Rr310405 Digital Communications

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Set No. 1

Code No: RR310405

III B.Tec B.Tech h I Seme Semeste sterr Supple Suppleme ment ntary ary Exami Examinat nation ions, s, Febr Februar uary y 2007 2007 DIGITAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆ ⋆⋆ ⋆⋆

1. (a) Explain the different types of Sampling.

[10]

(b) Obtain the Nyquist Nyquist rate and Nyquist Nyquist interv interval for the signal 10 Cos (2000 πt) Cos ( 4000πt) based on i. low pass sampling theory, and ii. band pass sampling theory.

[6]

2. Four signals Cos ω0 t, 0.2 Cos ω0t, 2 Cos 2 ω0 t and Cos 4 ω0t are to be multiplexed in a TDM system. Find the minimum sampling rate f , minimum interval and the associated commutator speed. If the commutator rotates at f /4, and f /8 revolutions per sec., determine the number of o/p samples of each signal per rotation. tation. Illustrate Illustrate and explain this process with neat schemat schematics ics for a commutator commutator switch rotating at a speed of f /4 revolutions per sec., showing the transmitting and rec recei eivin vingg sides. sides. Discus Discusss the nec necess essit ity y of sync synchro hroniz nizati ation on for this this case. case. [16] [16] s

s

s

s

3. (a) Why the the overal overalll transfer transfer function function of the duo-bina duo-binary ry filter filter is called as Half-cy Half-cycle cle cosine function. [8] (b) Give Give the Impulse Impulse response of the duo-binary duo-binary filter and sketc sketch h the amplitude amplitude and phase response. [8] 4. (a) Explai Explain n how how eye eye patte patterns rns are used for monitori monitoring ng the per p erfor forman mance ce of base base band PAM system. [8] (b) What What is the the nec necessit ssity y of shapi haping ng tran transsmitt mitteed sign signal al spe spectrum rum.

[8] [8]

5. (a) Prove Prove that the the mean value value of the quant quantizati ization on error is inve inversel rsely y proportional to the square of the number of quantization levels. (b) What is the function of a predictor in a DPCM system.

[10+6 10+6]]

6. (a) Explain Explain the workin workingg of delta modulation modulation system system with with neat block block diagram. diagram. (b) Clearly bring out the difference between granular noise and slope over load error. [8+8] 7. (a) Derive Derive an expressi expression on for probabilit probability y of bit-error bit-error for a binary binary non-cohe non-coheren rentt FSK demodulator. (b) Binary data is transmitted over a telephone line with usable bandwidth of 2400 Hz using the FSK signaling scheme. scheme. The transmit transmit frequencie frequenciess are 2025 & 2225 Hz, Hz, and the data rate is 300 bits/S bits/Sec ec.. The avera average ge signal signal to noise noise Seminar Topics - Scholarships - Admission/Entrance Exam Notifications


Set No. 1

Code No: RR310405

power ratio at the output of the channel is 6dB. Calculate Pe for the coherent and non coherent demo dulation schemes. [8+8] 8. (a) Explain enco ding operation in linear blo ck co des.

[8]

(b) Find the output code word word for the following following convoluti convolutional onal encoder for a message sequence 10011. {As shown in the Figure1 } [8]

Figure 1:

⋆ ⋆⋆ ⋆⋆

Seminar Topics - Scholarships - Admission/Entrance Exam Notifications


Set No. 2

Code No: RR310405

III B.Tec B.Tech h I Seme Semeste sterr Supple Suppleme ment ntary ary Exami Examinat nation ions, s, Febr Februar uary y 2007 2007 DIGITAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆ ⋆⋆ ⋆⋆

1. (a) Distinguish between between natural sampling sampling and flat top sampling with with neat neat schematschematics, listing out their merits and demerits. [8] (b) A signal m(t) = 4 Cos (60 πt) + 2 Cos (160 πt) + Cos (280 πt) is sampled at i. 150 Hz, ii. 75 Hz, iii. 300 Hz. Find the frequency components of the signal that appear at the output of an ideal ideal LPF with with cut off at 290 Hz., Hz., in each each case. case. What What is the Nyquis Nyquistt rate rate of sampling and Nyquist interval for m(t)? [8] 2. (a) Explai Explain n the princi principle pless of TDM. List out their their merits merits,, demer demerits its and applicaapplications. [8] (b) Explain the method of generation and detection of PAM signals with neat schematics. [8] 3. (a) What are the the two two key key functional functional operation operationss involv involved ed in correlat correlativ ivee coding? [8] (b) (b)

i. For the data strea stream m 001011 0010110, 0, fin find d the the du duo-b o-bin inar ary y decod decoded ed wav wave form form and give the electrical representation of the data obtained at stages of deco ding. [4] ii. Verify that Duo-binary decoding results in bandwidth reduction during transmission by assuming the message sequence to be transmitted is an alternative 1/0 sequence. [4]

4. (a) (a) What What is mean meantt by Sync Synchr hron oniz izat atio ion n in the the base base band band PAM PAM.. What What are are the the different metho ds to obtain Synchronization? [8] (b) The T1 carrier system used in digital telephony multiplexes multiplexes 24 voice channels based on 8-bit PCM. Each voice signal is usually put through a lowpass filter with cutoff frequenc frequency y of about 3.4 KHz. The filtered filtered voice signal is sampled at 8KHz. In addition addition,, a single single bit is added added at the end of the frame frame for the purpose of synchronization. synchronization. Calculate i. the duration of each bit ii. the resultant transmission rate and iii. the minimum required transmission bandwidth.

[8]

5. (a) Draw the the block block diagram of PCM PCM Generato Generatorr and explain explain each each block. Seminar Topics - Scholarships - Admission/Entrance Exam Notifications


Set No. 2

Code No: RR310405 (b) Determine the Transmission Bandwidth in PCM.

[8+8]

6. In a single-integration single-integration DM system, the voice voice signal is sampled at a rate of 64kHz.The maximum signal amplitude is Amax=1. (a) Determine the minimum minimum value of the step size size

σ

to avoid slope over load error.

(b) Determine Determine the granular granular noise power if the voice voice signals bandwidth bandwidth is 3.5 kHz. (c) Assuming Assuming that the voice signal is sinusoidal, sinusoidal, determine determine output signal power and SNR (d) Det Determine the minimum transmission bandwidth.

[4+4+4 4+4+4]

7. A statistically independent sequence of equiprobable binary digits is transmitted over a channel having finite band width using the rectangular signaling wave form is taken. The bit rate is rb and the channel noise has a psd G (f) given by n

G (f)=G0[1+(f/f1)2)]−1 n

(a) find the transfer function of the optimum receiver receiver and calculate the probability probability of error. [8] (b) If an integrate and dump receiver receiver is used instead of the optimum receiver, receiver, find the probability of error for the optimum receiver. [8] 8. (a) Explain Explain the table table lookup decoding decoding in linear linear block block codes codes using standard standard array array.. [8] (b) The generator polynomial of a (7,4) cyclic code is g (x)= 1+x+x 3 . Find the code vector for the message vector D= (1111) using systematic and nonsystematic form. [8] ⋆ ⋆⋆ ⋆⋆



Set No. 3

Code No: RR310405

III B.Tec B.Tech h I Seme Semeste sterr Supple Suppleme ment ntary ary Exami Examinat nation ions, s, Febr Februar uary y 2007 2007 DIGITAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) With neat neat diagrams, diagrams, explain explain the principle principle of working working a sample sample and hold hold circuit. circuit. List out its applications. [8] (b) Show that

∞

∞





R=−∞

x(RT s )

= f s

X (nf s ) , where x(t) is a signal band

n=−∞

limited to f 1 , X(f) is the spectrum of x(t), and f s = sampling rate.

1 T s

= 2f 1 is the Nyquist [8]

2. (a) Sketc Sketch h and explain the typical typical wav waveform eformss of PWM signals, for leading leading edge, trailing edge and symmetrical cases. (b) Compare the analog pulse modulation schemes schemes with CW modulation systems. systems. (c) Show Show that a PAM signal signal can be expresse expressed d as the convoluti convolution on of an instantainstantaneously sampled signal, and a rectangular pulse p(t) of the form 1, |t| ≤ τ /2 [5+5+6] p (t) = 0 elsewhere



3. (a) Explain Explain how the residual residual effect effect of the channe channell is responsible responsible for Inter Inter Symbol Symbol Interference (ISI). [8] (b) Explain about three tap preset equalizer.

[8]

4. (a) (a) What What is mean meantt by Sync Synchr hron oniz izat atio ion n in the the base base band band PAM PAM.. What What are are the the different metho ds to obtain Synchronization? [8] (b) The T1 carrier system used in digital telephony multiplexes multiplexes 24 voice channels based on 8-bit PCM. Each voice signal is usually put through a lowpass filter with cutoff frequenc frequency y of about 3.4 KHz. The filtered filtered voice signal is sampled at 8KHz. In addition addition,, a single single bit is added added at the end of the frame frame for the purpose of synchronization. synchronization. Calculate i. the duration of each bit ii. the resultant transmission rate and iii. the minimum required transmission bandwidth.

[8]

5. (a) Prove Prove that the the mean value value of the quant quantizati ization on error is inve inversel rsely y proportional to the square of the number of quantization levels. (b) What is the function of a predictor in a DPCM system.

[10+6 10+6]]



Set No. 3

Code No: RR310405

6. A Delta modulator system is designed to operate at five five times the Nyquist Nyquist rate for a signal with 3 kHz bandwidth. bandwidth. Determine Determine the maximum maximum amplitude of a 2 kHz input sinusoid for which the delta Modulator does not have slope overload. Quantization step size is 250mv .Derive the formula used. [16] 7. (a) With With a neat neat block block diagra diagram m expla explain in non noncoh coher eren entt detec detecti tion on of ASK receiv receiver, er, and hence derive the expression for probability of error Pe ≈ 1/2 exp (-A2 /8N0 ) if A2 >>N >>N0

[8]

(b) Binary data has to be transmitted over a telephone link that has a usable bandwidth at 3000Hz, and a maximum achievable signal-to-noise power ratio of 6 dB at its output. i. Determine Determine the maximum maximum signaling rate and Pe if a coherent coherent ASK scheme scheme is used for transmitting binary data through this channel. ii. If the data rate is maintained at 300 bits/Sec, calculate the error probability. [8] 8. (a) Design an encoder encoder for the the (7,4) (7,4) binary binary cyclic cyclic code code generate generated d by g(x) g(x) = 1+x+x 1+x+x 3 and verify its operation using the message vector (0101). (b) What are the differences differences between between block codes and the convo convolution lutional al codes? [8+8] ⋆⋆⋆⋆⋆



Set No. 4

Code No: RR310405

III B.Tec B.Tech h I Seme Semeste sterr Supple Suppleme ment ntary ary Exami Examinat nation ions, s, Febr Februar uary y 2007 2007 DIGITAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ t

2

   , estimate –

1. (a) (a) For the the sign signal al x(t) = Sa

2

i. The Nyquist sampling interval and frequency, frequency, ii. Bandwidth. Describe the effects of sampling at 10 rad./sec., explaining the requirements of ideal LPF and the resulting signal reconstruction. Take Sa( Sa (t) = Sint t . (b) Define the sampling sampling theorem theorem as applicable applicable to voice signals signals on telephone telephone lines. lines. [16] 2. (a) Sketc Sketch h and explain the typical typical wav waveform eformss of PWM signals, for leading leading edge, trailing edge and symmetrical cases. (b) Compare the analog pulse modulation schemes schemes with CW modulation systems. systems. (c) Show Show that a PAM signal signal can be expresse expressed d as the convoluti convolution on of an instantainstantaneously sampled signal, and a rectangular pulse p(t) of the form 1, |t| ≤ τ /2 [5+5+6] p (t) = 0 elsewhere



3. (a) (a) What What is mean meantt by Du Duoo-bi bina nary ry signal signalin ing? g? Giv Give the the sche scheme me of Du Duo-b o-bin inar ary y signaling and explain. [8] (b) A telephone signal with a cutoff frequency of 4 KHz is digitized in to 8 bit samples at the Nyquist sampling rate fs = 2w. Assuming raised cosine filtering with a roll-off factor unity. Calculate i. The transmission bandwidth and ii. The quantization signal to noise ratio. 4. (a) (a) Expl Explai ain n the the nece necess ssit ity y of Equa Equali liza zati tion on in base base band band PAM syst system ems. s.

[8] [8] [8]

(b) Find the duo-binary duo-binary encoded sequence sequence for the data sequence sequence 011010. How it is detected? [8] 5. A message signal m(t) is transmitted by binary PCM without compression .If the SNR is required to be at least 47dB, determine the minimum value of L required, assuming assuming that m(t) is sinusoid sinusoidal. al. Determine Determine the SNR obtained obtained with this minimum minimum L. Derive the formulae used. [16] Seminar Topics - Scholarships - Admission/Entrance Exam Notifications


Set No. 4

Code No: RR310405

6. A Delta modulator system is designed to operate at five five times the Nyquist Nyquist rate for a signal with 3 kHz bandwidth. bandwidth. Determine Determine the maximum maximum amplitude of a 2 kHz input sinusoid for which the delta Modulator does not have slope overload. Quantization step size is 250mv .Derive the formula used. [16] 7. (a) Write down down the modulation modulation wavefor waveforms ms for transmitt transmitting ing binary information information over baseband channels, for the following modulation schemes. i. ii. iii. iv.

ASK FSK PSK and Baseband Baseband pulse shaping followed followed by DSB modulation. modulation.

what are the advantages and disadvantages of digital modulation schemes? (b) Discuss base band transmission of M-ary data.

[6+10]

8. The parity check bits of a (8,4) block code are generated by c5 = d1 + d2 + d4 c6 = d1 + d2 + d3 c7 = d1 + d3 + d4 c8 = d2 + d3 + d4 , where d1 , d2, d3 , and d4 are the message bits. (a) Find the generator generator matrix and the parity parity check check matrix for this code. (b) Find the minimum minimum weight weight of this code (c) Find the error detecting detecting capabilitie capabilitiess of this code (d) Show through an example that this code can detect detect three errors per code word. [16] ⋆⋆⋆⋆⋆


Rr310405 Digital Communications

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