Modul 5 Fading Mitigation

Wireless Communication Systems

1

Faculty of Electrical and CommunicationInstitut Teknologi Telkom

Bandung 2012

10/29/2012

Subject

a. Diversity and Equalizationb. Channel Codingd. Teknik Multicarrier

2

a. Diversity and Equalizationb. Channel Codingd. Teknik Multicarrier

3 Typical Mobile Radio Propagation Channel

Fading channel manifestations4

Small-scale Fading: Mechanisms, Degradationcategories, and Effects5

6 Relationships among the channel correlationfunctions and power density functions

7 Fading mechanisms Frequency Dispersion

Time variations of the channel are caused by motion of the antenna Channel changes every half a wavelength Moving antenna gives Doppler spread Fast fading requires short packet durations, thus high bit rates Time variations poses requirements on synchronization and rate ofconvergence of channel estimation Interleaving may help to avoid burst errors

Time Dispersion Delayed reflections cause intersymbol interference (ISI) Channel Equalization may be needed. Frequency selective fading Multipath delay spreads require long symbol times Frequency diversity or spread spectrum may help

RSL Fluctuation Shadowing, obstruction, etc

Frequency Dispersion Time variations of the channel are caused by motion of the antenna Channel changes every half a wavelength Moving antenna gives Doppler spread Fast fading requires short packet durations, thus high bit rates Time variations poses requirements on synchronization and rate ofconvergence of channel estimation Interleaving may help to avoid burst errors

Time Dispersion Delayed reflections cause intersymbol interference (ISI) Channel Equalization may be needed. Frequency selective fading Multipath delay spreads require long symbol times Frequency diversity or spread spectrum may help

RSL Fluctuation Shadowing, obstruction, etc

8 Time Dispersion and Frequency Dispersion

Time Domain Channel variations Delay spreadInterpretation Fast Fading InterSymbol Interference

Correlation Distance Channel equalizationFrequency Doppler spread Frequency selective fadingDomain Intercarrier Interference Coherence bandwidthInterpretation

Frequency Dispersion Time Dispersion

Time Domain Channel variations Delay spreadInterpretation Fast Fading InterSymbol Interference

Correlation Distance Channel equalizationFrequency Doppler spread Frequency selective fadingDomain Intercarrier Interference Coherence bandwidthInterpretation

9 Effect of Fading

Freq.

Spec

tral den

sity

Bc

Bs

Freq. Selective Fading

TX BW > Channel BWBs > Bc

Freq.

Freq.

Spec

tral den

sity

Coherent BW, Bc

Bc

Bs

Freq. Flat Fading

TX BW < Channel BWBs < Bc

10 Statistical Fluctuations Area-mean power

is determined by path loss is an average over 100 m - 5 km

Local-mean power is caused by local 'shadowing'

effects has slow variations is an average over 40 (few

meters)

Area-mean power is determined by path loss is an average over 100 m - 5 km

Local-mean power is caused by local 'shadowing'

effects has slow variations is an average over 40 (few

meters) Instantaneous power

fluctuations are caused by multipath reception depends on location and frequency depends on time if antenna is in motion has fast variations (fades occur about every half a wave length)

11 Basic mitigation types

12 Fading Mitigation Techniques 3 techniques commonly used to combat the effectof fading without increasing TX Power and BW: Diversity : space/spatial, time, frequency Channel Encoding or Error protection coding Equalization

While Fading Margin and Power Control are usedto maintain a good signal reception at Receiver.

3 techniques commonly used to combat the effectof fading without increasing TX Power and BW: Diversity : space/spatial, time, frequency Channel Encoding or Error protection coding Equalization

While Fading Margin and Power Control are usedto maintain a good signal reception at Receiver.

13 FMT: Diversity Diversity exploits the random nature of radio propagation

by finding the independent signal paths. If one pathundergo a deep fade, another path may have a strongsignal.

Usually employed to reduce the depth and duration of fadeexperienced by receiver in flat fading channel.

Types of diversity: spatial, frequency, time, and polarization

Diversity exploits the random nature of radio propagationby finding the independent signal paths. If one pathundergo a deep fade, another path may have a strongsignal.

Usually employed to reduce the depth and duration of fadeexperienced by receiver in flat fading channel.

Types of diversity: spatial, frequency, time, and polarization

14 Spatial Diversity Use two or more receiving antenna While one antenna sees a null signal, the othersmay receive a peak signals.

The received signals are then combined andprocessed by an algorithm to get best reception.

Can be implemented in both BS and MS receiver

Use two or more receiving antenna While one antenna sees a null signal, the othersmay receive a peak signals.

The received signals are then combined andprocessed by an algorithm to get best reception.

Can be implemented in both BS and MS receiver

15 Spatial Diversity Antenna is spaced each

other by an odd integermultiply of /4, usually d >8 .

Spatial diversity canimprove SNR at receiver byas much as 20 dB to 30dB.

wow1

wK

ro(t)

r1(t)

rK(t)

y(t)d

Combining algorithm commonly used: Selective, Equal gain, andMaximal ratio combining.

Antenna is spaced eachother by an odd integermultiply of /4, usually d >8 .

Spatial diversity canimprove SNR at receiver byas much as 20 dB to 30dB.

wKProcessor

rK(t)

Diversity Combining Methods

Switching/selection Memilih sinyal terkuat dari dua sinyal sesaat (instantaneously):

~1 dB hysteresis saat pemilihan sinyal Menyebabkan pergeseran fasa random (random phase shifts)

Akan menjadi problem bagi yang menggunakan modulasi fasa sepertiIS-136, IS-95, where switch times between antennas is restricted to theboundaries of data bit fields

Struktur paling sederhana, dgn peningkatan C/(I+n) antara 1.5 sampai 4 dB Equal gain

Adaptive phase shift hardware digunakan untuk menggeser fasa salah satukanal , disamakan fasanya dengan fasa kanal yang lain, untuk kemudiandijumlahkan secara koheren

1.5 dB lebih baik dari switching diversity Maximal ratio

Seperti equal gain, tetapi sinyal yang lemah dikuatkan pada level rata-ratayang sama dengan sinyal yang kuat sebelum dijumlahkan

Paling kompleks , tetapi tipikalnya 2dB lebih baik dari switching diversity.10/29/2012 16

Switching/selection Memilih sinyal terkuat dari dua sinyal sesaat (instantaneously):

~1 dB hysteresis saat pemilihan sinyal Menyebabkan pergeseran fasa random (random phase shifts)

Akan menjadi problem bagi yang menggunakan modulasi fasa sepertiIS-136, IS-95, where switch times between antennas is restricted to theboundaries of data bit fields

Struktur paling sederhana, dgn peningkatan C/(I+n) antara 1.5 sampai 4 dB Equal gain

Adaptive phase shift hardware digunakan untuk menggeser fasa salah satukanal , disamakan fasanya dengan fasa kanal yang lain, untuk kemudiandijumlahkan secara koheren

1.5 dB lebih baik dari switching diversity Maximal ratio

Seperti equal gain, tetapi sinyal yang lemah dikuatkan pada level rata-ratayang sama dengan sinyal yang kuat sebelum dijumlahkan

Paling kompleks , tetapi tipikalnya 2dB lebih baik dari switching diversity.

17 Selective Combiner

G1SwitchingLogicor

Demodulator

output

Ant. 1

Ant. 2 G2

Gm

SwitchingLogicor

Demodulator

output

Variable gain

Ant. 2

Ant. m

18 Selective Combining

Receiver only select one strongest signal to detect. If average SNR of received signal in a branch = and threshold SNR

= , then probability that M branches of antenna receive signals withSNR below the threshold is:

P( i < ) = PM( 1 - e- In other word, probability that received signal SNR above the threshold

is :

P( i > ) = 1 - PM( 1 - e-

Receiver only select one strongest signal to detect. If average SNR of received signal in a branch = and threshold SNR

= , then probability that M branches of antenna receive signals withSNR below the threshold is:

P( i < ) = PM( 1 - e- In other word, probability that received signal SNR above the threshold

is :

P( i > ) = 1 - PM( 1 - e-

19 Selective CombiningExample: 4 antenna diversity is used. If average SNR is 20 dB,

determine the probability that SNR will drop below 10 dB (badreception), and also that good reception (SNR above 10 dB) will mostlytake place. Compare with single antenna receiver!

Answer:Threshold SNR = = 10 dB, = 20 dB, = 0.1

P4( i< 10 dB) = (1 e-0.1)4 = 0.000082, andP4( i> 10 dB) = 1- (1 e-0.1)4 = 0.999918 or 99.9918%

With single antenna:P ( i< 10 dB) = (1 e-0.1) = 0.095, andP ( i> 10 dB) = 1- (1 e-0.1) = 0.905 or 90.5%

Example: 4 antenna diversity is used. If average SNR is 20 dB,determine the probability that SNR will drop below 10 dB (badreception), and also that good reception (SNR above 10 dB) will mostlytake place. Compare with single antenna receiver!

Answer:Threshold SNR = = 10 dB, = 20 dB, = 0.1

P4( i< 10 dB) = (1 e-0.1)4 = 0.000082, andP4( i> 10 dB) = 1- (1 e-0.1)4 = 0.999918 or 99.9918%

With single antenna:P ( i< 10 dB) = (1 e-0.1) = 0.095, andP ( i> 10 dB) = 1- (1 e-0.1) = 0.905 or 90.5%

Improvement factor about 3 order in magnitude!

20 Selective CombiningPerbaikan SNR:

Pada contoh di atas:

M

k k11

Improvement factor about twice in SNR!

083.225.0333.5.011

1

M

k k

21 Equal Gain Combining

If weight of each branch is set to unity and co-phased, Max. ratio combining become equal gaincombining.

Less complex with slightly lower performance thanmax. ratio combining.

Without continuously adapt each weight ofbranches differently, it allows receiver to exploitreceived signals simultaneously.

If weight of each branch is set to unity and co-phased, Max. ratio combining become equal gaincombining.

Less complex with slightly lower performance thanmax. ratio combining.

Without continuously adapt each weight ofbranches differently, it allows receiver to exploitreceived signals simultaneously.

22 Max. Ratio Combiner

G1Co-phase

andSum

output

Ant. 1

Ant. 2MG2

Gm

Co-phaseandSum

output

Variable gain

Ant. 2

Ant. m

Detector

m

M

Adaptive control

23 Max. Ratio Combining Signals from each branch/antenna are co-phasedand individually weighted to provide coherentaddition to get optimal SNR.

Probability that received signal SNR belowthreshold is:

Probability of good reception:

M

k

kM keP 1

1/

)!1()/(1

Signals from each branch/antenna are co-phasedand individually weighted to provide coherentaddition to get optimal SNR.

Probability that received signal SNR belowthreshold is:

Probability of good reception:

M

k

kM keP 1

1/

)!1()/(1

M

k

kM keP 1

1/

)!1()/(

24 Maximal Ratio CombiningSNR improvement:

In the example above :

Probability (good signal)=e-0.1(1+0.1+0.12/2+0.13/6)=0.9999961531

M

MM

M

kM

1

SNR improvement:

In the example above :

Probability (good signal)=e-0.1(1+0.1+0.12/2+0.13/6)=0.9999961531

Improvement factor about four times in SNR!

4MM

25 Frequency Diversity Use two or more carrier frequency for transmission with spacing aboutUse two or more carrier frequency for transmission with spacing about22 5 % f5 % foo.. Need to employ two or more Transmitter and ReceiverNeed to employ two or more Transmitter and Receiver

Improvement factor :Improvement factor :

26 Time Diversity Interleaver

1 m+12 m+2

mrows

Read out bits to modulator one row at a time

M 2m nm

.

n columns

mrows

Read inCoded bitsfromencoder

27 Channel Encoding Channel encoding is done by encode the data into a special form, and

introduce redundancies in the transmitted data. It protects data/information from error and distortion introduced by the

channel. Redundant bits increase data rate hence the bandwidth, but improve

BER performance especially in fading channel. Reduce BW efficiency of the link in high SNR condition, but provide

excellent performance in low SNR condition Two types mostly used: Block Codes, Convolutional code and Turbo

Codes

Channel Coding meningkatkan kinerja hubungan small scale denganpenambahkan bit data dalam pesan yang dikirimkan sehingga jika terjadisuatu pelemahan seketika itu terjadi dalam saluran, data masih dapatdipulihkan pada penerima

Channel coding digunakan oleh penerima untuk mendeteksi ataumemperbaiki beberapa (atau semua) dari kesalahan terdapat padasaluran dalam urutan tertentu bit pesan

Channel encoding is done by encode the data into a special form, andintroduce redundancies in the transmitted data.

It protects data/information from error and distortion introduced by thechannel.

Redundant bits increase data rate hence the bandwidth, but improveBER performance especially in fading channel.

Reduce BW efficiency of the link in high SNR condition, but provideexcellent performance in low SNR condition

Two types mostly used: Block Codes, Convolutional code and TurboCodes

Channel Coding meningkatkan kinerja hubungan small scale denganpenambahkan bit data dalam pesan yang dikirimkan sehingga jika terjadisuatu pelemahan seketika itu terjadi dalam saluran, data masih dapatdipulihkan pada penerima

Channel coding digunakan oleh penerima untuk mendeteksi ataumemperbaiki beberapa (atau semua) dari kesalahan terdapat padasaluran dalam urutan tertentu bit pesan

28 Fading Margin

Kuat sinyal (dB) setelahditambah fading margin (FM)

t

Theshold FM

Fading margin depends upon target availability of the link/coverage.

Greater availability requires larger fading margin.Kuat sinyal (dB) setelah

ditambah fading margin (FM)

t

Theshold FM

29 Fading Margin

2FMerf2

121dm)m(p)Thm(PRP

mThTh

If fading margin FM applied to the link, then probabilitythat RSL at receiver separated at distance R above thethreshold can be written as:

2FMerf2

121dm)m(p)Thm(PRP

mThTh

Fading margin improve signal reception hence thelink performance, in an expense of increasingtransmission power.

30 Power Control

User 2

User 1

d1d2

Basestation

Pr2

Pr1

Pt2Pt1

Mitigating the effect of shadowing and near-far problemIf user 1 at 3 km from BTS transmitting with 100mWatt, how much power is needed by user 2 at9 km away from BTS using Okumura Hattamodel in urban area to achieve the same powerat the BTS with 10 m high above ground level?

User 2

User 1

d1d2

Basestation

Pr2

Pr1

Pt2Pt1

Answer: Path loss slope Hatta-Urban is( 44.9 6.55 log 10) =38.35.W2 = (d2/d1)3.835 W1 = 38.3 dBm =6.76 Watt

Power Control

t

Channelvariation(i)

est PCC biterrorDTpLoopdelay

_

+

+

e(i) 1 Base station

channel

Small Scale Fading Mitigation -31

31

Channelvariation(i) DTp

Loopdelay

pTpIntegrator

Transmitpower p(i)

Step size

+

+

+ _ Mobile station

channel

32 Power Control

0 50 100 150 200 250 300-30

-20

-10

0

10

20

30

Time slot (0.67 ms)

Signallevel(dB)

Received signal amplitudeControlled transmit powerControlled SIR (target = 10 dB)

Channel is estimated atthe receiver, then Tx isinstructed to adjust Txpower according to theestimated channel (e.g.SNR).Problem:Control rate >> fadingrateControl step sizesingle step or variablestepWhat is thebenefit/drawbacks ofsingle or variable stepsize ?

Rayleigh fading

0 50 100 150 200 250 300-30

-20

-10

0

10

20

30

Time slot (0.67 ms)

Signallevel(dB)

Received signal amplitudeControlled transmit powerControlled SIR (target = 10 dB)

Channel is estimated atthe receiver, then Tx isinstructed to adjust Txpower according to theestimated channel (e.g.SNR).Problem:Control rate >> fadingrateControl step sizesingle step or variablestepWhat is thebenefit/drawbacks ofsingle or variable stepsize ?

0 50 100 150 200 250 300 350 400 450 500-20

-15

-10

-5

0

5

10

Fadingamplitude[dB]

time x 0.67 msec

0b0b

e

I/E1I/E12

1

2/12/12

1BERP

Fading channel

Example for fading rate fd= 5o Hz ( vehiche speed 30 km/hr at 1.8 GHz).

Power ControlSmall Scale Fading Mitigation -33

330 50 100 150 200 250 300 350 400 450 500-20

-15

-10

-5

0

5

10

Fadingamplitude[dB]

time x 0.67 msec

0b0b

e

I/E1I/E12

1

2/12/12

1BERP

0

be N

E2QBERPAWGN channel

Example To achieve a satisfactory power control performance

when a vehicle moving at 30 km/h (carrier freq = 1.8GHz) the rate of power control is at least 30 timeshigher than the fading rates. Compute the minimum signalling rate required for power

control. If the voice channel is transmitted at 9.6 kbps, what

percentage of band width is lost due to power control with (a)fixed step algorithm (b) variable step with 3 bit algorithm

If the deepest fading is 30 dB below its average level, what isthe incremental power ajustment (step size) if fixed stepadjustment is employed to equalize the deepest fading.

Power ControlSmall Scale Fading Mitigation -34

34

Example To achieve a satisfactory power control performance

when a vehicle moving at 30 km/h (carrier freq = 1.8GHz) the rate of power control is at least 30 timeshigher than the fading rates. Compute the minimum signalling rate required for power

control. If the voice channel is transmitted at 9.6 kbps, what

percentage of band width is lost due to power control with (a)fixed step algorithm (b) variable step with 3 bit algorithm

If the deepest fading is 30 dB below its average level, what isthe incremental power ajustment (step size) if fixed stepadjustment is employed to equalize the deepest fading.

Antena Sektoral dan Smart Antenna Narrow sector akan mengurangi Co-channelinterference Mengijinkan pengulangan frekuensi yang lebih dekatsecara geografis

Sehingga: lebih banyak carrier per-sel lebih besarkapasitas

Tetapi sering back dan side lobe menjadiproblem Menghasilkan spot co-channel interference

Merupakan interferensi tak terduga yang sulitdiidentifikasi dan diatasi

smart antennas (adaptive phased arrays) dapatmengatasi persoalan ini lebih baik (tetapi high cost)

Mengatasi Large Scale Fading35

10/29/2012 35

Narrow sector akan mengurangi Co-channelinterference Mengijinkan pengulangan frekuensi yang lebih dekatsecara geografis

Sehingga: lebih banyak carrier per-sel lebih besarkapasitas

Tetapi sering back dan side lobe menjadiproblem Menghasilkan spot co-channel interference

Merupakan interferensi tak terduga yang sulitdiidentifikasi dan diatasi

smart antennas (adaptive phased arrays) dapatmengatasi persoalan ini lebih baik (tetapi high cost)

Representasi hexagon ideal idealnya tidak ada backantenna signal pada arah uplink maupun downlink

60 120Back ofblue sector

Back andside lobes

Antena Sektoral dan Smart AntennaMengatasi Large Scale Fading36

10/29/2012 36

6 sectors 3 sectorsReal sectored cells are non-ideal in several ways. One important difference:There is non-negligible power radiated in the back and side regions, andthe amount of such back and side lobe power is greater for narrow sectorsthan for wide angle sectors.

Front ofblue sector

Back ofblue sector RealSector

Teknik-Teknik Anti Frequency Selective FadingTeknik anti frequency selective fading diperlukan jika bandwidth sinyal lebihbesar dari bandwidth koheren kanal seperti yang sudah dijelaskan pada bagiansebelumnya.Teknik-teknik yang biasa dilakukan [PEI 97] adalah :

Decision Feedback Equalizer dengan RLS Algorithm (algoritmaKalman), Fast Kalman Algorithm, dan juga Tap Gain Interpolasi

Adaptive Array Antenna beamforming Rake Diversity untuk sinyal spread spectrum Multicarrier technique dll

Mengatasi Small Scale Fading -37

10/29/2012 37

Decision Feedback Equalizer dengan RLS Algorithm (algoritmaKalman), Fast Kalman Algorithm, dan juga Tap Gain Interpolasi

Adaptive Array Antenna beamforming Rake Diversity untuk sinyal spread spectrum Multicarrier technique dll

Pertanyaan :Sejauh mana unjuk kerja masing-masing perangkat tersebutdalam memperbaiki frequency selective fading ? Pelajarilahdan diskusikan dengan teman anda

InterSymbol Interference (ISI) Ketika multipath delay spread mulai lebih besar 20% dari

durasi symbol , ISI dapat menjadi problem. Untuk mengatasiISI...

Pertama, receiver terpasang dengan adaptive equalizer Adaptive equalizer (and also the similar RAKE receiver used for CDMA)

produces delayed copy/ies of the received signal waveform and use(s) thesecopy/ies to cancel the physically delayed radio signals

Equalixer ini mendeteksi/mengetahui efek multipath delay pada deretantraining bit yang diketahui, dan menggunakan informasi itu untuk mengatasiISI pada deretan bit informasi dengan cara memberikan replika delayinternal pada sinyal

Kedua, penggunaan error protection codes (channelcoding) untuk mendeteksi/mengkoreksi error (baik yangdisebabkan ISI ataupun fading)

You know ? . ISI tak dapat diatasi dengan penguatansinyal.

Mengatasi Small Scale Fading -38

10/29/2012 38

Ketika multipath delay spread mulai lebih besar 20% daridurasi symbol , ISI dapat menjadi problem. Untuk mengatasiISI...

Pertama, receiver terpasang dengan adaptive equalizer Adaptive equalizer (and also the similar RAKE receiver used for CDMA)

produces delayed copy/ies of the received signal waveform and use(s) thesecopy/ies to cancel the physically delayed radio signals

Equalixer ini mendeteksi/mengetahui efek multipath delay pada deretantraining bit yang diketahui, dan menggunakan informasi itu untuk mengatasiISI pada deretan bit informasi dengan cara memberikan replika delayinternal pada sinyal

Kedua, penggunaan error protection codes (channelcoding) untuk mendeteksi/mengkoreksi error (baik yangdisebabkan ISI ataupun fading)

You know ? . ISI tak dapat diatasi dengan penguatansinyal.

Attenuation, Dispersion Effects: ISI!

Inter-symbol interference (ISI)

39

multi-path propagationPath Delay

Powe

r

path-2path-2

path-3

path-1

path-1

Multipaths: Power-Delay Profile40

Base Station (BS) Mobile Station (MS)path-3

Channel Impulse Response:Channel amplitude |h| correlated at delays .Each tap value @ kTs Rayleigh distributed

(actually the sum of several sub-paths)

40

Inter-Symbol-Interference (ISI) due to Multi-Path Fading

Transmitted signal:

Received Signals:Line-of-sight:

Reflected:

The symbols add up onthe channel

Distortion!

41

Transmitted signal:

Received Signals:Line-of-sight:

Reflected:

The symbols add up onthe channel

Distortion!Delays

41

42 Types of Equalizer

Linear: Transversal filter (Zero forcing, LMS, RLS,fast RLS, Sq. root RLS)

Lattice Filter (Gradient RLS) Non Linear:

DFE (LMS, RLS, Fast RLS, Sq. root RLS) ML Symbol Detection MLSE

Linear: Transversal filter (Zero forcing, LMS, RLS,fast RLS, Sq. root RLS)

Lattice Filter (Gradient RLS) Non Linear:

DFE (LMS, RLS, Fast RLS, Sq. root RLS) ML Symbol Detection MLSE

Channel Equalizer

i index waktuV orde equalizerD index delay

z-1 z-1 z-1(i-D-v)(i-D) (i-D-V+1)(i)

Channel equalizer diperlukan untuk mengkompensasi ISI yang disebabkan kanalmultipath (Freq. Selective Fading Channel).

Karena multipath fading channel bersifat dynamic random equalizer hrs bersifatadaptif

43

10/29/2012 43

i index waktuV orde equalizerD index delayb0 bD+

vbV-1

(i)out

Adaptivealgorithm

(i)

Beamforming Beamforming adalah proses pembentukan beam menuju ke arah user

yang diinginkan serta menekan sinyal pengganggu dari arah lain.Dengan demikian, beamforming bisa dikatakan sebagai spatial filteringsinyal

Pembentukan beam ke arah sinyal yang diinginkan bisa dilakukandengan memberikan pembobotan dengan algoritma adaptif padaelemen antena pengganggu-1

user yangdiinginkan

pengganggu-2

user yangdiinginkan

10/29/2012 44

Beamforming dengan kriteria MMSE(Minimum Mean Squared Error)

MSE, E{|e(n)|2}diminimumkan. Disini e(n)adalah

Solusi optimum Wienerdiberikan oleh

x1(n)

x2(n)

w1*(n) y(n)=wH(n).x(n) )(.)()( nxwndne H

MSE, E{|e(n)|2}diminimumkan. Disini e(n)adalah

Solusi optimum Wienerdiberikan olehx2(n)

w2*(n)AlgoritmaAdaptif

- + d(n)e(n))( nd

xdxxopt rRw .1 )()( nxnxER Hxx )()( * ndnxEr xd

adalah matriks kovarians sinyal terima

adalah vektor kroskorelasi antara vektor sinyal terima x dansinyal referensi d .

45

46 CDMA RAKE Receiver

Correlator 2

Correlator 1

.

...

Int. DCr(t)IF or base bandCDMA signalwith multipathcomponents

Z1Z2 Z Z m(t)

2

Correlator m

.

...IF or base bandCDMA signal

with multipathcomponents Zm

m

Since chip rate of CDMA much greater than coherence BW, delay spreadmerely provide a multiple delayed version of signals at receiver. Instead ofcausing ISI, RAKE receiver attempts to collect multipath signals, process it byseparate correlator receiver, and combine the signals to have a better detection.

Multicarrier CDMA:

Gabungan OFDM dan CDMA

10/29/2012 47

OFDM membagi data serial

kecepatan tinggimenjadi data paralelkecepatan rendah

Data paralel tersebutdibawa oleh masing-masing subcarrier

Antar subcarrier satudengan yang lain salingorthogonal

-8 -6 -4 -2 0 2 4 6 8-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Normalized Frequency(FT)

NormalizedAmplitude

10/29/2012 48

membagi data serialkecepatan tinggimenjadi data paralelkecepatan rendah

Data paralel tersebutdibawa oleh masing-masing subcarrier

Antar subcarrier satudengan yang lain salingorthogonal

-8 -6 -4 -2 0 2 4 6 8-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Normalized Frequency(FT)

NormalizedAmplitude

Analog Carrier frequency is low enough to avoid problems

GSM Channel bit rate well above Doppler spread TDMA during each bit / burst transmission the channel is fairly

constant. Receiver training/updating during each transmission burst Feedback frequency correction

DECT Intended to pedestrian use: only small Doppler spreads are to be anticipated for Original DECT concept did not standardize an equalizer

IS95 Downlink: Pilot signal for synchronization and channel estimation Uplink: Continuous tracking of each signal

How do systems handle Doppler Spreads?Kasus-Kasus Fading Mitigation

10/29/2012 49

Analog Carrier frequency is low enough to avoid problems

GSM Channel bit rate well above Doppler spread TDMA during each bit / burst transmission the channel is fairly

constant. Receiver training/updating during each transmission burst Feedback frequency correction

DECT Intended to pedestrian use: only small Doppler spreads are to be anticipated for Original DECT concept did not standardize an equalizer

IS95 Downlink: Pilot signal for synchronization and channel estimation Uplink: Continuous tracking of each signal

How do systems handle delay spreads? fenomena ISIAnalog Narrowband transmissionGSM Adaptive channel equalization Channel estimation training sequenceDECT Use the handset only in small cells with small delay spreads Diversity and channel selection can help a little bit

pick a channel where late reflections are in a fadeIS95 Rake receiver separately recovers signals over paths with excessivedelays

Digital Audio Broacasting OFDM multi-carrier modulationThe radio channel is split into many narrowband (ISI-free) subchannels

Kasus-Kasus Fading Mitigation

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Analog Narrowband transmissionGSM Adaptive channel equalization Channel estimation training sequenceDECT Use the handset only in small cells with small delay spreads Diversity and channel selection can help a little bit

pick a channel where late reflections are in a fadeIS95 Rake receiver separately recovers signals over paths with excessivedelays

Digital Audio Broacasting OFDM multi-carrier modulationThe radio channel is split into many narrowband (ISI-free) subchannels

Typical Delay SpreadsMacroce lls TRMS < 8 sec GSM (256 kb it/s ) uses an equa lize r IS -54 (48 kb it/s ): no equa lize r In m oun tanous reg ions de lays o f 8 sec and m ore

occu rGSM has som e p rob lem s in Sw itze rland

M icroce lls TRMS < 2 sec Low an tennas (be low tops o f bu ild ings )P icoce lls TRMS < 50 nsec - 300 nsec Indoor: o ften 50 nsec is assum ed DECT (1 Mb it/s ) wo rks we ll up to 90 nsec

Outdoors , DECT has p rob lem if range > 200 .. 500 m

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Macroce lls TRMS < 8 sec GSM (256 kb it/s ) uses an equa lize r IS -54 (48 kb it/s ): no equa lize r In m oun tanous reg ions de lays o f 8 sec and m ore

occu rGSM has som e p rob lem s in Sw itze rland

M icroce lls TRMS < 2 sec Low an tennas (be low tops o f bu ild ings )P icoce lls TRMS < 50 nsec - 300 nsec Indoor: o ften 50 nsec is assum ed DECT (1 Mb it/s ) wo rks we ll up to 90 nsec

Outdoors , DECT has p rob lem if range > 200 .. 500 m

How to handle fast multipath fading?A n a l o g

U s e r m u s t s p e a k s lo w lyG S M

E r r o r c o r r e c t io n a n d in t e r le a v in g t o a v o idb u r s t e r r o r s

E r r o r d e t e c t io n a n d s p e e c h d e c o d in g F a d e m a r g in s in c e l l p la n n in g

D E C T D iv e r s i t y r e c e p t io n a t b a s e s t a t io n

I S 9 5 W id e b a n d t r a n s m is s io n a v e r a g e s c h a n n e l

b e h a v io u rT h is a v o id s b u r s t e r r o r s a n d d e e p f a d e s

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A n a l o g U s e r m u s t s p e a k s lo w ly

G S M E r r o r c o r r e c t io n a n d in t e r le a v in g t o a v o id

b u r s t e r r o r s E r r o r d e t e c t io n a n d s p e e c h d e c o d in g F a d e m a r g in s in c e l l p la n n in g

D E C T D iv e r s i t y r e c e p t io n a t b a s e s t a t io n

I S 9 5 W id e b a n d t r a n s m is s io n a v e r a g e s c h a n n e l

b e h a v io u rT h is a v o id s b u r s t e r r o r s a n d d e e p f a d e s

How to handle long fades when the user is stationary?Analog Disconnect userGSM Slow frequency hopping Handover, if appropriate Power controlDECT Diversity at base station Best channel selection by handsetIS95 Wide band transmission avoids most deep fades (at least in macro-cells) Power controlWireless LANs Frequency Hopping, Antenna Diversity

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Analog Disconnect userGSM Slow frequency hopping Handover, if appropriate Power controlDECT Diversity at base station Best channel selection by handsetIS95 Wide band transmission avoids most deep fades (at least in macro-cells) Power controlWireless LANs Frequency Hopping, Antenna Diversity

MengatasiLarge Scale

Fading

memperbesardaya kirim Tx

Antisipasi pengaruhnyaterhadap interferensi !!

Uplink

Downlink

Power control

FadingMargin

Mengatasi Large Scale Fading

Tidak dominan

Link budget calculation

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MengatasiLarge Scale

Fading

memperbaikikualitas

penerima Rx

Uplink

Downlink

FadingMargin

Diversitas

Perbaikansensitivitashandset

Sectoral &Smartantena

Catatan: dapat dikerjakan engineer

Mengatasi Small Scale Fading

MengatasiFlat Fading

Fast FadingRate simbol >rate fading

Atau, melaluidesain Fading

Margin

Masalahpenurunansinyal diatas

denganDiversitas

Fading dibuatmenjadi Slow

Modulator ygrobust yg tidak

perlu carriertracking error correction

coding daninterleaving

Karena Eb/Norequirement lebihkecil

Flat fading umumnyaFast

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MengatasiFlat Fading

Slow Fading

Power control

Catatan: dapat dikerjakan engineer

Atau, melaluidesain Fading

Margin

Untuk Fast Fading, responpower control mungkinterlambat thd fading rate

Masalahpenurunansinyal diatas

denganDiversitas

What next ?

Mengatasi Small Scale FadingFrequency Selective Fading, terjadi karena bandwidthsinyal lebih besar dari bandwidth koheren kanalSehingga persoalan fading frekuensi selektif terjadi padasistem broadband wireless

Persoalan sistembroadbandwireless

Masalahmultipath Frequencyselectivefading

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Persoalan sistembroadbandwireless

Masalahmultipath Frequencyselectivefading

Kompleksitasequalizer

Kesimpulan singkat, fading akan diatasi dengan berbagaicara : Fading Margin dalam desain cakupan RF Diversitas: space, time, frequency

Interleaving, suatu bentuk dari diversitas waktu Error protection coding, (atau channelcoding) dengan menambahkan bit-bitredundant

Receive antenna diversity: Fading jarang terjadi pada 2 lokasi secarasimultan, khususnya pada jarak kelipatan ganjil seperempat panjanggelombang

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Fading Margin dalam desain cakupan RF Diversitas: space, time, frequency

Interleaving, suatu bentuk dari diversitas waktu Error protection coding, (atau channelcoding) dengan menambahkan bit-bitredundant

Modul 5 Fading Mitigation SubjectTypical Mobile Radio Propagation ChannelFading channel manifestationsSmall-scale Fading: Mechanisms, Degradation categories, and EffectsSlide39Fading mechanismsTime Dispersion and Frequency Dispersion Effect of FadingStatistical FluctuationsSlide40Fading Mitigation TechniquesFMT: DiversitySpatial DiversitySpatial DiversitySlide115Selective CombinerSelective CombiningSelective CombiningSelective CombiningEqual Gain CombiningMax. Ratio CombinerMax. Ratio CombiningSlide34Frequency DiversityTime DiversityChannel EncodingFading MarginFading MarginPower ControlSlide83Power ControlSlide84Slide85Slide106Slide107Slide108Slide109Attenuation, Dispersion Effects: ISI!Slide98Inter-Symbol-Interference (ISI) due to Multi-Path FadingTypes of EqualizerSlide110BeamformingBeamforming dengan kriteria MMSE (Minimum Mean Squared Error)CDMA RAKE ReceiverSlide118OFDMSlide119Slide120Slide121Slide122Slide123Slide104Slide105Slide117Slide114