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Antennas and Propagation
Mobile Networks Module D-1
Slides adapted from Stallings, Wireless Communications & Networks, Second Edition, Chapter 5 © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4 1
1. Introduction 2. Propagation modes 3. Line-of-sight transmission 4. Fading
1. Introduction
n An antenna is an electrical conductor or system of conductors n Transmission - radiates electromagnetic
energy into space n Reception - collects electromagnetic
energy from space n In two-way communication, the same
antenna can be used for transmission and reception
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Radiation Patterns
n Radiation pattern n Graphical representation of radiation properties of
an antenna n Depicted as two-dimensional cross section
n Beam width (or half-power beam width) n Measure of directivity of antenna
n Reception pattern n Receiving antenna’s equivalent to radiation pattern
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Types of Antennas
n Isotropic antenna (idealized) n Radiates power equally in all directions
n Dipole antennas n Half-wave dipole antenna (or Hertz
antenna) n Quarter-wave vertical antenna (or Marconi
antenna)
n Parabolic Reflective Antenna
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Antenna Gain n Antenna gain
n Measure of the directionality of an antenna n Power output, in a particular direction,
compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
n Effective area n Related to physical size and shape of
antenna
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Free-space propagation
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)d
Omni-directional transmitting antenna Receiving antenna, area AR
where is an efficiency parameter
Transmitted power PT
Received power PR
Focusing capability: determined by the antenna size in wavelenths (the wavelength is λ):
where is the transmitting antenna efficiency factor
Directional antenna
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)d
Transmitting antenna, area AT Receiving antenna, area AR
Transmitted power PT Received power PR
Directional antenna à gain GT > 1
(
Similarly to what done on previous slide:
Hence: free-space received power: Effective radiated power: ERP = PT . GT
2. Propagation Modes
n Ground-wave propagation n Sky-wave propagation n Line-of-sight propagation
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Ground Wave Propagation
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Ground Wave Propagation n Follows contour of the earth
n The electromagnetic wave induces a current in hte earth’s surface è the waveform tilts downwards
n Diffraction
n Can Propagate considerable distances n Frequencies up to 2 MHz n Example
n AM radio 15
Sky Wave Propagation
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Sky Wave Propagation
n Signal reflected from ionized layer of atmosphere back down to earth
n Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
n Reflection effect caused by refraction n Examples
n Amateur radio n CB radio
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Line-of-Sight Propagation
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Line-of-Sight Propagation n Required above 30 MHz n Transmitting and receiving antennas must be
within line of sight n Satellite communication – signal above 30 MHz
not reflected by ionosphere n Ground communication – antennas within effective
line of site due to refraction
n Refraction – bending of microwaves by the atmosphere n Velocity of electromagnetic wave is a function of
the density of the medium n When wave changes medium, speed changes n Wave bends at the boundary between mediums
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Line-of-Sight Equations
n Optical line of sight
n Effective, or radio, line of sight
n d = distance between antenna and horizon (km)
n h = antenna height (m) n K = adjustment factor to account for
refraction, rule of thumb K = 4/3
hd 57.3=
hd Κ= 57.3
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Line-of-Sight Equations
n Maximum distance between two antennas for LOS propagation:
n h1 = height of antenna one n h2 = height of antenna two
( )2157.3 hh Κ+Κ
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3. Line-Of-Sight Transmission
n Attenuation and attenuation distortion n Free space loss n Noise n Atmospheric absorption n Multipath n Refraction n Thermal noise
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Attenuation
n Strength of signal falls off with distance over transmission medium
n Attenuation factors for unguided media: n Received signal must have sufficient strength so
that circuitry in the receiver can interpret the signal
n Signal must maintain a level sufficiently higher than noise to be received without error
n Attenuation is greater at higher frequencies, causing distortion
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Free Space Loss – Isotropic Antenna
n Free space loss, ideal isotropic antenna
n Pt = signal power at transmitting antenna n Pr = signal power at receiving antenna n λ = carrier wavelength n d = propagation distance between antennas n c = speed of light (≈ 3 × 10^8 m/s) n f = frequency where d and λ are in the same units (e.g., meters)
( ) ( )2
2
2
2 44cfdd
PP
r
t πλπ
==
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Free Space Loss– Isotropic Antenna
n Free space loss equation can be recast:
⎟⎠
⎞⎜⎝
⎛==λπd
PPLr
tdB
4log20log10
( ) ( ) dB 98.21log20log20 ++−= dλ
( ) ( ) dB 56.147log20log204log20 −+=⎟⎠
⎞⎜⎝
⎛= dfcfdπ
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Free Space Loss– Isotropic Antenna
Free Space Loss – Directional Antenna
n Free space loss for directional antennas
n Gt = gain of transmitting antenna n Gr = gain of receiving antenna n At = effective area of transmitting antenna n Ar = effective area of receiving antenna
( ) ( ) ( ) ( )trtrtrr
t
AAfcd
AAd
GGd
PP
2
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2
224===
λλ
π
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Note: is here at the denominator because the size of the antenna expressed in wavelength diminishes when the wavelength increases (and thus when the frequency decreases)
Free Space Loss – Directional Antenna
n Free space loss for directional antennas can be recast as:
( ) ( ) ( )rtdB AAdL log10log20log20 −+= λ
( ) ( ) ( ) dB54.169log10log20log20 +−+−= rt AAdf
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Categories of Noise
n Thermal Noise n Intermodulation noise n Crosstalk n Impulse Noise
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Thermal Noise
n Also called white noise n Due to agitation of electrons n Present in all electronic devices and
transmission media n Cannot be eliminated n Function of temperature n Particularly significant for satellite
communication
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Thermal Noise
n Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:
n N0 = noise power density in watts per 1 Hz of bandwidth n k = Boltzmann's constant = 1.3803 × 10-23 J/K n T = temperature, in kelvins (absolute temperature)
( )W/Hz k0 TN =
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Thermal Noise
n Noise is assumed to be independent of frequency
n Thermal noise present in a bandwidth of B Hertz (in watts):
or, in decibel-watts
TBN k=
BTN log10 log 10k log10 ++=BT log10 log 10dBW 6.228 ++−=
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Noise Terminology
n Intermodulation noise – occurs if signals with different frequencies share the same medium n Interference caused by a signal produced at a
frequency that is the sum or difference of original frequencies
n Crosstalk – unwanted coupling between signal paths
n Impulse noise – irregular pulses or noise spikes n Short duration and of relatively high amplitude n Caused by external electromagnetic disturbances,
or faults and flaws in the communications system 34
Expression Eb/N0
n Ratio of signal energy per bit to noise power density per Hertz
where S is the signal power, R the bitrate, k Boltzmann’s constant and T the temperature
n The bit error rate for digital data is a function of Eb/N0 n Given a value for Eb/N0 to achieve a desired error
rate, parameters of this formula can be selected n As bit rate R increases, transmitted signal power
must increase to maintain required Eb/N0
TRS
NRS
NEb
k/
00
==
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Other Impairments
n Atmospheric absorption – water vapor and oxygen contribute to attenuation
n Multipath – obstacles reflect signals so that multiple copies with varying delays are received
n Refraction – bending of radio waves as they propagate through the atmosphere
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Multipath Propagation
n Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal
n Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave
n Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
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Multipath Propagation
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The Effects of Multipath Propagation
n Multiple copies of a signal may arrive at different phases n If phases add destructively, the signal level
relative to noise declines, making detection more difficult
n Intersymbol interference (ISI) n One or more delayed copies of a pulse may
arrive at the same time as the primary pulse for a subsequent bit
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4. Fading in the mobile environment Fading: time variation of received signal power due to
changes in the transmission medium or path(s) Kinds of fading: n Fast fading n Slow fading n Flat fading à independent from frequency n Selective fading à frequency-dependent n Rayleigh fading à no dominant path n Rician fading à Line Of Sight (LOS) is dominating +
presence of indirect multipath signals
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Power in the dominant pathPower in the scattered paths
K =
K=0 : Rayleigh K=∞: Additive White Gaussian Noise
Error Compensation Mechanisms
n Forward error correction n Adaptive equalization n Diversity techniques
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Forward Error Correction
n Transmitter adds error-correcting code to data block n Code is a function of the data bits
n Receiver calculates error-correcting code from incoming data bits n If calculated code matches incoming code, no
error occurred n If error-correcting codes don’t match, receiver
attempts to determine bits in error and correct
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Adaptive Equalization
n Can be applied to transmissions that carry analog or digital information n Analog voice or video n Digital data, digitized voice or video
n Used to combat intersymbol interference n Involves gathering dispersed symbol energy
back into its original time interval n Techniques
n Lumped analog circuits n Sophisticated digital signal processing algorithms
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A known training sequence is sent periodically. The receiver fine tunes the coefficients accordingly.
Diversity Techniques n Diversity is based on the fact that individual
channels experience independent fading events
n Space diversity – techniques involving physical transmission path (e.g., multiple antennas)
n Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers
n Time diversity – techniques aimed at spreading the data out over time
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