Important 2 marks
EC2401 - WIRELESS COMMUNICATION – QUESTION BANK
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.Durkin’s 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.
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
computational complexity
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
90.Write advantages 2G over 1G.
i)Natural integration with the evolving digital wireless network
ii)Higher data rate
iii) Flexibility for capacity expansion
91.What are service offered by GSM?
i) Telephone services
ii) Bearer or Data services
iii) Supplementary services
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)?
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.
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.
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.
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 that’s 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
