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TE 4103 SISTEM KOMUNIKASI BERGERAK
Modul 6 Teknik-Teknik Mengatasi Fading
Jurusan Teknik Elektro Program StudiS1
INSTITUT TEKNOLOGI TELKOM2008
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Typical Mobile Radio Propagation Channel
TimeFrequency
Amplitude
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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
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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
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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
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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)
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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.
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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
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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
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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
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Selective Combiner
G1
G2
Gm
Switching Logic
or
Demodulator
output
Variable gain
Ant. 1
Ant. 2
Ant. m
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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- / )
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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!
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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
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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
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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.
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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.
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C(t)
C(t- 2)
C(t- n)
delay adj.
korelator
BTS
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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 :
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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
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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
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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
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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
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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.
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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.
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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
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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
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Pt
Pt
Pr1
Pr1
Menjadi sangat fital karena sharing resource yang sama, setiap userberprilaku sebagai random noise terhadap yang lain.
user 2user 1
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Power kontrol sangat bagus
10 km5 km
Subc-1Subc-2
Subc-1
Subc-2
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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.
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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
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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
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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
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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
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PATH LOSS
RAYLEIGH FADING (fading cepat)
LOGNORMAL FADING (fading lambat)
Distance (km)
SignalLeve
l(dBm)
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Koreksi
access probe
pertama
Koreksi
access probe
kedua
Waktu
MS
trans. power
initial
random random
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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
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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
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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
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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
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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
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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
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Some practical approaches
H t h dl f t lti th f di
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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 ?
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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?
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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