SCCS424 Wireless and Mobile Computing

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Computing-- Mahidol University, ICT Program --

Lecture 2Antennas and Radio Propagation

http://www.ict.sc.mahidol.ac.th/course/sccs424/

RF Propagation Properties

� Absorption� Reflection� Scattering� Refraction

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� Refraction� Diffraction� Loss or Attenuation� Free Space Path Loss� Multipath

Absorption� RF signal passing through the obstruction will be

absorbed and attenuated� Different materials of obstruction have different

degree of RF absorption

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Reflection� Reflection - occurs when signal encounters a

surface that __________ relative to the wavelength of the signal

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Scattering� Scattering – occurs when incoming signal hits

an object whose size in the order of the wavelength of the signal or ________

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Refraction� Refraction – Bending of RF signal as it passes through a medium

with a different __________, therefore causing of the direction of wave to change

� Three most common causes of refraction: water vapor, changes in air ______________, changes in air pressure

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Diffraction� ____________of RF signal around an object (whereas refraction is

the bending of RF signal as it pass through the medium)� Diffraction depends on shape, size and material of object as well as

the characteristics of RF signal (such as polarization, phase, amplitude)

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Attenuation or Loss

� Strength of signal falls off with distance over transmission medium

� Attenuation factors for unguided media:�Received signal must have sufficient strength so

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�Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal

�Signal must maintain a level sufficiently higher than noise to be received without error

�Attenuation is greater at higher frequencies, causing distortion

Material – Attenuation Comparison (at 2.4 GHz)

� Brisk, Concrete blocks, -15 dB� Metal obstacle, _______dB� Metal rock, -6 dB

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� Drywall, -3 dB� Glass window, _____ dB� Wood door, -3 dB� Cubical wall, -2 dB

Cubicle wall

Free Space Loss

� free-space loss is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection

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with no obstacles nearby to cause reflection or diffraction.

Free Space Loss

� Free space loss, ________ isotropic antenna

( ) ( )2

2

2

2 44

c

fdd

P

P

r

t πλπ ==

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� Pt = signal power at transmitting antenna� Pr = signal power at receiving antenna

� λ = carrier wavelength� d = propagation distance between antennas� c = speed of light (3 x 108 m/s)

where d and λ are in the same units (e.g., meters)

cPr λ

Free Space Loss

� Free space loss equation can be recast:

==λπd

P

PL

r

tdB

4log20log10

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λPr

( ) ( ) dB 98.21log20log20 ++−= dλ

( ) ( ) dB 56.147log20log204

log20 −+=

= dfc

fdπ

Free Space Loss

� Free space loss accounting for gain of other antennas

( ) ( ) ( ) ( )t

AAf

cd

AA

d

GG

d

P

P2

22

2

224 === λλ

π

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� Gt = gain of transmitting antenna� Gr = gain of receiving antenna� At = effective area of transmitting antenna� Ar = effective area of receiving antenna

trtrtrr AAfAAGGP 22===

λ

Free Space Loss

� Free space loss accounting for gain of other antennas can be recast a

( ) ( ) ( )rtdB AAdL log10log20log20 −+= λ

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( ) ( ) ( )rtdB AAdL log10log20log20 −+= λ

( ) ( ) ( ) dB54.169log10log20log20 +−+−= rt AAdf

Power Notation (trick from Aj. Siwaruk)� 0 dBm � 1 mW, -3 dBm � 0.5 mW, 3 dBm � 2 mW

� 10 dBm � 10 mW, -10 dBm � 0.1 mW

� 20 dBm � 100 mW, 30 dBm � 1000 mW

� 21 dBm = 30-3-3-3 dBm � 1000/2/2/2 ≈ 125 mW

� 22 dBm = 10 +3+3+3+3 dBm � 10*2*2*2*2 ≈ 160 mW

23 dBm = 20 + 3 dBm � 100 * 2 ≈ 200 mW

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� 23 dBm = 20 + 3 dBm � 100 * 2 ≈ 200 mW

� 24 dBm = 30-3-3 dBm � 1000/2/2 ≈ 250 mW

� 25 dBm = 10 +3+3+3+3+3 dBm� 10*2^5 ≈ 320 mW

� 26 dBm = 20+3+3 dBm � 100*2*2 ≈ 400 mW

� 27 dBm = 30-3 dBm � 1000/2 ≈ 500 mW

� 28 dBm = 10+3+3+3+3+3+3 dBm � 10*(2^6) ≈ 640 mW

� 29 dBm = 20 +3+3+3 dBm � 100*(2^3) ≈ 800 mW

Example� Source generates 600 Mhz signal with power of 5 mW.

Then, signal is amplified by a 28-dB amplifier, and then transmitted via a dipole antenna with 3 dB gain.

� What is the signal power at (100/π) meters away from the transmitter using a receiver with 3-dB gain dipole antenna?

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antenna?

AP

Power output5 mW

Antenna +3 dB

Amplifier

Gain +28 dB

Antenna +3 dB

π

600 MHz

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100/π

Example

� Source generates 600 Mhz signal with power of 5 mW. Then, signal is amplified by a 28-dB amplifier, and then transmitted via a dipole antenna with 3 dB gain.

� What is the signal power at (100/π) meters away from the transmitter using a receiver with 3-dB gain dipole antenna?

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antenna?�Path loss at 100/π m = 20 log(4πfd/c) dB

= 20 log(4π100/π *600e6/ 3e8) = 58 dB�5 mW = 10/2 mW � 10-3 dBm = 7 dBm� Rx Power at 100/π m = 7 + 28 +3 – 58 +3 = -17 dBm

= -20 +3 dBm � 0.01* 2 = 0.02 mW

Exercise!

� Source generates 300 Mhz signal with power of 4 mW. Then, signal is amplified by a 30-dB amplifier, and then transmitted via a dipole antenna with 5 dB gain.

� What is the signal power at (250/π) meters away from the transmitter using a receiver with 3-dB gain dipole antenna?

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�Path loss at 250/π m = 20 log(4πfd/c) dB = 20 log(4π300/π *250e6/ 3e8) = 40 dB

�4 mW = 2*2 mW � 3+3 dBm = 6 dBm� Rx Power at 250/π m = 6 + 30 +5 – 40 +3 = 4 dBm

= 10-3-3 dBm � (10/2)/2 = 2.5 mW

Multipath Propagations� Multipath is a propagation that results in _________

paths of a signal arriving at a receiving antenna at the same time or within nanoseconds of each other

� Mutipath may come from reflection, scattering, refraction, diffraction

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Multipath Propagation

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The Effects of Multipath Propagation� 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

� Intersymbol interference (ISI)� One or more delayed copies of a pulse may arrive at the

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� One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

Propagation Modes

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Low-frequencysignals Higher-frequency

signals

Very High-frequencysignals

Ground Wave Propagation

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� Follows contour of the earth� Can Propagate considerable distances� Frequencies up to 2 MHz� Example

�AM radio

Sky Wave Propagation

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� Signal reflected from ionized layer of atmosphere back down to earth

� Signal can travel a number of hops, back and forth between ionosphere and earth’s surface

� Reflection effect caused by refraction� Examples

� Amateur radio� CB radio

Line-of-Sight Propagation

� Transmitting and receiving antennas must be within line of sight� Satellite communication – signal ______________not reflected

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� Satellite communication – signal ______________not reflected by ionosphere

� Ground communication – antennas within effective line of site due to refraction

� Refraction – bending of microwaves by the atmosphere� Velocity of electromagnetic wave is a function of the density of

the medium� When wave changes medium, speed changes� Wave bends at the boundary between mediums

Line-of-Sight Equations

� Optical line of sight

� Effective, or radio, line of sight

hd 57.3=

h

d

Earth surface

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� Effective, or radio, line of sight

� d = distance between antenna and horizon (km)� h = antenna height (m)� K = adjustment factor to account for refraction,

rule of thumb K = ___________

hd Κ= 57.3

Line-of-Sight Equations

� Maximum distance between two antennas for LOS propagation:

( )2157.3 hh Κ+Κ

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� h1 = height of antenna one� h2 = height of antenna two

( )2157.3 hh Κ+Κ

LOS Wireless Transmission Impairments

� Attenuation and attenuation distortion� Free space loss� Noise

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� Atmospheric absorption� Multipath� Refraction� Thermal noise

Electromagnetic Spectrum

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Eight BandsBandBand RangeRange PropagationPropagation ApplicationApplication

VLFVLF 3–30 KHz Ground Long-range radio navigation

LFLF 30–300 KHz GroundRadio beacons and

navigational locators

MFMF 300 KHz–3 MHz Sky AM radio

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HF HF 3–30 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF VHF 30–300 MHz Sky andline-of-sight

VHF TV, FM radio

UHF UHF 300 MHz–3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF SHF 3–30 GHz Line-of-sight Satellite communication

EHFEHF 30–300 GHz Line-of-sight Long-range radio navigation

Antennas� An antenna is an electrical conductor or system of

conductors� Transmission - Antennas convert electrical energy into RF

waves � Reception – Antennas convert RF waves into electrical

energy

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energy

� In two-way communication, the same antenna can be used for transmission and reception

� The physical dimensions of an antenna, such as its length, are directly related to the frequency at which the antenna can propagate waves or receive propagated waves

Radiation Patterns

� Power radiated in all directions� Not same performance in all directions� Radiation Patterns

� Graphical representation of radiation properties of an antenna

� Radiation patterns represented in two points of view

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� Radiation patterns represented in two points of view� Top-down view or H-plane or Azimuth chart� Side view or E-plane or Elevation chart� Both charts are not represent the distance or any level of power

or strength.� The chart only represents the relationship of power between

different points on the chart

Radiation Patterns – Top-down view� Top-down view or H-plane or Azimuth chart� H-plane is perpendicular to the antenna element

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Antenna

Antenna

Radiation Patterns – Side view� Side view or E-plane or Elevation chart� E-plane is parallel to the antenna element

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AntennaAntenna

Antenna Beamwidth� Beamwidth is the measurement of how broad or narrow the

focus of an antenna is and is measured both horizontally and vertically

� Beamwidth measures from the center, or strongest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power

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and vertical axes where the signal decreases by half power (–3 dB)

P

P/2

P/2

Antenna Beamwidth

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http://www.satsig.net/pointing/antenna-beamwidth-calculator.htm

Antenna Types

� Omni-directional Antenna� Semi-directional Antenna� Highly-directional Antenna

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Omni-directional Antenna� Omni-directional antenna radiates its energy equally

in all directions around its axis� If an antenna radiates in all directions equally

(forming a sphere), it is called an isotropic antennawhich is idealized. Isotropic antenna is theoretical reference for antenna

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reference for antenna

Isotropic Antenna

Omni-directional Antenna� Dipole antenna is an Omni-directional antenna that

is commonly used in wireless LAN� The dipole radiates equally in all directions around

its axis, but does not radiate along the length of the wire itself - hence the doughnut pattern

42Dipole Antenna

(doughnut radiation pattern)

Top View

Side View

Omni-directional Antenna

� High-gain omni-directional antennas offer more horizontal coverage area, but the vertical coverage area is reduced

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Omni-directional Antenna

Half wave dipole antenna

Folded dipole antenna

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Rubber dipole antenna

Semi-directional Antenna

� Semi-directional antennas are designed to direct a signal in a specific direction

� It is common to use semi-directional antennas to provide a network bridge between two buildings in a campus environment or down the street from each other

� Examples: Patch or Panel Antenna, Yaki antenna

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Semi-directional Antenna

� Patch, Panel antenna

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Radiation Pattern

Semi-directional Antenna

� Yaki antennaRadiation Pattern

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Highly-directional Antenna� Highly-directional antennas emit the most narrow

signal beam of any antenna type and have the greatest gain of these three groups of antenna(Omini-directional, Simi-directional and Highly-directional)

� Highly-directional antennas are typically concave,

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� Highly-directional antennas are typically concave, dish-shaped devices. Some models are referred to as parabolic dishes.

Radiation Pattern

Antenna Gain� Antenna gain

� Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

� Antenna gain is specified in _________, which means decibels referenced to an isotropic radiator

� Sometimes, Antenna gain is specified in dBd, which

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� Sometimes, Antenna gain is specified in dBd, which means decibels referenced to a dipole antenna

� Rule: x dBd ���� x + 2.41 dBi

� The higher the antenna gain, the farther the wave will travel, concentrating its output wave more tightly so that more of the power is delivered to the destination (the receiving antenna) at long distances

Fading

�Slow Fading

�Fast Fading (Short-term fading)

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�Signal Strength�(dB)

�Distance

�Path Loss

�Slow Fading (Long-term fading)

Fading

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Types of Fading� Slow fading

� caused by large obstructions between transmitter and receiver

� Fast fading � channel changes rapidly w.r.t. data rate.� caused by scatterers in the vicinity of the transmitter.

� Flat fading (frequency non-selective fading)

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� Flat fading (frequency non-selective fading)� All frequencies components fade similarly

� Selective fading� __________________

� Non-LOS �Channel attenuations are random variables with zero-mean and complex Gaussian distribution.

� __________________� LOS � Channel attenuations are non-zero-mean and complex

Gaussian random variables.

Slow Fading

� The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). This fading caused by shadowing.

� Log-normal distribution:- The pdf of the received signal level is given in decibels by

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where M is the true received signal level m in decibels, i.e., 10log10m, M is the area average signal level, i.e., the mean of M, σ is the standard deviation in decibels

( )( )

,2

1 2

2

2σ

σπ

MM

eMp−−

=

Log-normal Distribution

2σ

p(M)

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MM

p(M)

The pdf of the received signal level

Fast Fading� The signal from the transmitter may be reflected from

objects such as hills, buildings, or vehicles.� When MS far from BS, the envelope distribution of received signal is

Rayleighdistribution. The pdf is

( ) 0,2

2

22

>=−

rer

rpr

σ

σ

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where σ is the standard deviation.� Middle value rm of envelope signal within sample range to be

satisfied by

� We have rm = 1.777 �( ) 0,

2>= rerp

σ

.5.0)( =≤ mrrP

Rayleigh DistributionP(r)

0.6

0.8

1.0

σ=1

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The pdf of the envelope variation

r2 4 6 8 10

0

0.2

0.4 σ=2

σ=3

Rayleigh Fading

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Fast Fading (Continued)

� When MS far from BS, the envelope distribution of received signal is Riciandistribution. The pdf is

( ) 0,02

2

2

22

≥

=+−

rr

Ier

rpr

σα

σσ

α

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where σ is the standard deviation,I0(x) is the zero-order Bessel function of the

first kind,α is the amplitude of the direct signal

σσ

Rician Distribution

Pdf

p(r

)

0.6

0.5

0.4

α = 2α = 1α= 0 (Rayleigh)

α = 3

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r

Pdf

p(r

)

r86420

0.3

0.2

0.1

0

σ = 1

The pdf of the envelope variation

Error Compensation Mechanisms

� Forward error correction� Adaptive equalization� Diversity techniques

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Compensate = repair

Forward Error Correction

� Transmitter adds error-correcting code to data block�Code is a function of the data bits

� Receiver calculates error-correcting code

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� Receiver calculates error-correcting code from incoming data bits� If calculated code matches incoming code, no

error occurred� If error-correcting codes don’t match, receiver

attempts to determine bits in error and correct

Exercise

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Exercise

RF components

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EIRP = Equivalent Isotropically Radiated Power

Exercise1

1. Given an access point with 100 mW of output power connected through a 50-foot cable with 3 dB of loss to an antenna with 10 dBi of gain, what is the EIRP at the antenna in mW?

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antenna in mW?

Exercise2

2. Given an access point with an output power of 20 dBm connected through a cable with a loss of 6 dB to an amplifier with a 10 dB gain, then through a cable with 3 dB of loss to an antenna with 6 dBi of gain, what is

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to an antenna with 6 dBi of gain, what is the EIRP in dBm?