HC VIN CNG NGH BU CHNH VIN THNG

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

TH MINH QU

K THUT CHUYN TIP (AMPLIFY AND

FORWARD) CA H THNG TRUYN THNG

A CHNG

CHUYN NGNH: K THUT VIN THNG

M s: 60.52.02.08

TM TT LUN VN THC S

H NI - NM 2013

Lun vn c hon thnh ti:

HC VIN CNG NGH BU CHNH VIN THNG

Ngi hng dn khoa hc: GS.TSKH. NGUYN NGC SAN

Phn bin 1:.

Phn bin 2:.

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:.gi..ngy.thng.nm..

C th tm hiu lun vn ti:

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

1

M U

Truyn thng d liu qua mng v tuyn l mt lnh vc ang dn nng cao c v

phng din k thut v tnh ng dng. y chnh l mi nhn trong ngnh thng tin v

truyn thng hin ti v trong tng lai. Tuy nhin, vic truyn dn thng tin qua cc knh

v tuyn khng c m bo chc chn bi nhiu l do nh thi tit, a hnh. Trong thc

t, tn hiu c truyn t my pht ti my thu theo nhiu ng khc nhau gy ra hin

tng thng ging ngu nhin v bin , pha v gc ti ca tn hiu thu c, hin tng

ny c gi l fading a ng. nh hng ca fading a ng ti cht lng truyn tn

hiu l rt ln. Vn ny nhn c rt nhiu s quan tm nghin cu v nhiu phng

php khc nhau c a ra hn ch nh hng ca fading ny nh s dng k thut

phn tp, MIMOTuy nhin, vi mi phng php u c tn ti nhng khuyt im.

Lun vn ny s trnh by v mt phng php khc lm gim nh hng ca

fading l k thut truyn thng a chng (Multi-hop Communication), y l mt k

thut kh mi. tng chnh ca k thut ny l s chia nh ng truyn gia node ngun

v node ch bng cch s dng cc node trung gian gia (relay) chuyn tip tn hiu.

Node relay ngoi nhim v chuyn tip tn hiu n cn lm nhim v khuch i v truyn,

gii m v truyn m rng phm vi ph sng, tng cht lng ca h thng. y cng

l vn ng c quan tm v nghin cu.

Vi l do trn v mc tiu cp nht nhng cng ngh, k thut mi nhm phc v

cho vic nghin cu, ging dy nn em chn ti K thut chuyn tip (Amplify and

Forward) ca h thng truyn thng a chng lm lun vn tt nghip. Tuy nhin, do

gii hn v mt thi gian v cn hn ch v mt kin thc nn phm vi ti ch nghin cu

v mt im trung chuyn c nh trong mng truyn thng a chng.

Phng php nghin cu ca lun vn

- Kho st cc nghin cu, ti liu lin quan thu thp thng tin v c s l thuyt

lin quan n ni dung nghin cu ca ti.

- Nghin cu l thuyt v k thut chuyn tip

- Xy dng m hnh h thng hai chng v a chng, nh gi cc tham s ca h thng.

Lun vn c mt s ng gp ni bt nh sau:

- Nghin cu v mng truyn thng a chng v cc phng php chuyn tip tn

hiu ti node relay

2

- Xy dng c hai m hnh: Truyn thng hai chng v truyn thng a chng.

- Tnh ton h s khuch i trong tng m hnh v tc c th t c ti a

trong mng chuyn tip AF

- Thit lp h thng m phng v kt qu m phng bng s

Theo nhng phng hng nh trn, ni dung ca lun vn c chia lm ba

chng nh sau:

Chng 1: Tng qut v h thng truyn thng a chng. Trong chng ny, lun

vn cp ti mt s yu t lin quan n cht lng ng truyn v tuyn v tp trung

ch yu vo phn tch mt s m hnh truyn thng a chng (hai chng, ba chng, 4

chng). So snh u nhc im ca phng php ny vi phng php truyn trc tip

(n chng).

Chng 2: K thut chuyn tip ca h thng truyn thng a chng. Trong chng

ny lun vn gii thiu v k thut chuyn tip, cc loi chuyn tip, yu cu ca node

trung gian trn ng truyn. Trong chng ny, lun vn cng ch yu tp trung vo k

thut khuch i v chuyn tip ti node relay ca h thng truyn thng hai chng, a

chng.

Chng 3: Kt qu m phng. Trong chng ny lun vn thit lp h thng m

phng (h thng truyn thng 2 chng, a chng). ng vi mi h thng lun vn a ra

kt qu m phng (bng s) chng minh u im ca phng php khuch i v chuyn

tip trong truyn thng a chng.

3

CHNG I: TNG QUT V H THNG

TRUYN THNG A CHNG

1.1 Gii thiu chng

Truyn dn a chng l mt s kt hp ca cc lin kt truyn dn ngn c th m

rng phm vi ph sng ca mng bng cch s dng thit b chuyn tip trung gian gia

my pht v my thu. Vic s dng truyn dn chuyn tip c nhiu u im, quan trng

nht l cng sut pht yu cu ca c hai pha pht v thu. Trong chng ny, lun vn s

phn tch u v nhc im ca mng a chng so vi n chng v mt s loi m hnh

truyn d liu qua mt s chng trung gian. Ngoi ra, s phn tch hiu qu ca cng sut

pht khi thay i s chng truyn dn v tuyn t ngun n ch trong mi trng khng

gian t do.

1.2 Nhng yu t lin quan n cht lng ng truyn

nh gi kh nng hot ng ca mt h thng thng tin v tuyn v m t chnh

xc knh truyn khng dy th phi xc nh c c tnh mi trng truyn. Sau y l

tm tt nhng yu t chnh c lin quan n cht lng ng truyn.

1.2.1 Hin tng fading

1.2.2 Cc yu t nh hng n fainh a ng

1.2.3 Xc sut thng k fainh

1.2.4 Nhiu giao thoa lin k t (ISI)

1.2.5 Phn tp

- Phn tp thi gian

- Phn tp tn s:

- Phn tp khng gian

Ngoi ba phng php phn tp c bn trn th mt gii php hiu qu l s dng

k thut truyn thng a chng. y l mt k thut mi trong truyn thng khng dy, cho

php ngi dng c th hot ng nh mt trm chuyn tip h tr truyn tn hiu n

ngi khc. My chuyn tip (my relay) cng vi my ngun hp tc truyn tn hiu n

my ch, to nn mt mng anten o mc d mi my ch c mt anten. Mng anten o

ny tng t nh mng anten vt l, c th gim nh hng ca hin tng fading v cho

4

php my thu thu c nhiu bn sao ca tn hiu t cc ng khc nhau tng t nh khi

my c nhiu anten.

1.3 Gii thiu v phng php truyn thng a chng

Truyn thng a chng l mt phng php hiu qu thit lp kt ni gia cc node

mng khi m truyn thng theo ng truyn trc tip l khng kh thi hoc do hiu sut

ca cng sut khng ti u. Trong truyn thng a chng, d liu truyn t ngun ti ch

tng ng s c gip bi mt s lng nht nh cc node trung gian. So vi truyn

thng mt chng, truyn thng a chng c hng t li knh truyn.

Trong h thng ny, tn hiu t node ngun s c truyn n node ch thng qua

mt s node trung gian (relay)

Hnh 1.6 M hnh mng truyn thng a chng

Mt mng truyn thng a chng gm c ba thnh phn c bn l:

- Trm gc BS (Base Station): C kh nng tr gip, giao tip vi nhiu im chuyn

tip nn BS cn c tn l MR (multihop relay)

- Trm chuyn tip RS (Relay Station): Nhn thng tin t trm gc a ti v truyn

ti ch SS (subscriber Station)

- Trm ch MS (Mobile Station)

1.3.1 M hnh h thng truyn trc tip n chng

Mng v tuyn di ng hin ti (nh GSM, CDMA, v IEEE 802.16) hot ng theo

cu trc lin kt im-a im, trong ch tn ti hai v ch hai loi thc th mng, l trm

gc (BS) v trm di ng (MS).

5

Hnh 1.7: M hnh truyn thng n chng

Trong m hnh truyn dn trc tip (n chng), ta c cng sut ca my thu trong

mi trng khng gian t do nh sau:

TxRx P

DP

2

4

(

)

(1.3)

Trong l bc sng truyn dn, D l khong cch t ngun n ch, PTx l

cng sut pht, PRx l cng sut thu. Trong qu trnh truyn dn tn hiu b suy hao:

(

)

(1.4)

T cng thc (1.4) ta c c cng sut pht nh sau:

RxTx PDP

2

24

(1.5)

t (

)

ta c c

Nh vy ta c cng sut pht ca tuyn truyn dn n chng nh sau:

(1.6)

1.3.2 Cc loi m hnh truyn d liu a chng

Mng chuyn tip a chng l mt s kt hp ca cc lin kt ngn c th ph

sng mt khu vc rng ln bng cch s dng thit b chuyn tip trung gian gia trm gc

(BS) v my thu (MS)

Hnh 1.9: M hnh truyn dn qua 2 chng, 3 chng, 4 chng

D Source Destination

da =D/2 Source Destination

db =D/2

D/3 D/3 D/3

D/4 D/4 D/4 D/4

6

Vi m hnh mng hai chng th cng sut pht c m t bi biu thc sau:

SecHopTxFirstHopTxHopTx PPP __2_ (1.7)

khi

th

(

)

(1.10)

(

)

(1.11)

Nh vy ta c cng sut mng ca mng hai chng c m t bi biu thc sau:

(

)

(1.12)

Trong trng hp tng qut, khong cch truyn dn c chia thnh n chng bng

nhau, mi chng c khong cch l

. T (1.12), ta xc nh c cng sut cho tuyn

truyn dn a chng nh sau

n

n

DKPnP RxMHTx 1;.

2

_ (1.13)

1.3.3 c trng c bn ca phng php truyn a chng

Mng cm bin v tuyn bao gm nhiu nt trung gian, cc nt trung gian ny c

xem l nh, gi thnh thp, phc tp khng cao v tiu th t nng lng. Trong phn

ny lun vn s trnh by c tnh ca nt ngun, nt trung gian ca h thng truyn thng

a chng.

Hnh sau l s khi m t cng sut tiu th ca nt trung gian.

Hnh 1.12: Cng sut tiu th ca nt trung gian

Mch in c cung cp mt cng sut l Pd, cng sut ny b tiu tan mt phn Pi

bi ti v suy hao thun ty trong cc b khuch i. H s ca mt b khuch i l

7

(1.15)

Trong l cng sut tn hiu nhn c t knh truyn a ti v h s khuch

i . Chng ta c th c tnh cng sut cung cp l

( ) (1.16)

T ta thy rng G ln hn rt nhiu so vi . Do cng sut tiu th Pc ti nt

trung gian l:

(1.17)

Mc nhiu my thu

Nhiu l thnh phn khng th trnh khi h thng truyn thng v tuyn. n

gin ha trong lun vn ny ch trnh by nhiu mt nt. Sau y l s khi n gin

in hnh cho mt nt.

Hnh 1.13: S khi ca nt chuyn tip trong pha u tin

u tin tn hiu cao tn nhn c t anten c a vo b lc v c khuch i

bi b khuch i tp m (LNA Low Noise Amplifier). Tn hiu sau khi khuch i c

lm sch v c a vo b trn. Tn hiu sau khi trn li c a vo b khuch i

trung gian (IFA intermediate frequency amplifier)

1.4 So snh phng php truyn n chng vi phng php truyn a chng.

1.4.1 u im ca mng a chng

+ Gim cng sut pht :

+ Tng dung lng h thng:

+ Dch v tc d liu cao hn:

+ Cn bng lu lng ti:

+ Gim hiu ng nt c chai:

+ M rng vng ph sng ca h thng:

8

+ Ci thin tin cy nh tuyn:

1.4.2 Nhc im ca mng a chng

+ H thng phc tp:

+ Bo mt km:

+ Thch thc th tc AAA

+ Tr

1.5 Kt lun chng

Trong chng 1, chng ta nghin cu mt cch tng qut v nhng yu t lin

quan n cht lng ng truyn trong mi trng truyn dn v tuyn. khc phc

nhng yu t trn v t c nhng yu cu v thng lng, phm vi ph sng rng

ln, cng nh vic cung cp cht lng dch v tt hn th trong chng 1 a ra mt

phng php truyn thng mi l truyn thng a chng. Trong chng ny cng nu

ra cc c trng c bn ca phng php truyn thng a chng v so snh c phng

php truyn thng a chng vi phng php truyn trc tip truyn thng.

K thut truyn thng a chng s dng nt chuyn tip chia ng truyn (vng

ph sng) ra thnh nhiu chng nh. K thut ny c nhiu u im nh: gim cng sut

pht, m rng vng ph sng ca mng, tng thng lng h thng lm r vn ny

th mt trong nhng yu cu ca node trung gian trn ng truyn l khuch i v chuyn

tip. hiu r hn v k thut chuyn tip, cc loi chuyn tip, k thut khuch i v

chuyn tip ti node trung gian trn ng truyn th vic xy dng m hnh truyn thng

a chng v tnh ton cc thng s l cn thit. Vn ny s c trnh by tip theo trong

chng II.

9

CHNG II: K THUT CHUYN TIP CA H THNG

TRUYN THNG A CHNG

2.1 Gii thiu chng

Thnh phn quan trng trong mng a chng l nt chuyn tip (Relay). c ci nhn

tng qut v v k thut chuyn tip, to tin cho vic phn tch hot ng ca nt

chuyn tip trong mng a chng. Trong chng ny s gii thiu khi nim c bn v nt

chuyn tip, nhng phng php chuyn tip thng c s dng trong mng truyn thng

a chng. Ngoi ra, trong chng cng s phn tch mt s loi chuyn tip v gii thiu hai

loi m hnh truyn thng hai chng v a chng.

2.2 Gii thiu v k thut chuyn tip

Vi mng truyn thng, vic tng cng dung lng, m rng phm vi ph sng v

gim chi ph vn hnh l mc tiu ca cc nh qun l mng vin thng. Mt s k thut

nng cao dung lng v m rng phm vi ph sng c gii thiu nh gii php a

anten, truyn dn a im phi hp. K thut chuyn tip c gii thiu l mt cng ngh

tin tin p ng v tha mn c nhng yu cu ny.

MS

Direct

link

Relay link

Access link

MS

BS

Relay

2.3 Cc loi chuyn tip

C hai loi chuyn tip c nh ngha trong tiu chun 3GPP LTE-Advanced l

chuyn tip loi 1 (Type-I) v loi 2 (Type-II). Trong WiMAX, hai loi ny c gi tng

ng l chuyn tip khng trong sut (Non Transparent Mode) v chuyn tip trong sut

(Transparent Mode).

Hnh 2.1 Minh ha k thut chuyn tip

10

Type-I

BS

MSMS

RelayRelay

Type-II

Hnh 2.3 Chuyn tip loi I v loi II

2.4 Yu cu ca node trung gian trn ng truyn

2.4.1 Khuch i v chuyn tip AF (Amplify and Forward) Non regenerative

Relay

Hnh 2.4: Phng php chuyn tip Amplify and Forward

K thut ny c s dng trong trng hp thi gian tnh ton hoc cng sut vn

c ca trm chuyn tip b gii hn, hay c thi gian tr hon. Trm chuyn tip nhn c

tn hiu b suy hao v cn phi khuch i ln trc khi truyn tip.

2.4.2 Gii m v truyn DF (Decode and Forward)- Regenerative Relay

11

Hnh 2.5: Phng php chuyn tip Decode and Forward

Phng php ny dng trong vic truyn tn hiu s. Tn hiu nhn c u tin

c gii m v sau m ha, v vy nhiu khng c khuch i trong tn hiu nhn

c. (c ti to li tn hiu: transparent mode)

2.4.3 Gii m, khuch i v truyn (Decode, Amplify and Forward)

u tin, nt chuyn tip s gii m tn hiu nhn c t node ngun sau m ha

li ri mi thc hin khuch i v truyn n ch. Phng php ny n gin v tr x

l thp, nhng khng th trnh c li lan truyn. Mt khc tn hiu c c tnh v

truyn ti ch

Hnh 2.6: Phng php Decode, Amplify and Forward

2.5 K thut khuch i v chuyn tip AF ca h thng truyn thng hai chng

Chuyn tip trong h thng khng ti sinh li c phn ra lm 2 loi l chuyn tip

h tr thng tin trng thi knh (CSI) v chuyn tip m (blind relay). H thng khng ti

sinh vi chuyn tip h tr thng tin trng thi knh s dng CSI tc thi ca chng u

tin kim sot li c a ra bi nt chuyn tip v khi phc cng sut truyn li

ca tn hiu. Ngc li, cc h thng vi chuyn tip m khng cn CSI tc thi, nhng s

dng b khuch i vi li c nh v do tn hiu c cng sut thay i ng ra ca

12

nt chuyn tip. H thng vi cc loi chuyn tip m nh vy c th khng vn hnh tt

bng cc h thng c trang b chuyn tip h tr CSI, nhng phc tp thp v d dng

trin khai khin chng tr nn hp dn hn t gc nhn thc t.

2.5.1 H thng hai chng khng hp tc vi nt chuyn tip c nh

2.5.1.1 M hnh h thng

Tn hiu c truyn t node ngun ti node ch thng qua mt node chuyn tip

relay. Gi s rng ti node ngun truyn mt tn hiu s vi mt cng sut trung bnh

node relay c th nhn c, tn hiu nhn c ti node relay c th c vit nh sau:

(2.1)

Hnh 2.7: M hnh h thng 2 chng khng hp tc

Trong : n1 l nhiu AGWN vi cng sut N01 u ra ca relay

l bin fading ca knh truyn t node ngun ti node relay, v

tun theo hm mt xc sut nh sau:

( )

(

) (2.2)

Vi l cng sut fading trung bnh chng th i v l nhiu AWGN vi

mt ph cng sut N01. Tn hiu nhn c ti node relay sau c khuch i ri

truyn ti node ch. Tn hiu nhn c ti node ch c biu din nh sau:

( ) (2.3)

Trong l bin fading ca knh gia node relay vi node ch v l nhiu

AWGN vi hm mt ph cng sut N02 u vo node ch. T biu thc (2.3) ta c

biu thc tc thi SNR node ch nh sau:

(2.4)

13

R rng t phng trnh trn cho thy rng vic chn li G s xc nh c t s

tn hiu trn nhiu SNR t u cui ti u cui ca h thng hai chng.

Xt trng hp: Node relay l loi h tr trng thi knh thng tin CSI

Khi li G l:

(2.5)

Trong : l cng sut tn hiu pht ng ra ca nt chuyn tip. Vic la chn

li ny nhm mc ch o ngc hiu ng fading ca knh u tin, v gii hn cng sut

ng ra ca chuyn tip nu bin b fading ca chng u tin l thp. Thay (2.5) vo

(2.4) ta c c:

(2.6)

Trong iii NhE 02

1 , (vi i = 1, 2) l SNR mi chng.

Nu ch xem xt hiu sut ca h thng khng ti sinh vi nt chuyn tip m th loi

chuyn tip ny s c li c nh vi tn hiu thu m khng quan tm n bin fading

chng u tin.

t )( 012

2 NGEC , cng thc (2.4) c th vit li nh sau:

2

21

2

Ceq

(2.7)

Vi C l hng s ca li c nh G. Hai chng c gi nh l c lp nhng

khng nht thit c phn b fading Rayleigh ging nhau.

2.5.2 H thng hai chng hp tc vi nt chuyn tip c nh

2.5.2.1 M hnh h thng

Xt m hnh h thng v tuyn hai chng hp tc nh hnh 2.8. Trm ch (D) lin

lc vi trm ngun (S) thng qua nt chuyn tip (R). Ngoi ra, trm ch (D) cn c

nhn tn hiu trc tip t trm pht (S). Nt chuyn tip l c s dng l nt chuyn tip

AF vi li c nh.

14

Hnh 2.8 M hnh h thng hai chng hp tc

Thng tin gia trm ngun (S) v ch (D) lin lc vi nhau thng qua knh chu nh

hng bi Rayleigh fading vi h s hS,D, nt chuyn tip chia tuyn truyn dn gia ngun

v ch ra thnh hai chng vi h s Rayleigh fading tng ng l hS,R v hR,D. Vi gi nh

rng, nhiu AWGN trn 3 tuyn (S-D, S-R, R-D) c mt ph cng sut l N0 v h s

fading (hS,D, hS,R , hR,D) l c lp vi nhau.

Vic truyn tn hiu din ra trong 2 pha do nt chuyn tip khng th truyn v thu tn

hiu ng thi.

+ Trong pha th nht, tn hiu t ngun c pht qung b ti ch v ti relay. Tn

hiu nhn c ch v relay nh sau:

(2.23)

(2.24)

Trong : P1 l cng sut pht ti ngun

hs,r v hs,d l h s knh gia nt ngun vi nt chuyn tip, gia nt ngun vi

nt ch v c phng sai l:

ns,r v ns,d l nhiu Gauss cng sinh v c phng sai l

+ Trong pha th hai: Node relay khuch i tn hiu nhn c t node ngun v

truyn ti node ch vi cng sut P2. Tn hiu nhn c node ch t relay a ti l:

| |

(2.25)

Thay biu thc trn vo ta c

| |

(2.26)

15

Vi

| |

(2.27)

Gi s rng l hai bin ngu nhin c lp v c phng sai

l: ( | |

| |

)

Nt ch nhn c hai bn sao ca tn hiu x qua hai ng: (S D) v (S-R-D). C

nhiu cch khc nhau kt hp hai tn hiu bn sao , chng ta quan tm ti phng php

MRC. T s tn hiu trn nhiu ti u ra ca b kt hp MRC l bng tng cc t s tn

hiu nhn c trn nhiu trn mi ng truyn. Gi s rng cng sut truyn P = P1 + P2.

Trong phng thc AF th node relay ngoi nhim v khuch i tn hiu t node

ngun a ti, n cn khuch i c tn hiu nhiu. Thnh phn nhiu trong pha 2 c

phng sai l: ( | |

| |

) . Vi nhng h s knh bit hs,d, hs,r, hs,d tn hiu ti

u ra ca b kt hp MRC nt ch c th c vit nh sau:

(2.28)

Nhng h s kt hp a1 v a2 nn c chn sao cho kt hp c SNR cc i:

v

| |

( | |

| |

)

(2.29)

Gi s tn hiu pht t ngun x c nng lng trung bnh l 1, SNR ti u ra MRC

l:

Trong : | | (2.30)

| | | |

| | | |

(2.31)

Khi ta c biu thc SNR cho hai chng l:

(2.32)

2.6 K thut khuch i v chuyn tip AF ca h thng truyn thng a chng

2.6.1. M hnh h thng

Xt h thng truyn thng v tuyn nh hnh 2.9. Tn hiu t ngun lan truyn qua n

chng trc khi n ch. Trong hn l bin fading ca chng th n, Gn l li ca

nt chuyn tip th n. Vi m hnh ny, h thng s dng loi chuyn tip khng ti sinh.

16

Hnh 2.9: M hnh h thng a chng

Trong trng hp trng thi knh thng tin CSI, ti nt chuyn tip c li G nh sau:

(2.40)

Trong : l nng lng ti ng ra ca nt chuyn tip th n v N0 l tn hiu

nhiu trng Gaussian ti ng vo ca mi nt chuyn tip.

Ta c SNR ton h thng c cho bi:

* (

) +

(2.41)

Nu trong h thng s dng loi chuyn tip m th

(2.45)

Trong l hng s, s dng hai phng trnh trn th biu thc biu thc t s tn

hiu trn nhiu t u cui ti u cui SNR u ra ca node ch c vit li nh sau:

(

)

*

+

(2.46)

Trong :

l SNR tc thi ca chng th j

2.7 Kt lun chng

Trong chng ny, lun vn a ra nhng khi nim c bn v k thut chuyn

tip, cc loi chuyn tip v cc yu cu ca node trung gian trn ng truyn ca mng

truyn thng a chng. phn tch c mng truyn thng a chng th da trn c s l

mng truyn thng hai chng. Tuy nhin trong chng ny, lun vn ch trnh by v k

thut khuch i v chuyn tip trong mi m hnh truyn thng.

S D

R1 R2 Rn-2 Rn-1 h

h

h

17

CHNG III: KT QU M PHNG

3.1 Gii thiu chng

Trong chng trc, lun vn trnh by k thut khuch i v chuyn tip AF ca

h thng truyn thng hai chng v truyn thng a chng. nhn nhn r rng hn v xc

thc mt s vn m l thuyt cp, chng ny s gii thiu chng trnh m phng

minh ha mt cch trc quan h s khuch i ca h thng vi biu nh gi BER so

vi SNR. Thng qua phn m phng nh gi cht lng h thng hai chng, a chng

vi nt chuyn tip c nh. Vic m phng c thc hin thng qua nh gi xc sut li

bt (BER) v xc sut rt ca h thng hai chng, a chng vi cc mc ngng 0 khc

nhau trn knh Rayleigh fading vi phng thc iu ch BPSK. Qua nh gi c s

ci thin BER ca h thng hai chng hp tc so vi ng truyn trc tip.

3.2 Thit lp h thng m phng

3.2.1 H thng truyn thng hai chng khng hp tc

3.2.2 H thng truyn thng qua hai chng hp tc

3.2.3 H thng truyn thng a chng

3.3 Kt qu m phng (bng s)

3.3.1 i vi h thng truyn thng qua hai chng khng hp tc

Hnh 3.5 m t xc sut u ra ca h thng hai chng vi mt nt chuyn tip AF

hot ng 2 ch khc nhau:

Khi nt chuyn tip c li c nh:

(

)

Khi nt chuyn tip c li bin thin:

hai ch trn th trong trng hp khi nt chuyn tip c li bin thin c

hiu sut tt hn. Tuy nhin khong cch chnh lch v hiu sut gia hai ch trn l

khng nhiu so vi phc tp ca relay c li bin thin. Thm ch rng i khi h

thng s dng relay c li c nh hot ng hiu qu hn trng hp relay c li

bin thin. iu ny ph thuc vo h s relay bin thin khi l qu nh. Ch

rng khi tng th phm vi t s tn hiu trn nhiu trung bnh trong trng hp h thng

c li c nh tt hn trong trng hp h thng c li bin thin.

18

Hnh 3.6 l so snh t l li bt trung bnh ca h thng hai chng khi s dng relay

c li c nh v li bin thin, tuy nhin lnh lch trong hai trng hp ny l

nh.

Hnh 3.5: Xc sut u ra ca h thng hai chng vi vic s dng relay hot ng ch

khc nhau

Hnh 3.6: T l bt li trung bnh ca h thng hai chng vi vic s dng relay hot ng

ch khc nhau

19

3.3.2 i vi h thng truyn thng qua hai chng hp tc

+ nh gi BER h thng hai chng hp tc:

Kt qu m phng hnh 3.7 cho thy t l li bit vi cc gi tr khc nhau ca 0 .

Kt qu cho thy BER ca h thng hai chng vi nt chuyn tip hp tc tt hn so vi

ng truyn trc tip. Khi mc ngng 0 tng t l li bt s c ci thin hn. S d c

c iu ny l do h thng t c li ch t c ch phn tp hp tc. Ngoi ta, kt qu

cng cho thy rng SNR cao hiu nng li ca c ch chuyn tip hp tc l c xu hng

song song vi ng truyn trc tip, iu ny cho thy h thng t c li ca mng

ng ten o nhng khng c li phn tp. Nguyn nhn l do ti SNR cao, ch s t khi

cn n s h tr ca vic truyn tn hiu t nt chuyn tip.

Hnh 3.7 BER cho h thng hai chng hp tc vi nt chuyn tip AF

0 5 10 15 20 25 3010

-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Es/No(dB)

BE

R

BER for dual hop with Cooperative Relay

Direct Transmission

Dual-hop g0=2.71

Dual-hop g0=4.77

Dual-hop g0=6.92

Dual-hop g0=9.09

20

+ nh gi xc sut rt h thng hai chng hp tc

Hnh 3.8: Xc sut rt cho h thng hai chng hp tc vi nt chuyn tip AF

3.3.3 i vi h thng truyn thng a chng

M hnh m phng BER h thng a chng (m hnh tuyn tnh) c s dng hnh

(3.4). Nt chuyn tip c s dng l nt AF. Gi nh cc terminal l c nh, khong

cch t ngun n ch l d. H thng a chng s dng )1( N nt chuyn tip chia h

thng thnh N chng c khong cch bng nhau )/( Nd .

Vi h thng N chng, tn hiu sau khi qua nt chuyn tip u tin c tip tc

chuyn n nt th 2, v nt chuyn tip th N-1, hnh thnh h thng N chng. Sau mi

nt chuyn tip, ta tnh ton SNR v BER. Sau tnh ton t s BER sau mi chng v so

snh vi ng trc tip.

Cc kt qu m phng c thc hin vi ngun vo l 1 triu bit ngu nhin, s dng

iu ch BPSK, QPSK cho cc knh Rayleigh fading. Nt chuyn tip c s dng l AF.

0 5 10 15 20 25 3010

-6

10-5

10-4

10-3

10-2

10-1

100

Es/No(dB)

Pout

Pout Dual-hop Cooperative with Amplify and Forward Relay

gamma0=2.71

gamma0=4.77

gamma0=6.92

gamma0=9.09

21

+ iu ch BPSK

Hnh 3.10 BER cho h thng a chng trn knh Rayleigh fading s dng phng

php BPSK

+ iu ch QPSK

Hnh 3.11 BER cho h thng a chng trn knh Rayleigh fading s dng phng

php QPSK

0 5 10 15 20 25 30 3510

-4

10-3

10-2

10-1

100

SNR per Hop

Bit

Err

or

Rate

BER for BPSK - Multihop over Rayleigh channel

2 Hops

3 Hops

4 Hops

5 Hops

6 Hops

Direct Link

0 5 10 15 20 25 30 3510

-4

10-3

10-2

10-1

100

SNR per Hop

Bit E

rror

Rate

BER - QPSK for multihop over Rayleigh channel

2 Hops

3 Hops

4 Hops

5 Hops

6 Hops

Direct Link

22

3.4 Nhng bn lun (trn c s kt qu)

M phng vi vic b sung thm nhiu chng vo h thng, theo kt qu m phng

trong hnh (3.10) v (3.11) ta thy rng, vi h thng 6 chng vi 5 nt chuyn tip cho

BER tt hn ng truyn trc tip. Ngoi ra, t l BER sau mi chng thm vo l tng so

vi chng trc nhng t l tng khng ging nhau, hay ni cch khc BER tng sau mi

chng l c hiu ng gim dn. Nguyn nhn tng BER l do h thng s dng nt chuyn

tip AF, nt chuyn tip s khuch i tn hiu v nhiu t knh u tin.

3.5 Kt lun chng

Chng ny trnh by kt qu m phng nh gi h thng hai chng hp tc v h

thng a chng mi m hnh tuyn tnh s dng vi nt chuyn tip AF. Kt qu m phng

vi cho thy c nh hng ca vic tng s chng ln t l li bit ca h thng. Vi h

thng hai chng hp tc, xc sut li bt c ci thin hn so vi truyn dn trc tip, nh

c ch phn tp hp tc gia ch v nt chuyn tip. Vi h thng a chng khng hp tc,

BER ca h thng a chng vi nt chuyn tip l tt hn so vi ng truyn trc tip.

Tuy nhin, hiu nng li sau mi chng thm vo l tng ln do nh hng ca fading v

nhiu. Vi m hnh ny. BER khng tng u m c hiu ng gim dn sau mi chng thm

vo.

23

KT LUN V KIN NGH

1. Tm tt kt qu nghin cu

Trong thng tin v tuyn, k thut truyn thng a chng vi nt chuyn tip l mt

cng ngh mi em li nhiu u im v li ch cho ngnh vin thng trong tng lai, c

th l p dng trong h thng thng tin di ng 4G. Ti cc khu vc ra cell, ni m tn hiu

thu c c SNR rt thp, h thng mng 4G khng p ng c nhu cu truyn ti d

liu tc cao, trong khi vic s dng k thut chuyn tip c th p ng c yu cu

v tc truyn ti d liu cho thu bao ti khu vc ra cell, m rng vng ph sng ca h

thng mng 4G.

Lun vn phn tch khi qut nhng u nhc im ca h thng a chng so vi

n chng, cng vi vic a ra mt s m hnh truyn d liu qua 2 chng, 3 chng, 4

chng. Ngoi ra lun vn cng trnh by c cc loi chuyn tip trung gian trn ng

truyn v yu cu ca node trung gian trn ng truyn.

nh gi cht lng ca mng truyn thng a chng, lun vn phn tch k

thut khuch i v chuyn tip ti node chuyn tip c nh trn ng truyn da trn m

hnh h thng ca mng hai chng v h thng a chng. Qua nh gi c SNR h

thng, t l li bit, xc sut rt ca h thng.

Kt qu thc nghim cho thy rng h thng a chng vi s h tr ca nt chuyn

tip l cn thit m rng vng sng, tng cng cht lng tn hiu vng ra ca cell.

Tuy nhin, lun vn cng cho thy rng khng phi lc no vic m rng thm nhiu chng

cng ph hp. Do cn phi cn bng gia vic m rng vng ph sng vi hiu nng li

ca h thng.

2. Hng nghin cu tip theo

Nghin cu ny em li kt qu v nhng ng gp nht nh v h thng truyn

thng a chng. Tuy nhin, vic nh cht lng ca h thng hai chng ch mi thc hin

vi mt nt chuyn tip c nh. Ngoi ra, vi h thng a chng, mi ch dng trng

hp cha c phn tp hp tc cng nh h thng a chng c nhiu nhnh, my thu nhn

c tn hiu t nhiu nhnh. Do cha nh gi ht nhng u im ca mng a chng.

nh gi c u im ca mng a chng, cn pht trin nghin cu thm nhng vn

sau:

- Vi h thng hai chng, phn tch nh gi cht lng mng vi s h tr ca nhiu

nt chuyn tip hay nhiu nhnh chuyn tip.

24

- Vi h thng a chng cn phn tch m hnh a chng c s hp tc ca nhiu

nhnh, nhiu nt chuyn tip.

- Nghin cu bi ton khi cc node relay trong mng truyn thng a chng c kt

ni vi nhau theo cu trc hnh li.

- K thut chuyn tip (Decode and forward) ca h thng truyn thng a chng.