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Communication Systems MCQs

Practice Communication Systems MCQs from fundamental concepts through intermediate, advanced and application-oriented questions. Each page contains up to 100 questions with answers and explanations where available.

Question 106
Communication SystemsFM Reception

The capture effect in FM refers to the tendency of an FM receiver to?

  • ADemodulate two equal signals perfectly at once
  • BPrefer the stronger of two signals at or near the same frequency
  • CConvert FM directly to PCM
  • DReject the stronger signal
Report an error
Correct Answer: B. Prefer the stronger of two signals at or near the same frequency
Explanation: When two FM signals compete on nearly the same channel, a sufficiently stronger signal can dominate the receiver output.
Reference: Standard communication systems: FM capture effect
Question 107
Communication SystemsReceivers

In a superheterodyne receiver, increasing the intermediate frequency generally improves?

  • AImage-frequency rejection for a given RF selectivity
  • BAudio output power automatically
  • CModulation index of the transmitter
  • DSampling resolution
Report an error
Correct Answer: A. Image-frequency rejection for a given RF selectivity
Explanation: A higher IF increases image separation from the desired RF signal, which can make image rejection easier for a given RF front-end selectivity.
Reference: Standard communication systems: IF trade-off
Question 110
Communication SystemsDSB-SC & SSB

The phase-shift method of SSB generation ideally requires two versions of the message signal differing in phase by?

  • A45 degrees
  • B90 degrees
  • C180 degrees
  • D360 degrees
Report an error
Correct Answer: B. 90 degrees
Explanation: The phase-shift method uses quadrature message components and quadrature carrier components so one sideband cancels while the other adds.
Reference: Standard communication systems: phase-shift SSB generation
Question 111
Communication SystemsVSB Modulation

A vestigial-sideband signal transmits?

  • ABoth complete sidebands with full carrier only
  • BOne complete sideband plus a vestige of the other
  • CCarrier only
  • DNo sideband components
Report an error
Correct Answer: B. One complete sideband plus a vestige of the other
Explanation: VSB retains one full sideband and a small portion of the other, reducing bandwidth while easing practical filtering.
Reference: Standard communication systems: VSB
Question 112
Communication SystemsVSB Modulation

VSB modulation is widely associated with television video transmission mainly because it?

  • ARequires zero bandwidth
  • BProvides a compromise between SSB bandwidth saving and practical filtering
  • CEliminates all noise
  • DNeeds no carrier recovery under any implementation
Report an error
Correct Answer: B. Provides a compromise between SSB bandwidth saving and practical filtering
Explanation: Video contains significant low-frequency content, making sharp SSB filtering difficult; VSB offers practical filtering with reduced bandwidth.
Reference: Standard communication systems: VSB application
Question 115
Communication SystemsFrequency Modulation

For narrowband FM, the modulation index beta is generally?

  • AMuch less than 1
  • BExactly 1 in every case
  • CMuch greater than 1
  • DInfinite
Report an error
Correct Answer: A. Much less than 1
Explanation: Narrowband FM is characterized by a small modulation index, beta<<1, so higher-order sidebands are negligible.
Reference: Standard communication systems: narrowband FM
Question 116
Communication SystemsAngle Modulation

In an ideal angle-modulated signal, the total transmitted power is essentially independent of?

  • ACarrier amplitude
  • BMessage-induced frequency or phase variation
  • CLoad resistance in every transmitter
  • DAntenna efficiency
Report an error
Correct Answer: B. Message-induced frequency or phase variation
Explanation: FM and PM have constant envelope in the ideal model, so total average signal power does not vary with modulation index when carrier amplitude is fixed.
Reference: Standard communication systems: constant-envelope power
Question 117
Communication SystemsReceivers

In a superheterodyne receiver, using a lower intermediate frequency generally makes it easier to obtain?

  • ABetter adjacent-channel selectivity
  • BLarger image separation
  • CHigher antenna gain
  • DHigher transmitted power
Report an error
Correct Answer: A. Better adjacent-channel selectivity
Explanation: A lower IF permits narrower practical IF filters and can improve selectivity, although it worsens image-frequency separation.
Reference: Standard communication systems: IF selectivity trade-off
Question 122
Communication SystemsNoise in Communication Systems

For cascaded stages, Friis’ formula shows that the overall receiver noise performance is strongly influenced by?

  • AThe noise factor and gain of the first stage
  • BOnly the last-stage output power
  • CAntenna color
  • DAudio-frequency volume setting
Report an error
Correct Answer: A. The noise factor and gain of the first stage
Explanation: In Friis' cascade formula, noise contributions of later stages are divided by preceding gains, making a low-noise high-gain first stage especially important.
Reference: Standard communication systems: Friis formula
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Question 127
Communication SystemsInformation Theory

The mutual information I(X;Y) is zero when X and Y are statistically?

  • AIdentical in every sample
  • BIndependent
  • CGaussian only
  • DPeriodic
Report an error
Correct Answer: B. Independent
Explanation: For statistically independent random variables, observing Y provides no information about X, so mutual information is zero.
Reference: Standard communication systems: mutual information
Question 128
Communication SystemsInformation Theory

For a discrete memoryless channel, channel capacity is the maximum of mutual information I(X;Y) over?

  • AInput probability distributions
  • BCarrier frequencies only
  • CReceiver temperatures only
  • DSampling phases only
Report an error
Correct Answer: A. Input probability distributions
Explanation: Channel capacity is obtained by maximizing the input-output mutual information over permissible input distributions.
Reference: Standard communication systems: channel capacity definition
Question 133
Communication SystemsSource Coding

The main objective of source coding is to?

  • AReduce statistical redundancy in the source representation
  • BAdd controlled redundancy for error correction
  • CIncrease receiver thermal noise
  • DCreate image-frequency interference
Report an error
Correct Answer: A. Reduce statistical redundancy in the source representation
Explanation: Source coding represents information efficiently by reducing removable redundancy.
Reference: Standard communication systems: source coding
Question 134
Communication SystemsChannel Coding

The main objective of channel coding is to?

  • ARemove every redundant bit
  • BIntroduce structured redundancy to improve error detection or correction
  • CSuppress the RF carrier only
  • DReduce sampling rate below Nyquist
Report an error
Correct Answer: B. Introduce structured redundancy to improve error detection or correction
Explanation: Channel coding deliberately adds controlled redundancy so the receiver can detect or correct transmission errors.
Reference: Standard communication systems: channel coding
Question 135
Communication SystemsSource Coding

A prefix code has the useful property that?

  • ANo codeword is a prefix of another codeword
  • BEvery codeword has exactly one bit
  • CAll source probabilities must be equal
  • DIt cannot be decoded sequentially
Report an error
Correct Answer: A. No codeword is a prefix of another codeword
Explanation: The prefix condition permits instantaneous decoding because a complete codeword cannot be confused with the beginning of another.
Reference: Standard communication systems: prefix code
Question 136
Communication SystemsSource Coding

Huffman coding assigns shorter codewords generally to symbols with?

  • ALower probabilities
  • BHigher probabilities
  • CZero probability only
  • DLonger analog durations
Report an error
Correct Answer: B. Higher probabilities
Explanation: Huffman coding minimizes average codeword length by assigning shorter words to more probable symbols.
Reference: Standard communication systems: Huffman coding
Question 140
Communication SystemsChannel Coding

For a systematic linear block code, a codeword contains?

  • AOnly parity bits
  • BOriginal information bits explicitly plus parity bits
  • COnly carrier samples
  • DNo redundancy
Report an error
Correct Answer: B. Original information bits explicitly plus parity bits
Explanation: In systematic form, the information bits appear unchanged in specified codeword positions and parity bits occupy the remaining positions.
Reference: Standard communication systems: systematic block code
Question 141
Communication SystemsChannel Coding

For an (n,k) block code, the code rate is?

  • An/k
  • Bk/n
  • Cn-k
  • D1/(n+k)
Report an error
Correct Answer: B. k/n
Explanation: A block code maps k information bits into n coded bits, so its rate is R=k/n.
Reference: Standard communication systems: code rate
Question 144
Communication SystemsChannel Coding

In polynomial representation of cyclic codes, multiplication by x followed by reduction modulo x^n-1 corresponds to?

  • AA cyclic shift
  • BAmplitude clipping
  • CSampling
  • DFrequency translation only
Report an error
Correct Answer: A. A cyclic shift
Explanation: Multiplication by x shifts polynomial coefficients; modulo x^n-1 wraps the end coefficient around, implementing a cyclic shift.
Reference: Standard communication systems: cyclic-code polynomial representation
Question 145
Communication SystemsChannel Coding

The Viterbi algorithm is widely used for?

  • AMaximum-likelihood sequence decoding of convolutional codes
  • BAM envelope detection
  • CFM generation by reactance modulation
  • DAnalog source compression only
Report an error
Correct Answer: A. Maximum-likelihood sequence decoding of convolutional codes
Explanation: The Viterbi algorithm searches a trellis efficiently for the most likely transmitted state sequence of a convolutional code.
Reference: Standard communication systems: Viterbi decoding
Question 146
Communication SystemsChannel Coding

In a trellis representation of a convolutional code, a path through the trellis represents?

  • AA possible encoder state and output sequence over time
  • BOnly receiver bandwidth
  • CA single analog carrier cycle
  • DThermal-noise temperature
Report an error
Correct Answer: A. A possible encoder state and output sequence over time
Explanation: Each trellis path corresponds to a possible sequence of encoder state transitions and associated coded outputs.
Reference: Standard communication systems: convolutional-code trellis
Question 147
Communication SystemsError Control

Interleaving in a digital communication system is especially useful for combating?

  • ABurst errors
  • BCarrier generation
  • CQuantization itself
  • DAntenna polarization mismatch only
Report an error
Correct Answer: A. Burst errors
Explanation: Interleaving spreads adjacent corrupted bits or symbols across different codewords so a burst can appear as more manageable separated errors to the decoder.
Reference: Standard communication systems: interleaving
Question 148
Communication SystemsError Control

A burst-error channel corrupts 12 consecutive coded bits. An idealized block interleaver primarily helps by?

  • ASpreading those errors across separated decoder positions
  • BIncreasing the RF carrier power automatically
  • CChanging every 0 into 1
  • DRemoving the need for channel coding
Report an error
Correct Answer: A. Spreading those errors across separated decoder positions
Explanation: The interleaver rearranges transmission order, so consecutive channel errors are dispersed after deinterleaving.
Reference: Standard communication systems: burst-error interleaving
Question 149
Communication SystemsError Control

Automatic repeat request (ARQ) primarily combines error detection with?

  • ARetransmission requests
  • BAnalog frequency modulation
  • CSource entropy maximization
  • DCarrier suppression
Report an error
Correct Answer: A. Retransmission requests
Explanation: ARQ detects erroneous or missing blocks and requests retransmission rather than relying solely on forward correction.
Reference: Standard communication systems: ARQ
Question 150
Communication SystemsError Control

Forward error correction (FEC) differs from basic ARQ because FEC aims to?

  • ACorrect errors at the receiver without requiring retransmission for each correctable error
  • BEliminate all redundancy
  • COperate only on analog AM signals
  • DMake channel noise exactly zero
Report an error
Correct Answer: A. Correct errors at the receiver without requiring retransmission for each correctable error
Explanation: FEC adds sufficient coding redundancy for the receiver to correct specified error patterns without requesting retransmission.
Reference: Standard communication systems: forward error correction
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Question 151
Communication SystemsChannel Coding

A convolutional encoder is characterized by the fact that its output depends on?

  • AOnly the present input bit
  • BPresent input and encoder memory state
  • COnly receiver noise
  • DCarrier amplitude alone
Report an error
Correct Answer: B. Present input and encoder memory state
Explanation: Convolutional coding has memory: coded outputs depend on the current input and previous input information stored in the encoder state.
Reference: Standard communication systems: convolutional coding
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