Modulation Techniques –Diversity And Antennas-UNIT-III
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Part-A
1.Define modulation.
Modulation may be defined as the process by which some parameters of a high frequency signal termed as carried, is varied in accordance with the signal to be transmitted.
2. What is demodulation?
Demodulation or detection is the process of recovering the original modulating signal from a modulated wave.
3. Write the advantages of digital
over analog modulation?
(i) Greater
noise immunity.
(ii) Robustness to channel impairments
(iii) Easier multiplexing of various forms of information. (iv) Greater security.
4. What are the digital
modulation techniques are available?
Digital modulation techniques
1. Amplitude shift keying (ASK)
2. Frequency shift
keying (FSK)
3. Phase shift keying (PSK)
5. Write a short note on
(a) Amplitude shift
keying (ASK)
If amplitude of the carrier is varied depending on the incoming digital signal, then it is called
amplitude shift keying (ASK)
(b) Frequency shift
keying (FSK)
If the frequency of the sinusoidal carrier varied depending on the incoming digital signal,
then
it is called frequency shift
keying (FSK).
(c) Phase shift
keying (PSK)
If phase of the carried is varied depending on the input digital signal, then it is called phase shift keying (PSK)
6. Define M-ary trammission
system.
In digital modulations instead of transmitting one bit at a time, two or more bits are transmitted
simultaneously. This is called M-ary transmission.
7. What is Quadrature modulation?
Some times two or more quadrature carriers are used for modulation. It is called quadrature modulation.
8. Define baud rate
Speed at which symbols (signals) are transmitted in a digital communications system. Simply no. of
symbols/second.
9. Define bit
rate.
Speed at which data (Bits) is transmitted in a digital communication system. Simply no. of bits/second.
10. What is QAM?
At high bit rates, a combination of ASK and PSK is employed in order to minimize the errors
in the received data. This method is known
as “quadrature amplitude modulation”.
11. Define QPSK.
QPSK is a multilevel modulation in which four phase shifts are used for representing four different symbols.
12. What is linear modulation?
In linear modulation techniques, the amplitude of the transmitted
(carrier) signal varies with the
modulating digital signal.
13. Write the merits of linear modulation.
(i) Bandwidth efficient.
(ii) Very attractive for use in wireless communication systems and
(iii) Accommodate more and
more
users within a limited spectrum.
14. Define nonlinear modulation.
In the non linear modulation the amplitude of the carrier is constant, regardless of the variation
in
the modulating signals.
Non-linear modulations may have either linear or constant envelops depending on whether
or not the base band waveform is
pulse shaped.
15. Mention the merits and demerits of non-linear modulation.
MERITS:
(i) Power efficient class C
amplifiers can be used without introducing
Degradation in the spectrum occupancy of the transmitted signal.
(ii) Low out of band radiation of the order of -60dB to -70 dB can be achieved.
(iii) Limiter-discriminator detection can be used, which simplifies receiver design
and provides high immunity against random FM noise and Signal fluctuations
due to
Rayleigh fading.
DEMERITS:
(i) Constant
envelope modulations, occupy a larger bandwidth than
linear modulation schemes.
(ii) In situations where
bandwidth efficiency
is more
important
than power efficiency,
constant envelope modulation is not well-suited
16. Why MSK is called as fast
FSK?
MSK is sometimes referred to as fast FSK, as the frequency spacing used for only half
as much as that
used
in conventional noncoherent FSK.
17. Mention some merits of MSK.
(i) Constant envelope
(ii)
Spectral efficiency
(iii) Good BER performance
(iv) Self-synchronizing capability
(v) MSK is a spectrally efficient modulation scheme and is particularly attractive for use
in
mobile radio communications systems.
18. What is the need of Gaussian filter?
Gaussian filter is used before the modulator to reduce the transmitted bandwidth of the signal. It uses less bandwidth than conventional FSK.
19. Write the formula for bit error rate in GMSK.
The bit error probability for GMSK is given by
20. Give some examples of linear modulation.
The most popular linear modulation techniques are
(i)
Pulse shaped QPSK
(ii)
OQPSK
(iii) Π/4QPSK
21. Define M-ary FSK.
In M-ary system, M=2N different symbols are used and N-number of bits per symbol. Every symbol
uses separate frequency for transmission. Such system is called M-ary FSK system.
22. Write the applications of MFSK and OFDM.
MFSK and OFDM modulation methods are used for high speed data connections as part of the IEEE 802.11a standards activities to provide 54Mbps WLAN connections, as well as for high speed line-of sight
and non-line-of-sight microwave connections for MMDS (Multichannel multipoint distribution service) Operation.
23. What are the techniques used to improve the received signal quality?
Equalization, diversity and channel coding.
24. What is the need of equalization?
Equalization can be used to compensate the inter symbol
interference (ISI) created by multipath within time dispersion channel.
25. Write the function of diversity.
Diversity is
used to
compensate
for
fading
channel
impairments,
and is
usually implemented by using two or more receiving antennas.
Diversity
improves
transmission performance by
making use of more
than
one
independently faded version of the transmitted signal.
26. Define spatial diversity?
The most common diversity technique is called spatial diversity, whereby multiple
antennas are strategically spaced and connected to a common receiving system. While one antenna sees s signal null, one
of the other
antennas may see a signal peak, and the
receiver is able to select the
antenna with the best
signals at
any time.
27. Define STCM.
Channel
coding
can
also
be
combined
with diversity (via
multiple antennas) a technique called space-time coded modulation (STCM).
The
space-time
coding
is a bandwidth and power
efficient method for wireless communication.
28. What is equalizer?
The device which equalizes the dispersive effect of a channel is referred to as an equalizer.
29. Define adaptive equalizer.
To combat ISI, the equalizer coefficient should change according to the channel status so as to track the channel variations. Such an equalizer is called an adaptive equalizer since it adapts to the channel variations.
30. Write the major classifications of equalizers
The major
classification
of
equalization
techniques are
linear
and nonlinear equalization.
Linear equalizers: If the output d(t) is not used in the feedback path to adapt the equalizer. This type of equalizers is called linear equalizer.
Non-linear equalizers: If the output d(t) is feedback to change the subsequent outputs of the
equalizer
31. Write the advantages of lattice equalizer.
(i) It is simplest and easily available. (ii) Numerical stability.
(iii) Faster convergence.
(iv) When
the channel becomes more time dispersive, the length of the equalizer
can
be increased
by the
algorithm without
stopping
the operation.
(v) Unique structure of the lattice filter allows the dynamic assignment.
32. What are the non-linear equalization methods are used?
Three very effective non-linear methods are used in most 2G and 3G systems. (i) Decision
feedback equalization(DFE)
(ii) Maximum likelihood sequence estimation(MLSE) (iii) Maximum likelihood symbol detection.
33. What are the factors
used in adaptive algorithms?
(i) Rate of convergence
(ii) Misadjustment
(i)
Computational complexity
34. Write the basic algorithms used for adaptive equalizations.
(i) Zero forcing (ZF) algorithm.
(ii) Least mean squares (LMS) algorithm.
(iii) Recursive least
square(RLS) algorithm.
35. Write the advantages of LMS algorithm.
(i) The LMS equalizer maximizes the signal to distortion at
its
output within the constraints of the equalizer filter length.
(ii) Low computational complexity and
(iii) Simple program.
36. Write the advantages of RLS algorithm. (i) Fast convergence
(ii) Good tracking ability. If smaller value of weighting coefficient λ, the
Equalizer has better tracking ability.
37.
Define diversity concept.
If one radio path undergoes a deep fade, another independent path may have a strong signal. By
having more than one path to select from, both the instantaneous and average SNRs at
the
receiver may be
improved, often by as much as 20dB to 30dB.
38. Write the types of diversity techniques.
The diversity techniques
are
classified into following categories. a)
Space or antenna diversity
b) Angle or direction diversity c) Polarization diversity
d) Time diversity.
39. Write the classification in space diversity reception method.
|
Space diversity reception method can be classified into four categories
40.
Define fading.
Fading is nothing but reduction in radio signal strength, usually caused by reflection or absorption of the signal.
41. Explain small –scale fading.
Small-scale fading or simply fading is used to describe the rapid fluctuations of the amplitudes, phases, multipath delays of a radio signal over a short period of time or travel distance.
42. What is multipath wave?
Fading is caused by interference between two or more versions of the transmitted signal
which
arrive at
the receiver
at slightly different times. These
waves
are called multipath
wave.
43. Write the effects of fading.
(i) Rapid changes in signal
strength over a small travel distance or time interval.
(ii) Random
frequency modulation
due
to varying
Doppler
shifts
on
different multipath
signals and ‘
(iii) Time dispersion (echoes) caused by multipath propagation delays.
44. What are the factors influencing small-scale fading?
Factors influencing small-scale fading
(i)
Speed of surrounding objects.
(ii) Multipath propagation
(iii) Speed of the mobile
(iv) Transmission bandwidth of the signal.
45. Define coherence bandwidth
The coherence bandwidth is related to the specific multipath structure of the channel. The coherence bandwidth is a measure of the maximum frequency difference for which signals are still strongly correlated in amplitude. This bandwidth is inversely proportional to the rms value of time delay spread (σz)
f = Vf
d c
46. What is coherence time (TC)?
Coherence time (TC) is usually defined as the required time interval to obtain an
envelope correlation of 0.9 or less.
It is inversely proportional
to
the maximum Doppler frequency
1
Tc =
fm
Where, fm = maximum Doppler frequency
The coherence time defines the static ness of the channel
47.
Define impulse response model.
The impulse response model is a wideband channel characterization of the channel, it may be
used
to predict and
compare
the performance of
many different mobile communication
systems and transmission bandwidths for
a
particular mobile channel condition.
Mobile radio channel maybe modeled as a linear filter with a time varying impulse response.
48. Mention some of the small-scale multipath measurement techniques.
Some of the techniques are,
(i) Direct
pulse measurements.
(ii) Spread spectrum sliding correlator measurement and
(ii)
Swept
frequency measurements.
49. Define power delay profile.
The power delay profile provides an indication of the dispersion or distribution of transmitter power over various paths of the multipath.
50. Write the advantages of direct RF pulse system.
(i) Lack of
complexity, because off the shelf equipment can be used.
(ii)
There also be a minimum resolvable delay.
PART-B
1. With necessary diagrams
explain Gaussian minimum
shift keying.
2. Describe
in detail about minimum shift keying.
3. Explain
in detail about BFSK.
4. Explain with
neat diagram the performance of RAKE receiver.
5. With neat
diagram explain about orthogonal frequency division multiplexing.
6. Explain in
detail about base station and mobile stations antennas.
7.
Explain any two diversity techniques, in detail.
8.
Explain about modulation techniques in detail.
9.
Explain MSK transmitter and receiver implementation with suitable diagram.
10Expalin
about combining methods
11.
Explain about base and mobile station antenna.
&�r s � �R� 05pt'>em grades the problem
into one of the four categories.
(a) Single diffraction edge
(b)Two diffraction edges
(c)Three diffraction edges
(d) More than three diffraction edges
PART B
1.
Explain in detail about the three basic propagation mechanisms.
2.
Write short notes on
(a) Factors influencing small scale fading.
(b) Doppler Shift.
3.
Explain in detail about impulse response model of a multipath channel.
4.
Describe the parameters of mobile multipath channels.
5.
Explain in detail about ground reflection model.
6.
Explain in detail about knife edge diffraction model
7.
Explain in detail about Radar cross section model.
8.
Explain in detail about log -distance path loss model.
9.
Explain in detail about outdoor propagation model.
10.
Explain time and frequency dispersion parameter of mobile multipath channel
11.
what do you understand by large scale fading? Explain the 2-ray ground
reflection model for path loss prediction.
12.
Derive and explain the free space propagation model to determine received power
at a distance
13.
Explain outdoor and Indoor propagation model
Mobile communicates with two or more cells at the same time and find which one is a strongest signal base station then it automatically transfer the call to that base station is
called soft handoffs.
42. Write the features of handoff.
a. Fast and lossless
b. Minimal number of control signal
exchanges.
c. Scalable with network size.
d. Capable of recovering from link failures.
e.
Efficient use of resources.
43. What is intracell interference?
Interference from other mobiles at the cell-site (base station) receiver in the same cell is
intracell interference.
44. What are the major types of cellular interference available?
The two major types of system-generated cellular interference are
(i) Co-channel
interference and
(ii) Adjacent channel
interference
45. What is breathing cell
effect?
When specific radio channels are in use, the CDMA system instead has a dynamic,
time
varying coverage region which varies depending on the instantaneous number of users on the CDMA radio channels.
This
effect is known as breathing cell effect.
46. Define the grade of service.
A measure of congestion which is specified as the probability of a call being blocked (for Erlang B) or the probability of a call being delayed beyond a certain amount of time(for Erlang C).
47. What is set-up time?
The time required to allocate a trunked radio channel
to
a requesting user.
48. Define blocked call.
Call which cannot be completed at time of request, due to congestion also referred to as
a lost
call.
49. Define holding time.
Average duration of
a typical call. It is denoted by H (in seconds).
50. Define traffic intensity.
Measure
of channel time
utilization,
which
is the
average
channel occupancy measured in Erlangs. This is a dimensionless quantity and may by used to measure the time utilization
of single or multiple channels. It is denoted by A.
PART- B
1. Explain elaborately about types of handoffs.
2. Explain in detail about
dropped call rate and cell splitting.
3. Explain the different techniques of improving coverage
and capacity in cellular system
4. Explain in detail about usage of repeater for coverage improvement.
5.
Explain in detail about the various trends in personal wireless communication
systems.
6. Narrate wireless
communication systems with the help of paging system.
7.
Write short notes on
(i) Cordless telephone system
(ii) Cellular telephone system
8.
Discuss briefly about Interference and system capacity in cellular systems.
9.
Explain in detail about trunking and grade of service in cellular systems
10. Explain in
detail about the various Multiple Access Schemes.
11. Explain in
detail about the handoff strategies with suitable diagram.
12. Explain in
detail about the umbrella cell concepts.
13. Discuss
briefly about Improving channel capacity in cellular systems.
14. Explain in detail about Co-channel
interference and system capacity.
15. Explain
spread spectrum with its types
16. what is need
for frequency reuse? Explain frequency reuse concept and show that N = i2 +
ij +j2.
Where N is the no of cells
17. derive
expression for signal to noise ratio for 7 cell cluster
18. Explain
channel assignment concept in detail
19. Explain
about CSMA protocols
20 . Explain about TDMA
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