Baseband Demodulation and Detection MCQs ( Digital Communications ) MCQs – Digital Communications MCQs
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Baseband Demodulation and Detection MCQs ( Digital Communications ) MCQs – Digital Communications MCQs
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Signals and Noises
1. The term heterodyning refers to
a) Frequency conversion
b) Frequency mixing
c) Frequency conversion & mixing
d) None of the mentioned
Answer: c
Explanation: The method heterodyning means frequency conversion and mixing and this results in a 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
Explanation: The main causes of error performance degradation are interference electrical noise effect of filtering and also due to the surroundings.
3. Thermal noise in the 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
Explanation: 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
Explanation: 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) All of the mentioned
d) None of the mentioned
Answer: b
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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) Thermal noise & Interference from other signals
d) None of the mentioned
Answer: c
Explanation: 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
Explanation: Wireless wide-band channels display multi-path due to time delayed reflections, diffraction and also scattering.
Intersymbol Interference-1
1. The method in which the tail of one pulse smears into adjacent symbol interval is called as
a) Intersymbol interference
b) Interbit interference
c) Interchannel interference
d) None of the mentioned
Answer: a
Explanation: Due to the effect of system filtering the received pulse can overlap on one and another. The tail of one pulse smears into the adjacent symbol interval thereby interfering the detection process. This process is called as intersymbol interference.
2. If each pulse of the sequence to be detected is in _____ shape, the pulse can be detected without ISI.
a) Sine
b) Cosine
c) Sinc
d) None of the mentioned
Answer: c
Explanation: The sinc shaped pulse is the ideal nyquist pulse. If each pulse in the sequence to be detected is in sinc shape the pulses can be detected without ISI.
3. What is symbol rate packing?
a) Maximum possible symbol transmission rate
b) Maximum possible symbol receiving rate
c) Maximum bandwidth
d) Maximum ISI value allowed
Answer: a
Explanation: A system with bandwidth Rs/2 can support a maximum transmission rate of Rs without ISI. Thus for ideal Nyquist filtering the maximum possible symbol transmission rate is called as symbol rate packing and it is equal to 2 symbols/s/Hz.
4. A nyquist pulse is the one which can be represented by _____ shaped pulse multiplied by another time function.
a) Sine
b) Cosine
c) Sinc
d) None of the mentioned
Answer: c
Explanation: A nyquist filter is one whose frequency transfer function can be represented by a rectangular function convolved with any real even symmetric frequency function and a nyquist pulse is one whose shape can be represented by sinc function multiplied by another time function.
5. Examples of nyquist filters are
a) Root raised cosine filter
b) Raised cosine filter
c) Root raised & Raised cosine filter
d) None of the mentioned
Answer: c
Explanation: The most popular among the class of nyquist filters are raised cosine and root raised cosine filter.
6. The minimum nyquist bandwidth for the rectangular spectrum in raised cosine filter is
a) 2T
b) 1/2T
c) T2
d) 2/T
Answer: b
Explanation: For raised cosine spectrum the minimum nyquist bandwidth is equal to 1/2T.
7. Roll off factor is the fraction of
a) Excess bandwidth and absolute bandwidth
b) Excess bandwidth and minimum nyquist bandwidth
c) Absolute bandwidth and minimum nyquist bandwidth
d) None of the mentioned
Answer: b
Explanation: The roll off factor is defined by a fraction of excess bandwidth and the minimum nyquist bandwith. It ranges from 0 to 1.
8. Which value of r (roll off factor) is considered as Nyquist minimum bandwidth case?
a) 0
b) 1
c) Infinity
d) None of the mentioned
Answer: a
Explanation: For the roll off factor of 0 an ideal rectangular nyquist pulse is obtained. This is called as nyquist minimum bandwidth case.
9. A pulse shaping filter should satisfy two requirements. They are
a) Should be realizable
b) Should have proper roll off factor
c) Should be realizable & have proper roll off factor
d) None of the mentioned
Answer: c
Explanation: A pulse shaping filter should provide the desired roll off and should be realizable, that is the impulse response needs to be truncated to a finite length.
10. Examples of double side band signals are
a) ASK
b) PSK
c) ASK & PSK
d) None of the mentioned
Answer: c
Explanation: ASK and PSK needs twice the transmission bandwidth of equivalent baseband signals. Thus these are called as double side band signals.
Intersymbol Interference-2
1. The likelihood ratio test is done between
a) Likelihood of s1 by likelihood of s2
b) Likelihood of s2 by likelihood of s1
c) Likelihood of s1 by likelihood of s1
d) None of the mentioned
Answer: a
Explanation: The likelihood ratio is given with the help of conditional probabilities.
2. According to the rule of minimizing the error probabilities, the hypothesis should be like if the priori probabilities are ________ than the ratio of likelihoods.
a) Lesser
b) Greater
c) Equal
d) None of the mentioned
Answer: a
Explanation: The rule of minimizing the error probabilities show that the hypothesis should be like if the ratio of likelihoods should be greater than the priori probabilities.
3. The detector that minimizes the error probability is called as
a) Maximum likelihood detector
b) Minimum likelihood detector
c) Maximum & Minimum likelihood detector
d) None of the mentioned
Answer: a
Explanation: For the signals that are equally likely, the detector used to minimize the error probability is called as maximum likelihood detector.
4. For a M-ary signal or symbol the number of likelihood functions are
a) M
b) M+1
c) M-1
d) 2M
Answer: a
Explanation: For a M-ary signal there will be M likelihood functions representing M signal classes to which a received signal might belong.
5. An error in binary decision making occurs when the channel noise is
a) Greater than the optimum threshold level
b) Lesser than the optimum threshold level
c) Greater or Lesser than the optimum threshold level
d) None of the mentioned
Answer: c
Explanation: An error will occur when s1(t) or s2(t) is sent and if the channel noise is greater or lesser than the optimum threshold level.
6. Optimum threshold value is given by
a) a0+a1/2
b) a0-a1/2
c) a0/2
d) a1/2
Answer: a
Explanation: The optimum threshold level value for error probability reduction is given by a0+a1/2.
7. The symbol of the probability under the tail of Gaussian pdf is called as
a) Complementary error function
b) Co error function
c) Complementary error & Co error function
d) None of the mentioned
Answer: c
Explanation: The co error function or complementary error function is a commonly used symbol for probability under the tail of Gaussian pdf.
8. Matched filter provides _____ signal to noise ratio.
a) Maximum
b) Minimum
c) Zero
d) Infinity
Answer: a
Explanation: A matched filter is a linear filter designed to give a maximum signal to noise ratio power at the output.
9. The impulse response of the filter is the ________ of the mirror image of the signal waveform.
a) Delayed version
b) Same version
c) Delayed & Same version
d) None of the mentioned
Answer: a
Explanation: The matched filter’s basic property is that the impulse response of the filter is the delayed version of the mirror image of the signal waveform.
10. Example for antipodal bandpass signaling is
a) BPSK
b) ASK
c) FSK
d) MSK
Answer: a
Explanation: One type of antipodal bandpass signaling is binary phase shift keying.
Equalization
1. Channel’s phase response must be a linear function of
a) Time
b) Frequency
c) Time & Frequency
d) None of the mentioned
Answer: b
Explanation: To achieve ideal transmission characteristics, the signal’s bandwidth must be constant and the channel’s phase response must be a linear function of frequency.
2. Amplitude distortion occurs when
a) Impulse response is not constant
b) Impulse response is constant
c) Frequency transfer function is constant
d) Frequency transfer function is not constant
Answer: d
Explanation: When modulus of channel’s transfer function is not constant within W then amplitude distortion occurs.
3. Phase distortion occurs when
a) Phase response is function of frequency
b) Phase response is not a function of frequency
c) All of the mentioned
d) None of the mentioned
Answer: b
Explanation: When channels phase response is not a linear function of frequency within W then phase distortion occurs.
4. Equalization process includes
a) Maximum likelihood sequence estimation
b) Equalization with filters
c) Maximum likelihood sequence estimation & Equalization with filters
d) None of the mentioned
Answer: c
Explanation: Equalization process can be divided into two major categories called as maximum likelihood sequence estimator and equalizer with a filter.
5. The maximum likelihood sequence estimator adjusts _______ according to _____ environment.
a) Receiver, transmitter
b) Transmitter, receiver
c) Receiver, receiver
d) None of the mentioned
Answer: a
Explanation: In maximum likelihood sequence estimator it measures the impulse response value and adjusts the receiver environment according to that of the transmitter.
6. The filters used with the equalizer is of ______ types.
a) Feed forward
b) Feed backward
c) Feed forward and feedback
d) None of the mentioned
Answer: c
Explanation: The filters used with equalizers can be linear devices that contain only feed forward elements or non linear devices with both feed forward and feed back elements.
7. Transversal equalizers are ________ and decision feedback equalizers are ______
a) Feed forward, feed back
b) Feed back, feed forward
c) Feed forward, feed forward
d) Feedback, feedback
Answer: a
Explanation: Linear devices with only feed forward elements are called as transversal equalizers and non linear devices with both feed forward and feed back elements are called as decision feedback equalizers.
8. Symbol spaced has ____ sample per symbol and fractionally spaced has ___ samples per symbol.
a) One, many
b) Many, one
c) One, one
d) Many, many
Answer: a
Explanation: Predetection samples are provided only on symbol boundaries. A condition in which only one sample per symbol are provided is called as symbol spaced and the condition in which multiple samples are provided per symbol is called as fractionally spaced.
9. The _______ of the opening of eye pattern indicates the time over which the sampling for detection might be performed.
a) Length
b) Width
c) X-axis value
d) Y-axis value
Answer: b
Explanation: An eye pattern is a display that results from measuring a system’s response to baseband signals in a prescribed way. The width of the opening gives the time over which the sampling for detection might be performed.
10. Range of time difference of the zero crossing gives the value of
a) Width
b) Distortion
c) Timing jitter
d) Noise margin
Answer: c
Explanation: The time difference of the zero crossing in the eye pattern gives the value of timing jitter.
Equalization and Non Coherent Detection
1. The range of amplitude difference gives the value of
a) Width
b) Distortion
c) Timing jitter
d) Noise margin
Answer: b
Explanation: In the eye pattern, the amplitude difference gives the value of distortion caused by ISI.
2. As the eye opens, ISI _______
a) Increases
b) Decreases
c) Remains the same
d) None of the mentioned
Answer: b
Explanation: As the eye closes, ISI increases and as the eye opens ISI decreases.
3. Pseudo noise signal has _______ and _______ SNR for the same peak transmitted power.
a) Larger, smaller
b) Smaller, larger
c) Larger, larger
d) Smaller, smaller
Answer: c
Explanation: A training pulse is applied to the equalizer and corresponding impulse response is observed. Pseudo noise is preferred as the training pulse as it has larger SNR value and larger average power value.
4. The index value n, in transversal filter can be used as.
a) Time offset
b) Filter coefficient identifier
c) Time offset & Filter coefficient identifier
d) None of the mentioned
Answer: c
Explanation: The index n can be used as both time offset and the filter coefficient identifier, which is the address in the filter.
5. The over-determined set of equations can be solved using
a) Zero forcing
b) Minimum mean square error
c) Zero forcing & Minimum mean square error
d) None of the mentioned
Answer: c
Explanation: The matrix x in transversal equalizer if non square with dimensions 4N+1 and 2N+1. Such equations are called as over-determined set. This can be solved by two methods called as zero forcing method and minimum mean square error method.
6. If the filter’s tap weight remains fixed during transmission of data, then the equalization is called as
a) Preset equalization
b) Adaptive equalization
c) Fixed equalization
d) None of the mentioned
Answer: a
Explanation: If the weight remains fixed during transmission of data then the equalization is called as preset equalization. It is a simple method which consists of setting the tap weight according to some average knowledge of the channel.
7. Equalization method which is done by tracking a slowly time varying channel response is
a) Preset equalization
b) Adaptive equalization
c) Variable equalization
d) None of the mentioned
Answer: b
Explanation: This method is implemented to perform tap weight adjustment periodically or continually. Equalization is done by tracking a slowly varying channel response.
8. Preamble is used for
a) Detect start of transmission
b) To set automatic gain control
c) To align internal clocks
d) All of the mentioned
Answer: d
Explanation: The receiver uses preamble for detecting the start of transmission, to set automatic gain control, and to align internal clocks and local oscillator with the received signal.
9. The disadvantage of preset equalizer is that
a) It does not requires initial training pulse
b) Time varying channel degrades the performance of the system
c) All of the mentioned
d) None of the mentioned
Answer: b
Explanation: The disadvantage of preset equalization is that it requires an initial training period that must be invoked at the start of any new transmission. Also time varying channel can degrade system performance due to ISI, since the tap weights are fixed.
10. For AWGN, the noise variance is
a) N0
b) N0/2
c) 2N0
d) N0/4
Answer: b
Explanation: The noise variance out of the correlator for AWGN is N0/2.
11. Performance of BFSK signal is ________ than BPSK.
a) 3db worse
b) 3db better
c) 6db worse
d) 6db better
Answer: a
Explanation: The performance of BFSK is 3db worse than BPSK signalling, since for a given signal power, the distance squared between orthogonal vectors is a factor of two less than the distance squared between orthopodal signals.
12. A Gaussian distribution into the non linear envelope detector yields
a) Rayleigh distribution
b) Normal distribution
c) Poisson distribution
d) Binary distribution
Answer: a
Explanation: The two output signals of Gaussian distribution yields Rayleigh and Rician distribution.
13. The non coherent FSK needs ________ Eb/N0 than coherent FSK.
a) 1db more
b) 1db less
c) 3db more
d) 3db less
Answer: a
Explanation: The non coherent receiver is easier to implement. The non coherent FSK needs 1db more Eb/N0 than coherent FSK.
14. The DPSK needs ________ Eb/N0 than BPSK.
a) 1db more
b) 1db less
c) 3db more
d) 3db less
Answer: a
Explanation: The DPSK system is easier to implement than PSK and it needs 1db more Eb/N0 than BPSK.
15. Coherent PSK and non coherent orthogonal FSK have a difference of ______ in PB.
a) 1db
b) 3db
c) 4db
d) 6db
Answer: c
Explanation: The difference of PB is approximately 4db for the best ( coherent PSK ) and the worst (non coherent orthogonal FSK).
16. Which is easier to implement and is preferred?
a) Coherent system
b) Non coherent system
c) Coherent & Non coherent system
d) None of the mentioned
Answer: b
Explanation: A non coherent system is desirable because there may be difficulty is establishing and maintaining a coherent reference.
17. Which is the main system consideration?
a) Probability of error
b) System complexity
c) Random fading channel
d) All of the mentioned
Answer: d
Explanation: The major system considerations are error probability, complexity and random fading channel. Considering all this non coherent system is more desirable than coherent.