CHAPTER 6 THE WIRELESS CHANNEL - Autenticação · PDF fileThe Wireless Channel...
Transcript of CHAPTER 6 THE WIRELESS CHANNEL - Autenticação · PDF fileThe Wireless Channel...
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Wireless Communication Networks and Systems
1st edition Cory Beard, William Stallings
© 2016 Pearson Higher Education, Inc.
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All material copyright 2016 Cory Beard and William Stallings, All Rights Reserved
CHAPTER 6 THE WIRELESS
CHANNEL
The Wireless Channel 6-1
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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
The Wireless Channel 6-2
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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
The Wireless Channel 6-3
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6.1 ANTENNA RADIATION PATTERNS
(a) Omnidirectional
A
B
(b) Directional
B
A
Antenna location
z
The Wireless Channel 6-4
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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
The Wireless Channel 6-5
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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
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6.2 SIMPLE ANTENNAS
l/2 l/4
(a) Half-wave dipole (b) Quarter-wave antenna
The Wireless Channel 6-7
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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)
The Wireless Channel 6-8
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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
The Wireless Channel 6-9
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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
The Wireless Channel 6-10
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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
The Wireless Channel 6-11
!!G =
4πAeλ2 =
4π f 2Aec2
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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)
The Wireless Channel 6-12
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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
The Wireless Channel 6-13
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PROPAGATION MODES
• Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation
The Wireless Channel 6-14
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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
The Wireless Channel 6-15
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GROUND WAVE PROPAGATION
• Follows contour of the earth • Can propagate considerable distances • Frequencies up to 2 MHz • Example
– AM radio
The Wireless Channel 6-16
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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
The Wireless Channel 6-17
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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
The Wireless Channel 6-18
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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
The Wireless Channel 6-19
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6.6 REFRACTION OF AN ELECTROMAGNETIC WAVE
Area of lowerrefractive index
Incidentdirection
Refracteddirection
Area of higherrefractive index
i
r
The Wireless Channel 6-20
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LOS WIRELESS TRANSMISSION IMPAIRMENTS
• Attenuation and attenuation distortion • Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise
The Wireless Channel 6-21
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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
The Wireless Channel 6-22
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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)
The Wireless Channel 6-23
2
2
2
2 )4()4(cGG
dfGGd
PP
rtrtr
t π
λ
π==
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FREE SPACE LOSS
• Free space loss (isotropic antenna) equation can be recast:
The Wireless Channel 6-24
!!LdB =10log
PtPr
=20log 4πdλ
⎛⎝⎜
⎞⎠⎟
( ) ( ) dB 98.21log20log20 ++−= dλ
( ) ( ) dB 56.147log20log204log20 −+=⎟⎠
⎞⎜⎝
⎛= dfcfdπ
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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
The Wireless Channel 6-25
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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
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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
The Wireless Channel 6-27
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PATH LOSS EXPONENT IN PRACTICAL SYSTEMS
The Wireless Channel 6-28
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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
The Wireless Channel 6-29
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CATEGORIES OF NOISE
• Thermal Noise • Intermodulation noise • Crosstalk • Impulse Noise
The Wireless Channel 6-30
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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
The Wireless Channel 6-31
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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)
The Wireless Channel 6-32
( )W/Hz k0 TN =
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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
The Wireless Channel 6-33
TBN k=
BTN log10 log 10k log10 ++=BT log10 log 10dBW 6.228 ++−=
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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
The Wireless Channel 6-34
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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
The Wireless Channel 6-35
TRS
NRS
NEb
k/
00
==
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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
The Wireless Channel 6-36
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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 Wireless Channel 6-37
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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
The Wireless Channel 6-38
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6.10 EXAMPLES OF MULTIPATH INTERFERENCE
(a) Microwave line of sight
(b) Mobile radio
The Wireless Channel 6-39
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6.11 SKETCH OF THREE IMPORTANT PROPAGATION MECHANISMS
R
R
D
S
Lamppost
The Wireless Channel 6-40
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REAL WORLD EXAMPLES
www.ihe.kit.edu/index.php
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6.12 TWO PULSES IN TIME-VARIANT MULTIPATH
ReceivedLOS pulse
Receivedmultipath
pulses
Time
Time
Transmittedpulse
Transmittedpulse
ReceivedLOS pulse
Receivedmultipath
pulses
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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
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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
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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
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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
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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
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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
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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):
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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
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CHANNEL CORRECTION MECHANISMS
• Forward error correction • Adaptive equalization • Adaptive modulation and coding • Diversity techniques and MIMO • OFDM • Spread sprectrum • Bandwidth expansion
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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
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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
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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
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6.16 LINEAR EQUALIZER CIRCUIT
v v v
∑
Algorithm for tapgain adjustment
Unequalizedinput
Equalizedoutput
C–2 C–1 C0 C1 C2
v
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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
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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
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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
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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
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6.19 MIMO SCHEME
Transmitter Receiver
AntennaObject
MIMOsignal
processing
MIMOsignal
processing
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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)
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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