wireless communication network lab. chapter 3 mobile radio propagation ee of niuchih-cheng tseng1...

Wireless Communication Network Lab.

Chapter 3Mobile Radio Propagation

EE of NIU Chih-Cheng Tseng 1

Prof. Chih-Cheng Tseng

[email protected]

http://wcnlab.niu.edu.tw

mailto:[email protected]

http://wcnlab.niu.edu.tw/




Speed, Wavelength, Frequency

2

Light Speed (c) = Wavelength (l) x Frequency (f)

= 3 108 m/s

System Frequency Wavelength

AC current 60 Hz 5,000 km

FM radio 100 MHz 3 m

Cellular 800 MHz 37.5 cm

Ka band satellite 20 GHz 15 mm

Ultraviolet light 1015 Hz 10-7 m

EE of NIU Chih-Cheng Tseng


Types of Radio Waves


Transmitter Receiver

Earth

Sky wave

Space wave (LOS)

Ground waveTroposphere

(0 - 12 km)

Stratosphere (12 - 50 km)

Mesosphere (50 - 80 km)

Ionosphere (80 - 720 km)

電離層

中氣層

平流層

對流層


Radio Frequency Bands


Classification Band Initials Frequency Range Characteristics

Extremely low ELF < 300 HzGround waveInfra low ILF 300 Hz - 3 kHz

Very low VLF 3 kHz - 30 kHz

Low LF 30 kHz - 300 kHz

Medium MF 300 kHz - 3 MHz Ground/Sky wave

High HF 3 MHz - 30 MHz Sky wave

Very high VHF 30 MHz - 300 MHz

Space wave (LOS)Ultra high UHF 300 MHz - 3 GHz

Super high SHF 3 GHz - 30 GHz

Extremely high EHF 30 GHz - 300 GHz

Tremendously high THF 300 GHz - 3000 GHz


5

Propagation Mechanisms

Reflection (反射 )• Propagation wave impinges on an object which is large as compared

to wavelength, e.g., the surface of the Earth, buildings, walls, etc. Diffraction (繞射 )

• Radio path between transmitter and receiver obstructed by surface with sharp irregular edges, e.g., waves bend around the obstacle, even when LOS (line of sight) does not exist.

Scattering (散射 )• Objects smaller than the wavelength of the propagation wave, e.g.

foliage, street signs, lamp posts.



6

Radio Propagation Effects


Transmitterd

Receiver

hb

hm

Diffracted Signal

Reflected Signal

Direct Signal

Building

STOP Scattered


Free-Space Propagation

Assuming that the radiated power is uniformly distributed over the surface of the sphere.

The received signal power at distance d:

• Pt is transmitting power

• Ae is effective area covered by the sender

• Gt is the transmitting antenna gain


24e t t

rA G P

Pd


Antenna Gain

For a circular reflector antenna, the antenna gain

• is the net efficiency (typically 0.55)

• D is the diameter

• c is the speed of light 3108 m/s Example:

• If the antenna diameter = 2 m and frequency = 6 GHz, then, wavelength = 0.05 m and G = 39.4 dB.

• If the frequency = 14 GHz and diameter = 2 m , then, the wavelength = 0.021 m and G = 46.9 dB

Higher the frequency, higher the gain for the same size antenna.


2 2D Df

Gc

Wireless Communication Network Lab. The received signal power:

• Gr is the receiver antenna gain,

• L is the propagation loss in the channel, i.e.,

L = LP LS LF


L

PGGP trt

r

Fast fading

Slow fading

Path loss

Land Propagation


Path Loss (Free-Space)

Definition of path loss LP :

• The signal strength decays exponentially with distance d between transmitter and receiver;

• The loss could be proportional to somewhere between d2 and d4 depending on the environment.

Path Loss in Free-space:

• fc is the carrier frequency.

This shows greater the fc, more is the loss.


,r

tP P

PL

10 10(dB) 32.45 20log (MHz) 20log (km)f cL f d


Path Loss (Land Propagation)

Simplest Formula:

• A and α are the propagation constants

• d is the distance between transmitter and receiver

• α is the value of 3 ~ 4 in typical urban area


PL Ad


Example of Path Loss (Free-space)


Path Loss in Free-space

70

80

90

100

110

120

130

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lf

(dB)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz


Path Loss in Different Areas (1)

Path loss in decreasing order:• Urban area (large city)

• Urban area (medium and small city)

• Suburban area

• Open area




Urban Area

where

• For a large city, the path loss is the same as the that for small- and medium cities under hb=50m and hm=1.65m.


10 10

10 10

(dB) 69.55 26.16log (MHz) 13.82log (m) (m)

44.9 6.55log (m) log (km)

PU c b m

b

L f h h

h d

10 10

2

10

2

10

(m)

1.1log (MHz) 0.7 (m) 1.56log (MHz) 0.8 , for large cities

8.29 log 1.54 (m) 1.1, for 200 MHz, for small & medium cities

3.2 log 11.75 (m) 4.97, for 400 MHz

m

c m c

m c

m c

h

f h f

h f

h f

correction factor for the mobile antenna height


Concept Used for Calculating a(hm)


Transmitter Distance dReceiver

hb

hm



Suburban

Open area


2

10

(MHz)(dB) (dB) 2 log 5.4

28c

PS PU

fL L

2

10

10

(dB) (dB) 4.78 log (MHz)

18.33log (MHz) 40.94PO PU c

c

L L f

f

課本 (3.14)式有誤


Path Loss in Urban Area in Large City

100

110

120

130

140

150

160

170

180

0 10 20 30

Distance d (km)

Pa

th L

oss

Lp

u (

dB

)

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

fc=150MHz

Path Loss --- Urban: Large City


hb=50m and hm=1.65m


Path Loss in Urban Area for Small & Medium Cities

100

110

120

130

140

150

160

170

180

0 10 20 30

Distance d (km)

Pat

h Lo

ss L

pu (

dB)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

Path Loss --- Urban: Medium and Small Cities


hb=50m and hm=1.65m


Path Loss in Suburban Area

90

100

110

120

130

140

150

160

170

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lp

s (d

B)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

Path Loss --- Suburban Area


hb=50m and hm=1.65m


Path Loss in Open Area

80

90

100

110

120

130

140

150

0 5 10 15 20 25 30

Distance d (km)

Pa

th L

oss

Lp

o (

dB

)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

Path Loss --- Open Area


hb=50m and hm=1.65m


Fading


Signal Strength

(dB)

Distance

Path Loss

Slow Fading(Long-term fading)

Fast Fading(Short-term fading)


Slow Fading

What is slow fading?• Long-term variation in the mean level of received signals.

• The channel impulse response changes at a rate much slower than the transmitted baseband signal.

Slow fading is also called shadowing or log-normal fading because its amplitude has a log-normal pdf.

The amplitude change caused by shadowing is often modeled using a log-normal distribution.



Shadowing

Log-normal distribution• The pdf of the received signal level is given in

decibels by

• M is the true received signal level m in decibels, i.e., 10log10m.

• is the area average signal level, i.e., the mean of M.

• is the standard deviation in decibels.


,2

1 2

2

2

MM

eMp

M


Log-normal Distribution


MM

2

p(M)

Wireless Communication Network Lab. What is fast fading?

• Short-term fading due to fast spatial variation.

• The channel impulse response changes rapidly than the transmitted baseband signal.

Fast Fading



Fast Fading --- Receiver Far From the Transmitter

Assume• There are no direct radio waves between transmitter and

receiver.

• The prob. distribution of signal amplitude of every path is a Gaussian distribution.

• The phase distribution of every path is uniformly distributed over (0,2p) radians.

The prob. distribution of the envelope for the composite signals is a Rayleigh distribution.



Rayleigh Distribution


r2 4 6 8 10

P(r)

0

0.2

0.4

0.6

0.8

1.0

=1

=2

=3

2

222

( ) , 0rr

p r e r

r is the envelope of the fading signals is the standard deviation


Fast Fading --- Receiver Close to the Transmitter

Assume• The direct radio wave between transmitter and receiver is

stronger than other radio waves.

• The prob. distribution of signal amplitude of every path is a Gaussian distribution.

• The phase distribution of every path is uniformly distributed over (0,2p) radians.

The prob. distribution of the envelope for the composite signals is a Rician distribution.



Rician Distribution


r86420

0.6

0.5

0.4

0.3

0.2

0.1

0

r

Pdf

p(r)

b = 2b = 1b= 0 (Rayleigh) = 1

b = 3

2 2

2202 2

( )rr r

p r e I

r is the envelope of the fading signals is the standard deviationb is the amplitude of the direct signalI0(x) is the zero-order Bessel function of the first kind


General Model of Fading Channel --- Nakagami

For Nakagami distribution, the pdf of the received signal envelop is

• is the Gamma function

• is the average power, i.e., the 2nd moment of the fading signal

is the fading term, where m≥ 0.5

• If m=1, Nakagami distribution becomes to Rayleigh distribution

• If m approach to infinity, the distribution becomes to an impulse, i.e., no fading.

• Nakagami is used to replace Rician since Bessel function is not required


22 12( ) for 0

( )

m mrmr mp r e r

m

( )m2[ ]E r

2

2 2( )E rm


Characteristics of Instantaneous Amplitude

Level Crossing Rate:• Average number of times per second that the signal envelope crosses

a specified level in positive going direction. Fading Rate:

• Number of times signal envelope crosses middle value in positive going direction per unit time.

Depth of Fading:• Ratio of mean square value and minimum value of fading signal.

Fading Duration:• Time for which signal is below given threshold.



Doppler Effect

When a wave source and a receiver are moving, the frequency of the received signal will not be the same as the source.• When they are moving toward each other, the frequency of the

received signal is higher than the source.

• When they are opposing each other, the frequency decreases. The frequency of the received signal is

• fC is the frequency of source carrier,

• fD is the Doppler frequency.


DCR fff


Doppler Shift

Doppler frequency (or Doppler shift)

• v is the moving speed,

• is the wavelength of carrier


cosv

fD

MS

Signal from sender

Moving direction of receiver with speed v

q


Moving Speed Effect


Time

V1 V2 V3 V4

Sig

nal s

tren

gth


Delay Spread

When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections.

This forces signal to follow different paths. Each path has different path length, so the time of

arrival for each path is different. This effect which spreads out the signal is called

“Delay Spread”, td.



Delay Spread


Delay

Sig

nal S

tren

gth

The signals from close by reflectors

The signals from intermediate reflectors

The signals from far away reflectors


Inter-Symbol Interference (ISI)


Caused by time delayed multipath signals Has impact on burst error rate of the channel If the transmission rate is R, a low bit-error-rate

(BER) can be obtained when1

2 d

R


Inter-Symbol Interference (ISI)


Time

Time

Time

Transmission signal

Received signal (short delay)

Received signal (long delay)

1

0

1

Propagation timeDelayed signals


Coherence Bandwidth

Coherence bandwidth Bc≈ 1/(2πd)

• A statistical measure of the range of frequencies over which the channel can be considered as “flat”.

Flat fading• If the bandwidth of Tx signal is lower than the channel

coherent bandwidth, nonlinear transformation could no occur.

Frequency-selective fading• If the bandwidth of Tx signal is higher than the channel

coherent bandwidth, nonlinearity is present.



Cochannel Interference

Cells having the same frequency interfere with each other. In a cellular system, to reuse the frequencies, the same

frequency is assigned to different cells.



Homework

P3.7 P3.11 P3.12 P3.18 P3.20


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