R5310406-digital Communications

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Code No: R5310406

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 DIGITAL COMMUNICATIONS (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. The specifications of a PCM signal of a digital telephone system is as follows: 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 i. Sinc pulse is used to generate PCM ii. Rectangular pulse shaping is used 2. (a) What is Delta modulation and compare with PCM? (b) Discuss different types of noise effects in delta modulation. 3. Explain the design and analysis of M-ary signaling schemes. List the waveforms in quaternary schemes. 4. (a) What is meant by ISI? Explain how it differs from cross talk in the PAM. (b) What is the ideal solution to obtain zero ISI and what is the disadvantage of this solution.

[16] [8+8] [16] [6+10]

5. Figure 1 illustrates a binary erasure channel with the transmission probabilities probabilities P(0|0) = P(1|1) = 1 − p and P(e|0) = P(e|1) = p. The probabilities for the input symbols are P(X=0) =α and P(X=1) =1- α.

Figure 1: Determine the average mutual information I(X; Y) in bits.

[16]

6. An Ideal channel lowpass channel of bandwidth B Hz with additive Gaussian white noise is used for transmitting of digital information. (a) Plot C/B versus S/N in dB for an Ideal System using this channel. (b) A practical signaling scheme on this channel uses one of two waveforms of duration Tb sec to transmit binary information. The signaling scheme transmits data at the rate of 2B bits/sec, the probability of error is given by P(error|1sent) =Pe . Plot graphs of i. C/B ii. Dt / B. where Dt is rate of information transmission over channel. [16] 7. Explain about block codes in which each block of k message bits encoded into block of n>k bits with an example. [16] 8. Explain various methods for describing Conventional Codes. ?????

[16]

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Code No: R5310406

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 DIGITAL COMMUNICATIONS (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Draw the block diagram of a PCM system and explain each block in detail. (b) Give the advantages of PCM over other digital techniques.

[12+4]

2. (a) Consider a DM system whose receiver does not include a low pass filter (LPF) i. Show that under the assumption of no slope overload distortion, the maximum output signal to quantization noise ratio increased by 6dB when the sampling rate is doubled. ii. What is the improvement that results from the use of a LPF at the receiver output? (b) A given DM system operates with a sampling rate fS and fixed size ∆. If the input to the system is m(t) = αt; for t > 0. Determine the value of ‘α’ for which slope overload occurs. [12+4] 3. (a) Draw the block diagram and explain the base band decoder used to recover the signal in a BPSK system i.e. DEPSK decoder. (b) The bit stream d(t) is to be transmitted using DEPSK. If d(t) is 001010011010, Determine b(t). Show that the data d(t) is recovered using the decoder of DEPSK. [8+8] 4. (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]

5. (a) What is discrete message? Explain. (b) Consider communication system in which allowable messages are m1,m2,......... with probabilities occurrence p1 ,p2 ,p3 ......... where p1 +p2 +p3 .........=1, and the transmitter select message is mk with probability pk . Further assumption is that receiver correctly identify messages. Find (a) Amount of information Ik . (b) Composite message pk pl with corresponding information content of messages mk and ml . [16] 6. Prove that the rate of information transfer over the channel capacity is given by Dt = [H(X) − H(X|Y )] rs bits/sec. [16] 7. Explain about block codes in which each block of k message bits encoded into block of n>k bits with an example. [16] 8. (a) What is a convolution code? How it is generated. (b) Explain in detail the convolution coder with a suitable diagram. ?????

[16]

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Code No: R5310406

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 DIGITAL COMMUNICATIONS (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. Two signals m1 (t) and m2 (t), each band limited to 4kHz are sampled at the Nyquist rate, PCM encoded and then time division multiplexed. The output SNR, including thermal noise effects of each multiplexed signal is to be at least 30dB. (a) Sketch the entire transmitter and receiver structure and Find the value of ‘N’. (b) Minimum Si /N that is required if operation is to be above threshold.

[10+6]

2. (a) What is Delta modulation and compare with PCM? (b) Discuss different types of noise effects in delta modulation.

[8+8]

3. Explain the design and analysis of M-ary signaling schemes. List the waveforms in quaternary schemes. [16] 4. (a) What is meant by ISI? Explain how it differs from cross talk in the PAM. (b) What is the ideal solution to obtain zero ISI and what is the disadvantage of this solution. [6+10] 5. (a) What is discrete message? Explain. (b) Consider communication system in which allowable messages are m1,m2,......... with probabilities occurrence p1 ,p2 ,p3 ......... where p1 +p2 +p3 .........=1, and the transmitter select message is mk with probability pk . Further assumption is that receiver correctly identify messages. Find (a) Amount of information Ik . (b) Composite message pk pl with corresponding information content of messages mk and ml . [16] 6. (a) Define the channel capacity in terms of average signal-power and noise power. (b) Mention the two important implications of Shannon-Hartley theorem?

[16]

7. State and prove the important theorem of cyclic code to generate code polynomial V(x) = r(x)+ xn−k D(x). [16] 8. Explain various methods for describing Conventional Codes. ?????

[16]

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Code No: R5310406

III B.Tech I Semester(R05) Supplementary Examinations, May 2009 DIGITAL COMMUNICATIONS (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain µ - law and A - law companding technique? (b) Approximate µ-law curve with µ = 100 by two-linear segments, one for 0<x≤ 0.1 and the other for 0.1 < x ≤ 1. Calculate S/Nq for S = -40, -10. Where x(t) is input signal. [8+8] 2. (a) Write short notes on i. Granular Noise ii. Slope overload distortion. (b) Derive an expression for quantization noise in Delta modulation.

[8+8]

3. (a) Explain with a neat block diagram, the generation and reception of BFSK. (b) The bit stream 001010011010 is to be transmitted using BFSK. Sketch the transmitted waveform. Assume fL = fb and fH = 2fb . [8+8] 4. (a) What is meant by ISI? Explain how it differs from cross talk in the PAM. (b) What is the ideal solution to obtain zero ISI and what is the disadvantage of this solution. [6+10] 5. Figure 1 illustrates a binary erasure channel with the transmission probabilities probabilities P(0|0) = P(1|1) = 1 − p and P(e|0) = P(e|1) = p. The probabilities for the input symbols are P(X=0) =α and P(X=1) =1- α.

Figure 1: Determine the average mutual information I(X; Y) in bits.

[16]

6. (a) Calculate the bandwidth limits of Shannon-Hartley theorem. (b) What is an Ideal system? What kind of method is proposed by Shannon for an Ideal system?[16] 7. Consider ¯ ¯ 1 ¯ G=¯ 0 ¯ 0 Find

a (6, 3) generator ¯ matrix. 0 0| 0 1 1 ¯¯ 1 0| 1 0 1 ¯ 0 1| 1 1 0 ¯

(a) all the code vectors of this data. (b) the parity check matrix for this code. (c) the minimum weight of this code.

[16]

8. Explain various methods for describing Conventional Codes. ?????

[16]