7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 1/31

Modul : 05 

TEKNIK TRANSMISI SELULER 

 

TekTransSel-Modul#5

 

Small Scale Fading 

Program Studi D3Jurusan Teknik Elektro – Institut Teknologi TelkomBANDUNG, 2012

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 2/31

Multipath

• At a receiver point – Radio waves generated from the same transmitted

signal may come• from different directions

• with different propagation delays

• with ossibl different am litudes random

 

TekTransSel-Modul#5

 

• with (possibly) different phases (random)

• with different angles of arrival (random).

 – These multipath components combine vectorially at

the receiver antenna and cause the total signal – to fade

 – to distort

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 3/31

Small Scale Multipath Propagation

• Effects of multipath – Rapid changes in the signal strength – Over small travel distances, or 

 – Over small time intervals

 – Random frequency modulation due to varying Doppler 

shifts on different multiples signals 

TekTransSel-Modul#5

 

 – Time dispersion (echoes) caused by multipathpropagation delays

• Multipath occurs because of 

 – Reflections – Scattering

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 4/31

Multipath Components

Component 2

Component 1

Radio Signals Arriving from different directions to receiver 

TekTransSel-Modul#5

Component N

Receiver may be stationary or mobile.

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 5/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 6/31

Mobility

• Other Objects in the radio channels maybe mobile or stationary• If other objects are stationary

• Motion is only due to mobile

• Fading is purely a spatial phenomenon (occurs only 

TekTransSel-Modul#5

 

• The spatial variations as the mobile moves will beperceived as temporal variations

t = d/v

• Fading may cause disruptions in thecommunication

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 7/31

Factors Influencing Small Scale Fading

• Multipath propagation – Presence of reflecting objects and scatterers causemultiple versions of the signal to arrive at the receiver 

 – With different amplitudes and time delays

 – Causes the total signal at receiver to fade or distort

• Speed of mobile 

TekTransSel-Modul#5

 

 – Cause Doppler shift at each multipath component

 – Causes random frequency modulation

• Speed of surrounding objects – Causes time-varying Doppler shift on the multipath

components

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 8/31

Factors Influencing Small Scale Fading

• Transmission bandwidth of the channel – The transmitted radio signal bandwidth and

bandwidth of the multipath channel affect thereceived si nal ro erties:

 

TekTransSel-Modul#5

 

 – If amplitude fluctuates of not

 – If the signal is distorted or not

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 9/31

Faktor-Faktor 

 Yang

 

• Gerak relatif antara Base Station dengan MSmenghasilkan modulasi frekuensi random berkaitandengan pergeseran frekuensi Doppler yang berbedauntuk tiap lintasan multipath.

• Doppler shift bisa positif dan negatif tergantung dariposisi pergerakan MS terhadap RBS

• Jika ob ek-ob ek ber erak dalam suatu kanal radio, 

Small Scale Fading

Kecepatan Obyek Pemantul

Kecepatan MS

empengaru

Small Scale

Fading

 maka akan menghasilkan pergeseran Doppler yangberubah terhadap waktu , yang berbeda untuk tiapkomponen multipath.

• Jika pergerakan benda lebih besar dibandingkan

gerakan MS sendiri, maka akan mendominasi smallscale fading

Pita frekuensi yang relatif lebih lebar dibandingkanbandwidth kanal multipath, akan mengalami frequency selective fading .

Lebar pita transmisi sinyal

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 10/31

v

• Variasi sinyal sesaat (small scale

variation) sinyal komunikasi bergeraksecara langsung berhubungan denganrespon impulse dari kanal radionya.

• Respon impulse ini merupakankarakteristik kanal yang memuat

Model Respon Impulse Kanal Multipath

C. Small Scale Fading

RBS

d

 

informasi sifat-sifat kanal radio.• Karakteristik kanal perlu diketahui untuk

mengetahui unjuk kerja sistemkomunikasi dalam kanal radio

• Kanal radio mobile memiliki sifat Linear Time Varying Channel 

h(t)x(t) y(t)

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 11/31

Amplitudo• Amplitudo sinyal terimatergantung dari lokasi

dan frekuensi

• Jika antena bergerak,maka lokasi x akan

berubah linear terhadap 

C. Small Scale Fading

Frekuensi Waktu

 

wa u  x = v 

Parameters:

• probability of fades

• duration of fades

• bandwidth of fades

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 12/31

Delay Spread Model dan Time Varying Model

• Fokus pada multipath fading, disebabkan 2 hal: – Time spreading sinyal Akibat sinyal datang

dengan delay yang berbeda-beda, dianalisisdengan Delay Spread Model 

 – Time varying of channel  akibat pergerakan, 

Karakterisasi Kanal Multipath

 

ana s s engan me ary ng o e

• Evaluasi/analisis biasa dilakukan dalam

 – Domain waktu, dan – Domain frekuensi

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 13/31

Time Varying Model

• Variasi kanal (karena gerakan) Doppler spread 

• Doppler spread f D channel coherence time T C .

• Channel coherence time adalah suatu selang waktudimana kanal diperhatikan (dapat dianggap) tidak berubah

terhadap waktu (time invariant). 

C. Small Scale Fading

• Dalam kata lain: Channel coherence time adalah waktudimana 2 sinyal terima memiliki korelasi amplitudo yangkuat

• Jika periode simbol (reciprocal BW ) lebih besar daricoherence time artinya kanal akan berubah selamaperiode simbol tersebut terjadi fast fading .

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 14/31

Doppler Effect

• When a transmitter or receiver is moving, thefrequency of the received signal changes, i.e. İtis different than the frequency of transmission.This is called Doppler Effect.

• The change in frequency is called Doppler Shift. 

TekTransSel-Modul#5

 

 – epen s on• The relative velocity of the receiver with respect to

transmitter 

• The frequenct (or wavelenth) of transmission

• The direction of traveling with respect to the direction of the arriving signal.

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 15/31

Pergeseran doppler 

• v = kecepatan pergerakan relatif • = panjang gelombang

frekuensi carrier 

• = sudut antara arah propagasi

sinyal datang dengan araher erakan antena 

 

 

cosv

 f  d 

 

• jika = 00, maka f d,max= f m= v/  

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 16/31

Doppler Spread dan Coherence Time

• Doppler shift (pergeseran doppler) adalah pergeseranfrekuensi yang disebabkan pergerakan penerima.

• Doppler shift meningkatkan bandwidth sinyal yangditransmisikan

Latar belakang : Pergeseran Doppler ( Doppler Shift )

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 17/31

Doppler Shift – Transmitter is moving

TekTransSel-Modul#5

The frequency of the signal

that is received in front of the

transmitter will be bigger 

The frequency of the signal

that is received behind the

transmitter will be smaller 

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 18/31

Doppler Shift – Recever is movingS

 

 

cos

cos

t vl 

d SY SX l 

 XY d 

TekTransSel-Modul#5

v

X Y

l

d    

  

  

 

cos2

1

cos2

2

v

t  f  

t vl 

d 

:frequency)inchangeapparent(TheshiftDoppler 

:signalreceivedtheinchangephaseThe

 A mobile receiver is traveling from point X to point Y

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 19/31

Doppler Shift

• The Dopper shift is positive – If the mobile is moving toward the direction of 

arrival of the wave

 

TekTransSel-Modul#5

 

 – If the mobile is moving away from the directionof arrival of the wave.

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 20/31

Impulse Response Model of a Multipath

Channel

• The wireless channel charcteristics can beexpressed by impulse response function

• The channel is time varying channel when the

receiver is moving. 

TekTransSel-Modul#5

 

• e s assume rs a me var a on ue s r c yto the receiver motion (t = d /v)

• Since at any distance d = vt, the received power 

will ve combination of different incoming signals,the channel charactesitics or the impulseresponse funcion depends on the distance d between trandmitter and receiver 

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 21/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 22/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 23/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 24/31

Coherence Bandwidth

• Adalah ukuran statistik suatu range frekuensi pada kanal yangdapat dianggap “flat” atau bandwidth diantara 2 frekuensi yang

memiliki potensi kuat dalam korelasi amplitudo.

• Semua kompunen spektrum dalam range bandwidth koherendapat diperhatikan (dapat dianggap) mendapatkan gain danfasa yang linier 

• Bandwidth koheren sebaiknya diukur , tetapi bisa didekati

atau

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 25/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 26/31

Intersymbol Interference (ISI):

• Ketika multipath delay spread lebih besar dari 20% durasi

simbol, ISI dapat menjadi problem. Untuk mengatasi ISI …• Pertama, receiver dipasangi dengan adaptive equalizer 

Equalizer ini menguji efek delay multipath pada deretan training bityang diketahui, selanjutnya menggunakan informasi hasil pengujianini untuk mengatasi efek delay multipath pada deretan bit-bit informasi

sesungguhnya

 • Kedua, menggunakan kode-kode proteksi error (channelcoding) untuk mendeteksi dan mengkoreksi error 

• Catatan : ISI tidak bisa diatasi dengan memperbesar kuat sinyal

!!

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 27/31

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 28/31

SMALL SCALE

FADING

FLAT FADING

Berdasarkan

atas multipath

Time Delay

SpreadFREQUENCY

SELECTIVE FADING

 

BW sinyal < BW koheren Delay spread < periode

simbol

BW sinyal > BW koheren

Delay spread > periode simbol

 

KlasifikasiSmall Scale

Fading 

 

Berdasarkan

atas Doppler 

Spread

 

SLOW FADING

 

Doppler spread >>

Coherence time < periode

simbol

Variasi kanal lebih cepat dari

variasi sinyal baseband

Doppler spread <<

Coherence time > periode

simbol

Variasi kanal lebih lambat dari

variasi sinyal baseband

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 29/31

Basic Theory

• Mobile RadioPropagation The received signal is a sum of 

real signals that haveexperience attenuation,

reflection, refraction and .

Mobile station movement

Rayleigh Distribution

 

  

 

00

02

exp2

2

2

r 

r r r 

r  p   

TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 30/31

Simulation Result (1)Signal Analysis on Rayleigh Channel

Mobile station speed 5km/hour,fd=4,023Hz

 

mobile station speed25km/hour, fd=20,27Hz

 

Mobile station speed 100km/hour, fd=80,52Hz

 

• Amplitude fluctuation (Fading) willincrease as the mobile station (MS) speedincrease, it can be seen as follow:

MS speed 5km/hour, fd=4,023Hz.

Received signal will achieve maximalattenuation 11db, amplitude fluctuation isnot occurs many times.

MS speed 25km/hour, fd=20,27Hz.

Received signal will achieve maximalattenuation 13dB, amplitude fluctuation ismore often than MS 25km/hour.

MS speed 100km/hour, fd=80,52Hz.

Maximal attenuation 17dB, amplitudefluctuation is the most often in this threeexperiment.TekTransSel-Modul#5

7/30/2019 Modul small scale fading

http://slidepdf.com/reader/full/modul-small-scale-fading 31/31

TekTransSel-Modul#5