Tìm hiểu và nghiên cứu mạng không dây Wimax.PDF

Chuang I: Gied thigu chung
ve WiMAX
CHUCING 1. GI61 THItU CHUNG VE WiMAX
1.1. GIOI THItU
CHIANG
WiMAX la mOt cong nghe cho phdp truy c#p bang Ong vo tuyen den
du
Si
nhu mOt phuong thirc thay the cho cap va dyeing day thue bao so DSL. WiMAX
cho phep ket
n6i
bang Ong vo tuyen c6 djnh, nomadic (nguOi sir dung co the di
dOng nhung
c6 djnh
trong 18c kat rthi), portable (ngutri sir dung co the di chuyen vOi
the do cham) vi cuOi
ding la di dOng ma khong
can a
trong tam nhin thing LOS
(Line-Of-Sight) tri,rc trap vii tram g6c BS (Base Station). WiMAX khac phuc duce
cac nhuqc diem cita cac phuong phap truy nhip hien tai, cung cap mOt phuong tien
truy nhip Internet khong day
ding
hqp co the thay the cho ADSL vi WiFi. He thtong
WiMAX c6 kha nang cung cap duemg truyen co the dO len den 70Mbit/s vi yid ban
kinh phit song cita mOt tram anten phat len den 50Km. MO hinh phu song
dm
mang
WiMAX tuong tu nhu mang dien thoai
to
bao. Ben canh d6, WiMAX cling host

dOng mem deo nhu WiFi khi truy cip mang.
m6i
khi may tinh mu& truy nhip
mang no
0
ur
Ong ket
n6i
den tram anten WiMAX gin nhat.
Trong chuong nay, chitng to se tim hien khai niem co ban ve WiMAX, cac chuan
WiMAX hien nay, cac bang tar
t
co the sir dung cho WiMAX, cac uu diem va lqi ich
cua
WiMAX mang lai, &mg thtd la tinh hinh trien khai WiMAX tren the gieri vi
tai
Viet Nam
1.2. ICHAI NEN
WiMAX (Worldwide Interoperability of Microwave Access) la he thOng truy
nhip vi ba co tinh tuong thich toin
cAu
dua ten co so tieu chuan IEEE 802.16
WirelessMAN (Wireless Metropolitan Area Network). Hq 802.16 nay dua ra nhUng
lieu chuin, chi tieu
ky
thujt nhim tip trung giai quyet the van de trong mang vo
tuyAn
bang Ong diem — da diem ve giao dien vo tuyen bao Lip dieu khien
truy cop mill trutmg (MAC) vi 16p v#t 1St (PHY).
WiMAX la mOt chuan khong day dang phat trien rat nhanh, hira hgn tao ra khi

nang ket not bang thong rung
the dO
cao cho ca mang c6 djnh Ian mang khong day
di dOng. Hai phien ban cua WiMAX duqc dua ra:
Fixed WiMAX (WiMAX co
d(nh):
Dva tren tieu chain IEEE 802.16-2004, duce
thiet ke cho loai truy nh#p c6 djnh va luu dOng. Trong phien ban nay sir dung
ky
-
1 -
Chuang 1: Gioi thieu chung v'J WiMAX
thuat ghep kenh phan chia theo tan s6 trvc giao OFDM (Orthogonnal Frequency
Division Multiple) host dOng trong ca m8i trutrng nhin thing — LOS (line-of-sight)
va kh8ng shin thing — NLOS (Non-line-of-sight). San pham dva tren tieu churn
nay hien tai da dopy
cap
chung chi va thuong mai hem.
Mobile WiMAX (WiMAX di dOng):
dva tren tieu chuan IEEE 802.16e, duqc thiet
ke cho loai truy cap xach tay vi di dOng. ve co bin, tieu chuan 802.16e duqc pith
trien tren co
so seta doe
tieu chuan IEEE 802.16-2004 de tOi uu cho cac kenh vo
tuy6n
di dOng, cung cap kha nang chuyin yang — handoff vi chuyen mang —
roaming. Tieu chuan nay sir dung phucmg thirc da truy cap ghep kenh chia
tan s6
tri,rc
giao OFDMA (Orthogonnal Frequency Division Multiple Access) — 11 skr ph6i

hqp dm
k9
thuat ghep kenh vi
ky
that phan chia tan s6 co tinh chit tri,rc giao, rat
phu hqp v6i moi toting truyen dan da &Ong nhim tang thong loqng cling nhu
dung loqng mang, tang dO linh host trong viec quin 19 tai nguyen, tan dung toi da
ph6 tan, cai thin kha nang phi song von ac loai dja hinh da
WiMAX da duqc phit trien va khic phuc duqc nh&ng nhirqc diem et' cac ding
nghe truy cap bang rung truck day, cu the:
Cau Vac mem deo: WiMAX h6 trq cac cau true he thOng bao gem diem — da
diem, cling nghe lutri (mesh) vi pha song khap nu:A nod. Dieu khien truy nhap mai
toting — MAC, phuong tien truyen din 1145 trq diem — da diem va djch vu rung khip
bed lap ljch mot khe thtri gian cho mOi tram di dOng (MS). Neu co duy nhat
WA
MS
trong mang, tram gOc (BS) se lien lac vori MS tren co sty diem — diem. Mot BS trong
mot eau
hinh diem — diem co the sir dung anten chum hyp ham de bao phu cac
khoirtg cach xa han.
Chat hung djch vu QoS: WiMAX co the duqc
t6i
uu dOng d6i yea Mn hqp lou
lucmg se duqc mang. Co 4 loai djch vu doge h6 trq: djch
vv cap
phat to nguyen
(UGS), djch vu h6i wing thin gian thvc (rtPS), djch vo h6i Wong khong theri gian
thvc (nrtPS), no
lye t6t 'that
(BE).

Trien khai nhanh, chi phi thap: So sinh yeti trien khai cac giai phip co day,
WiMAX you cau it hoac kh8ng co bat cir sv xay dung thiet lop ben ngoii. Vi du,
dao he) de tao ranh cac dutmg cap thi khong you au. Ngoii ra, dva ten cac chuin
ma cua WiMAX, se khOng co sv dOc guy& v'e tieu chuan nay, dan den viec earth
tranh cua nhieu nha sin xuat, lam cho chi phi du to mot he tho
t
ng giim ding ke.
-2-
Chuang 1: Geri thieu chung
ve WiMAX
Dich da mac: Cach thirc nai ma QoS duce phan phat not chung dva vao sv
them thuan mire dich vti (SLA - Service-Level Agreement) giui nha cung cap dich
vu va ngu&i sir dung cu6i cling. Chi tie[ hun, mOt nha cung cap djch vp c6 the cung
cap the SLA khic nhau tai cac thud bao !chic nhau, then chi tad nhang ngtrai dung
khac nhau sir dung cling MS. Cung cap troy nhap bang rang co djnh trong nheng
khu vrc do thi va ngoai o, nth chat Itrqng cap diing thi kern hoac dua vao kh6 khan,
khac phtic thiet bj s6 trong nheng viing mat do thap nai ma cac nhan M ding nghe
vi kinh to thvc hien phat trien bang rung rat thach that.
Tinh ttrcmg thich: WiMAX duqc xay dung de tra thanh mOt char' quok tot, tao ra
str
de
ding d6i vii nguai dimg cu6i cling de truyen tai vi sir dung MS cia hp
o
cac
vi tri khac nhau, hoac vii cac nha cung clip djch
v4
khac nhau. Tinh ttrcmg thich bao
ve str dau to ctia mOt nha van hanh ban dau vi no c6 the chpn Iva thiet bj to cac nhi
dai ly thiet bj.
Di dOng: IEEE 802.16e ba sung them cac dac diem chinh h8 trq khi flan di

dOng. Nhftng cai tien lap vat ly OFDM (ghdp kenh phan chia tan sit')
th
e
re giao) va
OFDMA (da truy nhap phan chia tan s6 Dix giao) de ha trq cac thiet bi vi cac djch
vu trong mOt moi truing di dOng. NhOng cai tien nay, bao g6m OFDMA ma ring
dtrqc, MIMO (Multi In Multi Out - nhieu dau vao nhieu dim ra), vi h8 trq d6i vii
che dO idle/sleep vi handoff, se cho phop khi thing di dOng day di 6 tic do ten 160
km/h. Mang WiMAX di dOng cho phdp ngtrai sir dung do the troy cap Internet
khong day bang thong rung tai bat ca tau co phi sang WiMAX.
Hoat dOng NLOS: Kha nang hpat dOng cia mang WiMAX ma khong del hei
tam
nhin thing giaa BS va MS. Kha thing nay cia no &dip cac sin pham WiMAX phan
phat dai thong Ong trong mOt mei truemg NLOS.
Phi song rung han: WiMAX ho trq dung nhieu mire dieu che, bao gom BPSK,
QPSK, 16QAM, 64QAM. Khi yeu cau yeti bO khuech dai cling suit cao vi hoot
dOng vii dieu che mire thap (vi du BPSK hoac QPSK). Cac he th6'ng WiMAX co
the phi sang mOt yang dia ly Ong khi &rang truyen gigra BS va MS khong bi can
tra. Ma Ong pham vi bj giai han hien tai cia WLAN cling cling (hotspot) den pham
vi rung (hotzone). nhang dieu kien Mt nhat co the dat duqc pham vi phi song 50
km vii tic do
da
lieu bi ha thap (melt vii Mbit/s), pham vi phi song dien hinh la
gan
5
km veri CPE (NLOS) trong nha va gin 15km vii mOt CPE duqc n6i vii mOt
anten ben ngoai (LOS).
-3-
Chuang 1:
Gieri

thieu chung
ve WiMAX
Dung liking cao: Co the dot dirge dung lugng 75 Mbit/s cho cac tram g6c vgi
met kenh 20 MHz trong cac dieu kiln truyen song tot nhit.
Tinh ma rOng: Chitin 802.16 -2004 h6 try cac dai thong kenh tin
s6
ve tuyen
(RF) mem deo va sir dung 1a1 cac kenh tar
t
.
' so nay nhtr la mot cach de tang dung
lugng ming. Chuan ding dinh re h6 try &Si yeti TPC (dieu khien cong suit phat) vi
cac phop do chat lugng kenh nhu cac cong cu them vao de hO try sir dung ph6 hieu
qua. Chuan da dugc thiet ke de dat tS le len ten hang tram tham chi hang nghin
ngtkri sir dung trong mOt kenh RF. HO try nhieu kenh cho phdp cac nha the tao thiet
bi cung cap met phuong tin de chit trcing vao pham vi sir dung ph6 vi nhUng quy
dinh cap phat duck not re beri cac nha van hank trong cac thi twang quiic
to
thay
dei khac nhau.
1.3. CAC CHUAN CIA WiMAX
13.1. Chitin IEEE 802.16 - 2001
Chuin IEEE 802.16-2001 duck hoar
.
' thanh vao thing 10/2001 va dugc cong b6
vao 4/2002, dinh nghia dac to Icy thuot giao dien kh8ng gian WirelessMANTm cho
cac mang yang do thi. Dk diem chinh dm IEEE 802.16 — 2001:
Giao din khong gian cho hg thong truy Whip kung day bang rOng co dinh hgat
dOng o dai tin 10 — 66 GHz, can thoa man tint nhin thing.
Lop vat IS

,
PHY: WirelessMAN-SC.
T6c dO bit: 32 —134 Mbps vol kenh 28 MHz.
Dieu the QPSK, 16 QAM va 64 QAM.
Cac dai thong kenh 20 MHz, 25 MHz, 28 MHz.
Ban !anti cell: 2 — 5 km.
Ket n6i
co dinh huOng, MAC TDM/TDMA, QoS, bao mat.
13.2. ChuAn IEEE 802.16a
Vi nhang kh6 khan trong
then
khai chart IEEE 802.16, Inking vao viec sir dtmg
tin
so
tir 10 — 66 GHz, mOt dy an sira &Si co ten IEEE 802.16a da dugc hoin thanh
vao thing 11/2002 va dugc cong b6 vao tang 4/2003. Chuan nay dugc ma king h6
.
try giao din khong gian cho ithang tan
s6
trong bang tan 2-11 GHz, bao g6m ca
-4-
Chuang 1: Gioi thieu chung ve WiMAX
nhang ph6
cAp
phep va khong cap phdp vi khong can thoa man dieu kien tam nhin
thing. Dac diem chinh ctia IEEE 802.16a:
B6 sung 802.16, cac hieu chinh MAC vi cac dac diem PHY them vao cho dal 2 —
11 GHz (NLOS).
T6c de bit: UM 75Mbps yeti kenh 20 MHz.
Dieu the OFDMA vai 2048 song mang, OFDM 256 song mang, QPSK, 16

QAM, 64 QAM.
Dai thong kenh co the thay deoi Ora 1,25MHz vi 20MHz.
Bin kinh cell: 6 — 9 km.
Lop It 13% PHY: WirelessMAN-OFDM, OFDMA, SCa.
Cac chirc nang MAC them vao: 116 trq PHY OFDM vi OFDMA, h6 trq ding
nghe Mesh, ARQ.
1.3.3. Chan IEEE 802.16
-
2004
Thing 7/2004, chitin IEEE 802.16 — 2004 hay IEEE 802.16d duqc chap thong
qua, kat hqp cua cic chitin IEEE 802.16 — 2001, IEEE 802.16a, img citing LOS 6
dai tan
s6 10 -
66 GHz vit NLOS o dal 2 - 11 GHz. Kha Tian vo
tuyen b6
sung nhir
la "beam forming" vi kenh con OFDM.
1.3.4. Chuin IEEE 802.16e
DAu
nam 2005, chuan khong day bang thong rung 802.16e van ten gni Mobile
WiMAX da duqc phe chuan, cho phep tram gtic
kAt n6i
toi nhOng thiet bi dang di
chuyen. Chuan nay giup cho cac thitt bi tir cac nha sin xuat nay co the lam viec,
ttrong thich tot veci cic thiet bi tir cic nhil sin xuat khac. 802.16e hnat dOng 0 cac
bang tan nho ham 6 GHz, t6c dO len tai 15 Mbps veri kenh 5 MHz, ban kinh cell tir 2
— 5 km.
WiMAX 802.16e co hit trq handoff vi roaming.
sa
dung SOFDMA, mOt cling

nghe dieu the da sang mang. Cic nhl cung cap dich vu ma
then
khai 802.16e cling
co the sir dung mang del cung
cAp
dich vu c6 dinh. 802.16e h6 trq cho SOFDMA
cho phep s8 song mang thay d6i, ngoli cic m8 hinh OFDM vi OFDMA. Sr phan
chia song mang trong mo hinh OFDMA (kw thiet ke de
tai
thieu anh hueng
cua
-5-
Chuang 1: Giol thieu chung ve WiMAX
nhiOu phia thiet bj nguen clUng veri anten da huerng. Cu the ham, 802.16e dua ra hO
trq cai
ties ho
trq MIMO va AAS, cling nhtr cac handoff cling vi mem. NO cling cai
lien cac kha nang tiet kiem cling suet cho cac thiet bi di dOng va cac die diem bao
mat linh host hon.
802.16
802.16-2004
802.16-2005
Tinh
trang
Hoin thie'n vao
thing 12-2001
Holm thie'n vao thang
6-2004
Hoan thin vao thing 12-
2005

Dii tin
10-66 GHz
2-11 GHz
2-11 GHz cho c6 djnh;
2-6 GHz cho di dOng
(J'ng
dyng
Co
dinh, tam
nhin thing (LOS)
Co djnh, lchting nhin
thing (NLOS)
C6 dinh vi di dOng,
khong nhin thing (NLOS)
Citu
true lop
MAC
Diem — da diem,
mang luai
Diem — da diem, mang
lueri
Diem — da diem, mang
lucid
Mo hinh
A
truyen
song
Don song mang
Don song mang, 256
OFDM, 2048 OFDM

Don sang mang, 256
OFDM hoac S-OFDM veri
128, 512, 1024, 2048 song
mang con.
Dieu chi
QPSK, 16QAM,
64QAM
QPSK, 16QAM,
64QAM
QPSK, 16QAM, 64QAM
Ting
tin
so
de lieu
32-134 Mbps
1-75 Mbps
1-75 Mbps
Ghip
Werth
Khii
Kh6i
Kh6i
TDM/TDMA
TDM/TDMA/OFDMA
TDM/TDMA/OFDMA
Song
cengs
TDD va FDD
TDD va FDD
TDD va FDD

DO rOn
g

20, 25, 28
1.75, 3.5, 7, 14, 1.25,
1.75, 3.5, 7, 14, 1.25, 5,
-6-
Chuang 1:
Glen
thiOu chung 14
.
WiMAX
kenh
truyan
(MHz)
5, 10, 15, 8.75
10, 15, 8.75
Giao
then
kheng
gian
WirelessMAN-
SC
WirelessMAN-SCa,
WirelessMAN-OFDM,
WirelessMAN-OFDMA
WirelessMAN-SCa,
WirelessMAN-OFDM,
WirelessMAN-OFDMA
Xir 1f

WiMAX
Khong
256-OFDM nhu 11
WiMAX co
dish
S-OFDMA nhu la
WiMAX di dOng
Bang 1.1.
Tom
kit cac clOc trung ca ban czia churn WiMAX[6]
1.4. PH6 WiMAX
1.4.1. Bang tan ding ky
Cac giii phap dang ky cung
cap
cac uu diem chit ltrqng dich vu duqc cai thien
cao ham cac giii phap kh6ng dang ky, chap &tan NLOS Mt han a cac tan M thip,
no co mot ngan qiiy cong sulk clueing xu6ng rang han va co the ho trq cac anten
trong nhit
tot
han. Giai phap dang ky cho phep kient soit qua cach sir dung ph6 va
nhieu.
Bang tin Mug ky 2,5 GHz
DA duqc
cAp
phat trong phan lan cac ntrac tren the giai, bao g6m bitc
My,
My
Latin, (long va tay
Au va
nhiau vimg cila chin

A -
thai birth &rang. M6i
quitoc
gia
throng cap phat dai khac nhau, vi vay ph6 duqc
cAp
phitt qua cac vimg co the tir
2,495 GHz din 2,690 GHz. Tong ph6 kha dung la 195 MHz, bao g6m cac dai
ph6ng ve va cac kenh MDS, gtla 2.495 GHz va 2.690 GHz. H6 trq FDD, TDD. Phi;
tren moi dang ky la 22.5 MHz, met block 16.5 MHz va met block 6 MHz,
tong
s6 8
dang
ky.
Bang tin dang
ky
3,5 GHz.
chau Au, vien chitin vien thong chau au da phan ph6i dai 3,5 GHz, bit dau
duac sir dung cho WPLL, cho cac giii phip WiMAX dang ky. Ring ph6 kha dung,
thay del theo qu6c gia nhung nal chung khoang 200MHz giaa 3,4 GHz va 3,8 GHz.
HO trq ca FDD va TDD, met vai quac gia chi sir dung FDD trong khi cac quo
t
c gia
-7-
Chuang 1: GUN thieu chung ve WiMAX
khac cho phdp sir dung FDD hoac TDD. Pho tren moi clang ky thay dei tir 2x5MHz
dEn 2x56 MHz.
1.4.2. BAng tan khOng clang k5T 5GHz
Phan 16n the quoc gia town the gieri da six dung phi)
.

5 GHz cho cac phircmg tin
lien lac kh8ng dang ky. Cac bang 5,15 GHz vi 5,85 GHz CIA duqc chi dinh nhu
khong dang Icy trong phap lent the gieri.
Cac gill phap kh8ng ding Icy cung cap melt vii thuitn lqi chinh ham cac giii phap
Bang ky, bao gem chi phi ban dAu thAp ham, rut ra nhanh han, vi mOt bang chung co
the
duqc sir dung 6 phan torn the gi6i. Cac lqi ich nay dang thu hat sq quan tam vi
co khA nang cho su chap nhan bang rOng nhanh chung.
Tuy nhien met giai phip khong Clang ky thi khA nang nhieu cao han, vi nhiEu su
canh tranh dal v6i cac nha kink doanh bat dOng sin cho viec trien khai. Mot giii
phap khong clang Icy se khong duqc xem nhu mot su thay the cho giii phap clang Icy.
Moi giii phap cung cap mOt thi twang khac nhau dua vao su them hie]) gifra chi phi
va QoS.
1.5. TRUYEN SoNG
Hien tai nhieu cong nghe !thong day bang th8ng rong chi co the cung cAp phu
song clang LOS, cong nghe WiMAX duqc thi uu cung cAp ca Icha nang phi' song
LOS va NLOS. Mac dit vay, ca 2 clang phii song deu bi anh htrerng it nhieu bed cac
dac tinh cda moi trutmg truyen, cac Mn that Iron throng din vi tai nguyen het n6i v8
tuyen.
MOt tin hieu trong lien lac LOS la mOt tin hieu ma duqc phat ten 1 duting truyEn
true tiep, khong co vat can tir may phat den mot may thu hay tir may pith den may
thu IA thin shin thing. lien lac LOS you au phi') 16n mien Fresnel khong bi ngan
can bai SS vat nao, neu you cau nay khong duce th6a man thi co su giam thieu
Bang ke cueing dO tin hieu gift tram thu vi tram phat. DO her Fresnel dtrqc yeu eau
bed tin s6 hog dOng cila lien lac vi kiting each gifra may phat vi may thu.
Trong lien lac NLOS, tin hieu den may thu bi phin xa, tin xa vi nhieu xa do
dtrtmg truyen bi cac vat can gay ra. Cac tin hiEu den may thu bao gem cac tin hiEu
tir dyeing true tiep (tin bleu chink), cac tin hiEu bi phin xa nhiEu Ian, nang 1uqng bi
-8-
INTERIMT

BACKBONE
MINAS 802.24
TRANSMITTER
LIRE OF SIGWI
WICKHAM
SIMM
T=ESTA
V
trOMI
LIMAS YEA
RSTWOINI
Chuang 1: Glut thieu chung ve WiMAX
tan xa va cac duemg truyen bi nhieu xa. Cac tin hieu nay co khoang tre khac nhau,
suy hao, phan cuc, DO chinh la nguyen lain gay ra nhieu ISI va meo tin hieu,
day kheing phai la yin de Ion del vol lien lac LOS, nhung veri LOS lai la van de
chinh.
Ngoai ra, hien tugng da duOng co the gay ra hieu tugng phan cuc. Khic phoc
tinh trang nay, viec sir dung lai tin
s6
nhu la mot Bien phap manh me cil ia LOS
nhung lai kho giii guy& trong ang clang NLOS.
MOt hg thing vo tuyen sir dung cac tin hieu da duemg hieu qua tao thuan Igi de
cung cap dich vu trong dieu kiln NLOS. MOt san phin chi don thuin tang cong suit
de xuyen qua cac vat can (doi khi ggi la "can tam nhin thing") thi kh8ng dugc coi la
cong nghe NLOS bed vi phucmg phap nay can dui vao duong truyEn true ti'ep
du
manh chic khong sir dung cac !tang lugng sinh ra trong cac tin hieu gian dep.
NLOS tao ra cac uu diem Clang ke, vi du nhu cac yeu au lap lich ke hoach chat
the va gieri han chieu cao anten ma thutmg thi quy Binh chieu cao anten ciia LOS
kh6ng cho phep. Vic

ha
thap chiEu cao ciia anten la met thuan Igi dE giam nhieu
kenn
chung gifra cac vi tri cell lien ke nai ma cac to bio tram g6c dugc then khai ke
nhau tren pham vi Ong kitten tar
'
s6
dugc sir dung co gieri hen.
Hinh 1.1. Minh hpa hpat 445ng WiMAX [1O]
- 9 -
Chuang 1: Giai thieu chung
ve WiMAX
Cong nghe NLOS cling giam phi ten cai dot bang each dat clued cac mai the thiet
bi CPE dung nhu nguyen ban va giam bat kh6 khan dinh vi tri cac dia diem dot
CPE
thich hqp. C6ng nghe cling giam bat nhu eau quan sat vi tri thiet bi phia throe va cal
thien dO chinh xac cua cac cong cv lap ke hooch NLOS.
C6ng nghe NLOS va Meng tinh nang duce fling cao trong WiMAX too !chit
nang sir dung thiet bi phia
dAu
Ichach hang (CPE) trong nha. Dieu nay co hai kh6
khan chinh; dau tien la khic phvc nhOng ten hao xuyen qua tea nhA va thir hai, phi)
song cac khoang each hqp IST vii cong suit truyen va cac anten th'Ap hen ma &rang
duqc ket hap vii the CPE trong nha.
Trartattry Antense
Hinh 1.2. Truyin song trong Thremg hqp NLOS 1101
1.6. UV DIEM VA NHUQC DIEM CUA CONG NGHE WiMAX
Mvc
tieu cua
t6

chirc IEEE khi phat trien tieu chuAn 802.16:
Xay clung mot pham vi chuAn di
a
clang cho sv phat trien va phei hap gift cac
nha san
xuAt,
nha cung
cap
va ca ngtrai sir dung.
Thilc dAy qua Wirth chimg nhan ph6i hap hoot dOng va tuan chit cho cac he thring
truy nhap vo tuyen bang rOng tren toan cAu.
Nhu vay, voi mvc tieu de ra, cac tieu chuAn cho WiMAX co Iqi ich het sue to lan
doi
voi cae nhA san xuat, cac nhi cung cap dich vv va ca ngtrai sir dung dich vv.
fled
voi nhi sin ausit:
Tren ca s6 tieu chuAn chung, nhi sin xuAt co the nhanh cheng phat trien cac san
phini ma it phai chi phi cho viec nghien ciru, too thanh phAn va (Lich 91 mai.
MOt nha sin xuAt co the top trung vao mot linh we (ching han tram g6c hay
CPE) ma kh6ng can thvc hien day du giii phip tir
du cued den dAu
-
10 -
Chuang 1: Giol thieu chung v*J WiMAX
Si ved nha cung tip djch vg:
Tren cu so nen tang chung cho phep nha cung cap djch vu giam gia thank, tang
kha nang canh tranh cling nhu khuyen khich sv ded mdi.
ICha nang giain cac chi phi vit mac dau to cho phep nha khai thic tang pham vi
phvc vv cia minh.
Nha khai thac kh8ng con phv thuOc vAo mot nha cung cap thiet bj rieng do cac

san pham rieng biet cua timg hang.
He' thong vo
tuyen
cho phep giam cac rii ro cho nha khai thac.
DOI vii nguriri sir dyng dich vy:
Ngtrai sir dung tai the khu vve Mem day chua dirge cung cap djch vv truy cop
bang rOng nay co the duqc sir dung nha kha nang phi song rOng cua WiMAX.
Nhieu nha cung cap dich vv tren thi truang tao dieu kien cho nguiri sir dung co
them nhieu Iva chon cho djch vv truy nh#p bang rOng.
Tao sv canh tranh co lqi cho nguai sir dung, giam cac chi phi djch vv.
Cic nhusrc diem cfia cong nghe WiMAX:
Dai tin WiMAX sir dung khong tucmg thich tai nhieu qu6c gia, lam han the sv
pile, hien cong nghe rOng rai.
Do cong nghe mai xuAt hien gAn day nen van con
met s610
h6ng bao mat.
Tuy duqc gqi la chuin cong nghe nhung th‘t su chua duce "chuan" do hien gin!
dang sir dung gin 10 chuin cong nghe khac nhau. Theo dien din WiMAX chi mai
co
khoang 12 hang phat then churn WiMAX duqc chung nhan bao g6m: Alvarion,
Selex Communication, Airspan, Proxim Wilreless, Redline, Sequnas, Siemens, SR
Telecom, Telsim, Wavesat, Aperto, Axxcelera.
C6ng nghe nay Icheri xuemg tir nude
My,
nhung thvc sv chua co thong tin chinh
thirc nao de cap den viec
My
sir dung WiMAX nhu the nao, khic phut hat qua str
c6 ra
sao. Ngay ca a Viet Nam, VNPT (voi nha thin nude ngoai la Motorola,

Alvarion) cling da
mien
khai o
met sof) tinh mien
nui phia Bat, cu the la a Lao Cai
nhung cling chi gi6i han la cac diem truy cap Internet tai Buu dien tinh,huyen
cho
chua c6 nhang ket luOn chinh thirc ve tinh hieu qua dang ke cua he th6ng.
Nhu vay, co the thAy rang kha nang cling nhu lqi ich ctia cac he
Meng
WiMAX
dva tren ho churn 802.16 la het sire to Ion mac dit no van tan tai met s6 han the. NO
Chuang 1: Gioi thieu chung
ve WiMAX
cho phep nha cung cap djch vv trien khai nhanh chong cac hg thOng mang cua minh,
tang kha nang earth tranh eking theri cho phip ngutri tieu dimg co them nhieu Iva
chon, fiat kiem hon trong cac chi phi. Dieu nay chinh la dOng lye MIK day cac this
san xuat t va cac nha cung cap djch vv phat trien hg thong WiMAX.
- 12 -
Chutmg 1: Gioi thieu chung IFJ
WiMAX
1.7. TiNH HiNH TRIEN KHAI WiMAX
1.7.1. Tinh hinh trien khai WiMAX tren the gild
Hien nay, tren the gioi, dA co cac mpg thu nghiem cong nghe WiMAX co djnh
va di dOng. Theo danh gia cea Maravedis Inc. thi thi trueng ten thong bang rOng
co djnh den nAm 2010 co doanh thu vugt 2 ty USD. Hien nay, thc dO tang tramg
hang nam la 30%.
Vic xuAt
hien met ding nghe truy cop khong day bang rOng mai
nhu WiMAX cho phep trien khai nhanh djch vv, se lim bung no thj tubing trong

nheng nAm
Den nay, dA c6
met s6 nuerc
da
di vao
trien
khai va khai thac the nghiem cac djch
vv tren nen Mobile WiMAX nhu My, Uc, Brazil
MOt sv kien co the coi la met buck ngoOt quan tong cea WiMAX — to ngay 15-
19/10/2007 — ca quan vien thOng
qu6c to
thuec lien hip (in& ITU di phe duyet
cong nghe bang rOng kh8ng day nay vao be chuin IMT-2000. Quyet dinh nay dA
dua WiMAX len ngang tam vol nhang
k9
thufit k'et
n6i
v8 tuyen hang du hien nay
trong be churn IMT-2000 gem co GSM, CDMA va UMTS. Dieu nay dam boo cho
cac nha khai thac vi quan 13% tren toan the giai yen tam du to vao Wing rOng di
dOng thvc sv dung WiMAX.
1.7.2. Tinh hinh
trien
khai WiMAX thin nghiem tai Viet Nam
VNPT trien khai the nghiem cong nghe WiMAX tai Lao Cai vao thang 10/2006
va dA nghiem thu thanh cong vao thang 4/2007.
NAm 2006, tai Viet Nam, dA co 4 doanh nghiep duqc Be Buu chinh Vien thong
cho phdp
cung cAp
thir nghiem djch vv WiMAX c6

djnh la Viettel, VTC, VNPT vi
FPT Telecom. Sau khi the nghiem xong, Be se Iva chon 3 nha cung
cAp chinh
the
cho loci hinh bang rOng khong day nay.
Ngay 1/10/2007, Chinh phu dA cep phep trier, khai djch vv thong tin di dOng 3G
ve dich vv truy nhOp bang Ong khong day WiMAX (theo cong van 5535/VPCP-CN
dm van phong Chinh pile). Deng that Phe the Wang dA deng y cep phep thir
nghiem djch vv WiMAX di dOng cho 4 doanh nghiep EVN Telecom, Viettel, FPT
vi VTC the nghiem tai bAng tan 2.3 - 2.4 GHz; VNPT the nghiem tai bang tin 2.5 —
2.69 GHz.
- 13 -
Chuang 1: Giol thieu chung ve WiMAX
1.8. KET LUAN CHIRING
Chuang 1 nay da khai plat duqc nhftng dac diem co ban caa WiMAX bao gam
khai nid'm, cac phien ban, ph* vi cac bang tar
' cluqc s* dung cho WiMAX. Ngoli
ra, chtrang nay cling net' len duce cac tru diem
va
nhtrqc diem hg thong sir dung
ding nghd WiMAX. Chuang nay se la nen tang cho cac chtrcmg
tier)
theo nhim tim
hieu sau ham vd hg th6ng WiMAX.
-14-
Chuang 2: Cac k
-
St thudt &roc ste dung trong WiMAX
CHUONG 2. CAC KY TI1U4T DU'QC DING TRONG WiMAX
2.1. GIell THItU CHVONG

Yeti kien triic ma, linh hoat, dung luqng lan va chi phi then khai thip dA lam cho
WiMAX tres thanh mot giai phap hang diu cho cac dich vu v8 tuyen bang rOng.
KM nang cung cap ket not bang 'tong veri nhieu kjch ban sir dung tir truy cap co
djnh va di dOng tren ding mot ha ring mang va vi the se tao nen mot can lit mang
linh hoat, mang lai cac dich vv truy Gip bang rang gia re va hira hen mot mo hinh
kinh doanh hip din d6i veri cac nha cung cap djch vu.
Veri cong nghe tuang to truerc day (FM, AM) va bieu
416
dieu the so h6a hieu
suit thip (PSK, BPSK va QPSK) duqc sir dung rOng rai trong cac mang ngay nay,
cong nghe bang Ong khong day yeu ciu sir dung cac bieu dE dieu the theo thU ig
cao han veri hieu qua trai phE
tot
hem. Tuy nhien, bleu dE dieu the theo thir
ig
cao
han nay Mt
a
1:1 tic dOng bed nhieu va hien tucmg da &rang clan.
Ca
hai yeu
to
nay
d'eu phE bien trong cac triEn khai mang kh8ng day c6 mAt khip nai va
s6 itrong
nguiii dung lem.
OFDM, OFDMA va S-OFDMA la nhftng cong nghe truy nhip mOi cai
tien hb'
trq kenh can thiEt de dat duqc hieu qua trai phi
tot han

va th6ng lucmg kenh cao
han. Nhang cong nghe 'my nhip meri nay la nen tang cho WiMAX va la lua chqn
cho cac he thEng bang rOng di ding tiep theo khac nhim cung cap nhieu loci hinh
dich vv truyen th6ng da phuang tien tic dO cao.
Trong chuang nay, chung to se khao sat tong quan cac
kg thuo
Wan tien
duqc ap
dung trong cong nghe WiMAX nhu la
icy digit
OFDM, OFDMA, he thEng anten
tien tien
- 15 -
011-1 >Pa ape nava
Frespoeuar
I-13MA epectesrmo
Chuang 2:
Cac
ky
thuat
&tor
sei dung trong WiMAX
2.2. Ki? THQAT OFDM
2.2.1. Khei nifm
Ky thuat OFDM la kji thuat ghip kenh phial chia theo ten s6 true giao
(Orthogonal Frequency Division Multiplexing). Trong OFDM, chuti du lieu toi Mu
phat thueng c6 the do
rat
cao. Deng
aft

lieu nay duqc chia thanh nhieu deng dOr lieu
song song the do thip ham nhe bo chuyen dti
not tiep-song
song (S/P). Moi deng
du
lieu song song sau do dtrqc di'eu the beri melt song mang, cac song mang nay
duqc chen true giao veri nhau de dim bio co the tach rieng timg luring chi lieu tai
dau thu. Ke den cac song mang nay dirge
tong
hqp lei va dua len tan se Oat.
Hinh 2.1. So scinh giito FDMA vn OFDM
Se lugng cac song mang con phy thuOc vio nhieu yeu
to
nhtr do Yong kenh va
mUc do nhieu. Con
s6 nay trong 1mg yen
kich thuem FFT. Chu'An giao tiep ve tuyen
802.16-2004 xac djnh 256 song mang con tucmg img FFT 256 diem, hinh thanh
chuan Fixed WiMAX, yen do rOng kenh co djnh. Chuin giao
tiep
802.16
-
2005 cho
phop kich cer
FFT tir 512 den 2048 phit hqp veri do rOng kenh 5MHz den 20MHz,
hinh thanh chusan Mobile WiMAX (Scalable OFDMA), de duy tri tuong &Si khoing
thei gian khong del cim cac ki hieu vi khoang clan each gift cac song mang veri dO
reng kenh.
CO the thiy re lqi ich caa OFDM khi xet qua kenh truyen. Neu luring
der lieu

goc
duqc chuyen true tiep len song mang va phat len kenh truyen, thi bang thong rOng
dm tin hieu phat se bj tae Ong chen lee an se. Bed vi, khi tin hieu truyen co bang
thong reing (do Mc do bit cao), cac tan M khic nhau se co do suy hao khic nhau khi
truyen qua kenh truyen vo tuyen. Dieu nay dim den viec kh6i phqc tin hieu tai may
- 16 -
Chuang 2: Ccic kfr thigh &or
sir citing trong WiMAX
thu se phut tap, dei hoi phai co be can bang. Trong OFDM,
(ding
dr! lieu duqc tach
thanh N luting
da Ru
Mc de thip, co bang thong hcp. Do 46, khi truyen, cac
da
lieu nay chju Fading phing cling de.
Tai may thu, ling
dii lieu
trate tien dirge dua v'e' bang
geic
bed be trOn. Luting
du lieu nay sau d6 duqc txich ra thanh N ling dii lieu Mc de thap, theo sau la be lqc
th6ng thap va be quy'et djnh.
2.2.2. Su
dO
khOi OFDM
Da
lieu

DO

lieu




Minh 2.2 Sc
cle;
khdi he thcing OFDM
Dau tien, dong
da
lieu vao yeti Mc de cao duqc chia thanh nhieu dong
dii
lieu
song song toc de thap hcm nhe be chuyen doi
not 4p-song
song (S/P). Mai dong
du lieu song song sau do duqc ma h6a (Coding) sir dung twat toan FEC (Forward
Error Correcting) va duqc sip xep (Mapping) theo met trinh to hon hqp. NhOng ky
to
hem hqp dirge dua den dau vao cua khoi IDFT (6 day de thuc hien phdp bien d6i
IDFT ngued to &mg thuat toan IFFT). Sau de khoing bio v'e duqc chen vao de gilun
nhieu xuyen 147 to (ISI), nhieu xuyen kenh (ICI) do truyen tren cac kenh vo tuyen di
Ong da clueing. Deng
dii
lieu song song lai dirge chuyen thanh
not
tiep nher be

chuyen dbi song song-n6i flap (P/S). Cuoi cling, Ix) A/D phia phat djnh clang tin
hieu thin gian lien tuc va chuyen did ten mien tan so cao de truyen di xa.
- 17 -
Chuang 2: Cdc 1c9 thu<it dupt sir dung trong WiMAX
Trong qua trinh truyen, tren cac kenh se co cac ngurin nhieu tac (long den nhu
nhieu Gausian tring cong (Additive White Gaussian Noise-AWGN).
phia thu, tin hieu thu dirge chuyen xu6ng tan s6 thap va tin hieu rtri rac nhan
dirge sau bo D/A thu. Khoang bao vg dirge loai be va cac mau dirge chuyen cl6i to
mien died gian sang mien din so bang phep bien d6i DFT dung thuat toan FFT (khoi
FFT). Sau do, thy vao sir c16 dieu the dirge sir dung, sr dich chuyen ve bien do va
pha cua cac song mang nhanh se dirge sip xtp ngugc tres lai vi dirge gill ma. Cuoi
cimg, chimg to than lai dirge clang dir lieu nil tiEp ban du.
2.2.3. Chugi bao vg trong ht' thOng OFDM
lfu diem
dm phuong phap dieu chE OFDM khong chi the hien 6 hieu qua sir
dung bang thong ma con co kha nang lam giam hay loai trir nhieu xuyen ki hieu ISI
TINY sir dung chutii bao ve (Guard Interval- GI). MOt matt tin hieu co t10 dai la Ts,
churai bao ve tucmg img la mOt chuSi tin hieu co dO daft
Too
phia sau duct sao chep
len phin phia fru& cila mail tin hieu nay nhu hinh ve sau (do de, GI con doge poi la
Cyclic Prefix-CP). Su sao chop nay co tac dung ch6'ng lai nhieu xuyen ki hieu ISI
do hieu img phan tap da dtrang.
Pluin tin hieu c6 ich

Ifinh 2.3. Khdi niem va chuiii bit° ve.
Nguyen tic nay giai thich nhu sau: Gia sir may phat di mOt khoang tin hieu co
chieu dai la Ts, sau khi Chen them chuai bao ve co chiEu dai
To
thi tin hieu nay co

chieu dai la T=T
s
+T
G
. Do hieu img da dutrng multipath, tin hieu nay se taxi may thu
theo nhiEu dtrimg khac nhau. Trong hinh ve mo to cluti day, hinh 2.4, tin hieu theo
&rang thir that khong co tre, cac throng thir hai va thir ba dEu bi tre mOt khoang
than gian so yeti dirtmg thin nhat.Tin hieu thu dirge 6 may thu se la tong hgp ciia tit
ca cac tuyen, cho thiy ki hieu &mg trink se chting lin vao ki hieu ngay sau d6, day
chinh la hien tugng 151. Do trong OFDM co sir dyng chub
z
i bao ye co do dai
To
se
a
ding loai be hien tugng nay. Trong trutmg hgp
T6 ?T tux nhu
hinh ve mo to thi
phan bi chOng lin ISI nim trong khoang cua chuoi bao ve, can thanh phan tin hieu
co ich van an toan.
6
phia may thu se gat be chutii Wm ye trutrc khi gin tin hieu den
- 18 -
pain
02
path
12
Received
,
Chuang 2: Ccic kfr thudt chew sir dung trong WiMAX

be giai dieu
the
OFDM. Do do, diEu kien can thief de cho hg thong OFDM khong
bi anh hut:mg beri ISI la:
T
G
r
max
V
ori Twocia
tre truyEn dim gi da cua kenh.
or.
V

al
path el
A
■
.
pall *3
Fteceieed
Time
IChang có GI
Time
Co GI
Hinh 2.4. ISI va
cyclic prefix
2.2.4. Nguyen tic gill dieu the OFDM
Cac biz& thgc hien 6 day deu ngtrqc Igi so vol phia may phat. Tin hieu thu se
&lac Lich thuoi bao vg, giai dieu the de khoi phgc bang tan

goc,
giai diEu
the
ey
the
song mang con, chuyen d6i matt tin hieu phac thinh clang bit (tin hieu s6) va
chuyen d6i song song
sang nal nap
2.2.5. Cic wu va nhurut diem ciia ki thuilt OFDM
Qua viec phan tich va ca ban kg thuat OFDM nhu tren, chimg to co thE rut ra met
s6 uu diem, nhugc diem chinh cua OFDM nhu sau:
Uu dam:
- 19 -
Chuang 2: Ccic kfr thuat dttqc
sie
citing trong WiMAX
Sir dung ph6 hieu qua nher ph6
tan s6
co clang gin nhu cira
s6
cha nhat
nett s6
song mang con da kin.
Loai bei !Neu xuyen 14 to (ISI) va nhieu giaa cac khung (IFI) nher sir d0ng tiEn
t6 yang CP (Cyclic Prefix).
Kha nang chring nhieu giaa cac kenh con rat tat nher viec sir dung cac song mang
con true giao.
Kyr thuat OFDM cho phop thong tin Mc el0 cao duqc truyen song song veri Mc dO
thap hem tren cac kenh bang hyp. Cac kenh con nay dugc coi la cac kenh fading
kh6ng lira chip tan so nen co the dung cac b0 can bang don gian

trong su6t
qui
trinh nhan th6ng tin. Noi nhu vay, he thEng OFDM cluing duerc anti huemg cira
fading lua chin
tan so.
K9
thuat OFDM la met phuang phap hieu qua de giai
quy6t
da duong, khang
nhieu bang hyp Mt vi nhieu nay chi anh hu?mg met 4 le nhe cac song mang con.
Thuc hien don gian trong mien tan
s6
bang cach dung giai thuat FFT. Dang thin
may thu don gian do khong can b0 kilter ICI va ISI nEu khoing du troy dti dai.
- 20 -
Chuang 2: Cdc Icy thudt duvc ste dung trong WiMAX
Nhuyrc diim:
OFDM la tap hqp cita tin hieu tren nhiEu song mang, dai dOng cea tin hieu Ion
nen cot
s6
ding suit dinh/trung binh tucmg doi len se lam han che hi'E'u suit cea
b0 khuech dai am tan.
Mit mat hieu suit pht do then khoing do MI.
Nhitu pha do so khong phti hqp gita cac be dao dOng 0 may phat va may thu,
co thE lam Anh hueng den chit lucmg he tilting.
Phil co so dang b0 chinh xac vE
tan s6 va
thin gian, dac bier la tit
'
so.

Nhu vay,
ky
thuat OFDM IA giiti phap rat pith hop cho truyen clan vo tuytn ttc
dO cao not chung va cho tong nghe Winn not rieng. Theo phan tich ve
ky
thuat
OFDM nhu tren , dung Wong cda
4
thting se duqc darth gia thong qua so luting cac
sang mang con dime diEu che. St lucmg the song mang con pho thuk vAo nhiEu
yeu to nhu dO rang kenh, mire do nhieu,
icieu diau cha
r
Con s6 nay (song mang
con) ttrcmg
(mg voi kich thuk FFT. Cu the nhu chuin 802.16-2004 xic Binh re 256
song mang con, ttrcrng Cmg veri kich thuerc FFT 256 dO
rtjng
kenh dpc lap, chuan
802.16e-2005 cung cap kich ca FFT tir 512 den 2048 tuong img vei dO rOng kenh
tir 5 MHz den 20 MHz dE duy tri khoing each trong clti khong dti cea ky hieu va
khoing dan each gift cac sang mang con d'Oc 1#p veri dO rOng kenh. Nhu vay, veri
tong nghe OFDM, nha sir kat hqp cua cac song mang con ow giao truyEn song
song veri cac
icy
hieu co khoang thtri gian dai dim bio rang luu luorng bang thong
rOng kh8ng bi han che
r
' do mei truing khong theo
tam

nhin thing NLOS va nhitu do
hien Wong da dutmg dn.
2.3. Kt THU*T OFDMA
2.3.1. IChai
nit'm
OFDMA (Orthogonal Frequency Division Multiple Access- Da truy nhap phin
tin troc giao) la met ding nghe da song mang phit
then
doa tren nen ki thuat
OFDM. Trong OFDMA, met so cac song mang con, khong nhAt thiEt phai nim ke
nhau, dugc gap Iai thanh mot kenh con (sub-channel) va cac user khi truy cap vao
tai nguyen se duqc
cap
cho met hay nhieu kenh con dt truyEn !Man thy theo nhu
cau luu Wong cu
the.
-21-
mom no
MO
M
ME
M
ME
Chuang 2: Cac
ky
thydt &roc szi dung trong WiMAX
2.3.2. D$c diem
OFDMA eel
met s6 uu
diem nhu la Ling kha fling linh hoot, thong Wong va tinh

on
djnh dune cai Mien. Viec an djnh cac kenh con cho cac thue bao cu the, viec
truyen nhan tir met s6 thue bao co the xay ra deng then ma kh6ng can sv can thiep
nao, do d6 se giiun thieu nhang tac d(mg nhu nhieu da truy xuat (Multi access
Interfearence - MAI)
Di p
a
l

ut pert
SOFDIAA
DI Dort

UL part
Userl
User?
User3
User4
111
users
Hinh 2.5. ODFM
va OFDMA
Hinh 2.6 mO to met vi du ve bang tin
s6 -
thai gian cua OFDMA, trong d6 co 7
ngueri dimg tir a den g va
in& ngueri
sir dung met phin xac djnh cua cac song mang
phy co sin, khac veri nhang ngu&i can lai.
- 22 -

Chuang 2: Ccic
kJ;
thrall dupe sze dung trong WiMAX
a
d
a
E
a
d
a
d
a
E a
d
a
c
e a
c
E
a c
e
a
c
e
a
c E
a
c e
b
e

g
b
E
g
b
e
g
b
e
g
b
E
g
b
e
g
b
f
g
b F
g
b
f
g
b
f
g
b
F
g

b
f
g
Hinh 2.6 Vi du czia b'iu do
tan
so,
then
■
gian
vai OFDMA
Thi du cv the nay thvc to la scr hill hqp cria OFDMA va TDMA beri vi mui ngueri
sir dung chi phat a mOt trong 4 khe thai gian, chira 1 hoc vai symbol OFDM. 7
ngu&i sir dung tin a den g deu duqc d4t c6 djnh (fix set) cho the sang mang theo bein
khe than gian.
2.3.3. OFDMA nhily tan
Trong vi du truck cda OFDMA, moi ngtrai sir dung deu c6 mOt sv sap (tat c6
dinh (fix set) cho song mang. Co the de ding cho phdp nhay cac song mang phu
theo khe tired gian nhu d uqc mo to trong birth
Vic cho phep nhay vai cac
mau
nhay khac nhau cho mui user lam bien dti thvc
sv
110
thOng OFDM trong h0 thtng CDMA nhay tan. Dieu nay co lqi la tinh phan
tap theo tan
s6
tang len beri vi
meii
user dung toan b0 bang thong co sin cling nhu lit
có lqi ve xuyen nhieu trung binh, dieu rat pht bien dti vai cac bi '6n the cda CDMA.

Bing each sir dung ma sira loi htreng di (Forward Error Correcting- FEC) tren cac
bulk nhay, hg thong co thE sera cho cac song mang phu khi 131 fading sau hay cac
sang mang b1 xuyen nhiZu beri cac user khk. Do dac tinh xuyen nhieu va fading
thay dOi yen mai btrerc nhiy, hg thtng phu thu(ic vao nang hrong tin hiOu nhan duqc
trung binh hon la
php thugc vito user vi nAng lugng nhieu trong truing hqp Vitt
Slat.
•
- 23 -
Chuang 2: Cac ky thudt duce sti dung trong WiMAX
tru
diem ca ban dm hg th6ng OFDMA nhay tan ham hin cac hg th6ng DS-
CDMA va MC-CDMA la tuang d6i
a
ding loci b6 ductc xuyen nhiZu trong mot to
bao bang each stir dvng cac
mau
nhay true giao trong mot to bao.
MOt vi dv cua viOc nhay tan nhu vOy doge mO to trong hinh 2.9 cho N sang mang
phv, n6 luon luon co the do ra N mau nhay true giao.
A
f
e
d
c
B
B
a
f
e

d
C
c
b
a
f
e D
D
c
b
a
f
e
E d
c
b
a
f
F
e
d
c
b
a

•
Hinh 2.
7.
6 mint nhay tan true giao voi 6 kin so nhay khac nhau
- 2 4 -

DA
IF
FT
Chen
GI
Kenh
truyen
•-1
Chen 4–
GI
IFFT
Chuang 2: Ccic ky thu it &tor sit dung trong WiMAX
2.3.4. Hg thimg
OFDMA
Dieu the
bang ten
g'c
Chen pilot
symbol
Giii dieu
the bang
4-
Can
bang
4-
tan geic
kenh
Tach pilot
symbol
Khoi phmc

kenh truyen
Hinh 2.8. Tang quan
he
Meng sir dung OFDMA
Ngu6n
tin hie'u ducc dieu the er bang ten ca sti thong qua ck phucmg phap dieu
the nhu QPSK, M-QAM Tin hieu din clueing (ban tin din duang, kenh hoa tieu -
pilot symbol) duce cher' vao
ngu6n
tin hieu, sau ci6 duk dieu the thanh tin hien
OFDMA thong qua bin d6i IFFT va chen chuOi bao vg GI. Luling tin hieu s6 duce
chuyen thanh tin hieu tucmg tu try& khi truyen tren kenh vo tuyen qua anten phat.
Tin hieu My se bi anh hung beri fading va nhieu tang AWGN (Addictive White
Gaussian Noise ).
Tin hieu den dtaing la mkt tin hieu ducm bitt truck er phis phat va phis thu, ducic
phat kem veri tin hieu co ich nham Ichoi phvc kenh truyen vi dong bet he tilting.
- 25 -