SPPU Electronics and Telecom Engineering (Semester 5)
Digital Communication
December 2014
Total marks: --
Total time: --
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

Answer any one question from Q1 and Q2
1 (a) Explain the flat-top sampling with functional diagram. Draw spectral diagram for the flat-top sampled signal and aperture effect.
8 M
1 (b) With the help of neat schematic, explain early-late synchronizer.
6 M
1 (c) A voltage V(t) which is a Gaussian Ergodic Random process with a mean of zero and a variance of 4 volt 2 , is measured by a DC meter, a true RMS meter and a meter which first squares V(t) and then reads its DC component. Find the output of each meter.
6 M

2 (a) Explain with neat schematic and mathematical analysis, a transmitter and receiver for DPCM.
8 M
2 (b) Derive the expression for power spectral density of polar NRZ signal.
6 M
2 (c) Define Random process. Explain various time averages associated with the random process.
6 M

Answer any one question from Q3 and Q4
3 (a) Derive the expression of SNR for integrator and dump filter and explain working of integrator and dump filter.
8 M
3 (b) Explain Gram - Schmitt procedure for orthogonalization.
8 M

4 (a) Derive the expression of probability error (Pe) for matched filter.
8 M
4 (b) Find decision threshold if conditional probability density functions after addition of noise are of Gaussian distribution and voltage V1 represents symbols S1 and V2 symbol S2 for noise case, show the threshold when apriori probabilities are equal and unipolar signal V1 = + V and V2 = 0.
8 M

Answer any one question from Q5 and Q6
5 (a) In a digital communication system, the bit rate of NRZ data stream is 5Mbps and carrier frequency of transmission is 100 MH2. Find.
a) Mathematical equation
b) Symbol rate
c) Band width
for the following modulation schemes.
ii) QPSK
iii) 16-ary PSK.
9 M
5 (b) Draw the block diagram of DPSK transmitter and explain its operation with proper waveforms.
6 M
5 (c) Explain the concept of OFDM.
3 M

6 (a) Given the input binary sequence 1100100010, sketch the waveforms of the in-phase and quadrature components of a modulated wave obtained by using the QPSIC scheme.
9 M
6 (b) Compare BPSK, QPSK and M-ary PSK with the help of equations, signal space representation, symbol rate and bandwidth.
9 M

Answer any one question from Q7 and Q8
7 (a) A spread spectrum system has the following parameters.
Information bit duration Tb=4.095 m sec.
PN chip duration Tc=1 μ sec.

Find the processing gain. What is the number of shift registers required? Also find the jamming margin if the Eb/No=10 for the BPSK scheme.
6 M
7 (b) Draw block diagram DSSS-PSK transmitter.
2 M
7 (c) What are the properties of maximal length sequences ? Give the graphical representation of auto correlation property of random data and a PN sequence and explain.
8 M

8 (a) Draw and explain FHSS spread spectrum system with transmitter and receiver section.
8 M
8 (b) Explain various wireless standards for Wi-Fi and WiMax.
4 M
8 (c) Write short note on Personal Communication Systems (PCS).
4 M

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