vel tech high tech dr.ranagarajan dr.sakunthala ...modulation is defined as the process by which...

Vel Tech High Tech Dr.Ranagarajan Dr.Sakunthala Engineering College – Department of ECE

Page | 1

Course Code: EC8491

Course Name: COMMUNICATION THEORY L-3 : T-0 : P-0 : C-3

COURSE OBJECTIVES:

To introduce the concepts of various analog modulations and their spectral characteristics

To understand the properties of random process

To know the effect of noise on communication systems

To study the limits set by Information Theory

COURSE OUTCOMES:

At the End of the Course Students undergoing this course are able to

CO

No Course Outcomes

Knowledge

Level

C212.1 Explain the principles of different analog modulation techniques K2

C212.2 To illustrate the spectral characteristics of angle modulation

techniques

K2

C212.3 Apply the concepts of Random Process to the design of

Communication systems

K3

C212.4 Compare noise performance of receivers K2

C212.5 To summarize the principles of sampling and quantization. K2

MAPPING OF COURSE OUTCOMES WITH PROGRAM OUTCOMES:

CO PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 PO10 PO11 PO12

C212.1 3 2 1 - - - - - - - - 1

C212.2 3 2 1 - - - - - - - - 1

C212.3 3 2 1 - - - - - - - - 1

C212.4 2 2 1 - - - - - - - - 1

C212.5 3 2 - - - - - - - - - 1

C.No PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 PO10 PO11 PO12

C212 3 2 1 - - - - - - - - 1

Mapping Relevancy

3 – Substantial (Highly relevant)

2 – Moderate (Medium)

1 – Slight (Low)

COURSE DELIVERY METHODS

Class room lecture - Black board

PPTs, Videos

Lab Demonstrations

Activities like In Plant Training, Live Demonstrations and Guest Lecture


Page | 2

ASSESSMENT METHODS

DIRECT ASSESSMENT INDIRECT ASSESSMENT

Continuous Internal Assessment(CIA)

End Semester Examination

Assignments

Seminars

Plickers

Formative

Course Exit Survey

Periodical Feedback

COURSE SYLLABUS

UNIT I AMPLITUDE MODULATION 9

Amplitude Modulation- DSBSC, DSBFC, SSB, VSB - Modulation index, Spectra, Power relations

and Bandwidth – AM Generation – Square law and Switching modulator, DSBSC Generation –

Balanced and Ring Modulator, SSB Generation – Filter, Phase Shift and Third Methods, VSB

Generation – Filter Method, Hilbert Transform, Pre-envelope & complex envelope –comparison

of different AM techniques, Superheterodyne Receiver

UNIT II ANGLE MODULATION 9

Phase and frequency modulation, Narrow Band and Wide band FM – Modulation index, Spectra,

Power relations and Transmission Bandwidth - FM modulation –Direct and Indirect methods, FM

Demodulation – FM to AM conversion, FM Discriminator - PLL as FM Demodulator.

UNIT III RANDOM PROCESS 9

Random variables, Random Process, Stationary Processes, Mean, Correlation & Covariance

functions, Power Spectral Density, Ergodic Processes, Gaussian Process, Transmission of a

Random Process Through a LTI filter.

UNIT IV NOISE CHARACTERIZATION 9

Noise sources – Noise figure, noise temperature and noise bandwidth – Noise in cascaded systems.

Representation of Narrow band noise –In-phase and quadrature, Envelope and Phase – Noise

performance analysis in AM & FM systems – Threshold effect, Pre-emphasis and deemphasis for

FM.

UNIT V SAMPLING & QUANTIZATION 9

Low pass sampling – Aliasing- Signal Reconstruction-Quantization - Uniform & non-uniform

quantization - quantization noise - Logarithmic Companding –PAM, PPM, PWM, PCM – TDM,

FDM.

TOTAL: 45 PERIODS

TEXT BOOKS

TB1.Taub & Schiling ―Principles of Communication Systems, Tata McGraw Hill 2007.

TB2.J.Das ―Principles of Digital Communication‖ New Age International, 1986.


Page | 3

REFERENCE BOOKS

RB1.Kennedy and Davis ―Electronic Communication Systems‖ Tata McGraw hill, 4th Edition,

1993.

RB2.Sklar ―Digital Communication Fundamentals and Applications― Pearson Education, 2001.

RB3.Bary le, Memuschmidt, Digital Communication, Kluwer Publication, 2004.

RB4.B.P.Lathi ―Modern Digital and Analog Communication Systems‖ Oxford University Press,

1998

COURSE DELIVERY PLAN

S.

No Date

Un

it Topic

Text/

Refer

ence

Books

Teaching

Methodol

ogy

Cour

se

Outc

ome

1 17.12.18 I

Amplitude Modulation- DSBSC,

DSBFC-Modulation index, Spectra,

Power relations and Bandwidth

TB1,

RB1

Class

room

lecture -

Black

board

CO1

2 18.12.18 I

Amplitude Modulation- SSB,VSB-

Modulation index, Spectra, Power

relations and Bandwidth

CO1

3 19.12.18 I AM Generation – Square law and

Switching modulator, CO1

Slip Test 1 CO1

4 20.12.18 I DSBSC Generation – Balanced and

Ring Modulator Class

room

lecture -

Black

board

CO1

5 21.12.18 I

SSB Generation – Filter, Phase Shift and

Third Methods, VSB Generation – Filter

Method

CO1

6 22.12.18 I Hilbert Transform, Pre-envelope &

complex envelope PPT CO1

Slip Test 2 CO1

7 24.12.18 I Superheterodyne Receiver Class

room

lecture -

Black

board,

Plickers

CO1

8 26.12.18 I Comparison of different AM techniques CO1

UEPS / CIA- 1 CO1

9 05.01.19 II Phase and frequency modulation

TB1

Class

room

lecture -

CO2

10 07.01.19 II

Narrow Band -Modulation index,

Spectra, Power relations and

Transmission Bandwidth

CO2

COURSE

INSTRUCTOR

Mrs.G.S.SIVAPRIYA

Ms.R.SRIVIDHYA FACULTY ID

HTS1133

HTS1322

CURSE NAME Communication Theory COURSE CODE EC8491

YEAR/SEM II/ IV MONTH & YEAR DEC- 2018


Page | 4

11 08.01.19 II

Wide band FM – Modulation index,

Spectra, Power relations and

Transmission Bandwidth

Black

board,

Plickers

CO2

Slip Test 3 CO2

12 09.01.19 II FM modulation –Direct method. Class

room

lecture -

Black

board

,PPT

CO2

13 10.01.19 II FM modulation - Indirect method. CO2

14 11.01.19 II FM Demodulation – FM to AM

conversion. CO2

Slip Test 4 CO2

15 12.01.19 II FM Discriminator - PLL as FM

Demodulator

Class

room

lecture -

Black

board

CO2

UEPS / CIA- 2

16 25.01.19 II Problems on phase and frequency

modulation CO2

17 28.01.19 III Random variables, Random Process

TB1

Class

room

lecture -

Black

board ,

Plickers

CO3

18 29.01.19 III Stationary Processes, Mean, Correlation CO3

19 30.01.19 III Covariance functions, Power Spectral

Density CO3

Slip Test 5 CO3

20 31.01.19 III Ergodic Processes. Class

room

lecture -

Black

board

,PPT

CO3

21 01.02.19 III Gaussian Process CO3

22 04.02.19 III Transmission of a Random Process

Through a LTI filter. CO3

Slip Test 6 CO3

23 05.02.19 III Problems on Stationary Processes,

Mean, Correlation

Class

room

lecture -

Black

board

CO3

24 06.02.19 III Revision of Ergodic Processes and

Gaussian Process CO3

UEPS / CIA- 3 CO3

25 14.02.19 IV Noise sources – Noise figure, noise

temperature and noise bandwidth.

TB1,

RB1

Class

room

lecture -

Black

board,

Plickers

CO4

26 15.02.19 IV Noise in cascaded systems. CO4

27 18.02.19 IV Representation of Narrow band noise –

In-phase and quadrature. CO4

Slip Test 7 CO4

28 19.02.19 IV Envelope and Phase Class

room

lecture -

CO4

29 20.02.19 IV Noise performance analysis in AM &

FM systems. CO4


Page | 5

30 21.02.19 IV Threshold effect. Black

board,PPT

CO4

Slip Test 8 CO4

31 22.02.19 IV Pre-emphasis and de-emphasis for FM Class

room

lecture -

Black

board

CO4

32 23.02.19 IV Problems on Noise figure, noise

temperature and noise bandwidth. CO4

UEPS / CIA- 4 CO4

33 06.03.19 V Low pass sampling ,quantization noise

TB2,

RB1

Class

room

lecture -

Black

board

PPT

CO5

34 07.03.19 V Signal Reconstruction-Quantization CO5

35 08.03.19 V Aliasing, Uniform quantization CO5

Slip Test 9 CO5

36 09.03.19 V Non-uniform quantization Class

room

lecture -

Black

board

PPT

,Plickers

CO5

37 11.03.19 V Logarithmic Companding – CO5

38 12.03.19 V PAM, PPM &PWM CO5

Slip Test 10 CO5

39 13.03.19 V PCM TDM, FDM. CO5

UEPS / CIA- 5 CO5

UNIT-I AMPLITUDE MODULATION

PART-A

1. What are the advantages of converting low frequency signal to high frequency

signal? (Nov 2016)

• Ease of transmission

• Multiplexing

• Reduced noise

• Narrow bandwidth Frequency assignment

• Reduce the equipments limitations.

2. Suggest a modulation scheme for the broadcast video transmission and justify. (Nov

2016)

VSB i.e. Vestigial Sideband Modulation is used for TV transmission. Vestige

means "PART". The part of side band is used to transmit the video signal and the

remaining is used for transmitting the voice signal.


Page | 6

3. Define modulation index of AM. (Nov 2014)

In AM wave, the modulation index (ma) is defined as the ratio of maximum

amplitude of modulating signal to the maximum amplitude of carrier signal. ma =

Vm/Vc

4. Compare linear and non linear modulators. May 2015

Linear modulators Non-linear modulators

Heavy filtering is required Heavy filtering is required

These modulators are used in high level

modulation

These modulators are used in low level

modulation

The carrier voltage is very much greater

than modulating signal voltage

The modulating signal voltage is very

much greater than the carrier signal voltage

5. What are the methods used to generate the SSB-SC-AM?

SSB-SC-AM waves can be generated in to two ways,

(i) Frequency discriminator or Filter method

(ii) Phase discriminator method

Phase discriminator method itself can be divided in to

two types. (i) Phase shift method

(ii) Modified phase shift method or Weavers method

6. Define VSB and bandwidth of VSB. (May 2014, Nov 17)

In VSB sideband modulation (VSB), the desired sideband is allowed to pass

completely. Whereas just a small portion (called trace or vestige) of the undesired

sideband is allowed. The transmitted vestige of the undesired sideband compensates for

the loss of the wanted sideband. Bandwidth of VSB = fm+fv

fm - message bandwidth fv – width of the vestigial sideband

7. Distinguish between single sideband and Double sideband.

Parameter Single Side Band (SSB) Double Side Band (DSB)

Bandwidth fm 2fm

Power saving for

sinusoidal 83.3% 66.66%

Power saving for

nonsinusoidal 75% 50%

Generation methods More difficult Not difficult

Detection methods More difficult Difficult


Page | 7

Sidebands One sideband Double sideband

Application Long range HF Commn

especially in Audio Commn

Short distance point to point

communication

8. Define modulation and mention its types.

Modulation is defined as the process by which some parameter (amplitude,

frequency or phase) of a high frequency signal termed as carrier is varied in

accordance with the information signal.

(i)Analog modulation – Continuous and pulse modulation (ii) Digital modulation

9. What is pre-envelope and complex envelope? (May 2016)

An analytic signal is a complex signal created by taking a signal and then

adding in quadrature its Hilbert Transform. It is also called the pre-envelope of the

real signal. Analytic signals are often shifted in frequency (down-converted) toward

0 Hz, Possibly creating [nonsymmetrical] negative frequency components is called

complex envelope.

10. Name the advantage of modulation.

Ease of transmission, Multiplexing, Reduced noise, Narrow bandwidth

Frequency assignment and Reduce the equipments limitations.

11. Draw the power spectrum of AM. (May 2015)

12. Mention the relationship between carrier power and total power.

Pt=Pc (1+ma2/2)

Where, Pt - Total power of the modulated wave Pc - Carrier power

ma - Modulation index


Page | 8

13. Compare low level and high level modulation.

Low Level Modulation: 1. The generation of AM wave take place at a low power

level.

2. With low level modulation, the modulation takes place prior to the output element of

the final stage of transmitter. 3. The linear amplifiers are required in order to avoid any

waveform distortion.

High Level Modulation: 1. In this method, the generation of AM wave take

place at high power levels. 2. The carrier and the modulating signal both are amplified

first to an adequate power level and the modulation takes place in the last RF amplifier

stage of the transmitter.

14. List some application of SSB in AM? (May 2017)

1. Point to point radio telephone communication

2. SSB telegraph system

3. Police wireless communication

4. UHF and VHF communication

PART-B

1. Derive the expression for amplitude modulated and explain any one method to generate

and demodulate it. (Nov’16, May’14)

Ref: “Fundamentals of Communication Systems” by J.G.Proakis, M.Salehi

[Page.no:134,145]

2. Explain the operation of balanced modulator to generate DSB-SC-AM. (May’16, 17,

Nov’16)

Ref: “Fundamentals of Communication Systems” by J.G.Proakis, M.Salehi [Page.no:147]

3. With a neat block diagram explain the super heterodyne receiver and its performance.

(May’17, May’16, Nov’15, 17, May’15, May’14, May’11)


4. Explain the filter and phase shift method to generate SSB-SC-AM generation. (Nov 17)

Ref:”Communication theory” by Dr.J.S.Chitode [Page.no:1-42]

5. Derive the expression for envelope waveform for AM and calculate power, current,

efficiency for the modulated wave. (Nov’16, May’14)


6. Explain the operation of envelope detector with a neat waveform. (Nov’15, 17, May’15,

May’11)


7. Derive the expression for SSB-SC and calculate power, current and efficiency. (May’15)

Ref:”Communication theory” by K.Muralibabu [Page.no:4.13]


Page | 9

8. Explain any two methods to generate DSB-SC-AM in detail. (Nov’15)

Ref: “Fundamentals of Communication Systems” by J.G.Proakis, M.Salehi

[Page.no:147,148]

9. With a neat block diagram explain the modified phase shift method or weavers’ method

to generate SSB. (Nov 17)


10. Explain the operation of balanced modulator and ring modulator to generate DSB.

(May 17) Ref: “Fundamentals of Communication Systems” by J.G.Proakis, M.Salehi

[Page.no:147,148]

11. What is the need for VSB? Explain the generation of VSB (Filter method) and give its

advantage and application. (May’17)


12. Comparison of Amplitude Modulation system with AM, DSB, SSB and VSB. (Nov’14)


UNIT II – ANGLE MODULATION

PART-A

1. Define modulation index of frequency modulation. (Nov’16)

Modulation index of FM is defined as the ratio of frequency deviation to

modulating frequency.

mf = frequency deviation / modulating frequency

In FM, the modulation index is directly proportional to the amplitude of the

modulating signal and inversely proportional to the frequency of the modulating signal.

mf = ∆f / fm = Kf Vm / fm mf – Modulation index (unit less)

Kf – Deviation sensitivity (radians per second per volts) Vm - Peak modulating

signal amplitude (volts)

fm - Maximum frequency of modulating signal

2. Name the methods of detecting FM signal.

1. Slope detector – Single tuned and stagger tuned discriminator

2. Phase discriminator – Foster –seeley and Ratio detector

3. Illustrate relationship between FM and PM, with block diagrams. (May’15)


Page | 10

4. State carson’s rule. (Nov’15, May 17)

Carson’s rule approximates the bandwidth necessary to transmit and angle modulated

wave as twice the sum of the peak frequency deviation and the highest modulating signal

frequency.

BW = 2(∆f + fm) Hz

Where, ∆f – Peak frequency deviation, fm – Modulating signal frequency

5. List the properties of Bessel’s function. (Nov’14)

(i) Jn (β)=(-1)nJ-n(β) for all n, both positive and negative.

(ii) For small values of the modulation index β ,we have J0 (β)=1

J1 (β)=β/2

Jn(β)=0,n>2.

(iii) Σ J2n (β)=1

6. Define phase locked loop.

A phase-locked loop (PLL) is an electronic circuit with a voltage or voltage-

driven oscillator that constantly adjusts to match the frequency of an input signal. PLLs

are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a

"noisy" communications channel where data has been interrupted.

7. What is the disadvantage of balanced slope detector? (Nov 14) (i) Even though

linearity is good, it is not good enough.

(ii) It is difficult to tune since the three tuned circuits are to be tuned at different

frequencies.

(iii) Amplitude limiting is not provided.


Page | 11

9. Compare NBFM and WBFM. (May’11, Nov 17)

WBFM NBFM

Modulation index is greater than 1 Modulation index less than 1

Frequency deviation 75 KHz Frequency deviation 5 KHz

Bandwidth 15 times NBFM Bandwidth 2fm

Noise is more suppressed Less suppressing of noise

10. Give the transmission bandwidth required for FM.

For narrow band FM is same as that of AM, which is equal to twice of modulating

frequency. BW = 2fm Hz

For wide band FM, the BW = 2∆f

11. Define frequency deviation. In FM, the deviation (∆f) is defined as the amount by which

the carrier frequency is varied from its un modulated value. The magnitude of the

frequency and phase deviation is proportional to the amplitude of the modulating signal.

(Vm)

∆f = Kf Vm (Hz)

12. What are angle modulation and its types? (Nov’15)

In angle modulation, the timing parameters such as phase or frequency of the

carrier are modulated according to amplitude of modulating signal.

(i) Frequency modulation

(ii) Phase modulation

13. What are the two methods to producing FM signal? (May’11)

(i) Direct method – Reactance modulators and Varactor diode modulators

(ii) Indirect method – Armstrong method

14. Define the modulation index of PM.

In PM, the modulation index is proportional to the amplitude of the modulating

signal, independent of its frequency.

mp = Kp Vm (radians)

Where Kp – Deviation sensitivity (radians/volt) Vm – Peak modulating signal

amplitude(volt)

8. How FM c an be converted to PM wave. ( May’17 )

PM

Carrier

Message

signal Differentiator

FM


Page | 12

PART-B

1. Derive an expression for a single tone FM signal with necessary diagram and draw its

frequency spectrum. (Nov’16, May’16, Nov’15, May’11)


2. Explain the direct method of FM generation. (Nov’16, May’15, 17, Nov 17)


3. Explain the Armstrong method of FM generation with a neat diagram. (May’16, 14)

Ref:”Communication systems” by Simon Haykin [Page.no:120]

4. With a neat diagram explain the operation of slope detector of FM. (May11, Nov 17)


5. An angle modulated wave is V(t)=100 Cos(2*106 Пt + 10 Cos 2000 Пt). Find power of

modulating signal, bandwidth and maximum frequency deviation. (May’16, 15, 11)


6. Explain the function of any FM detector circuit. (May’14)


7. Explain the working operation of balanced slope detector. (May’11)


8. Derive the expression for frequency modulation and phase modulation with a neat

waveform. Ref: “Fundamentals of Communication Systems” by J.G.Proakis, M.Salehi

[Page.no:170]

9. Write about the basic principle of FM detection and explain about ratio detector.

(Nov’16,

May 17)


10. Draw the circuit diagram of foster seely discriminator and explain its working with

relevant

Phasor diagram. (May’16, Nov’15, May’15)


11. Derive the expression for NBWM and WBFM and compare it. (May 17)


Page | 13



12. Make atleast 10 comparison of AM and FM system. (May’14) Ref:”Communication

theory” by K.Muralibabu [Page.no:1.24]

UNIT III – RANDOM PROCESS

PART-A

1. State central limit theorem. (Nov 16, Nov 17)

It provides mathematical justification for using Gaussian process for large number of

individual random events.

Requirements:

i) Xi are statistically independent.

ii) Xi have some probability distribution with mean µx and variance σx2.

2. Write Einstein weiner khintchine relation. (Nov’16, May 17)

a)Sx(f) =∫ Rx(τ)exp(-j2Πf τ)d τ

b)Rx(τ)=∫ Sx(f)exp(-j2Πf τ)d τ

3. What is auto correlation function? (May’16)

The auto correlation function of the process X(t) is the expectation of the

product of two random variables X(t1) and X(t2) obtained by observing the

process X(t) at times t1 and t2. Rx (t1, t2) = E[X(t1).X(t2)]

=∬ 𝑥1𝑥2𝑓𝑥(𝑡1)𝑥(𝑡2)(𝑥1, 𝑥2)𝑑𝑥1𝑑𝑥2

4. Define random variable. (Nov’15)

Random variable is defined as a rule or mapping from the original sample space

to a numerical sample space subjected to certain constraints. Random variable is also

defined as a function where domain is the set of outcomes ωεs and whose range is R,

is the real line.

5. List out the condition for a process to be ergodic in mean.

The time average µx(T) approaches the ensemble average µx when observation

interval T approaches infinity.

The variance of µx(T) approaches zero when the observation interval

T approaches infinity.

6. What is shot noise? (May’15)

These Noise are generally arises in the active devices due to the random

behavior of Charge particles or carries. In case of electron tube, shot Noise is

produces due to the random emission of electron form cathodes.


Page | 14

7. List out the property of power spectral density. (Nov’15)

(i) The PSD of a stationary process for zero frequency value is equal to the

total area under the graph of auto correlation function.

(ii) The mean square value of a stationary process equals the total area under

the graph of the PSD.

(iii) PSD of a stationary process is always non negative.

(iv) PSD of a read valued random process is an even function of frequency.

(v) The PSD, approximately normalized has the properties usually associated

with a probability density function.

8. List out the condition for a process to be ergodic in auto correlation function σ.

lim 𝑉𝑎𝑟[𝑅𝑥(Ʈ, 𝑇)] = 0

𝑇→∞

9. Define ergodic random process. (May’13, Nov 17)

A random process is said to be ergodic if the time averages of the process are equal to

the ensemble averages.

10. What is Gaussian random variable?

X(t) is said to be Gaussian process if every linear functional of X(t) is a Gaussian

random variable.

Probability density function of Y can be given as

fY

11. Write the formula for cross correlation.

The cross correlation function of X(t) and Y(t) are defined by

RXY (t,u) = E[X(t) . Y(u)] (or)

RYX (t,u) = E[Y(t) . X(u)]

12. What is central limit theorem? (May’16, Nov 17)

It provides mathematical justification for using Gaussian process for large number of

individual random events.

Requirements:

i) Xi are statistically independent.

ii) Xi have some probability distribution with mean µx and variance σx2.

13. Define stationary process. (May’11)

The statistical characterization of a process is independent of time at which

observation of the process is initiated. If such a process is divided in to number of

time interval, the various section of the process exhibit same statistical properties.

Such a process is said to be stationary process.


Page | 15

14. List out the properties of correlation function.

(i) Cross correlation is not an even function.

(ii) It does not have its maximum at origin.

(iii) It does obey a certain symmetry relationship as follows RXY (Ʈ) = RYX (-

Ʈ)

PART-B

1. Give a random process X(t)=A Cos(ωt+ µ) where A and ω are constants and µ is a

uniform distribution random variable. Show that X(t) is ergodic in both mean and

auto correlation. (May’16, Nov 17)


2. Explain in detail about the transmission of a random process through a linear time

invariant filter. (May’16, Nov’16, Nov 17)


3. What is gaussian random process and mention its properties. (Nov’16, May’14, Nov

17)


4. When random process is said to be SSS, WSS and ergodic process. (Nov’16, May’16,

May’15)



5. Explain about central limit theorem in detail. (May’12)


6. Define the term mean, correlation, covariance and ergodicity. (Nov’16,May

15,13,Dec 11,12)



7. Two random process x(t)=A Cos(ωt+θ) and y(t)=A Sin(ωt+ θ) where A and ω are

constants and θ is uniformly distributed random variable in (0, 2П). Find the cross

correlation function.

(May’16, 15)


8. Define auto correlation and prove the auto correlation properties.



Page | 16

9. Explain in detail about the transmission of a random process through LTI filter.

(May’16)


10. When random process is said to be ergodic and wide sense stationary process.

(Nov’15, May’15)



11. Define PSD. Prove the properties of power spectral density. (Nov’15)


12. State and prove the central limit theorem.



UNIT IV – NOISE CHARACTERIZATION

PART-A

1. Two resistors of 20K, 50K is at room temperature 290°K. For a bandwidth of 100

KHz. Calculate thermal noise voltage generated by resistor connected in series.

(Nov’16) Ref:”Communication theory” by Dr.J.S.Chitode [Page.no:4-9]

2. Define noise figure. (Nov’16,15)

It is ratio of the total noise power spectral density (Sno) at the output of the two port

network to the noise power spectral density (s’no) at the output, assuming the network is

noiseless.

3. Give the definition of noise equivalent temperature. (May’16, Nov 17)

Noise equivalent temperature of a cascade system Te is given as,

Te = Te1 + Te2 / Ga1 or

Pn=KTeB

KTeB= (F-1)KTb

Te=(F-1)T.

4. Compare noise performance of DSBSC receiver using coherent detection with AM

receiver using envelope detection.

The figure of merit of DSB-SC or SSB-SC receiver using coherent detection is

always unity, the figure of merit of AM receiver using envelope detection is always

less than unity. Therefore noise performance of AM receiver is always inferior to that

of DSB-SC due to the wastage of power for transmitting the carrier.

5. List out the various sources of noise. (Nov 17)

Noise source – External and Internal noise

External noise – Natural noise and manmade noise


Page | 17

Internal noise – Thermal noise, shot noise, partition noise, flickering noise, low

frequency noise

6. What is threshold effect? (Nov’15, May’15, May 17)

As the input noise power is increased the carrier to noise ratio is decreased the

receiver breaks and as the carrier to noise ratio is reduced further crackling sound is

heard and the output SNR cannot be predicted by the equation. This phenomenon is

known as threshold effect.

7. Define Thermal noise. (May’11)

This is the electrical noise which is arising from the random motion of electron

in a conductor is called thermal noise.

8. What is capture effect in FM? (May’16)

When the interference signal and FM input are of equal strength, the receiver

fluctuates back and forth between them .This phenomenon is known as the capture

effect.

9. Define noise figure and express in equation. (Nov’15)

It is ratio of the total noise power spectral density (Sno) at the output of the two

port network to the noise power spectral density (s’no) at the output, assuming the

network is noiseless. It is expressed in terms of equation as,

In terms of temperature F = 1+Te/T

10. What are the methods to improve FM threshold reduction? (May’11)

1. Pre-emphasis and de-emphasis.

2. FMFB (Frequency modulation with feedback)

11. List out the properties of inphase and quadrature phase components of narrow band

noise.

(i) nI(t) and nQ(t) have zero mean.

(ii) If n(t) is Gaussian, nI(t) and nQ(t) are jointly Gaussian.

(iii) If n(t) is stationary, nI(t) and nQ(t) are jointly stationary (iv) nI(t) and nQ(t)

have same variance as n(t)

12. What is Figure Of Merit?

Figure of merit is defined as ‘the ratio of output signal to noise ratio to channel

signal to noise ratio of the receiver’.

ɤ = (SNR)o / (SNR)c

13. Why the AM figure of merit is always less than 1?

The figure of merit of an AM receiver using envelope detection is always less

than unity. This is due to the wastage of transmitting power, which results from

transmitting the carrier as a component of the AM wave.


Page | 18

14. Define narrow band noise.

In most communication system we often dealing with band pass filtering of

signals. The band pass filters have narrow bandwidths in the sense that bandwidth is

small as compared to center frequency. We refer output of this kind of band pass filter

as narrow band noise.

PART-B

1. Derive the figure of merit for AM system. Assume Coherent detection.(Nov’16, 17,

May 17)


2. Explain the noise in FM receiver and calculate the figure of merit. (Nov’16)


3. Explain the envelope detection of AM system and calculate FOM.(May’15, 17)


4. Derive the formula for noise figure and noise temperature.


5. Discuss about narrow band noise interms of inphase and Quadrature noise

components. (May’16,11)


6. Explain about pre-emphasis and de-emphasis in detail. (May’16, 14, 17, Nov 17)


7. Explain the noise in DSB-SC receiver using synchronous or coherent detection and

calculate Figure of merit. (May’17, 16, 15, 11)


8. Derive the figure of merit for a non coherent detection receiver system noise

performance of AM. (May’15, 17)


9. Discuss about pre-emphasis and De-emphasis in detail. (May’16, 14)


10. Explain the noise performance of FM system and calculate the figure of merit.

(Nov’16)



Page | 19

11. Define noise and explain the various sources of noise. (Nov’14, May 17)


12. Assume the system in cascade; calculate the noise figure for a circuit connected in

cascade.

(May’15, 17)


UNIT V – INFORMATION THEORY

PART-A

1. Define entropy function. (May’11, Nov 17)

The entropy of a source is defined as the source which produces information

per individual message or symbol in a particular interval. It is also called as

comentropy.

Entropy H(S) =

2. Define rate of information or information rate. (May’13)

The rate of information (R) is defined as ‘the average number of bits of

information per second’. It is given as,

R = rH bits / sec

R – Rate at which message generated from the source

H – Average number of bits of information per message i.e., entropy

3. Mention the lossless data compression algorithms. (May’11)

(i) Prefix coding

(ii) Shannon Fano coding

(iii) Huffman coding

(iv) Lempel Ziv coding

4. State Shannon theorem. (Nov’15, May’14, May 17)

Shannon theorem stated as ‘Given a discrete memoryless source of entropy H, the

average codeword length L` for any distortion less source encoding is bounded as

L`≥H.’

5. A source generates 3 messages with probability 0.5, 0.25, 0.25. Calculate entropy.

(Nov’16, May’15)

Formula: Entropy H(S) =

6. Express the equation for channel capacity with respect bandwidth and SNR.

(Nov’16)

Information capacity can be expressed as,

C = B log2 [1 + (P/NoB)]


Page | 20

7. List the properties of entropy? (May 17)

(i) Entropy is zero, if the event is sure or it is impossible. H=0 if Pk =0 or 1

(ii) Entropy H=log2K, when the symbol are equally likely for K symbols, i.e.,

Pk = 1/K

(iii) Maximum upper bound on entropy is, Hmax≤log2K

8. Distinguish between lossless and lossy coding. (May’12)

LOSSLESS CODING LOSSY CODING

No loss of information Loss of information

Original data exactly recovered from

compressed data

Perceptual loss of information

reduced ( Controlled)

Lower compression ratio Higher compression ratio

Huffman coding, instantaneous

coding, Shannon fano coding are

lossless source coding techniques

PCM, DM, ADM, are lossy source

coding techniques

9. Define rate bandwidth and bandwidth efficiency.

Rate Bandwidth: It is defined as the amount of data transferred per

second. Bandwidth Efficiency: It is the ratio of the transmission bit rate

to the minimum bandwidth required for a particular modulation scheme.

Bɳ = Transmission bit rate (bps) / Minimum bandwidth (Hz)

10. State Shannon channel capacity theorem or Hartley theorem. (Nov’16, May’15)

The maximum of the mutual information between the channels input Xk and

channel output Yk over all distributions on the input Xk that satisfy the power

constraint.

C=B log2 (1+S/N)

B=channel bandwidth

S=signal power

N=total noise power within the channel bandwidth.

11. What is channel efficiency?

The transmission efficiency or channel efficiency is defined as the ratio of actual

transmission to the maximum transmission.

ɳ = Actual transmission / Maximum transmission

= I(X;Y) / max I(X;Y)

= I (X;Y) / C

12. A source is emitting symbol with probability 0.6, 0.3, 0.1. What is entropy of the

source?

(Nov’16, May’15)

Formula: Entropy H(S) =


Page | 21

13. Mention the properties of mutual information. (May’15)

(i) The mutual information of channel is symmetric; that is I (X;Y) = I (Y;X)

(ii) The mutual information is always non negative. I (X;Y)≥0

(iii) The mutual information of a channel is related to the joint entropy of the

channel input and the channel output by,

I (X; Y) = H(x) + H(Y) – H(X, Y)

14. Define information theory and amount of information.

Information theory deals with “mathematically modeling and analysis of a

communication system rather than with physical sources and physical channels”.

The amount of information Ik is related to the algorithm on the inverse of the

probability of occurrence of an event.

Ik = log2 (1 / Pk)

PART-B

1. For a DMS ‘S’ with 5 symbols construct a Huffman and Shannon fano coding and

also calculate the efficiency if the probability distribution is given as,

P(s1)=0.4,P(s2)=0.2, P(s3)=0.2, P(s4)=0.1, P(s5)=0.1. (Nov’16, Nov’15, Nov 17)


2. A DMS has an alphabet of seven symbols whose probability are 0.25, 0.125, 0.25,

0.125, 0.125, 0.0625, 0.0625. Construct Shannon fano code and also calculate the

efficiency. (Nov’16, Nov’15, May’14)


3. Derive the Shannon Hartley theorem for the channel capacity of a continuous

channel having an average power limitation. (Nov’16)


4. Explain how channel capacity could be improved. Explain the S/N trade off in detail.

(May’14 )


5. Calculate the channel capacity for bandwidth B=10 KHz and SNR is 20. For the

same channel capacity, find the bandwidth, if the SNR is increased to 40. Comment

on it.


6. A voice grade telephone channel has a bandwidth of 4000 Hz. If the SNR on the

channel is 30 dB, Determine the channel capacity.



Page | 22

7. A DMS has an alphabet of five symbols whose probability are 0.4, 0.19, 0.16, 0.15

and 0.15 Construct Shannon fano code and also calculate the efficiency. Nov’15,

May’15


8. For a discrete memoryless source with 5 symbols construct a Huffman and Shannon

fano coding and also calculate the efficiency if the probability distribution is given

as, s1=0.2, s2=0.4, s3=0.2, s4=0.1, s5=0.1. (Nov’16, Nov’15, May’14, May’16, May’15,

May’11, 17) Ref:”Communication theory” by Dr.J.S.Chitode [Page.no:5-47]

9. State the Shannon various theorem and explain. (Nov’16)


10. Write the properties of mutual information and prove two properties. (May’16, 11)


11. State and prove entropy properties in detail. (Nov 17)


12. Explain the steps involved in Huffman and Shannon fano coding theorem. (May’17)



B.E – DEGREE EXAMINATION, NOV / DEC 2017

Fourth Semester / Second Year

Electronics and Communication Engineering

EC 6402 – COMMUNICATION THEORY (Regulation 2013)

PART A (10*2=20)

1. Determine the Hilbert transform of Cos ωt.

2. What is VSB? Where is it used?

3. A Frequency modulated signal is given as s (t) = 20 Cos[2Пfct + 4 Sin(200Пt)]. Determine

the required transmission bandwidth.

4. How is Narrowband signal distinguished from Wideband signal?

5. State Central Limit Theorem.

6. What is meant by ergodic process?

7. Define the term Noise Equivalent Temperature.

8. List the external source of noise.

9. Using Shannon law determine the maximum capacity of 5 MHz, channel with Signal to

Noise Ratio of 10dB.

10. Define entropy.


Page | 23

PART B (5*13=65)

11. (a) (i) Explain the operation of envelope detector. (7)

(ii) Discuss the generation of single sideband modulated signal.

(6) (Or)

(b) Explain the operation of super heterodyne receiver with neat block diagram. Draw

signal at

the output of each block.

(13)

12. (a) (i) Explain the generation of frequency modulation signal using the direct

method. (8) (ii) List the advantage of frequency modulation over amplitude

modulation. (5)

(Or)

(b) Explain the FM demodulation process using Frequency discrimination process. (13)

13. (a) (i) Discuss the properties of Gaussian noise process. (6)

(ii) Derive the input and output relationship of a random process applied through

LTI filter. (7)

(Or) (b) (i) Consider a random

process defined as X(t) = A Cosωt where ω is a constant and A is random

uniformly distributed over [0, 1], Find the auto correlation and auto covariance of

X(t).

(7)

(ii) Distinguish between random variable and random process. Give example to each. (6)

14. (a) Obtain the expression for the figure of merit of the AM receiver. (13)

(Or)

(b) (i) Explain the operation of pre-emphasis and de-emphasis in the FM

communication system.(9)

(ii) An amplifier has three stages with gain 5dB, 20dB, 12dB. The noise figure of the

stages is 7dB, 13dB, and 12dB respectively. Determine the overall noise figure and the

noise equivalent

temperature. (4)

15. (a) A source emits one of the four symbols A, B, C and D with probability 1/3, 1/6, 1/4

and 1/4 respectively. The emission of symbols by the source is statistically independent.

Determine the average code length and code efficiency if the Shannon fano coding is used.

(13) (Or)

(b) (i) Discuss about discrete memoryless channels.

(6)

(ii) Explain the property of entropy. (7)

PART C (1*15=15)


Page | 24

16. (a) Which modulation will be suitable for transmitting your audio file? Assume your audio

frequency and obtain its spectrum response? Is there any transformation needed for

transmission.

Summarize the modulation analysis and explain why and how this modulation suits.

(Or)

(b) Compile your favorite song modulate it and favorite it. During the transmission what

are the noises may occur and how can you reduce noise at the receiver end. Obtain the

PSD of your signal.

B.E – DEGREE EXAMINATION, APRIL / MAY 2017

Fourth Semester / Second Year

Electronics and Communication Engineering

EC 6402 – COMMUNICATION THEORY (Regulation 2013)

PART A (10*2=20)

1. Do the modulation techniques decide the antenna height?

2. Define carrier swing.

3. State carson’s rule.

4. Distinguish the features of Amplitude modulation and Narrow band FM.

5. List the necessary and sufficient conditions for the process to be Wide sense stationary.

6. State Wiener Khintchine theorem.

7. Specify the cause of threshold effect in AM systems.

8. Comment the role of pre-emphasis and de-emphasis circuit in SNR improvement.

9. State the property of entropy.

10. What is shannon’s limit.

PART B (5*13=65)

11. (a) (i) Derive an expression for output voltage of a Balanced modulator to generate DSB-

SC and explain its working principle.

(5)

(ii) Discuss the detection process of DSB-SC and SSB-SC using coherent detector.

Analyze

the drawback of the suggested methodology.

(8)

(Or)

(b) (i) Comment the choice of IF selection and image frequency elimination.

(5)

(ii) Elucidate the working principle of super heterodyne receiver with a neat block

diagram. (8)


Page | 25

12. (a) (i) Obtain the mathematical expression for Wideband Frequency Modulation. Also

compare and contrast its characteristics with NBFM.

(6)

(ii) Suggest and discuss the method for the generation of FM using direct method.

(7) (Or)

(b) (i) Analyze and brief how the ratio detector suppressed amplitude variation caused by

the communication media without using amplitude limiter circuit.

(7) (ii) Explain the detection of FM wave using PLL detection.

(6)

13. (a) Consider two filters connected in cascade as shown in fig 1. Let X(t) be a stationary

process with a auto correlation function Rs(Ʈ), the random process appearing at the first

input filter is V(t) and the second filter output is Y(t).

(i) Find the auto correlation function of Y(t)

(ii) Find the cross correlation function Rxy(Ʈ) of V(t) and Y(t)

(13)

(Or)

(b) The Amplitude modulated signal is defined as XAM(t) = A m(t) Cos (ωct +θ) where

m(t) is the base band signal and A Cos (ωct +θ) is the carrier. The baseband signal m(t)

is modeled as a zero mean stationary random process with the auto correlation function

Rxx(Ʈ) and the PSD Gx(f).

The carrier amplitude A and the frequency ωc are assumed to be constant and the initial

carrier phase θ is assumed to be a random uniformly distribution in the interval (-П,

П). Furthermore, m(t) and θ are assumed to be independent. (i) Show that XAM(t) is

WSS (ii) Find PSD of XAM(t).

(13)

14. (a) (i) Classify the different noise source and its effect in real time scenario. (7)

(ii) Discuss the effect of noise in cascade system. (6)

(Or)

(b) Derive the expression for signal to noise ratio for an AM signal, with assumption that

the noise added in the channel in AWGN. Compare its performance with FM system.

(13)


Page | 26

15. (a) (i) Consider a binary memoryless source X with two symbols x1 and x2. Prove that

H(X) is maximum when both x1 and x2 equiprobable. (6)

(ii) Given a telegraph source having two symbols dot and dash. The dot duration

0.2 sec. The dash duration is 3 times the dot duration. The probability of the dots

occurring is twice that of the dash and the time between symbol is 0.2 sec. Calculate

the information rate of the telegraph source. (7)

(Or)

(b) (i) Find the channel capacity of the binary r=erasure channel as shown in fig.

(7)

(ii) A source is emitting equiprobable symbol. Construct a Huffman code for the

source. (6) PART C (1*15=15)

16. (a) The Amplitude modulated signal S(t) = Ac[1+Ka m(t)] Cos (2Пfct) is applied to the

system shown in fig. Assuming that modulus of Ka m(t) is less than 1 for all t and the

message signal m(t) is limited to the interval -W≤f≤W and that the carrier frequency fc

greater than 2W. Show that m(t) can be obtained from the square rooter output Vs(t).

Consider a square law detector; using a non linear device whose transfer

characteristics is defined by V2(t) = a1V1(t) + a2V12(t) where a1 and a2 constant, V1(t) is

the input and V2(t) is output. The input consists of the AM wave V1(t) = Ac[1+Ka

m(t)] Cos (2Пfct)

(i) Evaluate the output V2(t)

(ii) Find the condition for which message signal m(t) may be recovered from Vs(t).

(Or)

(b) The discrete hilbert transform is a process by which a signal negative frequencies

are phase advanced by 90 degrees and the positive frequencies are phase delayed by

90 degrees shifting the results of the Hilbert transform (+j) and adding it to the original

signal creates a complex signal as mentioned in equation. If mi(n) is the Hilbert

transform of mr(n) then mc(n)=mr(n)+jmi(n). Apply the concept of Hilbert transform

to generates and detect SSB-SC signal.

vel tech high tech dr.ranagarajan dr.sakunthala ...modulation is defined as the process by which...

Documents