UNIT I

1. Write some examples for wireless communication

system.
Cordless phones, handheld walkie-talkies, pagers,
mobiles, remote controllers For home entertainment.
2. What is base station?
A fixed station in mobile radio system used for radio
communication with mobiles. It has transmitter and
receiver section. It is located at the centre of coverage
area.
3. What is MSC?
Mobile switching centre coordinates the routing of
calls in large service area. It connects the base station
and mobiles to PSTN. It is also called as
MTSO(Mobile telephone switching office.
4. What do you mean by forward and reverse
channel?
Forward channel is a radio channel used for
transmission of information from base station to
mobile. Reverse channel is a radio channel used for
transmission from mobile to base station.
5. What is the function of control channel? What are
the types?
Control channel is used for transmission of call setup,
call request, call initiation & Control. Types are
forward control channel, reverse control channel
.6. Define cell
Each cellular base station is allocated to a group of
radio channels to be used within a small geographic
area called as cell.
7. What is foot print?
Actual radio coverage of a cell is called as footprint.
It is determined from the field measurements or
propagation prediction models.
8. What is channel assignment ? what are the types?
For efficient utilization of radio spectrum a frequency
reuse scheme with increasing capacity and
minimizing interference is required. For this channel
assignment is used Types : Fixed channel
assignment, dynamic channel assignment.
9. What is fixed channel assignment?
If the channels in each cell is allocated to the users
within the cell, it will be called as fixed channel
assignment. If all channels are occupied, the call
Will be blocked.
10. What is dynamic channel assignment?
If the voice channels are not allocated permanently in
a cell, it will be called as dynamic channel
assignment. In this assignment, channels are
dynamically allocated to users by the MSC.
11. What is hand off?
When a mobile moves into a different cell while
conversation in progress, the MSC automatically

transfers the call from one cell to other cell without

any interference.This is called as hand off.
12. Define dwell time.
The time over which the call may be maintained
within a cell without handoff is called as dwell time.
This time is governed by factors such as propagation,
interference, distance between subscribers and base
station.
13. What is soft handoff?
In CDMA system, MSC selects received signals from
a variety of base stations with the help of software.
This is called as soft handoff.
14. What is co channel interference?
The interference between the signals from co channel
cells is called as co cannel interference.
15. Define co-channel reuse ratio.
It is define as the ratio between the distance between
the centers of nearest co channel cells to the radius of
the cell. Q = D/R
16. Define adjacent channel interference.
Interference resulting from signals which are adjacent
in frequency to the desired signal is called adjacent
channel interference.
17. Define Grade of service.
It is defined as the measure of the ability of a user to
access a trunked system during the busiest hour.
18. What is blocked call clear system(BCC)?
In a system, a user is blocked without access by a
system when no channels are available in the system.
The call blocked by the system is cleared and the user
should try again .This is called BCC system.
19. What is blocked call delay system?
If a channel is not available immediately, the call
request may be delayed until a
channel becomes available.
20. Define cell splitting.
Cell splitting is the process of subdividing congested
cells into smaller cells each
with its own base stations and a corresponding
reduction in antenna height and transmitter power. It
increases the capacity of cellular system.
21. What is sectoring?
Sectoring s a technique for decreasing co-channel
interference and thus increasing the
system performance by using directional antennas.
UNIT II
22. What is propagation model?

Propagation models that predict the mean signal

strength for an arbitrary transmitter
receiver separation distance are useful in estimating
the radio coverage area of a transmitter.
23. Define large scale propagation model?
The propagation models that characterize the signal
strength over large T-R separation
distances (several hundreds or thousands of meters.
24. What is small scale model?
The propagation models that characterize the rapid
fluctuations of the received signal
strength over very short travel distances (a few
wavelengths) or short time duration.
25. What is free space propagation model?.
The free space propagation model is used to predict
received signal strength, when
unobstructed line-of-sight path between transmitter &
receiver.
26. Define EIRP.
EIRP of a transmitting system in a given direction as
the transmitter power that would be
needed, with an isotropic radiator, to produce the
same power density in the given direction EIRP=PtGt
Where Pt-transmitted power in w
Gt-transmitting antenna gain
27.Explain path loss?
The path loss is defined as the difference(in dB)
between the effective transmitted power
& the received power, &may or may not include the
effect of the antenna gains.
28.What is intrinsic impedance& Brewster angle?
It is defined by the ratio of electric to magnetic field
for a uniform plane wave in the
particular medium. The Brewster angle is the angle at
which no reflection occurs in the origin.
29. What is scattering?
When a radio wave impinges on a rough surface , the
reflected energy is spread out in all
directions due to scattering.
30. Define radar cross section?
Radar Cross Section of a scattering object is defined
as the ratio of the power density of
the signal scattered in the direction of the receiver to
the power density of the radio wave
incident upon the scattering object & has units of
squares meters

31. Name some of the outdoor propagation models?

some of the commonly used outdoor propagation
models are
1.Longely-Rice model
2.Durkins model
3.Okumura model.
32. What is the function of outdoor propagation
models?
The outdoor propagation models aim to predict signal
strength at a particular receiving
point or in a specific local area.
33. Define indoor propagation models?
The indoor propagation models are used to
characterizing radio propagation inside the
buildings.
34. Mention some indoor propagation models?
Some indoor propagation models are
1.Long distance path loss model
2.Ericession multiple break point model
3.Attenuation factor model.
35. Explain small scale fading?
Small scale fading is used to describe the rapid
fluctuations of the amplitudes, phases, or
multipath delays of a radio signal over a short period
of time or travel distance.
36. What are the factors influencing small scale
fading?
Factors influencing small scale fading are
1.Speed of surrounding objects
2.Multipath propagation
3.Speed of the mobile
4.Transmission bandwidth of the signal.
37.Define Doppler shift?
The shift in received signal frequency due to motion
is called the Doppler shift.
38. What flat fading?
If the mobile radio channel has a constant gain &
linear phase response over a bandwidth
which is greater than the bandwidth of the
transmitted signal, then the received signal will
undergo flat fading.
39. What is frequency selective fading?
If the channel possesses a constant gain & linear
phase response over a bandwidth that is
smaller than the bandwidth of the transmitted signal ,
then the channel creates frequency
selective fading on the received signal.

40. Define fast fading channel?

The channel impulse response changes rapidly within
the symbol duration. This type of a
channel is called fast fading channel.
41. Define slow fading channel?
The channel impulse response changes at a rate much
slower than the transmitted
baseband signal. This type of a channel is called slow
fading channel.
UNIT-III
42 Write the advantages of MSK over QPSK.
1. In QPSK the phase changes by 90degree or 180
degree .This creates abrupt amplitude
variations in the waveform, Therefore bandwidth
requirement of QPSK is more filters of other
methods overcome these problems , but they have
other side effects.
2.MSK overcomes those problems. In MSK the
output waveform is continuous in phase
hence there are no abrupt changes in amplitude.
43.Define M-ary transmission 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.
44.What is quadrature modulation?
Sometimes two or more quadrature carriers are used
for modulation. It is called quadrature modulation.
45.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.
46.Define QPSK?
QPSK is a multilevel modulation in which four phase
shifts are used for representing four
different symbols.
47. What is linear modulation?
In linear modulation technique the amplitude of the
transmitted signal varies linearly with
the modulating digital signal.
In general, linear modulation does not have a
constant envelope.
48. Define non linear 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 envelopes depending on
whether or not the baseband waveform is pulse
shaped.
49. 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.
50. Mention some merits of MSK
Constant envelope
Spectral efficiency
Good BER performance
Self-synchronizing capability
MSK is a spectrally efficient modulation scheme and
is particularly attractive for use in
mobile radio communication systems.
51. Give some examples of linear modulation?
Pulse shaped QPSK
OQPSK
52.What are the techniques used to improve the
received signal quality?
Equalization , Diversity , Channel coding
53.What is the need of equalization?
Equalization can be used to compensate the Inter
Symbol Interference created by
multipath within time dispersion channel.
54.What is 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.
55.Define spatial diversity?
The most common diversity technique is spatial
diversity, whereby multiple antennas
are strategically spaced and connected to a common
receiving system. While one antenna sees a signal
null, one of the other antenna may sees a signal peak,
and the receiver is able to select the antenna with the
best signals at any time.
56.Define STCM.
Channel coding can also be combined with diversity
a technique called Space-Time
Coded Modulation. The space-time coding is a
bandwidth and power efficient method for
wireless communication.

computational complexity
57.Define adaptive equalization?
To combine Inter Symbol Interference, the equalizer
coefficients should change
according to the channel status so as to break channel
variations. Such an equalizer is called an adaptive
equalizer since it adapts to the channel variations.
58.Define training mode in an adaptive equalizer?
First , a known fixed length training sequence is sent
by the transmitter then the
receivers equalizers may adapt to a proper setting of
minimum bit error detection where the training
sequence is a pseudo random binary signal or a fixed
and prescribed bit pattern.

59.What is tracking mode in an adaptive equalizer?

Immediately following this training sequence the user
data is sent and the adaptive
equalizer at the receiver utilizes a recursive algorithm
to evaluate the channel and estimate filter
coefficients to compensate for the distortion created
by multipath in the channel.
60.Write a short note on linear equalizers and non
linear equalizers?
Linear equalizers: If the output d(t) is not used in the
feedback path to adapt the
equalizer. his type of equalizers is called linear
equalizer.
Nonlinear equalizers: If the output d(t) is fed back to
change the subsequent outputs
of the equalizers is called non linear equalizers.
61.Why non linear equalizers are preferred?
The linear equalizers are very effective in equalizing
channels where ISI is not severe.
The severity of the ISI is directly related to the
spectral characteristics. In this case that there are
spectral noise in the transfer function of the effective
channel, the additive noise at the receiver input will
be dramatically enhanced by the linear equalizer. To
overcome this problem non linear equalizers are
used.
62.What are the nonlinear equalization methods
used?
Decision feedback equalization
Maximum likelihood symbol detection
Maximum likelihood sequence estimation
63.What are the factors used in adaptive algorithms?
Rate of convergence
Mis adjustments

64.Define MSE in equalizers

The mean square error is a error between the desired
equalizer output and the actual
equalizer output.LMS equalizers are used to
minimize to MSE.
65.Write the advantages of LMS algorithm.
The LMS equalizer maximizes the signal to
distortion at its output
within the constraints of the equalizer filter length.
Low computational complexity
simple program
66.What are the advantages of RLS algorithm.
1.Fast convergence.
2.Good tracking ability. If smaller value of weighting
coefficient The equalizer has better
tracking ability.
67.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.
UNIT-IV
68. What are the two types of linear predictive
coders?
i) Multipulse excited Linear predictive coder
ii)Stochastic or code excited linear predictive coder
69. What is the criterion for the selection of speech
coders for mobile communication?
Because of the limited bandwidth, it is required to
compress speech to maximize the
number of users on the system. Other criterion
includes end-to-end encoding delay, the
algorithmic complexity of the coder, dc power
requirements, compatibility, robustness.
70.What is multiple access?
Multiple access schemes are used to allow many
mobile users to share simultaneously a
finite amount of radio spectrum. It is required to
achieve high capacity by simultaneously
allocating the bandwidth to mulitple users.
71.What is frequency division duplexing?
It is duplexing done using frequency techniques.FDD
provides two distinct bands of

frequencies for every user. The forward band

provides traffic from th base station to the mobile,
and the reverse band provides traffic from the mobile
to the base station.
72.What are the multiple access techniques?
i)Frequency division multiple access
ii)Time division multiple access
iii)Code division multiple access
73.What is a wide band system?
In wideband system, the transmission bandwidth of a
single channel is much larger than
the coherence bandwidth of the channel. Thus
multipath fading does not greatly vary the
received signal power within a wideband channel.
74. What are the nonlinear effects in FDMA?
In FDMA, many channels share the same antenna at
the base station. The power
amplifiers are nonlinear which causes signal
spreading in the frequency domain and generate inter
modulation frequencies. It is undesirable and can
result in interference.
75.What is time division multiple access?
Time division multiple access systems divide the
radio spectrum into time slots and in
each slot only one user is allowed to either transmit
or receive.
76.What is fast and slow frequency hopping?
If the rate of change of the carrier frequency is
greater than the symbol rate, then the
system is referred to as fast frequency hopping. If the
rate of change of the carrier frequency is lesser than
the symbol rate, then the system is referred to as slow
frequency hopping.
77.Define capacity of cellular systems
It can be defined as the maximum number of
channels or users that can be provided in a
fixed frequency band.
78.Define forward channel interference
For a particular subscriber unti, the desired base
station will provide the desired forward
channel while the surrounding co-channel base
stations will provide the forward channel
interference.
79.Define adaptive channel allocation
Adaptive channel allocation in TDMA eliminates
system planning since it is not required
to plan frequencies for cells.

80.What are vocoders?

Vocoders are speech coders that are used for signal
compression. Vocoders are based on
priori knowledge about the signal to be coded, and
hence are signal specific.
81.What is narrow band system?
In narrow band multiple access system, the variable
radio spectrum is divided into a large
number of narrowband channels. The channels are
operated using FDD.
82.Define SDMA
Space division multiple access controls the radiated
energy for each user in space. It
serves different users by using spot beam antennas.
83.State some of the features of CDMA
* Users of CDMA share the same frequency.
* CDMA has soft capacity limit.
* Multipath fading may be substantially reduced
* Channel data rates are very high
84.Define efficiency of TDMA
The efficiency of a TDMA is a measure of the
percentage of transmitted data that
contains information as opposed to providing
overhead for the access scheme.
85.What are the features of TDMA?
* TDMA shares a single carrier frequency with
several users, where each user makes use
of non overlapping time slots.
* Data transmission occurs in bursts.
*Handoff process is much simpler
*Duplexers are not required, since transmission and
reception occurs at different time
slots.
86.What is time divison multiplexing?
TDD uses time instead of frequency to provide both a
forward and reverse link. Multiple
users share a single radio channel by taking turns in
the time domain.
87.What are the features of FDMA?
* FDMA channel carries only one phone circuit at a
time
* The bandwidth of FDMA channels are relatively
narrow as each channel supports only
one circuit per carrier.
88.Why the second generation was developed?
The second generation systems have been
developed to provide higher quality signals,

higher data rate for support of digital services and

greater capacity.
89.What are second generation are available?
i)Global System Mobile (GSM) in Europe
ii)Interim standard
iii)Pacific Digital Cellular
iv)Interim standard-95

The guard period is used to avoid overlapping with

other bursts due to different path
delays and to give the transmitter time to turn on and
off.
100.Why Dummy burst is used?
Dummy burst is used as filter information for unused
time slots on the forward link.

90.Write advantages 2G over 1G.

i)Natural integration with the evolving digital
wireless network
ii)Higher data rate
iii) Flexibility for capacity expansion

101.Define burst formatting in GSM.

Burst formatting adds binary data to the ciphered
blocks, in order to help
synchronization and equalization of the received
signal.

91.What are service offered by GSM?

i) Telephone services
ii) Bearer or Data services
iii) Supplementary services

102.What is the need of pilot channel?

The pilot channel is intended to provide a reference
signal for all MSS within a cell
provides the phase reference for coherent
demodulation.

92.What is the function of NSS in GSM?

The NSS managing the switching function of the
systems and allows the MSCs
to communicate with other networks such as PSTN
and ISDN.
93.Define Abis Interface.
The interface which connects a Base Transceiver
Station (BTS) to a Base Station
Controller (BSC) is called the Abis Interface.
94.Define A Interface.
The interface between a BSC and a MSC is called the
A interface , which is standardized
within GSM
95.What is the function of VLR?
The VLR is a database which temporarily stores the
IMSI and customer information for
each roaming subscriber who is visiting the coverage
area of a particular MSC.
96.What are the basic channels available in GSM?
i)Traffic Channels (TCHs)
ii)Control Channels (CCHs)

97.Define the bursts.

Data is transmitted small portions called bursts.
98.Write types of TCH channels of GSM?
i)Full-rate TCH
ii)Half-rate TCH
99.What is the need guard period(space)?

103.What are the supervisory signals are used

AMPS?
The supervisory signals are the supervisory audio
tone(SAT) and signaling tone(ST).
104.What are the advantages of N-AMPS over
AMPS?
N-AMPS provided three uses in a 30kHz AMPS by
using FDMA and 10kHz channel and
provided three times the capacity of AMPS.
105.Define Piconet.
The simplest Bluetooth networks called piconet can
have from two to eight nodes.
Piconet is a collection of Bluetooth devices which are
synchronized to the same hopping
sequence.
106.What is Bluetooth?
Bluetooth is an open specification for the short range
wireless voice and data
communication thats was originally developed for
cable replacement in personal area
networking to operate all over the world.
107.What is Scatternet?
One Bluetooth devices can operate simultaneously on
two piconet acting as a bridge
between the two. A conglomeration of two or more
piconet is called a scatter net.