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Code No: 35095

Set No - 1

1. (a) What is Sequential Decoding?

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(b) What is the advantage of Sequential Decoding?

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III B.Tech I Semester Supplimentary Examinations,Nov/Dec 2009 DIGITAL COMMUNICATIONS Common to Electronics And Telematics, Electronics And Communication Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????

(c) Explain about Threshold in sequential decoding with appropriate figure. [16]

2. Explain about block codes in which each block of k message bits encoded into block of n>k bits with an example. [16]

or

3. (a) Explain with neat block diagram the generation and receiving of BFSK signals.

(b) Evaluate the magnitude spectrum for an FSK with alternating 1 and 0 data. Assume that the mark frequency is 50kHz, the space frequency is 55kHz and the bit rate is 2400bits/sec. Find the first null-to-null bandwidth. [10+6]

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4. (a) Explain about analog communication channel.

(b) What are the various characteristics analog communication channel? Explain. (c) Draw and explain analog portion of the communication channel.

[16]

5. (a) With a neat block diagram, explain the operation of delta modulation system. (b) A DM system is designed to operate at 3 times the Nyquist rate for a signal with 3kHz bandwidth. The quantizing step size is 250mV.

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i. Determine the maximum amplitude of a 1kHz input sinusoid for which the delta modulator does not show slope overload. ii. Determine the post-filtered output signal to quantizing noise ratio for the signal of part (i) above. [8+4+4]

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6. Let X is a discrete random variable with equally probable outcomes x1 =A, and x2 =- A and conditional pdf’s p(y/xi ), i=1, 2 be the Gaussian with mean xi and variance σ 2 . Calculate average mutual information I(X; Y). [16] 7. (a) Derive the expression for the probability of error in terms of the signal parameters, noise power spectral density, and the receiver parameters H(f) and threshold settings. Also mention the assumption made in deriving the expression for Pe . (b) n(t) is a zero gaussian white noise with a power spectral density of η2 . no (Tb ) is related to n(t) by R no (Tb )= T b n(t)s(t)dt 0

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Code No: 35095

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Set No - 1

Where s(t)= 0 for ’t’ outside the interval [0,Tb ] and  R Tb 2 s (t)dt = Es Show that E {no (Tb )} = 0 and E [no (Tb )]2 = ηEs /2. 0

[8+8] 8. (a) Explain µ - law and A - law companding technique?

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(b) Approximate µ-law curve with µ = 100 by two-linear segments, one for 0
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nt

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or

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2

Code No: 35095

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Set No - 2

1. Explain various methods for describing Conventional Codes.

in

III B.Tech I Semester Supplimentary Examinations,Nov/Dec 2009 DIGITAL COMMUNICATIONS Common to Electronics And Telematics, Electronics And Communication Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????

[16]

ld .

2. (a) Mention the drawbacks in duo - binary coding and how these drawbacks can be overcome by using modified duo - binary coding.

(b) Derive an expression for error probability of modified duo - binary PAM system. [8+8]

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3. (a) What are the various characteristics of ideal system? Explain.

(b) Does an Ideal system proposed by Shannon can be implemented in practice? Justify. [16] 4. The specifications of a PCM signal of a digital telephone system is as follows:

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Audio voice frequency : 300Hz to 3400Hz Sampling frequency : 8k samples/sec.

And each sample is represented by 8 bits. Then find (a) Bit rate, the first null bandwidth and Signal power to quantization power ratio (b) Minimum bandwidth required if

[16]

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i. Sinc pulse is used to generate PCM ii. Rectangular pulse shaping is used

5. (a) What are the problems encountered in linear delta modulation? Explain in detail.

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(b) Explain with a neat block diagram the operation of a continuously variable slope delta modulator (CVSD). [8+8]

6. (a) Define Information rate. (b) One of four possible messages Q1 , Q2 , Q3 , Q4 having probabilities 1/8, 3/8, 3/8 and 1/8 respectively is transmitted. Calculate average Information per message. [4+12]

7. Consider using 4-ary FSK (a) Show that if the frequencies are separated by ‘fS ’, then they are orthogonal signals and also find the bandwidth for this condition.

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Code No: 35095

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(b) Determine the ratio of bandwidths of 2-ary FSK and 4-ary when the frequencies are separated by ‘fS ’. (c) Find power spectral density of BFSK.

[8+4+4]

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8. An-error-correcting code is said to be a perfect code if it is possible to form standard array with all error patterns of t or fewer errors and no others as coset leaders Show that a (7, 4) linear code is generated by g(x) =1+x+x3 is perfect code. [16]

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nt

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4

Code No: 35095

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Set No - 3

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III B.Tech I Semester Supplimentary Examinations,Nov/Dec 2009 DIGITAL COMMUNICATIONS Common to Electronics And Telematics, Electronics And Communication Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Draw the block diagram of duo - binary PAM system and explain.

ld .

(b) Mention the drawbacks in duo - binary correlative level coding with neat waveforms. [10+6] 2. (a) Explain with a neat block diagram, the generation and reception of BFSK.

or

(b) The bit stream 001010011010 is to be transmitted using BFSK. Sketch the transmitted waveform. Assume fL = fb and fH = 2fb . [8+8] 3. (a) What is Delta modulation and compare it with PCM?

(b) Discuss different types of noise effects in delta modulation.

[8+8]

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4. What is amount of Information? Mention its four properties by considering event S = sk with probability pk . [16] 5. (a) Explain Shannon’s theorem? Mention its important feature? (b) What is the negative statement associated with Shannon’s theorem? Explain. [16] 6. Explain about the following special classes of cyclic codes. (a) Majority logic decodable codes.

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(b) Shortened cyclic codes.

[16]

Aj

7. Figure 7 describer the encoder which has four states a, b, c, d corresponding to M1, M2 = 00, 01, 10, 11 respectively, draw the State Diagram. [16]

5

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Set No - 3

or

Figure 7

ld .

in

Code No: 35095

8. The threshold value of the input signal power to noise ratio (S/N )i in PCM system is defined as the value of (S/N )i for which the value of (S/N)o is 1dB below its maximum.

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(a) Show that the threshold occurs when Pe ≈ 1/[16(22N )]. (b) Plot Pe Versus N, for N = 2, 4, 6 and 8.

(c) Sketch the threshold values of (S/N )i Versus N for which N = 2, 4, 6 and 8 (Assume that a PSK signaling scheme is used) [6+5+5]

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nt

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6

Code No: 35095

R05

Set No - 4

in

III B.Tech I Semester Supplimentary Examinations,Nov/Dec 2009 DIGITAL COMMUNICATIONS Common to Electronics And Telematics, Electronics And Communication Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????

1. (a) Define the channel capacity in terms of average signal-power and noise power.

[16]

ld .

(b) Mention the two important implications of Shannon-Hartley theorem?

2. (a) Draw the block diagram of adaptive delta modulation and explain in detail with neat waveforms.

or

(b) Find the step size ‘δ’ required to prevent slope overload noise for the case when the input signal is m(t) = A Sinωm t. [8+8] 3. (a) Write down the modulation waveform for transmitting binary information over base band channels, for the following modulation schemes: ASK, PSK, FSK and DPSK. (b) What are the advantages and disadvantages of digital modulation schemes?

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(c) Discuss base band transmission of M-ary data.

[4+6+6]

4. (a) Prove that “all the 2k n-tuples of co-set have same syndrome and syndromes of different co-sets are different. (b) Describe the procedure for the decoding of one-to-one correspondence between co-set leader and a syndrome. [16]

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5. Draw and explain Tree diagram of convolutional encoder shown in figure 5 with rate=1/3, L=3. [16]

Figure 5

7

Code No: 35095

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Set No - 4

6. (a) Draw the block diagram of band pass binary data transmission system and explain each block.

7. (a) What is the function of predictor in DPCM?

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in

(b) A band pass data transmitter used a PSK signaling scheme with s1 (t) = A coswc t; 0 ≤ t ≤ Tb s2 (t) = +A coswc t; 0 ≤ t ≤ Tb Where Tb = 0.2msec; wc = 10π /Tb . The carrier amplitude at the receiver input is 1mV and the power spectral density of the additive white gaussian noise at the input is 10−11 w/Hz. Assume that an ideal correlation receiver is used. Calculate the average bit error rate of the receiver. [8+8]

or

(b) A 2-bit uniform quantizer is used in the feed forward path of a DPCM coder, and a one-top predictor is used in the feedback path. The input signal variance is unity; the auto-correlation of the input for one sampling period delay is 0.95. Assume that a 4σ loading of the quantizer. Obtain the optimum value of the predictor gain a1 . Also determine the prediction gains with and without the quantizer. [6+10] 8. Source emits independent sequences of symbols from source alphabet containing five symbols with probabilities 0.4, 0.2, 0.2, 0.1 and 0.1,

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(a) Compute entropy of a source. (b) Calculate information rate.

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8

[16]