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Basics o f Smal l Scale

Fading : Towards choice

o f PHY

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Basic Quest ions

Desert Metro Street Indoor

What will happen if the transmitter

- changes transmit power ?

- changes frequency ?- operates at higher speed ?

What will happen if

the receiver moves?

What will happen if we conduct

this experiment in different types

of environments?

Channel effects

Effect of mobility

Transmit power, data rate,

signal bandwidth, frequency

tradeoff

Tx

Rx

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Review of basic concepts

Channel Impulse response

Power delay profile Inter Symbol Interference

Coherence bandwidth

Coherence time

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Channel Impu lse Response

)(tx

Channel

)(ty

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Power delay Prof i le

ReceivedSigna

lLevel(dBm)

-105

-100

-95

-90

-90

0 50 100 150 200 250 300 350 400 450

Excess Delay (ns)

RMS Delay Spread () = 46.4 ns

Mean Excess delay () = 45 ns

Maximum Excess delay < 10 dB = 110 ns

Noise threshold

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Examp le (Power delay p ro f i le)

-30 dB

-20 dB

-10 dB

0 dB

0 1 2 5

Pr()

(s)

s 38.4

]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1(_

2

2222_

2 07.21]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1( s

s

37.1)38.4(07.21 2

1.37 s

4.38 s

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RMS Delay Spread : Typ ical values

10ns 50ns 150ns 1s 2s 5s 10s 25s500ns

Office building 1

San Francisco

Manhattan

Suburban

Office building 2 SFO

Delay spread is a good measure of Multipath

3m 15m 45m 150m 300m 600m 3Km 7.5Km

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Inter Symbo l In terference

-30 dB

-20 dB

-10 dB

0 dB

0 1 2 5

Pr()

(s)

1.37 s

4.38 s

0 1 2 5 (s)

Symbol time

4.38

Symbol time > 10* --- No equalization required

Symbol time < 10* --- Equalization will be required todeal with ISI

In the above example, symbol time should be more than 14s to avoid ISI.

This means that link speed must be less than 70Kbps (approx)

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Coherence Bandw idth

)(tx

Time domain view

High correlation of amplitudebetween two different freq.

components

Range of freq over

which response is flat

Bc

delay spread

)( fX

Freq. domain view

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RMS delay spread and coherence b/w

RMS delay spread and

coherence b/w (Bc) are

inversely proportional

1

cB

.501cB For 0.9 correlation

.5

1

cBFor 0.5 correlation

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Time dispersive nature of channel

RMS delay spread () Coherence b/w (Bc)

Time domain view Freq domain view

Delay spread and coherence bandwidth are parameters which

describe the time dispersive nature of the channel.

channel 1

channel 2

channel 3

Signal

Cha

nnel

Symbol Time (Ts) Signal bandwidth (Bs)

signal 1

signal 2

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Revis i t Examp le (Power delay p ro f i le)

-30 dB

-20 dB

-10 dB

0 dB

0 1 2 5

Pr()

(s)

s 38.4_

2

_2 07.21 s

s

37.1

1.37 s

4.38 s

kHzBcoherence c 146

.5

1)%50(

Signal bandwidth for Analog Cellular = 30 KHz

Signal bandwidth for GSM = 200 KHz

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Doppler Shif t

cosvfv

Doppler shift

Example

- Carrier frequency fc= 1850 MHz (i.e. = 16.2 cm)

- Vehicle speed v= 60 mph = 26.82 m/s

- If the vehicle is moving directly towards the transmitter

- If the vehicle is moving perpendicular to the angle of arrival of the

transmitted signal

Hzf 165162.0

82.26

0f

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Coherence Time

Time domain view Frequency domain view

Coherence Time: Time interval

over which channel impulse

responses are highly

correlated

Tc

signal bandwidth

symbol time

fc+fdfc-fd

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Doppler sp read and coherence t ime

Doppler spread and

coherence time (Tc) are

inversely proportional mc

f

T1

m

cf

T 423.0 Rule of thumb

mc fT 16

9

For 0.5 correlation

fmis the max doppler shift

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Time varying nature of channel

Coherence Time (TC) Doppler spread (BD)

Symbol Time (TS) Signal bandwidth (BS)

Time domain view Freq domain view

Doppler spread and coherence time are parameters which

describe the time varying nature of the channel.

channel 1

channel 2

channel 3

Signal

Cha

nnel

signal 1

signal 2

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Small scale fading

Multi path time delay

Doppler spread

Flat fading BC

BS

Frequency selective fadingB

C

BS

TC

TSSlow fading

Fast fading TC

TS

fading

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PHY Layer Design Choices ?

Required Data Rates

Determines channel : frequency selective or flat fading; fastor slow fading

Required QoS at the PHY: bit-error-rate (BER),

packet-error-rate (PER), Frame-error-rate (FER)

May be determined by application needs (higher layers)

Affected by Interference and Noise levels

PHY layer choices include selection of

Modulation/Demodulation

Techniques to mitigate fading: diversity, equalization, OFDM,

MIMO

Techniques to mitigate interference (if necessary)

Error correction Coding