HC VIN CNG NGH BU CHNH VIN THNG

---------------------------------------

BI QUC VNG

NH GI HIU NNG H THNG TRUYN

THNG KHNG DY LAI GHP RF/ FSO

Chuyn ngnh: K thut Vin thng

M s: 60.52.02.08

TM TT LUN VN THC S

H NI 2016

Lun vn c hon thnh ti:

HC VIN CNG NGH BU CHNH VIN THNG

Ngi hng dn khoa hc: PGS. TS. ng Th Ngc

Phn bin 1: PGS. TS. Bi Trung Hiu

Phn bin 2: TS. Phm Mnh Lm

Lun vn s c bo v trc Hi ng chm lun vn thc s ti

Hc vin Cng ngh Bu chnh Vin thng

Vo lc: 9 gi 45 ngy 11 thng 03 nm 2017

C th tm hiu lun vn ti:

- Th vin ca Hc vin Cng ngh Bu chnh Vin thng

1

M U

Trong nhng nm gn y chng ta c chng kin nhu cu ngy cng tng ca

ngi dng u cui v bng thng trong truyn thng khng dy, nhm h tr cc dch v

bng rng nh: truyn hnh nt cao (HDTV), dch v VideoCall, truy cp internet tc

cao, hi ngh truyn hnh trc tuyn vv. p ng cc nhu cu ny, i hi bng thng

truyn dn rng hn, tc nhanh hn.

Trong mng truy nhp, mt s cng ngh v ang c s dng gii quyt nhu

cu truyn dn ca ngi dng u cui bao gm cc cng ngh da trn cp ng, lai ghp

cp si quang cp ng trc (HFC), cp quang ti tn nh, cc cng ngh khng dy bng

rng sng cao tn RF. Trong tng lai gn, mt thc t cc k r rng l cc cng ngh cao

tn RF vi nhng hn ch nh gii hn bng thng, khan him ph tn s, cc vn an

ninh, giy php v chi ph cao trong vic lp t v kh nng tip cn, khng th p ng

nhu cu sp ti.

Gn y, cng ngh truyn dn quang khng dy (FSO) ra i v c xem nh

gii php hiu qu, thay th v b sung cho cng ngh truyn thng khng dy sng cao tn

(RF). Vi nhng u im vt tri nh tc cao, chi ph thp, khng yu cu cp php tn

s, trin khai nhanh v linh hot. H thng ny hin nay c th truyn ti mt lng ln d

liu ln n 10 Gb/s. Tuy nhin, cng ngh FSO cng gp phi mt s hn ch nht nh

nh b nh hng mnh bi tp m, nhiu lon khng kh v iu kin thi tit xu nh gi,

sng m, khi, tuyt .

tn dng cc u im v khc phc cc hn ch ca c hai cng ngh RF v FSO,

h thng truyn thng lai ghp RF/FSO c xut. y c xem nh mt gii php

linh hot v hiu qu cho cc kt ni trong mng truy nhp v backhaul trong tng lai.

2

CHNG 1: TNG QUAN V H THNG TRUYN THNG

QUANG KHNG DY FSO

Gii thiu h thng FSO 1.1

Khc cc h thng khng dy thng thng, FSO l mt cng ngh truyn quang qua

khng gian t do. u im ca h thng ny khi s dng l khng yu cu phi ng k ph

hay phi hp tn s vi nhng nh khai thc khc, ng thi nhiu gia cc thit b c

hn ch. Mt u im na l tn hiu laser im im rt kh can thip, v th tnh

bo mt cao. Tc d liu c th so snh vi truyn dn qua si quang v t l li thp.

Ngoi ra vic s dng nhng chm laser c rng ph hp m bo cho kh nng lp t

nhiu cc b thu, pht cng mt a im.

Hin nay cc nh cung cp dch v khng ngng a ra cc ng dng nn nhu cu v

dch v bng thng rng trong cc mng th l rt ln. Tuy nhin, s chnh lch v tc

truyn dn gia mng li v mng truy nhp to ra mt s mt cn bng. S mt cn

bng ny thng c gi l nghn c chai. Hin tng nghn c chai xut hin khp

ni trong cc mng th hin nay.

Hnh 1.1: Hin tng nghn c chai trong mng

Mt la chn khc l cng ngh khng dy s dng tn s v tuyn RF (Radio

Frequency). Cng ngh ny cho php truyn i trong khong cch xa hn FSO, nhng cc

mng da trn RF yu cu s u t ln khi phi ng k di ph. Hn na, cc cng ngh

RF gp phi kh khn khi mun m rng ln dung lng cao. Khi so snh vi FSO, RF

khng m bo hiu qu kinh t cho cc nh cung cp dch v ang trng i vo s m

rng dung lng ca cc mng quang.

La chn th ba l cng ngh da trn dy cp ng (nh Ethernet, T1s hay

3

DSL). Mc d h tng cp ng c mt khp ni v s cc ta nh s dng cp ng l

cao hn si quang nhng y vn khng phi l mt gii php tha ng trong vic gii

quyt hin tng nghn c chai do tr ngi ln nht chnh l rng bng thng. Cng

ngh cp ng c th gii quyt mt s vn ngn hn, nhng hn ch v bng thng ch

mc 2Mbit 3Mbit/s khin cho cng ngh ny ch l mt gii php tm thi.

Gii php th t v thch hp nht chnh l FSO. Cng ngh ny l mt gii php ti

u nh nhng u im ca cng ngh quang nh rng bng thng, tc trin khai (vi

gi so vi vi tun hoc vi thng), mm do (c th ti trin khai v chuyn dch), hiu

qu kinh t cao (trung bnh chi ph lp t ch bng 1/5 so vi lp t cp quang).

Mi trng truyn dn ca h thng FSO l khng gian t do, ph thuc vo phm vi truyn

dn m h thng FSO c phn loi thnh 2 loi chnh nh sau: FSO trong nh (Indoor

FSO) v FSO ngoi tri (Outdoor FSO).

i vi FSO trong nh th mi trng truyn dn chnh l khong khng trong cc ta

nh. y l mi trng kh n nh, khng chu nhiu bin i, chi phi ca kh hu bn

ngoi.

FSO ngoi tri th ngc li. Mi trng truyn dn ca loi ny chnh l bu kh

quyn. L mi trng c tnh n nh km do chu nh hng mnh m t cc yu t mi

trng nh sng m, ma v nhiu t nh sng bn ngoi

Cc li th v thch thc ca h thng FSO 1.1.1

Cc li th ca FSO

- Khng yu cu giy php ph tn v tuyn.

- Khng b nh hng ca nhiu in t.

- D dng trin khai lp t.

- Kh nng bo mt cao.

Cc thch thc i vi h thng FSO

Gii hn c bn ca FSO do mi trng truyn dn gy ra. Ngoi vic tuyt v ma

c th lm cn tr ng truyn quang, FSO chu nh hng mnh bi sng m v s

nhiu lon ca khng kh. Nhng thch thc chnh trong vic thit k cc h thng FSO:

- Sng m

- S nhp nhy

- S tri bp

- Gi thng hng pht-thu khi ta nh dao ng

4

- S an ton cho mt

ng dng ca cng ngh FSO 1.1.2

Cc gii php FSO cho php loi b:

- Tc nghn v lu lng.

- Yu cu xin php v cp giy php.

- Vic o rnh, cng v t cp.

- Vn lin quan ti hp ng thu (cho thu) ta nh.

- Tn thi gian lp t.

- Chi ph cao.

H thng FSO v vn an ninh mng

Mc d FSO l cng ngh khng dy nhng n khng pht qung b ti bt k ngi

no v tt c mi ngi. FSO pht bp sng nh sng hp, tn s rt cao ti mt ni xc

nh. Do rt kh cho mt c nhn no c th thu trm thng tin m khng b pht

hin.

M hnh h thng FSO 1.2

H thng FSO gm ba phn: b pht, knh truyn v b thu.

Hnh 1.2: M hnh h thng FSO

5

B pht 1.2.1

D liu u vo pha ngun c truyn ti mt ch xa. Pha ngun c c ch

iu ch sng mang quang ring, chng hn nh iu ch laser, tn hiu quang sau s

c truyn i qua knh kh quyn. Cc tham s ca h thng pht quang l kch c, cng

sut v cht lng bp sng, cc tham s ny xc nh cng laser v gc phn k nh

nht c th t c t h thng. Phng thc iu ch c s dng rng ri ti b pht l

iu ch cng (IM), trong cng pht x ca ngun quang s c iu ch bi

s liu cn truyn i. Vic iu ch c thc hin thng qua vic thay i trc tip cng

ca ngun quang ti b pht hoc thng qua b iu ch ngoi nh b giao thoa MZI.

Vic s dng mt b iu ch ngoi nhm m bo tc d liu t c cao hn so vi

b iu ch trc tip. Cc thuc tnh khc ca trng bc x quang nh pha, tn s v trng

thi phn cc cng c th c s dng iu ch vi cng vi d liu/thng tin thng

qua vic s dng b iu ch ngoi.

Nguyn l cu to chung ca mt b pht laser gm c: bung cng hng cha hot

cht laser, ngun nui v h thng dn quang. Trong bung cng hng vi hot cht

laser l b phn ch yu. Bung cng hng cha hot cht laser, l mt cht c bit c

kh nng khuch i nh sng bng pht x cng bc to ra tia laser. Khi mt photon

ti va chm vo hot cht ny th ko theo l mt photon khc bay ra theo cng hng

vi photon ti. Mt khc bung cng hng c 2 mt chn hai u, mt phn x hon ton

cc photon khi bay ti, mt kia cho mt phn photon qua mt phn phn x li lm cho cc

ht photon va chm lin tc vo hot cht laser nhiu ln to mt photon ln. V th

cng chm laser c khuch i ln nhiu ln.

Hnh 1.3: Cu to v nguyn l hot ng ca Laser

6

Ngoi ra, vic to tn hiu pht cng c thc hin bi cc loi ngun khc nhau

c m t trong bng 1.2.

B thu 1.2.2

B thu h tr vic khi phc cc d liu c pht i t pha pht. B thu bao

gm cc thnh phn sau:

Khu thu Tp hp v tp trung cc pht x quang ti b tch sng quang. Khu

( m) ca b thu ln s gip tp hp c nhiu pht x quang vo b tch sng

quang.

B lc thng di quang B lc thng di lm gim lng bc x nn.

B tch sng quang PIN hoc APD chuyn i trng quang n thnh tn hiu

in. Cc b tch sng quang thng c dng trong cc h thng truyn thng quang hin

nay c tm tt trong bng 1.3.

Mch x l tn hiu C chc nng khuch i, lc v x l tn hiu m bo tnh

chnh xc cao ca d liu c khi phc.

C hai loi b thu quang c bn: b thu khng kt hp v b thu kt hp. B thu

khng kt hp tch trc tip cng sut tc thi ca trng quang tp trung khi chng ti b

thu, do thng c gi l b thu tch trc tip hoc tch cng sut. Loi b thu ny l

loi n gin nht trong vic thc hin v c th c s dng bt c khi no thng tin

truyn i xut hin s bin i cng sut (v d IM) ca trng quang. i vi b thu kt

hp, hay cn gi l b thu heterodyne, trn ln trng sng nh sng cc b pht ra vi

trng nh sng thu c, v sng kt hp ny s c tch photon. Loi b thu ny thng

c s dng khi thng tin c iu ch da trn sng mang quang s dng iu bin

(AM), iu tn (FM) hoc iu pha (PM), v l cn thit cho tch sng FM hoc PM.

Qu trnh tch ca cc trng quang b tc ng bi nhiu loi ngun nhiu khc

nhau xut hin ti b thu. Ba loi ngun nhiu ch yu trong truyn thng FSO l: nh sng

nn xung quanh, b tch photon gy ra nhiu, v nhiu nhit trong mch in t.

7

Cc yu t nh hng ln hiu nng ca h thng FSO 1.3

Nhiu lng t 1.3.1

Nhiu nhit 1.3.2

Nhiu dng ti v nhiu nn 1.3.3

Cc yu t khc 1.3.4

Kt lun chng 1 1.4

Ni dung chng 1 gii thiu khi qut v h thng truyn thng quang qua khng

gian t do cng nh m hnh ca h thng FSO, cc yu t nh hng n hiu nng ca h

thng. Tng lai ngy cng i hi phi c cc gii php truyn dn tc cao p ng

yu cu ca cc doanh nghip, t chc v c nhn. Cc gii php cng cn phi c chi ph

hiu qu, trin khai nhanh, truyn dn thng tin mt cch an ton v tin cy. FSO c th p

ng cc yu cu ny v s c s dng ngy cng nhiu trong tng lai.

8

CHNG 2: CC M HNH KNH TRUYN THNG KHNG DY

Suy hao trong h thng RF v FSO 2.1

Suy hao trong h thng RF 2.1.1

Suy hao trong h thng FSO 2.1.2

Fading trong h thng RF 2.2

Hin tng a ng (Multipath) 2.2.1

Hiu ng Doppler 2.2.2

Cc dng knh truyn 2.2.3

Cc m hnh knh fading c bn 2.2.4

M hnh knh fading Reyleigh

Trong nhng knh v tuyn di ng, phn b Rayleigh thng c dng m t

bn cht thay i theo thi gian ca ng bao tn hiu fading phng thu c hoc ng

bao ca mt thnh phn a ng ring l. Chng ta bit rng ng bao ca tng hai tn

hiu nhiu Gauss trc giao tun theo phn b Rayleigh. Phn b Rayleigh c hm mt

xc sut:

2

2 2exp (0 )

( ) 2

0 ( 0)

r rr

p r

r

(2.10)

Vi l gi tr rms (hiu dng) ca in th tn hiu nhn c trc b tch ng

bao (evelope detection). 2 l cng sut trung bnh theo thi gian.

Xc sut ng bao ca tn hiu nhn c khng vt qua mt gi tr R cho

trc c cho bi hm phn b tch ly:

2

2

0

( ) ( ) ( ) 1 exp2

r

RP R P r R p r dr

(2.11)

Gi tr trung bnh rmean ca phn b Rayleigh c cho bi:

0

[ ] 1.25332

meanr E r rp r dr

(2.12)

V phng sai (cng sut thnh phn ac ca ng bao tn hiu):

2 2 2 2 2 2

0

[ ] 2 0.42922 2

r E r E r rp r dr

(2.13)

Gi tr hiu dng ca ng bao l 2 (cn bc hai ca gi tr trung bnh bnh

2

r

9

phng). Gi tr median ca r tm c khi gii phng trnh:

0

1( ) 1.177

2

medianr

medianp r dr r (2.14)

M hnh knh fading Ricean

Trong trng hp fading Rayleigh, khng c thnh phn tn hiu n trc tip b thu

m khng b phn x hay tn x (thnh phn light-of-sight) vi cng sut vt tri. Khi c

thnh phn ny, phn b s l Ricean. Trong trng hp ny, cc thnh phn a ng ngu

nhin n b thu vi nhng gc khc nhau c xp chng ln tn hiu light-of-sight.

Hm mt phn b xc sut ca phn b Ricean:

2 2( )

2202 2

( 0, 0)( )

0 0

r Ar Ar

e I A rp r

r

(2.15)

A: Bin nh ca thnh phn light-of-sight.

Io: L hm Bessel sa i loi 1 bc 0.

Phn b Ricean thng c m t bi thng s k c nh ngha nh l t s gia

cng sut tn hiu xc nh (thnh phn light-of-sight) v cng sut cc thnh phn a

ng:

2

22

Ak

(2.16)

Hay vit di dng dB:

2

2( ) 10log

2

Ak dB dB

(2.17)

k xc nh phn b Ricean v c gi l h s Ricean.

Khi A 0, k 0 (dB) thnh phn light-of-sight b suy gim v bin , phn b

Ricean tr thnh phn b Rayleigh. Hnh 2.5 m t hm mt xc sut ca phn b

Ricean.

Nhiu lon trong h thng FSO 2.3

Nhiu lon khng kh 2.3.1

Chm tia quang truyn qua kh quyn chu tc ng ca nhiu lon kh quyn vi pha

v bin bin thin ngu nhin. Nhiu lon l mt trng thi ri lon ca dng kh quyn

gy ra bi s thay i nhit trong kh quyn. Nhiu lon kh quyn bao gm nhiu khu

vc dng xoy hnh cu vi ng knh v ch s khc x khc nhau. Cc chm tia quang

truyn qua kh quyn khng gian v thi gian khc nhau vi chit sut khc nhau, cc ch

s ny khng ng nht cc quy m khc nhau. S khng ng nht vi quy m ln s to

ra hin tng khc x khin chm tia pht i lch so vi hng truyn ban u. Do ,

quy m ln th hiu ng ch yu l lm sai lch pha ca sng truyn i. S khng ng nht

10

vi quy m nh to ra hiu ng nhiu x v lm sai lch bin ca sng gy ra s bin

thin ca bin .

Mi trng truyn dn 2.3.2

Knh truyn dn quang khc so vi knh nhiu Gauss thng thng, tn hiu u vo

ca knh, x(t), th hin cng sut ch khng phi l bin . iu ny dn ti hai iu kin

rng buc trn tn hiu c truyn: x(t) phi khng m v gi tr trung bnh ca x(t) khng

c vt qu mt gi tr quy nh.

max1

lim2

T

T T

x t dtT

(2.18)

Tri ngc vi knh thng thng, ni m SNR t l vi cng sut, trong cc h thng

quang cng sut in thu c v phng sai ca nhiu n t l tng ng vi Ad2 v Ad;

trong Ad l kch thc vng tch sng quang. Do trong h thng truyn dn quang b

hn ch bi nhiu n, SNR t l vi Ad. iu ny c ngha l vi cng sut pht cho, c

th t c SNR cao hn bng vic s dng b tch sng c vng tch sng ln. Tuy

nhin, in dung ca b tch sng cng tng theo Ad dn n hn ch v bng thng.

Knh truyn quang khng dy bao gm cc loi kh nh trong bng 2.3. Ngoi ra,

trong knh truyn cn bao gm ma, sng m v cc loi khc ca hi nc. Lng hi

nc c trong knh truyn ph thuc vo v tr (kinh , v ) v theo tng ma. S tp

trung hi nc ln nht gn b mt tri t nm trong tng i lu.

Vi s phn b v kch thc ca cc di thnh phn kh quyn t micromet ti

centimet, mt trng quang i qua kh quyn s b tn x hoc hp th v gy ra suy hao.

Lch hng pht-thu 2.3.3

Li nh hng (s lch hng) l tng dch gia tm chm tia v tm khu

thu. S lch hng c tng qut gm 2 yu t: s lch hng c nh v s lch hng

ngu nhin.

Trong ng truyn thng ca h thng FSO, chnh xc nh hng l mt vn

quan trng trong vic xc nh hiu nng ng truyn v tin cy. Tuy nhin, gi v s

gin do nhit dn ti s rung lc ta nh, iu ny gy ra s lch hng v fading tn

hiu ti pha thu. Thu c mt m hnh thng k mi cho s lch hng, m xc nh kch

thc khu thu, rng chm tia, v phng sai jitter.

11

Hnh 2.6: M hnh lch hng ca chm tia

i vi chm tia Gauss, s phn b chun ha theo khng gian ca cng tn

hiu pht ti khong cch z t b pht xc nh theo:

2

2 2

22; expbeam

z z

I z

(2.19)

Vi l vec t bn knh t tm chm tia, v z l rng bp quang (bn knh tnh ti

2 ) ti khong cch z.

12 2

0 2

0

1zz

(2.20)

Vi 0 l rng bp quang ti z = 0, 2 2

0 01 2 / ( )z v

3 52 2

0 ( ) 0.55 nz C k z

l

di kt hp.

Vi mt khu thu ca bn knh a v mt tham s chm tia Gauss ti pha thu beamI

, c ch ra hnh 2.6. S suy hao do gin rng hnh hc vi lch r c xc nh:

; ;p beamA

h r z I r z d (2.21)

Vi .ph k hiu cho t l cng sut thu c bi b thu v A l din tch vng thu.

M hnh knh nhiu lon khng kh 2.3.4

Trng bc x (cng ) trong mi trng nhiu lon l 2

I A r trong khi

cng trong khng gian t do (khng c s nhiu lon) c cho bi cng thc

12

2

0 0I A r , v cng theo hm log c cho bi cng thc:

2

0

log 2eA r

lA r

(2.33)

Phng sai chun ha ca cng , thng c bit n di tn gi l ch s

nhp nhy (S.I.) v c biu din nh sau:

2

2 2

2

0

. . exp 1IN lS II

(2.41)

Nh vy, m hnh knh log chun ch ph hp cho knh nhiu lon yu.

M hnh knh nhiu lon Gamma-Gamma

Bc x chun ha I=XY c th c phn chia thnh cc h s hot ng ging nh

qu trnh iu ch khi X v Y xut hin tng ng, t cp ln (large- scale) v cp

nh (small-scale) ca hiu ng khng kh. Gi s rng c hai cp mnh v cp yu

ca s dao ng bc x c hnh thnh bi cc phn b gamma:

1( )( ) exp( ), 0, 0

( )X

Xp X X X

(2.42)

1( )( ) exp( ), 0, 0

( )Y

Yp Y X Y

(2.43)

Hm phn b tch ly CDF (cummulative distribution function) ca hm mt xc

sut Gamma-Gamma PDF c th c vit dng gn ng.

Hm phn phi tch ly CDF c lin kt vi phn b Gamma-Gamma c vit

thnh:

0( ) ( )

sin ( ) ( ) ( )

TI

P I I p I dIT

2

( )( ; 1, 1; )

( 1)

TTl

IF I

2

( )( ; 1, 1; )

( 1)

TTl

IF I

(2.48)

13

S phn b log-normal khng th m t hiu ng nhp nhy trong ch nhiu lon

mnh. Trong khi , thit lp m hnh knh, mt phn b Gamma-Gamma c s

dng cho m hnh fading trong kh quyn. Trong trng hp ny, PDF ca ha c cho l:

/2

12

2( ) ( ) 2

( )ah a a af h h K h

(2.51)

Trong (.)K l hm Bessel ci tin loi hai, v 1/ v 1/ l phng sai ca cc

nhiu lon quy m nh v ln tng ng. Ngi ta chng minh rng PDF Gamma-

Gamma ph hp vi cc php o di nhiu iu kin nhiu lon.

Kt lun chng 2 2.4

Ni dung chng 2 trnh by khi nim v m hnh gii tch ca cc tham s

ng truyn FSO cng nh RF. Trong , FSO bao gm suy hao ng truyn, nhiu lon

khng kh yu, mnh v s lch hng. RF bao gm suy hao knh truyn v fading. Da

trn m hnh knh nu chng 2, vic phn tch, nh gi hiu nng ca h thng lai ghp

RF/FSO di nh hng ca nhiu lon v lch hng s c trnh by trong chng 3.

14

CHNG 3: NH GI HIU NNG H THNG TRUYN THNG

KHNG DY LAI GHP RF/FSO

3.1 Gii thiu v h thng lai ghp RF/FSO

Trong khi nhu cu v mng khng dy RF tip tc tng v cha p ng c nhu

cu ca ngi s dng, cng ngh FSO bt u ni ln nh mt gii php ti u cho vin

thng. Tuy nhin, mt trong nhng hn ch ln ca cng ngh FSO l i hi duy tr tm

nhn thng (LOS), v tuyn FSO b nh hng mnh bi iu kin thi tit bt li nh

sng m v tuyt ri nng. Nhng c tnh b sung ca RF v FSO thc y nghin cu h

thng lai ghp RF/FSO, trong nhng im yu ca tng loi lin kt s c gim thiu.

H thng truyn thng khng dy lai ghp RF/FSO c xem nh mt gii php ti u cho

vic truyn d liu tc bit cao qua mi trng nhiu lon mnh v thi tit xu [8].

Chng ny s trnh by v hiu nng ca h thng ny vi cc m hnh knh thch hp v

tnh ton xc sut di ngng (dng) cng nh t l li bit (BER).

M hnh knh Rayleigh s c s dng cho tuyn RF trong mi trng c nhiu i

tng lm phn tn cc tn hiu v tuyn trc khi n n b thu. M hnh knh phn b

Gamma-Gamma thch hp nht cho vic m hnh ha cc bc x ca tuyn FSO trong mi

mi trng t nhiu lon mnh ti yu.

3.2 M hnh h thng

Hiu nng ca lin kt RF/FSO bt i xng vi gi nh rng tuyn RF v tuyn FSO

c fading Rayleigh v fading Gamma-Gamma. K thut chuyn tip dng c ch khuch

i v chuyn tip (AF) vi h s khuch i c nh s c xem xt.

Da trn k thut chuyn tip, h thng c thit k bng cch s dng tuyn FSO ti

mt chng v mt tuyn RF ti chng khc. Nt ngun S kt ni vi im n, nt D, thng

qua mt nt chuyn tip R nh trong hnh 3.1 [8].

Hnh 3.1: M hnh h thng lai ghp RF/FSO

15

Vi 1P l cng sut pht t ngun S. Nu nt chuyn tip c khuch i c nh,

ta c th t 2

01

1C

G N , SNR c vit li thnh:

1 2

2 C

(3.6)

Vi 2

1 11

01

h P

N v

2 2

2

02

I

N

ln lt l SNR ca tuyn RF v FSO.

3.2.1 M hnh knh lin kt RF

Fading Rayleigh l mt m hnh c s dng khi trong mi trng c nhiu i

tng lm phn tn cc tn hiu v tuyn trc khi n n b thu. V vy, gi nh rng

tuyn lin kt S-R (RF) c phn b Rayleigh [6] vi hm mt xc sut (PDF):

1

1

1 1

1expf

(3.7)

Vi 1 l SNR trung bnh ca tuyn RF.

3.3.2 M hnh knh lin kt FSO

Trong lun vn ny, m hnh ha tuyn FSO trong iu kin nhiu lon mnh v

va, ta dng phn b fading Gamma Gamma, hm PDF ca n c cho bi [4], [6]:

2 1

22

2 , 0I

I

IIf I K I

I I

(3.8)

Mt m hnh c ngha cho vic c lng cc tham s bin ng c xut bi

Hufnagel v Stanley. Biu thc ton hc ca n nh sau:

2 1 30 0expnC h K h h h (3.14)

Vi 0K l ch s nhiu lon, ph thuc vo a im lp t tuyn quang khng dy c th,

h l cao.

Mt m hnh khc cho vic c tnh cc thng s nhiu lon kh quyn, 2

nC , l m

16

hnh SLC ban ngy v ban m, ch ph thuc vo chiu cao ni tuyn FSO hot ng.

Lu rng cc gi tr c s dng ch yu ca alpha v beta ( = 4,2, = 1,4) cho

nhiu lon mnh v ( = 4, = 1,9) cho nhiu lon va vi mt bc sng khong 785 nm

v mt phm vi trong khong t 20 m n 5 km.

3.3 M hnh ton hc nh gi hiu nng ca h thng

3.3.1 M hnh ton hc tnh ton xc sut di ngng (dng)

Xc sut di ngng c nh ngha l [8]:

1 2min ,out r thP P (3.17)

Vi th l ngng SNR, 1 v 2 ln lt l SNR tc thi ca tuyn FSO v RF, 1 v 2

c lp vi nhau.

Cng thc 3.17 c th c vit thnh [7]:

2

1 22 2

20

out r thP P f dC

(3.18)

Ta c:

21

1

21 22 1

2 2 10

1 exp

Cth

th

r th

CP f d

C

(3.19)

Xc sut di ngng (dng) c vit:

2

4

1 2

1 exp thout thP E

(3.20)

Vi tch phn ca thE c vit thnh:

4 1 22221 20

exp 2ththC

E K d

(3.21)

3.3.2 T l li bit trung bnh

Tng t, cc knh RF v FSO c gi nh vi phn b fading Rayleigh v

17

Gamma-Gamma. Nu coi P (e | ) biu th xc sut li bit c iu kin trong mt knh

AWGN, xc sut li trung bnh c th c th hin nh sau [8]:

0

eP P e f d

(3.22)

Xc sut li c iu kin c cho bi:

P e Q (3.23)

Vi l 2 v 1 ln lt cho iu ch BPSK v QPSK. Th 3.23 vo 3.22 ta c:

(3.24)

y, F l hm phn phi tch ly (CDF) ca bin . t 2x t , t l li trung

bnh c vit li thnh:

0

1

2 2e

x dxP F

x

(3.25)

Theo nh cng thc 3.20, s dng hm CDF v thay bin th bng ta c xc sut

di ngng c vit thnh:

2

4

1 2

1 exp thF E

(3.26)

Thay cng thc 3.26 vo 3.25, ng thi i bin x

, t l li trung bnh c

vit rng ra thnh:

2

4

10 2

11 exp exp

22 2

the

x x dxP E

x

1 2, ,L H H

(3.27)

2 2 2

0 0

1 1exp exp

2 22 2e

t t tP f dtd F dt

18

3.3.3 Kho st hiu nng h thng khi c s lch hng

SNR ca h thng khi c lch hng

Hm PDF ca SNR c vit thnh:

2

23,0 2

2 1,3

2

2 12, ,

f G

(3.31)

Vi v l thng s fading ca nhiu lon kh quyn, cc thng s ny cng thp th

nhiu lon kh quyn cng mnh. l t l gia bn knh chm tia tng ng vi lch

chun (jitter) ca my thu.

Xc sut di ngng (dng) ca h thng khi c lch hng

Xc sut di ngng khi c s lch hng c vit thnh:

16,0

1 1,6

21 1

1 exp thout thKB

P A GK

(3.32)

Vi:

2 2

1 2

22

1

2

1 11; , , , , ,0

2 2 2 2 2 2

2;

8 16

K K

CA B

Trong trng hp khng c s lch hng, 0 , ta s c outP c tnh nh cng

thc (3.20).

T l li bit (BER) ca h thng khi c lch hng

T cng thc 3.32, ta vit c hm CDF ca t l li bit (BER) thnh:

16,01 1,621 1

1 exp th thKB

F A GK

(3.33)

Vi:

2

1

2

2

12

1 1, , , , ,0

2 2 2 2 2

K

K

2

2

2

21 8

16

A

CB

19

PDF ca BER trong gn ng c vit di dng hm Meijers G thnh:

15,0 2 6,01 1 0,5 1 1,63 21 1 1 1

exp 2 22

th

KA B Bf G G

K K

(3.34)

Vi 31 1

, , , ,02 2 2 2

K

Trong trng hp khng c s lch hng, BER trung bnh ca h thng l:

0

eP P e f d

(3.35)

Pe cng c th vit theo mt cch khc [9, Eq 12]:

1

0

exp2

pp

e

qP q F d

p

(3.36)

Thay cng thc 3.33 vo cng thc 3.35, dng [1, Eq. (7.813.1)], ta c BER trung

bnh ca cc phng php iu ch nh phn c vit thnh:

1

2

1

6,11

2,6

11

1 ,1

2 12 1

p

e thp

p KK

Aq p BP G

qq

(3.37)

Vi p v q l cc thng s ca cc kiu iu ch nh phn, c th hin trong bng 3.1.

3.4 Cc kt qu kho st

Phn ny trnh by cc kt qu kho st xc sut di ngng, t l li bt (BER) v

s lch hng ca h thng lai ghp RF/FSO [10].

20

3.4.1 Xc sut di ngng

Hnh 3.2: Xc sut di ngng ca h thng trong iu kin nhiu lon mnh v va vi th = 0

dB [10].

Hnh 3.2 kho st xc sut di ngng ca h thng trong phn b Gamma-Gamma

vi = 4,2 v = 1,4 cho nhiu lon mnh, = 4 v = 1,9 cho nhiu lon va phi

cho thy c cc gi tr SNR khc nhau [10].

21

3.4.2 T s li bit (BER)

Hnh 3.7: So snh BER gia hai iu ch BPSK v QPSK trong nhiu lon mnh ( = 4,2 v =

1,4) vi C = 1 [10].

Hnh 3.7 kho st BER trung bnh ca h thng trong mi trng nhiu lon mnh

vi iu ch BPSK v QPSK. Ta c th thy rng, trong iu kin nhiu lon mnh, BER

trung bnh vi iu ch BPSK l tt hn rt nhiu so vi iu ch QPSK, c bit khi SNR

trung bnh cng tng cao th s chch lch cng r rt [10].

3.4.3 Kho st h thng trong nh hng ca lch hng

Xc sut di ngng

22

Hnh 3.8: Xc sut di ngng ca h thng trong nhiu lon mnh ( = 4,2 v = 1,4) vi

khc nhau v 10th dB [10].

Hnh 3.8 kho st xc sut di ngng ca h thng trong nhiu lon mnh v c

lch hng. Ta c th thy khi tng ln th tc ng ca s lch hng gim xung, outP

gim v ngc li. Khi tin ti 1, tc ng ca s lch hng ln h thng l rt ln, c

bit khi SNR ln hn 15 dB [10].

T l li bit (BER)

y, ta la chn iu ch BPSK 0,5; 1p q kho st BER trung bnh ca

h thng trong nhiu lon mnh.

23

Hnh 3.9: BER trung bnh khi c lch hng trong mi trng nhiu lon mnh ( = 4,2 v

= 1,4) vi khc nhau v C = 1 [10].

Hnh 3.9 kho st BER trung bnh ca h thng bng iu ch BPSK. Tc ng ca s

lch hng c quan st thy l s lch hng (gi tr ca tng, tc ng ca s lch

hng gim) tng, BER gim v ngc li [10].

3.5 Kt lun chng 3

Chng ny tm hiu v phn tch hiu nng h thng lai ghp RF/FSO qua mi

trng nhiu lon khng kh v lch hng. Cc kt qu tnh ton t l li bit (BER) v xc

sut di ngng (dng) khi khng c lch hng v c lch hng l kh quan, cho thy

tim nng rt ln ca h thng ny.

24

KT LUN

Lun vn trnh by mt cch tng quan nht v h thng truyn thng quang

khng dy lai ghp RF/FSO, cc u im v thch thc i vi ca h thng. Trnh by cc

nguyn nhn chnh lm suy gim hiu nng ca h thng, cng nh cc m hnh knh c

bn nh gi hiu nng h thng trong cc iu kin nhiu lon khc nhau.

Khi nim v m hnh gii tch ca cc tham s ng truyn FSO cng nh RF

c trnh by. Trong , FSO bao gm suy hao ng truyn, nhiu lon khng kh yu,

mnh v s lch hng. RF bao gm suy hao knh truyn v fading. Da trn cc m hnh

knh c nu ra, m hnh ton hc tnh ton hiu nng ca h thng lai ghp RF/FSO di

nh hng ca nhiu lon v lch hng cng c phn tch v trnh by, y l phn

trng tm ca lun vn.

Lun vn cng tm hiu, tham kho v phn tch cc kt qu tnh ton t l li bit,

xc xut di ngng v nh hng ca lch hng ln hiu nng h thng. Cc kt qu

cho thy c nhng s liu kh quan trong vic ci thin hiu nng h thng trong cc iu

kin nhiu lon khc nhau, cng nh cc phng php iu ch thch hp cho h thng ny.

Kt qu cho thy tim nng ln ca h thng lai ghp ny trong vic s dng lin kt FSO

b sung cho h thng RF trong tng lai.