Wcn-2
Topic:on algorithms.
1)Which of the following factors could not determine the performance of the algorithm?
a) Structural properties
b) Rate of convergence
c) Computational complexity
d) Numerical properties.
Answer: a
Explanation: The performance of an algorithm is determined by various factors. These
factors are rate of convergence, computational complexity and numerical properties. The
performance of algorithm does not depend on structural properties.
2)Rate of convergence is defined by _____ of algorithm.
a) Time span
b) Number of iterations
c) Accuracy
d) Complexity.
Answer: b
Explanation: Rate of convergence is required as number of iterations required for the
algorithm to converge close enough to the optimum solution. It enables the algorithm to
track statistical variations when operating in non stationary environment.
3)Computational complexity is a measure of ________
a) Time
b) Number of iterations
c) Number of operations
d) Accuracy
Answer: c
Explanation: Computational complexity is the number of operations required to make
one complete iteration of the algorithm. It helps in comparing the performance with other
algorithms.
4)Choice of equalizer structure and its algorithm is not dependent on ________
a) Cost of computing platform.
b) Power budget.
c) Radio propagation characteristics.
d) Statistical distribution of transmitted power.
Answer: d
Explanation: The cost of the computing platform, the power budget and the radio
propagation characteristics dominate the choice of an equalizer structure and its
algorithm. Battery drain at the subscriber unit is also a paramount consideration.
5. Coherence time is dependent on the choice of the algorithm and corresponding rate of
convergence.
a) True
b) False
Answer: a
Explanation: The choice of algorithm and its corresponding rate of convergence depends
on the channel data rate and coherence time. The speed of the mobile unit determines
the channel fading rate and the Doppler spread, which is directly related to coherence
time of the channel.
6. Which of the following is an algorithm for equalizer?
a) Zero forcing algorithm
b) Least mean square algorithm
c) Recursive least square algorithm
d) Mean square error algorithm
Answer: d
Explanation: Three classic equalizer algorithm are zero forcing (ZF) algorithm, least
mean squares (LMS) algorithm and recursive least squares (RLS) algorithm. They offer
fundamental insight into algorithm design and operation.
7) Which of the following is a drawback of zero forcing algorithm?
a) Long training sequence
b) Amplification of noise
c) Not suitable for static channels
d) Non zero ISI
Answer: b
Explanation: The zero forcing algorithm has the disadvantage that the inverse filter may
excessively amplify noise at frequencies where the folded channel spectrum has high
attenuation.
8. Zero forcing algorithm performs well for wireless links. State whether True or False.
a) True
b) False
Answer: b
Explanation: ZF is not often used in wireless links as it neglects the effect of noise
altogether. However, it performs well for static channels with high SNR, such as local
wired telephone links.
9. LMS equalizer minimizes __________
a) Computational complexity
b) Cost
c) Mean square error
d) Power density of output signal
Answer: c
Explanation: LMS equalizer is a robust equalizer. It is used to minimize mean square
error (MSE) between the desired equalizer output and the actual equalizer output.
10. For N symbol inputs, LMS algorithm requires ______ operations per iterations.
a) 2N
b) N+1
c) 2N+1
d) N2
Answer: c
Explanation: The LMS algorithm is the simplest algorithm. For N symbol inputs, it
requires only 2N+1 operations per iteration.
11. Stochastic gradient algorithm is also called ________-
a) Zero forcing algorithm
b) Least mean square algorithm
c) Recursive least square algorithm
d) Mean square error algorithm
Answer: b
Explanation: The minimization of the MSE is carried out recursively, and it can be
performed by the use of stochastic gradient algorithm. This more commonly called the
least mean square (LMS) algorithm.
12. Convergence rate of LMS is fast. State whether True or False.
a) True
b) False
Answer: b
Explanation: The convergence rate of the LMS algorithm is slow. It is slow due to the
fact that it uses only one parameter i.e. step size that control the adaptation rate.
13. Which of the following does not hold true for RLS algorithms?
a) Complex
b) Adaptive signal processing
c) Slow convergence rate
d) Powerful
Answer: c
Explanation: Recursive least square (RLS) algorithm uses fast convergence rate as
opposed to LMS algorithms. They are powerful, albeit complex, adaptive signal
processing techniques which significantly improves the convergence of adaptive
equalizer.
14. Which of the following algorithm uses simple programming?
a) LMS Gradient DFE
b) FTF algorithm
c) Fast Kalman DFE
d) Gradient Lattice DFE
Answer: a
Explanation: Advantages of LMS gradient DFE algorithm are low computational
complexity and simple programming. While fast transversal filter (FTF) algorithm, Fast
Kalman DFE and gradient lattice DFE uses complex programming.
TOPIC ON:EQUALIZATION:
This set of Wireless & Mobile Communications Multiple Choice Questions & Answers
(MCQs) focuses on “Fundamentals of Equalization”.
1. Which of the following is not used to improve received signal quality over small scale
times and distance?
a) Modulation
b) Equalization
c) Diversity
d) Channel coding
View Answer
Answer: a
Explanation: Equalization, diversity and channel coding are the three techniques
which are used to improve received signal quality and link performance over small
scale times and distance. But, the approach, cost, complexity and effectiveness varies
in wireless communication system.
2. Equalization is used to compensate __________
a) Peak signal to noise ratio
b) Intersymbol interference
c) Channel fading
d) Noises present in the signal
View Answer
Answer: b
Explanation: Equalization compensate the intersymbol interference (ISI) created by
multipath within time dispersive channels. An equalizer within a receiver compensates
for the average range of the expected channel amplitude and delay characteristics.
3. Training and tracking are the operating modes of _________
a) Diversity techniques
b) Channel coding techniques
c) Equalization techniques
d) Demodulation techniques
View Answer
Answer: c
Explanation: General operating modes of an adaptive equalizer includes training and
tracking. A known fixed length training sequence is sent by the transmitter so that the
receiver’s equalizer may adapt to a proper setting for minimum bit error rate detection.
4. An equalizer is said to be converged when it is properly _______
a) Trained
b) Tracked
c) Installed
d) Used
View Answer
Answer: a
Explanation: When an equalizer has been properly trained, it is said to have
converged. Equalizers are commonly used in digital communication systems where
user data is segmented into short time blocks or time slots.
5. Time for convergence of an equalizer is not a function of _______
a) Equalizer algorithm
b) Equalizer structure
c) Time rate of change of multipath radio channel
d) Transmitter characteristics
View Answer
Answer: d
Explanation: The timespan over which an equalizer converges is a function of the
equalizer algorithm, the equalizer structure and the time rate of change of the
multipath radio channel. Equalizers require proper retraining in order to maintain
effective ISI cancellation.
6. Equalizer is usually implemented in __________
a) Transmitter
b) Baseband or at IF in a receiver
c) Radio channel
d) Modulator stage
View Answer
Answer: b
Explanation: An equalizer is implemented at baseband or at IF in a receiver. Since,
the baseband complex envelope expression can be used to represent bandpass
waveform, the channel response, the demodulated signal and adaptive equalizer
algorithm are usually simulated and implemented at baseband.
7. Equalizer is ________ of the channel.
a) Opposite
b) Same characteristics
c) Inverse filter
d) Add on
View Answer
Answer: c
Explanation: An equalizer is actually an inverse filter of the channel. The goal of
equalization is to satisfy that the combination of the transmitter, channel and receiver
be an all pass channel.
8. ______ controls the adaptive algorithm in an equalizer.
a) Error signal
b) Transmitted signal
c) Received signal
d) Channel impulse response
View Answer
Answer: a
Explanation: The adaptive algorithm is controlled by the error signal. The error signal
is derived by comparing the output of the equalizer and some signal which is either an
exact scaled replica of the transmitted signal or represents a property of transmitted
signal.
9. The adaptive algorithms in equalizer that do not require training sequence are called
________
a) Linear adaptive algorithms
b) Blind algorithms
c) Non-linear adaptive algorithms
d) Spatially adaptive algorithms
View Answer
Answer: b
Explanation: Blind algorithms exploit the characteristics of the transmitted signal and
do not require training sequence. These type of algorithm are able to acquire
equalization through property restoral techniques of transmitted signal.
10. Which of the following is a blind algorithm?
a) Linear adaptive algorithms
b) Constant modulus algorithm
c) Non-linear adaptive algorithms
d) Spatially adaptive algorithms
View Answer
Answer: b
Explanation: Blind algorithm technique uses algorithms such as the constant modulus
algorithm (CMA) and the spectral coherence restoral algorithm (SCORE). CMA is
used for constant envelope modulation and forces the equalizer weights to maintain a
constant envelope on the received signal.
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This set of Wireless & Mobile Communications Multiple Choice Questions & Answers
(MCQs) focuses on “Equalization Techniques”.
1. Equalization techniques can be categorised into _______ and ______ techniques.
a) Linear, non linear
b) Active, passive
c) Direct, indirect
d) Slow, fast
View Answer
Answer: a
Explanation: Equalization techniques can be classified into linear and non linear
techniques. These categories are determined from how the output of a adaptive
equalizer is used for subsequent control of the equalizer.
2. Equalization is linear, if an analog signal is fed back to change the subsequent
outputs of the equalizer. State whether True or False.
a) True
b) False
View Answer
Answer: b
Explanation: If the analog signal is not used in the feedback path to adapt the
equalizer, the equalization is linear. On the other hand, if it is fed back to change the
subsequent ouputs of the equalizer, the equalization is non-linear.
3. In the context of equalizers, LTE stands for ________
a) Long transversal equalizer
b) Least time-varying equalizer
c) Linear transversal equalizer
d) Linear time-varying equalizer
View Answer
Answer: c
Explanation: The most common equalizer structure used for equalization is linear
transveral equalizer (LTE). It is made up of tapped delay lines, with the tappings
speed a symbol period apart.
4. Which of the following is not a characteristic of FIR filter?
a) Many zeroes
b) Poles only at z=0
c) Transfer function is a polynomial of z-1
d) Many poles
View Answer
Answer: d
Explanation: Finite impulse response filter (FIR) has many zeroes but poles only at
z=0. The transfer function of the filter is a polynomial of z-1. It is also referred to as
transversal filter.
5. Which of the following is not an advantage of lattice equalizer?
a) Simple structure
b) Numerical stability
c) Faster convergence
d) Dynamic assignment
View Answer
Answer: a
Explanation: The structure of lattice equalizer is more complicated than a linear
transversal equalizer. But, numerical stability and faster convergence are two
advantage of laatic equalizer. Also, its unique structure allows dynamic assignment of
the most effective length of lattice equalizer.
6. Non-linear equalizers are used in applications where channel distortion is not severe.
a) True
b) False
View Answer
Answer: b
Explanation: Non-linear equalizers are used in applications where the channel
distortion is too severe for a linear equalizer to handle. They are most commonly used
in practical wireless communication.
7. Which of the following is not a non-linear equalization technique?
a) Decision feedback equalization
b) Maximum likelihood symbol detection
c) Minimum square error detection
d) Maximum likelihood sequence detection
View Answer
Answer: c
Explanation: Decision feedback equalization, maximum likelihood symbol detection
and maximum likelihood sequence detection offers non-linear equalization. They offer
improvements over linear equalization techniques and are used in most 2G and 3G
systems.
8. For a distorted channel, LTE performance is superior to DFE. State whether True or
False.
a) True
b) False
View Answer
Answer: b
Explanation: LTE is well behaved when channel spectrum is comparatively flat. But if
the channel is severely distorted or exhibits null in the spectrum, the performance of
LTE deteriorates and MSE of DFE is better than LTE.
9. Which of the following does not hold true for MLSE?
a) Minimizes probability of sequence error
b) Require knowledge of channel characteristics
c) Requires the statistical distribution of noise
d) Operates on continuous time signal
View Answer
Answer: d
Explanation: Matched filter operates on the continuous time signal, whereas maximum
likelihood sequence estimation (MLSE) equalizer and channel estimator rely on
discretized samples. MLSE is optimal in the sense that it minimizes the probability of
sequence error.
10. MLSE decodes each received signal by itself.
a) True
b) False
View Answer
Answer: b
Explanation: Rather than decoding each received signal by itself, MLSE tests all
possible data sequences by using a channel impulse response simulator within the
algorithm. It chooses the data sequence with maximum probability as the output.
This set of Wireless & Mobile Communications Multiple Choice Questions & Answers
(MCQs) focuses on “Diversity Techniques”.
1. Diversity requires training sequence. State whether True or False.
a) True
b) False
View Answer
ANS:FALSE
2. Diversity decisions are made by ______
a) Receiver
b) Transmitter
c) Channel
d) Adaptive algorithms
View Answer:RECEIVER
3. Small scale fades are characterized by _____ amplitude fluctuations.
a) Large
b) Small
c) Rapid
d) Slow
View Answer ANS:RAPID
4. ______ is used to prevent deep fade for rapidly varying channel.
a) Modulation
b) Demodulation
c) Macroscopic diversity technique
d) Microscopic diversity technique
View Answer:D)MICROSCOPIC DIVERSITY TECHNIQUE.
5. Large scale fading can be mitigated with the help of _________
a) Modulation
b) Demodulation
c) Macroscopic diversity technique
d) Microscopic diversity technique
View Answer ANS: Macroscopic diversity technique
6. Space diversity s also known as ________
a) Antenna diversity
b) Time diversity
c) Frequency diversity
d) Polarization diversity
View Answer:Antenna diversity
7. Which of the following is not a category of space diversity technique?
a) Selection diversity
b) Time diversity
c) Feedback diversity
d) Equal gain diversity
View Answer
Answer: b
Explanation: Space diversity reception methods can be classified into four categories.
They are selection diversity, feedback diversity, maximal ratio combining and equal
gain diversity.
8. In selection diversity, the gain of each diversity branch provides different SNR. State
whether True or False.
a) True
b) False
View Answer :FALSE
9. Polarization diversity uses the ________ as the diversity element.
a) Modulation index
b) Carrier frequency
c) Reflection coefficient
d) Coherence time
View Answer
Answer: c
Explanation: Decorrelation of the signal in each polarization is caused by multiple
reflections in the channel between mobile and base station antenna. Reflection
coefficient for each polarization is different, which results in different amplitudes and
phases for each reflection.
10. Which of the factor does not determine the correlation coefficient?
a) Polarization angle
b) Cross polarization discrimination
c) Offset angle from the main beam direction
d) Coherence time
View Answer
Answer: d
Explanation: The correlation coefficient is determined by three factors, polarization
angle, offset angle from the main beam direction of the diversity antenna, and the
cross polarization discrimination. The correlation coefficient generally becomes higher
as offset angle becomes large.
11. Frequency diversity is implemented by transmitting information on more than one
_____
a) Carrier frequency
b) Amplitude
c) Phase
d) Modulation scheme
View Answer
Answer: a
Explanation: Frequency diversity is implemented by transmitting information on more
than one carrier frequency. Frequency diversity is often employed in microwave line of
sight links which carry several channels in frequency division multiplex mode.
12. Frequency diversity uses ________ as a diversity element.
a) Correlation coefficient
b) Coherence time
c) Coherence bandwidth
d) SNR
View Answer
Answer: c
Explanation: The rationale behind the frequency diversity is that frequencies
separated by more than the coherence bandwidth of the channel will be uncorrelated.
Thus, they will not experience the same fade.
13. Frequency diversity is good for low traffic condition. State whether True or False.
a) True
b) False
View Answer
Answer: b
Explanation: Frequency diversity is not good for low traffic conditions. This technique
has a disadvantage that it not only requires spare bandwidth but also requires that
there be as many receivers as there are channels used for frequency diversity.
However, for critical traffic, the expense may be justified.
14. Time diversity repeatedly transmits information at time spacings that exceed _____
a) Coherence bandwidth
b) Dwell time
c) Run time
d) Coherence time
View Answer
Answer: d
Explanation: Time diversity repeatedly transmits information at time spacings that
exceed coherence time of the channel. Thus, multiple repetitions of the signal will be
received with independent fading conditions, thereby providing for diversity.
15. In maximal ratio combining, the output SNR is equal to _______
a) Mean of all individual SNRs
b) Maximum of all SNRs
c) Sum of individual SNR
d) Minimum of all SNRs
View Answer
Answer: c
Explanation: Maximal ratio combining produces an output SNR equal to the sum of
the individual SNRs. Thus, it has the advantage of producing an output with an
acceptable SNR even when none of the individual signals are themselves acceptable.
16. In time diversity
a. Multiple versions of signals are transmitted at different time instants
b. The signal is transmitted using multiple channels
c. Signal is transmitted with different polarization
d. All of the above
ANS:-A
17. Working of Adaptive Equalizers include
a. Training
b. Tracking
c. Modulation
d. Both a and b
e. All a, b and c are correct
ANSWER: Both a and b
18) The time span for which the equalizer converges depends upon
1. Equalizer algorithm
2. Equalizer structure
3. Rate of change of multipath radio channel
4. Amplitude of signal
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All the four are correct
ANSWER: 1, 2 and 3 are correct
19) The Linear Equalizer may be implemented as
a. FIR filter
b. Lattice filter
c. Low pass filter
d. Both a and b
e. Both a and c
ANSWER: Both a and b
20) Linear equalizer is also known as
a. Transversal filter
b. Lattice filter
c. Low pass filter
d. None of the above
ANSWER: Transversal filter
21) The methods used for non linear equalization are
a. Decision Feedback Equalization
b. Maximum Likelihood Symbol Detection
c. Maximum Likelihood Sequence Estimation
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. None of the above
ANSWER: 1, 2 and 3 are correct
22) The performance of algorithms for Adaptive Equalization are given by
1. Rate of convergence
2. Computational complexity
3. Numerical properties
4. Frequency change
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All are correct
ANSWER: 1, 2 and 3 are correct
23) Computational complexity of an algorithm refers to the
a. Number of operations for one iteration of algorithm
b. Inaccuracies in the mathematical analysis
c. Noise produced during one complete iteration of algorithm
d. All of the above
ANSWER: Number of operations for one iteration of algorithm
24) The algorithms acquired for adaptive equalization are
1. Zero forcing algorithm
2. Least mean squares algorithm
3. Recursive least squares algorithm
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. None of the above
ANSWER: 1, 2 and 3 are correct
25) Fractionally spaced equalizer acts as
a. Matched filter
b. Equalizer
c. Demodulator
d. Both a and b
e. All a, b and c are correct
ANSWER: Both a and b
26) Diversity employs the decision making at
a. Transmitter
b. Receiver
c. Transmitter and receiver
d. Communication channel
ANSWER: Receiver
27) The diversity schemes are based on
1. Time diversity
2. Frequency diversity
3. Space diversity
4. Polarization diversity
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All the four are correct
ANSWER: All the four are correct
28) In time diversity
a. Multiple versions of signals are transmitted at different time instants
b. The signal is transmitted using multiple channels
c. Signal is transmitted with different polarization
d. All of the above
ANSWER: Multiple versions of signals are transmitted at different time instants
29) RAKE receiver is
1. Several sub receivers
2. Several correlators
3. Fingers
4. Equalization based
a. 1 and 2 are correct
b. 1, 2 and 3 are correct
c. 2 and 3 are correct
d. All the four are correct
ANSWER: 1, 2 and 3 are correct
30) The RAKE receiver involves the steps
a. Correlator, estimation of transmitted signal, demodulation, bit decision
b. Estimation of transmitted signal, correlator, demodulation, bit decision
c. Estimation of transmitted signal, demodulation, correlator, bit decision
d. Estimation of transmitted signal, demodulation, bit decision, correlator
ANSWER: Correlator, estimation of transmitted signal, demodulation, bit decision
31) Search window of a RAKE receiver is
a. Frequency band of the channel
b. Range of the time delays
c. Range of noise
d. All of the above
ANSWER: Range of the time delays
32) Speech Coders are categorized on the basis of
a. Signal compression techniques
b. Frequency of signal
c. Bandwidth of the signal
d. All of the above
ANSWER: Signal compression techniques
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This set of Wireless & Mobile Communications Multiple Choice Questions & Answers
(MCQs) focuses on “Rake Receiver”.
1. In CDMA spread spectrum systems, chip rate is less than the bandwidth of the
channel. State whether True or False.
a) True
b) False
View Answer
Answer: b
Explanation: In CDMA spread spectrum systems, the chip rate is typically much
greater than the flat fading bandwidth of the channel. Whereas conventional
modulation techniques require an equalizer to undo intersymbol interference between
adjacent channels.
2. A RAKE receiver collects the __________-versions of the original signal.
a) Time shifted
b) Amplitude shifted
c) Frequency shifted
d) Phase shifted
View Answer
Answer: a
Explanation: RAKE receiver attempts to collect the time shifted versions of the original
signal. It is due to the fact that there is useful information present in the multipath
components.
3. RAKE receiver uses separate _________ to provide the time shifted version of the
signal.
a) IF receiver
b) Equalizer
c) Correlation receiver
d) Channel
View Answer
Answer: c
Explanation: RAKE receiver uses separate correlation receivers to provide the time
shifted version of the original signal for each of the multipath signal. CDMA receivers
combine these time shifted versions of the original signal to transmission in order to
improve the signal to noise ratio of the receiver.
4. Each correlation receiver in RAKE receiver is adjusted in______
a) Frequency shift
b) Amplitude change
c) Phase shift
d) Time delay
View Answer
Answer: d
Explanation: Each correlation receiver may be adjusted in time delay, so that a
microprocessor controller can cause different correlation receivers to search in
different time windows for significant multipath.
5. The range of time delays that a particular correlator can search is called ________
a) Search window
b) Sliding window
c) Time span
d) Dwell time
View Answer
Answer: a
Explanation: The range of time delays that a particular correlator an search is called a
search window. RAKE receiver attempts to collect the time shifted version of the
original signal by providing a separate correlation receiver for each of the multipath
signal.
6. RAKE receiver is used for _______ technique.
a) CDMA
b) TDMA
c) FDMA
d) OFDM
View Answer
Answer: a
Explanation: RAKE receiver is essentially a diversity receiver which is used
specifically for CDMA. It uses the fact that the multipath components are practically
uncorrelated from one another when their relative propagation delays exceed a chip
period.
7. A RAKE receiver uses ____ to separately detect the M strongest signals.
a) Single correlator
b) Multiple correlator
c) Single IF receiver
d) Multiple IF receivers
View Answer
Answer: b
Explanation: A RAKE receiver uses multiple correlators to separately detect the M
strongest multipath components. Demodulation and bit decisions are then based on
the weighted ouputs of the M correlators.
8. In a RAKE receiver, if the output from one correlator is corrupted by fading, all the
other correlator’s output are also corrupted. State whether True or False
a) True
b) False
View Answer
Answer: b
Explanation: In a RAKE receiver, if the output from one correlator is corrupted by
fading, the others may not be. And the corrupted signal may be discounted through
weighting process.
9. A RAKE receiver uses ____
a) Equalization
b) Channel coding
c) Diversity
d) Encryption
View Answer
Answer: c
Explanation: RAKE receiver is a diversity receiver. Diversity is provided by the fact
that the multipath components are practically uncorrelated from one another when
their relative propagation delays exceed chip period.
10. Interleaving is used to obtain _____ diversity.
a) Time
b) Frequency
c) Polarization
d) Antenna
View Answer
Answer: a
Explanation: Interleaving is used to obtain time diversity in a digital communication
system without adding any overhead. It provides rapid proliferation of digital speech
coders which transform analog voices into efficient digital messages.
