DIGITAL COMMUNICATION UQ 2011

DIGITAL COMMUNICATION-PREVIOUS YEARS UQ

GCE, NAMAKKAL Page 30

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2010 VI SEMESTER

B.E. ELECTRONICS AND COMMUNICATION ENGINEERING EC341 – DIGITAL COMMUNICATION

Time: Three Hours Maximum : 100 Marks Answer All Questions PART – A (10 x 2 = 20 Marks) 1. If x(t) = cos (2pt / T) sin (4 pt / T) is to be sampled, what is the maximum sampling interval? 2. Find the step size S when a signal m(t) = 5 sin(2pft) is quantized using uniform quantizer to achieve an SNR= 1536. 3. What is inter symbol interference? Explain. 4. What is raised cosine pulse? Explain. 5. Obtain the signal constellation of a QPSK signal. 6. Compare the bandwidth efficiency of the M-ary FSK and M-ary PSK modulation schemes. 7. Obtain the parity cheque matrix of a (5,1) repetition code whose generator matrix is G = [11111] 8. Give the Hamming bound of (n,k) linear block code and hence explain perfect code. 9. List the applications of spread spectrum systems. 10. List the properties of PN sequence and explain. PART – B (5 x 16 = 80 Marks) 11. It is required to transmit data at the rate of 2Mbps through a channel whose bandwidth is 2MHz and No = 10-14 W/Hz. i) Suggest a suitable modulation scheme and find out the amplitude of the transmitted carrier to achieve a BER of 10-5 at the receiver and the bandwidth efficiency ? of the system and ii) If the required ? is 0.5, what modulation scheme would you choose for transmission and draw the block diagram of the modulator and demodulator for the same.



12.a) With the help of a neat block diagram, explain the encoder and decoder of differential PCM system. Also obtain an expression for prediction coefficient of first order predictor. (OR) 12.b)i) Explain the encoder and decoder for Delta Modulation. ii) What are slope overload and granular noise? How to overcome them. 13.a) Derive and plot the power spectra of NRZ unipolar and bipolar format signals. (OR) 13.b) Draw the block diagram of duobinary encoder and decoder and explain. What are the draw backs of this scheme? Suggest a method to overcome the same. 14.a) For(7,4) Hamming code with the generator binomial g(D) = 1+D2 + D3 (i) Form the generator matrix in systematic form. (ii) Obtain the code word polynomial for a message polynomial m(D) = 1+D+D3. (iii) Design the encoder for the above code. (OR) 14.b) For a (2,1,3) convolution code with g1 = ( 1 0 1 1) and g2 = ( 1 1 1 1), design the encoder and find the following. (i) Generator matrix (ii) Transfer function matrix Compute the coded output using both the methods assuming the input u = (101101). 15.a) Explain with the help of block diagrams and waveforms, the following techniques of spread spectrum communication. (i) Direct sequence (ii) Frequency hopping (OR) 15.b)i) Derive an expression for Jamming margin for direct sequence spread spectrum system with BPSK modulation. ii) An PN sequence is generated using a feed back register of length m = 4. The chip rate is 107 chips per sec. Find the length, chip duration and the period of the PN sequence.



B.E./B.Tech. DEGREE EXAMINATION, NOVEMBER/DECEMBER 2010 Fifth Semester

Electronics and Communication Engineering EC 2301 — DIGITAL COMMUNICATION

(Regulation 2008)

Time : Three hours Maximum : 100 Marks Answer ALL questions PART A — (10 × 2 = 20 Marks) 1. Which parameter is called figure of merit of a digital communication system and why? 2. What is meant by distortion less transmission? 3. Why is prefiguring done before sampling? 4. Define quantization noise power. 5. Define Hamming distance and calculate its value for two code words 11100 and 11011. 6. Draw the NRZ and RZ code for the digital data 10110001. 7. What is the need for a demodulator in case of base band signaling when the received waveforms are already in pulse like form? 8. How does pulse shaping reduce inter symbol interference? 9. Define QAM and draw its constellation diagram. 10. A binary frequency shift keying system employs two signaling frequencies 1 f and 2 f . The lower frequency 1 f is 1200 Hz and signaling rate is 500 Baud. Calculate 2 f . PART B — (5 × 16 = 80 Marks) 11. (a) Draw a neat block diagram of a typical digital communication system and explain the function of the key signal processing blocks. (Marks 16) Or (b) (i) Distinguish between base band and bandpass signaling. (Marks 6) (ii) Explain Binary symmetric channel and Gaussian channel with their mathematical models. (Marks 10) 12. (a) 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 [ ] m m - f < f < +f (Marks 16) Or (b) Explain in detail the various source coding techniques for speech signal and compare their performance. (16)

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13. (a) For (6,3) systematic linear block code, the code word comprises 1 I , 2 I , 3 I , 1 P , 2 P , 3 P where the three parity check bits 1 P , 2 P and 3 P 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. Or (b) (i) Explain how encoding is done by convolution codes with a suitable example. (Marks 10) (ii) Explain tree diagram, trellis diagram and state transition diagram of convolution codes. (Marks 6) 14. (a) (i) Define a matched filter and compare its functioning with a correlation. (Marks 10) (ii) Explain how a matched filter can maximize SNR for a given transmitted symbol. (Marks 6) Or (b) What does the term equalization refer to? Explain how it is carried out by using transverse filters. (Marks 16) 15. (a) (i) Distinguish coherent and non-coherent detection. (Marks 4) (ii) Explain non-coherent detection methods of binary frequency shift keying scheme.

(Marks 12) Or (b) (i) Explain Binary PSK and QPSK with corresponding equations and constellation diagrams. (Marks 8) (ii) Obtain the probability of bit error for coherently detected BPSK and compare its probability of bit error performance with QPSK scheme. (Marks 8)















































Department of ECE MODEL QP

Subject: Digital Communication Subject code: EC1351 Max. Marks: 100

Branch/Semester: ECE/VI Time : 3 Hrs

Answer ALL questions

PART A-(10X 2=20)

1. Define Pulse modulation scheme with a suitable diagram. 2. What is meant by aliasing effect? 3. Define Quantization error. 4. What is the use for Eye pattern? 5. Differentiate coherent and non coherent receiver. 6. What do you understand by continuous phase frequency shift keying? 7. What are the advantages and disadvantages of differential phase shift keying? 8. What is meant by constraint length of a convolution code? 9. What is meant by direct sequence and frequency hopped spread spectrum? 10. List any two applications of spread spectrum modulation.

PART B (5X16=80)

11. a) Explain the process of quantization, encoding and decoding in pulse code modulation?

In what way differential PCM is better than PCM? (16) Or

b) (i)For a uniform quantizer,discuss the way in which the number of quantization levels(L) influence the bandwidth and the quantization noise. (3) (ii)Discuss the need for Non-uniform quantization of speech signal. (2) (iii)Outline the principles of compander used for speech signal. (4) (iv)Give the bit rate of a delta modulator fed with samples at a rate of 40 K samples /sec. (2) (v) How does adaptive delta modulation help in alleviating the problems associated with delta modulation scheme? (5)

12. a)(i)Derive the expression for matched filter and prove anyone property. (8) (ii)Draw the block diagram of adaptive equalization and explain. (8)

Or b) Draw the block diagram of the duo binary and modified signaling scheme and explain with and without pre coder. (16)



13. a)(i)Draw the block diagram of MSK transmitter and receiver and discuss in detail with required waveform. (12) (ii)What are advantages and disadvantages of MSK? (4)

Or b) (i)In a PSK system, the received waveforms S1(t)=Acoswt and S2(t)= -Acoswt are

coherently detected with a matched filter. The value of A is 20 Mv, and the bit rate is1Mbps.Assume that the noise power spectral density h/2=10-11W/Hz. Find the

probability of error Pe. (6) (ii)Enumerate on carrier and symbol synchronization. (5+5)

14. a) The generator matrix for a (6,3) block code is given below. Find all code vectors for this

code. 100 : 011 G= 010 : 101 001 : 110 (16)

Or

b) Let g(x) is the generator polynomial of a cyclic code C. Find a scheme for encoding the data sequence(d0,d1….dk-1) into (n,k) systematic code C. (16)

15. a)Describe the following: (i)signal space dimensionality and processing gain. (ii)Maximum length and Gold codes. (8+8)

Or

b)(i)Discuss the configuration of a DSSS System with binary PSK in detail. (12) (ii)A slow FH/MFSK system has the following parameters. The number of bits per MFSK symbol=4 The number of MFSK symbols per hop=5 Calculate the processing gain of the system. (4)



Department of ECE MODEL QP

Subject: Digital Communication Subject code: EC1351 Max. Marks: 100 Branch/Semester: ECE/VI Time : 3 Hrs

Answer ALL questions

PART A-(10X 2=20) 1. State Sampling theorem. 2. What is the SNR of PCM system if the number of quantization levels is 28 . 3. State various types of line codes. 4. A message has zero mean value and a peak value of 10 V. It is to be quantized using a step size

of 0.1V with one level coinciding to 0V. Find number of bits required for encoding the quantized signal.

5. What is the error probability of MSK and DPSK? 6. Highlight the major difference between a QPSK signal and a MSK signal. 7. Define linear block code. 8. What is meant by BCH code? 9. Write down the properties of PN sequences. 10. What do you mean by jamming?

PART B (5X16=80)

11. a) Describe the following systems by presenting appropriate diagrams. (i)TDM (ii) Delta modulation (16)

Or b) (i) A PCM system uses a uniform quantizer followed by a 7-bit binary encoder. The bit rate of the system is equal to 50X106 b/s. (a) What is the maximum message bandwidth for which the system operates satisfactorily? (b) Determine the output signal-to-(quantization) noise ratio when a full-load sinusoidal modulating wave of frequency 1MHz is applied to the input. (8) (ii) Derive the expression for the quantization noise and signal to noise ratio in a PCM system using uniform quantizer . (8)

12. a)Discuss on the following: (i)Baseband M-array PAM transmission (ii)Adaptive equalization (8+8)

Or b)What do you understand by the term inter-symbol interference(ISI)?Discuss in detail the Nyquist criterion for minimizing ISI.Explain the difficulties in implementing it in a practical system. (16)



13. a)Explain BPSK signal transmission and coherent BPSK reception with suitable diagrams.Derive an expression for the probability of symbol error for the scheme.(16)

Or b)(i) Design the encoder for the (7,4) cyclic code generated by G(p)=p3+p+1 and verify its operation for any message vector. Find all the code vectors for the code in nonsystematic form. (10) (ii)Explain the viterbi decoding algorithm. (6)

14. a)(i)Enumerate on carrier and symbol synchronization. (10) (ii)Write short notes early-late gate type of symbol synchronizer. (6)

Or

b)Consider a rate -1/2, non systematic convolution code with g(1)={1,0,1} and g(2)={1,1,1} Determine the encoder output corresponding to the data sequence {10101}.If the first and the fourth bits of the encoded sequences are affected during transmission, demonstrate the error correcting capability of the viterbi algorithm. (16)

15. a)Explain the direct sequence spread spectrum modulation with coherent binary phase shift keying. (16)

Or b)(i)Draw the block diagram of FHSS system and explain. (10) (ii)Explain the generation of maximum length sequence. (6)

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DIGITAL COMMUNICATION UQ 2011