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Communications MCQ Set 1

1. Phase locked loops consists of
a) Phase detectors
b) Loop filter
c) Voltage controlled oscillator
d) All of the mentioned

View Answer

Answer: d [Reason:] Phase locked loops have three basic components – phase detector, loop filter and voltage controlled oscillator. It measures the phase difference between the incoming signal and local replica.

2. In VCO the output frequency is a linear function of its input
a) Frequency
b) Voltage
c) Time period
d) None of the mentioned

View Answer

Answer: b [Reason:] A VCO is an oscillator whose output frequency is a linear function of its input voltage over some range of input and output.

3. PLL designs have
a) First order loop
b) Second order loop
c) Higher order loop
d) None of the mentioned

View Answer

Answer: b [Reason:] Most of the PLL designs have second order loop as they can be made unconditionally stable.

4. A cycle slip occurs when the magnitude of the original phase error exceeds
a) Π radians
b) 2π radians
c) π/2 radians
d) π2 radians

View Answer

Answer: b [Reason:] A cyclic slip occurs when the magnitude of the original phase error exceeds 2π radians.

5. When phase noise and phase jitter is doubled, the phase variance increases by
a) Two times
b) Four times
c) 0.5 times
d) 8 times

View Answer

Answer: b [Reason:] In PLL, when the phase jitter and phase noise increases by 2, the phase variance increases by 4 than that of the original signal.

6. Acquisition is a ______ operation.
a) Linear
b) Non linear
c) Linear & Non linear
d) None of the mentioned

View Answer

Answer: b [Reason:] Acquisition is accomplished by external signal or circuits or sometimes it might be unaided ( self acquisition). This acquisition process is a inherently non linear operation.

7. To avoid the loop to be driven to lock, the sweep rate should be
a) Smaller
b) Larger
c) It does not depend on it
d) None of the mentioned

View Answer

Answer: a [Reason:] The sweep rate must not be too large, or the loop will be driven through the lock point so fast that it will fail to acquire.

8. Which method is more accurate?
a) Closed loop synchronizer
b) Open loop synchronizer
c) Closed & Open loop synchronizer
d) None of the mentioned

View Answer

Answer: a [Reason:] Closed loop synchronizers are more accurate, but they are much more complex and costly.

9. What is the function of edge detector?
a) Differentiation
b) Rectification
c) Differentiation & Rectification
d) None of the mentioned

View Answer

Answer: c [Reason:] The main operation of the edge detector is differentiation and rectification by use of square law detector.

10. Continuous phase modulation has
a) More bandwidth efficiency
b) More smoothness
c) More bandwidth efficiency & smoothness
d) None of the mentioned

View Answer

Answer: c [Reason:] Continuous phase modulation is an important signalling technique because of their bandwidth efficiency. Bandwidth efficiency is obtained by increasing the smoothness of the signal.

11. CPM has
a) Narrower bandwidth
b) Wider bandwidth
c) More bandwidth requirement
d) None of the mentioned

View Answer

Answer: a [Reason:] The increase in smoothness in the signals concentrate the signal energy in narrower bandwidth, reducing the amount of bandwidth required.

12. The number of filters in CPM receiver structure is
a) LM
b) ML
c) M/L
d) None of the mentioned

View Answer

Answer: b [Reason:] The number of filters in CPM receiver structure is given by ML.

Communications MCQ Set 2

1. Which signals are function of time?
a) Random signal
b) Deterministic signal
c) Random & Deterministic signal
d) None of the mentioned

View Answer

Answer: b [Reason:] Deterministic signals are function of time.

2. Auto-correlation function is a
a) Even function
b) Odd function
c) Even & Odd function
d) None of the mentioned

View Answer

Answer: a [Reason:] Auto-correlation function is an even function of time.

3. Shot noise occurs in
a) Transistors
b) Valves
c) Transistors & Valves
d) None of the mentioned

View Answer

Answer: c [Reason:] Shot noise occurs in both valves and transistors.

4. Source coding reduces
a) Redundancy
b) Average bit rate
c) Redundancy & Average bit rate
d) None of the mentioned

View Answer

Answer: c [Reason:] Source coding reduces both average bit rate and reduces redundancy.

5. Delay element in delta modulation acts as
a) First order predictor
b) Second order predictor
c) Third order predictor
d) Fourth order predictor

View Answer

Answer: a [Reason:] Delay element act as first order predictor.

6. Non uniform quantization includes
a) Compression
b) Expansion
c) Compression & Expansion
d) None of the mentioned

View Answer

Answer: c [Reason:] Compression and expansion gives the feature of non uniform quantization.

7. The quantization will be finer when
a) Smaller the number of discrete amplitudes
b) Larger the number of discrete amplitudes
c) Does not depend on amplitudes
d) None of the mentioned

View Answer

Answer: b [Reason:] Larger the number of discrete amplitudes, finer will be the quantization.

8. Different cases of sampling includes
a) Ideal impulse sampling
b) Flat-topped sampling
c) Sampling with rectangular pulses
d) All of the mentioned

View Answer

Answer: d [Reason:] The three cases of sampling are ideal impulse sampling, sampling with rectangular pulses and flat topped sampling.

9. Transmitted pulse becomes distorted due to
a) Ideal transmission characteristic
b) Non ideal transmission characteristic
c) Both of the mentioned
d) None of the mentioned

View Answer

Answer: b [Reason:] A transmitted pulse gradually becomes distorted due to non ideal transmission characteristic of the channel.

10. In which mixing is easier?
a) Analog signal
b) Digital signal
c) Analog & Digital signal
d) None of the mentioned

View Answer

Answer: b [Reason:] Mixing of digital signals are easier than that of mixing analog signals.

Communications MCQ Set 3

1. The minimum nyquist bandwidth needed for baseband transmission of Rs symbols per second is
a) Rs
b) 2Rs
c) Rs/2
d) Rs2

View Answer

Answer: c [Reason:] Theoretical minimum nyquist bandwidth needed for the baseband transmission of Rs symbols per second without ISI is Rs/2.

2. The capacity relationship is given by
a) C = W log2 ( 1+S/N )
b) C = 2W log2 ( 1+S/N )
c) C = W log2 ( 1-S/N )
d) C = W log10 ( 1+S/N )

View Answer

Answer: a [Reason:] The capacity relationship from Shannon-hartley capacity theorem is given by C = W log2 ( 1+S/N ).

3. Which parameter is called as Shannon limit?
a) PB/N0
b) EB/N0
c) EBN0
d) None of the mentioned

View Answer

Answer: b [Reason:] There exists a limiting value for EB/N0 below which they can be no error free communication at any information rate. This EB/N0 is called as Shannon limit.

4. Entropy is the measure of
a) Amount of information at the output
b) Amount of information that can be transmitted
c) Number of error bits from total number of bits
d) None of the mentioned

View Answer

Answer: a [Reason:] Entropy is defined as the average amount of information per source output.

5. Equivocation is the
a) Conditional entropy
b) Joint entropy
c) Individual entropy
d) None of the mentioned

View Answer

Answer: a [Reason:] Shannon uses a correction factor called equivocation to account for uncertainty in the received signal. It is defined as the conditional entropy of the message X given Y.

6. For a error free channel, conditional probability should be
a) Zero
b) One
c) Equal to joint probability
d) Equal to individual probability

View Answer

Answer: a [Reason:] For a error free channel conditional probability should be zero, because having received Y there is complete certainty about the message X.

7. Average effective information is obtained by
a) Subtracting equivocation from entropy
b) Adding equivocation with entropy
c) Ratio of number of error bits by total number of bits
d) None of the mentioned

View Answer

Answer: a [Reason:] According to Shannon the average effective information is obtained by subtracting the equivocation from the entropy of the source.

8. Turbo codes are
a) Forward error correction codes
b) Backward error correction codes
c) Error detection codes
d) None of the mentioned

View Answer

Answer: a [Reason:] Turbo codes are a class of high performance forward error correction codes.

9. Components used for generation of turbo codes are
a) Inter leavers
b) Punching pattern
c) Inter leavers & Punching pattern
d) None of the mentioned

View Answer

Answer: c [Reason:] There are many instances of turbo codes, using different component encoders, input/output ratios, inter leavers, punching patterns.

10. Decoders are connected in series.
a) True
b) False

View Answer

Answer: a [Reason:] Two elementary decoders are connected in serial connection for decoding the turbo codes.

11. The inter leaver connected between the two decoders is used to
a) Remove error bursts
b) Scatter error bursts
c) Add error bursts
d) None of the mentioned

View Answer

Answer: b [Reason:] An inter leaver installed between the two decoders connected in series is used to scatter error bursts.

12. In soft decision approach what does -127 mean?
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Very likely one

View Answer

Answer: b [Reason:] The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here -127 represents certainly zero.

13. In soft decision approach 100 means?
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Very likely one

View Answer

Answer: d [Reason:] The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here 100 represents very likely one.

14. In soft decision approach 0 represents
a) Certainly one
b) Certainly zero
c) Very likely zero
d) Could be either zero or one

View Answer

Answer: d [Reason:] The decoder front end produces an integer for each bit in the data stream. This integer is the measure of how likely it is that the bit 0 or 1 and is called as soft bit. It ranges from -127 to 127. Here 0 represents ‘could be either zero or one’.

Communications MCQ Set 4

1. Multiple isolated peaks in multipath components is called as
a) Fingers
b) Returns
c) Fingers & Returns
d) None of the mentioned

View Answer

Answer: c [Reason:] The received signal usually consists of several discrete multipath components causing multipath intensity profile to exhibit multiple isolated peaks also called as fingers or returns.

2. If delay time is greater than symbol time, _______ fading occurs.
a) Frequency selective
b) Time selective
c) Frequency non selective
d) None of the mentioned

View Answer

Answer: a [Reason:] When delay time is greater than symbol time, frequency selective fading occurs.

3. If delay time is lesser than symbol time, ______ fading occurs.
a) Frequency non selective
b) Flat
c) Frequency non selective & Flat
d) None of the mentioned

View Answer

Answer: c [Reason:] If delay time is lesser than the symbol time, frequency non selective or flat fading occurs.

4. When channel coherence bandwidth is greater than the transmitted signal bandwidth ______ occurs.
a) Time selective
b) Frequency selective
c) Frequency non selective
d) None of the mentioned

View Answer

Answer: c [Reason:] Frequency non selective or flat fading occurs if channel coherence bandwidth is greater than the transmitted signal bandwidth.

5. Channel’s time spreading properties are
a) Coherence bandwidth
b) Signal dispersion
c) Coherence bandwidth & Signal dispersion
d) None of the mentioned

View Answer

Answer: c [Reason:] Signal dispersion and coherence bandwidth characterizes the channel’s time spreading properties in a local area.

6. When channel coherence time is less than the time duration of a transmission symbol, then ______ fading occurs.
a) Fast
b) Slow
c) Time selective
d) None of the mentioned

View Answer

Answer: a [Reason:] Fast fading occurs when the channel coherence time is less than the time duration of a transmission symbol.

7. Minimum time required for magnitude change or phase change is called as
a) Shift time
b) Coherence time
c) Delay time
d) None of the mentioned

View Answer

Answer: b [Reason:] Coherence time is the minimum time required for magnitude or phase change of the channel to become uncorrelated from the previous value.

8. When a user’s moving, the user’s velocity causes shift in frequency. This is called as
a) Doppler shift
b) Frequency shift
c) Velocity shift
d) None of the mentioned

View Answer

Answer: a [Reason:] When a user’s moving, the user’s velocity causes shift in frequency of the transmitted signal along each signal path. This is called as Doppler shift.

9. Coherence time is _______ to Doppler spread.
a) Directly proportional
b) Inversely proportional
c) Not dependent
d) None of the mentioned

View Answer

Answer: b [Reason:] Coherence time is inversely proportional to Doppler spread.

10. Block fading occurs in
a) Frequency domain
b) Time domain
c) Frequency & Time domain
d) None of the mentioned

View Answer

Answer: c [Reason:] Block fading can occur in both frequency domain and time domain. This is called as doubly block fading.

11. If coherence bandwidth is smaller than the bandwidth of the signal, _____ fading occurs.
a) Flat
b) Frequency selective
c) Fast fading
d) Time selective

View Answer

Answer: b [Reason:] Frequency selective fading occurs when coherence bandwidth is smaller than the bandwidth of the transmitted signal.

12. Flat fading can be avoided or stopped by
a) Error coding
b) Equalization
c) Adaptive bit loading
d) All of the mentioned

View Answer

Answer: d [Reason:] Flat fading can be combated by means of error coding, equalization and adaptive bit loading.

13. The effect of Doppler shift can be counterattacked by
a) OFDM
b) By using two receivers with separate antennas
c) By using diversity receiver
d) All of the mentioned

View Answer

Answer: d [Reason:] The effect of Doppler shift can be removed by using diversity scheme such as orthogonal frequency division multiplexing, by using two receivers with separate antennas or by using specially designed diversity receiver.

14. When transmitter and receiver moves towards each other, the magnitude of frequency shift is
a) Positive
b) Negative
c) Zero
d) Infinity

View Answer

Answer: a [Reason:] When transmitter and receiver moves towards each other, the magnitude of frequency shift is positive, where as if it moves away from each other then it is negative.

15. If signalling rate is less than fading rate, _____ fading occurs.
a) Slow
b) Fast
c) Time selective
d) None of the mentioned

View Answer

Answer: b [Reason:] A channel is said to be fast fading if symbol rate or signalling rate is less than fading rate.

16. Frequency tracking loop can minimize irreducible error rate by using
a) GMSK
b) DMSK
c) MSK
d) QAM

View Answer

Answer: b [Reason:] Frequency tracking loop can minimize the irreducible error rate in a mobile system by using differential minimum shift keying modulation.

Communications MCQ Set 5

1. In digital communication system, signals in different frequency bands are
a) Orthogonal
b) Non orthogonal
c) Orthogonal & Non orthogonal
d) None of the mentioned

View Answer

Answer: a [Reason:] In digital communication system, signals from different frequency bands are orthogonal thus interference won’t occur.

2. Properties of impulse function are
a) Symmetry
b) Time scaling
c) Shifting
d) All of the mentioned

View Answer

Answer: d [Reason:] Some of the properties of impulse function are symmetry, time scaling and shifting.

3. Properties of Fourier transform are
a) Duality property
b) Time shifting property
c) Modulation property
d) All of the mentioned

View Answer

Answer: d [Reason:] Some of the properties of Fourier transform are duality property, time scaling property, time shifting property, modulation property and many more.

4. A base-band signal can be up-converted using
a) Sine wave
b) Cosine wave
c) Filtering
d) None of the mentioned

View Answer

Answer: b [Reason:] A base-band signal can be up-converted to band-pass filter by applying cosine wave.

5. A band-pass signal can be down-converted using
a) Sine wave
b) Cosine wave
c) Time delayed wave
d) None of the mentioned

View Answer

Answer: b [Reason:] For down-conversion of a band-pass signal also cosine signal is used and multiplied with it.

6. In down-conversion multiplication with cosine wave is followed by
a) Low pass filter
b) High pass filter
c) Bandpass filter
d) Bandstop filter

View Answer

Answer: a [Reason:] Downconversion of bandpass signal includes multiplication with sine wave followed by low pass filtering.

7. ADSL has _____ information channels.
a) One
b) Three
c) Four
d) None of the mentioned

View Answer

Answer: b [Reason:] Asymmetric digital subscriber line has three information channels – a high speed downstream channel, ISDN channel and medium speed duplex channel.

8. Fourier transform of a signal gives the
a) Frequency content
b) Bandwidth
c) Frequency content & Bandwidth
d) None of the mentioned

View Answer

Answer: c [Reason:] Fourier transform of a signal give the frequency content and also determines the bandwidth of the signal.

9. Random things in a communication system are
a) Timing offset
b) Device frequency
c) Attenuation
d) All of the mentioned

View Answer

Answer: d [Reason:] Some of the random things in the communication system are noise in the channel, attenuation, fading, channel filtering, device frequency, phase and timing offset.

10. Which can be used for periodic and non periodic?
a) Fourier series
b) Fourier transforms
c) Fourier series & transforms
d) None of the mentioned

View Answer

Answer: b [Reason:] Fourier series is limited to only periodic signals where as Fourier transforms and laplace transforms can be used for both periodic and non periodic signals.

11. A band-pass signal has a Fourier transform equal to
a) One
b) Zero
c) Infinity
d) Cannot be determined

View Answer

Answer: b [Reason:] A band-pass signal has a Fourier transform equal to zero for all value in both frequency and time domain.

12. A band-pass signal has
a) DC component
b) No DC component
c) No side lobes
d) Cannot be determined

View Answer

Answer: b [Reason:] A band-pass signal has no DC components and has Fourier transform equal to zero. Outside the band it will not be exactly zero. Thus this results in presence of side lobes.

13. Which are orthonormal signal representation?
a) Sine and cosine at same frequency
b) Wavelets
c) Complex sinusoids at different frequency
d) All of the mentioned

View Answer

Answer: d [Reason:] Some of the common orthonormal signal representations are sine and cosine at same frequency, Fourier serier, sinc functions centered at sampling times, wavelets etc.

14. Constellation diagram is plotted in
a) Constellation space
b) Signal space
c) Orthogonal space
d) Boundary space

View Answer

Answer: b [Reason:] The constellation diagram is plotted in a space called as signal space.

15. Cumulative distributive function is
a) Non negative
b) Non decreasing
c) Non negative & decreasing
d) None of the mentioned

View Answer

Answer: c [Reason:] Cumulative distribution function is non negative and non decreasing function.

16. Which are non negative functions?
a) PDF
b) PMF
c) PDF & PMF
d) None of the mentioned

View Answer

Answer: c [Reason:] PDF, PMF and CDF are non negative functions.