Digital Communications Laboratory Viva Questions - Part 1

Theory_Questions:

1. An ASK system transmits data with certain rate and signal is effected by white noise with a one sided spectral density of 10^-6 Watts/Hz. the amplitude of the received sinusoidal wave for digit ‘1’ is 500mv and digit ‘0’ is 0v . Determine the bit rate show that the probability of error equal to 10^-5. [ Q(x) = 10^-5 only when x = 4.25 ]

a) 3.46 Kbps

b) 1.73 Kbps

c) 6.92 Kbps

d) 2.46 Kbps

2. Consider a digital communication system with ‘1’ and ‘0’ transmission, when the binary symbol ‘1’ is transmitted the voltage at the input of the threshold comparator can be any value between 0v and 1v with equal probability. When the binary symbol ‘0’ is transmitted the input to the threshold comparator can be any value between -0.25v to +0.25v with equal probability. The threshold value used at the comparator is 0.2. Then find the probability of error when ‘1’ is transmitted.

a) 0.8

b) 0.4

c) 0.1

d) 0.2

3. In the above problem, find the probability of error when ‘0’ us transmitted.

a) 0.8

b) 0.4

c) 0.1

d) 0.2

4. In the above problem digit ‘1’ occur with probability 0.25 and digit ‘0’ occur with probability 0.75. Find the average probability of error of digital communication system.

a) 0.175

b) 0.125

c) 0.15

d) 0.2

5. Identify which of the following signal constellations given below are required to construct a 16 –level QAM signal constellation.

a) 2 octal phase shift keying systems with different radius

b) 2 QPSK systems with different energies

c) 3 QPSK systems with different energies

d) 1 octal phase shift keying and 2 QPSK systems with different energies

6. Consider two digital modulation techniques binary Phase shift keying (BPSK) and amplitude shift keying (ASK) are used to transmit digital information through AWGN channel at two different data rates 10 Kbits/s and 100 Kbits/s respectively. Determine the ratio of the signal amplitudes of carrier used in BPSK and ASK when the probability of error for the two coherent systems are equal?

a) 0.3162

b) 0.1581

c) 0.6324

d) 0.4471

7. Suppose that binary FSK is used for transmitting information over AWGN with two sided power spectral density of 10^-10 W/Hz. Then the modulate wave is passed through matched filter to maximize the Signal to Noise Ratio (SNR). It is observed that the probability of error achieved is 10^-6. Then find the Signal Noise Ratio (SNR) at the output of the matched filter? [Q(x) = 10^-6 only when x = 4.74]

a) 44.93

b) 22.46

c) 89.87

d) 11.23

8. The constellation diagram of a modem is shown in the figure given below, what does the modem use?

a) QAM

b) PSK

c) QPSK

d) Both QAM and QPSK

9. The constellation diagram of a QAM is show in the figure given below. The minimum distance between adjacent points is 2A. Determine the average transmitted power for QAM.

a) 6 A²

b) 9 A²

c) A²

d) A²

10. In the octal Phase shift keying the distance between two adjacent points in the constellation is ‘d’. determine the transmitted energy per symbol.

a)

b)

c)

d)

Answers:

1. a

2. d

3. c

4. b

5. d

6. b

7. a

8. a

9. d

10. b