Hall Ticket No Question Paper Code: AEC009
INSTITUTE OF AERONAUTICAL ENGINEERING
(Autonomous)
Four Year B.Tech V Semester End Examinations (Regular) November, 2018
Regulation: IARE R16
DIGITAL COMMUNICATIONS
Time: 3 Hours Max Marks: 70
Answer ONE Question from each Unit
All Questions Carry Equal Marks
All parts of the question must be answered in one place only
UNIT I
1. Define Quantization. What is the need for Companding in PCM system and show that the mean
square quantization error in PCM is
2/12.
Illustrate with waveforms, how PPM is generated using PWM.
2. Explain the encoding scheme used in DPCM scheme with block diagram description and mathematical
modeling.
What are the noise present in Delta Modulation. Explain the modulation scheme to avoid these
noise effects
UNIT II
3. Explain the generation of QPSK modulation scheme using constellation diagram.
Determine the amount of probability of error in matched filter Receiver.
4. How does the phase of the carrier vary for message in BPSK and DPSK.
Draw the block diagram of QPSK receiver.
A binary data has to be transmitted over a telephone link that has a usable bandwidth of 3000Hz
and a maximum achievable signal to noise power of 6dB at its output.
Determine the maximum signaling rate and probability of error if a coherent ASK is used for
transmitting binary data through this channel
if the data rate is maintained at 300 bits/sec. Find the error probability.
UNIT III
5. Write down the Unipolar, Polar and Bipolar Line Coding Schemes.
Explain Eye Diagram with neat diagram and how to draw eye diagram for ASK.
6. What is pulse shaping? Why it is useful in baseband transmission? Explain in detail.
A line coding scheme uses Unipolar NRZ encoding with rectangular pulses. Sketch the signal
corresponding to the bit sequence "101100". Compute its Power spectral density.
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UNIT IV
7. Define Mutual Information and Prove the relation .
A Guassian channel has 5MHz bandwidth. Calculate the channel capacity if the signal power to
noise spectral density ratio is 106 Hz. Discuss the trade off between bandwidth and S/N ratio.

8. Describe spread spectrum modulation techniques in detail.
A source emits letters from an alphabet with probabilities
P(a5)=0.04 and P(a6)=0.3
Find a Huffman code for this source.
Find the average length of the code.
UNIT V
9. What is a convolutional code? How it is different from a block code?
Show that if ci and cj are two code vectors in an linear block code, then their sum is also
a code vector.
10. Explain how generator and parity matrices are obtained for cyclic codes with an example.
Decode the Received code word [10 11 10 using viterbi algorithm and find the correct transmitted
code word using the given convolutional encoder.
Figure 1
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