redes inalámbricas – tema 2.a the radio channel
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Redes Inalámbricas – Tema 2.A The radio channel. Antennas Bands Characteristics of the wireless channel Fading Propagation models Power budget. Redes Inalámbricas – Tema 2.A The radio channel. Antennas Bands Characteristics of the wireless channel Fading Propagation models - PowerPoint PPT PresentationTRANSCRIPT
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Redes Inalámbricas – Tema 2.AThe radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Redes Inalámbricas – Tema 2.AThe radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
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Antenna Radiation
An antenna is just a passive conductor carrying RF currentRF power causes the
current flowCurrent flowing radiates
electromagnetic fieldsElectromagnetic fields
cause current in receiving antennas
TX RX
Width of banddenotes current
magnitude
Zero currentat each end
Maximum currentat the middle
Current induced inreceiving antennais vector sum of
contribution of everytiny “slice” of
radiating antenna
each tiny imaginary “slice”
of the antennadoes its shareof radiating
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Antenna Polarization The orientation of the antenna is called
its polarization. RF current in a conductor causes
electromagnetic fields that seek to induce current flowing in the same direction in other conductors.
Coupling between two antennas is proportional to the cosine of the angle of their relative orientation
To intercept significant energy, a receiving antenna must be oriented parallel to the transmitting antenna A receiving antenna oriented at right
angles to the transmitting antenna is “cross-polarized”; will have very little current induced
Vertical polarization is the default convention in wireless telephony
In the cluttered urban environment, energy becomes scattered and “de-polarized” during propagation, so polarization is not as critical
Handset users hold the antennas at seemingly random angles…..
TX
ElectromagneticField
current almostno
current
Antenna 1VerticallyPolarized
Antenna 2Horizontally
Polarized
RX
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Antenna Polarization (continued)
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Antennas basic types Isotropic Radiator
Truly non-directional -- in 3 dimensions
Difficult to build or approximate physically, but mathematically very simple to describe
Provides a reference point for representing the gain of an antenna
Usually expressed in dB isotropic (dBi)
Dipole Antenna Non-directional in 2-dimensional
plane only The smallest, simplest, most
practical type of antenna that can be made
But that also exhibits the least amount of gain
Has a fixed gain over that of an isotropic radiator of 2.15 dB
For microwave and higher frequency antennas
Gain is usually expressed in dB dipole (dBd)
YAGI Directional Antenna
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Decibels The decibel (dB) is a logarithmic unit of measurement
that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless unit. Gains adds instead of multiply
Example: computing the T-R attenuation PT=100, PR=10 [PT/PR]dB = 10 log(PT/PR) = 10 log(10) = 10 dB
Useful values: [2/1]dB ~ 3 dB [1000/1]dB = 30 dB
Expressing absolute values: [n mW]dBm = [n/mW]dB Ej.: [1mW]dBm = 0 dBm [n W]dBW = [n/W]dBEj.: [1 mW]dBW = -30 dBW
From decibels to power: P = 10dB/10
An interesting web page: http://www.phys.unsw.edu.au/jw/dB.html
)log(log10log10 121
210
1
2 pppp
pp
dB
dbm watt0 0.001
10 0.0120 0.130 140 10
log102 ~ 0,3
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Radiation Patterns
An antenna’s directivity is expressed as a series of patterns
The Horizontal Plane Pattern graphs the radiation as a function of azimuth (i.e..,direction N-E-S-W)
The Vertical Plane Pattern graphs the radiation as a function of elevation (i.e.., up, down, horizontal)
Typical ExampleHorizontal Plane Pattern
0 (N)
90 (E)
180 (S)
270 (W)
0
-10
-20
-30 dB
Notice -3 dB points
Front-to-back Ratio
10 dBpoints
MainLobe
a MinorLobe
nulls orminima
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Long reach antenas
Yagi antenna (13,5 dBi)Reach: 6 Km at 2 Mb/s
2 Km at 11 Mb/s
Parabolic Antenna (20 dBi)Reach: 10 Km at 2 Mb/s
4,5 Km at 11 Mb/s
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How Antennas Achieve Their Gain
Quasi-Optical Techniques (reflection, focusing) Reflectors can be used to concentrate radiation
technique works best at microwave frequencies, where reflectors are small
Examples:corner reflector used at cellular or higher
frequenciesparabolic reflector used at microwave
frequenciesgrid or single pipe reflector for cellular
Array techniques (discrete elements) Power is fed or coupled to multiple antenna
elements; each element radiates Elements’ radiation in phase in some directions In other directions, a phase delay for each
element creates pattern lobes and nulls
In phase
Out of phase
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Types Of Arrays Collinear vertical arrays
Essentially omnidirectional in horizontal plane
Power gain approximately equal to the number of elements
Nulls exist in vertical pattern, unless deliberately filled
Arrays in horizontal plane Directional in horizontal plane:
useful for sectorization Yagi
one driven element, parasitic coupling to others
Log-periodic all elements driven wide bandwidth
All of these types of antennas are used in wireless
RF power
RF power
CollinearVerticalArray
Yagi
Log-Periodic
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Sector Antennas
Typical commercial sector antennas are vertical combinations of dipoles, yagis, or log-periodic elements with reflector (panel or grid) backing Vertical plane pattern is
determined by number of vertically-separated elements
varies from 1 to 8, affecting mainly gain and vertical plane beamwidth
Horizontal plane pattern is determined by:
number of horizontally-spaced elements
Vertical Plane PatternUp
Down
Horizontal Plane PatternN
E
S
W
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An example: SECTOR VP Micro Strip (1/2)
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An example: SECTOR VP Micro Strip (2/2)
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Wall mounted antennas
For walls (8,5 dBi)Reach: 3 Km at 2 Mb/s
1 Km at 11 Mb/s
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Antennas?
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Redes Inalámbricas – Tema 2.AThe radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
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Bands without license in USA
Industrial, Scientific, and Medical (ISM) 902 – 928 MHz band.
Currently not being used for WLAN 2400 – 2483.5 MHz ISM band.
Unlicensed National Information Infrastructure (UNII): 5.15 – 5.25 GHz. 5.25 – 5.35 GHz. 5.725 – 5.850 GHz ISM band.
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Bands without license in Europa
Bands approved by the CEPT (European Conference of Postal and Telecommunications Administrations)
2400 – 2483.5 MHz, based on ISM. 5.15 – 5.35 GHz. 5.470 – 5.725 GHz.
ExtremelyLow
VeryLow
Low Medium High VeryHigh
UltraHigh
SuperHigh
Infrared VisibleLight
Ultra-violet
X-Rays
AudioAM Broadcast
Short Wave Radio FM BroadcastTelevision Infrared wireless LAN
Cellular (840MHz)NPCS (1.9GHz)
2.4 - 2.4835 GHz83.5 MHz
(IEEE 802.11)
5 GHz(IEEE 802.11)
HyperLANHyperLAN2
U N - 51 Aplicaciones ICM por encima de 2,4 GHzBandas de frecuencias designadas para aplicaciones industriales,
científicas, y médicas (Aplicaciones ICM, no servicios de radiocomunicaciones).•2400 a 2500 MHz (frecuencia central 2450 MHz)•5725 a 5875 MHz (frecuencia central 5800 MHz)•24,00 a 24,25 GHz (frecuencia central 24,125 GHz)•61,00 a 61,50 GHz (frecuencia central 61,250 GHz)
Los servicios de radiocomunicaciones (notas UN-85, 86, 130 y 133) que funcionen en las citadas bandas deberán aceptar la interferencia perjudicial resultante de estas aplicaciones.
La utilización de estas frecuencias para las aplicaciones indicadas se considera uso común.
http://www.mityc.es/telecomunicaciones/Espectro/Paginas/CNAF.aspx
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Details about the 5 GHz band
Europe19 Channels(*assumes noantenna gain)
1W200mW
5.15 5.35 5.470 5.725 5.8255 GHzUNII Band
5.25
UNII-1: Indoor Use, antenna must be fixed to the radioUNII-2: Indoor/Outdoor Use, fixed or remote antennaUNII-3: Outdoor Bridging Only (EIRP limit is 52 dBm if PtP)
UNII-140mW
(22 dBm EIRP)
UNII-2200mW
(29 dBm EIRP)
US (FCC)12 Channels(*can use up to
6dBi gain antenna)
UNII-3800mW
(35 dBm EIRP)
4 Channels
*if you use a higher gain antenna, you must reduce the transmit power accordingly
4 Channels 4 Channels11 Channels
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Redes Inalámbricas – Tema 2.AThe radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
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Characteristics of the wireless channel
The wireless channel suffers basically from the effects of the following two phenomena: Distance Path attenuation Multipath or scattering over time due to the differing paths of the signal
Other effects: diffraction, obstruction, reflection
Ref.: “Wireless Communications : Principles and Practice”, Theodore S. Rappaport .
The green signal travels 1/2 more than the yellow line. The receiver receives the red line.
For f = 2,4 GHz, = c/f = 12.5cm
T R
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More technically: Fading
“Path attenuation” and “Multipath” are also referred to using the terms slow and fast fading. They refer to the rate at which the magnitude and phase of the signal
change due to the channel. Slow (large-scale) fading arises when the coherence time
of the channel is large relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel
can be considered roughly constant over the period of use. Example: a large obstruction such as a hill or large building obscures the
main signal path between the transmitter and the receiver. Fast (small-scale) fading occurs when the coherence time
of the channel is small relative to the delay constraint of the channel. In this regime, the amplitude and phase change imposed by the channel
varies considerably over the period of use. Examples:
Multipath: Multiple copies of the signal arrive at destinationDoppler shift of the carrier frequency: relative motion of the receiver
and transmitter causes Doppler shifts
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Fading effects comparison
Distance
Powe
r
10-100 m(1-10 secs)
0.1 -1 m(10-100 msecs)
Exponencial
Slow Fading
Fast Fading
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Delay Spread
The difference between the first wave's arrival and the last arrival is indicated as the delay spread. Receivers can pick through the noise to find the signal, but only if the delay spread is not excessive. Some vendors also quote the maximum delay spread on their data sheets. Table below reports the delay spread for three of the cards listed above.
Cards rated for higher delay spreads are capable of dealing with worse multipath interference. The Cisco Aironet 350 was an extremely capable card for its day, capable of dealing with over twice the time-smearing as the Hermes-based card.
Delay spread (in ns) for various cards Card 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps Cisco 350 140 300 400 500 Orinoco Gold (Hermes) 65 225 400 500 Cisco CB-21 (a/b/g); 802.11b performance only 130 200 300 350
Ref.: “802.11 Wireless Networks: The Definitive Guide,” Matthew Gast
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Module 2.The radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
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Free space propagation
Computing the received power when LOS between T and R “signal attenuation without considering all the effects of diffraction,
obstruction, reflection, scattering.”
Friis formula:
222
2
)4()(
dPK
LdGGPdP trtt
r
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Path loss
Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.
Computing path loss: PL(d) = PL (d0)+10nlog(d/d0) (dB)
PL(d0) is obtained from Friis formula considering Gt=Gr=L=1:
0
20
2
2
0)4(log20
)4(log10log10)( d
dPPdPL
r
t
T Rd
d0df
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Path loss: a few examples Given: d=10km, f=900MHz,
=c/f = 3*108/9*108 = 1/3m d0=1km
PL(d0) = 20log(41000/) = 91,5 dB free space n=2
PL(d) = PL (d0)+10nlog(d/ d0) = 91,5 + 10*2*log(10000/1000) = 111,5 dB
Urban area n=3.5 PL(d) = PL (d0)+10nlog(d/ d0)
= 91,5 + 10*3.5*log(10000/1000) = 126,5 dB
Environment n
Free space 2
Urban area 2.7-3.5
Shadowed urban area 3-5
Indoor LOS 1.6-1.8
Indoor no LOS 4-6
T Rd
d0df
REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA
Module 2.The radio channel
AntennasBandsCharacteristics of the wireless channel
FadingPropagation modelsPower budget
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Power Budget
Prx = Ptx+Gpa-Gtxl+Gtxa-Lpath+Grxa+Gra-Grxl Ptx[dBm]=Power generated by TX Gpa[dB]=Gain of the Power Amplifier Gtxa[dBi]=Gain of TX antenna Gtxl[dB]=Gain (loss) of transmission line Lpth[dB]=Loss of the transmission medium Grxa[dBi]=Gain of RX antenna Gra[dB]=Gain of the Receive Amplifier Grxl[dB]=Gain (loss) of receiving line Prx[dBm]=Power received Sr[dBm]=Sensivity of receiver Gtxl
Must hold the condition Prx > Sr
EIRP (Effective Isotropically Radiated Power) = Ptx+Gpa+Gtxa-Gtxl
TX PA
RXRA
Ptx Gpa GTXA Lpath Grxa Gra Sr
Gtxl Grxl
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Power budget: graphic representation