Digital Communications Laboratory Viva Questions - Part 6

Theory_Questions:

1. The binary data are transmitted over a microwave link at the rate of 10⁶ bits per second. This data is transmitted using 4-PSK modulation scheme what is the required channel bandwidth
   1. 1 MHz
   2. 0.5 MHz
   3. 0.25 MHz
   4. 0.7 MHz

2. The binary data are transmitted over a microwave link at the rate of 10⁶ bits per second and the power spectral density of the noise at the receiver input is 10^-10 watts per hertz. Find the average carrier power required to maintain an average probability of error Pe ≤2x10^-4 for coherent binary PSK

Erfc(2.7) = 2x10^-4

Erfc(2.5)=4x10^-4

1. 1.25 mW
2. 2.5 mW
3. 1.46 mW
4. 2.92 mW

3. In the above problem a binary PSK signal is applied to a correlated supplied with a phase reference that differs from the exact carrier phase by 60⁰.determine the average probability of error Pe≤1x10^-4

1. 5 mW
2. 2.92 mW
3. 5.84 mW
4. 2.5 mW

4. An PSK system transmits binary data at the rate of 5x10⁶ bits per second during the course of transmission white Gaussian noise of zero mean and power spectral density 10^-20 watts per Hertz is added to the signal. In the absence of noise the amplitude of the received sinusoidal wave for digit 1 or 0 is 1µV. Determine the average probability of symbol error assuming coherent detection

(Notes : use the approximation erfc(x)≈ exp(-x²)/ x

1. 0.85 x10^-3
2. 0.65 x10^-3
3. 0.75 x 10^-3
4. 0.5 x10^-3

5. The Constellation diagram of 4-ary of QAM is similar to that of
   1. 4-PSK
   2. QPSK
   3. MSK
   4. 4-PAM

6. The signal x( t)=S_i(t)+W(t) is given to the correlators then what is the variance of the correlated output x_j

Where Si(t)= and w(t) is zero mean white Gaussian noise process with power spectral density N0/2

1. N_o
2. 2N_o
3. N_o/2
4. 4N_o

7. In the above problem what is the mean of the correlator output x_j
   1. S_i1
   2. S_i2
   3. 0
   4. S_ij

8. In the octa-phase shift keying what is the phase difference between adjacent points
   1. π/2
   2. π/8
   3. π/4
   4. π/16

9. In M-ary PSK signal which is defined by S_i(t)=. What is minimum distance between any two message points.
   1. 2
   2. 
      
   3. 2
   4. 
      

10. The BPSK waveforms defined by Si(t)=± where + or – corresponding to transmitting 1 or 0. Find out the average probability of symbol error for coherent detection in BPSK
    1. 2 Q( )
    2. Q )
    3. Q )
    4. Q )

Answers:

1. B
2. A
3. C
4. A
5. B
6. C
7. D
8. C
9. C
10. C