Fading dan

Mitigasi

Pendahuluan

• Kenapa penting untuk memahami secara memadai tentang

karakteristik channel wireless ?

– To facilitate a suitable signal design

– To develop smart transmission/reception technologies

– In multiuser communication, channel access scheme must be

shared efficiently.

– In cellular system, signal coverage need to be computed as just

necessary (excess power will create interference).

– In mobile celluler, threshold level needs to be determined to

maintain connection while traveling from cell to cell.

• Teknologi yang berbeda menunjukkan kelakuan yg berlainan pada

wireless channel.

Kanal Idel (AWGN)

• Ideal Channel

• Ideal Channel pass all signal spectral without distortion (channel

BW is infinite, frequency response is flat for all frequencies)

• The only impairment is the AWGN

• The received signal is deterministic using the statistics of AWGN

(Gaussian distribution)

Transmitted

bit

Ideal channel

AWGN

detection

Kanal real (fisik)

• Physical Channel

• Physical channel (cable) has a limited bandwidth

• Only significant component of signal spectral ispassed through the channel distortion

• Signal BW must equal or less than Channel BW How to make signal BW <= Channel BW ??

Transmitted

bit

Physical Channel

AWGN

detection

Fourrier Series/Transformation

• Physical channel is designed to have a large BW (twisted

pair, coaxial, fiber optic, microwave link)

• Signal BW is confined to meet the channel BW (equalizer)

• Wave shaping filter (Sync function)

Fourier

Problem of Wireless Channel

• If a large portion of signal spectral component cannot be passed through the channel, signal will experience distortion due to limited Channel BW.

• The receiver can be designed to detect the distorted received signal (introduced by the channel plus AWGN).

• A more severe distortion is introduced by the wireless channel due to multipath propagation ISI

• Without sophisticated technologies, the detector will be unable to detect the received signal because the received signal from wireless channel is very bad

Wireless mobile radio channel

Air interface (the main problem is located here)

Delay spread

The signal transmitted through a wireless

channel will undergoe multipath propagation.

As a result, multiple copies will arrive at the

receiver at different times causing intersymbol

interference

Wireless Propagation Mechanism

Free Space Loss

Which is the direct path (line of sight path) = very easy to predict using the free space loss formula)

Reflection

Which is the indirect path(s) that arrive at the receiver after reflected by other object. This can be multiple reflections which contribute to a large delay.

Diffraction

Propagation through large object that create a secondary source such as hill tops, etc

Scattering

Propagation through small and rough object that causes multiple reflection to different direction

Wireless Propagation Mechanism

Effect of multipath propagation:

Large scale path loss

Small scale propagation

Large scale path loss

Large attenuation in average

The received signal power decay in proportion to the nth

power of distance (2 < n < 5)

Use prediction models to predict the path loss

Small scale

Fast signal fluctuation around the average level

Doppler spread which corresponds to fading rate

Time dispersive due to different time delay

The Use of Propagation Analysis

• Prediction of large scale Propagation:

– Network coverage

– Hand-off between cells

– Link budget (interference calculation)

• Small scale (multipath fading)

– Signal design

– Sophisticated transmission, diversity, beamforming.

– Improve signal detection (smart receiver),

equalisation, receive diversity, beamforming

Multipath fading

• Large scale and small scale fading

Characterisation of Multipath Fading

• Focus on Multipath Fading

– Time spreading of signal

– Time varying of channel

• Time spreading of Signal

– Frequency selective fading

– Frequency nonselective (flat) fading

• Evaluation/analysis

– Time domain

– Frequency domain

Delay spread Model

• Channel sounding (by impulse response)

• Multipath delay is discretized into N bins each of which is .

• The model is used to analyzed signal with BW <= 1/(2 ).

• The total received power is the sum of all multipath component if

they can be resolved.

• If the signal BW << channel BW multipath can be resolved

• If signal BW >> channel BW multipath cannot be resolved.

• SIRCIM (Simulation of Indoor Radio Channel IMpulse response)

• SMRCIM (Simulation of Mobile Radio Channel Impulse resp.)

Multipath channel

Delay spread Model

Example

• Assume SMRCIM/SIRCIMat urban radio channel has max excess delay of

96 us, at microcell channel has max excess delay of 5 us, and at indoor has

max exces delay of 50 ns. Assume the multipath is discretized into 24 bins

in that all paths within each bin is considered as a single resolvable path.

Calculate the max bandwidth of signal that can be analyzed or represented

for each of the channel model above.

• Model channel SMRCIM/SIRCIM pada daerah urban, daerah mikrocell, dan

propagasi indoor diasumsikan memiliki max excess delay masing-masing

96 us, 5 us, dan 50 ns. Jumlah komponen multipath dibagi dalam 24

kelompok path yg dapat dianggap sebagai sebuah path yg dapat

diresolusikan. Hitung BW max dari signal impulse yg bisa dianalisis atau

representatip untuk masing-masing model channel tsb.

Delay spread Model

Jawab:

• Urban = 96/24=4 us Signal BW <= 1/(2*4 us)=0.125

MHz

• Mikrocell = 5/24= Signal Bw <=

• Indoor =

Delay spread Parameter

• To determine the BW signal that can be passed through the channel

without ISI must know Channel Coherence BW.

• Channel Coherence BW = W0 is related to rms delay spread m.

• Channel coherence BW for freq correlation of >= 90 % is defined as

W0 = 1/(50 m), while for freq correlation of >= 50 % is W0 = 1/(5 m).

• Example:

Environment Frequency (MHz) Rms delay m

Urban 800 - 900 30 – 110 us

Suburban 800 - 900 200 – 350 ns

Indoor 800 – 900 50 – 100 ns

Indoor 1500 – 1800 20 – 100 ns

Delay spread Parameter

Example

• Compute the rms delay spread for the channel with power-delay profile as follow:

• Compute the channel coherence BW with 90 % frequency correlation. Is the channel suitable for GSM application without using equalizer ?

0 dB

- 10 dB

-- 20 dB

-- 30 dB

0 10 20 30 40 50 us

Channel Coherence Band Width

• Answer:

Characterisation of Multipath Fading

• Time spreading of signal

• Delay spread determines channel coherence BW

| ( f)|S( )

Delay spread, m Channel coherence bandwidth, W0

W0 1/ m

Effect of Fading/Mitigation

• Manifestation of signal time-spreading====================================================

Characterisation Frequency selective Flat fading

====================================================

• Time domain m >> Ts m << Ts

• Frequency domain W0 << W W0 > >W

• Signal degradation ISI, loss of SNR. Loss of SNR.

• Mitigation Channel equalization, Diversity, error spread spectrum (rake) control, power error control control, Orthogonal

modulation (OFDM).

Time Varying Model

• Channel variation (by motion) Doppler spread

• Doppler spread fD channel coherence time T0 .

• Channel coherence time is the time over which the channel is

considered time invariant.

• Channel coherence time is the time over which two received signals

have a strong amplitudo correlation.

• If the reciprocal BW of the transmitted signal is greater than the

coherence time the channel will change during one symbol period

fast fading.

• For 50 % of time correlation function To = 9/(16 fD) where fD is the

maximum Doppler spread or fD = v/ .

• Using popular geometric mean T0 = sqrt[9/(16 fD2)] = 0.423/fD.

Time Varying of Channel

Example

• Sebuah vehicle melaju pada v = 36 km/jam menerima sinyal

multipath pada frekuensi 900 MHz menjauh dari pemancar.

– Hitung channel coherence time

– Hitung perioda sample untuk mendapatkan sample yang masih

berkorelasi tinggi

– Hitung jumlah sample serta berama lama pengukuran sample dalam

jarak tempuh 25 m.

– Berapa Doppelr spread dari channel tersebut

Time Varying of Channel

• Jawab:

Effect of Fading/Mitigation

• Time varying of channel

• Time correlation of signal determines Doppler spread

S( )( t)

Channel coherence time, T0

T0 1/fD

Doppler spread

fc - fD fc fc + fD

Effect of Fading/Mitigation

============================================================

Characterisation Fast fading Slow fading

======================================================

Time domain T0 << Ts T0 >>Ts

Frequency domain fD >> W fD << W

Signal degradation Loss of SNR, pulse Loss of SNR.

distortion, and synchronization

problem

Mitigation Error control/interleaving Diversity, error

robust modulation control, power control

Classification of fading channel

=========================================

Channel models Ts >> T0 Ts << T0

=========================================

W >> W0 time-frequency frequency-selective

selective fading time-nonselective

W << W0 time-selective time-frequency

frequency nonselective nonselective

The awful The Bad The worse

Fading Simulator

• Analytical evaluation of communication behavior in fading

channel is very difficult.

• Field measurement is very costly and time consuming.

• Fading simulator

– Mathematical model

– Realization

– Computer simulation

Fading Simulator

• Most popular fading simulator from Jakes (jakes method).function y = fading(len, fd, T)

N = 34;

N0 = (N/2 - 1)/2;

alpha = pi/4;

xc = zeros(len,1);

xs = zeros(len,1);

sc = sqrt(2)*cos(alpha);

ss = sqrt(2)*sin(alpha);

ts = 0:len-1;

ts = ts'.*T + round(rand(1,1)*10000)*T;

wd = 2*pi*fd;

xc = sc.*cos(wd.*ts);

xs = ss.*cos(wd.*ts);

for lx =1:N0

wn = wd*cos(2*pi*lx/N);

xc = xc + (2*cos(pi*lx/N0)).*cos(wn.*ts);

xs = xs + (2*sin(pi*lx/N0)).*cos(wn.*ts);

end;

y = (xc + i.*xs)./sqrt(N0+1);

Fading Simulator

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

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

-15

-10

-5

0

5

10

Fa

din

g a

mp

litu

de

[d

B]

time x 0.67 msec

Ringkasan

• The major challenge in Mobile Cellular Communications ishow to combat or mitigate the effect of wireless channel.

• Wireless channel is characterised by amplitude fluctuation(fading) and delay spread (frequency selectivity)

• Most Channel in CDMA is frequency selective due to the largebandwidth of the spread signal CDMA can resolvemultipath (frequency diversity) using RAKE receiver.

• CDMA require tight power control to combat the near-farproblem.