Antennas and Propagation Antennas Propagation Modes Line of Sight - - PowerPoint PPT Presentation

Lecture 3:

Antennas and Propagation

 Antennas  Propagation Modes  Line of Sight Transmission  Fading in the Mobile Environment

CMPE 477 – Wireless and Mobile Networks

Introduction

 An antenna is an electrical conductor or system of

conductors for radiating/collecting electromagnetic energy

 Transmission - radiates electromagnetic energy into

medium

 Reception - collects electromagnetic energy from

medium

 In two-way communication, the same antenna can be

used for transmission and reception

 Radiation: An antenna radiates power in all directions,

however does not perform well or the same in all directions.

Radiation Patterns

 Radiation pattern

 Graphical representation of radiation properties of an

antenna

 The simplest pattern is produced by Isotropic Antenna

 Ideal antenna that radiates power the same in all directions.

 Depicted as two-dimensional cross section

 Reception pattern

 Receiving antenna’s equivalent to radiation pattern

Antennas: simple dipoles

 Real antennas are not isotropic radiators but, e.g.,

dipoles with lengths /4 (Marconi) on car roofs or /2 as Hertzian dipole

 shape of antenna proportional to wavelength

 Example: Radiation pattern of a simple Hertzian dipole

side view (xy-plane) x y side view (yz-plane) z y top view (xz-plane) x z

simple dipoles

/4 /2

Antennas: directed and sectorized

side view (xy-plane) x y side view (yz-plane) z y top view (xz-plane) x z top view, 3 sector x z top view, 6 sector x z

Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

directed antenna sectorized antenna parabolic antenna

Antenna Gain

 Antenna gain Power output, in a particular direction, compared to

that produced in any direction by a perfect

• mnidirectional antenna (isotropic antenna)

 Effective area Related to physical size and shape of antenna Simply increasing the size of antenna does not

guarantee an increase in effective area; however,

• ther factors being equal, antennas with higher

maximum effective area are generally larger.

Antenna Gain Relationship between antenna gain and effective area

G = antenna gain Ae = effective area f = carrier frequency c = speed of light  = carrier wavelength

2 2 2

4 4 c A f A G

e e

    

Type of Antenna Effective Area Power Gain Isotropic 2/4ᴨ 1 Half-wave dipole 1.52/4ᴨ 1.5 Parabolic, Face Area A 0.56A 7A/2

Propagation Modes

In wireless networks, the signal has no wire to

determine the direction of propagation

Three basic routes are followed by the wireless

signals:

Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation

 Signals follow the contour of the earth and propagate

long distances

 Found in signals up to 2MHz  Why?  Wavefront of the signal near the earth is due to the

current produced by the electromagnetic wave on the earth’s surface

 Diffraction  Example: AM radio

Sky Wave Propagation

Sky Wave Propagation

 Signal reflected from ionized layer of atmosphere back

down to earth

 Why?

 Caused by refraction: Mediums at different densities

 Signal can travel a number of hops, back and forth

between ionosphere and earth’s surface, travelling thousands of km.

 Examples

 Amateur radio  CB (Citizens' Band) radio  International Broadcasts, BBC Voice of America

Line-of-Sight Propagation

Line-of-Sight Propagation

Transmitting and receiving antennas should be

within line of sight

Satellite communication – signal above 30 MHz not

reflected by ionosphere

LOS Wireless Transmission Impairments

 Impairments cause the received signal to be different than the

transmitted signal or degrade the signal quality

 Result: Bit errors are introduced  Impairments  Attenuation and attenuation distortion  Free space loss  Noise  Atmospheric absorption  Multipath  Refraction

Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media:

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

Free Space Loss

 Even if there are no other source of attenuation or

impairment , the signal attenuates with the distance

 Expressed in ratio  For the ideal isotropic antenna

   

2 2 2 2

4 4 c fd d P P

r t

    

Pt = signal power at transmitting antenna Pr = signal power at receiving antenna  = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s) where d and  are in the same units (e.g., meters)

Free Space Loss

Free space loss equation can be recast in decibels:

         d P P L

r t dB

4 log 20 log 10

   

dB 98 . 21 log 20 log 20     d 

   

dB 56 . 147 log 20 log 20 4 log 20           d f c fd 

Free Space Loss

Free space loss accounting for gain of other antennas

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

       

t r t r t r r t

A A f cd A A d G G d P P

2 2 2 2 2 2

4      

Free Space Loss

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

     

r t dB

A A d L log 10 log 20 log 20    

     

dB 54 . 169 log 10 log 20 log 20     

r t A

A d f

Categories of Noise

The received signal will consist of transmitted signal, modified by the various impairments and plus additional unwanted signals, referred as noise

 Thermal Noise  Intermodulation noise  Crosstalk  Impulse Noise

Thermal Noise

 Thermal noise due to agitation of electrons  Present in all electronic devices and transmission

media

 Cannot be eliminated, puts an upper bound on system

performance

 Uniformly distributed over the frequency spectrum Referred as white noise  Function of temperature  Particularly significant for satellite communication

Thermal Noise

Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:

 N0 = noise power density in watts per 1 Hz of bandwidth  k = Boltzmann's constant = 1.3803 ´ 10-23 J/K  T = temperature, in kelvins (absolute temperature)

Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts):

 

W/Hz k T N 

TB N k 

Noise Terminology

Intermodulation noise – occurs if signals with

different frequencies share the same medium

Interference caused by a signal produced at a

frequency that is the sum or difference of original frequencies

Crosstalk – unwanted coupling between signal

paths

Impulse noise – irregular pulses or noise spikes

Short duration and of relatively high amplitude Caused by external electromagnetic disturbances,

• r faults and flaws in the communications system

Expression Eb/N0

 Already discussed: SNR  Related to SNR, quality of the digital communication

performance

 Ratio of signal energy per bit to noise power density

per Hertz

 The bit error rate for digital data is a function of Eb/N0

 Given a value for Eb/N0 to achieve a desired error rate,

parameters of this formula can be selected

 As bit rate R increases, transmitted signal power must

increase to maintain required Eb/N0

TR S N R S N Eb k /  

Other Impairments

Atmospheric absorption – water vapor and oxygen contribute to attenuation, peak attenuation around 22GHz. Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Interference

Refraction

Multipath Propagation

Multipath Propagation Reflection - occurs when signal encounters a surface that is large relative to the wavelength

• f the signal

Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave Scattering – occurs when incoming signal hits an

• bject whose size in the order of the

wavelength of the signal or less

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 same time as the primary pulse for a subsequent bit

Types of Fading fast fading

 rapid changes in strength over half wavelength distances

 eg. 900MHz wavelength is 0.33m see 20-30dB

slow fading

 slower changes due to user passing different height

buildings, gaps in buildings etc.

 over longer distances than fast fading

flat fading

 affects all frequencies in same proportion

selective fading

 different frequency components affected differently

Review

• Antenna
• Antenna gain
• Attenuation
• Crosstalk
• Diffraction
• Dipole
• Fading
• Fast Fading
• Flat fading
• Free Space Loss
• Ground Wave Propagation
• Isotropic Antenna
• Multipath
• Reflection
• Refraction
• Scattering
• Selective Fading
• Sky Wave Propagation
• Slow Fading
• Line of Sight
• Noise