unit i sampling & quantization part a sem.pdf12. a 64 kbps binary pcm polar nrz signal is passed...

LATHA MATHAVAN ENGINEERING COLLEGE Alagarkovil, Madurai – 625301

Department of Electronics and Communication Engineering

EC6501 DIGITAL COMMUNICATION Question Bank

FACULTY : M.Anandan, ASP/ECE SEMESTER : V SEM

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UNIT I - SAMPLING & QUANTIZATION

PART A

1. What is aliasing? (EC6501 June 2016)

2. What is Companding? Sketch the input-output characteristics of a compressor

and an expander. (EC6501 June 2016)

3. An analog waveform with maximum frequency content of 3 kHz is to be transmitted over an M-ary

PCM system, where M=16. What is the minimum number of bits/sample that should be used in

Digitizing the analog waveform? (The quantization error is specified not to exceed ± 1 % of

the peak -to-peak analog signal) (EC2301 June 2016)

4. State Sampling theorem? (June 2011, 2015)

5. Define Non uniform quantization? (June 2015) 6. What is the need for non-uniform quantization? (June 2014)

7. What is difference between natural and flat top sampling (Nov 2014)

8. State any two non-uniform quantisation rules. (June2013)

9. What is natural sampling? (June, Nov 2013)

10. Write a law of compression? (Nov 2013) 11. What is quantization error? (June 2011)

12. Define quantization noise power. (Dec 2010)

13. A message has zero mean value and a peak value of 10V. It is to be quantized using a

Step size of 0.1V with one level coinciding with 0V.Find the number of bits required for

Encoding the quantized signal. (June 2007)

14. A signal x (t) = 5 Cos (1000πt) is sampled and quantized using 8 bit PCM system.

Find the signal to quantization noise ratio. (Dec 2007)

15. Plot the magnitude spectrum of the ideally sampled version of the signal

M (t) = 2 Cos (200πt) + 40 Sin (290πt).Assuming the sampling rate is 1 KHz. (June2008)

PART B

1. (i) State the low pass sampling theorem and explain reconstruction of the signal from its samples.(9)

(ii) The signal x(t) = 4 cos 400 Пt + 12 cos 360 Пt is ideally sampled at a frequency of 300 samples

Per second. The sampled signal is passed through a unit gain low pass filter with a

cut off frequency of 220 Hz. List the frequency components present at the output of the

low pass filter ? (7) (EC6501 June 2016)

2. (i) Explain the Pulse Code Modulation system with neat block diagram.(10)

(ii) What is TDM ? Explain the difference between analog TDM and digital TDM.(6)(EC6501 June 2016)

3. Explain Non - Uniform quantization technique. (8) (June 2011, 2014)

4. Explain the principal of quantization and obtain the expression for the signal to quantization noise

for the case of uniform quantizer (16) (Dec 2014)

5. (i) Explain Nyquist sampling theorems and how the message can be reconstructed from

its samples with an example. (Dec 2014)

(ii) Explain the practical limitation in sampling and reconstruction. (Dec 2014)

6. Explain a PCM system. Derive the expression for quantization noise of a PCM system

With uniform quantizer. (16) (June 2013)

7. Explain the process of quantization and obtain an expression for signal to quantization ratio

in the case of a uniform quantizer. (16) (Dec 2013)

8. State the Nyquist sampling theorem. Demonstrate its validity for an analog signal x (t) having a

Fourier transform x(f) which is zero outside the interval [-f m < f < +f m] (16) (Dec 2010)

9. Explain with a neat sketch Time Division Multiplexing (8) (June 2007)

10. Explain the process of quantization, encoding and decoding in PCM. In what way DPCM is

Better than PCM? (16) (June 2007)





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UNIT II - WAVEFORM CODING PART A

1. What are the advantages of Delta Modulator? (EC6501 June 2016)

2. What is a linear predictor? On what basis are the predictor coefficients determined? (EC6501 June 2016)

3. Differentiate the principles of temporal waveform coding and model based coding. (EC2301 June 2016)

4. Mention the merits of DPCM.

5. What is the main difference in DPCM and DM?

6. What is meant by Adaptive Delta Modulation?

7. What is the advantage of delta modulation and Disadvantages of Delta Modulation? (Nov2008)

8. What should be the minimum bandwidth required to transmit a PCM channel? (Dec 2007)

9. What is the advantage of delta modulation over PCM? (Dec 2007)

10. What are the two limitations of delta modulation? (May 2004)

11. How does Granular noise occurs? (May 2004)

12. A 64 kbps binary PCM polar NRZ signal is passed through a communication system with a raised

Cosine filter with roll-off factor 0.25. Find the bandwidth of a filtered PCM signal. (NOV2012)

PART B

1. (i) Draw the block diagram of ADPCM system and explain its function.(10)

(ii) A delta modulator with a fixed step size of 0.75 V, is given a sinusoidal message signal.

If the sampling frequency is 30 times the Nyquest rate, determine the maximum permissible

Amplitude of the message signal if slopes overload is to be avoided. (6) (EC6501 June 2016)

2. ( i) Draw the block diagram of an Adaptive Delta Modulator with continuously variable step size

And explain. (10)

(ii) Compare PCM system with delta Modulation system (6) (EC6501 June 2016)

3. Explain a DPCM system. Derive the expression for slope overload noise of the system. Show that

SNR of DPCM is better than that of PCM. (EC2301 June 2016)

4. (i) Explain sub band coding. (8)

(ii) Compare the performance of various speech encoding methods. (8) (EC2301 June 2016)

5. Describe the linear predictive coding technique in speech encoding (June 2014)

6. (i) Draw the block diagram of Adaptive Delta Modulation system and explain (Dec 2007)

(ii) Explain the two types of quantization noise in Delta Modulation system (Dec 2007)

7. Explain the slope overload and granular noise in delta modulation

8. Explain with neat diagram of Delta Modulation system

9. Compare various Digital Modulation Techniques

10. Compare various speech encoding Techniques

UNIT III - BASEBAND TRANSMISSION

PART A

1. What are line codes? Name some popular line codes. (EC6501 June 2016)

2. What is ISI and what are the causes of ISI? (EC6501 June 2016)

3. What is the use of eye pattern? (EC2301 June 2016)

4. Give four applications of eye pattern? (June 2011, 2015) 5. What are requirement for line code? (June 2014)

6. What is ISI? (June, Nov 2014)

7. What is the function of the equalizing filter? (Nov 2014)

8. What is Manchester coding? What is its advantage? (Nov 2014)

9. Define transparency of a line code. Give two examples of line codes which are not transparent? (June2011, 2013) 10. ‘ISI’ cannot be avoided justify the statement. (June 2013)

11. What is the use of eye pattern? (Nov 2013)





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12. How does pulse shaping reduce inter symbol interference? (Dec 2010)

13. Draw the NRZ and RZ code for the digital data 10110001. (Dec 2010) 14. What is the need for a demodulator in case of base band signalling when the received

Waveforms are already pulse like form? 15. Define the principle of adaptive equalization?

PART-B

1. (i) Draw the power spectra of (a) Polar NRZ and (b) bipolar RZ signals. (8)

(ii) Compare the various line coding techniques and list their merits and demerits. (8) (EC6501 June 2016)

2. (i) Draw the block diagram of Duo binary signalling scheme without and with precoder and explain.(9)

(ii) Explain the adaptive equalization with block diagram. (7) (EC6501 June 2016)

3. Explain the Nyquist first criterion for ISI elimination (June2015, 2016)

4. (i) Describe any one method for ISI control (June 2014)

(ii) List the inference made from the eye pattern (June 2014)

5. Derive and draw the power spectra of a NRZ, (i) Polar coded waveform.(8) (June 2013)

(ii) Bipolar coded waveform. (8) (June 2013)

6. (i) Describe with a diagram the functioning of a correlator type receiver. (Dec 2013)

(ii) Explain the equivalence between correlator and matched filter receiver. (Dec 2013)

7. Draw the power spectra of polar codes and on-off codes. Discuss their characteristics (16) (June2011)

8. What does the term equalization refer to? Explain how it is carried out by using transverse filters. (16) (Dec 2010)

9. Draw the block diagram of adaptive equalization and explain (Dec 2007)

10. Discuss the following: (i) Adaptive equalization

(ii) Base band M- ary PAM transmission (June 2007)

UNIT IV - DIGITAL MODULATION SCHEME

PART A

1. Distinguish between coherent and non coherent reception. (EC6501 June 2016)

2. What is QPSK? Write the expression for the signal set of QPSK. (EC6501 June 2016)

3. What are the draw backs of binary PSK system. (EC2301 June 2016)

4. What are coherent and non-coherent receivers? (June 2015, 2016) 5. What are the advantages of QPSK over PSK? (June 2014)

6. Draw the signal space diagram for QAM signal for M=8? (June 2014)

7. Draw constellation diagram of QAM? (Nov 2014)

8. Define QAM and draw its constellation diagram. (Dec 2010) 9. A binary frequency shift keying system employs two signalling frequencies f1 and f2.

The lower frequency f1 is 1200 Hz and signalling rate is 500 Baud. Calculate f2. (Dec 2010)

10. Plot the power spectrum of a BPSK signal operated with a carrier frequency of 140MHz,

Modulated by data bits at a rate of 2400 bits/sec. What is the bandwidth requirement?

PART B

1. Explain the generation and detection of Coherent binary PSK signal and derive the Power

Spectral density of binary PSK signal and plot it (16) (EC6501 June 2016)

2. Explain the non-coherent detection of FSK signal and derive the expression

for probability of error.(16) (EC6501 June 2016)

3. (i) Derive the functional block diagram of modulator for QAM and explain its operation

(ii) Draw the block diagram of QPSK transmitter and receiver. Explain the function

of various block. (EC2301 June 2016) 4. (i) Draw the functional block diagram of Modulator for QAM and explain its operation.(8)

(ii) Derive the expression for error probability of QAM system. (8) (EC2301 June 2016)





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5. (i) Explain Gram- Schmidt orthogonalisation procedure. (12)

(ii) State and explain the dimensionality theorem. (4) (EC2301 June 2016)

6. Derive the bit error probability of QPSK receiver (16) (June 2013, 2015)

7. (i) Define basis set. How they are useful in representing a signal?(8) (June 2014)

(ii) With an example, Explain how the basis set is determined by GSOP.(8) (June 2014)

8. (i) Describe the following modulation method (i) BPSK (ii) QAM (10) (June 2014)

(ii) Compare the performance of various digital modulation schemes (6) (June 2014)

9. Discuss about the coherent detection of QPSK and non-coherent detection of DPSK (June 2014)

10. (i) Explain geometric representations of signals? (8) (Nov 2014)

(ii) Explain different mathematical modes of a communication channels? (8) (Nov 2014)

UNIT V - ERROR CONTROL CODING

PART A

1. What is linear code? (EC6501 June 2016)

2. What is meant by constraint length of a convolutional encoder? (EC6501 June 2016)

3. Define Hamming distance and Hamming Weight? (EC2301 June 2016, 2015)

4. Define constraint length of a convolutional coder? (EC2301 June 2016, 2015) 5. What is need for error control codes? (Nov 2014)

6. Define code rate of a block code? (Nov2013) 7. State the significance of minimum distance of a block code. (June 2013)

8. State the principle of maximum likelihood detector. (June 2013)

9. Define Hamming distance and calculate its value for two code words 11100 and 11011. (Dec 2010)

10. Define the Channel coding

11. Differentiate Source coding from the Channel coding.

12. What is syndrome?

13. Define Code rate.

14. Define CRC.

15. Find the Hamming distance between 101010 and 010101. If the minimum Hamming distance of a

(n,K) linear block code is 3, what is the minimum Hamming weight ?

PART B

1. Consider a linear block code with generator Matrix (3+3+6+4)

(i) Determine the parity check matrix.

(ii) Determine the error detecting and capability of the code.

(iii) Draw the encoder and syndrome calculation circuits.

(iv) Calculate the syndrome for the received vector r = [ 1 1 0 1 0 1 0 ]. (EC6501 June 2016)

2. (i) The generator polynomial of a (7,4) cyclic code is 1 + X + X3 . Develop encoder and

Syndrome calculator for this code. (8)

(ii) Explain the Viterbi decoding algorithm for convolutional code. (8) (EC6501 June 2016)

3. Explain the steps involved in the generation of linear block codes. Define and explain the

Properties of syndrome. (16) (EC2301 June 2016)

4. (i) Explain how convolutional codes are generated? Illustrate with an example. (8)

(ii) For a convolutional encoder of constraint length 3 and rate ½, obtain the encoded

Output for the input message 10011. (8) (EC2301 June 2016)

5. Explain Viterbi algorithm to decode a convolutional coded message with an example? (June2013, 2015)





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6. (i) Brief about any one decoding procedure of linear block codes (8) (June 2014)

(ii) Find a generator for a (7,4) cyclic code hence find the code word for [1 0 0 0] (8) (June 2014)

7. Describe coherent detection using maximum likelihood detector. (Dec 2013)

8. Assume a (2, 1) convolutional coder with constraint length 6. Draw the tree diagram, State diagram

and Trellis diagram for the assumed coder. (16) (June 2011)

9. Find the (7, 4) linear systematic block code word corresponding to 1101.Assume a suitable

Generator matrix. (16) (June 2011)

10. For (6, 3) systematic linear block code, the code word comprises I1, I2, I3, P1, P2, P3 where the

Three parity check bits P1, P2 and P3 are formed from the information bits as follows: (16)

P1 = I1 < I2 , P2 = I1 < I3,P3 = I2 < I3

Find (i) The parity check matrix (ii) The generator matrix (iii) All possible code words. (iv) Minimum weight

and minimum distance and (v) The error detecting and correcting capability of the code.(vi) If the received

Sequence is 10,000. Calculate the syndrome and decode the received sequence. (Dec 2010)

Unit – 1 Two Marks with Answer

1. What is meant by aliasing effect?

Aliasing effect takes place .when sampling frequency is less than Nyquist rate under such condition,

The spectrum of the sampled signal overlaps with itself. Hence higher frequencies take the form of lower frequencies.

This interference of the frequency components is called as aliasing effect.

2. State Sampling theorem. (June 2011 ,2015) A band limited signal of finite energy, which has no frequency components higher than W Hz, may be completely

recovered from the knowledge of its samples taken at the rate of 2W samples per second.

3. The signal to quantization noise ratio in a PCM system depends on what criteria?

The signal to quantization noise ratio in PCM is given as, (S/N)db ≤(4.8+6v) dB . Here v is the number of bits used to

represent samples in PCM. Hence signal to quantization noise ratio in PCM depends upon the number of bits or

quantization levels

4. What is meant by quantization? (June 2012) While converting the signal value from analog to digital, quantization is performed. The .analog value is assigned to

nearest digital value. This is called quantization. The Quantized value is then converted into equivalent binary value.

The quantization levels are fixed depending upon the number of bits. Quantization is performed in every Analog to

Digital Conversion.

5. What you mean by Non- uniform quantization?

Step size is not uniform. Non uniform quantizer is characterized by a step size that increases as the separation from the

origin of the transfer characteristics is increased. Non- uniform quantization is otherwise called as robust quantization

6. What is the disadvantage of Uniform quantization over the Non-uniform quantization?

SNR decreases with decrease in input power level at the uniform quantizer but non-uniform quantization maintain

a constant SNR for wide range of input power levels. This type of quantization is called as robust quantization.

7. What is Natural sampling? (June, Nov 2013) A band limited signal of finite energy, which has no frequency components higher than W Hz, may be completely

recovered from the knowledge of its samples taken at the rate of 2W samples per second.

8. What is Quantization error? (June 2011) The difference between the message signal x(t) and quantized signal x q(t) is called as Quantization error or

Quantization noise. e = xq (t) - x(t) .Where, x(t) –message signal and x q(t) - quantized signal

9. Compare uniform and Non-uniform quantization (NOV 2011)

S No UNIFORM QUANTIZATION NON QUANTIZATION

1. The quantization step size remains

same throughout the dynamic range

The quantization step size varies with the amplitude

of the input signal of the signal

2. SNR ratio varies with input signal

amplitude

SNR ratio can be maintained constant





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10. What is TDM?

Time Division Multiplexing (TDM) is a type of digital multiplexing. In TDM each signal can occupy the entire

bandwidth of the channel. i.e. the multiple signal take turns transmitting over the single channel. Each signal is

allowed to use the channel for a fixed period of time one after the other.

11. What is frame?

One byte of data may be transmitted during the time interval assigned to a particular channel. On transmission of each

channel completes one cycle of operation called a frame.

Sixteen Marks with Hints

1. Explain sampling theorem. Sampling process converts a continuous time varying signal to a discrete time varying signal.

Sampling theorem states that a band limited signal of finite energy having the highest frequency component Fm

Hz can be represented and removed completely from a set samples taken at a rate of Fs sampling per seconds

provides that Fs ≥ 2Fm

2. Draw the block diagram of compander? Mention the types of companding?

Block diagram: Input Compressor uniform quantizer expander output signal Transmitter receiver

Types of companding: 1. μ law companding 2. A law companding





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3. Explain PCM in detail.





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9





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11





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1. Mention the merits of DPCM.

1. Bandwidth requirement of DPCM is less compared to PCM.

2. Quantization error is reduced because of prediction filter

3. Numbers of bits used to represent one sample values are also reduced compared to PCM.

2. What is the main difference in DPCM and DM? DM encodes the input sample by one bit. It sends the information about + δ or -δ, ie step rise or fall.

DPCM can have more than one bit of encoding the sample. It sends the information about difference

Between actual sample value and the predicted sample value.

3. What is meant by Adaptive Delta Modulation?

In adaptive delta modulation, the step size is adjusted as per the slope of the input signal. Step size is made high if

slope of the input signal is high. This avoids slope overload distortion.





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4. What is the advantage of delta modulation over pulse modulation schemes?

Delta modulation encodes one bit per samples. Hence signaling rate is reduced in DM

5. What should be the minimum bandwidth required to transmit a PCM channel?

The minimum transmission bandwidth in PCM is given as, BT = vW, Here v is the number of bits

used to represent one pulse. W is the maximum signal frequency.

6. What is the advantage of delta modulation over PCM?

Delta modulation uses one bit to encode on sample. Hence it rate of delta modulation is low compared to PCM.

7. What are the two limitations of delta modulation?

1 Slope of overload distortion. 2. Granular noise.

8. What are the advantages and disadvantages of Delta Modulation? (Nov2008)

Advantages: 1. One bit is transmitted per sample. So speed will be high.2.Because of single bit per sample

Bandwidth is small 3.Less complicated transmitter and receiver circuit when compared to PCM circuits.

Disadvantages: 1.Two distortions are produced Slope overload distortion and Granular noise.2.Slope overload

Distortion will be much higher than that of PCM.

9. A 64 kbps binary PCM polar NRZ signal is passed through a communication system with a raised

Cosine filter with roll-off factor 0.25. Find the bandwidth of a filtered PCM signal. (NOV2012)

Given, FB = 64 kpbs, B0 = FB/2 = 32 kpbs, α = 0.25

The bandwidth of a filtered PCM signal, B = B0 (1+α) = 32(1+0.25) = 40 kHz

10. What are the two fold effects of quantizing process?

The peak-to-peak range of input sample values subdivided into a finite set of decision levels or

Decision thresholds. The output is assigned a discrete value selected from a finite set of representation

Levels are reconstruction values that are aligned with the treads of the staircase.


1. Explain the slope overload and granular noise in delta modulation

Slope-overhead distortion: If the slope of the signal x(t) is much higher than that of the approximate signal x’(t)

over will not follow x(t) at all as shown in figure.

The difference between x(t) and x’(t) is called the slope-overload distortion or the slope overload error. Thus,

slope-overload error occurs when the slope of x(t) is much higher than x’(t). The slope overload error can be

reduced by increasing the step size δ or by increasing the sampling frequency. However with the bit rate and

bandwidth requirement will increase.

Granular noise: When the input signal x(t) is relatively constant in amplitude, the approximated signal x’(t) will

hunt above and below x(t) . This leads to a noise called granular noise. It increases with in step size δ. To reduce

granular slope-overload distortion.

2. Explain about DPCM

DPCM is a combination between DM and PCM techniques. In DPCM scheme shown in , the input signal to the

quantizer is defined by ( ) = ( ) − ( ) the difference between the input sample m(nTs) and a prediction

of it, denoted by which is produced by using a prediction filter . The difference signal e(nTs) is called the

prediction error, since it is the amount by which the prediction filter fails to predict the incoming message signal

exactly. By encoding the quantizer output, we obtain a variation of PCM, which is known as differential pulse-code

modulation (DPCM).





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3. Differentiate DM and ADM.

S

N Delta Modulation Adaptive Delta Modulation

1. Is a digital modulation in which one

bit per sample is transmitted i.e. step

size is constant.

A system which uses variable step size

is known as a adaptive data

modulation.

2. Reduces the BW requirement to a

great extent.

BW requirement is increased.

3. There are two types distortion in DM.

Slope overload & granular Noise.

In ADM there is reduction in slope

overload and distortion.

4. Wide dynamic range of analog signal

cannot be used

Wide dynamic range of analog signal

can be used due to variable step size.

5.

S/N ratio is high

Improvement in S/N ratio

4. Explain with neat diagram of Delta Modulation system

Delta modulation (DM) is implemented by applying a sampled version of the incoming message signal to a

transmitter that consists of a comparator, quantizer, and accumulator connected together as shown in Figure 2 (a). The

comparator computes the difference between its two inputs.

The quantizer consists of a hard limiter with an input output characteristic that is a scaled version of the signum

function. The accumulator operates on the quantizer output so as to

Produce an approximation to the message signal.

Fig.2 DM system (a) Transmitter (b) Receiver





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5. Compare all the digital modulation techniques.


1. What is linear code?

A code is linear if the sum of any two code vectors produces another code vector.

2. What is ISI? (June, Nov 2014)

The presence of outputs due to other bits interference with the output of required bit this effect is called inter symbol

interference (ISI)

3. Differentiate Noise & fading

Noise is an unwanted random addition to a wanted signal; it is called noise as a generalization of the audible noise

heard when listening to a weak radio transmission Fading - random interference between multiple arriving image

signals

4. Give the importance in calculating the spectral density of a signal.

The spectral density of a signal characterizes the distribution of the signal’s energy or power in the frequency domain.

It is important when considering filtering communication system. It is used for evaluating the signal and noise at the

filter.

5. State NRZ polar format?

Binary 1 is represented by a positive pulse and binary 0 is represented by a Negative pulse.





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6. Draw the NRZ and RZ code for the digital data 10110001

7. State NRZ bipolar format?

Binary 0 is represented by no pulse and binary one is represented by the alternative positive and negative pulse.

8. State Manchester format? (Nov 2014)

Binary 0 -> The first half bit duration negative pulse and the second half bit duration positive pulse.

Binary 1-> First half bit duration positive pulse and the second half bit duration negative pulse. 9. Define the principle of adaptive equalization?

The filters adapt themselves to the dispersive effects of the channel that is the coefficients of the filters are

changed continuously according to .the received .data. The filter coefficients are changed in such a way that

the distortion in the data is reduced. 10.What is meant by prediction error?

The difference between the actual sample of the process at the time of interest and the predictor output is

called a prediction error. 11. What is the use of eye pattern? (Nov 2013)

Eye pattern is used to study the effect of ISI in baseband transmission.1) Width of .eye .opening defines the .interval

over .which the .received wave can be sampled without error from ISI.2) The sensitivity of the system to timing error

is determined by the rate of closure of the eye as the sampling time is varied.

3) Height of the eye opening at sampling time is called margin over noise

12. Give two applications for eye pattern. (June 2011)

1. To determine an interval over which the received wave can be sampled without error due to ISI.

2. To determine the sensitivity of the system to timing error.

3. The margin over the noise is determined from eye pattern.

13. What you mean by correlative level coding

Correlative level coding .allows the signal scaling rate of 2Bo in the channel of bandwidth Bo this is made physically

possible by allowing ISI in the transmitted signal in controlled manner this ISI is known to the receiver

14. Draw the block diagram of base band binary data transmission system

Input

binary

data

15. Distinguish between baseband and band pass transmission.

Data

encoding Transmitti

ng filter

Channel Receivi

ng filter

Design

Device





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1. List and explain the properties of line codes (Dec 2011)

DC Component

Self-synchronization

Error detection

Band width compression

Differential Encoding

Noise Immunity

Spectral Compatability With Channel

2. Explain with neat diagram diagram of M-ary FSK transmitter and receiver.


3. Derive and draw the power spectra of a NRZ, (i) Polar coded waveform.(8) (June 2013)

(ii) Bipolar coded waveform. (8) (June 2013)

Line coding: Line coding refers to the process of representing the bit stream (1’s and 0’s) in the form of voltage or

current variations optimally tuned for the specific properties of the physical channel being used.

Unipolar (Unipolar NRZ and Unipolar RZ):

Unipolar is the simplest line coding scheme possible. It has the advantage of being compatible with TTL logic.

Unipolar coding uses a positive rectangular pulse p(t) to represent binary 1, and the absence of a pulse (i.e., zero

voltage) to represent a binary 0. Two possibilities for the pulse p(t) exist

Non-Return-to-Zero (NRZ) rectangular pulse and Return-to-Zero (RZ) rectangular pulse. The difference between

Unipolar NRZ and Unipolar RZ codes is that the rectangular pulse in NRZ stays at a positive value (e.g., +5V) for the

full duration of the logic 1 bit, while the pule in RZ drops from +5V to 0V in the middle of the bit time. A drawback

of unipolar (RZ and NRZ) is that its average value is not zero, which means it creates a significant DC-component at

the receiver (see the impulse at zero frequency in the corresponding power spectral density (PSD) of this line code.

Polar (Polar NRZ and Polar RZ):





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In Polar NRZ line coding binary 1‟s are represented by a pulse p(t) and binary 0‟s are represented by the

negative of this pulse -p(t) (e.g., -5V). Polar (NRZ and RZ) signals .Using the assumption that in a regular

bitstream a logic 0 is just as likely as a logic 1,polar signals (whether RZ or NRZ) have the advantage that

the resulting Dccomponent is very close to zero when a long sequence of 0’s or 1’s is transmitted

Manchester encoding:

In Manchester code each bit of data is signified by at least one transition. Manchester encoding is therefore

considered to be self-clocking, which means that accurate clock recovery from a data stream is possible. In

addition, the DC component of the encoded signal is zero. Although transitions allow the signal to be self-

clocking, it carries significant overhead as there is a need for essentially twice the bandwidth of a simple

NRZ or NRZI encoding.

POWER SPECTRA OF LINE CODES:

Unipolar most of signal power is centered around origin and there is waste of power due to





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DC component that is present. Polar format most of signal power is centered around origin and they are

simple to implement. Bipolar format does not have DC component and does not demand more bandwidth,

but power requirement is double than other formats. Manchester format does not have DC component but

provides proper clocking. Unit – 4 Two Marks with Answer

1. Why do we go for Gram-Schmidt Orthogonalization procedure?

Consider a message signal m. The task of transforming an incoming message mi i=1,2,…..M, into a modulated wave

Si(t) may be divided into separate discrete time & continuous time operations. The justification for this separation lies

in the Gram-Schmidt orthogonalization procedure which permits the representation of any set of M energy signals,

{Si(t)}, as linear combinations of N orthonormal basis functions, where N<M .

2. What are Coherent and Non Coherent Receivers? (June 2013)

Receivers in which the carrier used in the receiver is of same frequency and phase of transmitted one is called

Coherent Receivers. Receivers in which the carrier used in the receiver is of different frequency and phase of

transmitted one is called Coherent Receivers.

3. Define: BER

BER: Bit Error Rate. It is a measure of the performance of a communication system. The number of bits received in

error divided by the total number of bits.

4. Distinguish between Coherent and Non coherent detection.

In coherent detection the local carrier generated at the receiver is phase locked with the carrier at the

transmitter. Hence it is also called synchronous detection. In non-coherent detection the local carrier generated at the

receiver not be phase locked with the carrier at the transmitter. It is simple, but it has higher probability of error.

5. Explain how QPSK differs from PSK in term of transmission bandwidth and bit information it carries?

For a given bit rate 1/Tb, a QPSK wave requires half the transmission bandwidth of the corresponding

binary PSK wave. Equivalently for a given transmission bandwidth, a QPSK wave carries twice as many bits

of information as the corresponding binary PSK wave

6. Define QPSK.

QPSK is Quadrature phase shift keying. In QPSK the phase of the carrier takes on one of the four equally

spaced values Such as π/4 , 3π/4, 5π/4 and 7π/4.

7. Draw the signal space diagram for QAM signal for M=8? (June 2014)

8. Draw constellation diagram of QAM? (Nov 2014)





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9. What is QAM? (June 2013)

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog

message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the

amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme.

10. What are coherent systems? (Nov2013)

In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The

detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a

synchronous detection.

11. What is a non-coherent detection system? (June 2011)

Non coherent detection:

In this method, the receiver carrier need not be phase locked with transmitter carrier. Hence it is called envelope

detection.

12. What is meant by DPSK?

In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence be b(t).

Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes

level when d(t)=0. When b(t) changes level, phase of the carrier is changed. And as stated above, b(t)

changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence

the technique is called Differential PSK 13. A binary frequency shift keying system employs two signalling frequencies f1 and f2. The lower frequency

f1 is 1200 Hz and signalling rate is 500 Baud. Calculate f2. (Dec 2010)

Given , For binary FSK baud=Fb ,Fb=500Hz , Fs=f1=1200HZ

Consider the FN modulation index(h) of 1 in FSK

Wkt , Fm-Fs/ Fb = h=1 , Fm-Fs=Fb , Fm-1200Hz=500Hz

Fm=1700Hz, f2=fm=1700Hz


1. Explain geometric representations of signals? (8) (Nov 2014)

Analog signal: If the magnitudes of a real signal s(t) over its range of definition, T1≤ t ≤ T2, are real numbers (there

are infinite such values) within a finite range, say, Smin ≤ S(t) ≤ Smax, the signal is analog. A digital signal s(t), on

the contrary, can assume only any of a finite number of values.

Orthogonally and Signal spaces

To show that signals sharing the same channel don’t interfere with each other, we need to show that they are

orthogonal. This means, in short, that a receiver can uniquely separate them. Signals in different frequency bands are

orthogonal. We will study orthogonal signals, and learn an algorithm to take an arbitrary set of signals and output a set

of orthogonal signals with which to represent them.

Orthonormal

Two signals x(t) and y(t) are orthonormal if they are orthogonal and they both have norm 1,

i.e., ║x(t)║ = 1 ║y(t)║ = 1

Orthonormal Signal Representations

We consider how to take a set of arbitrary signals and represent them as vectors in an orthonormal basis. We refer to

the set of arbitrary signals as: S = {s0(t), s1(t), . . . , sM−1(t)}

A transmitter may be allowed to send any one of these M signals in order to convey information

to a receiver. We’re going to introduce another set called an orthonormal basis: B = {φ0(t), φ1(t), . . . , φN−1(t)}

Each function in B is called a basis function. Let B as an unambiguous & useful to represent the signals from the set S.

2. Describe the following modulation method (i) BPSK (ii) QAM (10) (June 2014)

(i) BPSK: In BPSK the distance d between signals is 2√PsTb = 2√Eb Where Eb is PsTb is the energy

Contained in a bit duration. The distance d is inversely proportional to the probability that we make an error, in the

presentation noise. At end of receiver b(t) is being received.





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(ii) QAM QAM is a form of digital modulation where the digital information is contained in both the amplitude and phase

of the transmitted carrier signal. Noise immunity of the system can be improved by allowing the signal to differ

not only in phase but also in amplitude is called as QAM. The bit stream b(t) is applied to the series to parallel

converter, operating on a clock which has a period of Ts , which is the symbol duration. The bits b (t) are stored

by the converter and then presented in the parallel form. The four bit symbols are bk+3, bk+2, bk+1, bk.

Out of these four bits, the first two bits are applied to a D/A converter and the other two bits are applied to the

second D/A convertor. The output of the first converter is Ae(t), which is modulated by the carrier √2Pscosωct

whereas the output of the second D/A converter, A0(t) is modulated by the carrier √2Pssinωct in the balanced

modulators .The balanced modulator output are added together to get the QASK output signal

Which is expressed as,

Diagram of QAM Modulator

3. Draw and explain the DPSK receiver.

Here the received signal and the received signal delayed by the bit time Tb are applied to a balanced modulator.

The balanced modulator output is given by, b (t) b (t-Tb) (2Ps) cos ωc (t+θ) cos [ωc(t-Tb) + θ]

= b(t)b(t-Tb)Ps{ cosωcTb + cos [2ωc(t-Tb/2) + 2 θ] }

The output of the balanced modulator is applied to the integrator which suppressed the double frequency term.

The first term [b(t)b(t-Tb)Ps cosωcTb ] is the required signal and ωcTb is selected in such a way so that ωcTb = 2nπ

(where n is integer) so that cosωcTb = +1 and the signal output will be as large as possible.

Further with this selection, the bit duration encompasses an integer number of clock cycle of the double

frequency term is exactly zero. The transmitted data bit d(t) can be determined from the product b(t)b(t-Tb).

If there is no phase change between b(t) and b(t-Tb) then d(t) = 0.

In this case, b(t)b(t-Tb) = -1. Then d(t) = 1.





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4. Explain in detail about BPSK and obtain an expression for its probability of error.

i. Block Diagram of Transmitter and Receiver.

ii. Explanation of Transmitter and receiver.

iii. Signal Space Diagram

iv. Calculation of Probability of Error

5. Explain in detail about QPSK and obtain an expression for its probability of error.

i. Block Diagram of Transmitter and Receiver.

ii. Explanation of Transmitter and receiver.

iii. Signal Space Diagram

iv. Calculation of Probability of Error


1. State Shannon’s Channel Coding Theorem.

Shannon's channel coding theorem states that "the error rate of data transmitted over a bandwidth limited

Noisy channel can be reduced to an arbitrary small amount if the information rate is lower than the

Channel capacity."

2. What are Golay codes?

Golay code is the (23,12) cyclic code whose generating polynomial is, G(p) = p11+p9+p7+p6+p5+p+1

This code has minimum distance of dmin = 7. This code can correct upto 3 errors. But

Golay code cannot be generalized to other combinations of n and k.

3. What is meant by systematic and non-systematic codes?

In a Systematic block code, message bits appear first and then check bits. In the non-systematic code, message

and check bits cannot be identified in the code vector.

4. What is meant by linear code?

A code is linear if modulo-2 sum of any two code vectors produces another code vector. This means any

Code vector can be expressed as linear combination of other code vectors.

5. What are the error detection and correction capabilities of hamming codes?

The minimum distance (dmin) of hamming codes is „3‟. Hence it can be used to detect double errors or

Correct single errors. Hamming codes are basically linear block codes with dmin =3.

6. What is code rate?

Code rate is the ratio of message bits (k) and the encoder output bits (n).It is defined by r. (i.e.) r = k/n

7. What is meant by cyclic codes?

Cyclic codes are the subclasses of linear block codes. They have the property that a cyclic shift of one code word

Produces another code word.

8. What is hamming distance?

The hamming distance between two code vectors is equal to the number of elements in which they differ.

For example, let the two code words be, X = (101) and Y= (110). These two code words differ in second and

third bits. Therefore, the hamming distance between X and Y is two.

9. How syndrome is calculated in Hamming codes and cyclic codes?

In hamming codes the syndrome is calculated as, S=YHT

. Here Y is the received and HT is the transpose of

parity check matrix.

10. What is convolution code? What are its advantages?

Fixed number of input bits is stored in the shift register & they are combined with the help of mod 2 adders.

This operation is equivalent to binary convolution coding. Advantages: 1 .The decoding delay is small in

Convolutional codes since they operate o smaller blocks of data.2. .The storage hardware required by

Convolutional decoder is less since the block sizes are smaller.





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1. Explain in detail about Shannon Coding Theorem.

i. Statement of Theorem. ii. Discussion of theory. iii. Derivation of the Theorem.

2. Explain the construction of Block Code and explain how error syndrome is calculated

i. Representation of Block Code. ii. Generator Matrix.

iii. Generation of Code words. iv. Generation of Parity Check Matrix.

v. Calculation OF Error Syndrome. 3. Explain in detail about Golay Codes, Reed Solomon Codes and BCH Codes.

i. Definition of Golay Codes, Reed Solomon Codes and BCH Codes. ii. Explanation of Golay Codes

iii Explanation of Reed Solomon Codes iv Explanation of BCH Codes.

4. Draw the code tree of a Convolutional code of code rate r=1/2 and Constraint length of K=3 starting

from the state table and state diagram for an encoder which is commonly used.

a. Draw the state Diagram. b. Draw the state Table. c. Draw the code Tree

5. Decode the given sequence 11 01 01 10 01 of a convolutional code with a code rate of r=1/2 and

Constraint length K=3, using viterbi decoding algorithm.

a. Draw the state Diagram. b. Draw the state Table.

c. Draw the code Tree d. Decode the given sequence using trellis diagram

unit i sampling & quantization part a sem.pdf12. a 64 kbps binary pcm polar nrz signal is passed...

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