Antennas and Propagation

▪ Agenda
o Antennas

o Propagation Modes

o Line-of-Sight Transmission

o Fading in the Mobile Environment

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 What is Antenna

▪ Antenna: is an electrical conductor or system of conductors used

either for radiating electromagnetic energy or for collecting
electromagnetic energy.
o Transmission - radiates electromagnetic energy into space
o Reception - collects electromagnetic energy from space

▪ In two-way communication, the same antenna can be used for

transmission and reception.

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Antennas Samples

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 Radiation Pattern

▪ Radiation Pattern
o Graphical representation of radiation properties of an antenna as a
function of space coordinates.
o Depicted as two-dimensional cross section of the 3Dpattern.
▪ Beam Width (or half-power beam width)
o Measure of directivity of antenna
o The angle within which the power radiated by the antenna is at least
half of what it is in the most preferred direction.
▪ Reception Pattern
o Receiving antenna’s, equivalent to radiationpattern

▪ Main, Back and Side Lobes

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Radiation Pattern

▪ Omni Directional
o Radiates in every direction on one
of the principal planes

▪ Directional
o Radiates mainly in one direction

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Antenna Types
I. Isotropic Antenna (Idealized)
o Theoretical reference antenna
o Radiates power equally in all directions
II. Dipole Antenna
o Half-wave dipole antenna (or Hertz antenna)
• The length of the antenna is one-half the wavelength of the signal that can be transmitted
most efficiently.
• A half-wave dipole has omni-directional radiation pattern in one dimension and a figure
eight pattern in the other two dimensions.
• More complex antenna configurations can be used to produce a directional beam.
o Quarter-wave vertical antenna (or Marconi antenna)
• Commonly used for automobile radio
and portable radios

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Antenna Types
III. Parabolic Reflective Antenna
o Used in terrestrial microwave and
satellite applications.
o If a source of electromagnetic
energy is placed at the focus of
the paraboloid, and if the
paraboloid has a reflecting
surface, then the wave will bounce
back in lines parallel to the axis of
the paraboloid.
o If incoming waves are parallel to
the axis of the reflecting
paraboloid, the resulting signal
will be concentrated at the focus.
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Antenna Gain
▪ Antenna Gain
o Power output, in a particular direction, compared to that produced in any
direction by a perfect omni-directional antenna (isotropic antenna)
o A measure of the directionality of an antenna
o Unit = dBi = Decibel Isotropic
▪ Effective Area
o Related to physical size and shape of antenna
▪ Relationship between antenna gain and effective area
4𝜋𝐴𝑒 4𝜋𝑓 2 𝐴𝑒
𝐺= =
𝜆2 𝑐2
• G = antenna gain
• Ae = effective area
• f = carrier frequency
• c = speed of light (» 3 x108 m/s)
•  = carrier wavelength
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Antenna Gain

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Antenna Installation
▪ Antenna Installation Parameters
o Height (h)
o Azimuth Angle (θ) – direction with reference to the north
o Tilt Angle (Φ) – radiation pattern incline
• Mechanical (incline the antenna to incline the pattern)

• Electrical (inclining the pattern without inclining the antenna)

o Polarization

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 Smart Antenna
▪ A smart antenna system consists of an array of antennas.
▪ Automatically changes the radiation pattern (Adaptive).
▪ Directs the transmission/reception beam towards the desired user, and
puts a Null in the direction of Interferer.
▪ This method of transmission and reception is called beamforming and is
made possible through smart (advanced) signal processing.
▪ Reduces interference levels and improves the system capacity.
▪ Can determine the Direction of Arrival (DoA)
▪ Exploit multipath instead of mitigating it

Wi-Fi Access Point

with Smart Antenna

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 MIMO
▪ Isthe use of multiple antennas (with adaptive signal processing
techniques) at both the transmitter and receiver to improve
communication performance.
▪ It is one of several forms of Smart Antenna technology
▪ Benefits: Spatial Multiplexing , Diversity, and Interference Reduction
▪ First introduced at Stanford University (1994)
▪ Exploit multipath instead of mitigating it
▪ MIMO – used in current and future broadband
wireless access
o WiFi – 802.11n/ac
o WiMAX – 802.16e
o 3G / 4G / 5G

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Propagation Modes
▪ A signal radiated from an antenna travels along one of three
routes: ground wave, sky wave, or line of sight (LOS)
▪ Ground Wave Propagation
o Follows the earth’s curvature
o Factors:
• The electromagnetic wave induces a current in the earth‘s surface, the result of
which is to slow the wavefront near the earth, causing the wavefront to tilt
downward and hence follow the earth's curvature
• Diffraction
o Can propagate considerable distances
o Frequencies up to 2 MHz
o Example:
• AM Radio

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 Propagation Modes
▪ Sky Wave Propagation
o Signal is reflected from the ionized layer of atmosphere (Ionosphere)
back down to earth.
o Signal can travel a number of hops, back and forth between
ionosphere and earth’s surface
o Reflection effect caused by refraction
o Frequencies: 2 MHz to 30 MHz
o Examples:
• International Radio Broadcast
• Long distance aircraft and ship navigation

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 Propagation Modes
▪ Line-of-Sight (LOS) Propagation
o Transmitting and receiving antennas mustbe within line-of-sight
• Satellite communication – signal not reflected by ionosphere
• Ground communication – antennas within effective line-of-site due to refraction
o Frequencies: Above 30 MHz
o Examples:
• Mobile Communication
• Satellite Communication
• Wi-Fi, Wi-MAX ….

Note: in practice, this mode can be used for

non-line of sight communication because the
signal can penetrate some obstacles and
because of signal reflections.

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Propagation Modes
▪ Refraction
o Velocity of electromagnetic wave is a function of the density of the medium.
o In a vacuum, an electromagnetic wave travels at the speed of light c.
o In air, water, glass, and other transparent or partially transparent media,
electromagnetic waves travel at speeds less than c.
o When wave changes medium, speedchanges
o Wave bends at the boundary between mediums (toward the more dense medium)
o Under normal propagation conditions, the refractive index of the atmosphere decreases
with height so that radio waves travel more slowly near the ground than at higher
altitudes. The result is a slight bending of the radio waves toward the earth.

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 Optical and Radio LOS
▪ Line-of-Sight Equations
o Optical line of sight
d = 3.57 h
o Effective, or radio, line of sight

d = 3.57 h
• d = distance between antenna and horizon (km)
• h= antenna height (m)
• K = adjustment factor to account for refraction, rule of thumb K = 4/3

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Optical and Radio LOS
▪ Line-of-Sight Equations
o Maximum distance between two antennas for effective LOS propagation:

3.57 ( h1 + h 2 )
• h1 = height of antenna one, h2 = height of antenna two

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 Line-of-Sight Transmission
▪ With any communications system, the signal that is received will
differ from the signal that is transmitted, due to various transmission
impairments.
▪ For analog signals, these impairments introduce various random
modifications that degrade the signal quality. For digital data, bit
errors are introduced: A binary 1 is transformed into a binary 0,
and vice versa.
▪ Significant Impairments:
o Attenuation and attenuation distortion
o Free space loss
o Noise
o Atmospheric absorption
o Multipath
o Refraction

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 Attenuation
▪ Strength of signal falls off with distance over transmission medium
o For guided media, this reduction in strength, or attenuation, is expressed as a
constant number of decibels per unit distance.
o For unguided media, attenuation is a more complex function of distance,
frequency, and the makeup of the atmosphere

▪ Attenuation factors for unguided media:

o Received signal musthave sufficient strength so that circuitry in the receiver
can interpret thesignal
o Signal mustmaintain a level sufficiently higher than noise to be received
without error.
o Attenuation is greater at higher frequencies, causing distortion

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 Free Space Loss
• For any type of wireless communication, the signal disperses with
distance.
• Free space loss, ideal isotropic antenna

Pt (4d )2 (4fd )2
= =
Pr 2 c2
• Pt= signal power at transmitting antenna
• Pr= signal power at receiving antenna
•  = carrier wavelength
• d = propagation distance between antennas
• c = speed of light (3 x 108 m/s)
where d and  are in the same units (e.g., meters)

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Free Space Loss
▪ The expression Pt (4d )2 (4fd )2 actually encapsulates two effects.
= =
Pr 2 c2
o Firstly, the spreading out of electromagnetic energy in free space is
determined by: Ps
• PS: is the power per unit area or power spatial density (in w/m2) at distance d.
• Pt:is the total power transmitted (in watts).
** Note that this is not a frequency-dependent effect.
o Thesecondeffect is that of the receiving antenna's aperture, which describes
how well an antenna can pick up power from an incoming electromagnetic
wave. For an isotropic antenna, this is given by:

• Where Pr is the received power.

** Note that this is entirely dependent on wavelength, which is how the frequency-
dependent behavior arises.

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Free Space Loss

Pt (4d )2 (4fd )2
= =
Pr 2 c2

▪ Free Space Loss:

o Proportional to d2
o Inversely proportional to λ2
(proportional to f2)

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Free Space Loss
▪ Free space loss accounting for gain of other antennas

Pt (4 )2 (d )2 (d )2 (cd )2

= = =
t 
2
Pr Gr G Ar At f 2 Ar A
t

• Gt = gain of transmitting antenna

• Gr = gain of receiving antenna
• At = effective area of transmitting antenna
• Ar = effective area of receiving antenna

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 Noise
▪ Thermal Noise
o Thermal noise due to agitation of electrons
o Present in all electronic devices and transmission media
o Cannot be eliminated
o Function of temperature
o Particularly significant for satellite communication
o Amount of thermal noise to be found in a bandwidth of 1Hz in any device or
N 0= kT (W/Hz)
conductor is:

• N0 = noise power density in watts per 1 Hz of bandwidth

• k = Boltzmann's constant = 1.3803 x10-23 J/K
• T= temperature, in kelvins (absolute temperature)
o Noise is assumedto be independent of frequency (White Noise)
o Thermal noise present in a bandwidth of BHertz (in watts):
N = kTB
o or, in decibel-watts: NdBW = 10 log k + 10 log T + 10 log B
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 Noise
▪ Inter-modulation Noise
o Occurs if signals with different frequencies share the same medium
o Interference caused by a signal produced at a frequency that is the sumor difference of
original frequencies
o Produced when there is some nonlinearity in the transmitter, receiver, or transmission
system
▪ Crosstalk
o If hear another conversation while using the telephone system
o Unwanted coupling between signal paths
o Unwanted signals picked by microwave antennas
o Dominates in the unlicensed (e.g.: ISM) bands
▪ Impulse Noise
o Irregular pulses or noise spikes
o Short duration and of relatively high amplitude
o Caused by external electromagnetic disturbances, or faults and flaws in the
communications system
o Minor annoyance for analog data, but primary source of error for digital data

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The Expression Eb/N0
▪ A parameter related to SNR
▪ A standard quality measure for digital communication system
performance.
▪ Ratio of signal energy per bit to noise power density per Hertz
▪ Consider a signal that contains binary digital data transmitted at
a certain bit rate R
▪ Recalling that 1 watt = 1 J/s
Eb=STb = S/R
Eb S / R S
= =
N0 N0 kT R

S: SignalPower R:Bit Rate, R =1ITb

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 The Expression Eb/N0
▪ Thebit error rate (BER)for digital data is a (decreasing) function of Eb/N0

▪ There is not a single unique curve that expresses the dependence of BERonEb/N0.
▪ BERversus EblN0also depends on the way in which the data is encoded onto the signal
(modulation technique)
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The Expression Eb/N0
▪ As bit rate Rincreases, transmitted signal power must increase to
maintain required Eb/N0

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 Expression Eb/N0
▪ We can relate Eb/N0 to the SNRas follows:

Thenoise in a signal with bandwidth BTis

N = NoBT

Shannon’s:

Equating BTwith Band Rwith C

This is a useful formula that relates the achievable spectral efficiency C/B to Eb/N0
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Other Impairments
▪ Atmospheric Absorption
o Water vapor and oxygen contribute to attenuation
o Rain and fog causes scattering of radio waves that results in attenuation
▪ Multipath
o Obstacles reflect signals so that multiple copies with varying delays are
received
o Thereceiver my capture only reflected signals and not the direct signal
o For mobile telephony, multipath considerations can be paramount.

S1
S2

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Other Impairments
▪ Refraction
o Bending of radio waves as they propagate through the
atmosphere
o Weather conditions may lead to variations in speed with height
that differ significantly from the typical variations. This may result
in a situation in which only a fraction or no part of the line- of-
sight wave reaches the receiving antenna.

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Fading in the Mobile Environment
▪ Fading: time variation of received signal power caused by
changes in the transmission medium or path(s).
o In a fixed environment, fading is affected by changes in atmospheric
conditions, suchas rainfall.
o In a mobile environment, where one of the two antennas is moving relative to
the other, the relative location of various obstacles changes over time, creating
complex transmission effects.
o Themost challenging technical problem facing communicationsEngineers
Received Signal Level

Distance

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 Multipath Propagation
▪ A phenomenon that results in radio signals reaching the receiving
antenna by two or more paths
▪ Causes:
o Reflection - occurs when signal encounters a surface that is large
relative to the wavelength of the signal.
• e.g.: the surface of the Earth, buildings, walls, etc.
Reflection
o Diffraction - occurs at the edge of an impenetrable body that is
large compared to wavelength of radio wave.
• Waves propagate in different directions with the edge as the source
o Scattering - occurs when incoming signal hits an object whose
size in the order of the wavelength of the signal or less.
• An incoming signal is scattered into several weaker outgoing signals.
• e.g.: traffic signs, lampposts

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 Multipath Propagation
• Shadowing - occurs when there are physical
obstacles including hills and buildings between the Shadowing
transmitter and receiver. Theobstacles create a
shadowing effect which can decrease the received
signal strength.

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 Doppler Shift
▪ A frequency variation of the received signal due to the relative
movement of the receiver with respect to the transmitter.
o When they are moving toward each other, the frequency of the received
signal is higher than the source.
o When they are moving away from each other, the received frequency
decreases.
o Thus,the frequency of the received signal is
fr = f c – fd
• fC is the frequency of sourcecarrier
• fd is the Doppler frequency (DopplerShift).

• v is the moving speed, λ is the wavelength of carrier,

• cos θ represents the velocity component of the receiver in the direction of the sender.
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 Effects of Multipath Propagation
▪ Positive:
o Helps receive signals even without line of sight.

o With smart Antennas, can substantially increase the usable received

power
▪ Negative:
o Multiple copies of a signal may arrive at different phases
• If phases add destructively, the signal level relative to noise declines,
making detection more difficult and increaseserrors.
o Inter-symbol Interference (ISI)
• One or more delayed copies of a pulse may arrive at the same time as the
primary pulse for a subsequentbit

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 Effects of Multipath Propagation
▪ ISI:

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 Types of Fading
▪ Fast Fading: Rapid variations in signal strength occur over distances of
about one-half a wavelength
• Slow Fading: A change in the average received power level about which the
rapid fluctuations occur. (over long distances)
o Also known as shadowing or log-normal fading.

Fast Slow

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 Types of Fading
Signal Level

Slow Fading

Fast Fading

Distance

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 Types of Fading
• Flat Fading: All frequency components of the received signal fluctuate in the
same proportions simultaneously

• Selective Fading: Affects unequally the different frequency components of

the received signal.

frequency

Channel Frequency Response

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The Fading Channel
▪ Additive White Gaussian Noise (AWGN) Channel
o Thedesired signal is degraded by thermal noise
o Thismodel is fairly accurate in somecases,suchas space communications and
somewire transmissions, suchas coaxial cable
▪ Rayleigh Fading
o Occurs when there are multiple indirect paths between transmitter and
receiver and no distinct dominant path, suchas an LOSpath.
o Thisrepresents a worst case scenario
o Used in difficult outdoor environments, suchas downtown urbansettings.
▪ Rician Fading
o Occurs when there is a direct LOSpath in addition to a number of indirect
multipath signals.
o Used in indoor environment, smaller cells or in more open outdoor
environments.

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 The Fading Channel
▪ The channels can be characterized by
a parameter K (Rician K–factor),
defined as follows:

▪ When K = 0 the channel isRayleigh

▪ When K = ∞, the channel isAWGN

▪ Worst: Rayleigh + Selective

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 Radio Propagation Model
▪ An empirical mathematical formulation for the characterization of radio
wave propagation as a function of frequency, distance, and other
conditions.
▪ Used to predict the received power or path loss
▪ To determine the coverage area of the transmitter
▪ Most famous model: Okumura-Hata
o Okumura made extensive measurements (empirical model – plots)
o Hata transformed Okumura’s plots to an analytical model
o Valid for 150-1500 MHz
o Model takes the effect of
• Transmitter height
• Receiver height
• Frequency fc in MHz
• Distance d in km
• Different environments (Terrain, Building Profile, Roads...)
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Error Compensation Mechanisms
I. Forward Error Correction
o Thereceiver, using only the received bits (data plus error-correcting
code), detects and corrects bit errors in the data
o Backward error correction: the receiver merely detects the presence of
errors and then sends a request back to the transmitter for
retransmission.
• Not practical in many wireless applications
o Typically in mobile wireless applications, the ratio of total bits sent to
data bits sent is between 2 and3.

II. Adaptive Equalization

o Used to combat inter-symbol interference (ISI)
o Involves gathering dispersed symbol energy back into its original time
interval
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Error Compensation Mechanisms
Adaptive (DSP based) Equalizer

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Error Compensation Mechanisms
III. Diversity Techniques
▪ Utilizing two or more copies of a signal with varying degrees of noise/
interference effects to achieve, by selection or a combination scheme,
higher degree of message-recovery performance than that achievable
by any one of the individual copies separately.
o Space Diversity – techniques involving physical transmission path
• Multiple nearby antennas may be used to receive the message, with the
signals combined in some fashion to reconstruct the most likely transmitted
signal
o Frequency Diversity – techniques where the signal is spread out
over a larger frequency bandwidth or carried on multiple
frequency carriers
o Time Diversity – techniques aimed at spreading the data out
over time (interleaving)
o Multipath Diversity, Polarization Diversity, Code Diversity …
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Error Compensation Mechanisms

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