Digital Communications Systems

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Noise”.

1. SQNR can be improved by _______ sampling rate.
a) Increasing
b) Decreasing
c) Does not depend
d) None of the mentioned

2. Which offers better SQNR?
a) Delta modulation
b) PCM
c) Delta modulation & PCM
d) None of the mentioned

3. Two functions are called as orthogonal if on integrating the product we get
a) One
b) Zero
c) Infinity
d) None of the mentioned

4. Eigen functions are not orthogonal.
a) True
b) False

5. Shot noise is produced by
a) Electrons
b) Photons
c) Electrons & Photons
d) None of the mentioned

6. Shot noise is avoidable when current is
a) Zero
b) One
c) Infinity
d) None of the mentioned

7. Shot noise is
a) Stochastic process
b) Poisson process
c) Stochastic & Poisson process
d) None of the mentioned

8. Which occurs due to equilibrium fluctuations?
a) Thermal noise
b) Johnson noise
c) Thermal & Johnson noise
d) None of the mentioned

9. Flicker noise is called as
a) White noise
b) Pink noise
c) Brown noise
d) None of the mentioned

10. Which has the same power spectral density?
a) White noise
b) Brown noise
c) White & Brown noise
d) None of the mentioned

11. Which noise is useful for dithering?
a) White noise
b) Pink noise
c) Brown noise
d) None of the mentioned

12. Capacity of a channel can be increased by
a) Increasing channel bandwidth
b) Increasing signal power
c) Increasing channel bandwidth &signal power
d) None of the mentioned

13. Capacity of the channel can be increased by reducing in band noise power.
a) True
b) False

14. Noise has
a) Infinite energy
b) Infinite power
c) Infinite energy & power
d) None of the mentioned

15. Thermal noise is
a) Energy signal
b) Power signal
c) Energy & Power signal
d) None of the mentioned

16. Thermal noise is a wide sense stationary process.
a) True
b) False

17. The maximum likelihood function is
a) Positive
b) Negative
c) Any of the mentioned
d) None of the mentioned

18. Matched filtering operation includes
a) Spectral phase matching
b) Spectral amplitude matching
c) Spectral phase & amplitude matching
d) None of the mentioned

19. Matched filter is
a) Linear
b) Non linear
c) Linear & Non linear
d) None of the mentioned

20. Which consists of less ISI?
a) Nyquist filter
b) Raised cosine filter
c) Nyquist & Raised cosine filter
d) None of the mentioned

21. _____________ refers to any spurious or undesired disturbances that mask the received signal in a communication system.
a) Attenuation
b) Noise
c) Dispersion
d) Bandwidth

22. How many types of noise are observed because of the spontaneous fluctuations in optical fiber communication systems?
a) One
b) Four
c) Two
d) Three

23. ______________ is caused due to thermal interaction between the free electrons and the vibrating ions in the conduction medium.
a) Thermal noise
b) Dark noise
c) Quantum noise
d) Gaussian noise

24. A small leakage current still flows from the device terminals even if there is no optical power incident on the photo detector.
a) True
b) False

25. ___________ distribution provides the description the random statistics of light emitted in black body radiation.
a) Poisson
b) Cumulative
c) Probability
d) Bose-Einstein

26. The probability of zero pairs being generated when a light pulse is present is given by which of the following equation?
a) P(0/1) = exp(-Zm)
b) P(x) = exp (Zm)
c) P(y) = x (0) + x(1)
d) P(z) = P(-Zm)

27. The minimum pulse energy needed to maintain a given bit-error-rate (BER) which any practical receiver must satisfy is known as ___________
a) Minimal energy
b) Quantum limit
c) Point of reversed
d) Binary signaling

28. A digital optical fiber communication system requires a maximum bit-error-rate of 10-9. Find the average number of photons detected in a time period for a given BER.
a) 19.7
b) 21.2
c) 20.7
d) 26.2

29. For a given optical fiber communication system, P(e) = 10-9, Zm = 20.7, f = 2.9×1014, η = 1. Find the minimum pulse energy or quantum limit.
a) 3.9×10-18
b) 4.2×10-18
c) 6.2×10-14
d) 7.2×10-14

30. An analog optical fiber system operating at wavelength 1μmhas a post-detection bandwidth of 5MHz. Assuming an ideal detector and incident power of 198 nW, calculate the SNR (f = 2.99×1014Hz).
a) 46
b) 40
c) 50
d) 52

31. The incident optical power required to achieve a desirable SNR is 168.2nW. What is the value of incident power in dBm?
a) -37.7 dBm
b) -37 dBm
c) – 34 dBm
d) -38.2 dBm

32. In the equation given below, what does τstands for?

Zm = ηP0τ/hf

a) Velocity
b) Time
c) Reflection
d) Refractive index

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