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## Communications MCQ Set 1

1. The term heterodyning refers to
a) Frequency conversion
b) Frequency mixing
c) Frequency conversion & mixing
d) None of the mentioned

Answer: c [Reason:] The method heterodyning means frequency conversion and mixing and this results in spectral shift.

2. The causes for error performance degradation in communication systems are
a) Interference
b) Electrical noise
c) Effect of filtering
d) All of the mentioned

Answer: d [Reason:] The main causes of error performance degradation are interference electrical noise effect of filtering and also due to the surroundings.

3. Thermal noise in communication system due to thermal electrons
a) Can be eliminated
b) Cannot be eliminated
c) Can be avoided upto some extent
d) None of the mentioned

Answer: b [Reason:] Thermal noise which cannot be eliminated is caused by the motion of thermal electrons causes degradation in system.

4. White noise has _______ power spectral density.
a) Constant
b) Variable
c) Constant & Variable
d) None of the mentioned

Answer: a [Reason:] The AWGN has constant power spectral density.

5. Which are called as hard decisions?
a) Estimates of message symbol with error correcting codes
b) Estimates of message symbol without error correcting codes
c) Both of the mentioned
d) None of the mentioned

Answer: b [Reason:] If error correcting codes are not present, the detector output consists of estimates of the message symbol which is also called as hard decisions.

6. The filter which is used to recover the pulse with less ISI is called as
a) Matched filter
b) Correlator
c) Matched filter & Correlator
d) None of the mentioned

Answer: b [Reason:] The optimum filter used to recover the pulse with best possible signal to noise ratio and less or no ISI is called as correlator or matched filter.

7. The composite equalizing filter is the combination of
a) Receiving and equalizing filter
b) Transmitting and equalizing filter
c) Amplifier and equalizing filter
d) None of the mentioned

Answer: a [Reason:] The functions of both receiving and equalizing filter can be performed by only the equalizing filter alone. Thus equalizing filter is the combination of equalizing and receiving filter.

8. The sample from the demodulation process consists of sample which is _______ to energy of the received symbol and _____ to noise.
a) Directly and inversely proportional
b) Inversely and directly proportional
c) Both directly proportional
d) Both inversely proportional

Answer: a [Reason:] The output symbol of the sampler consists of sample which is directly proportional to the energy of the received signal and inversely proportional to the noise.

9. The average noise power of white noise is
a) 0
b) Infinity
c) 1
d) None of the mentioned

Answer: b [Reason:] White noise is a idealized process with two sided spectral density equal to constant N0/2 and frequencies varying from minus infinity to plus infinity. Thus the average noise power is infinity.

10. The channel may be affected by
a) Thermal noise
b) Interference from other signals
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] A channel can be modelled as a linear filter with additional noise. The noise comes from thermal noise source and also from interference from other signals.

11. Channels display multi-path due to
a) Scattering
b) Time delayed reflections
c) Diffraction
d) All of the mentioned

Answer: d [Reason:] Wireless wide-band channels display multi-path due to time delayed reflections, diffraction and also scattering.

## Communications MCQ Set 2

1. The main sources of corruption are
a) Sampling and quantizing effects
b) Channel effects
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] The analog signal obtained from sampling quantization and transmitted pulses will have corruption from several sources in which the two main sources are quantizing and sampling effect and channel effect.

2. The distortion in quantization is called as
a) Round off error
b) Truncation error
c) Round off & Truncation error
d) None of the mentioned

Answer: c [Reason:] After sampling and quantization of input signals, the ouput sampled sequence consists of some distortion which can be called as round off error or truncation error.

3. In quantization process, the amount of quantization noise is _______________ to number of levels.
a) Directly proportional
b) Inversely proportional
c) Independent
d) None of the mentioned

Answer: b [Reason:] The distortion introduced to approximate the analog signal is called as quantization noise. The amount of this noise is inversely proportional to number of levels employed in quantization process.

4. Saturation noises can be avoided or reduced by
a) Automatic gain control
b) Amplifying
c) Filtering
d) None of the mentioned

Answer: a [Reason:] When the difference between input and output signal increases, we say that analog to digital converter is working in saturation. This introduces saturation noise or error. This can be reduced by using automatic gain control.

5. Timing jitter can be reduced by
a) Good power supply isolation
b) Stable clock reference
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] Jitter occurs when there is a slight position change in the sampled signals. This timing jitter can be controlld by power supply isolation and clock reference.

6. The reasons for threshold effect are
a) Thermal noise
b) Interference from other users
c) Interference from circuit switching transients
d) All of the mentioned

Answer: d [Reason:] The channel noise which is caused by thermal noise interference from other users and circuit switching transients is called as threshold effect.

7. When channel bandwidth is greater than the pulse bandwidth , it causes
a) Intersignal interference
b) Intersymbol interference
c) Bandwidth error
d) None of the mentioned

Answer:b [Reason:] When channel bandwidth is greater than pulse bandwidth, the signal widens and expands exceeding the symbol duration which causes intersymbol interference.

8. The _____________ corresponds to average quantization noise power.
a) Mean
b) Variance
c) Probability density function
d) None of the mentioned

Answer: b [Reason:] The variance corresponds to average quantization noise power. It is calculated assuming the quantization noise and probability distribution function.

9. Signal to noise ratio increases as ___________ increases .
a) Quantization level
b) Square of quantization level
c) Square root of quantization level
d) None of the mentioned

Answer: b [Reason:] On calculating the number of levels , quantization error and power and also signal to noise ration we can find that signal to noise ratio depends directly on square of number of quantization levels.

10. Signal to noise ratio is infinite when
a) Quantization noise is zero
b) Number of levels are infinite
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] In the limit L tends to infinity and signal to quantization noise ratio tends to infinity when quantization levels are infinite and quantization noise is zero.

11. The ratio of average signal power and quantization noise is
a) 3L2
b) L2/3
c) 2L3
d) L3/2

Answer: a [Reason:] On calculating the signal power and the quantization noise , and on taking its ratio it depends on the number of quantization level L and we get as 3L2.

## Communications MCQ Set 3

1. Some advantages of spread spectrum are
a) Low susceptibility
b) Immunity to jamming
c) Reduced interference
d) All of the mentioned

Answer: d [Reason:] Advantages of spread spectrum are low susceptibility, reduced interference, immunity to jamming and co existence of multi path system.

2. Processing gain is the ratio of message bandwidth to signal bandwidth.
a) True
b) False

Answer: b [Reason:] Processing gain is the ratio of signal bandwidth to message bandwidth.

3. Which is better for avoiding jamming?
a) Direct sequence spread spectrum
b) Frequency hopping spread spectrum
c) Time hopping spread spectrum
d) None of the mentioned

Answer: b [Reason:] Frequency hopping spread spectrum is better for avoiding jamming.

4. Which is more bandwidth efficient?
a) Direct sequence spread spectrum
b) Frequency hopping spread spectrum
c) Time hopping spread spectrum
d) None of the mentioned

Answer: c [Reason:] Time hopping spread spectrum is more bandwidth efficient.

5. Which is more simpler to implement?
a) Direct sequence spread spectrum
b) Frequency hopping spread spectrum
c) Time hopping spread spectrum
d) None of the mentioned

Answer: a [Reason:] Direct sequence spread spectrum is more simpler to implement.

6. Which uses orthogonal codes?
a) Synchronous CDMA
b) Asynchronous CDMA
c) Synchronous & Asynchronous CDMA
d) None of the mentioned

Answer: a [Reason:] Synchronous CDMA uses orthogonal codes and asynchronous CDMA uses pseudorandom codes.

7. Which is more suitable when large number of transmitters are used?
a) Synchronous CDMA
b) Asynchronous CDMA
c) Synchronous & Asynchronous CDMA
d) None of the mentioned

Answer: b [Reason:] Asynchronous CDMA is more suitable for large number of transmitters.

8. CDMA rejects
a) Narrow band interference
b) Wide band interference
c) Narrow & Wide band interference
d) None of the mentioned

Answer: a [Reason:] CDMA effectively rejects narrow band interference.

9. Frequency planning is very essential in
a) FDMA
b) TDMA
c) FDMA & TDMA
d) None of the mentioned

Answer: c [Reason:] Frequency planning is necessary for both TDMA and FDMA.

10. CDMA uses
a) Hard hand off
b) Soft hand off
c) Hard & Soft hand off
d) None of the mentioned

Answer: b [Reason:] CDMA uses soft hand off which provides more reliable communication.

## Communications MCQ Set 4

1. Structured sequences are
a) Block codes
b) Convolutional codes
c) Turbo codes
d) All of the mentioned

Answer: d [Reason:] Structured sequences are those which add structured redundancy to source code. Examples are turbo codes, block codes, and convolutional codes.

2. In binary symmetric channel, in the demodulator _____ decision is made.
a) Hard
b) Soft
c) Hard & Soft
d) None of the mentioned

Answer: c [Reason:] In binary symmetric channel the demodulator consists of only two discrete elements. Thus it makes hard or firm decision.

3. In Gaussian channel demodulator ______ decision is made.
a) Hard
b) Soft
c) Hard & Soft
d) None of the mentioned

Answer: b [Reason:] In Gaussian channel demodulator consists of continuous alphabet or quantized approximation thus soft decision is made.

4. Which decoders are less complex?
a) Soft decision decoder
b) Hard decision decoder
c) Hard & Soft decision decoder
d) None of the mentioned

Answer: b [Reason:] The design of soft decision decoder is more complex than the design of hard decision decoder.

5. The ratio of redundant bits to data bits is called as
a) Code rate
b) Redundancy rate
c) Symbol rate
d) Transmission rate

Answer: b [Reason:] The ratio of redundant bits to data bits is called as redundancy of the code and the ratio of data bits to total bits is called as code rate.

6. If the parity bit takes value one then the summation of code-word gives
a) Even result
b) Odd result
c) Even & Odd result
d) None of the mentioned

Answer: a [Reason:] The parity bit takes on the value of one or zero to ensure that the summation of all the bits in the code-word yields even or odd result.

7. The summation operation is performed using _____ logical operation.
a) EX-OR
b) AND
c) OR
d) EX-NOR

Answer: a [Reason:] The summation operation is performed using modulo 2 arithmetic or exclusive OR operation.

8. The probability of message error is written as
a) Block error
b) Word error
c) Block & Word error
d) None of the mentioned

Answer: c [Reason:] We can write the probability of message error also called as block error or word error.

9. In a real time communication system addition of redundant bits leads to
a) More bandwidth requirement
b) Faster rate of transmission
c) Both of the mentioned
d) None of the mentioned

Answer: c [Reason:] In a real time communication system the addition of redundant bits dictates a faster transmission rate and which means more bandwidth.

10. Coding gain is defined as
a) Reduction in Eb/N0
b) Increase in Eb/N0
c) Same as Eb/N0
d) None of the mentioned

Answer: a [Reason:] For a given bit error probability coding gain can be defined as reduction in Eb/N0 that can be realized through the use of code.

11. The capacity per cell is ______ Eb/N0.
a) Directly proportional
b) Inversely proportional
c) Double the
d) Half the

Answer: b [Reason:] The capacity that is the number of users per cell is inversely proportional to Eb/N0.

## Communications MCQ Set 5

1. Which is based on orthogonality?
a) TDM
b) FDM
c) TDM & FDM
d) None of the mentioned

Answer: b [Reason:] FDM technique is based on orthogonality of sinusoids.

2. Which provides constant delay?
a) Synchronous TDM
b) Non synchronous TDM
c) Synchronous & Non synchronous TDM
d) None of the mentioned

Answer: a [Reason:] Synchronous time division multiplexing provides constant bandwidth and constant delay.

3. Random access is
a) Distributed
b) Fault tolerant
c) Distributed & Fault tolerant
d) None of the mentioned

Answer: c [Reason:] Random access is simple, distributed and very fault tolerant.

4. Example for reservation system is
a) Synchronous TDM
b) Non synchronous TDM
c) Synchronous & Non synchronous TDM
d) None of the mentioned

Answer: a [Reason:] Synchronous TDM is an example for reservation system.

5. Reservation systems are useful when
a) Delay is small
b) Delay is large
c) Delay is small or large
d) None of the mentioned

Answer: b [Reason:] Reservation systems are useful when delays are large.

6. In slotted system, access is
a) Synchronous
b) Asynchronous
c) Synchronous & Asynchronous
d) None of the mentioned

Answer: b [Reason:] In slotted system, access is asynchronous and requires addressing information inside.

7. In CSMA, collision window is _____ to cable length.
a) Equal
b) Greater
c) Lesser
d) None of the mentioned

Answer: b [Reason:] The collision window is twice the cable length.

8. Which isolates collision?
a) Bridges
b) Routers
c) Bridges & Routers
d) None of the mentioned

Answer: c [Reason:] Bridges and routers isolates collision.

9. Minimum packet size increases as
a) Speed increases
b) Distance increases
c) Speed & Distance increases
d) None of the mentioned