CHAPTER 6 THE WIRELESS CHANNEL - Autenticação · PDF fileThe Wireless Channel 6-5 . ISOTROPIC RADIATOR • Radiation and reception of electromagnetic waves, coupling of ... Parabola

Wireless Communication Networks and Systems

1st edition Cory Beard, William Stallings

© 2016 Pearson Higher Education, Inc.

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CHAPTER 6 THE WIRELESS

CHANNEL

The Wireless Channel 6-1

ANTENNAS

•  An antenna is an electrical conductor or system of conductors – Transmission - radiates electromagnetic energy

into space – Reception - collects electromagnetic energy from

space •  In two-way communication, the same antenna

can be used for transmission and reception


RADIATION PATTERNS

•  Radiation pattern –  Graphical representation of radiation properties of an antenna –  Depicted as two-dimensional cross section

•  Beam width (or half-power beam width) –  Measure of directivity of antenna

•  Reception pattern –  Receiving antenna’s equivalent to radiation pattern

•  Sidelobes – Extra energy in directions outside the mainlobe

•  Nulls – Very low energy in between mainlobe and sidelobes


6.1 ANTENNA RADIATION PATTERNS

(a) Omnidirectional

A

B

(b) Directional

B

A

Antenna location

z


TYPES OF ANTENNAS

•  Isotropic antenna (idealized) – Radiates power equally in all directions

•  Dipole antennas – Half-wave dipole antenna (or Hertz antenna) – Quarter-wave vertical antenna (or Marconi antenna)

•  Parabolic Reflective Antenna •  Directional Antennas

– Arrays of antennas •  In a linear array or other configuration

–  Signal amplitudes and phases to each antenna are adjusted to create a directional pattern

– Very useful in modern systems


ISOTROPIC RADIATOR

•  Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission

•  Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna

•  Real antennas always have directive effects (vertically and/or horizontally)

•  Radiation pattern: measurement of radiation around an antenna

z y

x

z

y x ideal isotropic radiator

6.2 SIMPLE ANTENNAS

l/2 l/4

(a) Half-wave dipole (b) Quarter-wave antenna


6.3 RADIATION PATTERN IN THREE DIMENSIONS

x

y

z

y

x

z

x

y y

z x

z

Side view (zy-plane)

(a) Simple dipole

(b) Directed antenna

Side view (zy-plane)

Top view (xz-plane)

Top view (xz-plane)

Side view (xy-plane)

Side view (xy-plane)


6.4 PARABOLIC REFLECTIVE ANTENNAS

y

a

ab

bc

f f

c

x

Dir

ectr

ix

Focus

(a) Parabola (b) Cross section of parabolic antennashowing reflective property

(c) Cross section of parabolic antennashowing 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)

•  Effective area – Related to physical size and shape of antenna


ANTENNA GAIN

•  Relationship between antenna gain and effective area

•  G = antenna gain •  Ae = effective area •  f = carrier frequency •  c = speed of light (≈ 3 × 108 m/s) •  λ = carrier wavelength


!!G =

4πAeλ2 =

4π f 2Aec2

SPECTRUM CONSIDERATIONS

•  Controlled by regulatory bodies – Carrier frequency – Signal Power – Multiple Access Scheme

•  Divide into time slots –Time Division Multiple Access (TDMA)

•  Divide into frequency bands – Frequency Division Multiple Access (FDMA)

•  Different signal encodings – Code Division Multiple Access (CDMA)


SPECTRUM CONSIDERATIONS

•  Industrial, Scientific, and Medical (ISM) bands – Can be used without a license – As long as power and spread spectrum regulations

are followed •  ISM bands are used for

– WLANs – Wireless Personal Area networks –  Internet of Things


PROPAGATION MODES

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


6.5 WIRELESS PROPAGATION MODES

Earth

Ionosp

here

(b) Sky wave propagation (2 to 30 MHz)

Transmitantenna

Receiveantenna

Signal

propagation

Earth

(a) Ground wave propagation (below 2 MHz)

Transmitantenna

Receiveantenna

Signalpropagation

Earth

(c) Line-of-sight (LOS) propagation (above 30 MHz)

Transmitantenna

Receiveantenna

Signalpropagation


GROUND WAVE PROPAGATION

•  Follows contour of the earth •  Can propagate considerable distances •  Frequencies up to 2 MHz •  Example

– AM radio


SKY WAVE PROPAGATION

•  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 above 30 MHz 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


FIVE BASIC PROPAGATION MECHANISMS

1.  Free-space propagation 2.  Transmission

–  Through a medium –  Refraction occurs at boundaries

3.  Reflections –  Waves impinge upon surfaces that are large compared to

the signal wavelength 4.  Diffraction

–  Secondary waves behind objects with sharp edges 5.  Scattering

–  Interactions between small objects or rough surfaces


6.6 REFRACTION OF AN ELECTROMAGNETIC WAVE

Area of lowerrefractive index

Incidentdirection

Refracteddirection

Area of higherrefractive index

i

r


LOS WIRELESS TRANSMISSION IMPAIRMENTS

•  Attenuation and attenuation distortion •  Free space loss •  Noise •  Atmospheric absorption •  Multipath •  Refraction •  Thermal noise


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

•  Free space loss (Friis Model):

•  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 ×108 m/s) •  Gt = transmitter antenna gain (=1 for isotropic antenna) •  Gt = receiver antenna gain (=1 for isotropic antenna)

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


2

2

2

2 )4()4(cGG

dfGGd

PP

rtrtr

t π

λ

π==

FREE SPACE LOSS

•  Free space loss (isotropic antenna) equation can be recast:


!!LdB =10log

PtPr

=20log 4πdλ

⎛⎝⎜

⎞⎠⎟

( ) ( ) dB 98.21log20log20 ++−= dλ

( ) ( ) dB 56.147log20log204log20 −+=⎟⎠

⎞⎜⎝

⎛= dfcfdπ

6.8 FREE SPACE LOSS

601 5 10

Distance (km)

Los

s (d

B)

f = 30 MHz

f = 300 MHz

f = 3 GHz

f = 30 GHz

f = 300 GHz

50 100

70

80

90

100

110

120

130

140

150

160

170

180


DERIVATION OF THE FRIIS EQUATION

•  Power Flux Density: power spread over the sphere’s surface:

–  𝑝= 𝑃↓𝑡 /4𝜋𝑟↑2  𝐺↓𝑡 

•  Antenna’s Apperture or Effective Area:

–  𝐴↓𝑒𝑓𝑓 = 𝜆↑2 /4𝜋 𝐺↓𝑟 = 𝑃↓𝑜 /𝑝  –  Where 𝑃↓𝑜 ≡ 𝑃↓𝑟  is the antenna’s output power that feeds

the receiver circuit’s load.

–  Note: Antenna’s apperture efficiency (0≤ 𝑒↓𝑎 ≤1):

•  𝑒↓𝑎 = 𝐴↓𝑒𝑓𝑓 /𝐴↓𝑝ℎ𝑦𝑠   , where 𝐴↓𝑝ℎ𝑦𝑠  is the physical apperture of e.g., parabolic dish or horn

•  Friis Equation:

–  𝑃↓𝑟 =𝑝∙ 𝐴↓𝑒𝑓𝑓 

PATH LOSS EXPONENT IN PRACTICAL SYSTEMS

•  Practical systems – reflections, scattering, etc. •  Beyond a certain distance, received power

decreases logarithmically with distance – Based on many measurement studies

Pt

Pr

= 4πλ

⎛⎝⎜

⎞⎠⎟

2

d n = 4πfc

⎛⎝⎜

⎞⎠⎟

2

d n

LdB = 20log f( ) +10n log d( )−147.56 dB


PATH LOSS EXPONENT IN PRACTICAL SYSTEMS


MODELS DERIVED FROM EMPIRICAL MEASUREMENTS

•  Need to design systems based on empirical data applied to a particular environment –  To determine power levels, tower heights, height of mobile

antennas •  Okumura developed a model, later refined by Hata

–  Detailed measurement and analysis of the Tokyo area –  Among the best accuracy in a wide variety of situations

•  Predicts path loss for typical environments –  Urban –  Small, medium sized city –  Large city –  Suburban –  Rural


CATEGORIES OF 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 •  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)


( )W/Hz k0 TN =

THERMAL NOISE

•  Noise is assumed to be independent of frequency •  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 ++−=

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, or faults

and flaws in the communications system


EXPRESSION Eb/N0

•  Ratio of signal energy per bit to noise power density per Hertz

•  The bit error rate (i.e., bit error probability) 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


TRS

NRS

NEb

k/

00

==

6.9 GENERAL SHAPE OF BER VERSUS Eb/N0 CURVES

Prob

abili

ty o

f bi

t err

or (

BE

R)

(Eb/N0) (dB)

Worseperformance

Betterperformance


OTHER IMPAIRMENTS

•  Atmospheric absorption – water vapor and oxygen contribute to attenuation

•  Multipath – obstacles reflect signals so that multiple copies with varying delays are received

•  Refraction – bending of radio waves as they propagate through the atmosphere


THE EFFECTS OF MULTIPATH PROPAGATION

•  Reflection, diffraction, and scattering •  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

•  Rapid signal fluctuations – Over a few centimeters


6.10 EXAMPLES OF MULTIPATH INTERFERENCE

(a) Microwave line of sight

(b) Mobile radio


6.11 SKETCH OF THREE IMPORTANT PROPAGATION MECHANISMS

R

R

D

S

Lamppost


REAL WORLD EXAMPLES

www.ihe.kit.edu/index.php

6.12 TWO PULSES IN TIME-VARIANT MULTIPATH

ReceivedLOS pulse

Receivedmultipath

pulses

Time

Time

Transmittedpulse

Transmittedpulse

ReceivedLOS pulse

Receivedmultipath

pulses


6.13 TYPICAL LARGE-SCALE AND SMALL-SCALE FADING IN AN URBAN MOBILE ENVIRONMENT

50 10 15

Position (m)

Rec

eive

d po

wer

(dB

m)

20 25 30

–130

–120

–110

–100

–90

–80


TYPES OF FADING

•  Large-scale fading – Signal variations over large distances – Path loss LdB as we have seen already – Shadowing

•  Statistical variations – Rayleigh fading – Ricean fading


TYPES OF FADING

•  Doppler Spread –  Frequency fluctuations caused by movement – Coherence time Tc characterizes Doppler shift

•  How long a channel remains the same – Coherence time Tc >> Tb bit time à slow fading

•  The channel does not change during the bit time – Otherwise fast fading

•  Example 6.11: Tc = 70 ms, bit rate rb = 100 kbs – Bit time Tb = 1/100 × 103 = 10 µs –  Tc >> Tb? 70 ms >> 10 µs? – True, so slow fading


TYPES OF FADING •  Multipath fading

–  Multiple signals arrive at the receiver –  Coherence bandwidth Bc characterizes multipath

•  Bandwidth over which the channel response remains relatively constant •  Related to delay spread, the spread in time of the arrivals of multipath signals

–  Signal bandwidth Bs is proportional to the bit rate –  If Bc >> Bs, then flat fading

•  The signal bandwidth fits well within the channel bandwidth –  Otherwise, frequency selective fading

•  Example 6.11: Bc = 150 kHz, bit rate rb = 100 kbs –  Assume signal bandwidth Bs ≈ rb, Bs = 100 kHz –  Bc >> Bs? 150 kHz >> 100 kHz? –  Using a factor of 10 for “>>”, 150 kHz is not more than 10 ×100 kHz –  False, so frequency selective fading


6.14 FLAT AND FREQUENCY SELECTIVE FADING

B-B

A

Signal Spectrum

B-B

A

Signal Spectrum

B-B

0.1

Flat FadingChannel

B-B

0.1

Frequency selectivechannel

B-B

0.1 A

Flat fading outputfrom the channel

B-B

0.1 A

Frequency selective outputfrom the channel


6.15 THEORETICAL BIT ERROR RATE FOR VARIOUS FADING CONDITIONS

(Eb/N0) (dB)

0 5 10 15 20 25 30 3510–4

10–3

10–2

10–1

1

Prob

abili

ty o

f bi

t err

or (

BE

R)

Flat fadingand slow fadingRayleigh limit

Frequency–selective fading orfast fading distortion

Rician fading

K = 16

Rician fading

K = 4

Additive w

hite

Gaussian noise


FRESNEL ZONES

•  1st Fresnel Zone –  Obstruction must be <20% in order to result in propagation loss

equivalent to free space.

•  Radius of the nth Fresnel Zone at point P (d1, d2, lambda in meters):

TWO-RAY MODEL

•  𝑑< 𝑑↓𝑐 : Friis model •  𝑑> 𝑑↓𝑐 : 𝑃↓𝑟 = 𝑃↓𝑡 ∙ 𝐺↓𝑡 ∙ 𝐺↓𝑟 ∙ (ℎ↓𝑡 )↑2 ∙ (ℎ↓𝑟 )↑2 /𝑑↑4 ∙𝐿 

•  Crossover distance: 𝑑↓𝑐 =(4𝜋∙ ℎ↓𝑡 ∙ ℎ↓𝑟 )/𝜆

CHANNEL CORRECTION MECHANISMS

•  Forward error correction •  Adaptive equalization •  Adaptive modulation and coding •  Diversity techniques and MIMO •  OFDM •  Spread sprectrum •  Bandwidth expansion


FORWARD ERROR CORRECTION

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

•  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 •  Subject of Chapter 10


10.5 FORWARD ERROR CORRECTION PROCESS

Data

No

erro

r or

corr

ecta

ble

erro

r

Det

ecta

ble

but n

otco

rrec

tabl

e er

ror

Codeword’

FECdecoder

Errorindication

k bits

Data

Codeword

FECencoder

n bits

Transmitter Receiver


ADAPTIVE EQUALIZATION

•  Can be applied to transmissions that carry analog or digital information –  Analog voice or video –  Digital data, digitized voice or video

•  Used to combat intersymbol interference •  Involves gathering dispersed symbol energy back into

its original time interval •  Techniques

–  Lumped analog circuits –  Sophisticated digital signal processing algorithms


6.16 LINEAR EQUALIZER CIRCUIT

v v v

∑

Algorithm for tapgain adjustment

Unequalizedinput

Equalizedoutput

C–2 C–1 C0 C1 C2

v


ADAPTIVE MODULATION AND CODING (AMC)

•  The modulation process formats the signal to best transmit bits – To overcome noise – To transmit as many bits as possible

•  Coding detects and corrects errors •  AMC adapts to channel conditions

–  100’s of times per second – Measures channel conditions –  Sends messages between transmitter and receiver to

coordinate changes


DIVERSITY TECHNIQUES •  Diversity is based on the fact that individual channels

experience independent fading events •  Space diversity – techniques involving physical

transmission path, spacing antennas •  Frequency diversity – techniques where the signal is

spread out over a larger frequency bandwidth or carried on multiple frequency carriers

•  Time diversity – techniques aimed at spreading the data out over time

•  Use of diversity –  Selection diversity – select the best signal –  Combining diversity – combine the signals


MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) ANTENNAS

•  Use antenna arrays for – Diversity – different signals from different antennas – Multiple streams – parallel data streams – Beamforming – directional antennas – Multi-user MIMO – directional beams to multiple

simultaneous users •  Modern systems

–  4 × 4 (4 transmitter and 4 reciever antennas) –  8 × 8 – Two dimensional arrays of 64 antennas –  Future: Massive MIMO with many more antennas


6.18 FOUR USES OF MIMO

Diversity for improvedsystem performance

Beam-forming for improved coverage(less cells to cover a given area)

Spatial division multiple access(“MU-MIMO”) for improved capacity

(more user per cell)

Multi layer transmission(“SU-MIMO”) for higher data rates

in a given bandwidth


6.19 MIMO SCHEME

Transmitter Receiver

AntennaObject

MIMOsignal

processing

MIMOsignal

processing


CHANNEL CORRECTION MECHANISMS

•  Orthogonal Frequency Division Multiplexing (OFDM) – Chapter 8 –  Splits signal into many lower bit rate streams called subcarriers –  Overcomes frequency selectivity from multipath –  Spaces subcarriers apart in overlapping yet orthogonal carrier

frequencies •  Spread spectrum (Chapter 9)

–  Expand a signal to 100 times its bandwidth –  An alternative method to overcome frequency selectivity –  Users can share the channel by using different spreading codes

•  Code Division Multiple Access (CDMA)


SIGNAL PROPAGATION RANGES •  Transmission range

–  communication possible –  low error rate

•  Detection range –  detection of the signal

possible –  no communication

possible •  Interference range

–  signal may not be detected

–  signal adds to the background noise

•  Warning: figure misleading – bizarre shaped, time-varying ranges in reality!

distance

sender

transmission

detection

interference

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CHAPTER 6 THE WIRELESS CHANNEL - Autenticação · PDF fileThe Wireless Channel 6-5 . ISOTROPIC RADIATOR • Radiation and reception of electromagnetic waves, coupling of ... Parabola