Redes Inalámbricas – Tema 2.A The radio channel

REDES INALÁMBRICAS Máster de Ingeniería de Computadores-DISCA

Redes Inalámbricas – Tema 2.AThe radio channel

AntennasBandsCharacteristics of the wireless channel

FadingPropagation modelsPower budget

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

102

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: [1/2]dB -3 dB [1/1000]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

10log1/2

http://www.phys.unsw.edu.au/jw/dB.html

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

104

Antennas: basic concepts

Non Isotropic Antenna Radiates power only in a section of

the surrounding space.

Isotropic Antenna In free space radiates power

equally in all direction.

YAGI Directional Antenna

d• d- distance directly away from the antenna.• is the azimuth, or angle in the horizontal plane.• is the zenith, or angle above the horizon.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

105

Antennas: Simple Dipoles

Not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole

Example: Radiation pattern of a simple Hertzian dipole shape of antenna proportional to wavelength

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simpledipole

/4 /2

Thanks to: Professor Z Ghassemlooy, School of Computing, Engineering and Information Sciences University of Northumbria, U.K.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

106

Antennas: directed and sectorized

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

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

zDirected

top view, 3 sector

x

z

top view, 6 sector

x

z

Sectorized


RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

107

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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

108

More examples

For walls (8,5 dBi)Reach: 3 Km at 2 Mb/s

1 Km at 11 Mb/s

Horizontal Radiation

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

109

Antennas?

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1011 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.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1012 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/Secciones/Espectro/cnaf/

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1013 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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1015 Wireless Communication System

User SourceDecoder

ChannelDecoder

Demod-ulator

Estimate ofMessage signal Estimate of

channel code word

ReceivedSignal

Channel code word

Source SourceEncoder

ChannelEncoder

Mod-ulator

Message SignalModulated

Transmitted Signal

Wireless Channel


RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1016 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, reflectionTheodore S. Rappaport , “Wireless Communications : Principles and Practice”, Prentice

Hall,

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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1017 Diffraction

Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle.

It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings

The complex patterns resulting from the intensity of a diffracted wave are a result of the superposition, or interference of different parts of a wave that traveled to the observer by different paths

TR

1st Fresnel zone

Obstruction

Numerical approximation of diffraction pattern from a slit of width four wavelengths with an incident plane wave. The main central beam, nulls, and phase reversals are apparent.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1018 Fading

The terms slow and fast fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes.

Slow 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 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.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1019 Comparison

Distance

Powe

r

10-100 m(1-10 secs)

0.1 -1 m(10-100 msecs)

Exponencial

Slow Fading

Fast Fading

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1020 Coherence time

Temporal coherence is the measure of the average correlation between the value of a wave at any pair of times, separated by delay τ.

In other words, it characterizes how a wave can interfere with itself at a different time. The delay over which the phase or amplitude wanders by a significant amount (and hence the correlation decreases by significant amount) is defined as the coherence time τc.

At τ=0 the degree of coherence is perfect whereas it drops significantly by delay τc.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1021 Coherence time and doppler spread

The coherence time of the channel is also related to a quantity known as the Doppler spread of the channel.

When a user is moving, the user's velocity causes a shift in the frequency of the signal transmitted along each signal path. This phenomenon is known as the Doppler shift. Signals travelling along different paths can have different Doppler shifts. The difference in Doppler shifts between different signal components

contributing to a single fading channel tap is known as the Doppler spread.

Channels with a large Doppler spread have signal components that are each changing independently in phase over time. Since fading depends on whether signal components add constructively or destructively, such channels have a very short coherence time.

In general, coherence time is inversely related to Doppler spread, typically expressed as

where Tc is the coherence time, Ds is the Doppler spread, and k is a constant taking on values in the range of 0.25 to 0.5.

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1022 Delay Spread

When radio waves bounce off objects, several echos of the wave will converge on the receiver. The difference between the first wave's arrival and the last arrival is 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

(from 802.11 Wireless Networks: The Definitive Guide, Matthew Gast)


Module 2.The radio channel



RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1024 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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1025 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:

d

dPPdPL

r

t )4(log20)4(

log10log10)( 22

2

0

T Rd

d0df

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1026 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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1027 Log-normal shadowing

The amplitude change caused by shadowing is often modeled using a log-normal distribution with a standard deviation according to the log-distance path loss model.

Where Xs is a Gaussian variable with (m=0, s)

Therefore:

ss XddndPLXdPLdBdPL

00 log10)()(])[(

])[(][])[( dBdPLdBmPdBmdP tr


Module 2.The radio channel



RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1029 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

RED

ES IN

ALÁ

MB

RIC

AS

MIC

200

9/20

1030 Power budget: graphic representation

Redes Inalámbricas – Tema 2.A The radio channel

Documents

Transcript of Redes Inalámbricas – Tema 2.A The radio channel