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IT 2202 PRINCIPLES OF COMMUNICATION DEPARTMENT OF ECE

IT 2202 PRINCIPLES OF COMMUNICATION DEPARTMENT OF ECE Page 1

DEPARTMENT OF INFORMATION TECHNOLOGY

SUBJECT : PRINCIPLES OF COMMUNICATION CODE: IT 2202

UNIT I : FUNDAMENTALS OF ANALOG COMMUNICATION

1. What is the need for modulation?It is extremely difficult to radiate low frequency signals through earths

atmosphere in the form of electromagnetic energy.

At low frequency, the antenna size required becomes impractical.

Information signals often occupy the same frequency band. Signals from

two or more sources would interfere if they are not modulated and

translated to a different frequency band.

2. With reference to AM define modulation index (or) depth of modulation.c

m

EEm

E c = peak amplitude of the unmodulated carrier voltage

E m= peak amplitude of modulating signal voltage

3. A broadcast radio transmitter radiates 5 kW power when the modulationpercentage is 60%. How much is the carrier power?

Wm

PP

mPP

tc

ct

28.4237

2

6.01

5000

21

21

22

2

4. What is the relationship between total power in AM wave and unmodulatedcarrier power?

indexulationm

powertotalp

powercarrierulatedunP

mPP

t

c

ct

mod

mod

21

2

5. What is the relationship between total current in AM wave andunmodulated carrier current?

indexulationm

cuurentcarrierulatedunI

currenttotalI

mII

c

t

ct

mod

mod

21

2

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6. An unmodulated carrier is modulated simultaneously by three modulatingsignals with coefficients of modulation m1 = 0.2, m2 = 0.4, m3 = 0.5.

Determine the total coefficient of modulation.

67.05.04.02.0 222232

2

2

1 mmmmt

7. Define selectivityof a radio receiver.It is the ability of the receiver to accept a given band of frequencies

and reject all others.

11. Define sensitivityof a radio receiver.

It is the ability of the receiver to pick up weak signals and amplify

them.

It is defined in terms of the voltage that must be applied to the receiverinput terminals to give the standard output power. It is usually

expressed in micro volts.

12.Define the term fidelity of a radio receiver.

Fidelity is a measure of the ability of a receiver to reproduce an exact

replica of the original source information.

i.e., fidelity is the ability of a receiver to reproduce faithfully all frequency

components present in the base band signal

8. Define band width improvement factor and noise fi gure improvement.Band width improvement factor ( BI ) is the ratio of RF bandwidth and IF

bandwidth.

BandWidthIF

BandWidthRFBI

Noise figure improvement = 10 log ( BI )

9. Define Noise fi gureNoise figure is a parameter commonly used to indicate the quality of a

receiver.

Lower the Noise figure value, better is the performance.

Noise figure = 10 log ( F )Where, F = noise factor

Noise factor is the ratio of input SNR and output SNR.

output

input

SNRSNRFactorNoise

SNR ( signal to noise ratio )is the ratio of signal power to noise power.

powerNoise

powerSignalSNR

Define Equi valent Noise TemperatureEquivalent noise temperature ( Te) is a parameter used to indicate the

quality of a receiver. Lower the value of Te , better the quality.

Te= T ( F1 )

T = Temperature in Kelvin

F = Noise factor16. What is Amplitude modulaltion?

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Amplitude Modulation is the process of changing the amplitude of a

relatively high frequency carrier signal in proportion with the instantaneous value

of the modulating signal.

17. What is Frequency modulation?

Frequency of carrier is varied in accordance with amplitude of modulatingsignal.

18.What is Phase modulation?

Phase of carrier is varied in accordance with the amplitude of modulating

signal.

19.What is Bandwidth of AM wave?

Band width is difference between highest upper side frequency and

lowest lower side frequency. B.W = 2fm(max).

20. What is over,under,critical modulation?

If m >1, has severe distortion.This codn is Over modulation. If m=1, has

greatest output and codn is Critical modulation. If m< 1 ,has no distortion

and codn is Uunder modulation.

21.With reference to FM, define modulation index.

Modulation index is the ratio of frequency deviation and modulating

signal frequency

m = f / fm

f = frequency deviation in Hz

fm = modulating signal frequency in Hz

22.Define deviation ratio.

It is the worst-case modulation index which is the ratio of maximum

permitted frequency deviation and maximum modulating signal

frequency.

Deviation ratio = f(max) / fm(max)

23.State Carsons rule for determining approximate Band Width of FM

signal.

Band Width = 2 [ f + fm(max)] Hz

f = frequency deviation in Hz

fm(max) = highest modulating signal frequency in Hz

24.A carrier is frequency is frequency modulated with a sinusoidal signal of 2

KHz resulting in a maximum frequency deviation of 5 KHz. Find the

approximate band width of the modulated signal.

f = frequency deviation in Hz = 5 KHzfm(max) = highest modulating signal frequency in Hz = 2 KHz

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Band Width = 2 [ f + fm(max)] Hz = 14 KHz

25.Determine the modulation index of a FM system with a maximum

frequency deviation of 75 KHz and maximum modulating frequency of 10

KHz.

m = f / fm = 75 KHz / 10 KHz = 7.5

26.Distinguish between narrow band FM and wide band FM.

Narrow band FM Wide band FM

Frequency deviation in carrier

frequency is very small

Frequency deviation in carrier

frequency is large

Band width is twice the

highest modulating frequency

Band width is calculated as per

Carsons rule

27.What are the advantages of FM over AM?

The amplitude of FM is constant. Hence transmitter power

remains constant in FM where as it varies in AM.

Since amplitude of FM is constant, the noise interference is

minimum in FM. Any noise superimposing on modulated carrier

can be removed with the help of amplitude limiter.

The depth of modulation have limitation in AM. But in FM, the

depth of modulation can be increased to any value.

Since guard bands are provided in FM, there is less possibility of

adjacent channel interference.

Since space waves are used for FM, the radius of propagation islimited to line of sight( LOS ) . Hence it is possible to operate

several independent transmitters on same frequency with

minimum interference.

Since FM uses UHF and VHF ranges, the noise interference is

minimum compared to AM which uses MF and HF ranges.

28. What is the advantage and disadvantage of Angle modulation?

Advantages: 1. Noise Reduction

2. Improved system fidelity

3. More effective use of power

Disadvantage: 1. Require more Bandwidth2. Use more complex circuits in both transmitter and

receiver

29. With respect to PM define modulation index?m = k Em =

30. Define peak phase deviation?

k Em =

where k is deviation sensitivity in rad/volt

and Em is amplitude of modulating signal.

31.Define frequency deviation?

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f = k1 Em/2 in Hz

Where k1 is deviation sensitivity rad/volt-sec

32. Define percent modulation?

Percent modulation = [actual frerquency deviation/max allowable frequency

deviation] x100

33.What are the types of modulation Index?

1.Low index modulation (m1)

34.Obtain total angle modulated wave power?

Pt = Po + P1 +P2 +.Pn

35.With reference to AM define modulation index.

It is defined as the ratio of maximum amplitude of the modulating voltage to themaximum amplitude of carrier voltage

=Am/Ac (or) =ka Am

36.Why is modulation necessary?

Ease of radiation

Multiplexing

To overcome hardware limitation

To reduce noise and interference

Frequency Assignment

37.The carrier of a 100% modulated wave is suppressed Find the percentage

of power savingPower saving in DSBSC compared to AM is 66.6%

38.Define selectivity and sensitivity of a radio receiverSelectivity: It is defined as the ability to select a particular frequency from different

frequency

Sensitivity: It is defined as the ability of receiver to give maximum output even

though i/p is very low

39.A Tx supplies 8KW to the antenna when unmodulated. Determine the totalpower when amplitude modulate to 30%.

Pt=Pc(1+ma2/2)

=8x103(1+0.32/2)

40.What is the main difference b/w frequency modulation and phase modulation

Frequency modulation:It is the form of angle modulation in which

instantaneous frequency fI(t) is varied linearly with the base band signal m(t)

Where,fI(t)=fc+kfm(t) fc unmodulated carrier

kfFrequency sensitivity of the modulator

m(t)-Base band signalintegrating above equation w. r .t time and multiplying with 2

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i(t)= 2 fct+2 K f m(t) dt

s(t)=Ac cos i(t)

s(t)= Ac cos(2 fct+2 K f m(t) dt)

Phase modulation :

It is that form of Angle modulation in which angle i(t) is varied linearlywith the base band signal m(t) as as shown by

i(t)= 2 fct+Kpm(t)

s(t)=Ac cos i(t)

s(t)= Ac cos(2 fct+Kpm(t)

41.Define modulation index for AM and FM

=Am/Ac---------------------------AM

= f/f M

Ratio of frequency deviation and modulating signal frequency

42.With reference to FM define modulation index and frequency deviation

modulation index = f/fMFrequency deviation :It represent the maximum departure of instantaneous frequency

of FM wave from the carrier frequency fc.Frequency deviation is proportional to the

amplitude of modulating wave and is independent of the modulation frequency f=Kf

Am

43.Determine the modulation depth of FM system with a maximum

frequency deviation of 75 KHz and the maximum modulating frequency of 10KHz

= f /f M

=75 x103/10 x103

=7.5

44.Write down the expression for FM signal with sinusoidal modulationFrequency modulation :It is the form of angle modulation in which

instantaneous frequency fI(t) is varied linearly with the base band signal m(t)

Where,fI(t)=fc+kfm(t) fc unmodulated carrier

kfFrequency sensitivity of the modulator

m(t)-Base band signal

integrating above equation w. r .t time and multiplying with 2

i(t)= 2 fct+2 K f m(t) dt

s(t)=Ac cos i(t)

s(t)= Ac cos(2 fct+2 K f m(t) dt)

45.What are the main difference b/w AM Rx and FM RxThe basic difference is AM demodulator replaced by FM demodulator such as limiter

and discriminator.

Typical freq parameters of commercial AM valueRF carrier range 0.535-1.605 MHz

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Mid band freq of IF station =.455 MHz

IF band width =10 KHz

Typical freq parameters of commercial FM value

RF carrier range 88-108 MHz

Mid band freq of IF station =.10.7 MHz

IF band width =.2 MHz

In an FM system the message information is transmitted by variation of the

instantaneous frequency of sinusoidal carrier wave and its amplitude is

maintained constant .Therefore any variation of the carrier amplitude at the R

x i/p must result from the noise or interference The amplitude limiter before

discriminator remove amplitude variation.

46.Distinguish b/w direct method and indirect method of producing FM waveIN indirect method the instantaneous frequency of the carrier wave is varied

directly in accordance with the base band signal by means of a device known as vco

e.g Hartley oscillatorDisadvantage is that carrier frequency is not obtained from a highly stable oscillator

In indirect method the base band signal m(t) is first integrated used to phase modulate

a crystal controlled oscillator .Hence stable carrier frequency is obtained

47.Why is FM system preferred over AM system.PM system can provide better discrimination against noise and interference than

AM

48.What is the main difference between natural sampling and instantaneous

sampling

Instantaneous sampling:

Consider an arbitrary signal g(t) of finite energy which is specified for alltime as shown in fig a .Suppose that we sample the signal g(t) instantaneously and at

uniform rate once every T seconds .As a result of this sampling process we obtain an

infinite sequence of numbers spaced Tsseconds apart and denoted by g(nTs) where n

takes all a possible integer values .Ts is the sampling period.

Natural sampling:This is applied in PDM(pulse duration modulation ).Samples of

message signal are used to vary the duration of individual pulses .

49.Define Modulation index and percent modulation for an AM wave.Modulation index is a term used to describe the amount of amplitude

change present in an AM waveform .It is also called as coefficient of modulation.

Mathematically modulation index ism = Em/Ec

Where m = Modulation coefficient

Em = Peak change in the amplitude of the output waveform voltage.

Ec = Peak amplitude of the unmodulated carrier voltage.

Percent modulation gives the percentage change in the amplitude of the output

wave when the carrier is acted on by a modulating signal.

49.Define direct frequency modulation.In direct frequency modulation , frequency of a constant amplitude carrier

signal is directly proportional to the amplitude of the modulating signal at a rateequal to the frequency of the modulating signal.

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50.Define indirect frequency Modulation.In indirect frequency modulation phase of a constant amplitude carrier is

directly proportional to the amplitude of the modulating signal at a rate equal to

the frequency of the modulating signal.

51.Define instantaneous frequency deviation.

The instantaneous frequency deviation is the instantaneous change in the

frequency of the carrier and is defined as the first derivative of the instantaneous

phase deviation.

52.Define frequency deviation.Frequency deviation is the change in frequency that occurs in the carrier

when it is acted on by a modulating signal frequency.Frequency deviation is

typically given as a peak frequency shift in Hertz( f).The peak to peak frequency

deviation (2 f) is sometimes called carrier swing.The peak frequency deviation is

simply the product of the deviation sensitivity and the peak modulating signalvoltage and is expressed mathematically as f=K1 Vm Hz

53.State Carson rule.Carson rule states that the bandwidth required to transmit an angle

modulated wave as twice the sum of the peak frequency deviation and the highest

modulating signal frequency. Mathematically carsons rule is

B=2( f +fm) Hz.

54.Define Deviation ratio.Deviation ratio is the worst case modulation index and is equal to the

maximum peak frequency deviation divided by the maximum modulating signal

frequency. Mathematically, the deviation ratio is

DR= f (max)/fm(max)

Unit II : DIGITAL COMMUNICATION

1.Obtain the Bandwidth for narrow band FM?

For narrowband FM bandwidth is 2fm.where fm is modulating frequency.

2. For commercial FM broadcast band transmitter frequency deviation is

75KHz. If the given modulating signal produce 35KHz frequency deviation.

What is the percent modulation?percent modulation = 35/75 = 46.66

3.Determine peak phase deviation for PMmodulator with deviation sensitivity

k = 2.5 rad/v and em = 2cos(22000t).

k Em =

= 2.5(2) = 5rad/v

4.What is a constellation diagram? Draw the constellation diagram for BPSK.

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Constellation diagram is used to show the relative positions of the peaks of the

phasors. It is similar to a phasor diagram except that the entire phasor is not

drawn(peak of the phasors are shown).

5.What is the minimum bandwidth required for an FSK system?

frequencyspaceffrequencymarkfratebitft

tffrequiredwidthband

smb

b

b

sm

;;1

2

6.What is the primary advantage of DBPSK and what is its disadvantage?

Advantage: simple implementation. No carrier recovery circuit needed for

detection.

Disadvantage:It requires between 1 dB and 3 dB more signal to noise ratio to

achieve the same BER as that of standard absolute PSK

7.What are the advantages of M-ary signaling schemes?

M-ary signaling schemes transmit multiple bits at a time.

Bandwidth requirement of M-ary signaling schemes is reduced.

8.Compare binary PSK with QPSK.

BPSKBinary Phase Shift Keying

QPSKQuadrature Phase Shift Keying

One bit form a symbol Two bits form a symbolTwo possible symbols Four possible symbols

Minimum bandwidth required = f

b

where fbis bit rate

Minimum bandwidth required = fb/

2

where fbis bit rate

9.What are the advantages of QPSK as compared to BPSK?

For the same bit rate, the bandwidth required by QPSK is reduced to

half as compared to BPSK.

Because of reduced bandwidth, the information transmission rate of

QPSK is higher.

10.What happens to the probability of error in M-ary PSK as the value of M

increases?As the value of M increases, the Euclidean distance between the

symbols reduces. Hence the symbols are closer to each other. This increases

the probability of error in M-ary systems.

11.What is the minimum bandwidth required for BPSK, QPSK, 8-PSK, 8-QAM

and16-QAM systems if the bit rate is 10 Mbps?

system Minimum band width Minimum band width

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required

if fb = bit raterequired

if fb = 10 Mbps

BPSK fb 10 MHz

QPSK fb/ 2 5 MHz

8 - PSK fb/ 3 3.33 MHz

8- QAM fb/ 3 3.33 MHz

16 - QAM fb/ 4 2.5 MHz

12.What is difference between coherent and non coherent detection?

Coherent detection Non- Coherent detection

Carrier which is in perfect

coherence with that used in

transmitter is used for

demodulation.

Carrier recovery circuit is

needed for detection

No carrier recovery circuit needed

for detection.

Relatively complex Simple implementation

13.Define Bandwidth efficiency. What is the bandwidth efficiency of BPSK

and 8-PSK system?It is the ratio of the transmission bit rate to the minimum bandwidth required for a

particular modulation scheme.

)(min

)(

Hzwidthbandimum

bpsrateontransmissiefficiencywidthBand

For BPSK , transmission rate = fb and minimum bandwidth = f b

Band width efficiency = 1For 8-PSK , transmission rate = fb and minimum bandwidth = f b/3

Band width efficiency = 3

14.What is the difference between probability of error P(e) and bit error rate

BER?P(e) Probability of error is a theoretical (mathematical) expectation of

the bit error rate for a given system.

BER is an empirical record of a systems actual bit error performance.

For Example, if a system has a P(e) of 10 -5 , this mean that, you can expect

one bit error in every 100,000 bits transmitted.

If a system has a BER of 10 -5 , this mean that, there was one bit error for

every 100,000 bits transmitted.

BER is measured and then compared to the expected probability of error to

evaluate the systems performance.

15.What is the probability of error for (i) non-coherent FSK and (ii) coherent

FSK? Compare their error performance.

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ratiodensitypowernoisetobitperenergyN

E

FSKcoherentforerrorofyprobabilitN

EerfceP

FSKcoherentnonforerrorofyprobabilitN

EeP

b

b

b

0

0

0

)(

2exp

2

1)(

For a given energy per bit to noise power density ratio, probability of error for non-

coherent FSK is greater than that of coherent FSK.

16. Define ( Eb/ N0 ) Energy per bit to Noise power density ratio.Energy per bit to noise power ratio is used to compare two or more digital

modulation systems that uses different bit rates and modulation schemes.

It is the product of carrier to noise power ratio and the noise band width to bit

rate ratio. This is equivalent to signal to noise ratio.

b

b

b

bb

b

f

B

N

C

N

E

HzinwidthbandnoiseB

wattinpowernoisethermalNwhere B

NdensitypowerNoiseN

indurationbitT

wattinpowercarrierCwhere

TCbitperEnergyE

ratiodensitypowernoisetobitperenergyN

E

0

0

0

sec

UNIT III : DIGITAL MODULATION TECHNIQUES

1.State sampling Theorem.If a finite energy signal g(t) contains no frequency component higher than W Hz,

it is completely determined by specifying its ordinates at a separation of points

spaced 1/ 2W seconds apart.

If a finite energy signal g(t) contains no frequency component higher than W Hz,

it is completely recovered from its ordinates at a separation of points spaced 1/

2W seconds apart.

2.Methods of Sampling1. Ideal Sampling (or) Instantaneous Sampling2. Natural Sampling3. Flat-top Sampling

3.What is Aliasing or Foldover?

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Aliasing occurs when the signal is sampled at a rate less than Nyguist rate(2W

samples/ sec). It is prevented by using

Guard Bands

Pre-alias Filter

2. Define Nyquist rate and Nyquist interval.According to sampling theorem, a continuous time signal can be completely

represented in its samples and recovered back if the sampling frequency is fS 2W.

Here fS is sampling frequency and W is the highest frequency component of the

signal.

Nyquist rate: The minimum sampling rate of 2W samples per second is called

Nyquist rate.

i.e., fS= 2W Nyquist rate

Nyquist interval: Reciprocal of 2W is called the Nyquist interval.

Nyquist interval = 1/2W

3. What is aliasing?When the continuous time signal g(t) is sampled at the rate less than Nyquist

rate, frequencies higher than W takes on the identity of the low frequencies in

sampled signal spectrum . This is called aliasing.

The use of a low pass reconstruction filter, with its pass band extending from

W to W will not yield an undistorted version of the original signal. Aliasing can be

reduced by sampling at a rate higher than Nyquist rate.

4. Give the practical procedure for the sampling of a signal whose

spectrum is not strictly band limited.i) Prior to sampling, a low pass pre-alias filter ( anti-alias filter) of high

enough order is used to attenuate those frequency components of the signal that do not

contribute significantly to the information content of the signal.

ii) The filtered signal is sampled at a rate slightly higher than the Nyquist rate

2W, where W is the cutoff frequency of the pre-alias filter.

5. Define aliasing error. Give the upper bound for the aliasing error.Let {g(n/fS)} denote the sequence obtained by sampling an arbitrary signal g(t)

at the rate fS samples per second. Let gi(t) denote the signal reconstructed from this

sequence by interpolation;

That is, gi(t)= g( ) sinc ( fSt - n)

The absolute error = | g(t) gi(t) | is called the aliasing error.

The aliasing error is bounded as

2 | G(f) | df

6. Given the signal m(t)=10 cos (2000 t) cos (8000 t ) , what is the minimum

sampling rate based on the low pass uniform sampling theorem?The equation shows that m(t) is generated by multiplication of two signals.

We know that cosAcosB = [cos (A-B)+cos (A+B)]

There fore, m(t) = (10/2) [ cos (6000 t) + cos (10000 t)]

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The two frequencies in m(t) are 3000 Hz and 5000 Hz and the highest frequency

present in m(t) is 5000 Hz.

Minimum sampling rate is 2 (5000) = 10000 samples per second.

7. What is Inter symbol Interference (ISI) ?ISI arises because of imperfections in the overall frequency response of the

system.

When a short pulse of duration Tb seconds is transmitted through a band

limited system, the frequency components constituting the input pulse are

differentially attenuated and, more significantly, differentially delayed by the system.

Consequently the pulse appearing at the output of the system is dispersed over an

interval longer than Tbseconds.

Thus when a sequence of short pulses are transmitted through the system, one

pulse every Tb seconds, the dispersed responses originating from different symbol

intervals will interfere with each other, there by resulting in ISI.

8. What is eye pattern?When the received signal in a data transmission system is applied to the

vertical deflection plates of an oscilloscope and saw tooth wave( frequency = symbol

rate) is applied to the horizontal deflection plates of an oscilloscope, the resulting

display is called an eye pattern.

The display pattern is called an eye pattern because of its resemblance to the

human eye. An eye pattern provides a great deal of information about the performance

of the data transmission system. Eye pattern is particularly useful in studying ISI

problem.

9. List out the information provided by eye pattern about the system

performance.

i)The width of the eye opening defines the time interval over which the received wave

can be sampled without error from ISI. The preferred time for sampling is the instant

of time at which the eye is open widest.

ii) The sensitivity of the system to timing error is determined by the rate of closure ofthe eye as the sampling time is varied.

iii) The height of the eye opening , at a specified sampling time, defines the margin

over noise.

iv) Completely closed eye shows that the effect of ISI is severe.

10. Compare base band binary PAM and base band M-ary PAM.

In the baseband binary PAM system , the out put of the pulse generator takeson one of two possible amplitude levels. In a baseband M-ary PAM system,

the out put of the pulse generator takes on one of M possible amplitude

levels.

An M- ary PAM system transmit information at a rate that is log2M fasterthan the corresponding binary PAM for a fixed channel band width. That is

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for a fixed bit rate, an M-ary PAM system requires less channel bandwidth

than a binary PAM.

In order to realize the same average probability of symbol error , an M-ary

PAM system requires more transmitted power.

12. State Nyquist criterion for zero ISI.

P( f n Rb) = Tb

Tb= bit duration in seconds ; Rb= bit rate = 1/Tb

p(t ) = received pulse

P(f) = Fourier Transform of p(t)

13. What is adaptive equalization?

Equalization is the process of correcting channel induced distortion. This process is

said to be adaptive when it adjusts itself continuously during data transmission byoperating on the input signal.

In adaptive equalization, filters adapt themselves to the channel

characteristics. That is , the filter coefficients are adjusted in such a way that the

distortion in the data is reduced.

14. What is PAM?

It is a process in which amplitudes of regularly spaced pulses are varied in

proportion to the corresponding sample values of continuous message signal.

15.What do you mean by Aperture Effect?It is nothing but amplitude distortion occurring at PAM due to the sinc function. It

is overcome by using a Equalizer whose transfer function is

| H(f)| = T 1 sinc(fT)

16. What is PPM?It is the process in which the position of a pulse relative to its unmodulated time of

occurrence is varied in accordance with message signals.

17. What is PWM?It is the process in which the samples of message signal are used to vary the

duration of individual pulses in the carrier.

18. What is Quantization?

It is the process in which the analog sample of the original signal is converted into

a digital form.

19. What is Sampling?It is the process in which the original analog signal is converted into a discrete

time and continuous amplitude signal.

20. Classify Quantizers.

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Uniform QuanatizerRepresentation levels are uniformly spaced

Non-Uniform QuanatizerRepresentation levels are non-uniformly spaced

21. What is Quantization Noise?The difference between the output analog sample and the discrete output

quantized signal gives raise to an error called Quantization Noise.

22. What are the two types of Quantization Noise?1. Slopeoverload distortion2. Granular Noise

23. What is Slope- Overload Distortion?

When the message signal varies steeply, the quantized approximation cannot

follow the message signal and this results in slope-overload distortion. It can be

reduced by increasing the step-size. To minimize the distortion, we need to have

/ Ts max | d(x(t)) / dt |

24. What is Granular Noise?When the input waveform has a flat segment, then the step-size is

larger when compared to the input. Therefore the approximation hunts

around the segment resulting in Granular noise.

This noise can be reduced by decreasing the step-size.

25. What is Companding?It is the combined process of compressing and Expanding used for improving the

dynamic range of signal and also to increase the SNR of low level signals.

26. List the advantages of Digital pulse modulation systems.

Immunity to

3. Transmission distortion.4. Interference5. Noise

27.What is meant by hunting with reference to delta modulation?

The up & down movement of the approximated signal m (t) above & below the

input signal m (t) because of the step-wise approach during delta modulation is calledhunting.

28. What is the principle of adaptive delta modulation?The principle underlying ADM:

a) If successive errors are of opposite polarity, then the delta modulator isoperating in its granular mode. In this case the step size is reduced.

b) If successive errors are of same polarity, then the delta modulator is operatingin its slope overload mode. In this case the step size is increased.

28. What are the limitations of delta modulation?

The major limitations of delta modulation are:a) Slope overload error

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b) Granular noise

29.State the difference between DPCM and DM.

Parameter DPCM DM

Number of Bits

Levels, Step size.

Bandwidth of

transmission channel.

Bits can be more than one

but less than PCM.

Fixed numbers of levels are

used.

Bandwidth required is

lesser than PCM.

It can use 4, 8 or 16 bits per

sample.

Step size is fixed and

cannot be varied.

Lower bandwidth is

required.

30. Define slope overload error?The signal m (t) over a extended time has a very large slope which cannot be

approached by the approximated signal m (t) In short time.as a result the error m (t) -

m (t) becomes progressively larger i.e., exceeding s/2 this is known as slope overload

error.

31. Define granular noise?Even though the signal m(t) remains unvarying the approximated signal m(t)

swinging up or down above & below m(t) . It is because of step-wise approach of m

(t) to the m(t).This process is referred as hunting & it gives the noise known as

granular noise.

32. Define Time division multiplexing.In time division multiplexing common cable/medium is shared by different

channels on time basis Each channel is assigned a fixed rtime slot .Every Channel

transmits in its own time slot.In TDM

33. What are the advantages of digital transmission?The advantage of digital transmission over analog transmission is noise

immunity. Digital pulses are less susceptible than analog signals to

variations caused by noise.

Digital signals are better suited to processing and multiplexing than analog

signals.

Digital transmission systems are more noise resistant than the analog

transmission systems.

Digital systems are better suited to evaluate error performance.

34. What are the disadvantages of digital transmission?_ The transmission of digitally encoded analog signals requires

significantly more bandwidth than simply transmitting the original

analog signal._ Analog signal must be converted to digital codes prior to

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transmission and converted back to analog form at the

receiver, thus necessitating additional encoding and

decoding circuitry.

35. Define pulse code modulation.

In pulse code modulation, analog signal is sampled and converted to fixed length,

serial binary number for transmission. The binary number varies according to

the amplitude of the analog signal.

36. What is the purpose of the sample and hold circuit?

The sample and hold circuit periodically samples the analog input signal and

converts those samples to a multilevel PAM signal.

37. What is the Nyquist sampling rate?

Nyquist sampling rate states that, the minimum sampling rate is equal to twice

the highest audio input frequency.

38. Define and state the causes of fold over distortion.The minimum sampling rate(fs) is equal to twice the highest audio input

frequency(fa).If fs is less than two times fa ,distortion will result. The distortion is

Called aliasing or fold over distortion.

The side frequencies from one harmonic fold over into the sideband of another

harmonic. The frequency that folds over is an alias of the input signal hence ,the

names aliasing or fold over distortion

.

39. Define overload distortion.If the magnitude of sample exceeds the highest quantization interval, overload

distortion occurs.

40. Define quantization.Quantization is a process of approximation or rounding off. Assigning PCM codes

to absolute magnitudes is called quantizing.

41. Define dynamic range.

Dynamic range is the ratio of the largest possible magnitude to the smallestpossible magnitude. Mathematically, dynamic range is

DR= Vmax/Vmin

42. Define slope overload. How it is reduced.The slope of the analog signal is greater than the delta modulator can maintain,

and is called slope overload. Slope overload is reduced by increasing the clock

frequency and by increasing the magnitude of the minimum step size.

43.Define granular noise.How it is reduced.When the original input signal has relatively constant amplitude, the

reconstructed signal has variations that were not present in the original signal.This is called granular noise.

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Granular noise can be reduced by decreasing the step size.

44. Define peak frequency deviation for FSK.Peak frequency deviation (_f) is the difference between the carrier rest frequency

and either the mark or space frequency and either the mark or space frequency.

45. Define bit rate.

In digital modulation, the rate of change at the input to the modulator is called the

bit rate (fb) and has the unit of bits per second (bps).

46. Define Baud rate.

The rate of change at the output of the modulator is called baud.

47. Define QAM.Quadrature amplitude modulation is a form of digital modulation where the

digital information is contained in both the amplitude and phase of the transmitted

carrier.

48. Write the relationship between the minimum bandwidth required for an

FSK system and the bit rate.

The minimum bandwidth can be approximated as

B=2_f +2fb

Where B=minimum bandwidth (hertz)

_f=minimum peak frequency deviation (hertz)

Fb=bitrate

49.What is PAM?

It is a process in which amplitudes of regularly spaced pulses are varied in proportion

to the corresponding sample values of continuous message signal.

50.What do you mean by Aperture Effect?

It is nothing but amplitude distortion occurring at PAM due to the sinc

function. It is overcome by using a Equalizer whose transfer function is

| H(f)| = T1

sinc(fT)

51.What is PPM?It is the process in which the position of a pulse relative to its unmodulated

time of occurrence is varied in accordance with message signals.

52.What is PWM?

It is the process in which the samples of message signal are used to vary the duration

of individual pulses in the carrier.

53. List the advantages of Digital pulse modulation systems.Immunity to--- Transmission distortion, Interference & Noise

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54. What is the minimum bandwidth requirement of a PAM system?

Nfm N:No.of signals to be multiplexed fm:Highest frequency of the signal

55. How is synchronization achieved in PAM system?

In a PAM system, marker pulses are introduced in each frame. These pulses are

identified by making its amplitude greater than all possible message pulses and the

marker pulse is identified at the receiver by applying the pulses to the slicer network.

56.What are the corrective measures taken to avoid aliasing effect?A low pass anti-alias filter is used to attenuate high frequency of the signal that lie

outside the frequency band of interest prior to sampling. The filtered signal is sampled

at a slightly higher rate than Nyquist rate.

57. Mention the advantages of digital communication over analog

communication.

i)It gives ruggedness to transmission noise and interference.

ii) Efficient regeneration of coded signal along transmission path.

iii)Possible to have a uniform format for different kinds of baseband signal.

58. How many bits required to sample audio signals with 128 quantization levels

?

7.

59.What is hunting as related to delta modulation?

The initial large discrepancy between the message signal and its stepwise approach is

called hunting or startup response resulting in redundancy (repetition of same pattern

of bits for almost nil variation in slope in message signal).

60. State the differences between DPCM and DM.

DM uses only one bit information for transmission.

Replacement of the prediction filter in DPCM by a single delay element

constitutes DM system.

61.PCM requires larger BW as compared to DM for the same performance.State true or false. False.

62. What is meant by DPSK method?It is a non-coherent binary modulation technique. This is a modified technique of

binary phase shift keying. DPSK avoids the need to provide a synchronous carrier

required at the demodulator for detecting BPSK signal.

63. What is meant by Binaryphase- shift keying?If the transmitted signal is sinusoid of fixed amplitude then it is called as Binary

phase- shift keying. It has one fixed phase when the data is at one level & when thedata is at another level the phase is different by 180

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64. How the BPSK signal is generated?The BPSK signal is generated by applying the waveform Cos(uo t) as a carrier to a

balanced modulator and applying the base band signal b(t) as the modulating

waveform,

in this sense BPSK can be thought of as an AM signal.

65. What is the significance of DPCM?In DPCM instead of quantizing the individual sample values, the difference between

the successive samples is quantized. As the difference between sample values {M(k)

M(k-1)} will be smaller than the sample value themselves fewer levels will be

required to quantize the difference than that required to quantize M(k) and

correspondingly fewer bits will be needed to encode the levels.

UNIT IV : SPREAD SPECTRUM AND MULTIPLE ACCESS TECHNIQUES

1. What is the primary advantage of spread spectrum communication?The primary advantage of spread spectrum communication system is its ability

to reject interference whether it be the unintentional interference of another user

simultaneously attempting to transmit through the channel or the intentional

interference of a hostile transmitter attempting to jam the transmission.

2. What are the applications of spread spectrum modulation?Protection against jammers in military applications

Code division multiple access (CDMA)

Multipath interference suppression in mobile communication

3. Define Pseudo Noise sequence.A Pseudo Noise (PN) sequence is coded sequence of 1s and 0s with

certain autocorrelation properties.

It is periodic a sequence of 1s and 0s repeats itself exactly with a

known period

PN sequence is a noise like high frequency signal. The sequence is not

truly random but it is generated by a well defined logic.This is used in spread spectrum communication.

4. What are the properties of maximum length PN sequence?Balance property: In each period of a maximum length sequence, the

number of 1s is always one more than the number of 0s.

Run property: Among the runs of 1s and 0s in each period of a

maximum length sequence, one half the runs are of length one, one

fourth are of length two, one eighth are of length three and so on.. as

long as these fractions represent meaningful number of runs

Correlation property: The auto correlation function of a maximum

length sequence is periodic and binary valued.

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5. Define processing gain. What is the processing gain for DSSS?It is defined as the ratio of spreaded signal bandwidth to unspreaded signal

bandwidth.

6. Define chip rate.Number of chips per second.7. What is the difference between slow frequency hopping spread spectrum

technique and fast frequency hopping spread spectrum technique?

Slow frequency hopping: Symbol rate RS of the MFSK signal is an

integer multiple of the hop rate R h. i.e., several symbols are transmitted in

each hop.

Fast frequency hopping:The hop rate R h is an integer multiple of the

MFSK symbol rate RS. i.e., carrier frequency will change or hop several

times during the transmission of one symbol.

8. What are the different multiple access techniques ( Fixed assignment multipleaccess techniques)used in wire less communication?

TDMATime Division Multiple Access

FDMA- Frequency Division Multiple Access

CDMACode Division Multiple Access.

9. Compare FDMA and TDMA.FDMA - Frequency Division Multiple

Access

TDMA - Time Division Multiple Access

All users access the channel by

transmitting simultaneously but using

disjoint frequency bands

All users occupy the same RF band width

of the channel, but they transmit

sequentially in time

Fixed assignment multiple access

technique

Fixed assignment multiple access

technique

Well suited for analog communication Well suited for digital communication

10.List the merits of CDMA over TDMA.CDMA does not require an external synchronization network which is an

essential feature of TDMA.

CDMA offers a gradual degradation in performance as the number of useris increased. It is there fore relatively easy to add new users to the system.

CDMA offers an external interference rejection capability.

11.What is CDMA?CDMA Code Division Multiple Access. In CDMA , all users transmit

simultaneously and occupy the same RF bandwidth. Each user is assigned a

code which perform the DSSS or FHSS modulation

12. What is meant by spread spectrum?

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Spread spectrum is a means of transmission in which the data of interest occupies a

bandwidth in excess of the minimum bandwidth necessary to send the data

13. What are the applications of spread spectrum?

It is used in military communications systems.

It allows the transmitter to transmit a message to a receiver without themessage being detected by a receiver for which it is not intended.

It decreases the transmitted power spectral density so that it lies well belowthe thermal noise level of any unfriendly receiver.

It turns out not to be possible to conceal the transmission.

14. What is DS spread spectrum?

DS spread spectrum signal is one in which the amplitude of an already modulated

signal is AM modulated by a very high rate NRZ binary stream of digits.

15. Define FH spread spectrum.FH spread spectrum is an FM or FSK technique while DS is a AM or PSK technique.

The signal to be FH is usually a BFSK signal although M-ary FSK, MSK or TFM can

be employed.

16. What is meant by pseudo random noise?The pseudo random noise is the nose which presents in the DS spread spectrum, if the

DS spread spectrum signal is V(t)=g(t)S(t)=2Psd(t)cos ot. Here g(t) is a pseudo

random noise binary sequence having the values 1. A pseudo noise sequence is

defined as a coded sequence of 1s and 0s with certain autocorrelation properties. The

class of sequences used in spread spectrum communication is usually periodic in that

a sequence of 1s and 0s repeats itself exactly with a known period

17.Write the advantages of FH over direct sequence spread spectrum. Processing gain is more Spread of the transmitted signal is spread sequentially rather than instantaneously Greater transmission bandwidth and more chips per bit.

UNIT V : SATELLITE AND OPTICAL FIBRE COMMUNICATIONS

1. State Keplers first law.Keplers first law states that the satellite will follow an elliptical path in its orbit

around the primary body. An ellipse has two focal points as F1 and F2.

2. State Keplerssecond & third law.It states that for equal time intervals the satellite sweeps out equal areas in the orbital

plane, focused at the baycenter.

Third law states that the square of the periodic time of orbit is proportional to the

cube of the mean distance between the two bodies.

3. What is apogee and perigee?

Apogee is the point farthest from the earth. Perigee is the point of closest approachto the earth.

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4. What are the classifications of satellites?Satellites are generally classified as a low earth orbit (LEO),Medium earth orbit

(MEO),or Geosynchronous earth orbit(GEO) .

5. What is major axis?The line joining the perigee and the apogee through the centre of earth.

6. What is geo-stationary satellite?

Geo-stationary satellite is one that appears to be stationary relative to the earth.

7. What is ascending node?The point where the orbit crosses the equatorial plane from south to north. The point

where a polar or inclined orbit crosses the equatorial plane traveling from north to

south is called ascending node.

8. What are the advantages of optical fibre?Optical fibers are free from electromagnetic radiation and other electrical

interference. Optical fibres have a very small attenuation .Optical fibers have large

bandwidth capacity.

9. What is meant by the term numerical aperture of an fiber?Numerical aperture is used as figure merit for the fiber .It is given as

N A = n12n22 /no

Where no is refractive index of medium through which light is launched.

n1 is refractive index of fiber core glass.n2 is refractive index of fiber

cladding.

10. What is the difference between TDM and FDM?In TDM ,fixed time slot is given to each channel .In FDM fixed frequency slot is

given to each channel.Thus in FDM ,bandwidth is shared ,whereas in TDM time is

shared.

11. What is meant by dispersion and what is it unit of measurement?The time dispersion of the pulse takes place during transmission. The changes timing

characteristics of the pulse. Because of dispersion ,the pulse is widened and amplitudeis further reduced, This may cause overlapping of adjacent pulses. Dispersion is

measured in nanoseconds per kilometers.

12. Distinguish between graded index and step index multimode fiber?

In step index fiber the refractive index of the fiber core remains same throughout its

bulk. There is sudden transition of refractive index at the interface at the interface of

core and cladding .Because of the constant refractive index of the core material the

light rays travel straight and reflected from the walls of the core. Hence the light path

is zig zag straight segments.

In graded index fiber the refractive index is maximum at the core axis and goes

decreasing gradually towards the wall of the core. The acceptance cone of the gradedindex fiber is more than the step index fibers.

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13.What is acceptance cone half angle for an optical fiber with core and

cladding refractive index of n1=1.48 and n2=1.46 respectively.The acceptance cone angle is given by Qo=sin-1[sqrt(n12n22/no ] (no=1) =14

14. Define satellite.Satellite is a celestial body that orbits around a planet.In aerospace terms,

a satellite is a space vehicle launched by humans and orbits earth or another

celestial body.

15. State Keplers third law.The third law states that the square of the periodic time of orbit is

proportional to the cube of the mean distance between the primary and the

satellite

.

16. Define orbital satellite.Orbital satellites are also called as non synchronous satellite. Non synchronous

satellites rotate around earth in an elliptical or circular pattern. In a circular orbit, the

speed or rotation is constant however in elliptical orbits the speed depends on the

height the satellite is above the earth.

17. Define posigrade orbit.If the satellite is orbiting in the same direction as earths rotation and at an

angular velocity greater than that of earth, the orbit is called a prograde (or)

posigrade orbit.

18. Define retrograde orbit.If the satellite is orbiting in the opposite direction as the earths rotation or

in the same direction with an angular velocity less than that of earth, the orbit is

called a retrograde orbit.

19. Define Geo synchronous satellite.

Geo synchronous or geo stationary satellites are those that orbit in a

circular pattern with an angular velocity equal to that of Erath. Geosynchronous

satellites have an orbital time of approximately 24 hours, the same as earth; thus

geosynchronous satellites appear to be stationary as they remain in a fixed

position in respect to a given point on earth.

20. Define angle of inclination.The angle of inclination is the angle between the earths equatorial plane

and the orbital plane of a satellite measured counterclockwise at the point in the

orbit where it crosses the equatorial plane traveling from south to north.

21. Define Descending node.The point where a polar or inclined orbit crosses the equatorial plane

traveling from south to north. This point is called descending node.

22. Define line of nodes.

The line joining the ascending and descending nodes through the center ofearth is called line of nodes.

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23. Define angle of elevation.Angle of elevation is the vertical angle formed between the direction of

travel of an electromagnetic wave radiated from an earth station antenna pointing

directly toward a satellite and the horizontal plane.

24. Define Azimuth angle.

Azimuth is the horizontal angular distance from a reference direction,

either the southern or northern most point of the horizon.

25. What are the advantages of optical fiber communication?

Greater information capacity

Immunity to crosstalk

Immunity to static interference

Environmental immunity

Safety & Security

26. Define a fiber optic system.

An optical communications system is an electronic communication system

that uses light as the carrier of information. Optical fiber communication systems

use glass or plastic fibers to contain light waves and guide them in a manner

similar to the way electromagnetic waves are guided through a waveguide.

27. Define refractive index.The refractive index is defined as the as the ratio of the velocity of

propagation of light ray in free space to the velocity of propagation of a light

ray in a given material.

28. Define critical angle.Critical angle is defined as the minimum angle of incidence at which a

light ray may strike the interface of two media and result in an angle of refraction

of 90or greater.

29. Define single mode and multi mode propagation.If there is only one path for light to take down the cable, it is called single

mode. If there is more than one path, it is called multimode.

30. Define acceptance angle.It defines the maximum angle in which external light rays may strike the

air/fiber interface and still propagate down the fiber with a response that is no

greater than 10 dB below the maximum value.

31. Define numerical aperture.

Numerical aperture is mathematically defined as the sine of the maximum

angle a light ray entering the fiber can have in respect to the axis of the fiber and

still propagate down the cable by internal reflection.

32. Define modal dispersion.

Modal dispersion or pulse spreading is caused by the difference in thepropagation times of light rays that take different paths down a fiber. Modal

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dispersion can occur only in multimode fibers. It can be reduced by using single

mode step index fibers and graded index fibers.

33.What are the advantages of heterojunction LEDs?a. The increase in current density generates a more brilliant light spot.

b. The smaller emitting area makes it easier to couple its emitted light into Fiber.c. The small effective area has a smaller capacitance, which allows the

planar heterojunction LED to be used at higher speeds.

34. What are the disadvantages of injection laser diode?_ ILDs are typically on the order of 10 times more expensive than LEDs

_ Because ILDs operate at higher powers, they typically have a much shorter

life time than LEDs.

_ ILDs are more temperature dependent than LEDs.

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