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

22

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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