Wireless Communication Channels: Large-Scale Pathloss.
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Transcript of Wireless Communication Channels: Large-Scale Pathloss.
© Tallal Elshabrawy 2
Three basic mechanisms that impact propagation in a mobile communication system
Reflection A propagating electromagnetic wave impinges upon an object which has
very large dimension compared to the wavelength Reflection occurs from surface of earth and from buildings and walls
Diffraction The radio path between the transmitter and receiver is obstructed by a
surface that has sharp irregularities
Scattering When the medium consists of objects with dimensions that are small
compared to the wavelength
The Three Basic Propagation Mechanisms
© Tallal Elshabrawy 4
Reflection from Dielectrics
Snell’s Law
i t1 1 2 2sin 90 sin 90
i r r i t iE E E E1
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
Hi Hr
Et
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
HiHr
Et
Normal IncidenceParallel Incidence
© Tallal Elshabrawy 5
Reflection from Dielectrics
t ir
i t i
E
E2 1
2 1
sin sin
sin sin
i tr
i i t
E
E2 1
2 1
sin sin
sin sin
First Medium is Free space and μ1 = μ2
r i r i
r i r i
2
2
sin cos
sin cos
i r i
i r i
2
2
sin cos
sin cos
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
Hi Hr
Et
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
HiHr
Et
Normal IncidenceParallel Incidence
© Tallal Elshabrawy 6
First Medium is Free space and μ1 = μ2 , θi approaches 0
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
Hi Hr
Et
i r
t
Ei
1 1 1, ,
2 2 2, ,
Er
HiHr
Et
Normal IncidenceParallel Incidence
1 1 This shows that the ground may be modeled as a perfect reflector with a reflection coefficient of unit magnitude when an incident wave grazes the earth regardless of the polarization or dielectric properties
Reflection from Dielectrics
© Tallal Elshabrawy 7
Ground Reflection (Two-Ray) Model
A single direct path between the base station and a mobile is seldom the only means for propagation
Friis equation is most likely inaccurate Two ray ground reflection model has
been found to be reasonably accurate for predicting large scale strength over distances of several Kms for mobile radio systems that use tall towers
TOT TOTR
E EW m
2 2
2
120
Power Flux Density
ht
hr
ELOS
EiEr= Eg
i r
Transmitter
Receiver
ETOT=ELOS+Eg
d
© Tallal Elshabrawy 8
Ground Reflection (Two-Ray) Model
c
E d dE d t t d d
d c0 0
0, cos ,
LOS c
E d dE d t t
d c0 0 '
', cos'
g c
E d dE d t t
d c0 0 ''
'', cos''
Assuming grazing incidence Γ=-1 for normal incidence
TOT LOS g
TOT c c
E E E
E d E dd dE d t t t
d c d c0 0 0 0' ''
, cos 1 cos' ''
© Tallal Elshabrawy 9
Ground Reflection (Two-Ray) Model
ht
ELOS
EiEr= Eg
i r
Transmitter
Receiver
ETOT=ELOS+Eg
d
ht
hr
hr
d’
d’’
© Tallal Elshabrawy 10
Ground Reflection (Two-Ray) Model
t r t rt r t r
t r t rt r t r
h h h hd h h d d d d h h
d d
h h h hd h h d d d d h h
d d
xwhere x for x
2 22 2
2 2
2 22 2
2 2
'' 1 1 ,2
' 1 1 ,2
1 1 12
t r ch hd d
d c'' ' 2 ,
+
<<
<<
<<
© Tallal Elshabrawy 11
Ground Reflection (Two-Ray) Model
TOT c c
E d E dd dE d t t t
d c d c0 0 0 0' ''
, cos 1 cos' ''
TOT c
E d E dd d dE d t
c d c c d0 0 0 0'' '' '
, cos 1 cos 0' ''
Let evaluate ETOT at t=d’’/c
Slide 9
E d
d0 0
'
E d
d0 0
''
ETOT
E d
d0 0 sin'
E d E d
d d0 0 0 0 cos'' '
© Tallal Elshabrawy 12
c c ct t t2 sin sin cos cos2 2
Ground Reflection (Two-Ray) Model
E d E d E d
d d d0 0 0 0 0 0
' ''
Important Note:The significance of the path difference Δ appears in the phase difference θΔ between the two waves (LOS and Reflected Wave)
TOT
TOT
TOT
E d E dE d
d d
E dE d
d
E dE d
d
2 22 20 0 0 0
0 0
0 0
cos 1 sin
2 2cos
2 sin2
Assume for large d
Note that rather than using phasors, we could also use
© Tallal Elshabrawy 13
Ground Reflection (Two-Ray) Model
Under the assumption
t r
t r t r
rads
h hrad
dh h h h
d
0.3 sin2 2 2
20.3
220 20
3
TOT
t rTOT
E dE d
d
E d h hE d
d d
0 0
0 0
2 sin2
22
t r
TOT t rR
E d h h E dE d d h hd
d
2 20 0 0 0
2 2
22
4
120 120
© Tallal Elshabrawy 14
Ground Reflection (Two-Ray) Model
t r
TOT t rR
E d h h E dE d d h hd
d
2 20 0 0 0
2 2
22
4
120 120
But from LOS analysis T T
E dP Gd
d
2
0 0
2120 4
TOT t rT TR
R R e R R edir iso
T T R t rt rT TR R
E h hP G
dd
P d A G A
P G G h hh hP GP d G
dd d
2 2
2
22 2
2 4
4
4
4
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