
core and the cladding interface are sharply defined in step it means that the core has
one constant index of refraction N1 while the cladding has another constant index of
refraction N2.
17. Define Graded indexfiber
Graded indexfiber: In graded index fiber the index of refraction of the core is
not constant instead, the index of refraction varies smoothly & continuously over the
diameter of the core.
18. Define single mode fiber.
Single mode fiber: in single mode fiber, light follows a single path through the
core.
19. Define multimode fiber.
Multimode fiber: in this the light takes many paths through the fiber.
20. What are the types of optical fibers?
Single mode step index fiber
Multi-mode step index fiber
Multi-mode graded index fiber
21. Expand LASER.
Light Amplification by Stimulated Emission of Radiation
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22. What is the advantage of multimode step index fiber?
The main advantage of a multimode step index fiber is the large size. Typical
core diameters are in 50-1000um range. Such large diameter cores excellent gathering
light & transmitting it efficiently. It is an inexpensive light source to produce the light
source.
23. What are the mechanisms responsible for attenuation in optical fiber?
Absorption loss&Scattering loss
Bending loss
Core & cladding loss
24. What are the types of absorption losses?
Ultraviolet absorption
Infrared absorption
Ion resonance absorption
25. What is ion resonance absorption?
During manufacturing, minute quantities of water molecules are trapped in the
glass. These water molecules make OH impurity ion in the material. These ions absorb
light energy. This is called ion resonance absorption.
26. Define infrared absorption.
When light propagates in the fiber, the photons of light energy are absorbed by
the atoms within the glass molecules. It is then converted to random mechanical
vibrations, with a result that heat is produced. It is called infrared absorption.
27. Define ultraviolet absorption.
When light propagates in the fiber of pure fused silica, the valance electrons can
be ionized into conduction band. The energy for this ionization is drawn from the
light, propagate through it. It produces a transmission loss called ultraviolet
absorption.
28. What are the types of scattering losses?
Linear scattering losses
Rayleigh scattering
Mie scattering
Non-linear scattering losses
Stimulated brilliouin scattering
Stimulated remand scattering
29. What is bending loss? Give the types of bending losses.
Whenever the optical fiber contains bends, then the bends produce radiative
losses.
Types: 1) Micro bending losses
2) Macro bending losses.
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30. What is intermodal dispersion?
Intermodal dispersion arises due to the variation of group velocity for each
mode at a single frequency
31. What is dispersion? Mention its types.
A pulse of light with a given width & amplitude transmitted into one end of a
fiber should theoretically arrive at the far end with its shape & width unchanged. But
its amplitude is reduced by losses. Types of dispersions are: Intermodal dispersion
&intramodel dispersion
32. What is intramodel dispersion?
Intramodel dispersion arises due to the dependence of group velocity on the
wavelength. Further it increases with the increase of special width of the optical
source. This spectral width is the range of wavelengths emitted by the optical source.
33. What is material dispersion?
Material dispersion is sometimes referred to as chromatic dispersion, because it
produces the same effect by which a prism spreads out a spectrum. This causes a
wavelength dependence of the group velocity of any given mode, that is pulse
spreading occurs even when different wavelength follow the same path.
34. What is waveguide dispersion?
The waveguide dispersion arises due to the finite frequency bandwidth and the
dependence of the mode group velocity with frequency of light. Higher the frequency
bandwidth of the transmitted pulse, higher will be the waveguide dispersion.
35. What is LED?
A light emitting diode works by the process of spontaneous emission when it is
forward biased and conducting current.
36. What are the types of diodes or detectors used in communication applications?
PIN photodiode & Avalanche photodiode
37. What is PIN photodiode?
An ordinary “PN” diode may be used as a photo detector. It has sufficient
speed, but its sensitivity is very low. The sensitivity of the pn photodiode can be
improved by including a lightly doped n layer between the junction and the more
heavily doped n-contact region to form the pin diode.
38. What are the disadvantages of fushion splicing techniques?
During fusion chemical reaction may take place which causes a change in the
refractive index of two fibers
It develops thermal stresses at the fusioned region
39. What is APD?
It consists of four regions p+
-i –p –n+ in order to develop a very high electric
field in the intrinsic region. It is used to produce new electron pairs by impact
ionization. This impact ionization leads to avalanche breakdown in the reverse biased
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diode. So the APD‟s have high sensitivity & high response over pin diodes due to the
avalanche multiplication.
40. What are the types of fiber coupling devices?
Fiber splices (Permanent joint)
Fiber connectors (Temporary joint)
41. Define connector & splices.
Two fibers are connected by a permanent bond called splice, connected by a
demountable joint called connector.
42. What are the types of couplers?
Diffusion couplers
Radiative couplers
Area splitting couplers
Beam splitter coupler
43. What are the types of splicing techniques?
Fusion splice, V-groove splice
Elastic tube splice,Rotary splice
Mechanical tube splice
44. What are the advantages of fushion splicing techniques?
Low splice loss
There is perfect bonding between two fibers
45. What are the light sources used in optical fiber system?
Light emitting diodes & Semiconductor lasers are used in optical fiber system.
46. Mention advantageof ILD over LED.
The radiant output power from an ILD is greater than that for a LED.
47. How scattering loss occurs?
Linear scattering may cause the attenuation of operating mode by means of
transferring power to leaky or radiation mode. Nonlinear scattering occurs when the
refractive index of the medium depends upon the optical intensity of the signal.
48. Mention the principle of LASER.
The principle of operation of Laser action is the result of 3 key processes: they are:
Photon absorption
Spontaneous emission
Stimulated emission
49. What is lasing?
The ends of ILD are highly polished. The mirror like ends trap the photons in te
active region and as they reflect back and forth, stimulate free electrons to recombine
with holes at a higher than normal energy level. This is called lasing.
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50. What are the requirements of good connector?
Low coupling losses & Interchangeability
Ease of assembly & Ease of connection
Low environmental sensitivity
Low cost & reliable construction
51. What is fiber splice & fiber coupler?
The permanent joint formed between two individual optical fibers in the field or
factory is known as fiber splice.
The fiber couplers are branching devices that split all the light from a main fiber
into two or more fibers.
52. What is the function of photo detector?
The photo detector converts the current changes into voltage changes.
53. Outline the primary building blocks of fiber optical system.
The primary building blocks of fiber optic system are the transmitter, receiver
and optical fiber cables.
54. What are the disadvantages of APD?
Carriers in the intrinsic region have a long transit time to the junction, which
slows the response as in the pin diode.
The avalanche multiplication factor tends to fluctuate randomly to add noise to
the signal.
55. What are the applications of optical fiber communications?
Military, civil, networking, industry, telecommunications, government, CATV,
computer, business/retail offices & automotive.
56. Mention any two advantages of optical fiber.
An optical fiber has very high information transfer rate-gigabits/sec.
The advantages of fiber are low loss, very wideband, flexible & light weight.
57. Mention the advantage of V-groove splice technique.
No thermal stress
Refractive index does not change between the two fibers
UNIT-IV SATELLITE COMMUNICATION
1. Define satellite communication.
The process of sending information around the word by means of satellites is
known as satellite communication.
2. Define uplink frequency. Mention its frequency range.
The original signal being transmitted from the earth station to the satellite is
called the uplink. The typical uplink frequency is 6GHz.
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3. Define downlink frequency. Mention its frequency range.
The retransmitted signal from the satellite to the receiving station is called
downlink. The downlink frequency is about 4GHz.
4. Define Kepler’s first law.
Kepler‟s first law states that the satellite will follow an elliptical path in its orbit
around the primary body. The eccentricity of the ellipse is defined as
e = 𝑎2−𝑏2
𝑎; a- semi major axis & b- semi minor axis
5. Define Kepler’s second law.
Kepler‟s second law states that for equal time intervals the satellite sweeps out
equal areas in the orbital plane, focused at the Barry center.
6. Define Kepler’s third law.
Kepler‟s third law states that the square of the periodic time of orbit is
proportional to the cube of the mean distance between bodies. The mean distance is
equal to the semi minor axis α.
α=µ/n2; n= mean motion of the satellite in radians per second
µ=earth‟s geocentric gravitational constant
7. Define PROGRADE orbit.
In circular orbit, satellite rotates the earth by counter clockwise. This angular
velocity 𝜔𝑠 is greater than earth‟s angular velocity𝜔𝑒 . This is known as prograde orbit.
8. What is WARC?
World Administrative Radio Conference (WARC) distributes radio frequencies
required for the satellite to all countries in the world.
9. What are the types of orbits?
Inclined Elliptical orbit
Polar Circular orbit
Geostationary orbit
10. What are the common satellite orbital patterns?
Low earth orbit, Medium Earth orbit & Geostationary orbit.
11. What is the angle of inclination of a satellite in polar satellite?
The inclination angle is about 63.4° and covers the Polar Regions for about 8 to
12 hours daily.
12. What is the disadvantage of inclined elliptical orbit?
As the transmit time is the longest at the apogee, making the satellite visible for
a relatively long period of time over these regions. The inclined elliptical orbit does
not permit continuous contact with the satellite from the fixed spot on the earth.
13. Define ascending node.
The point where the orbit crosses the equatorial plane going from south to north
is known as ascending node.
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14. Define apogee.
The point farthest from the earth is called apogee.
15. What is the advantage of inclined elliptical orbit?
The main advantage of this is, it provides coverage of the polar region. The
apogee or highest point of the orbit is arranged to occur over the region requiring most
coverage. This puts the satellite at its greatest height and therefore gives the greatest
earth coverage of this region.
16. What is geostationary synchronous orbit?
A geostationary satellite is one that appears to be stationary relative to the earth.
There is only one geostationary satellite, but this is occupied by a large number of
satellites. This satellite is placed in the orbit at the height of 36000km from the earth‟s
equatorial plane is called geostationary orbit. It is visible for continuous time.
17. What is the height of geosynchronous orbit?
The earth‟s equatorial radium is approximately 6378km, and hence the height of
the geostationary orbit above the earth is
h=42164-6378 = 35786km.
This value is rounded up to 36000km. There can be only one geostationary orbit.
18. What are the advantages of geostationary synchronous satellite?
Tracking equipment is not required at the earth stations.
The satellites are available to all the earth stations within their shadow 100% of
the time.
No breaks in transmission.
The effect of Doppler shift is negligible.
19. What are the disadvantages of geostationary synchronous satellite?
High altitude geosynchronous satellites introduce much longer propagation
delays
It requires higher transmit powers & more sensitive receivers because of the
longer distances & greater path losses.
20. Define Sidereal Period.
The sidereal period is the time taken for a satellite to make a complete
revolution in an orbit.
21. Define perigee.
The point of closest approach to the earth is perigee.
22. Define active satellite.
The signal received by the satellite is retransmitted rather than being simply
reflected back on board to earth. It contains highly transmitting & receiving antennas
& complex interconnecting circuits.
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23. Define attitude control.
In addition to maintaining the position of the satellite in orbit, some means must
be provided to position the satellite for optimum performance. This is called attitude
control.
24. Define passive satellite.
It is a simple reflector. Whatever be the signal transmitted from the earth, the
same signal is retransmitted without any conversion to the earth. This does not have
any electronic system. The signal is simply reflected back to the earth by means of
antenna systems.
25. Define earth eclipse of satellite.
A satellite is said to be in eclipse when the earth prevents sunlight from
reaching it, that is, when the satellite is in the shadow of the earth. For
geosynchronous satellites, eclipses occur during two periods that begin 23 days before
equinox (about march 21 & about September 23) and end 23 days after equinox.
26. Define transponders.
The transmitter receiver combination in the satellite is known as transponder.
The basic function of a transponder is amplification and frequency translation.
27. What is the reason for frequency translation?
The reason for frequency translation is that the transponder cannot transmit &
receive on the same frequency. Transponders are wide bandwidth units so that they
may receive and retransmit more than one signal.
28. What are the types of subsystem in space segment?
Communication subsystem- repeaters & antennas for primary functions
Common subsystem-power, stabilization, propulsion, station keeping, attitude
control, TTC & thermal control
29. How much power is produced by solar cell array?
Solar arrays on satellite are built up by attaching solar cells about 2 square cm
to a panel. To generate 100W power, more than 2000 cells are needed a square meter
surface.
30. Define TTC subsystem.
The telemetry system provides a data stream to the ground that reports on each
of the satellite subsystems independent of another communication ink. This is
achieved by the sensors mounted on the satellite.
31. What are the important factors of TTC subsystem?
The transmission of housekeeping information and status of the satellite to the
ground control station.
Carrying out angular & range measurement to permit localization of the satellite.
Receiving command signals from the ground control station, to initiate attitude &
station keeping process & operation on board equipment.
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32. Define earth segment.
The earth segment of a satellite communication consists of transmit & receive
earth stations. The satiations which are placed on the ships at sea, commercial military
land & aeronautical mobile stations functions for logistic support of satellites such as
providing Telemetry, Tracking & Command.
33. Expand TTC.
Telemetry, Tracking and Command.
34. What are the parameters used in designing antenna?
Mount, steer ability, gain, beam width, side lobe levels, pointing accuracy &
wind speed are the parameters taken for designing an antenna.
35. What are the several methods used for tracking the satellites?
Manual tracking, Auto tracking, Program tracking, and Step Tracking.
36. Define step tracking.
In step tracking system, the antenna is stepped intermittently in azimuth &
elevation.
37. Define station keeping.
The control routine necessary to keep the satellite in position is known as
station keeping.
38. Define GPS.
Global Positioning System is useful satellite system. Its primary application is
navigation. The receivers use the signals & compute the exact position of the receiver
on earth. The receiver output is displaying the latitude, longitude & altitude of the
receiver.
39. What are the features of Mobile Satellite?
Excellent solutions for emergency management and first response teams.
Satellite communications for special events, live, video feeds & multicasting
available.
Move, relocate & establish connectivity at moment‟s notice.
40. Mention the microwave devices used in microwave communication.
Tunnel diodes, gunn diodes, transferred electron devices & avalanche
transit-time devices.
41. Define parametric amplifier.
A parametric device is one that uses a nonlinear reactance or a time varying
reactance. The parametric is derived from the parametric excitation since the
capacitance or inductance which is a reactive parameter can be used to produce
capacitive or inductive excitation.
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42. Define microwave communication.
Microwaves are generally electromagnetic waves with frequencies range from
300MHz to 300GHz. Because of high frequencies it has shorter wavelength. It can
propagate at a velocity of 3X108m/sec.
43. What are the advantages of parametric amplifier?
No resistors or semiconductor devices involved there is no source of noise.
It amplifies at microwave region.
44. Expand TWT.
Travelling Wave Tube.
45. What are the limitations of parametric amplifier?
Up frequency is restricted by pump frequency & varactor capacitance.
Because of tuned circuits bandwidth is small.
The lower frequency is decided by cut off frequencies of microwave
components.
46. Define TWT.
Travelling wave tube is an amplifier which makes use of a distributed
interaction between electron beam & a travelling wave. The TWT is an O-type parallel
field linear beam device & here the wave propagates with same speed.
47. What are the reasons for position changing of a satellite in station keeping?
The external forces will act on satellite to alter its position & orientation with
respect to earth.
The various forces are earth‟s gravity, solar radiation, magnetic force, field
force, meteorite bombardment etc.
48. What is MSAT?
Meteorological Satellite (MSAT) is used in terrestrial surveying. This is a
mobile satellite communication system. It is 10 times powerful than INMARSAT. It
uses smaller antenna since its EIRP is 57.3dBW.
49. What is INTELSAT?
INTELSAT-Indian National Satellite System used as multipurpose & multi agency
system. It has 3 national services.
TV radio broadcasting
Long distance Communication
Meteorology
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UNIT-V MOBILE COMMUNICATION
1. Which pattern is used for maximum coverage in cellular communication?
The hexagon shape was chosen because it provides the most effective
transmission by approximating a circular pattern while eliminating gaps inherently
present between adjacent circles.
2. What is forward voice channel & reverse voice channel?
The channels used for voice transmission from the base station to mobiles are
called forward voice channels. The channels used for voice transmission from mobiles
to the base station are called reverse voice channels.
3. What is micro cell & macro cell?
Microcells are used most often in high density areas such as found in large
cities and inside buildings.
Macro cells may overlay clusters of microcells with slow moving mobile units
using the micro cells & faster moving units using the macro cells.
4. What is a pico cell?
In well shielded areas or areas with high levels of interference, cellular radio
signals are too weak to provide reliable communications indoor. In these areas very
small cells called Pico cells are used.
5. Define adjacent channel interference.
The interference from different frequency channels used within a cluster whose
side lobes overlap is called adjacent channel interference.
6. Define MSC & MTSO.
A basic cellular system consists of mobile stations, base stations and a mobile
switching center (MSC). The mobile switching center is sometimes called Mobile
Telephone Switching Office (MTSO), since it is responsible for connecting all
mobiles to the PSTN in a cellular system.
7. Define frequency reuse.
The same spectrum can support multiple users separated by a distance is the
primary approach for efficiently using the spectrum. The reusing of available
spectrum is called frequency reuse.
The design process of selecting and allocating channel groups for all base
stations within cellular base stations within a system is called frequency reuse or
frequency planning.
8. Define co-channel interference.
The interference due to using the same frequencies in cells of different clusters
is referred to as co-channel interference. The cells that use the same set of frequencies
or channels called co-channels.
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9. What are the different ways for minimizing adjacent channel interference?
Adjacent channel interference can be minimized through careful filtering and
channel assignments. Each cell is given only a fraction of the available channels, a cell
is need not be assigned channels which are adjacent in nature.
10. How to improve coverage & capacity in cellular systems?
The techniques such as cell splitting, sectoring and coverage zone approaches
are used to expand the capacity of cellular systems.
11. Define foot print.
The hexagonal shaped model of radio coverage for each base station has the
actual radio coverage of a cell is known as the footprint.
12. Define cell splitting.
Cell splitting is the process of subdividing highly congested cells into smaller
cells, each with its own base station and a corresponding reduction in antenna height
and transmitted power. The purpose is to increase the channel capacity and improve
the availability and reliability of a cellular telephone network.
13. Mention the drawbacks of cell splitting.
The major drawback of cell splitting is that it results in more base station
transfers (handoffs) per call and a higher processing load per subscriber.
14. Define sectoring.
In cell splitting method the cell radius is decreased and keeping the co-channel
reuse ratio D/R unchanged, which will increase the number of channels per unit area.
In another way to increase capacity is to keep the cell radius unchanged and decreased
the D/R ratio is called sectoring.
15. Define roaming.
When a mobile unit moves from its own service area to another service area is
called roaming. This allows the subscribers to operate in service areas other than the
one which service is subscribed.
16. Define handoff.
Once a call is on progress, the MSC adjusts the transmitted power of the mobile
and changes the channels of the mobile unit and base stations in order to maintain call
quality as the subscriber moves in and out of range of each base station. This is called
handoff.
17. What are the various stages of handoff?
A hand off consists of four stages:
Initiation
Resource reservation
Execution
Completion
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18. Define FDMA.
In FDMA, all users share the satellite at the same time, but each transmits in its
own unique frequency band. This is most commonly employed with analog
modulation, where signals are present all the time. The available transponder
bandwidth is divided among the users and all can transmit simultaneously.
19. Define TDMA.
In TDMA, only one user transmits at any time and that user can use the entire
available bandwidth, so the instantaneous data rate is proportional to the available
bandwidth.
20. Define CDMA.
In CDMA, many earth stations simultaneously transmit orthogonally coded
spread- spectrum signals that occupy the same frequency band. That is in CDMA;
users can transmit simultaneously and also share the frequency allocation.
21. What is the use of CDMA?
CDMA is more suitable for military tactical communications environment
where many small groups of mobile stations are briefly at regular intervals.
22. Define digital cellular system.
Cellular systems which use digital modulation technique are called digital
cellular systems. Digital systems offer large improvement in capacity and system
performance.
23. What is GSM?
Global system for mobile (GSM) is a second generation cellular system
standard. GSM was the world‟s first cellular system to specify digital modulation and
network level architecture and services. It is the most popular 2G (second generation)
technology.
24. What is the maximum transmission speed in GPRS network?
The maximum transmission speed in the GPRS network is theoretically
1722Kbit/sec. the devices available on the market offer a maximum transmission
speed of 586 Kbit/sec.
25. What are the types of GSM services?
Telephone services
Bearer services or data services
Supplementary ISDN services
26. What are the primary subsystems used in GSM architecture?
Base station subsystem (BSS)
Network and switching subsystem (NSS)
Operation subsystem (OSS)
27. Expand GPRS.
General Packet Radio Service
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28. Mention some of the basic parameters of GSM?
GMSK modulation&50MHz bandwidth
890MHz to 915MHz mobile transmit band
FDMA/TDMA accessing & 200KHz traffic channel
Call diversion, call identification & SMS services
29. Define BSS.
The BSS, also known as the radio subsystem, provides and manages radio
transmission paths between the mobile stations and mobile switching center (MSC). It
also manages the radio interface between the mobile stations and all other subsystems
of GSM.
30. Define NSS.
The NSS manages the switching functions of the system and follows the MSC‟s
to communicate with other networks such as PSTN and ISDN.
31. Define OSS.
The OSS supports the operation and the maintenance of GSM and allows
system engineers to monitor, diagnose and trouble shoot all aspects of the GSM
system. This subsystem interacts with the other GSM subsystems.
32. Define GPRS.
GPRS (General Packet Radio Service) is a service within the GSM network,
just like the two most popular services of SMS and voice connections. GPRS is used
for transmitting data in the GSM network in the form of packets.
33. What are the different databases in NSS?
Home location register (HLR),Visitor location register (VLR)
Authentication center (AUC)
34. Define Abis interface.
The interface which connects a BTS to BSC is called Abis interface.
35. Define A interface.
The interface between a BSC and a MSC is called the A interface.
36. Expand GSM, MIN, ESN and SCM.
GSM-Global system for mobile
MIN-Mobile Identification Number
ESM-Electrical Serial Number
SCM-Station Class Mark
37. Expand TDMA, FDMA & CDMA.
TDMA- Time Division Multiplexing Access
FDMA- Frequency Division Multiplexing Access
CDMA - Code Division Multiplexing Access