Cellular Comm-EC3036-Notes-Module-2.pptx

EC 3036
Cellular Communication

Module 2
RF Propagation & Multi-path Model

Free Space Propagation Model
 The free space propagation model is used to predict received signal strength when the transmitter and
receiver have a clear, unobstructed line-of-sight (LOS) path between them (satellite link, microwave link etc.).
The free space power received by a receiver antenna which is separated from a radiating transmitter antenna by
a distance d, is given by the FRIIS free space equation,
Pt, Gt power transmitted and gain of the transmitting antenna and Pr, Gr are the power received and gain of the
receiving antenna at a distance ‘d’ apart. ‘L’ is the system loss factor related to the propagation. Gain of the
antenna has relation with effective aperture area Ae as
The miscellaneous losses L (L ≥1) are usually due to transmission line attenuation, filter losses, and
antenna losses in the communication system. A value of L = I indicates no loss in the system hardware.

Free Space Propagation Model
 An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions, and is
often used to reference antenna gains in wireless systems. The effective isotropic radiated power (EIRP) is
defined as EIRP = Pt Gt which represents the maximum radiated power available from a transmitter in the
direction of maximum antenna gain, as compared to an isotropic radiator.
The path loss for the free space model when antenna gains are included is given by
When antenna gains are excluded, antennas are assumed to have unity gain, and path loss is given by
FRIIS transmission formula is valid for far field (Fraunhofer zone). In mobile radio systems, it is not
uncommon to find that may change by many orders of magnitude over a typical coverage area of several square
kilometers.

Three Basic Propagation Mechanisms
 Reflection, diffraction, and scattering are the three basic propagation mechanisms which impact propagation in a
mobile communication system.
Reflection: Reflection occurs when a propagating electromagnetic wave impinges upon an object which has
very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the
surface of the earth and from buildings and walls.
Diffraction: Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a
surface that has sharp irregularities (edges). The secondary waves resulting from the obstructing surface are
present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle,
even when a line-of-sight path does not exist between transmitter and receiver. At high frequencies, diffraction, like
reflection, depends on the geometry of the object, as well as the amplitude, phase, and polarization of the incident
wave at the point of diffraction.
Scattering: Scattering occurs when the medium through which the wave travels consists of objects with
dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is
large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel. In
practice, foliage, street signs, and lamp posts induce scattering in a mobile communications system.

Cellular Comm-EC3036-Notes-Module-2.pptx

Log-distance Path Loss Model (Outdoor Propagation)
 Both theoretical and measurement-based propagation models indicate that average received signal power
decreases logarithmically with distance, whether in outdoor or indoor radio channels.
The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by
using a path loss exponent, n.
or
 Here, d0 is the close-in reference distance which is determined from measurements close to the transmitter, and
d is the T-R separation distance. The bars in equations denote the ensemble average of all possible path loss
values for a given value of d [Ensemble average is averaged quantity of a many identical systems at a certain
time].
 The value of n depends on the specific propagation environment. For example, in free space, n is equal
to 2, and when obstructions are present, n will have a larger value.

Log-normal Shadowing
 The log-distance path loss equation does not consider the fact that the
surrounding environmental clutter may be vastly different at two different locations having the same T-R
separation.
 Measurements have shown that at any value of d, the path loss PL(d) at a particular location is random and
distributed log-normally (normal in dB) about the mean distance dependent value, that is,
and
where Xσ, is a zero-mean Gaussian distributed random variable (in dB) with standard deviation σ (also in
dB).
The log-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 log-normal shadowing. Log-normal Shadowing is path loss formula in
presence of clutter/shadowing.

Outdoor Propagation Models
Radio transmission, in a mobile communications system often takes place over irregular
terrain. The terrain profile of a particular area needs to be taken into account for estimating the path
loss. The terrain profile may vary from a simple curved earth profile to a highly mountainous profile.
The presence of trees, buildings, and other obstacles also must be taken into account. A number of
propagation models are available to predict path loss over irregular terrain, mainly based on
measurements.
Widely used propagation models are Okumura model and Hata model.

Okumura Model
Okumura's model is wholly based on measured data and does not provide any analytical explanation.
Okumura's model is considered to be among the simplest and best in terms of accuracy in path
loss prediction for mature cellular and land mobile radio systems in cluttered environments.
Okumura's model is one of the most widely used models for signal prediction in urban areas.
This model is applicable for frequencies in the range 150 MHz to 1920 MHz (although it is typically
extrapolated up to 3000 MHz) and distances of 1 km to 100 km. It can be used for base station antenna
heights ranging from 30 m to 1000 m.
Okumura developed a set of curves giving the median attenuation relative to free space (Amu), in
an urban area over a quasi-smooth terrain with a base station effective antenna height (hte) of 200
m and a mobile antenna height (hre) of 3 m.
These curves were developed from extensive measurements using vertical omni-directional
antennas at both the base and mobile, and are plotted as a function of frequency in the range 100
MHz to 1920 MHz and as a function of distance from the base station in the range 1 km to 100 km.

Okumura Model
In Okumura‘s model, the free space path loss between the points of interest is first determined, and
then the value of Amu(f, d) (as read from the curves) is added to it along with correction factors to account
for the type of terrain. The model can be expressed as
where L50 is the 50th percentile (i.e., median) value of propagation path loss, LF is the free space
propagation loss, Amu(f, d) is the median attenuation relative to free space, G(hte) is the base station
antenna height gain factor, G(hre) is the mobile antenna height gain factor, and GAREA is the gain
due to the type of environment. The antenna height gains are strictly a function of height and have
nothing to do with antenna patterns.

Okumura Model
Plots of Amu(f, d) & GAREA for a wide range of frequencies are shown in Figures.
Okumura found that G(hte) varies at a rate of 20 dB/decade and G(hre) varies at
a rate of 10 dB/decade for heights less than 3 m.

Hata Model
The Hata model is an empirical formulation of the graphical path loss data provided by Okumura, and is
valid from 150 MHz to 1500 MHz. Hata presented the urban area propagation loss as a standard formula.
The standard formula for median path loss in urban areas is given by
fc is the frequency (in MHz) from 150MHz to 1500MHz, hie is the effective transmitter (base station) antenna height
(in meters) ranging from 30m to 200m, hre is the effective receiver (mobile) antenna height (in meters) ranging
from 1m to 10m, d is the T-R separation distance (in km), and a(hre) is the correction factor for effective mobile
antenna height which is a function of the size of the coverage area.
For a small to medium sized city, mobile antenna correction factor
For a large city it is given by
For path loss in suburban area standard Hata formula in equation is modified as
For path loss in open rural areas, the formula is modified as

PCS Extension of Hata Model
The predictions of the Hata model compare very closely with the original Okumura model, as long as d exceeds
1 km. This model is well suited for large cell mobile systems, but not personal communications systems
(PCS) which have cells of the order of 1km radius.

Indoor Propagation Models
The indoor radio channel differs from the traditional mobile radio channel in two aspects — the distances
covered are much smaller, and the variability of the environment is much greater for a much smaller range
of T-R separation distances.
It has been observed that propagation within buildings is strongly influenced by specific features such as
the layout of the building, the construction materials, and the building type.
Indoor radio propagation is dominated by the same mechanisms as outdoor: reflection, diffraction, and
scattering. However, conditions are much more variable.
For example, signal levels vary greatly depending on whether interior doors are open or closed inside a
building.
Buildings have a wide variety of partitions and obstacles which form the internal and external structure.
Hard partitions (part of building structures) & soft partitions (made of wood etc. with open ceiling) have
different effects in wave propagation.

Log-distance Path Loss Model (Indoor Propagation)
Indoor path loss has been shown by many researchers to obey the distance power law as
where the value of n depends on the surroundings and building type, and Xσ represents a normal
random variable in dB having a standard deviation of σ dB.
Notice that equation is identical in form to the log-normal shadowing model.

Large-scale Fading
Fading is used to describe the rapid fluctuation of the amplitude of a radio signal at the receiving end.
Fading is caused by interference between two or more versions of the transmitted signal which arrive at the
receiver at slightly different times.
Large-scale Fading (Long-term Fading): Large-
scale fading represents average signal power
attenuation due to movement over large areas
having irregular terrain configuration (mountain,
valley etc.) between receiver and transmitter and it
happens over a long distance (several hundreds
meters).
Under certain circumstances, the fluctuation of a
large-scale fading, caused by the terrain configuration
may cause log-normal distribution because of the
statistical nature of fluctuation.

Small-scale Fading
Small-scale Fading (Short-term Fading): Small-scale Fading is used
to describe the rapid fluctuation of the amplitude of a radio signal
over a short period of time or travel distance.
These waves, called multipath waves, combine at the receiver
antenna to give a resultant signal which can vary widely in amplitude
and phase, depending on the distribution of the intensity and relative
propagation time of the waves and the bandwidth of the transmitted
signal.
The cause of small-scale fading is mainly due to local scatterers
like, houses, flats and other human-made structures.
Random frequency modulation due to varying Doppler shifts on
different multipath signals.
Time dispersion (echoes) caused by multipath propagation
delays.
Multi-path propagation occurs mainly in NLOS situations but even a
line-of-sight exists, multipath still occurs due to reflections from the
ground and surrounding structures.

Small-scale Fading
Doppler Shift: Consider a mobile moving at a constant velocity v, along a
path segment having length ‘d’ between points X and Y, while it receives signals
from a remote source S. The difference in path lengths traveled by the wave
from source S to the mobile at points X and Y is ∆l= dcosθ = v∆tcosθ. where ∆t
is the time required for the mobile to travel from X to Y, and θ is assumed to be
the same at points X and Y since the source is assumed to be very far away.
The phase change in the received signal due to the difference in path
lengths is therefore ∆ϕ=2π ∆l/λ = 2π v∆tcos θ /λ
Apparent change in frequency or Doppler shift is given by
fd =(1/2π) ∆ϕ/∆t = (v/λ) cosθ
The incoming radio waves arrive from different directions with different propagation delays. The signal received by
the mobile at any point in space may consist of a large number of plane waves having randomly distributed
amplitudes, phases, and angles of arrival. These multipath components combine vectorially at the receiver antenna,
and can cause the signal received by the mobile to distort or fade.
The physical factors in the radio propagation channel influence small-scale fading are Multipath propagation,
Speed of the mobile, Speed of surrounding objects, Transmission bandwidth of the signal.

Probability Density Functions (PDFs)

Level Crossing Rate (LCR) & Average
Duration of Fade (ADF)

Flat-Flat Fading Channel
T-Signal Duration
W-Signal Bandwidth
F-Doppler Spread
δ-Time Spread

Time-Dispersive Channel//Frequency-
Selective Fading Channel//Time-Flat
Fading Channel
T-Signal Duration
W-Signal Bandwidth
F-Doppler Spread
δ-Time Spread

Frequency-Dispersive Channel//Time-
Selective Fading Channel//Frequency-
Flat Fading Channel
T-Signal Duration
W-Signal Bandwidth
F-Doppler Spread
δ-Time Spread

Delay Spread
When a signal is transmitted from transmitter, at the receiving end it reaches at different times due
multiple reflections and the signal information reaches at the receiver over a time period instead of a
particular time. This stretching of signal in time domain is known as Delay Spread resulting echo of signal.
In the mobile radio environment, as a result of the multipath reflection phenomenon, the signal
transmitted from a cell site and arriving at a mobile unit will be from different paths, and since each path
has a different path length, the time of arrival for each path is different.
Delay Spread is a time dispersion parameter.
For an impulse transmitted at the cell site, by the time this impulse is received at the mobile unit, it is no
longer an impulse but rather a pulse with a spread width that we call the delay spread.

Coherence Bandwidth
Coherence bandwidth BC is a statistical measure of the range of frequencies over which the channel can
be considered "flat" (i.e., a channel which passes all spectral components with approximately equal gain and
linear phase).
In other words, coherence bandwidth is the range of frequencies over which two frequency components have a
strong potential for amplitude correlation.
The coherence bandwidth is the defined as the bandwidth in which either the amplitudes or the phases of two
received signals have a high degree of similarity.
The delay spread is a natural phenomenon, and the coherence bandwidth is related to the delay spread (∆). A
coherence bandwidth for two fading amplitudes of two received signals is BC = 1/2π∆,

Doppler Spread & Coherence Time
Delay spread and coherence bandwidth are parameters which describe the time dispersive nature of the
channel in a local area. However, they do not offer information about the time varying nature of the channel
caused by either relative motion between the mobile and base station, or by movement of objects in the channel.
Doppler spread and coherence time are parameters which describe the time varying nature of the channel in a
small-scale region.
Doppler spread BD is a measure of the spectral broadening caused by the time rate of change of the mobile
radio channel.
When a pure sinusoidal tone of frequency fc is transmitted, the received signal spectrum, called the Doppler
spectrum, will have components in the range fc — fd to fc + fd where fd is the Doppler shift. The amount of spectral
broadening depends on which is a function of the relative velocity of the mobile, and the angle 9 between the
direction of motion of the mobile and direction of arrival of the scattered waves.

 If the baseband signal bandwidth is much greater than Doppler Spread BD, the effects of Doppler spread
are negligible at the receiver. This is a slow fading channel.
 In a slow fading channel, the channel may be assumed to be static over one or several reciprocal
bandwidth intervals. In the frequency domain, this implies that the Doppler spread of the channel is much
less than the bandwidth of the baseband signal.
Coherence time TC, is the time domain dual of Doppler spread and is used to characterize the time
varying nature of the frequency dispersiveness of the channel in the time domain. The Doppler spread and
coherence time are inversely proportional to one another. That is,
TC = 1/fm
Where, fm is the maximum Doppler shift v/λ [ from fd =(v/λ) cosθ].
In a fast fading channel, the channel impulse response changes rapidly within the symbol duration.
That is, the coherence time of the channel is smaller than the symbol period of the transmitted
signal.
Fast fading only deals with the rate of change of the channel due to motion.
Doppler Spread & Coherence Time

Inter Symbol Interference (ISI)
Due to the Delay Spread when stretched signal ( in digital form
symbol) reaches at the receiving end, it is possible that the next
transmitted signal will also suffer Delay Spread resulting superposition
of two symbols partially. This is symbol interference and is known as
Inter Symbol Interference (ISI).
Inter Symbol Interference (ISI) is a distortion in communication and
mainly happens due to time delay spread.
There are several methods of mitigation of Inter Symbol
Interference, some of which are:
 Control of transmitted symbol data rate.
Channel-based Adaptive equalization.
Use of effective error correcting codes.

Cellular Comm-EC3036-Notes-Module-2.pptx

More Related Content

Similar to Cellular Comm-EC3036-Notes-Module-2.pptx (20)

Recently uploaded (20)

Cellular Comm-EC3036-Notes-Module-2.pptx