Modul 4 Small Scale Fading : Doppler Effect

Transkripsi

1 Wireless Communication System Modul 4 Small Scale Fading : Doppler Effect Faculty of Electrical Engineering Bandung Modul 4 Small Scale Fading : Doppler Effect

2 2 Subject a. Doppler Effect

3 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 Time dispersion (echoes) caused by multipath propagation delays Multipath occurs because of Reflections Scattering

4 4 Mobility Other Objects in the radio channels may be mobile or stationary If other objects are stationary Motion is only due to mobile Fading is purely a spatial phenomenon (occurs only when the mobile receiver moves) The spatial variations as the mobile moves will be perceived as temporal variations Dt = Dd/v Fading may cause disruptions in the communication

5 5 Small Scale Fading Amplitudo Amplitudo sinyal terima tergantung dari loasi dan freuensi Jia antena bergera, maa loasi x aan berubah linear terhadap watu t (x = v t) Freuensi Watu Parameters: probability of fades duration of fades bandwidth of fades

6 6 Small Scale Fading Fator-Fator Yang Mempengaruhi Small Scale Fading Modul 4 Small Scale Fading Kecepatan MS Gera relatif antara Base Station dengan MS menghasilan modulasi freuensi random beraitan dengan pergeseran freuensi Doppler yang berbeda untu tiap lintasan multipath. Doppler shift bisa positif dan negatif tergantung dari posisi pergeraan MS terhadap RBS Kecepatan Obye Pemantul Jia obye-obye bergera dalam suatu anal radio, maa aan menghasilan pergeseran Doppler yang berubah terhadap watu, yang berbeda untu tiap omponen multipath. Jia pergeraan benda lebih besar dibandingan geraan MS sendiri, maa aan mendominasi small scale fading Lebar pita transmisi sinyal Pita freuensi yang relatif lebih lebar dibandingan bandwidth anal multipath, aan mengalami frequency selective fading.

7 7 Small Scale Fading Model Respon Impulse Kanal Multipath RBS d v Variasi sinyal sesaat (small scale variation) sinyal omuniasi bergera secara langsung berhubungan dengan respon impulse dari anal radionya. Respon impulse ini merupaan arateristi anal yang memuat informasi sifat-sifat anal radio. Karateristi anal perlu dietahui untu mengetahui unju erja sistem omuniasi dalam anal radio Kanal radio mobile memilii sifat Linear Time Varying Channel x(t) h(t) y(t) Modul 4 Small Scale Fading

8 8 Small Scale Fading multipath signal analysis Assumptions : Vehicles move affected Doppler effect v Signals received from MS that no movement can be expressed : e r N 1 t a cos2f t For the mobile MS, f 0 f, because the frequency received for each of the different paths 0 e r N 1 t a cos2f t where, f v cos 8/18/ Complicated but interesting!

9 9 Small Scale Fading Time Varying Model Variasi anal (arena geraan) Doppler spread Doppler spread f D channel coherence time T C. Channel coherence time adalah suatu selang watu dimana anal diperhatian (dapat dianggap) tida berubah terhadap watu (time invariant). Dalam ata lain: Channel coherence time adalah watu dimana 2 sinyal terima memilii orelasi amplitudo yang uat Jia periode simbol (reciprocal BW) lebih besar dari coherence time artinya anal aan berubah selama periode simbol tersebut terjadi fast fading.

10 10 Doppler Effect When a transmitter or receiver is moving, the frequency of the received signal changes, i.e. İt is different than the frequency of transmission. This is called Doppler Effect. The change in frequency is called Doppler Shift. It depends 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.

11 11 Doppler Shift Recever is moving S d XY l SX SY d cos l vt cos The phase change in the received l D 2 2vt cos signal : Dl Doppler shift (The apparent change in frequency) : X d Y f d 1 D 2 Dt v cos v A mobile receiver is traveling from point X to point Y

12 12 Small Scale Fading multipath signal analysis sudut f f v cos v cos cos For details of the Doppler spectrum, see the : Parson, David, The Mobile Radio Propagation Channel, Pentech Press,1992 8/18/

13 13 Pergeseran doppler f d v cos v = ecepatan pergeraan relatif = panjang gelombang freuensi carrier = sudut antara arah propagasi sinyal datang dengan arah pergeraan antena jia = 0 0, maa f d,max = f m = v/

14 14 Doppler Spread dan Coherence Time Latar belaang : Pergeseran Doppler ( Doppler Shift ) Doppler shift (pergeseran doppler) adalah pergeseran freuensi yang disebaban pergeraan penerima. Doppler shift meningatan bandwidth sinyal yang ditransmisian

15 15 Doppler Shift Transmitter is moving The frequency of the signal that is received behind the transmitter will be smaller The frequency of the signal that is received in front of the transmitter will be bigger

16 ec t16 Doppler Shift The Dopper shift is positive If the mobile is moving toward the direction of arrival of the wave The Doppler shift is negative If the mobile is moving away from the direction of arrival of the wave.

17 17 Small Scale Fading Multipath channel parameter - Doppler Effect Doppler spread, f m, is the maximum Doppler shift maximum, cos = 1 Coherence Time, T C : If the speed of symbol is greater than 1/T C, then the signal is not distorted due to movement of the user channel

18 18 Small Scale Fading Multipath channel parameters - time dispersion parameters Typical Delay Spreads Macrocells T RMS < 8 sec GSM (256 bit/s) uses an equalizer IS-54 (48 bit/s): no equalizer In mountanous regions delays of 8 sec and more occur GSM has some problems in Switzerland Microcells T RMS < 2 sec Low antennas (below tops of buildings) Picocells T RMS < 50 nsec nsec Indoor: often 50 nsec is assumed DECT (1 Mbit/s) wors well up to 90 nsec Outdoors, DECT has problem if range > m

19 19 Small Scale Fading Multipath channel parameters - time dispersion parameters

20 20 Impulse Response Model of a Multipath Channel The wireless channel charcteristics can be expressed by impulse response function The channel is time varying channel when the receiver is moving. Lets assume first that time variation due strictly to 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 impulse response funcion depends on the distance d between trandmitter and receiver

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30 30 Coherence Bandwidth Adalah uuran statisti suatu range freuensi pada anal yang dapat dianggap flat atau bandwidth diantara 2 freuensi yang memilii potensi uat dalam orelasi amplitudo. Semua ompunen spetrum dalam range bandwidth oheren dapat diperhatian (dapat dianggap) mendapatan gain dan fasa yang linier Bandwidth oheren sebainya diuur, tetapi bisa dideati dengan persamaan : atau

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33 33 Intersymbol Interference (ISI): Ketia multipath delay spread lebih besar dari 20% durasi simbol, ISI dapat menjadi problem. Untu mengatasi ISI Pertama, receiver dipasangi dengan adaptive equalizer Equalizer ini menguji efe delay multipath pada deretan training bit yang dietahui, selanjutnya menggunaan informasi hasil pengujian ini untu mengatasi efe delay multipath pada deretan bit-bit informasi sesungguhnya Kedua, menggunaan ode-ode protesi error (channel coding) untu mendetesi dan mengoresi error Catatan : ISI tida bisa diatasi dengan memperbesar uat sinyal!!

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35 35 Small Scale Fading Mean Excess Delay Time Dispersion Parameter RMS Delay Spread Maximum Excess Delay Spread

36 36 C. Small Scale Fading Multipath channel parameters - time dispersion parameters Mean Excess Delay : The first moment of power delay profile a 2 a 2 P( P( ) ) RMS Delay Spread : Square root of second central moment of power delay profile 2 () 2 RMS Delay Spread is the standard deviation of excess delay and The average deviation from the mean excess delay 2 a 2 a 2 2 P( P( ) ) 2

37 37 C. Small Scale Fading Multipath channel parameters - time dispersion parameters Maximum Excess Delay Spread time delay during multipath energy down by X db (typically 10 db), below the maximum value

38 38 C. Small Scale Fading Multipath channel parameters - time dispersion parameters Example: Calculate the mean excess delay and rms delay spread of a multipath channel. Multipath profile is given in the following figure. Give a recommendation whether the multipath channel is suitable for AMPS and GSM without using equalizer? Answer: Mean excess delay, Pr() 0 db -10 db -20 db -30 db (s) 2 a 2 a 2 P( P( ) (1)(5) (0.1)(1) (0.1)(2) (0.01)(0) ( ) Second moment of delay profile, (1)(5) 2 (0.1)(1) 2 ( ) (0.1)(2) ) 2 (0.01)(0) s s 2

39 39 C. Small Scale Fading Multipath channel parameters - time dispersion parameters RMS delay spread, (4.38) Coherence bandwidth, 1 B C 146 Hz 5 Thus, s For AMPS (RF channel BW = 30 Hz), RF channel BW <BW so it does not require coherent equalizer For GSM (RF channel BW = 200 Hz), RF channel BW> BW coherent, thus requiring equalizer