Code: A-15                                                        Subject: COMMUNICATION ENGINEERING

Time: 3 Hours                                                                             Flowchart: Alternate Process: December 2005                        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 best alternative in the following:                                         (2x10)

       

a.       Error signals associated with the sampling process are called

 

                   (A)  Fold over distortion.                     (B)  Aliasing

(C)    Nyquist rate                                 (D)  A and B

       

b.      When the message and the BCC are transmitted as separate parts within the same transmitted code, it is called

 

(A)    Systematic code                           (B)  CRC

(C)  (n,k) cyclic code                          (D)  Interleaved code

            

c.       E1 channel corresponds to                                    

 

(A)    24 8-bit PCM voice channels      

(B)    32 8-bit PCM voice channels

(C)    24 16-bit PCM voice channels

(D)    32 16-bit PCM voice channels

 

             d.   Aperture effect is associated with

 

(A)    Instantaneous sampling                (B) Natural sampling

(C)  Flat-topped sampling.                  (D) Ideal sampling 

 

             e.   For an FM system, the effect of decrease in output noise power as the carrier power increases is known as

                  

(A)     Noise quieting                              (B)  Threshold

(C)  Capture effect                              (D)  Aperture effect

 

             f.    For tone modulation in AM, the modulation index is 0.3 and the message signal is 20 cos1000  t. If modulation index is increased from 0.3 to 0.7, the improvement in the output SNR in dBs will be

 

(A)     3.3                                               (B)  6.6

(C)  9.9                                               (D)  12.5

 

 

 

             g.   A channel described by a channel matrix with only one non-zero element in each column is known as a

 

(A)     Deterministic channel                    (B)  Loss less channel

(C)  Noiseless channel                         (D)  Noisy channel

 

             h.   A linear code C of minimum distance dmin can correct up to t errors if

 

(A)    dmin <  t + 1                                  (B) dmin  t + 1

(C) dmin   t + 1                                  (D) dmin = t + 1

 

             i.    Monitoring of the number of telephone calls arriving at a switching center during various intervals of time will most likely correspond to

 

(A)   Guassian distribution.                    (B) Binomial distribution

(C) Rayleigh distribution                       (D) Poisson distribution

 

             j.    A communication channel of bandwidth 75 kHz is required to transmit binary data at a rate of 0.1Mbps using raised cosine pulses. The roll of factor α is given by

 

(A)  1.5                                               (B)  2.5

(C)  5.5                                               (D)  0.5

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

  Q.2     a.   Prove that the Dirac Delta function  is a limiting form of the Gaussian pulse.                  (5)

       

             b.   Explain how the carrier frequency can be acquired in an AM-DSBSC system using Costas loop.                                                               (5)

 

             c.    A received single tone sinusoidally modulated  SSB signal  has a normalized power of 0.5 W. The signal is to be detected by carrier reinsertion technique. Determine the amplitude of the carrier to be reinserted so that the power in the recovered signal at the demodulator output is 80% of the normalised power. The DC component may be neglected.                                                                   (6)

 

  Q.3     a.   Explain mathematically, how an FM signal can be demodulated using a PLL.                      (7)                  

 

             b.   An FM signal has a frequency deviation of 5 kHz and a modulating frequency of 1 kHz. The SNR at the input to the receiving detector is        20 dB. Calculate the approximate SNR at the detector output.                                                           (5)                                                             

 

             c.   Explain the threshold and the capture effects with reference to FM.                     (4)

       

 

  Q.4     a.   Show that there is an improvement in SNR with companding in a PCM system.                  (8)

 

             b.   A signal at the input to a μ-law compressor is positive with its voltage one half the maximum value. What proportion of the maximum output voltage is produced? Find the maximum dynamic range for a linear PCM system using 16-bit quantizer.                                                                             (4)

 

             c.   Distinguish between a DS-1 signal and a T1 carrier.                                            (4)

 

  Q.5     a.   Derive an expression for SNR in a Delta modulated signal.                                 (6)

       

             b.   Show that for a fixed bandwidth, the performance of a Delta modulator is inferior to that of PCM.                                                            (4)

            

             c.   A signal is to be transmitted using DM and is of the form                                        s (t)=10 cos 1000πt + 5 cos 1500πt. Choose an appropriate sampling rate, step size and also find the SNR.            (6)

 

  Q.6     a.   What do you understand by the term ISI in digital communication systems? Explain the methods of reducing ISI.                                                     (10)

 

             b.   Explain bit and frame synchronization as applied to digital communication systems.              (6)

 

  Q.7     a.   Derive the radar range equation and discuss the parameters that affect the maximum radar range?                                                              (8)   

 

             b.   Explain a color TV receiver using a block diagram clearly mentioning the functions of every block.                                                             (8)

 

  Q.8     a.   State and prove Shannon’s channel capacity theorem.  Derive the Shannon’s limit (bound).             (10)

 

             b.   An FSK system transmits binary data at the rate of 2.5 Mbps. During the course of transmission, while Guassian noise of zero mean and power spectral density 10-20 W/Hz is added to the signal. In the absence of noise, the amplitude of the received sinusoidal wave for digit 1 or 0 is 1 μV. Determine the average probability of symbol error assuming coherent detection.                                                    (6)

 

  Q.9           Write explanatory notes on:

 

(i)                  FM stereophonic broadcasting.

(ii)                ADM

(iii)               Optimum Receiver

(iv)              Block codes                                                                    (4x4=16)