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SPPU Electronics and Telecom Engineering (Semester 5)
Digital Communication
December 2016
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

Solve any one question from Q.1(a,b,c) & Q.2(a,b,c)
1(a) Draw blcok diagram of Adaptive Delta modulator and explain the same. What are the advantages of Adaptive delta modulator over Delta Modulator.
8 M
1(b) Explain Inter symbol interference. Explain its causes and remedies to avoid it.
6 M
1(c) A random process X (t) is expressed as X (t) = M. Cos(ω0t)+N.Sin (ωt) where ω0 is constant while M and N are random variables.
i) Prove that the necessary condition for X (t) to be stationary is, E [M] = E[N]=0.
ii) Prove that X (t) is wide sense stationary if M and N only if and are uncorrelated and have equal variance.i.e. E [MN]= 0 and E[M2]= E[N2]=σ2.
6 M

2(a) Draw and explain CCIT hierarchy of multiplexing.
6 M
2(b) An audio signal with highest frequency component 3300 Hz is pulse code modulated with a sampling rate 8000 samples/sec. The required signal-to- quantization noise ratio is 40dB.
i) What is the minimum number of uniform quantising levels needed?
ii) What is the minimum number of bits per sample needed? < br>iii) Calculate the minimum number of bits per sample needed.
8 M
2(c) Define mean, correlation and covariance function for random process. Write down mathematical expression for the same.
6 M

Solve any one question from Q.3(a,b) & Q.4(a,b)
3(a) Derive expression for signal to noise ratio of Intergrate and Dump filter.
8 M
3(b) Explain Geometrical representation of signal and Garam-Schmidt procedure.
8 M

4(a) Derive the expression for signal to noise ratio and error probability of a matched filter in the presence of white gaussian noise.
8 M
4(b) Explain the principle of Maximum Likelihood receiver with the help of various methods of detection of signal.
8 M

Solve any one question from Q.5(a,b) & Q.6(a,b)
5(a) Find the bit error probability for a BPSK system having a bit rate of 1 Mbits/s. The receiver receives the waveforms S1 (t) = A. Cos(ω0t) and S2(t)=-A. Cos (ω0t). The received signals are co-herently detected using a matched fiter. If A = 10mV and single sided noise power spectral density is No = 10-11 W/Hz. Assume that the signal power and energy per bit are normalized. Assume if necessary erfc [ 2.24]= 41×10-5 and erf [ 3.1] = 0.9999.
8 M
5(b) Give mathematical representation of QPSK signal. Draw signal space diagram of QPSK signal. Write the expression of all message points in the diagram.
8 M

6(a) Exlain M-ary PSK transmitter with suitable block diagram. What are the advantages of M-ary PSK over M-ary FSK.
8 M
6(b) Draw signal space of 16-QAM system and comments on Eucliden distance and probability of error for 16-QAM signals.
8 M

Solve any one question from Q.7(a,b,c) & Q.8(a,b,c)
7(a) What is PN sequence ? Verify the three properties of PN sequence with the help of shift register.
6 M
7(b) A spread spectrum communication system system is characterised by the following parameters. Duration of each information bit Tb= 4.095 ms, Chip duration of a PNN sequecncy Tc = 1&muS. Calculate the processing gain and jamming margin if (Eb/N0)=10 and the average probability of error Pe= 0.5×10-5.
6 M
7(c) Explain in brief frequency reuse schemes and cell splitting in mobile communication system.
6 M

8(a) Draw the block diagram of FH-SS systems transmitter and receiver. Write the functional names inside the blocks and input output signals of each block.
6 M
8(b) What are multiple access techniques? Explain WCDMA in detail.
6 M
8(c) Write a short note on Cellular Telephone system.
6 M

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