Mobile Radio Propagation-UNIT-II
Part-A
Propagation models have traditionally focused on predicting the average received signal strength at a given distance from the transmitter, as well as the variability of the signal strength
in
close spatial
proximity to a particular location.
Propagation models that
predict the mean signal
strength for an arbitrary transmitter- receiver (T-R) separation
distance are useful in estimating the radio coverage area of a transmitter.
2. Define large-scale propagation models.
The propagation models that characterize the signal strength over large
T-R separation distance (several
hundreds or thousands of meters).
3. What is a small-scale model?
Propagation models that
characterize the rapid fluctuations of the received signal strength over very short travel distances (a few wavelengths) or short time duration (seconds) are
called small-scale or fading.
4. Explain free space propagation model.
The free space propagation model is used to predict received signal
strength, when unobstructed
line-of-sights path between transmitter and receiver.
The free space power received by a receiver antenna which is separated from
a radiating transmitter antenna by a distance d.
PG G λ 2
P (d ) = t t r
r (4Π)2 d 2 L
Where, Pr (d) is the received power which is a function of the T-R Separation
Pt is the transmitted power
Gt is the transmitter antenna gain.
Gr is the receiver antenna gain
d is the T-R separation distance in meters.
L is the system loss factor
5.
Define
effective area
The effective area of an antenna can be defined as
Where, Aeff – Effective
area of the antenna in m2
Pr – Power delivered to the receiver in W. PD-
Power density of the wave in w/m2
6. What is Isotropic radiator?
An isotropic radiator is an ideal
antenna which radiates power with unit gain uniformly in
all
directions.
7. 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
= Pt
Gt
Where, Pt is the transmitted power in W
and
Gt is the transmitting
antenna gain
8. Explain path loss.
The path loss is defined as the difference (in dB) between the effective transmitted power
and
the received power, and may or may not
include the effect
of the antenna gains.
9. What is far-field region?
The far-field or fraunhofer region, of
a transmitting antenna is defined as the region beyond
the
far-field distance dr, which is related to the largest
linear dimension of the transmitter antenna aperture and the carrier wavelength.
10. Write the name of three basic propagation mechanisms.
The three basic propagation mechanisms which impact propagation in a mobile communication systems are
(i) Reflection
(ii) Diffraction and
(iii) Scattering
11.
Write a note on
(i) Plane of incidence
(ii) E-field in the plane of incidence
(iii) E-field normal to the plane of incidence.
Plane of incidence: the plane of incidence is defined as the plane containing the incident,
reflected and transmitted rays.
E-Field in the plane of incidents: E-field polarization is parallel
with
the plane of incidence.
E-field normal to the plane of incidence: E-field polarization is perpendicular to the plane
of incidence.
12. What is intrinsic impedance?
It
is
defined by the radio of electric to magnetic field for a uniform plane wave in the particular
medium.
µi
ηi = ε
i
13.
Define Brewster angle.
The Brewster angle is the angle at
which no reflection occurs in the medium of origin.
14. What is the need of ground reflection (two-ray) model?
The two-ray model is reasonably accurate for predicting the large-scale signal strength over distances of several
kilometers for
mobiles radio systems that
use tall towers(height which exceed 50m), as
well as for line-of-sight
micro cell
channels in urban environments.
15. What is the use of method of images?
The method of images is used to find the path difference between the line-of-sight and
the
ground reflected paths.
16. Write the expressions for both received power and path loss in two-ray model.
PL (dB) = 40log d – (10log Gt + 10log Gr + 20 log ht + 20 log hr)
17. How diffraction will
occur?
Diffraction
occurs
when the
radio
path between the
transmitter
and receiver is obstructed by a surface that has sharp irregularities (edges).
18. Define excess path length (EPL).
The difference between the direct path and diffracted path called excess path length.
19. What are Fresnel zones?
The concentric circles on the transparent plane located between a transmitter and receiver represent the loci of the origins of secondary wavelets which propagate to the receiver such that the total path length increases by λ/2 for successive circles. These circles are
called fresnel
zones.
20. Explain knife-edge diffraction model.
Knife-edge is the simplest
of diffraction models, and the diffraction loss can be readily
estimated using the classical fresnel
solution for the field behind a knife edge.
21. What is scattering?
When a radio wave impinges on rough surfaces, the reflected energy is spread out
(diffused) in all directions due to scattering.
22. Define radar cross section.
Radar cross section (RCS) of a scattering object is defined as the radio of the power
density of the signal scattered in the of the receiver to the power density of the radio wave incident upon the scattering object and has units of square meters.
23. What is the need of bistatic radar equation?
The bistatic radar equation may be used to compute the received power due to scattering in the far field in the urban mobile radio systems.
24. What are the path loss models available in link budget design?
a) Long-distance path loss model and b)
Long-normal
shadowing model
25. What is the need of path loss models in link budget design?
The path loss models are used to estimate the received signal level as a function of distance
it
becomes possible to predict the SNR for a mobile communication system.
26. What are major drawbacks in long-distance path loss model?
The long
distance
path
loss model equation
does
not
consider the
surrounding environment clutter may be vastly different
at two different.
27. What is long normal shadowing?
The long-normal distribution describes the random shadowing effects which occur over a large number
of
measurement locations which
have the same T-R separation, but
have different levels of clutter on the propagation path. This phenomenon is referred to as long- normal shadowing.
28. Name some of the outdoor propagation models.
Some of the commonly used outdoor propagation models are
(i) Longely-rico model
(ii) Durkin’s model
(iii) Okumura model.
29. 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.
30. What are modes available in longly rice model?
The longly-rice model operates in two modes. (i)Point-to-Point
mode
prediction.
(ii)Area mode
prediction.
31. What is point-to-point prediction?
When a detailed terrain path profile is available, the path specific parameters can be easily determined and the prediction is called a point-to-point
mode
prediction
.
32. Define area mode prediction.
If
the terrain
path profile is
not
available, the
Longley-rice method provides techniques to estimate the path-specific parameters and such prediction are called an area mode
prediction.
33. What are the drawbacks available in Longley-rice models?
Longley-rice
model
does
not
provide a way of determing correction
due
to environmental factors in the immediate vicinity (neighbors) of the mobile receiver. Here multipath is also not considered.
34. Explain Durkin’s model?
Durkin’s model describes a computer simulator for predicting field strength contours over
irregular terrain, for the estimation of effective mobile radio coverage areas.
35. Mention some of the terrain related parameters.
The terrain undulation, (a way motion or form) height (∆h), isolated ridge height, average slope of the terrain and the mixed land-sea parameter, are the some of the important terrain related parameters.
36. Write the advantages of Okumura model.
(i) It is simplest
and
best in terms of accuracy in path loss prediction.
(ii) It is very practical and become a standard for system
planning in modern land mobile radio systems in Japan.
(iii) This model is fairly good in urban and suburban areas.
(iv) All the correction factors are also available as Okumura curves.
37. What are the drawbacks available in Okumura models?
(i) Its slow response to rapid
changes in terrain, so its not good in rural areas.
(ii) This model is fully based on measured data and does not provide any analytical explanation.
38. Write the hata model standard formula for calculate medium path loss.
The standard formula for
medium path loss in urban areas is given by
L50 (urban) (dB) = 69.55 + 26.16 log fc – 13.82 log the – a (hre)
+ (44.9 – 6.55 log the) log d
Where, fc is the frequency
hre is the effective receiver (mobile) antenna height.
d is the T-R se[parathion distance (in km)
a (hre) is the correction factor for effective mobile antenna height
Which is a function is the size of the coverage area
39. What are the advantages of hata model?
a) The predictions of the hata model compare very closely with the original Okumura model, as long as d exceeds 1 km.
b) This model is well
suited for large cell
mobile systems.
40. Define indoor propagation models.
The indoor propagation is used to characterizing radio propagation inside the buildings.
41. How indoor radio channel
differs from the traditional mobile radio channel?
The indoor radio channel differs from
the
traditional mobile radio channel
in
two aspects.
a) The distance covered is much smaller.
b) The variability of the environment is much greater
for a much smaller range of T-R
separation distances.
42. What are the mechanisms available in the indoor propagation models?
Reflection, diffraction, and scattering are the propagation mechanisms available in the indoor
radio
propagation.
43. Write the classification in the indoor
channels.
Generally indoor channels may be
classified either
as
line-of-sight (LOS) or obstructed (OBS) with varying degrees of
clutter.
44. Mention some indoor propagation model.
Some of the indoor propagation models are a) Long-distance path loss model
b) Ericsson multiple break point model c) Attenuation factor model
45. Define hard partitions
Partitions
that are formed as part of the building structure are called hard partitions.
46. Define soft partitions.
Partitions
that may be moved and which do not span to the ceiling are called soft partitions.
47. State Huygen’s principle.
Hurgen’s principle states that all points on a wave front
can be considered as point sources for the
production of secondary wavelets, and that these wavelets combine to produce
a new waterfront
in
the direction of propagation.
48. Mention the drawback of long-distance path loss model.
The long-distance
path
loss model equation
does
not
consider the
fact
that
the surrounding environment clutter may be greatly different at two different locations having the same T-R separation. This leads to measured signals which are vastly different than the average value predicted by the model.
49. What are the demerits of Longley-rice model?
a) Longley-rice model does not provide a way of determing corrections due to environmental factors in the immediate vicinity of the mobile receiver and
b) Multipath is also not considered.
50. Mention the problems due to non-LOS.
For the case of non-LOS, the system 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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