# Mumbai University Previous year question papers

## Digital Communication and Coding Techniques

N.S. (1) Question NO.1 is compulsory.

(2) Attempt any four questions out of remaining six questions.

(3) Assume suitable data wherever necessary and justify it.

1. (a) Explain the following questions with proper illustration

(i) BFSK transmission can be orthogonal or non orthogonal.

(ii) Phase continuity is maintained in MSK signal.

(b) Define the following terms

(i) Hamming weight and Hamming distance

(ii) Systematic and non systematic code

(iii)’ Code rate

(iv) Entropy and redundancy.

2. (a) Define the term Random Variable and explain the meaning of the terms discrete RV and continuous RV.

(b) A three digit message is transmitted over a noisy channel having a probability of error P(E)=5 per digit. Find out the corresponding PDF and CDF. Also find the probabilityof morethan one error.

(c) State and explain shannon Heartley theorem. Also explain bandwidth SIN  trade off.

3. (a) An analog signal is band limited to 4 KHz. It is sampled at Nyquist rate and the samples are quantized into 4 levels. The quantization levels °1, °2′ Q3 and °4  are independentmessagesand have probabilitiesP

1= P2= 1/8and P3= P4 = 3/8.

Find the informationrate of the source. .

(b) Findthe Huffmancode for a source alphabetof{A,B,.., H} with the probabilitiesof

11111111

2″4′ ..”I ..,,’ “”,’ “,,’ “”,'””,’ “1=.1 Ir’-‘ “I..~ ~’.’~””g'” ,,”rlolonnt~efficiency.

(c) Explainthe necessityof line codesfor data transmission.Statedifferent types of line codes.

4. (a) For a systematic linear blockcode, the three paritycheck digits,C4.Csand Cs are given by :

C4 = d1 Ef)d2 Ef)d3

Cs = d1 Ef)d2

Cs = d1 Ef) d3

(i) Construct generator matrix .

(ii) Construct code generated by this matrix

(iii) Determine error detecting and correcting capability.

(iv) Decode the received word 10 11 00,

(b) For the convolutional encoder shown below sketch the code tree and trellis diagram. Obtain the convolution code for the input bit sequence 11 0011.

5. (a) Explain the Nyquist criteria for distortionlessbaseband binary transmission. 10

(b) What isduo-binaryencoding? Explainwith the neat block diagram. Howthe duo-binary encoding reduces the bandwidth requirements?

6. (a) Draw block diagram of a generator of DPSK. Also show how data is recovered from DPSK signal.

(b) Explain 4-ary PSK alongwith the following points –

(i) Offset and Non-offset QPSK

(ii) Modulation andcdemodulation blockcdiagram  of offset QPSK

(iii) Plot of power spectral density

7. Write short notes on any four :-

(a) PN sequence generator

(b) Viterbi algorithm