AMIETE – ET (OLD SCHEME)

 

Code: AE21                                                               Subject: DIGITAL COMMUNICATIONS

Flowchart: Alternate Process: DECEMBER 2009Time: 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:                                   (2x10)

                                

a.       Which of the following requires a synchronizing signal?  

 

                   (A)  single channel PPM system           (B)  PAM

(C)  PDM                                           (D)  all of the above

       

b.      In delta modulation, which of the following drawbacks are existing?

                                                                       1. Slope overload        2. Serration noise        3. Granular noise   

 

(A)    1 and 2 only                                 (B) 2 and 3 only

(C)  1 and 3 only                                 (D) 1,2 and 3

            

             c.   In an PCM the amplitude levels are transmitted in a 7 unit code. The sampling is done at the rate of 10 Hz. The bandwidth should be

            

                   (A)  5 kHz                                           (B)  5 MHz

(C)  70 kHz                                         (D)  35 kHz

 

             d.   The purpose of source coding is to 

                                                                                                                              

                   (A) Increase the information Transmission rate

                   (B) Decrease the information Transmission rate

(C) Decrease the S/N ratio

(D) Decrease the probability of error

 

             e.   If carrier modulated digital bit stream had one of the possible phases of 0, 90, 180 and 270 degrees, then the modulation scheme used is

 

(A)    BPSK                                         (B) QPSK

                   (C)   QAM                                          (D) MSK

 

             f.    Aperture effect in the reconstructed signal from samples is due to loss of    

 

(A) High frequency components           (B)  midrange frequency components         

                   (C) low frequency components            (D) None of these

 

 

             g.   Which of the following gives the minimum probability of error        

 

                   (A) ASK                                             (B) FSK

                   (C) DPSK                                           (D) PSK   

            

             h.   A DSSS signal is designed so that PR/PN at the intended receiver is 10-2. If the desired Eb/No is 10 for acceptable performance, determine the minimum value of processing gain.  

 

                   (A)  20                                                (B)  1000

                   (C)  100                                              (D)  50

       

             i.    The generator matrix for a (7, 4) linear block code is given by:

                           Then the code word for the message vector[1 0 1 1] will be                    

 

                   (A) 1011001                                       (B)  1000110

                   (C)  1010101                                      (D)  1010111

 

             j.    In a matched filter probability of error depends on

 

                   (A) signal wave shape                          (B) signal energy

                   (C) Both A and B                                (D) None of these 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   State and prove Sampling Theorem used for the reconstruction of original signal.                 (8)

 

             b.   Derive an expression for signal to quantization noise ratio for a PCM system which employs linear quantization technique. Given that input to the PCM system is a sinusoidal signal                 (8)

 

  Q.3     a.   Draw and explain the block diagram of Adaptive Delta Modulation.                    (9)          

             b.   A delta modulation system is designed to operate at 3 times the Nyquist rate for a signal with a 3kHz bandwidth. The quantizing step size is 250mV.                                                    (7)

                   (i)   Determine the maximum amplitude of a 1kHz input sinusoid for which the delta modulator does not show slope overload distortion.

                   (ii)  Determine the post filtered output signal to quantizing noise ratio for the signal of part (i).

 

 

 

  Q.4     a.   Prove that the output signal of a matched filter is proportional to a shifted version of the auto correlation function of the input signal to which the filter is matched.                                                     (6)

 

 

             b.   Explain the effect of Inter Symbol Interference in base band transmission. How ISI can be reduced?                                                                      (10)

 

  Q.5     a.   Explain the detection of BPSK modulated signal.                                                (8)          

 

             b.   Describe error performance analysis for QPSK, MSK and DPSK signal.           (8)

 

  Q.6     a.   Explain the necessity of syndrome decoding. Also calculate the syndrome vector.                (8)

 

             b.   For a (n, k) linear block code which corrects single error per code vector, prove that, and hence design a linear block code with a minimum distance of three and a message block size of eight bits.                         (8)

       

 

  Q.7     a.   Explain the Viterbi algorithm for decoding of convolutional codes.                       (8)          

             b.   The generator polynomial of a (7,4) cyclic code is G(p)=p3+p+1. Find all the code vector for the code in systematic form.                                   (8)

       

  Q.8     a.   Explain the operation of FHSS system with block diagram.. Also mention the limitations of DS-SS and FH-SS systems.                                         (12)

 

 

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

 

  Q.9     a.   With the help of example, explain the DPSK modulation technique of digital transmission.               (8)

 

             b.  Explain PN sequence.  Also, describe three randomness properties which makes it to be appear as random.                                                                                                                         (8)