View
222
Download
0
Category
Preview:
Citation preview
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
2/37
http://www.ebook.edu.vn 2
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
3/37
http://www.ebook.edu.vn 3
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
4/37
http://www.ebook.edu.vn 4
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
5/37
http://www.ebook.edu.vn 5
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
6/37
http://www.ebook.edu.vn 6
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
7/37
http://www.ebook.edu.vn 7
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)
8/8/2019 Nhieu Trong Thong Tin Di Dong
8/37
http://www.ebook.edu.vn 8
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
9/37
http://www.ebook.edu.vn 9
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:
8/8/2019 Nhieu Trong Thong Tin Di Dong
10/37
http://www.ebook.edu.vn 10
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
11/37
http://www.ebook.edu.vn 11
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
12/37
http://www.ebook.edu.vn 12
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
13/37
http://www.ebook.edu.vn 13
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
14/37
http://www.ebook.edu.vn 14
( )
1if
2
0
d
du
f ffP f
8/8/2019 Nhieu Trong Thong Tin Di Dong
15/37
http://www.ebook.edu.vn 15
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:
8/8/2019 Nhieu Trong Thong Tin Di Dong
16/37
http://www.ebook.edu.vn 16
[ ]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
8/8/2019 Nhieu Trong Thong Tin Di Dong
17/37
http://www.ebook.edu.vn 17
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:
8/8/2019 Nhieu Trong Thong Tin Di Dong
18/37
http://www.ebook.edu.vn 18
( ) ( ){ }( )
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
=
8/8/2019 Nhieu Trong Thong Tin Di Dong
19/37
http://www.ebook.edu.vn 19
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
20/37
http://www.ebook.edu.vn 20
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
21/37
http://www.ebook.edu.vn 21
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
22/37
http://www.ebook.edu.vn 22
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
23/37
http://www.ebook.edu.vn 23
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
24/37
http://www.ebook.edu.vn 24
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
25/37
http://www.ebook.edu.vn 25
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
26/37
http://www.ebook.edu.vn 26
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:
8/8/2019 Nhieu Trong Thong Tin Di Dong
27/37
http://www.ebook.edu.vn 27
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:
8/8/2019 Nhieu Trong Thong Tin Di Dong
28/37
http://www.ebook.edu.vn 28
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
29/37
http://www.ebook.edu.vn 29
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
30/37
http://www.ebook.edu.vn 30
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
31/37
http://www.ebook.edu.vn 31
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
32/37
http://www.ebook.edu.vn 32
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
8/8/2019 Nhieu Trong Thong Tin Di Dong
33/37
http://www.ebook.edu.vn 33
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
34/37
8/8/2019 Nhieu Trong Thong Tin Di Dong
35/37
http://www.ebook.edu.vn 35
8/8/2019 Nhieu Trong Thong Tin Di Dong
36/37
http://www.ebook.edu.vn 36
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.
8/8/2019 Nhieu Trong Thong Tin Di Dong
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.
Recommended