Sixth Semester B.E. Degree Examination, May/June 2010

Digital Communication

Note:Answer any FIVEfull questions, selectingat least TWO questionsfrom each part.

PART-Aa. With a block diagram, explain the basic signal processing operations involved in a digital

communication system. (07 Marks)

b. Explain 'flat-top' sampling, using waveforms and equations. (07 Marks)

c. The signal x(t)= 2cos400nt+ 6cos640nt is ideally sampled at fs = 500 Hz. If the sampled

signal is passed through an ideal low pass filter with cut off frequency of 400 Hz:i) Determine the spectrum of the sampled signal and sketch.ii) What frequency components will appear in the filter output? (06 Marks)

b. Show that the signal to quantization noise power ratio of a uniform quantizer is[SNRlm = 1.8+6n andn = number of bits/sample. (06 Marks)

c. What is the necessity of non uniform quantization? Explain compounding. (06 Marks)

d. A telephone signal band limited to 4 KHz is to be transmitted by PCM. The signal toquantization noise power ratio is to be at least 40 dB. Find the number of levels into whichthe signal has to be encoded. Also find the transmission band width. (04 Marks)

a. With neat diagrams, explain the operation of DPCM. (07 Marks)

b. A DM system is tested with a 10KHz sinusoidal signal with 1 V peak to peak at the i/p. It issampled at10 times the Nyquist rate. What is the step size required to preventslopeoverload? (04 Marks)

c. Explain T1- carrier system. (05 Marks)

d. For the binary bit sequence 10110100, draw the waveforms using: (04 Marks)

i) Unipolar NRZ ii ) Unipolar RZ iii) Polar NRZ iv) Bipolar NRZ.

a. Describe Nyquist's criteria for distortionless baseband transmission. (06 Marks)

b. Explain the need for a precoder in a duobinary signaling. For i/p binary data 1011101, obtainthe o/p precoder and o/p of duobinary coder. Explain how data can be detected at thereceiver. (08 Marks)

c. What is equalization? Explain adaptive equalization for data transmission. (06 Marks)

5 a. Explain the working of:i) Coherent BFSK transmitter and ii) QPSK transmitter. (10 Marks)

b. The bit stream 1011100011 is to be transmitted using DPSK technique. Determine theencoded sequence and transmitted phase sequence. Also write the block diagram of themodulator and demodulator for the same and explain. (10 Marks)

10f2

6 a. A binary data is transmitted using ASK over a AWON channel at a rate of 2.4 Mbps. Thecarrier amplitude at the receiver is 1 ~V. Noise spectral densityNJ2 is 10-15 Watt/Hz. Findthe average probability of error if the detection is coherent. Take Erfc(5)::: 3xl 0-

6. (06 Marks)

b. With a diagram, explain the model of digital communication system. (08 Marks)

c. Explain geometric interpretation of signals. (06-Marks)

7 a. Explain the detection of known signals in noise. (10 Marks)

b. Write a note on minimum mean square error estimate. (04 Marks)

c. A polar NRZ waveform is to be received by a matched filter. Binary 1 is represented by arectangular positive pulse and binary 0 is represented by a rectarlgular negative pulse. Findout the impulse response of the matched filter and sketch it. (06 Marks)

a. Mention the applications of spread spectrum system.sequence spread spectrum system.

b. Explain the frequency hopped spread spectrum system.

c. A slow FHIMFSK system has the following parameters:The number of bits / MFSK symbol= 04The number of MFSK symbol/hop = 05Calculate the processing gain of the system in decibels.

Explain the principle of direct(08 Marks)

(08 Marks)

Sixth Semester B.E. Degree Examination, May/June 2010

Digital Communication

Note:Answer any FIVEfull questions, selectingat least TWO questionsfrom each part.

PART-Aa. With a block diagram, explain the basic signal processing operations involved in a digital

communication system. (07 Marks)

b. Explain 'flat-top' sampling, using waveforms and equations. (07 Marks)

c. The signal x(t)= 2cos400nt+ 6cos640nt is ideally sampled at fs = 500 Hz. If the sampled

signal is passed through an ideal low pass filter with cut off frequency of 400 Hz:i) Determine the spectrum of the sampled signal and sketch.ii) What frequency components will appear in the filter output? (06 Marks)

b. Show that the signal to quantization noise power ratio of a uniform quantizer is[SNRlm = 1.8+6n andn = number of bits/sample. (06 Marks)

c. What is the necessity of non uniform quantization? Explain compounding. (06 Marks)

d. A telephone signal band limited to 4 KHz is to be transmitted by PCM. The signal toquantization noise power ratio is to be at least 40 dB. Find the number of levels into whichthe signal has to be encoded. Also find the transmission band width. (04 Marks)

a. With neat diagrams, explain the operation of DPCM. (07 Marks)

b. A DM system is tested with a 10KHz sinusoidal signal with 1 V peak to peak at the i/p. It issampled at10 times the Nyquist rate. What is the step size required to preventslopeoverload? (04 Marks)

c. Explain T1- carrier system. (05 Marks)

d. For the binary bit sequence 10110100, draw the waveforms using: (04 Marks)

i) Unipolar NRZ ii ) Unipolar RZ iii) Polar NRZ iv) Bipolar NRZ.

a. Describe Nyquist's criteria for distortionless baseband transmission. (06 Marks)

b. Explain the need for a precoder in a duobinary signaling. For i/p binary data 1011101, obtainthe o/p precoder and o/p of duobinary coder. Explain how data can be detected at thereceiver. (08 Marks)

c. What is equalization? Explain adaptive equalization for data transmission. (06 Marks)

5 a. Explain the working of:i) Coherent BFSK transmitter and ii) QPSK transmitter. (10 Marks)

b. The bit stream 1011100011 is to be transmitted using DPSK technique. Determine theencoded sequence and transmitted phase sequence. Also write the block diagram of themodulator and demodulator for the same and explain. (10 Marks)

10f2

6 a. A binary data is transmitted using ASK over a AWON channel at a rate of 2.4 Mbps. Thecarrier amplitude at the receiver is 1 ~V. Noise spectral densityNJ2 is 10-15 Watt/Hz. Findthe average probability of error if the detection is coherent. Take Erfc(5)::: 3xl 0-

6. (06 Marks)

b. With a diagram, explain the model of digital communication system. (08 Marks)

c. Explain geometric interpretation of signals. (06-Marks)

7 a. Explain the detection of known signals in noise. (10 Marks)

b. Write a note on minimum mean square error estimate. (04 Marks)

c. A polar NRZ waveform is to be received by a matched filter. Binary 1 is represented by arectangular positive pulse and binary 0 is represented by a rectarlgular negative pulse. Findout the impulse response of the matched filter and sketch it. (06 Marks)

a. Mention the applications of spread spectrum system.sequence spread spectrum system.

b. Explain the frequency hopped spread spectrum system.

c. A slow FHIMFSK system has the following parameters:The number of bits / MFSK symbol= 04The number of MFSK symbol/hop = 05Calculate the processing gain of the system in decibels.

Explain the principle of direct(08 Marks)

(08 Marks)

Sixth Semester B.E. Degree Examination, May/June 2010

Digital Communications

Note: 1.Answer any FIVE full questions.2. Assume suitable datafor numerical problems.

1 a. What are the two primary channel characteristics? Explain different types of digitalcommunication channels. (07 Marks)

b. What is quadrature sampling? Explain the generation and reconstruction of quadraturesampled bandpass signals. (08 Marks)

c. Find the Nyquist rate for the analog signal given by x (t)x (t) = 3 Cos (50nt) + 10 Sin (300nt) - Cos (100nt).. (05 Marks)

2 a. What is flat-top sampling? Derive the expression for the spectrum of flat-top sampled signal.(08 Marks)

b. A flat-top sampling system samples a signal of maximum 1Hz with 2.5 Hz samplingfrequency. The duration of the pulse is 0.2 seconds. Calculate the amplitude distortion due toaperture effect at highest signal frequency. Also find out the equalization characteristic.

(08 Marks)c. Explain the concept ofTDM. (04 Marks)

3 a. Explain the blocks of basic elements ofPCM system. (08 Marks)- - - - ---- - - ----- - ----- - -

b. Show that for!J. = A, the !J. -law and A -law have the same companding gain. (06 Marks)

c. A modulating signal is given by x (t)= A sin (Pt). Find the maximum value of A for noslope over load distortion. Assume step size 8= 2 mV and a sampling rate of fs = 10kHz.

(06 Marks)

4 a. Explain T1 carrier multiplexing system. Also show that how the higher carrier multiplexingtakes place for high bit rates. (06 Marks)

b. Derive the expression for NRZ polar waveform PSD (Power Spectral Density).(08 Marks)

c. The binary data stream 0010110 is applied to the input of a duo binary system. Construct theduo binary coder output and corresponding receiver output. Assume that there is'a pre coderat the input. (06 Marks)

5 a. Explain the operation of binary phase shift keying (BPSK) transmitter and coherent BPSKreceiver. (06 Marks)

b. Obtain the signal space diagram for binary frequency shift keying (BFSK) and hence prove

that the average probability of symbol error is given by Pe= ~ erfc~ Eb . (08 Marks)2 2No

c. Input binary sequence to QPSK modulator is given by{b i} = {O 1 0 0 1 0 1 1 0 1 1O}. Sketch the transmitted phase of the carrier as a function oftime. (06 Marks)

lof2

6 a. Explain the two stage Gram-Schmidt orthogonalization procedure to find orthonormal basisfunctions. (10 Marks)

b. Derive the equation for maximum likelihood tofind maximum likelihood estimation.• rOlVfilrkS) ••

7 a. Explain the properties of maximum length sequences. (06 Marks)

b. Explain the operation of direct sequence spread coherent PSK transmitter and receiver.(08 Marks)

c. In direct sequence spread spectrum modulation scheme, a 14 - stage feedback linear shiftregister is used to generate the PN code sequence. Find the period of the code sequence andprocessing gain. (06 Marks)

8 a. Explain the idealized model of baseband spread spectrum with the illustration of waveformsin the transmitter for a data sequence of 0011101. (10 Marks)

b. With a neat block diagram, explain the working of frequency hopped transmitter andreceiver combination for M - ary frequency shift keying. (10 Marks)

Sixth Semester B.E. Degree Examination, May/June 2010

Digital Communications

Note: 1.Answer any FIVE full questions.2. Assume suitable datafor numerical problems.

1 a. What are the two primary channel characteristics? Explain different types of digitalcommunication channels. (07 Marks)

b. What is quadrature sampling? Explain the generation and reconstruction of quadraturesampled bandpass signals. (08 Marks)

c. Find the Nyquist rate for the analog signal given by x (t)x (t) = 3 Cos (50nt) + 10 Sin (300nt) - Cos (100nt).. (05 Marks)

2 a. What is flat-top sampling? Derive the expression for the spectrum of flat-top sampled signal.(08 Marks)

b. A flat-top sampling system samples a signal of maximum 1Hz with 2.5 Hz samplingfrequency. The duration of the pulse is 0.2 seconds. Calculate the amplitude distortion due toaperture effect at highest signal frequency. Also find out the equalization characteristic.

(08 Marks)c. Explain the concept ofTDM. (04 Marks)

3 a. Explain the blocks of basic elements ofPCM system. (08 Marks)- - - - ---- - - ----- - ----- - -

b. Show that for!J. = A, the !J. -law and A -law have the same companding gain. (06 Marks)

c. A modulating signal is given by x (t)= A sin (Pt). Find the maximum value of A for noslope over load distortion. Assume step size 8= 2 mV and a sampling rate of fs = 10kHz.

(06 Marks)

4 a. Explain T1 carrier multiplexing system. Also show that how the higher carrier multiplexingtakes place for high bit rates. (06 Marks)

b. Derive the expression for NRZ polar waveform PSD (Power Spectral Density).(08 Marks)

c. The binary data stream 0010110 is applied to the input of a duo binary system. Construct theduo binary coder output and corresponding receiver output. Assume that there is'a pre coderat the input. (06 Marks)

5 a. Explain the operation of binary phase shift keying (BPSK) transmitter and coherent BPSKreceiver. (06 Marks)

b. Obtain the signal space diagram for binary frequency shift keying (BFSK) and hence prove

that the average probability of symbol error is given by Pe= ~ erfc~ Eb . (08 Marks)2 2No

c. Input binary sequence to QPSK modulator is given by{b i} = {O 1 0 0 1 0 1 1 0 1 1O}. Sketch the transmitted phase of the carrier as a function oftime. (06 Marks)

lof2

6 a. Explain the two stage Gram-Schmidt orthogonalization procedure to find orthonormal basisfunctions. (10 Marks)

b. Derive the equation for maximum likelihood tofind maximum likelihood estimation.• rOlVfilrkS) ••

7 a. Explain the properties of maximum length sequences. (06 Marks)

b. Explain the operation of direct sequence spread coherent PSK transmitter and receiver.(08 Marks)

c. In direct sequence spread spectrum modulation scheme, a 14 - stage feedback linear shiftregister is used to generate the PN code sequence. Find the period of the code sequence andprocessing gain. (06 Marks)

8 a. Explain the idealized model of baseband spread spectrum with the illustration of waveformsin the transmitter for a data sequence of 0011101. (10 Marks)

b. With a neat block diagram, explain the working of frequency hopped transmitter andreceiver combination for M - ary frequency shift keying. (10 Marks)