Modul 6.Fading Mitigation

8/6/2019 Modul 6.Fading Mitigation

1/49

TE 4103 SISTEM KOMUNIKASI BERGERAK

Modul 6 Teknik-Teknik Mengatasi Fading

Jurusan Teknik Elektro Program StudiS1

INSTITUT TEKNOLOGI TELKOM2008


2/49

Typical Mobile Radio Propagation Channel

TimeFrequency

Amplitude


3/49

Fading Mechanisms Time 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 dispersion poses requirements on synchronization and rate of convergence

of channel estimation

Interleaving may help to avoid burst errors

Frequency 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


4/49

Time Dispersion and Frequency Dispersion

Time Domain Channel variations Delay spread

Interpretation Fast Fading InterSymbol Interference

Correlation Distance Channel equalization

Frequency Doppler spectrum Frequency selective fading

Domain Intercarrier Interference Coherence bandwidth

Interpretation

Time DispersionFrequency Dispersion


5/49

Effect of Fading

Freq.

Freq.

Spectrald

ensity

Spectral

density

Coherent BW, Bc

Bc

Bs

Bs

Freq. Selective Fading

Freq. Flat Fading

TX BW > Channel BW

Bs > Bc

TX BW < Channel BWBs < Bc


6/49

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)

d (m)

P (dB)

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)


7/49

Fading Mitigation Techniques

Three techniques commonly use to

combat the effect of fading without

increasing TX Power and BW: diversity,

channel encoding, and equalization.

While Fading margin and Power control

are used to maintain a good signal

reception at Receiver.


8/49

Fading Mitigation Technique:

( Diversity )

Diversity exploits the random nature of radio

propagation by finding the independent signal

paths. If one path undergo a deep fade, another

path may have a strong signal. Usually employed to reduce the depth and

duration of fade experienced by receiver in flat

fading channel.

Types of diversity: spatial, frequency, time, and

polarization


9/49


10/49


11/49

Spatial Diversity

Use two or more receiving antenna

While one antenna sees a null signal, the

others may receive a peak signals.

The received signals are then combined and

processed by an algorithm to get best

reception.

Can be implemented in both BS and MS

receiver


12/49

Spatial Diversity

Combining algorithm commonly used: Selective, Equal gain,

and Maximal ratio combining.

Antenna is spaced

each other by an odd

integer multiply of

/4, usually d > 8 .

Spatial diversity can

improve SNR at

receiver by as much

as 20 dB to 30 dB.

wo

w

1

wK

Processor

ro(t)

r1(t)

rK(t)

y(t)d


13/49

Selective Combiner

G1

G2

Gm

Switching Logic

or

Demodulator

output

Variable gain

Ant. 1

Ant. 2

Ant. m


14/49


15/49

Selective Combining

Receiver only select one strongest signal to detect.

If average SNR of received signal in a branch = , and thresholdSNR = , then probability that M branches of antenna receivesignals with SNR 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 (1 - e- / )


16/49

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, and

P4(

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!


17/49

Max. Ratio Combiner

G1

G2

Gm

Co-phase

and

Sum

output

Variable gain

Ant. 1

Ant. 2

Ant. m

Detector

1

2

m

M

Adaptive control


18/49

Max. Ratio Combining Signals from each branch/antenna are co-phased and

individually weighted to provide coherent addition to getoptimal SNR.

Probability that received signal SNR below threshold is:

Probability of good reception:

( ) =

=

M

k

k

MkeP 1

1/

)!1(

)/(

11

( ) =

=

M

k

k

M

keP

1

1/

)!1(

)/(1

G C


19/49

Equal Gain Combining

If weight of each branch is set to unity and

co-phased, Max. ratio combining becomeequal gain combining.

Less complex with slightly lower

performance than max. ratio combining. Without continuously adapt each weight of

branches differently, it allows receiver to

exploit received signals simultaneously.


20/49

CDMA RAKE Receiver

Correlator m

Correlator 2

Correlator 1

.

.

.

.

Int. DCr(t)

IF or base band

CDMA signal

with multipathcomponents

Z1

Z2

Zm

Z Z m(t) 1

2

m

Since chip rate of CDMA much greater than coherence BW, delay spread

merely provide a multiple delayed version of signals at receiver. Instead ofcausing ISI, RAKE receiver attempts to collect multipath signals, process it by

separate correlator receiver, and combine the signals to have a better detection.


21/49

C(t)

C(t- 2)

C(t- n)

delay adj.

korelator

BTS


22/49

Frequency Diversity

TX

TX

RX

RX

RF

Branching

Network

Combiner

F2

F3

F4

F1

TX

TX

RX

RX

RF

Branching

Network

Combiner

F2

F3

F4

F1

Use two or more carrier frequency for transmission with spacing aboutUse two or more carrier frequency for transmission with spacing about

2 5 % f2 5 % foo..

Need to employ two or more Transmitter and ReceiverNeed to employ two or more Transmitter and Receiver Improvement factor :Improvement factor :


23/49

Channel Equalizer

i time index

Vequalizer order

D delay index

b0

z-1 z-1 z-1

bD+vbV-1

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

(i)o

ut

Adaptive

algorithm

(i)

(i)

Channel equalizer is employed to compensate ISI. Since multipath fading channel is dynamic, equalizer must

be adaptive


24/49

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


25/49

Time Diversity

1 m+1

2 m+2

M 2m nm

.

n columns

m

r

o

w

s

Read in

Coded bits

from

encoder

Read out bits to modulator one row at a time


26/49

Channel Encoding

Channel encoding is done by encode the data into aspecial form, and introduce redundancies in thetransmitted 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 fadingchannel.

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

Two types mostly used: Block Code and Convolutionalcode


27/49

Fading Margin

K u a t s i n yd i t a m b a h f

Fading margin depends upon target availabilityof the link/coverage.

Greateravailabilityrequires larger fading margin.


28/49

Fading Margin

( )

===

2

FMerf

2

1

2

1dm)m(p)Thm(PRP

mTh

Th

If fading margin FM applied to the link, then probability

that RSL at receiver separated at distance R above the

threshold can be written as:

Fading margin improve signal reception hence the

link performance, in an expense of increasing

transmission power.


29/49

Power Control

User 2

User 1

d1

d2

Basestation

Pr2

Pr1

Pt2Pt1

Mitigating the effect of shadowing and near-far problem

If user 1 at 3 km from BTS transmitting with 100

mWatt, how much power is needed by user 2 at

9 km away from BTS using Okumura Hatta

model in urban area to achieve the same power

at the BTS with 10 m high above ground level?

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

P C t l


30/49

Power Control

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0- 3 0

- 2 0

- 1 0

0

1 0

2 0

3 0

T im e s l o t ( 0 . 6 7 m s )

Signallevel

(dB)

R e c e iv e d s i g n a l a m p l i t u d eC o n t r o l le d t r a n s m i t p o w e rC o n t r o l le d S I R ( ta r g e t = 1 0 d B )

Channel is estimated at the

receiver, then Tx is instructedto adjust Tx power according to

the estimated channel (e.g.

SNR).

Problem:

Control rate >> fading rateControl step size single step

or variable step

What is the benefit/drawbacks

of single or variable step size ?

Rayleigh fading


31/49

Pt

Pt

Pr1

Pr1

Menjadi sangat fital karena sharing resource yang sama, setiap userberprilaku sebagai random noise terhadap yang lain.

user 2user 1


32/49

Power kontrol sangat bagus

10 km5 km

Subc-1Subc-2

Subc-1

Subc-2


33/49

User 2

User 1 S/N = 1/10

S/N = 10

frekuensi

power Distribusi powerdi penerima

Kualitas voice user 2 >> user 1, fenomena ini disebut problem

near-far. Dari dua user tadi kalau ditambahkan user ketiga

akan menurunkan kualitas baik user 3 maupun dua user yang

sudah ada.


34/49

Mengatasi problem near-far sehingga Pr(i) di BTS (up-link)

akan sama. Jika syarat S/N tiap user = 1/10, maka kapasitas

akan menjadi 11 user (meningkat signifikan).

Pt

PtPr1 Pr2

user 1

user 2

user 3user M

Pr3

PrM


35/49

frekuensi

power

User 1, S/N = 1/10

User 2, S/N = 1/10

User 4, S/N = 1/10

User 3, S/N = 1/10

User 5, S/N = 1/10

User 6, S/N = 1/10

User 8, S/N = 1/10

User 7, S/N = 1/10

User 9, S/N = 1/10

User 10, S/N = 1/10

User 11, S/N = 1/10

Dengan power kontrol kapasitas dimaksimalkan


36/49

MS mengirim Pt maksimum ; pasti akan bisa diterima oleh

BTS, namun interferensi akan meningkat MS mengirim Pt minimum ; ada kemungkinan tidak diterima

oleh BTS tetapi interferensi sangat kecil IS-95 memberi solusi dengan :

Open loop mekanisme initial transmit power tanpa bantuan BTS

Close loop mekanisme pengendalian transmit power dengan

bantuan BTS


37/49

Untuk mengatasi log-normal shadowing Dengan access probe Pt MS diestimasi dengan Pr MS

PtPr

1

n

Access probe

Pt(1) = -Pr - 73 +NOM_PWR +INIT_PWR IS-95


38/49

PATH LOSS

RAYLEIGH FADING (fading cepat)

LOGNORMAL FADING (fading lambat)

Distance (km)

SignalLeve

l(dBm)


39/49

Koreksi

access probe

pertama

Koreksi

access probe

kedua

Waktu

MS

trans. power

initial

random random


40/49

Untuk mengatasi fluktuasi receive power karena Rayleigh

fading (fading cepat). BTS memeriksa kualitas up-link secara kontinyu. Jika kualitas jelek, BTS akan mengirim command(PCB)via

down-link untuk menaikan Pt MS.

Jika kualitas terlalu bagus akan dilakukan hal sebaliknya. Eb/N0 digunakan sebagai indikator kualitas link. PCB = +1dB jika Pt MS harus dinaikan dan -1dB jika

sebaliknya Karena digunakan untuk mengatasi fading cepat, maka

proses pengendalian harus berlangsung cepat sehingga PCB

disisipkan pada kanal traffik arah down-link


41/49

VOCODER KONVOLUSI MUX SPREADING

Power control (PCB)

800 bps

19,2 kbps9,6 kbps 19,2 kbps 1,2288 Mcps

Power control

bit position

0 atau 1 tanpa proteksi


42/49

PCG0 PCG1 PCG2 PCG3 PCG12 PCG13 PCG14 PCG15

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

PCG7

1 0 1 1

(11)10

20ms

1,25 ms

Probabilitas meletakan PCB

PCB = 0 MS harus naik 1 dB

PCB = 1 MS harus turun 1 dB

CONTROL POSITION


43/49

UL

DL

PCG0 PCG1 PCG2 PCG3 PCG4 PCG5 PCG6 PCG7 PCG8

PCG0 PCG1 PCG2 PCG3 PCG4 PCG5 PCG6 PCG7 PCG8 PCG9

Eb/N0 diukur di BTS

Selama menerima PCG8BTS memutuskan kirim 1 atau 0

BTS MENGIRIM

PCB


44/49

RX

DEMESTIMASI

FERTh. Eb/N0

DECISION

EST >Th PCB = 1

EST < Th PCB = 0

ESTIMASI

Eb/N0

FTC TRANSMITTER

PCB

BASE STATION

DARI MS

KE MS


45/49

TRANSMITTER RTC

DECISION

PCB = 0 , +1dB

PCB = 1, -1dB

(-1)

RXDEM

ESTIMASI

RX TOTALPr-Pr

K= -73 (dB)

NOM_PWR (dB)

INIT_PWR (dB)

Pt yang harus dipakai

selama 1,25 ms

TCH

+1 dB atau -1 dB

MOBILE STATION


46/49

Some practical approaches

H t h dl f t lti th f di


47/49

How to handle fast multipath fading

Analog

User must speak slowly

GSM

Error correction and interleaving to avoidburst errors

Error detection and speech decoding

Fade margins in cell planning

DECT

Diversity reception at base station

IS95

Wideband transmission averages channelbehaviour

This avoids burst errors and deep fades

(CDMA2000)

H d t h dl D l d ?


48/49

How do systems handle Doppler spreads?

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 Only small Doppler spreads are to be anticipated for

Original DECT concept did not standardize an equalizer

IS95 (CDMA2000) Downlink: Pilot signal for synchronization and channel estimation

Uplink: Continuous tracking of each signal

How do systems handle delay spreads?


49/49

How do systems handle delay spreads?

Analog

Narrowband transmission

GSM

Adaptive channelequalization

Channel estimation training sequence

DECT Use the handset only in small cells with small delay spreads

Diversity and channel selection can help a little bitpick a channel where late reflections are in a fade

IS95

Rake receiver separately recovers signals over paths with excessive dela

Digital Audio Broacasting

OFDM multi-carrier modulationThe radio channel is split into manynarrowband (ISI-free)subchannels

Documents

Modul 6.Fading Mitigation