AMIETE – ET (OLD SCHEME)

 

Code: AE21                                                                Subject: DIGITAL COMMUNICATIONS

Flowchart: Alternate Process: JUNE 2010Time: 3 Hours                                                                                                     Max. Marks: 100

 

 

NOTE: There are 9 Questions in all.

·      Question 1 is compulsory and carries 20 marks. Answer to Q.1 must be written in the space provided for it in the answer book supplied and nowhere else.

·      Out of the remaining EIGHT Questions, answer any FIVE Questions. Each question carries 16 marks.

·      Any required data not explicitly given, may be suitably assumed and stated.

 

 

Q.1       Choose the correct or the best alternative in the following:                                  (210)

                                

a.       In a DM system, the granular noise occurs when the modulating signal is ___________.

  

                   (A)  increases rapidly                          

                   (B)  decreases rapidly

(C)  remains constant                          

(D)  the nature of modulating signal has nothing to do with this noise.

       

b.      Viterbi’s algorithm uses ____________.

  

(A)    sequential decoder                        (B) maximum likelihood detector

(C)  matched filter detector                  (D) correlation detector

            

             c.   Bandwidth of MSK is _________ that of QPSK.

            

                   (A)  higher than                                    (B)  lower than

(C)  equal to                                        (D)  less than or equal to

 

             d.   The length of PN sequence for a 8 stage feedback shift register is ____.  

                                                                                                                              

                   (A) 127                                               (B) 256

(C) 255                                               (D) 128

 

             e.   The signal space diagram given in Fig.1 corresponds to:
 
 

 

 

 

 

 


                  

 

 

 

                  

 

 

(A)    BPSK                                         (B) BFSK

                   (C)   BASK                                        (D) QPSK

 

             f.    The Hamming distance between two binary codes  and  is ______.     

 

(A) 9                                                   (B) 5         

                   (C) 6                                                   (D) 7

                  

             g.   For M >4, the signal constellation of M-ary PSK is _______.

 

                   (A) circular                                          (B) rectangular

                   (C) line                                                (D) elliptical

       

             h.   In a SEC Hamming Code, the number of message bit in a block is 4.  The number of check bits in the block will ______.   

 

                   (A)  3                                                  (B) 4

                   (C)  5                                                  (D) 7

       

             i.    In QPSK, is

             

                   (A)                                            (B)

                   (C)                                            (D) Es

 

             j.    BFSK is inferior to BPSK by

 

                   (A) 3 dB                                             (B) 6 dB

                   (C) 0 dB                                             (D) 2 dB   

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Derive the expression of SNR (in dB) of a DM system.  Hence, if 25 dB (SNR)q is to be maintained in a DM circuit.  Determine the quantized bit rate for an analog signal with a bandwidth of 3.4 kHz.       (8)

 

             b.   Consider a duo-binary signalling system.  If an input binary sequence of 001101001 is applied to this system what will be the duo-binary encoded output and the corresponding decoder output at receiver.       (8)

 

  Q.3     a.   In polar signalling the signal is +1 volts pulse during the interval (0, T).  Additive white Gaussian noise with a power spectral density is added to the signal.  Determine the maximum bit rate with a bit error probability. Given.                                                 (8)          

 

             b.   Determine the expression of transfer function and impulse response of matched filter.                      (8)

                                                                                                                                           

Q.4       a.   Binary data is to be transmitted at a rate of  bps over a channel having bandwidth of 3 MHz.  Assume that noise PSD at the receiver is.  Find the average carrier power required as the input for coherent BPSK to maintain an error probability of.                                                              (8)

 

 

             b.   For the data input 1011011100.  Determine the output of DPSK transmitter.  Also find the detector output.                                                           (8)

 

 

  Q.5     a.   Explain the need of companding. Compare A-Law companding and -Law companding.             (8)       

 

             b.   Explain the MSK modulation and demodulation schemes.  Also, show that it is modified form of QPSK and BFSK.                                                (8)

 

  Q.6     a.   For the (7, 4) Hamming Code with the parity check matrix given by

                   (i)   Construct the generator matrix.

                   (ii)  The code word that begin with 1010….

                   (iii) Suppose that the received codeword is 0111100.

 

                   Determine whether a received code vector is erroneous, locate the error and decode this received code.                                                                (10)

 

             b.   Consider the Convolutional encoder.

 

 

 

 

 

 

 

 


                   (i)   Find the impulse response of encoder.

                   (ii) Using impulse response found above, for input data d=101, find the               output.                 (6)

       

 

  Q.7     a.   Show that the code  is a linear cyclic code.           (4)          

 

             b.   Find a generator matrix G for.                              (4)

 

             c.   Consider (7, 4) cyclic code with .

                   (i)   Let data word d = 1010.  Find the corresponding codeword.

                   (ii)  Let codeword C = 1100101.  Find the corresponding data.                          (4)                      

 

             d.   Explain BCH code and compare it with other linear codes.                                 (4)          

       

  Q.8     a.   How is spread spectrum (SS) communication system different from the other communication system?  Mention the application of SS system.  (6)

 

 

             b.   Determine the processing gain and Jamming margin in a DS-SS system. Given.  Assume a maximum of is.  Also calculate the number of feedback stages required. (4)

 

c.       Explain a FH-SS system.  Mention the advantages and limitation of FH-SS system.            (6)

 

  Q.9     a.   Determine, probability of error and bandwidth of Binary phase shift keying.                (6)       

 

             b.   Design a PCM multiplexer system using a 256 level signal quantizer for the transmission of 3 signals,  and  band limited to 5 kHz, 10 kHz and 5 kHz, respectively.  Assume each signal is sampled at Nyquist rate.  Find

                   (i)  Maximum bit duration.

                   (ii) Channel BW required for transmission.

                   (iii) Commutator speed.                                                                                     (6)

 

             c.   Write short note on linear prediction filter.                                                           (4)