Polarization Diversity for Base Station

8/2/2019 Polarization Diversity for Base Station

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Polarization Diversity for Base Station

Antennas

Martin Nilsson,Allgon System

Allgon System



2

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



3

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?



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



5

Antenna model

The channelvectors a ,b areprojected onto

the polarizationellipse of axialratio χ0.5



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 .



7

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



8

Geometry of the simulated antennas

Aperture Coupled Patch overan infinite groundplane

Slanted dipoles over aninfinite groundplane



9

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



10

Simulated Patterns, cont.Phase between horizontal and vertical

far-field components



11

Far-field coupling

The scalar product of the normalized far-fields of

the two channels: <a,b> = (a,b*)



12

Calculated Output PowerCorrelation for Rayleigh Distributed

Incident Fields

Aperture Coupled Patch overan infinite groundplane

Slanted dipoles over aninfinite groundplane



13

Geometry of the

two Measured

Base Station

Antennas

•Dual polarizedantenna arrays of 8elements.•Aperture CoupledPatch elements are

symmetrical andcentred•Dipole elements aredisplaced to increaseisolation



14

Measured Radiation Patterns:

Co- and Cross-Polar

ACP antenna Slanted dipole antenna



15

Measured Radiation Patterns:

Vertical and Horizontal Polarizations

ACP antenna Slanted dipole antenna



16

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



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



19

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.

Polarization Diversity for Base Station

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