Art.sobre Fading

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TechZine Alcatel-Lucent

KEY COUNTERMEASURES AGAINST SELECTIVE FADING

By: Roberto Valtolina| February 28, 201 | Article!,Technolo"y

#i"hli"ht!

#i"her $o%ulation increa!e! !en!iti&ity to !electi&e 'a%in"

A (o)er'ul e*uali+er i! a ey counter$ea!ure a"ain!t !electi&e 'a%in"

(ace %i&er!ity %i"ital !i"nal co$binin" i! $ore e''ecti&e than !)itchin"

electi&e 'a%in", e!(ecially at lo)-to-$e%iu$ 're*uencie!, can be the li$itin" 'actor )hen %eter$inin" the $a.i$u$ lin len"th 'or

(oint-to-(oint $icro)a&e lin!/ icro)a&e !y!te$!that o''er the ri"ht counter$ea!ure! a"ain!t !electi&e 'a%in" increa!e the $a.i$u$

lin len"th 'or lon"-haul lin!/ They al!o hel( !er&ice (ro&i%er! re%uce the nu$ber o' lin! )ith e.(en!i&e !(ace %i&er!ity or 2-

antenna con'i"uration!/

3F34546 A6A54T 3L37T5V3 FA546

A! the $icro)a&e in%u!try a%o(t! hi"her $o%ulation !che$e!, e''ecti&e counter$ea!ure! a"ain!t !electi&e 'a%in" beco$e e.tre$elyi$(ortant/ er&ice (ro&i%er! are alrea%y %e(loyin" 102 ua%rature A$(litu%e o%ulation 9102 A !y!te$!/ An% 0;< A i! on

the near hori+on/ The!e hi"her $o%ulation !che$e! are $uch $ore !en!iti&e to !electi&e 'a%in"than tra%itional 128 A/

e'en%in" a"ain!t !electi&e 'a%in" i! (articularly i$(ortant in lon"-haul lin!/ =ith (oint-to-(oint ra%io lin!, $any (eo(le thin >the

hi"her the tran!$it (o)er, the lon"er the ra%io lin/? Thi! i! true, but tran!$it (o)er i! only one o' the 'actor! that !houl% be con!i%ere%in lon"-haul lin!/ There are !i"ni'icant (ro(a"ation i$(air$ent! that cannot be $iti"ate% by @u!t increa!in" the tran!$it (o)er/

i''erent technolo"ie!, !uch a! (o)er'ul e*uali+er!, are nee%e% to co$bat %i!tortion/

n%er!tan%in" the 'actor! that a''ect $a.i$u$ lin len"th an% the ey counter$ea!ure! a"ain!t !electi&e 'a%in" hel(! !er&ice (ro&i%er!

e&aluate $icro)a&e !y!te$!/

FA7TR T#AT AFF37T AC5 L54D L346T#

Rain an% !no) are the $ain 'actor! that a''ect $a.i$u$ lin len"th/ The $ore inten!e the rain, the hi"her the !i"nal attenuation, creatin"lon"-haul lin %e(loy$ent challen"e!/ An% the lon"er the lin, the hi"her the attenuation e''ect! o' the rain/ Thi! ty(e o' attenuation i!

co$$only calle% 'lat attenuation, or 'lat 'a%in", becau!e the !a$e %e"ree o' 'a%in" occur! acro!! the entire 're*uency/

Attenuation %ue to rain increa!e! at hi"her 're*uencie!/ For e.a$(le, in a rain rate o'


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Figure 1. Selective fading affects frequency in different ways

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Technolo"ie! !uch a! (o)er'ul e*uali+er! co$(en!ate 'or an% reco&er the %i!tortion that $ulti(ath ray! intro%uce/ But not all e*uali+er!

are e*ual in ter$! o' ca(abilitie!/ 5t i! i$(ortant to $ea!ure the e''ecti&ene!! o' the e*uali+er! an% to co$(are their (er'or$ance/

The conce(t o' a !i"nature, !o$eti$e! calle% a =-cur&e[2], i! u!e% to co$(are e*uali+er (er'or$ance/ The area o' the !i"nature hel(! to

%eter$ine the $a.i$u$ lin len"th/ The !$aller the area o' the !i"nature, the better the (er'or$ance o' the e*uali+er, an% the lon"er the

$a.i$u$ lin len"th/

The !i"nature !ho)n in Fi"ure 2 in%icate! that the e*uali+er can co$(en!ate 'or 'a%in" u( to 0 %B )ithout error!:

The . a.i! !ho)! the !y!te$ "ain inclu%in" antenna "ain/

The y a.i! !ho)! the !i"nature %e(thJ the "reater the &alue, the !$aller the !i"nature area/

icro)a&e !y!te$! )ith e*uali+er! that (ro&i%e a 0 %B !i"nature 9in 28 h+ channel !(acin" are the $o!t a%&ance% a&ailable to%ay/

Figure 2. The smaller the signature, the better the performance of the equalizer

y!te$ "ain (lu! notch %e(th %eter$ine! the $a.i$u$ lin len"th/ 5' the lin !y!te$ "ain i! e(t con!tant, an e*uali+er that i$(ro&e!

the !i"nature 'ro$ 20 %B to 0 %B increa!e! the $a.i$u$ lin len"th $ore than


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Figure . !et system gain plus notch depth determines ma"imum lin# length

IA73 5V3R5T #A IR A4 74

=hile e*uali+er! hel( to i$(ro&e the !i"nature area an% increa!e $a.i$u$ lin len"th, there are ca!e! )here they are in!u''icient/ For

e.a$(le, )ith lon"-haul lin! that are $ore than H0 $ lon" an% are o&er )ater, $ulti(ath (ro(a"ation e''ect! cannot be !u''iciently

re%uce% by an e*uali+er/

5n the!e ca!e!, !(ace %i&er!ity, )hich i! !o$eti$e! calle% antenna %i&er!ity, i! a $ore e''ecti&e counter$ea!ure a"ain!t !electi&e 'a%in"/

=ith !(ace %i&er!ity, the tran!$itte% !i"nal i! !i$ultaneou!ly recei&e% at t)o !(atially !e(arate% antenna! the $ain antenna an% the

%i&er!ity antenna/ The techni*ue i! e''ecti&e becau!e the correlation bet)een the 2 !i"nal! i! lo) i' the antenna! are )ell !e(arate%/ 5' the

!i"nal recei&e% by one antenna i! o' (oor *uality, the !i"nal recei&e% by the other antenna i! liely "oo% enou"h/

#o)e&er, )hile !(ace %i&er!ity i$(ro&e! lin a&ailability, it re*uire! %ouble the nu$ber o' antenna!/ ore antenna! $ean hi"her ca(ital

an% o(erational e.(en%iture! 'or !er&ice (ro&i%er!/

A"ain, the (o)er o' the e*uali+er in the $icro)a&e !y!te$ i! &ery i$(ortant/ 5n $any ca!e!, an e*uali+er that (ro&i%e! a !i"nature o' 0%B let! !er&ice (ro&i%er! a&oi% u!in" !(ace %i&er!ity/ They can achie&e ;;/;;;K a&ailability on lon"-haul lin! in the 0 to 0 $ ran"e

u!in" a !in"le antenna/

7B54546 5 T#3 53AL IA73 5V3R5T 3T#

There are !till lin! )here !(ace %i&er!ity i! the only )ay to "uarantee lin a&ailability/ For the!e ca!e!, $icro)a&e &en%or! o''er 2

%i''erent $etho%! to (roce!! the (air o' recei&e% !i"nal!:

The !)itchin" $etho% choo!e! the better o' the 2 !i"nal!, i%eally at e&ery in!tant, to $ini$i+e the bit error rate/

The co$binin" $etho% (roce!!e! the 2 %i&er!ity !i"nal!, blen%in" their !a$(le! )ith intelli"ent al"orith$!/

The %i"ital !i"nal co$binin" $etho% i! a $ore e''ecti&e a((roach becau!e it u!e! the @oint contribution 'ro$ the 2 !i"nal!/ The !)itchin"

$etho% coul% be con!i%ere% )a!te'ul becau!e it u!e! only 1 o' the 2 !i"nal at any (oint in ti$e/

A !(ace %i&er!ity !y!te$ that u!e! %i"ital !i"nal co$binin": 7ontribute! u( to a %B i$(ro&e$ent in the !i"nal-to-noi!e ratio to counter 'lat 'a%in"/

ut(er'or$! the !)itchin" techni*ue/ 3&en i' 1 o' the 2 !i"nal! !u''er! 'ro$ H %B o' 'a%in", the co$bine% !i"nal i! error 'ree/

electin" only 1 o' the 2 !i"nal! $ean! the !y!te$ cannot be error-'ree/ A! illu!trate% in Fi"ure 2 abo&e, to%ayG! $o!t a%&ance%

e*uali+er! (ro&i%e a !i"nature %e(th o' 0 %B/ Thi! $ean! that H %B 'a%in" cannot be 'ully eli$inate%/ A! a re!ult, H %B o' 'a%in" on

a !electe% channel in a !)itche% !y!te$ )ill create error!/

3VALAT3 T3 7AR3FLL

icro)a&e !y!te$! that (ro&i%e e''ecti&e counter$ea!ure! a"ain!t !electi&e 'a%in" o''er an i$(ortant %i''erentiator/ The!e !y!te$! "i&e

!er&ice (ro&i%er! a %e(loyable 102 A lon"-haul linJ !y!te$! that ha&e !i$(ly a%%e% (oint! to their A con!tellation to reach

102 A %o not/

3*uali+er! are al!o critical(oint! o' co$(ari!on)hen !electin" $icro)a&e !y!te$!, not nice-to-ha&e 'eature!/ 3*uali+er! can eli$inate

the nee% to %e(loy 2-antenna !(ace %i&er!ity !y!te$!/ To"ether, e*uali+er! an% %i"ital !i"nal co$biner! i$(ro&e (er'or$ance an% lin

len"th in con%ition! )here !(ace %i&er!ity i! re*uire%/

To contact the author or re*ue!t a%%itional in'or$ation, (lea!e !en% an e-$ail to tech+ine/e%itorMalcatel-lucent/co$/

FT4T3

1/ [1]The Ru$$ler $o%el i! na$e% a'ter =// Ru$$ler, the re!earcher )ho (ro(o!e% thi! $o%el in the late 1;N0! a'ter e.ten!i&e$ea!ure$ent! on a 0-$,

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