Viva Questions on Mobile Radio Propagation
Viva Questions on Mobile Radio Propagation
Short Answer type Viva Questions on Mobile Radio Propagation
Q.1. What are the two basic types of communication channels?
Ans.
Basically, there are following two types of communication channels
(i)
wired channels
(ii)
radio or wireless channels
The wired channels are stationary and predictable
whereas the radio channels are extremely random and do not offer easy analysis.
Even the speed of motion affects how rapidly the signal level fades as a mobile
terminal moves in space. Therefore, it has been most difficult to model a radio
channel in mobile radio system design. However, the model of a radio channel is
achieved in a statistical manner which is based upon the measurement observed
for the desired communication system or spectrum allocation.
Q.2.
What is bandwidth?
Ans.
Bandwidth is the range of frequencies and represented as BW = f2 – f1 where f1 and f2 are the first and last frequencies of the signal
graph.
Q.3.
Define attenuation.
Ans.
It is nothing but reduction in signal strength mainly at higher frequency
ranges and at receiving end this attenuated signal has reduced voltage levels.
Q.4.
Differentiate guided and unguided mediums.
Ans.
In a guided medium, the electromagnetic waves are guided by the transmission medium
e.g., Copper coaxial cable.
In an unguided medium, the electromagnetic wave is not
guided by the medium. e.g., the radiated energy is not guided in wireless
communication.
Q.5.
Give the significance
of Eb/No.
Ans.
The
bit energy to output noise ratio is important in mobile communication where the
bit error rate of data. It is generally expressed in decibel.
Q.6.
What is known as multipath?
Ans.
When a signal from a cell phone emerges, due to many obstacles it radiates in
many directions and finally reaches its base station receiver and vice versa.
Such multiple signals take multipaths to reach the base station or the cell
phone.
Q.7.
What is the propagation mechanism?
Ans.
(a) Reflection
(b) Diffraction
(c) Scattering
Q.8.
What are types of fading?
Ans.
(a) Fast
fading
(b) Slow
fading
(c) Flat
fading
(d) Selective
fading
Q.9.
What are the advantages of radio
transmission?
Ans.
(a) It
is cheap and simple in design.
(b) Infrared
transmission is not interfered with by electrical devices.
(c) The
radio transmission can cover a larger geographical region.
Q.10.
Define refractive index.
Ans.
At the interface of two materials (m1 and m2) of different densities, there
will be bending or refraction taking place.
Q.11.
Write a short note on reflection.
Ans.
Reflection is the act of reflection or a means to turn back. This reflection
with respect to electromagnetic waves will occur whenever an incident wave hits
a boundary of any two media. In this case, some of the total incident power of the signal will not enter the second media.
Q.12.
When does Rayleigh fading occur?
Ans.
Rayleigh fading will take place when there are many paths between the
transmitter and receiver.
Q.13.
What is dBm0?
Ans.
It is dBm referred to as zero (0) transmission level point.
Q.14.
What is known as scattering?
Ans.
When the radio wave impinges on a surface that is rough, then the reflected
energy spreads out in all directions and it is known as scattering. It
generally occurs when1 the medium of wave travel comprises many objects with
smaller dimensions when compared to the signal wavelength (λ) and also if the
number of objects per unit volume is large it increases the scattering effect.
Q.15.
List the material properties over which
the Fresnel zone reflection coefficient depends on.
Ans.
The reflection coefficient (G) depends on the material's properties namely,
(i)
Wave polarization
(ii)
Frequency of propagation of the wave
(iii)
Angle of incidence
Q.16.
Write a short note on reflection.
Ans.
The reflection mechanism takes place when an electromagnetic wave impinges on
an object that has larger dimensions than the wavelength (λ) of the propagating
wave. Reflection occurs from buildings, walls, and surfaces of the earth.
Q.17.
Write a short note on diffraction.
Ans.
Diffraction is a propagating mechanism that will take place when the radio path
available between transmitter and receiver (T-R) is obstructed by any surface
with sharp irregularities or edges and it creates secondary waves due to
diffraction.
Q.18.
When the reflection of the radio signal will
take place?
Ans.
The reflection mechanism takes place when the electromagnetic wave travels from
one medium to another medium that has different electrical properties. For example, if a wave enters from medium 1 to medium 2 and in case medium 2 is a
perfect conductor then the entire radio energy will get reflected back into
medium 1.
Q.19.
What is the plane of incidence?
Ans.
The plane of incidence is defined as a plane containing reflected, incident and
also the transmitted rays.
Q.20.
Give two advantages of diffraction
mechanism.
Ans.
(i)
Diffraction allows radio signals to
travel around curved surfaces and to propagate beyond obstructions.
(ii)
Even when the receiver moves into a shadowed
region the diffraction field will exist.
Q.21.
What is the main cause for diffraction
effect?
Ans.
Diffraction is caused by the propagation of secondary wavelets into the
shadowed regions. Also, the field strength of the diffracted wave will be a
vector sum of all the electric field components of the secondary
wavelets.
Q.22.
List few outdoor propagation models.
Ans.
1. Durkins
model
2. Longley-Rice
model
3. Okumura
model
4. PCS
extension model
5. Hata
model
6. Wideband
PCS microcell model
Q.23.
Mention any three indoor propagation
models.
Ans.
(i)
Log-distance path loss model
(ii)
Ericsson multiple breakpoint model
(iii)
Partition losses considering the same floor
of a building
Q.24.
What will be the level of attenuation for
reinforced concrete partitions and wooden partitions?
Ans.
If the partitions of a multistoried building are concrete (permanent), the
attenuation from 1 room to another will be high whereas for wood like movable
partitions in a building the level of attenuation of mobile radio propagation will
be less.
Q.25.
List any three factors influencing small
scale fading.
Ans.
(i)
Speed of mobile unit
(ii)
Multipath propagation
(iii)
Transmission bandwidth of the signal
Q.26.
What are the two types of fading of
multipath delay spread?
Ans.
1. Time
dispersion fading
2. Frequency
selective fading
Q.27.
What are the two types of fading of
Doppler spread?
Ans.
1. Frequency
dispersion fading
2. Time
selective fading
Q.28.
What are the fading effects of Doppler
spread?
Ans.
1. Fast
fading
2. Slow
fading
Q.29.
What are level crossing rates and Rayleigh
fading in multipath fading channels?
Ans.
The level crossing rate (LCR) and Rayleigh fading signal predictions are
important for designing error controls codes and diversity schemes that have to
be implemented. The level crossing rate is also taken as an expected rate at
which the Rayleigh fading envelope crosses a specified level in the positive
going signal directed and the number of level crossings is denoted as Nr.
Q.30.
What is slow and fast fading?
Ans.
Under slow fading case, the channel impulse response varies at a rate much
slower rate when compared to a transmitted baseband signal.
In the fast-fading case, the impulse response of the
channel varies rapidly within the respective symbol duration. In other words, the
channel's coherence time period is smaller than the symbol time period of the
signal being transmitted.
Q.31.
List two parameters that describe a time
varying nature of the channel.
Ans.
1. Doppler
spread
2. Coherence
time
Q.32.
What is coherence bandwidth?
Ans.
The coherence bandwidth represents a range of frequencies over which the channel is almost flat In other words it is the range of frequencies over which
the frequency components have enough potential for proper amplitude
correlation.
Q.33.
What is a small-scale fading model?
Ans.
The radio propagation models that characterize the fluctuations in received
signals over short distances of short time periods are called as small scale
fading model.
Q.34.
What is a free space propagation model?
Ans.
It is a propagation model that can predict the received signal strength
whenever the transmitter and receiver (T-R) have a clear line of sight (LOS) path between them.
Q.35.
Define path loss.
Ans.
The signal attenuation is represented as path loss (PL). It is
defined as the difference between the power transmitted and power received by
the antenna and it is expressed in decibels (dB).
Q.36.
What are the components of propagation
path characteristics?
Ans.
The propagation path characteristics can be divided into three components in
indoor and outdoor environments. They are:
(i)
Shadowing
(ii)
Multipath fading
(iii)
Path loss with respect to distance.
Q.37.
What are the demerits of LoS wireless
transmissions?
Ans.
(a) Free
space loss
(b) Noise
(c) Multipath
fading
(d) Refraction
(e) Distortions
Q.38.
Define multipath-propagation.
Ans.
In cellular mobile communication, the received signal arrives through delayed
multiple paths (due to obstacles in its wave travel) and this is termed as
multipath propagation.
Q.39.
What are the effects of 'fading'?
Ans.
As fading occurs,
(a) The error rate of the transmission system will increase
(b) There
will be a significant fluctuation in BER value
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