Chapter 3

Mobile Radio Propagation

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Outline
• Free space propagation
• Basic Propagation models
 Reflection
 Diffraction
 Scattering
• Path Loss and Shadowing Models

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Wave Propagation
 Radio, microwave, infrared and visible light portions of the spectrum can all be used to
transmit information
 By modulating the amplitude, frequency, or phase of the waves.
 The amount of information a wireless channel can carry is related to its bandwidth
 Wavelength dictates the optimum size of the receiving antenna

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Characteristics of Radio Waves
 Easy to generate
 Can travel long distances
 Can penetrate buildings
 Used for indoor and outdoor communication
 Can be narrowly focused at high frequencies (greater than
100MHz) using parabolic antennas (like satellite dishes)
 Subject to interference from other radio wave sources

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Characteristics of Radio Waves(cont.)
Properties of radio waves are frequency dependent
 At low frequencies, they can pass through obstacles , but the power falls
off sharply with increase in distance from the source
 At high frequencies, they tend to travel in straight lines and bounce of
obstacles (they can also be absorbed by big objects)
LOS path

Reflected Wave

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Communication Channels
 Wired Channel
o Stationary
o Predictable
 Wireless channel
o Random
o Difficult to analyze
o Susceptible to noise, interference, other time varying channel impairments

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Channel models for Wireless Communication
 Physical models: Considers exact profile of the propagation environment.
 Modes of propagation considered: Free-space or LOS, reflection, and
diffraction.
 Statistical models: Takes an empirical approach.
 The model is developed on measuring propagation characteristics in a variety of
environments. They are easy to describe and use than physical models.

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Need for Propagation models

Propagations models can be used to determine

 Coverage area of a transmitter
 Transmit power requirement
 Battery lifetime
 Modulation and coding schemes required to improve
the channel quality
 Maximum achievable channel capacity of the system
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Propagation models
Large-scale propagation models
o Characterize signal strength for large Transmitter -Receiver separation (several
hundreds or thousands of meters)
o Compute local average received power by averaging signal measurements over a track of
5 to 40
o Received signal decrease gradually
o Useful for estimating the coverage area of transmitters
Small-scale propagation models
o Characterize rapid fluctuations in the received signal strength over very short travel
distances (a few wavelengths)
o Signal is the sum of many contributors coming from different directions. Thus phases of
received signals are random and the sum behave like a noise (Rayleigh fading)
o Received power may vary by as much as 3 or 4 orders of magnitude (30 or 40 dB)

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Small-Scale and Large-Scale Fading

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Free-Space Propagation Model
 Predict the received signal strength when transmitter and receiver have clear, unobstructed LOS
path between them.
 Ex: Satellite communication system, microwave LOSsystem

 The received power decays as a function of Transmitter-Receiver separation raised to some power.
 Free space power received by a receiver antenna is given by Friis free-space equation

oPt is transmitted power o Pr(d) is the received power

oGt, Gr is the Tx, Rx antenna gain o d is Tx-Rx separation distance in meters
(dimensionless quantity) o  is wavelength in meters
o L is system loss factor not related to propagation (L≥1). L= 1 indicates no loss in system
hardware (we consider L = 1 in our calculations)

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Free-Space Propagation Model (cont.)
 The gain of an antenna G is related to its affective aperture Ae by G = 4Ae / 2 where
• Ae is related to the physical size of theantenna
•  is related to the carrier frequency ( = c/f = 2c / c ) where
o f is carrier frequency in Hertz
o c is speed of light in meters/sec
o c is carrier frequency in radians per second
 Isotropic radiator generally considered a reference antenna in wireless systems; radiates power
with unit gain uniformly in all directions.
 Effective isotropic radiated power (EIRP) is the amount of power that a theoretical isotropic
antenna emits to produce peak power density in the direction of maximum antenna gain.
 EIRP = PtGt
 Antenna gains are given in units of dBi (dB gain with respect to an isotropic antenna)

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What is Decibel (dB)
 A logarithmic unit used to describe a ratio between two values of a physical
quantity (usually measured in units of power or intensity)
oThe ratio of two values 𝑃1and 𝑃2 in dB is
 10 log (𝑃/ 𝑃2)dB

 dB unit is generally used to describe ratios of numbers with modest size.

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dBm

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dBW

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Free-Space Path Loss

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Reference Distance, 𝑑0

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Radio propagation mechanisms

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Radio propagation mechanisms, cont..

Radio Frequency and Wireless Communications

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Reflection
 Reflection occurs when wave impinges upon an obstruction much larger in size compared to the wavelength of
the signal
oExample: reflections from earth and buildings

 Reflected waveform may interfere with the original signal constructively (Positively) or destructively
(Negatively)
 When a radio wave propagating in one medium impinges upon another medium having different electrical
properties, the wave is partially reflected and partially transmitted
oPerfect dielectric:
Part of the energy is transmitted into the second medium and part of the energy is reflected back into the
first medium
 no loss of energy in absorption
o Perfect conductor:
 All incident energy is reflected back into the first medium
 No loss of energy.

 The fraction that is reflected is described by the Fresnel equation and is dependent upon the incoming light's
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polarization and angle of incidence. (Reading assignment on Fresnel equation derivation )
Reflection (cont.)
 Electromagntives waves are transmitted in two orthogonal dimensions, referred to as
polarizations. Two commonly used orthogonal sets of polarizations are
o Horizontal and Vertical polarization
• Vertical polarization is commonly used in terrestrial mobile radio communication. In
VHF band, vertical polarization produces a higher field strength near the ground. Also,
mobile antennas for vertical polarization are more robust and convenient to implement.
o Left-hand and right-hand circular polarization
• Often used in satellite communication. Can be used together for well-designed
communication links to double the transmission capacity in a given frequency band.
 In a mobile radio channel, a single direct path between the Base Station and a mobile is
seldom the only physical means for propagation and the Free space propagation model is
inaccurate in most cases when used alone. Two-ray model is
• Based on geometric optics and it considers both the direct path and a ground reflected
propagation path
• Reasonably accurate for predicting the large scale signal strength over distances of several
kilometers for mobile radio systems that use tall towers. 21
Ground Reflection Model (cont.)

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Ground Reflection Model (cont.)

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Diffraction
 Diffraction occurs when radio wave is obstructed
by an impenetrable body or a surface with sharp
irregularities (edges)
 Due to bending of radio waves it enables
communication between devices with no line-
of-sight path
 Secondary waves are present throughout the
space including the space behind the obstacle
due to bending of waves around the obstacle.
 Enables communication even when a line of
sight path does not exist between transmitter
and receiver.
 At high frequencies, diffraction depends on the
geometry of the object, as well as the amplitude,
phase and polarization of the incident wave at the
point of diffraction

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Diffraction
Huygens’Principal
 All points on a wavefront can be considered as point sources for
producing secondary wavelets
 Secondary wavelets combine to produce new wavefront in the
direction of propagation
 Diffraction arises from propagation of secondary wave front into shadowed
area
 Field strength of diffracted wave in shadow region =  electric field
components of all secondary wavelets in the space around the obstacle
 Consider a transmitter-receiver pair in free space
 Obstacle of effective height h with infinite width is placed between Tx and Rx
• distance from transmitter = d1
• distance from receiver = d2
 LOS distance between transmitter & receiver is 𝑑 =𝑑1+𝑑2
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Diffraction (cont.)

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Diffraction Fresnel zones

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Diffraction Fresnel zones (cont.)
 Fresnel Zones explains the concept of
diffraction loss as a function of path difference.
 Secondary waves in successive regions
have a path length n/2 greater than LOS
path.
• nth region is the region where path length of
secondary waves is n/2 greater than that of
LOS path length
 Regions form a series of ellipsoids with
attentions at Tx & Rx antennas

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Diffraction Fresnel zones (cont.)

2(d1  d 2 ) 2d1d2
v= h  
d1d2 (d1 d 2 )

TX RX
d1 d2 d1 h d2
TX RX 
If h = 0, then  and v are 0

If  and v are negative, then h

is negative
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Diffraction Fresnel zones (cont.)

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Diffraction Fresnel zones (cont.)

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Diffraction Fresnel zones (cont.)

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Diffraction፡ Diffraction Loss
• Diffraction Loss is caused by blockage of secondary (diffracted) waves
 Partial energy from secondary waves is diffracted around an obstacle
 obstruction blocks energy from some of the Fresnel zones and only a portion of
transmitted energy reaches receiver
 Received energy is vector sum of contributions from all unobstructed Fresnel zones
 depends on geometry of obstruction
 phase of secondary (diffracted) E-field is indicated by the Fresnel Zones
 Obstacles may block transmission paths causing diffraction loss
 construct a family of ellipsoids between TX & RX to represent Fresnel zones
 join all points for which excess path delay is multiple of 𝜆 / 2
 compare geometry of obstacle with Fresnel zones to determine diffraction loss (or
gain)

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Diffraction፡ Diffraction Loss (cont.)
 Place ideal, perfectly straight screen between Tx and Rx
 If top of screen is well below LOS path then
screen will have little effect
 the Electric field at Rx = 𝐸𝐿𝑂𝑆 (free space
value)
 As screen height increases, Electric field will  If (55 to 60)% of 1st Fresnel zone is clear
vary as screen blocks more Fresnel zones than further Fresnel zone clearing does not
 The amplitude of oscillation increases until significantly alter diffraction loss
the screen is just in line with Tx and Rx  For free-space transmission condit-
 field strength = ½ of unobstructed field ions,1st Fresnel Zone is kept unblocked
strength

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Diffraction፡Knife Edge Diffraction Model
 Diffraction Losses
 estimating attenuation caused by diffraction over obstacles is
essential for predicting field strength in a given service area
 not possible to estimate losses precisely
 theoretical approximations typically corrected with empirical
measurements
 Computing Diffraction Losses
 for simple terrain: expressions have been obtained
 for complex terrain: computing diffraction losses is complex

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 Huygens
secondary

Diffraction source

Knife Edge Diffraction Model (cont.)

 Knife-edge model is the simplest model that
provides insight about magnitude of diffraction loss
 Diffraction losses are estimated using the classical Fresnel solution
for field behind a knife edge
 Useful for shadowing caused by 1 knife edge object
 Considers receiver Ris located in shadowed region
 E-field strength at R is vector sum of all fields due to secondary
Huygens’ sources in the plane above the knife edge

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Diffraction
Knife Edge Diffraction Model (cont.)
 The diffraction gain due to the presence of knife edge, as compared to
the free space E-field

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Diffraction: Knife Edge Diffraction Model (cont.)

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Diffraction
Multiple Knife-Edge Diffraction Model
Bullington's model
owith more than one obstruction: compute total diffraction loss is by
replacing multiple obstacles with one equivalent obstacle
ouse single knife edge model
Disadvantage:
ooversimplifies problem
ooften produces overly optimistic estimates of received signal strength

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Scattering
 Scattering occurs when obstacle size is less than or of the order of the
wavelength of propagating wave
 Causes the transmitter energy to be radiated in many directions
 Occur due to small objects, rough surfaces, and other irregularities of the
channel.
 Number of obstacles are quite large
 Scattering follows the same principles as diffraction
 Received signal strength is often stronger than that predicted by
reflection/diffraction models alone
 The EM wave incident upon a rough or complex surface is scattered in
many directions and provides more energy at a receiver
 Energy that would have been absorbed is instead reflected to the receiver
o flat surface → EM reflection (one direction)
o rough surface → EM scattering (many directions) 40
Scattering (cont.)
 Critical height for surface protuberances ℎ𝑐
for given incident angle

 Let ℎ be the maximum

protuberances, then surface is
considered
o smooth if ℎ < ℎ𝑐
o rough if ℎ > ℎ𝑐

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 Reflection, Diffraction, and Scattering
 As a mobile moves through a region, these mechanisms have an
impact on the instantaneous received signal strength
 In case LOS path exists between the devices, diffraction and
scattering will not dominate the propagation.
 If device is at a street level without LOS path, then
diffraction and scattering will probably dominate the
propagation.

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Path Loss Models
 Radio Propagation models are derived using a combination of empirical and
analytical methods.
 These methods implicitly take into account all the propagation factors both
known and unknown through the actual measurements.
 Path loss models are used to estimate the received signal level as a
function of distance.
 With the help of this model we can predict SNR for a mobile
communication system.
 Path loss estimation techniques
o Log - Distance Path Loss Model
o Log - Normal Shadowing

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Path Loss Models, cont.
Log-distance path loss model Path loss exponent for different environments
 Average large scale path loss is
d 
PL(dB)  PL(d 0 ) 10n log  
 d0 
 Where:

 PL: is ensemble average of all possible path loss

values for given value of d

 On log-log scale path loss is a straight line

with slope equal to 10 n dB/decade

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Path Loss Models (cont.)

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Path Loss Models
Log-Normal Shadowing Model

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Path Loss Models
Outdoor Propagation
 We will look to the propagation characteristics of the three outdoor environments
o Propagation in macrocells
o Propagation in microcells
o Propagation in street microcells

Outdoor Propagation: Macrocells

 Base stations at high-points
 Coverage of several kilometers
 The average path loss in dB has normal distribution
 Average path loss is a result of forward scattering over a large number of obstacles each
contributing a random multiplicative factor. On changing to dB, it is a sum of random
variables
 Sum is normally distributed because of central limit theorem 47
Outdoor Propagation: Longley-Rice Propagation Prediction Model
 Point-to-point communication in frequency range 40 MHZ to 100GHz
 Also referred as irregular terrain model (ITM)
 Predicts median transmission loss, takes terrain into account, uses path geometry, calculates diffraction losses
 Inputs of computer program of Longley-Rice model :
o Frequency
o Path length
o Polarization and antenna heights
o Surface refractivity
o Effective radius of earth
o Ground conductivity
o Ground dielectric constant
o Climate

 Disadvantages

o Does not take into account details of terrain near the receiver

o Does not consider Buildings, Foliage, Multipath

 Original model modified by Okamura for urban terrain (include extra term called urban factor)

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Outdoor Propagation: Okumura Model
 In early days, the models were based on emprical studies
 Okumura did comprehesive measurements in 1968 and came up with a model.
oDiscovered that a good model for path loss was a simple power law where the exponent 𝑛is a function of
the frequency, antenna heights, etc.
o It is one of the most widely used models for signal prediction in urban areas,
oValid for frequencies in: 150 MHz – 1920 MHz for distances: 1km – 100km

𝐿50(𝑑)(𝑑𝐵) = 𝐿𝐹(𝑑) + 𝐴𝑚𝑢(𝑓, 𝑑) – 𝐺(ℎ𝑡𝑒) – 𝐺(ℎ𝑟𝑒) – 𝐺area

• 𝐿50: 50th percentile (i.e. median) of path loss

• 𝐺area: gain due to different type of environment

• 𝐿𝐹 (𝑑): free space propagation path loss

• 𝐴𝑚𝑢(𝑓, 𝑑): median attenuation relative to free space

• ℎ𝑡𝑒: transmitter antenna height

• ℎ𝑟𝑒: receiver antenna height

• 𝐺(ℎ𝑡𝑒): base station antenna height gain factor

• 𝐺 ℎ𝑡𝑒 and 𝐺 ℎ𝑟𝑒 are determined for different antenna height 49

Outdoor Propagation Okumura Model (cont.)

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Outdoor Propagation
Okumura Model (cont.)
 Advantage
 Okumuras’ model is considered to be among the simplest and
best in terms of accuracy in path loss prediction for mature
cellular and land mobile system in a cluttered environment.
 Disadvantage
 Low response to rapid changes in terrain

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Outdoor Propagation, Hata Model

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Outdoor Propagation Hata Model (cont.)
 For small to medium sized city:
 For large city:

 In sub urban areas, path loss is:

 In open rural areas, path loss is:

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Outdoor Propagation Hata Model (cont.)
 No path specific corrections
 Suitable for large cell mobile system (d >1 km)
 Not suitable for Personal computer service (PCS)

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Outdoor Propagation: PCS Extension of Hata Model

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Path Loss Models: Microcells
Propagation differs significantly
 Milder propagation characteristics
 Small multipath delay spread and shallow fading imply the feasibility of higher
data-rate transmission
 Mostly used in crowded urban areas
 If transmitter antenna is lower than the surrounding building than the signals propagate
along the streets: Street Microcells
 Macrocells versus Microcells

Item Macrocell Microcell

Cell Radius 1 to 20km 0.1 to 1km
Tx Power 1 to 10W 0.1 to 1W
Fading Rayleigh Nakgami-Rice
RMS Delay Spread 0.1 to 10s 10 to 100ns
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Max. Bit Rate 0.3 Mbps 1 Mbps
Path Loss Models: Street Microcells
 Most of the signal power propagates along the street
 The signals may reach with LOS paths if the receiver is along the same street
with the transmitter
 The signals may reach via indirect propagation mechanisms if the
receiver turns to another street

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 Path Loss Models: Street Microcells
D
Building Blocks

B C
A
Breakpoint

received power (dB) received power (dB)

A A n=2
n=2 Breakpoint 15~20d
B
C n=4 B
n=4~8 D
log (distance)
log (distance)
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Indoor Propagation
 Indoor channels are different from traditional mobile radio channels in two
different ways:
• The distances covered are much smaller
• The variablity of the environment is much greater for a much smaller range of T-R
separation distances.
 The propagation inside a building is influenced by:
• Layout of the building
• Construction materials
• Building type: sports arena, residential home, factory, etc
 Indoor path loss models are less generalized
o Environment comparatively more dynamic
 Significant features are physically smaller
o Smaller propagation distances
 Less assurance of Far-field for all receiver locations and antenna types. 59
Path Loss Models, Indoor Propagation (cont.)
 Indoor propagation is dominated by the same mechanisms as outdoor: reflection, scattering, diffraction.
o However, conditions are much more variable
 Doors/windows open or not
 The mounting place of antenna: desk, ceiling, etc.
 The level of floors

 Indoor channels are classified as

o Line-of-sight (LOS)
o Obstructed (OBS) with varying degrees of clutter.
 Buiding types
o Residential homes in suburban areas
o Residential homes in urban areas
o Traditional office buildings with fixed walls (hard partitions)
o Open plan buildings with movable wall panels (soft partitions)
o Factory buildings
o Grocery stores
o Retail stores
o Sport arenas
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Indoor propagation, Events and parameters
 Temporal fading for fixed and moving terminals  Indoor propagation Models
• oMotion of people inside building causes Ricean
Fading for the stationary receivers  Log-distance path loss model
• oPortable receivers experience in general:  Same floor partition losses
 Rayleigh fading for obstructed propagation paths • Hard partitions (cannot be moved) /soft partitions (can be
 Ricean fading for LOS paths. moved)
 Multipath Delay Spread • Internal walls & external walls

o Buildings with fewer metals and hard-partitions  Partition loss between floors
typically have small rms delay spreads: 30-60ns. • Determined by the dimensions/materials
 Can support data rates excess of several Mbps used/surroundings, including number of windows) /floor
without equalization attenuation
o Larger buildings with great amount of metal and  Ericsson multiple breakpoint model
open aisles may have rms delay spreads as large as
• Obtained by measurements in a multiple office building
300ns.
 Can not support data rates more than a few • Has 4 breakpoints and has upper & lower bound on the PL
hundred Kbps without equalization. • Model assumes 30 db attenuation at do=1m, for f=900Mhz,
unity gain antenna

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