polarization diversity for base station
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8/2/2019 Polarization Diversity for Base Station
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Polarization Diversity for Base Station
Antennas
Martin Nilsson,Allgon System
Allgon System
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Outline
• Problem formulation• Model for the radio channel and the antenna reception
• Derivation of the relation between power correlation and
far-field coupling
• Simulated antennas: Dual polarized Aperture Coupled
Patch and slanted dipoles
• Measured radiation patterns of base station antennas and
calculated correlation
• Far-field coupling from amplitude only measurements
• Correlation and diversity gain
• Slant ±45° vs. vertical/horizontal polarization
• Conclusion
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Problem Formulation• Base station antenna used in a Rayleigh fading
environment
• We wish to use polarization diversity with equal meanpower on both branches; thus the two antenna channels
should be symmetrical• We assume un-correlated envelopes of vertical and
horizontal incident field components
What is the output signal correlation from the antenna?
Is there a difference between different antenna configurations?
What is the impact in terms of diversity gain of using different types of
base station antennas?
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Measurements of the radio chan
E n v i r o n m e n t a n d s o u r c e M o b i l e O r i e n t a t i o n d B F r e q u e n c y C o r r e l a
U r b a n 1 V e r t i c a l c a r a n t e n n a 4 - 7 9 2 0 M H z m e d i a
U r b a n 2 3 0
o n l a r g e g r o u n d p l a n e 7 4 6 3 M H z - 0 .
S u b - u r b a n 2 1 2 0 .
U r b a n & s u b - u r b a n 3 0
1 0 1 7 9 0 M H z 0 . 7 f
4 5
4 . 6 - 6 . 3 0 . 7 f
U r b a n 4 7 0 1 5
i n - a n d o u t d o o r 1 - 4 1 8 2 1 M H z 0 . 2 f
S u b - u r b a n 4 2 - 7 0 . 1 f
U r b a n & s u b - u r b a n 5 0
4 - 7 1 8 4 8 M H z 0 . 5 f
4 5
0 0 . 5 f
U r b a n 6 C a r m o u n t e d m o n o p o l e 7 : 6 2 : 1 9 7 0 M H z 0 : 0 9
1 S . K o z o n o , T . T s u r u h a r a , a n d M . S a k a m o t o , B a s e s t a t i o n p o l a r i z a t i o n d i v e r s i t y r e c e p t i o n f o r
T r a n s . V e h . T e c h n o l . , v o l . 3 3 , p p . 3 0 1 3 0 6 , N o v . 1 9 8 4 .
2 R . G . V a u g h a n , P o l a r i z a t i o n d i v e r s i t y i n m o b i l e c o m m u n i c a t i o n s , " I E E E T r a n s . V e h . T e c h n
1 8 6 , A u g . 1 9 9 0 .
3 A . M . D . T u r k m a n i , A . A . A r o w o j o l u , P . A . J e o r d , a n d C . J . K e l l e t t , A n e x p e r i m e n t
p e r f o r m a n c e o f t w o b r a n c h s p a c e a n d p o l a r i z a t i o n d i v e r s i t y s c h e m e s a t 1 8 0 0 M H z , " I E E E T r a n s . V
p p . 3 1 8 3 2 6 , M a y 1 9 9 5 .
4 F . L o t s e , J . - E . B e r g , U . F o r s s e n , a n d P . I d a h l , B a s e s t a t i o n p o l a r i z a t i o n d i v e r s i t y r e c e p t
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Antenna model
The channelvectors a ,b areprojected onto
the polarizationellipse of axialratio χ0.5
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Derivation of Power Correlation from
Far-field CouplingI n c i d e n t e l d :
E
= E
̂ = r
t e
, j
t
̂ 1
E
= E
̂
= r
t e
, j
t
̂
. 2
w h e r e
p r =
r
2
e
, r
2
= 2
2
3
A n t e n n a r e p r e s e n t a t i o n b y f a r - e l d v e c t o r f u n c t i o n s :
a = a ; ̂ a ; 4
b = b ;
̂
b ; 5
I f w e d e n e a m a t r i x :
A =
a
h ̂ ; ̂a i a
h
̂
; ̂a i
b
h ̂ ;
̂
b i b
h
̂
;
̂
b i
6
t h e n t h e o u t p u t f r o m t h e a n t e n n a i s
V
a
V
b
= A
E
E
o r y = A . 7
T h e c o v a r i a n c e m a t r i x o f t h e i n p u t s i g n a l i s
T h e c o m p l e x n o r m a l i z e d c r o s s - c o v
c
=
C
2 ; 1
y
q
C
1 ; 1
y
C
2 ; 2
y
=
C
1 ; 2
y
q
C
1 ; 1
y
C
a n d f o r t h e c i r c u l a r l y s y m m e t r i c R a y
p o w e r
= j
c
j
2
=
j C
2 ; 1
y
j C
1 ; 1
y
C
y
F o r t h e u n - p o l a r i z e d c a s e w i t h e q u a
t h e v e r t i c a l a n d h o r i z o n t a l c o m p o n e n
c
= h ̂a ;
̂
b i
e
, j a r g f a b
g
S o 1 1 r e d u c e s t o :
p o w e r
= j h ̂a ;
̂
b i j
2
:
T h u s , t h e p o w e r c o r r e l a t i o n i s e q u a l
t h e f a r - e l d c o u p l i n g .
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Output CorrelationIdeal ±45º Slanted Dual Polarized Antenna
Environment
(XPD in dB)
Received Polarization
Statistical Distribution
Output Correlation
Coefficient ((ρρpower))
0 (indoor-microcell)
3 (urban)
6 (urban-suburban)
9 (rural)
0.00
0.11
0.36
0.77
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Geometry of the simulated antennas
Aperture Coupled Patch overan infinite groundplane
Slanted dipoles over aninfinite groundplane
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Simulated Patterns(HP-Momentum and λ / 2 dipole theory)
Aperture Coupled Patch overan infinite groundplane:
HBW = 72 degrees
Slanted dipoles over aninfinite groundplane:
HBW = 75 degrees
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Simulated Patterns, cont.Phase between horizontal and vertical
far-field components
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Far-field coupling
The scalar product of the normalized far-fields of
the two channels: <a,b> = (a,b*)
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Calculated Output PowerCorrelation for Rayleigh Distributed
Incident Fields
Aperture Coupled Patch overan infinite groundplane
Slanted dipoles over aninfinite groundplane
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Geometry of the
two Measured
Base Station
Antennas
•Dual polarizedantenna arrays of 8elements.•Aperture CoupledPatch elements are
symmetrical andcentred•Dipole elements aredisplaced to increaseisolation
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Measured Radiation Patterns:
Co- and Cross-Polar
ACP antenna Slanted dipole antenna
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Measured Radiation Patterns:
Vertical and Horizontal Polarizations
ACP antenna Slanted dipole antenna
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Simulated Output Envelope Correlation fromMeasured Radiation Patterns: 10000 samples
ACP antenna:
ρenvelope ~= 0.3 at -60 degrees
Slanted dipole antenna:
ρenvelope = 0.8 at -60 degrees
Both antennas: ρenvelope = 0.38 at boresight
due to projection onto the polarization ellipse
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Far-field coupling from
amplitude-only radiation patt
P r o j e c t a a n d b o n t o t h e v e r t i c a l a n d h o r i z o n t a l
p o l a r i z a t i o n s :
a = a
v
̂ v + a
h
̂
h 1
b = b
v
̂ v + b
h
̂
h . 2
N o w , i f t h e r e i s a s y m m e t r y i n t h e r a d i a t i o n
p a t t e r n s w i t h r e s p e c t t o t h e v e r t i c a l a x i s , i . e :
b
v
= e
, j
a
v
3
b
h
= , e
, j
a
h
, 4
t h e F a r - e l d c o u p l i n g h a ; b i c a
h a ; b i = a ; b
= a
v
̂ v + a
h
̂
h e
j
a
v
̂ v
= e
j
j a
v
j
2
, j a
h
j
2
s i n c e h ̂ v ;
̂
h i = 0 .
F o r t h e u n p o l a r i z e d c a s e
p
h e n c e t h e o u t p u t p o w e r c o r r e l a t
p o w e r
= j a
v
j
2
, j a
h
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• Mobile at -60 degrees azimuth (cell border):ρenvelope = 0.3 for ACP and 0.8 for slanted dipole antenna
• Radio channel XPD (vert./ hor. power) = 6 dB
Impact of correlation on diversity gain
a) Selection diversity
(Schwartz, Bennett, Stein 1966)
Dipoles:ρpower = 0.8 = 0.92
ρpower = 0
~2.5 dB
ρpower = 0.8 = 0.92
b) Maximum Ratio Combining
(Yongbing Wan, J.C. Chen 1995)
ρpower = 0
~2.8 dB
Note: ρenv ∼=ρpower = ρ2 for Rayleigh signals
ACP:ρpower = 0.3 =
= 0.552
ACP: ρpower = 0.3 = 0.552
= loss of diversity gain
1% level
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Slant ±45° vs.
vertical/horizontal polarization
Pre-detection combining:
•With orthogonal far-fields of the two channels, all power isreceived at the antenna and thus all the information in both cases•We can change slant ±45° to vertical/ horizontal using loss-less,reciprocal networks•The eigen-values of the covariance matrix and thus the probability
density function are identical in both cases⇒
no difference between the two with optimal combining (MRC)
digital signaldetectionof
RF-signal
non-linear!
RF signals MRC
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Conclusions
• A closed form expression for the output correlation as a function of far-field patterns has been shown.
• The output correlation is a function of the antenna far-field coupling as
well as the XPD of the environment.
• For an un-polarized environment (XPD = 0 dB) the output correlation
equals the square of this coupling.
• Symmetrical antenna designs with equal patterns for vertical and
horizontal polarizations provide orthogonal far-fields <=> low far-field
coupling.
• The aperture coupled patch provides the lower output correlation in all
investigated cases.
• For symmetrical radiation patterns, the far-field coupling can becalculated from amplitude-only patterns.
• A high far-field coupling, i.e. poor orthogonality, could result in a loss
of 2-3 dB diversity gain for selection or MR combining.