Nhieu Trong Thong Tin Di Dong

8/8/2019 Nhieu Trong Thong Tin Di Dong

1/37

http://www.ebook.edu.vn 1

MC LCMC LC.................................................................................................... 1LI MU.............................................................................................. 21.Nhiu trng................................................................................................ 3

1.1. Khi nim v nhiu trng................................................................... 31.2. Cc php biu din ton hc ca nhiu trng..................................... 41.3. Ph cng sut ca nhiu trng c bng tn gii hn: ......................... 4

2. Nhiu lin k t ISI (Inter symbol interference) ...................................... 62.2. Cc bin php khc phc nhiu ISI.................................................... 7

2.2.1. B lc cos nng: .......................................................................... 72.2.2. B lc ngang p khng: ............................................................... 9

3. Nhiu lin knh ICI (Interchannel Interference)..................................... 11

3.1. Nhiu lin knh ICI.......................................................................... 113.2.Nhiu xuyn knh trong OFDM knh thng tin di ng................ 123.2.1. Gii thiu.................................................................................. 123.2.2. OFDM knh thay i theo thi gian....................................... 133.2.3. Phn tch nhiu xuyn knh (ICI) .............................................. 153.2.4 Kt lun. ..................................................................................... 19

3.3.ISI v ICI trong h thng FDM......................................................... 194. Nhiu ng knh (Co-Channel Interference) ......................................... 23

4.1. Khi nim chung.............................................................................. 234.2.Ti s dng tn s ............................................................................. 24

4.2.1.Khi nim ti s dng tn s ...................................................... 244.3.Cc mu ti s dng tn s ............................................................... 29

4.3.1. Mu 3/9...................................................................................... 294.3.2.Mu 4/12..................................................................................... 304.3.3.Mu 7/21..................................................................................... 31

4.3. Nhn xt........................................................................................... 314.3.1. So snh gia cc mu s dng tn s: ....................................... 314.3.2. Dung lng v t s C/I. ............................................................ 31

5. Nhiu a truy nhp (Multiple Access Interference)................................ 32

KT LUN................................................................................................ 37


2/37


LI MUNhiu l mt vn rt quan trng trong thng tin di ng, nh

hng n cht lng ca tn hiu, khi x l tn hiu v khi truyn tn hiu

tn hiu lm gy mo tn hiu hoc xut hin cc tp m trong cc thit b

ti to li tn hiu

V vy ta phi gim st c chng v tm bin php khc phc tn

hiu nhiu n mc ti a tng cht lng ca ca tn hiu.

Bi tiu lun di y s trnh by mt cch ngn gn l thuyt v 05

loi nhiu trong thng tin di ng. Nm vng l thuyt v nhiu v tm

hiu, nghin cu cc bin php khc phc chng s gip mng thng tin di

ng nng cao cht lng, hiu qu.

B cc ca bi tiu lun gm c 05 phn, tng ng vi 05 loi

nhiu, l:

1.Nhiu trng ( White Gaussian Noise)

2.Nhiu xuyn m ( Intersymbol Interference)3.Nhiu xuyn knh ( Interchannel Interference)

4.Nhiu ng knh ( Cochannel Interference)

5.Nhiu a truy nhp ( Multiple access Interference).

Nhm chng ti xin chn thnh cm n thy Nguyn Vn c

hng dn, gip chng ti hon thnh bi tiu lun ny.


3/37


1.Nhiu trng

1.1. Khi nim v nhiu trngNhiu trng l qu trnh xc xut c mt ph cng sut phng(

khng i ) trn ton b di tn

Nhiu trng l mt loi nhiu c hm mt xc sut tun theo phn

b Gauss.

Nhiu trng c th do nhiu ngun khc nhau gy ra nh thi tit,do

b khuch i my thu,do nhit ,hay do con ngi.Tn hiu thu do vy

c vit li nh sau:

y(t)=x(t)*h()+n(t)

h( )H(j )

+

n(t)

y(t)x(t)

Tn hiu pht

M hnh knh

Tn hiu thu

Hinh 1. Mi trung truyn dn vi sc mt ca nhiu trng


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1.2. Cc php biu din ton hc ca nhiu trngV mt ton hc,ngun nhiu trng n(t) c th m hnh bng mt

bin xc sut Gauss vi gi tr k vng =0 v lch chun l 2

=E[x]=0

2 =E[(x- )2]

Hnh 2. Hm phn bGauss

Do k vng bng khng nn lch chun cng bng phng sai ca

bin ngu nhin x. C th hn nhiu trng c cng sut khng i 2

1.3. Ph cng sut ca nhiu trng c bng tn gii hn:V mt l thuyt ,nhiu trng c bng tn v hn v cng sut nhiu

l u n mi tn s .V mt thc t khng c h no c bng tn v

hn m b gii hn mt bng tn no . Do vy mt ph cng sut

ca nhiu cng b gii hn nhhnh 3


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Hnh 3. Mt phcng sut nhiu

hnh trn ta gi s l h thng c bng tn gii hn B = 2g vi

chu k ly mu l ta.. Mt ph cng sut ca nhiu nh hnh1 c vit

li nh sau:

Tt c cc bin ngu nhin u khng tn ti php bin i Fourier

m ch tn ti hm t tng quan v hm mt cng sut,trong hm

mt cng sut l php bin i Furie ca hm t tng quan. phng

trnh trn nn(j) l hm mt cng sut nhiu cn nn() l hm t tng

quan ca nhiu vi nh ngha :

nn() = E[n(t)n(t+ )] = 2

si g

Theo phng trnh hm t tng quan l bin i Furie ngc ca

hm mt ph cng sut.Do hm mt ph cng sut c dng hnh ch

nht nhhnh 1, kt qu bin i Furie ngc ca hm hnh ch nht

cho ta hm s Si().

Cng sut ca nhiu c th tnh c bng c t hm mt cng

sut nhiu hoc hm t tng quan ca nhiu nh sau:

Pn=E[n2(t)] = nn(0) =1/2 +

nn(j)d = 2 t


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Khi t s tn hiu trn tp m c tnh theo cng thc sau:

SNR=Ps/Pn

Vi Ps l cng sut tn hiu c ch.T s ny quyt nh cht lng

tn hiu v dung lng knh

2. Nhiu lin k t ISI (Inter symbol interference)

Trong mi trng truyn dn a ng, nhiu xuyn k t (ISI) gy

bi tn hiu phn x c thi gian tr khc nhau t cc hng khc nhau t

pht n thu l iu khng th trnhkhi. nh hng ny s lm bin dng

hon ton mu tn hiu khin bn thu khng th khi phc li c tn hiugc ban u. Cc k thut s dng tri ph trc tip DS-CDMA nh trong

chun 802.11b rt d bnh hng bi nhiu a ng v thi gian tr c

th vt qu khong thi gian ca mt k t. OFDM s dng k thut

truyn song song nhiu bng tn con nn ko di thi gian truyn mt k t

ln nhiu ln. Ngoi ra, OFDM cn chn thm mt khong bo v (guard

interval - GI), thng ln hn thi gian tr ti a ca knh truyn, gia hai

Hnh 4. M hnh gy nhiu


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k t nn nhiu ISI c th b loi b hon ton.2.1. c im:Di thng

tuyt i ca cc xung nhiu mc nh phng l v hn. Nu cc xung ny

c lc khng ng khi chng truyn qua mt h thng thng tin th

chng s tri ra trn min thi gian v xung cho mi k hiu s chn vo cc

khe thi gian bn cnh gy ra nhiu gia cc k hiu (ISI).ISI l hin tng

nhiu lin k hiu. ISI xy ra do hiu ng a ng, trong mt tn hiu

ti sau s gy nh hng ln k hiu trc .

Chng hn nh hnh 4 pha trn, ta thy r tn hiu phn x

(reflection) n my thu theo ng truyn di hn so vi cc tn hiu cn

li.

2.2. Cc bin php khc phc nhiu ISITrong cc h thng n sng mang, ISI l mt vn kh nan gii.

L do l rng bng tn t l nghch vi khong thi gian k hiu, do vy,

nu mun tng tc truyn d liu trong cc h thng ny, tc l gim

khong k hiu, v hnh chung lm tng mc tri tr tng i. Lc ny

h thng rt nhy vi tri tr. V vic thm khong bo v kh trit tiu ht

ISI.

gim nhiu xuyn m ngi ta phi lm th no hn ch di thng

m vn khng gy ra ISI. Khi di thng b gii hn, xung s c nh trn

thay v nh phng.

Mt trong nhng phng php loi b nhiu ISI l dng b lc

cos nng v b lc ngang p khng (phng php Nyquist I).

2.2.1. B lc cos nng:

Hnh 5. S b lc cos nng

Ngun s To xung Lc pht Lc thu Nhn tinQuytnh

mk (t-kT) T( ) n(t) R( ) kak

s(t) s(t)


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Tn hiu t ngun gm c M phn t, song chng ta hn ch ch

kho st trng hp khi cc phn t si(t) ca tp tn hiu ch khc nhau v

bin , tc l ta s hn ch ch xt h thng iu ch bin xung PAM.

Thc t h thng ny c th xem nh gn cho mi mt tin mk mt hng s

akm bin ca xung u ra ca b to xung sc nhn vi n.

Ta hy gi s rng b to xung cho ra cc xung Dirc ti cc thi

im t=kTs. Cc xung dng dirac ny, c bin thay i tu theo s thay

i cc gi tr mk, qua b lc T() s ti knh truyn. Phn my thu trn

hnh 2.2 l my thu ti u, thu lc phi hp, mch quyt nh thc hin ly

mu v so ngng. Hm truyn tng cng ca h thng (c tnh tn s

tng cng ca h thng) l tch ca hai c tnh ca hai b lc pht v thu

C()=T().R(). By gichng ta s tm kim lp cc c tnh lc C()

sao cho vic truyn chui tn hiu qua h thng s khng c ISI. Vic

truyn c coi l khng c ISI nu vo thi im quyt nh tn hiu ly

mu th k, ch c phn ng xung ca tn hiu th k l khc khng cn phn

ng ca cc tn hiu khc u bng khng.

Theo nh l Nyquist, rng bng tn truyn dn nh nht c th

truyn c khng mo tn hiu bng gc l B=1/2.T. rng bng y

c ngha l di tn m ngoi n gi tr hm truyn ng nht bng khng.

Tn s 1/2T c gi l tn s Nyquist. Do vy chng ta s xt cc c tnh

lc c rng thng tn ti thiu l 1/2T (hay /T tnh theo tn s gc).

Trc tin ta hy xem xt trng h p C() l c tnh ca b lc thng

thp l tng, tc l p tuyn pha ca b lc th tuyn tnh cn p tuyn

bin |C()| c dng:

( )

>

=

0

0

;0

0;1

C


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B lc ny c phn ng xung l:

t

t

tc0

0sin

)( =

C gi tr cc i bng 1 ti t=0 v c gi tr bng 0 to t=k/0.

Gi s rng u vo b lc l tng ny c tn hiu c to bi b

to xung nh trn hnh 2.2, tc l tn hiu li vo b lc T()c cho bi:

( )kTtak =

=

-k

s(t)

Trong trng hp ny, phn ng xung u ra s khng gy nn ISI

nu tn s ct ca b lc l f0=0/2=1/2T.

Do n gin trong tnh ton, hm s cong dng cosine thng a

c s dng phn tch cc b lc ny. Hm truyn tng cng khi c

dng:

V phn ng xung c dng:

222 /41/cos

//sin)(

TtTt

TtTttc

=

Hm truyn lin tc th c bin gn sng suy gim theo lu tha 3

ca bin t. Do vy ngay c khi ng b khng l tng th gi tr ca phn

ng xung u ra ca cc b lc ny s b chn. Do , ISI s nh ngay c

khi ng b khng l tng.

2.2.2. B lc ngang p khng:


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Theo hnh 6 ta c p ng tn s ca ton h thng t pht n thu l

H0(f)= HT(f). Hc(f)HE(f)

Vi p ng xung tng hp

h0(t)= f-1

[H0(f) ]

tho mn iu kin khng c nhiu lin k hiu ISI th

constT

kfH =

+0

Tn s ly mu tn hiu bn thu l 1/T. Theo th mt cn bng

l tng zero- ISI n gin l mt b lc nghch o p ng tn s ca bn

pht v knh truyn. B lc o ny thng c xp x bi mt b lc FIRnh hnh v di

C-N CN

D

C0

D

C-N+1

D D

tm

Pcq(t)

Hnh 7. B lc cn bng knh

Ngun sliu

B lcpht

B lc cnbng knh

Knh

truyn

tm=mT+t

V(tm)

V(t)

Hnh 6. V tr b lc cn bng knh.


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p ng xung ca b lc cn bng knh l:

( )nTtCN

N

nE = =

-n

(t)h

p ng tn s tng ng l:

nTfjN

N

nE eC2

-n

(f)H

==

Vn ca b lc o chnh l la chn cc h s ca b lc sao cho

xp xc iu kin zero- ISI.Trong mi trng truyn dn a ng, nhiu xuyn k t (ISI) gy

bi tn hiu phn x c thi gian tr khc nhau t cc hng khc nhau t

pht n thu l iu khng th trnh khi. nh hng ny s lm bin dng

hon ton mu tn hiu khin bn thu khng th khi phc li c tn hiu

gc ban u. Cc k thut s dng tri ph trc tip DS-CDMA nh

trong chun 802.11b rt d bnh hng bi nhiu a ng v thi gian tr

c th vt qu khong thi gian ca mt k t. OFDM s dng k thut

truyn song song nhiu bng tn con nn ko di thi gian truyn mt k t

ln nhiu ln. Ngoi ra, OFDM cn chn thm mt khong bo v (guard

interval - GI), thng ln hn thi gian tr ti a ca knh truyn, gia hai

k t nn nhiu ISI c th b loi b hon ton.

3. Nhiu lin knh ICI (Interchannel Interference)3.1. Nhiu lin knh ICI

Nhiu xuyn knh gy ra do cc thit b pht trn cc knh lin nhau


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Hnh 8

Nhiu lin knh thng xy ra do tn hiu truyn trn knh v tuyn

b dch tn gy can nhiu sang cc knh k n.

loi b nhiu xuyn knh ngi ta phi c khong bo v (guard

band) gia cc di tn.

3.2.Nhiu xuyn knh trong OFDMknh thng tin ding

3.2.1. Gii thiu

Trong phng php a truy nh p phn chia theo tn s trc giao

(OFDM), bng thng truyn c chia thnh nhiu knh nh, v c

truyn song song vi nhau. Do , gii hn ca k t tng ln v nhiu lin

k t (ISI) gy ra mi trng fading theo thi gian b loi b. Tuy nhin,

vi nhng gii hn k t di hn, nhiu xuyn knh (ICI) gy ra bi

Dopplerknh thng tin di ng li tng ln. Hiu ng Doppler c nh

hng n h thng OFDM. y, chng ta nhn c gii hn ca ICI,

tnh ton d dng hn v hu ch hn. Gii hn bao gm c gii hn chung

v ring. Gii hn chung ch ph thuc vo tn s Doppler ln nht ( df ) v


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thi gian k t ( sT). Gii hn ring cng ph thuc vo bin ca ph

Doppler.

3.2.2. OFDM knh thay i theo thi gianGi s tn hiu OFDM min thi gian l

( ) [ ] 2 kj f t k

x t s k e = vi 0 st T (3.1)

Vi 0kf f k f = + l tn s ca k knh con (subchannel) v1

s

fT

= .

Tn hiu truyn qua k (knh con), s[k], c coi l c lp vi cc knh con

khc. Tn hiu nhn c, sau khi truyn qua knh thay i theo thi gianvi p ng xung ( , )h t , l:

( ) ( ) ( )~

,x t h t x t d = Vi nhiu knh (channel noise) v nhiu ng knh (co-channel

interference) khng tnh ti.

Trong iu kin fading phng, p ng xung knh c tht l:

( ) ( ) ( ),h t t =

Tn hiu nhn c s l:

( )t l qu trnh wide-sense stationary stochastic vi dy s 0 v bin n

v. Vi ph Doppler nguyn thy, mt ph ca ( )t l:

( )2

1 1if

11

0

d

d

J

d

f ffP f

f


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( )

1if

2

0

d

du

f ffP f


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3.2.3. Phn tch nhiu xuyn knh (ICI)

Tn hiu c iu ch li c th bao gm c tn hiu nhn c ( )~

t l:

[ ] ( )~ ~

2

0

1ss

m

T

j f t

s

m x t e dt T

= (3.4)

n gin, s hi nhp (intergration) s dng thay th hm chuyn i

Fourier ri rc (DFT). Bi vy, kt qu s bao gm c phn a vo trng

hp s song mang khng xc nh. Tuy nhin, so snh vi trng hp s

sng mang xc nh th s sai khc l khng ng k.

a.Tnh ton chnh xc

n gin, u tin chng ta nhn c bin ICI cho knh fading

phng, v sau mrng kt qu cho mi trng c tn s bt k khc. T

(3.1), (3.2) v (3.4), tn hiu c iu ch li trthnh:

[ ] ( ) [ ]~

2 2

0

1s

s

k m

T

j f t j f t

ks

m t s k e e dt T

=

( ) [ ]2 ( )

0

1 s m kT

j f f t

k s

t e dt s k T

=

[ ] [ ]0 m km k

ICI

a s m a s k

= + 14243

(3.5)

Vi la c nh ngha l:

( ) 2

0

1 sT j t ft l

s

a t e dt T

= (3.6)

0a biu din s suy gim v pha ca tn hiu mong mun, cn vi biu

din h s khuch i knh ca tn hiu nhiu. Vi thi gian bt k trn

knh, 0la , v ra kt qu ca ICI.

Nng lng ca ICI c nh ngha l:


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[ ]2

0

ICI l

l

P E a s m l

=

Vi OFDM c s sng mang con khng xc nh, ICIP c th biu

din thng qua hm tng quan ( )r l:

( ) ( )( )1

1

1 1ICI sP x r T x dx

= (3.7)

Hoc thng qua mt ph Doppler:

( ) ( )

21 sind

d

f

ICI sf

P P f c fT df

=

(3.8)

C (7) v (8) u cho OFDM vi s sng mang con xc nh, ta c

th thy ch c sai s rt nh, ta c th dng cho OFDM vi s sng mang

xc nh. Do , cng thc (7) v (8) c th dung tnh gn chnh xc nng

lng ICI cho OFDM vi s sng mang xc nh.

Bit mi hm tng quan trong min thi gian ca knh thay i theo

thi gian, ta c th tnh nng lng ca ICI dng cng thc (3.7). Biu dintheo kiu thng thng:

( ) ( )1

0

1

1 1 2ICI d sP x J f T x dx

= (3.9)

Biu din theo kiu ng dng v kiu 2 ng:

( )

2

1 os(2 ) 2 (2 )1

2

d s d s d sICI

d s

c f T f T Si f T P

f T

= (3.10)

V ( )21 sinICI d sP c f T = (3.11)

b. Gii hn

S dng cc cng thc pha trn, nng lng ICI c th tnh chnh

xc. Tuy nhin, cc cng thc tnh chnh xc kh phc t p v khng d


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dng. Hn na, trong nhiu trng h p, hm tng quan theo min thi

gian chnh xc hay ph cng sut l khng th tnh. y, chng ta nhn

c gii hn trn v di v gii hn chung ca cng sut ICI. Nhng gii

hn ny t phc tp hn v d tnh ton hn.

u tin, a 1 ( )sr T x (3.7) vo ph cng sut, P(f), ta c:

( )( )21 ( ) 1d

s

d

f

j fT x

s

f

r T x P f e df

= ( ) ( )( )0

2 1 os 2df

sP f c fT x df =

D thy

2 4 21 1 11 os2 24 2

c

Hn na,

( )2 211 ( ) 2

2s d sr T x f T x

Vi i=1, 2 ..c nh ngha l

2 2

2 2

0

1 2( ) ( )

d d

d

f f

k k

k k k

d df

f P f df f P f df

f f

= =

Hng s1 v 2 d dng tnh ton v a ra kt qu nh bng 1.

Thay vo bt ng thc (7), chng ta tnh c gii hn ca cng sut ICI.

( ) ( )2 41 22 2

12 360ICI d s d sP f T f T

(3.12)

V ( )21 2

12ICI d sP f T

(3.13)

Nu khng bit ph Doppler, 1 v 2 cng c th tnh uowcj thng

qua cch tnh gn ng khc. V d, c th chng minh nh sau:

( ) ( ){ } ( ) ( )2 2 22

d

d

fkk k

f

E t f P f df

=

Vi ( ) ( )( )kkk

d tt

dt

= . Suy ra:


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( ) ( ){ }( )

2

22

k

k k

d

E t

f

=

Cch tnh c phn n gin hn khi lin quan n ph Doppler.

Bng 1Tnh ngha 1 , r rng 1 1 . Thc t, cng vi (3.13), ta c th tnh gii

hn trn ca cng sut ICI, ch ph thuc vo d sf T ,

( )21 2

12ICI d sP f T

(3.14)

Gii hn trn c th dung cho h thng OFDM vi ph Doppler, bao

gm h thng OFDM vi tn s off-set. Bi v 1 thng nh hn 1 nn

gii hn trn rng hn gii hn (3.13). Tuy nhin, (3.14) d tnh ton hn

v ch ph thuc d sf T . Vi dng hai dng, 1 1 = v gii hn chung cng l

gii hn hp.

Nh ni trn, chng ta c gii hn chung v ring cho knh

fading phng. h thng OFDM knh fading thi gian phn tn, thng th

ta s gi s rng phn m rng theo chu k c thm vo gia k t

OFDM trnh nhiu xuyn khi. Vi gi thuyt , ta s c

( )2

2

0

siTj l f

j l ft

l i

i s

ea t e dt

T

=


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Bi v ( )i t c lp vi I v c cng thng k, { }2lE a ging vi c knh

phng v knh fading phn tn. Hn na, cng thc chnh xc tnh ICIP v

gii hn khc cng c th tnh knh phn tn.

3.2.4 Kt lun.

i su vo tnh chnh xc cc gii hn, chng ta so snh gii hn

trn v gii hn di v gii hn chung vi gi tr chnh xc ca cng sut

ICI ( ICIP ). Ch rng vi kiu hai ng, gii hn trn v di l xc nh.

Gii hn ring rt gn tnh chnh xc ICIP . Khi thit k h thng OFDM,

nu chu k ca k t, Ts, phi c chn d sf T rt nh, do nh hng

ca ph Doppler s khng ng k.

3.3.ISI v ICI trong h thng FDMTrong h thng a sng mang (FDM), vn va ng v fading

la tn c gi quyt. Tn hiu c chia thnh N dng song song v truyn

trn N sng mang con vi tc nh hn. tit kim bng thng, ngi

ta a vo h thng OFDM, trong cc sng mang con l trc giao ln

nhau, tc l cho php cc sng mang con ny chng ph, nh vy tng c

hiu qu s dng ph.


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Hnh 9

Cc sng mang con c di tn hp, khong k hiu di hn nn tra

ng tng i l nh. Do vy, kh hon ton nhiu do a ng,

ngi ta thm vo k hiuOFDM mt khong bo v. Khong ny c th l

khong trng. Tuy nhin, mt vn mi li ny sinh. l hin tngnhiu lin knh (ICI) gia cc sng mang con ca OFDM. Nguyn nhn

vn hiu ng a ng, cng thm v khong bo v trng.

ICI sinh ra do s chng ph gia cc sng mang, gy ra xuyn m.

Trong cc h thng n sng mang, ICI thng xut hin cc h thng

lm vic ti cc di tn k nhau. Trong cc h thng ny, ngi ta thng

a vo gia c di tn lm vic mt khong ph nhm trnh xuyn m.

Trong h thng OFDM ni trn, ph cc sng mang con vn

chng ln nhau, tuy nhin vn khng gy xuyn m, l do cc sng

mang con l trc giao nhau theo ngha ton hc. S trc giao ny s mt i

nu trong khong k hiu ( chnh xc hn l trong khong tch phn FFT) c


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mt (hay nhiu) sng mang con khng tn ti chnh xc mt s nguyn ln

chu k.

Hnh 10

Khi , ngi ta thay khong bo v trng bng khong bo v lp,

tc l mrng chu k mu tn hiu

Hnh 11


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H thng OFDM do vy rt c k vng cho cc ng dng a hnh

phc t p, nh cc thnh ph (vi nhiu nh cao tng) hay vng nng

thn(nhiu i ni). Theo mnh bit th kthut ny hin c trong cc chun

pht thanh s (DAB), truyn hnh s (DVB), IEEE802.11a,g v

IEEE802.16 vi tc truyn d liu rt hp dn, c th ln ti 54Mbps

thay v 11Mbps nh trong CDMA. Tuy nhin, vn trin khai h thng

ny cn gp kh nhiu kh khn, ch yu l do vn chi ph lp t, v

hin ti cng khng c nhiu ng dng i hi tc truyn d liu qu

cao, cn vi cc ng dng thp hn, cc h thng hin nay qu

cung cp dch v.

Nhiu la chn tn s cng l mt vn gy nh hng ln n

cht lng truyn thng tn hiu. Tuy nhin, OFDM cng mm do hn

CDMA khi gii quyt vn ny. OFDM c th khi phc li knh truyn

thng qua tn hiu dn ng (Pilot) c truyn i cng vi dng tn hiu

thng tin. Ngoi ra, i vi cc knh con suy gim nghim trng v tn s

th OFDM cn c mt la chn na gim t l li bit l gim bt s btm ho cho mt tn hiu iu ch ti tn s.

Do vy, OFDM khng phi khng c nhc im, l n i hi

kht khe v vn ng b v s sai lch v tn s, nh hng ca hiu ng

Doppler khi di chuyn v lch pha s gy ra nhiu giao thoa tn s

(Intercarrier interference - ICI) m kt qu l ph b s trc giao gia cc

tn s sng mang v lm tng t s bt li (BER). Tuy nhin OFDM cng

c th gim bt s phc tp ca vn ng b thng qua khong bo v

(GI). S dng chui bo v (GI) cho php OFDM c thiu chnh tn s

thch hp mc d vic thm GI cng ng ngha vi vic gim hiu qu s

dng ph tn s. Ngoi ra OFDM chu nh hng ca nhiu xung, c ngha

l mt xung tn hiu nhiu c th tc ng xu n mt chm tn hiu thay

v mt s k t nh trong CDMA v iu ny lm tng t l li bit ca


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OFDM so vi CDMA.

4. Nhiu ng knh (Co-Channel Interference)

4.1. Khi nim chungNhiu ng knh xy ra khi c hai my pht trn cng mt tn s

hoc trn cng mt knh. My thu iu chnh knh ny s thu c c hai

tn hiu vi cng ph thuc vo v tr ca my thu so vi hai my pht.

Hnh 12: Nhiu ng knh

Nhiu ng knh thng g p trong h thng thng tin s cellular,

trong tng hiu sut s dng ph bng cch s dng li tn s. Nh

vy c th coi nhiu ng knh trong h thng cellular l nhiu gy nn do

cc cell s dng cng 1 knh tn s.

Nhiu ng knh lin quan ti vic s dng tn s. C th v d trong

mng GSM:Trong mng GSM, mi trm BTS c cp pht mt nhm tn


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s v tuyn. Cc trm thu pht gc BTS ln cn c cp pht cc nhm

knh v tuyn khng trng vi cc knh ca BTS lin k.

c trng cho loi nhiu ny l t s sng mang trn nhiu (C/I). T

s ny c nh ngha l cng tn hiu mong mun trn cng tn

hiu nhiu sau lc cao tn v n th hin mi quan h gia cng tn

hiu mong mun so vi nhiu ng knh t cc BTS khc.

C/I = 10log (Pc/Pi)

Yu cu l C/I khong cch cc cell cng tn s phi

ln.

4.2.Ti sdng tn s

4.2.1.Khi nim ti s dng tn s

Mng t bo hot ng trn nguyn tc phn chia vng ph sng dch v

thnh cc phn vng hoc t bo, c ring mt tp hp ti nguyn hoc

knh ngi s dng mng truy nhp.


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Thng thng, cc vng ph sng t bo c xy dng theo cu trc

t bo lc gic. Mng t bo b gii hn v bng thng hot ng trn

nguyn l ti s dng tn s. iu ny cho thy, cng mt nhm tn s

c s dng li trong cc t bo m c gi khong cch vi nhau

mt khong c ly ln sao cho khng gy tc hi ln nhau m th hiu

s giao thoa ng knh. i vi mt cu trc t bo lc gic, ta c th co

gn cc t bo thnh cm m bo khng c hai t bo ln cn no dng

chung mt tn s.

Hnh 13: M hnh ti sdng tn s

S dng li tn s l vic cp pht cng mt nhm tn s v tuyn ti

cc v tr a l khc nhau trong mng m khng lm nh hng n cht

lng kt ni ti giao din v tuyn do nhiu ng knh v nhiu knh ln

cn gy nn.

a. Nguyn l ti sdng tn s

Mt h thng t ong lm vic da trn vic s dng li tn s. Nguyn

l cbn khi thit k h thng t ong l cc mu s dng li tn s. Tng

bng thng c trn mng c phn chia gia cc t bo trong mt cm.

Cm ny sau c thc s dng xc nh s cuc gi c thc


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h trtrong mi t bo. Bng vic gim s lng cc t bo trong mt cm,

dung lng ca h thng c th tng ln, v c th c thm nhiu knh hn

trong mi t bo. Tuy nhin mi ln gim kch thc cm s gy nn mt

ln gim khong cch s dng lp tn, do vy, h thng rt c nguy ctr

thnh giao thoa ng knh.

Theo nh ngha s dng li tn s l vic s dng cc knh v tuyn

cng mt tn s mang ph sng cho cc vng a l khc nhau. Cc

vng ny phi cch nhau mt c ly ln mi nhiu giao thoa ng

knh (c th xy ra) chp nhn c. T s sng mang trn nhiu C/I ph

thuc vo v tr tc thi ca thu bao di ng do a hnh khng ng nht,

s lng v kiu tn x. Phn b t s C/I cn thit h thng xc nh s

nhm tn s F m ta c th s dng. Nu ton b s knh quy nh N c

chia thanh F nhm th mi nhm s cha N/F knh. V tng s knh N l c

nh nn s nhm tn s F nh hn s dn n nhiu knh hn mt nhm

v mt i trm. V vy, vic gim s lng cc nhm tn s s cho php

mi i trm tng lu lng nh s gim s lng cc i trm cn thitcho ti lu lng nh trc. Ta bit rng s dng li tn scc cell khc

nhau th b gii hn bi nhiu ng knh C/I gia cc cell nn C/I s l

mt vn chnh cn c quan tm.

D dng thy rng, vi mt kch thc cell nht nh, khong cch s

dng li tn s ph thuc vo s nhm tn s N. Nu N cng ln, khong

cch s dng li tn s cng ln v ngc li.

b. Cc thng s ti sdng tn s

Vic s dng lp tn s c thc xc nh theo phng trnh sau cho

mi kch thc cm t bo:


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Trong :

D l khong cch trung bnh s dng lp tn.

R l bn knh t bo. N l kch ccm.

Hnh 14: Khong cch ti sdng tn s

Cc mu ti s dng tn sc k hiu tng qut : mu M/N

Trong :

M= tng s site / cluster

N = tng s cell / cluster

l H s s dng li tn s: 1/N

=> Mi cell c cp pht 1/N tng s knh tn s v tuyn trong 1 cluster

.

Trong mt mi trng tn x di ng mt t, cng cng sut my thu

c ti mt khong R t thit bj pht lin quan n biu thc sau:


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Trong l mt hng s lin quan ti mi trng mt t,

thng ly bng 4.

i vi mt cu hnh gm 7 t bo s dng lp tn, t s sng mang giao thoa xy ra do mt my mobile trong vng 6 t bo nm trong

khong cch D so vi my mobile s dng lp tn ti thiu, ngha l, bn

ngoi vnh ai th nht ca mu cm t bo s dng lp tn, c cho bi

phng trnh:

= =

q: l tham s suy gim giao thao ng knh, c cho bi phng trnh:

Biu thc trn gi thit rng cng sut pht ra bi tt c cc t bo l

tng ng v s giao thao ng knh tn hiu nhn c t cc t bo

hot ng da trn cng loi tn s ti vnh ai th hai ca cm t bo c

thc lc b. Do vy, i vi , mt mu cm 7 t bo c th cho

mt t s C/I l 18dB. gim thiu hiu ng ca vic giao thoa ng

knh, s dng cc k thut iu khin cng sut ti im u cui mobile

v trm csm bo cht lng dch v Ngoi ra ta cn c cng thc

tnh C/I ti my di ng MS nh sau:

Trong x l h s truyn sng.


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4.3.Cc mu ti sdng tn s

4.3.1. Mu 3/9

Mu ti s dng tn s 3/9 c ngha cc tn s s dng c chia

thnh 9 nhm n nh trong 3 v tr trm gc. Mu ny c khong cch gia

cc i ng knh l D = 5.2R.

Hnh 15

Cc tn smu 3/9

n nh tn s

A1 B1 C1 A2 B2 C2 A3 B3 C3

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 34 35 36

37 38 39 40

Thng thng cm 9 cell c t s C/I khong 9dB. Vi t s ny, cc

my di ng c th hot ng c nhvic GSM cung cp cc phng

php o lng c bit c th lm gim nh hng ca nhiu. Cc


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phng php o lng ny gm nhy tn, iu khin cng sut v truyn

dn gin on.

4.3.2.Mu 4/12Mu s dng li tn s 4/12 c ngha l cc tn s s dng c chia

thnh 12 nhm tn sn nh trong 4 v tr trm gc. Khong cch gia cc

trm ng knh khi D = 6R.

Cc tn smu 4/12.

n nh tn s

A1 B1 C1 A2 B2 C2 A3 B3 C3 A4 B4 C4

1 2 3 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32 33 34 35 36

37 38 39 40

Ta thy mi cell c th phn b cc i l 4 sng mang.

Trong mu 4/12 s lng cc cell D sp xp theo cc cch khc nhau

nhm phc v cho cc cell A, B, C. Hiu qu ca vic iu chnh ny l

m bo hai cell cnh nhau khng s dng hai sng mang lin nhau

(khc vi mu 3/9). Hn na, s dng mu ny cng m bo cc cell s

dng cc sng mang ging nhau c phn cch bi khong cch ti s

dng.Trong phn trc, chng ta bit rng cm 12 cell c t s C/I khong 12dB.

y l t s thch hp i vi h thng GSM v nh vy vic s dng k

vng tn s, iu khin cng sut ng, truyn dn gin on thc cht

khng cn thit.


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4.3.3.Mu 7/21

Mu 7/21 c ngha l cc tn s s dng c chia thnh 21 nhm n

nh trong 7 trm gc. Khong cch gia cc trm ng knh D = 7.9R.

Cc tn smu 7/21: mi cell chc phn b ti a 2 sng mang.

n nh tn s

A

1

B

1

C

1

D

1

E

1

F

1

G

1

A

2

B

2

C

2

D

2

E

2

F

2

G

2

A

3

B

3

C

3

D

3

E

3

F

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

4.3. Nhn xt

4.3.1. So snh gia cc mu s dng tn s:

Khi s nhm tn s tng N/21, N/12, N/9, ngha l s tn s c th

dng cho mt i trm tng th khong cch gia cc i ng knh D s

gim 7.9R, 6R, 5.2R. iu ny ng ngha vi nhiu trong h thng cng s

tng ln. Vi s nhm tn s tng th s thu bao c phc v cng s tngln. Nh vy, vic la chn mu s dng li tn s phi da trn cc c

im a l vng ph sng, mt thu bao ca vng ph sng v tng s

knh N ca mng.

Mu 3/9: s knh trong mt cell l ln, tuy nhin kh nng nhiu cao.

M hnh ny c p dng cho vng c mt my di ng cao.

Mu 4/12: s dng cho nhng vng c mt lu lng trung bnh. Mu 7/21: s dng cho nhng khu vc mt lu lng thp.

4.3.2. Dung lng v t s C/I.

Vi tng s knh m ti nguyn h thng cho php l M knh, nu

chia u cho N nhm knh th s c s knh trong mt nhm knh hay mt

cell l M/N. Ty ta s tnh ton c dung lng phc vng vi cp


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phc v GOS nht nh qua bng Erlang. Nh nhn xt trn, s nhm tn

s cng nh th s lng knh trn mt nhm cng ln v s thu bao c th

c phc v cng cao, ngha l phn nh hiu qu trung k tt hn. Nhng

N nh li cho t s C/I nh, nhiu ng knh tng.

Vi N cho trc, th dung lng trn mt cell s l cnh. Nh

bit, khu vc cell t l thun vi bnh phng bn knh cell. Do vy, mt

dung lng trong mt n v din tch l t l nghch vi khu vc cell. Vy

nu ta chia cell nh c bn knh bng cell c th vi N cho trc dung

lng s tng ln 4 ln. Tuy nhin, trnh nhiu ng knh th khng th

lun s dng cng mt nhm tn s cho cc cell nh v iu nny s l

gim i u im ca vic gim kch ccell s lm gim cht lng.

5. Nhiu a truy nhp (Multiple Access Interference).Nhiu a truy nhp l nhiu do cc tn hiu ca cc user giao thoa vi

nhau,l yu tnh hng trc tip n dung lng ca h thng


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Trong cc h thng a truy nhp:

TTDDMMAA::aa ttrruuyy nnhhpp pphhnn cchhiiaa tthheeoo tthhii ggiiaann

Trong TDMA l s giao thoa ca cc tn hiu khe thi gian ny vi khe

thi gian khc do s khng hon ton ng b gy ra

Ngi ta phi c khong bo v (guard time) gim xc sut ngi dng

b giao thoa nhng cng ng thi lm gim hiu sut s dng ph

FDMA: a truy nhp phn chia theo tn s

Cc hiu ng Doppler lm dch ph tn s dn n c s giao thoa gia cc

di tn con

Guard band gim xc xut giao thoa gia cc knh k nhau =>gim hiu

sut s dng ph

CDMA: a truy nhp phn chia theo m

Trong CDMA ngi ta s dng tnh trc giao ca m nn hu nh khng

c nhiu gia cc user.

DS CDMA: a truy nhp phn chia theo m chui trc tip.

Theo nhng nghin cu gn y, phng thc a truy nhp phn chia theom chui trc ti p DS-CDMA (Direct Sequence Code Division

Multiplexing Access) da vo vic tri ph dng d liu bng cch s dng

mt m tri ph c n nh cho mi ngi s dng trong min thi

gian.Kh nng gim thiu nhiu a truy nh p MAI (Multiple Access

Interference) da vo tnh tng quan cho ca m tri ph. Trong trng

hp truyn a ng i hi rt kht khe ca vin thng di ng, kh nng

phn bit mt tn hiu thnh phn t nhiu thnh phn khc trong tn hiu

thu tng hp c cung cp bi tnh t tng quan ca m tri ph. My

thu RAKE c cha nhiu b tng quan, mi b tng quan c ni vi

mt dng dn c kh nng phn gi khc nhau. V vy hot ng ca h

thng DS-CDMA s ph thuc nhiu vo s lng ngi s dng thc t,

c trng ca knh v s lng cc nhnh c dng trong my thu RAKE.


34/37


35/37



36/37


kh MAI ngi ta thng dng cc phng php trc giao nhng

trn thc t khng th c s trc giao hon ton. Do MAI vn tn ti

trong cc h thng a truy nhp.


37/37

KT LUNTrn y nhm trnh by mt cch cbn 05 loi nhiu trong thng

tin di ng, l

Nhiu trng ( White Gaussian Noise)

Nhiu xuyn m ( Intersymbol Interference)

Nhiu xuyn knh ( Interchannel Interference)Nhiu ng knh ( Cochannel Interference)

Nhiu a truy nhp (Multiple access Interference)

Trc nhu cu ngy cng ln v thng tin lin lc ni chung v thng tin

di ng ni ring, vic nghin cu p dng cc cng ngh tin tin cng

nh tm kim cc gii php mi nhm nng cao v cht lng- dung lng

cng nh cc loi hnh dch v thng tin lin lc c ngha ht sc quan

trng v thit thc.

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