bao cao(sua)

7/29/2019 bao cao(sua)

1/22

M KHI KHNG GIAN THI GIAN ALAMOUTI(ALAMOUTI STBC)

1.1. Gii thiu v h th MIMO v h h i thi i

i vi mt h thng c mt slng ln anten pht v thu, mt knh fading phng

c lp nh bit ti my thu, dung lng s tng mt cch tuyn tnh vi s lng

anten ti thiu.

Mt cch thc t v hiu qun gn dung lng ca cc knh v tuyn MIMO

(Multiple-input multiple-output) l thc hin m ha khng gian-thi gian ST (Space-

time). M ha khng gian-thi gian l mt k thut m ha c thit k s dng vi

nhiu anten pht. M ha c thc hin trong chai min khng gian v thi gian

a vo s tng quan gia cc tn hiu c pht t cc anten khc nhau ti cc khethi gian khc nhau. S tng quan khng gian-thi gian c s dng khai thc

fading knh MIMO v ti thiu ha li truyn my thu. M ha khng gian-thi gian c

tht c li phn tp pht v li cng sut hn cc h thng khng c m

ha khng gian thi gian m vn khng lm hao ph bng thng.

Tu theo phng php m ho, m khng gian - thi gian c th chia thnh hai

loi: m khi khng gian-thi gian (STBC - SpaceTime Block Code) v m li khng

gian-thi gian (STTCSpace Time Trellis Code). M STBC c u im thit k v tch

tn hiu n gin. Tuy nhin, m STBC ch cung cp li phn tp pht m khng cung

cp li m ho (coding gain). Ngc li vi m STBC, m STTC cho php thu c

c li phn tp pht v li m ho nhng vic thit k v gii m li phc tp.

Trong bo co ny ch nghin cu n m STBC.

1.2. Cc h th h h i - thi i

Ta xt mt h thng truyn thng c m ha khng gian thi gian bng tn gc

vi nTanten pht v nRanten thu, nh c trnh by trn hnh 1.1.


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Hnh 1.1: Mt m hnh h thng bng tn gc

D liu pht c m ha bi mt b m ha khng gian-thi gian. Ti mi thi

im t, mt khi m symbol thng tin nhphn, c k hiu bi:

ct = (c1

t,ct2,...,ct

m) (1.1)

c a vo ng vo ca b m ha khng gian-thi gian. B m ha khng gian-thi gian s nh x khi m symbol nh phn ng vo ny thnh nTsymbol iu ch t mt

b tn hiu M = 2mim. D liu c m ha sc a vo b chuyn i t ni

tip sang song song (S/P-serial to parallel) xut ra mt chui nT symbol song song,

c sp xp thnh mt vecto ct nT1.

1 2( , , )tn H

t t t t x x x x ( 1.2)

Trong , Hc ngha l chuyn v ca mt ma trn,nTng ra song song c phtng thi bi nT anten pht khc nhau, nhvy m symbol xi (1 i nT), c pht bi

anten i v tt ccc symbol c pht u c cng mt khong thi gian T (s).Vector

symbol iu ch m ha tcc anten khc nhau, nh c ch ra (1.2), c gi l

symbol khng gian-thi gian (space-time symbol).

n 1.2: tng


3/22

Hiu sut ph ca h thng l :

br mB

(bit/sHz)(1.3)

Trong , rb l tc bit v B l bng thng knh truyn. Hiu sut ph trong (1.3)bng vi hiu sut ph ca mt h thng khng m ha vi mt anten pht. Nhiu anten

c my pht v my thu to ra mt hMIMO. i vi thng tin di ng v tuyn, mi

lin kt t mt anten pht n mt anten thu c thc m hnh ha bi fading phng,

nu ta gi s rng knh truyn l khng nh.

Trong mt knh fading nhanh, cc h s fading l hng s trong mi chu k symbol

v thay i t mt symbol ny sang symbol khc. Ti my thu, tn hiu ti mi nR anten

thu l mt s chng nhiu ca nT tn hiu pht b suy bin bi fading knh. Ti thi im

t, tn hiu thu c ti antenj, j= 1, 2, , nR, c k hiu bi rj

t,c cho bi:

,

1

Tnj t i i

t j i t t

i

r h x n

(1.4)

Vi itn l thnh phn nhiu ca anten thu j ti thi im t. N l mt mu c lp ca

bin ngu nhin Gaussian phc trung bnh 0 vi mt ph cng sutN0. Ta miu t cc

tn hiu thu c tnR anten thu ti thi im t bng mt vect ct nR1.

1 2

( , ,..., )Rn T

t t t t r r rr (1.5)

Nhiu ti my thu c thc m t bi mt vect ct nR 1, c k hiu bi nT

1 2( , ,..., )Rn T

t t t t n n n n (1.6)

Trong , mi thnh phn s chn mt mu nhiu ti mt anten thu. Do , vect

tn hiu thu c c thc biu din nh sau:

yt= Htxt+ nt (1.7)

Ta gi s rng b gii m ti my thu s s dng mt thut ton maximum-

likelihood khi phc chui thng tin pht v rng my thu c thng tin trng thi

knh CSI l tng trn knh MIMO. Mt khc, my pht li khng c thng tin v knh.

Ti my thu, s metric quyt nh c tnh ton da trn bnh phng khong cch

Euclidean gia chui thu gi thit v chui thu thc s nh sau:


4/22

2

,

1 1

T Tn ni t i

t j i t

t j i

r h x

(1.8)

B gii m s chn mt t m vi s metric quyt nh nh nht lm chui gii m.

1.3. Alamouti STBC

1.3.1. H th Auti TC te ht te thu

Hnh........... m ts phn tp pht khng gian-thi gian do Alamouti xut nm1998 sdng hai anten pht v mt anten tu. Png pp pn tp ny cn c gil m khi khng gian-thi gian (STBC: Space- Time Block Code) Alamouti, mc d v

mt bn cht n khng hem li li m ha.


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Png pp m a: Ti mt chu k tn hiu (symbol period) cho trc hai du tn

hiu Sfc v Sk+1 c m ha c v khng gian v thi gian. Ti khe thi gian kanten

pht th nht pht i Sktrong khi anten pht thhai pht i s/c+1- Ti khe thi gian tip

theo, k+ 1, anten pht th nht pht i -4+1 trong khi anten thhai pht is. Bng

cch ny vic m ha c thc hin theo c khng gian v thi gian.

Png pp gii m: S dng phng php tch MD tch c tn hiu.

1.3.2. H thng Alamouti vi nhiu anten thu

H thng Alamouti c thc ng dng cho mt h thng vi 2 anten pht v nR

anten thu. Qu trnh m ha v pht ca cu hnh ny cng ging vi trng hp mt

anten thu n. Cc tn hiu thu c r1j v r2

j t anten thu thj ti thi im tv t+ T

tng ng:

r1j=h1,jx1 + h2,jx2 + n1

j( 1.9)

r2j=-h1,j+ h2,j+ n2j (1.10)

Trong hi,j , i = 1, 2; j= 1, 2, ,nR l p ng knh truyn t anten pht i n

anten thu j v n1j , n2

j l cc tn hiu nhiu ca anten thu j ti thi im tv t+T tng

ng. My thu xy dng 2 i lng thng k quyt nh da trn s kt hp tuyn tnh

ca cc tn hiu nhn c. Cc i lng thng k quyt nh 1x v 2x c cho bi:

*

1 1, 1 2, 2

1

2 2 **

, 1 1, 1 2, 2

1 1 1

**

2 2, 1 2, 2

1

2 2 *

, 2 2, 1 1, 2

1 1 1

R

R R

R

R R

nj j

j j

j

n nj j

i j j j

i j j

nj j

j j

j

n nj j

i j j j

i j j

x h r h r

h x h n h n

x h r h r

h x h n h n

(1.11)

Cc quy tc gii m maximum likelihoodcho 2 tn hiu c lpx1 vx2c chobi :

1

2 2 2 2

1 1, 2, 1 1 1

1

arg min 1 ,Rn

j jx S

j

x h h x d x x

(1.12)


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2

2 2 2 2

2 1, 2, 2 2 2

1

arg min 1 ,Rn

j jx S

j

x h h x d x x

(1.13)

i vi iu ch MPSK, tt c tn hiu trong chm sao u c nng lng bng

nhau. Cc quy tc gii m maximum likelihood tng ng vi trng hp ch c mtanten thu n.

1.3.3. M Auti STBC te ht te thu

channel estimation-knh c lng

combiner-b kt hp

n ....... ki b m a kng gian ti gian Alamouti


7/22

a.

y cc bit thng tin c iu ch theo kiu iu ch a mc. B m ha sau

ly mt khi hai k t iu ch x1 v x2 trong mi ln m ha v a ra anten pht theo

ma trn m sau:

Ti mt chu k tn hiu cho trc hai du tn hiu v x2 c m ha c v

khng gian v thi gian c m t nh bng di.

Anten 1 Anten 2

he thi gian th nht x1 x2

he thi gian th hai Trong khe thi gian th nht, hai mu k hiu x1 v x2 c pht i t anten 1 v

anten 2. Trong khe thi gian th hai, anten 1 pht i v anten 2 pht i . Trong l lin hp phc ca x2, cn l lin hp phc ca x1. Bng cch ny vic m hac thc hin c theo khng gian v thi gian.

Gi ht,r l p ng knh phc gia anten pht t v anten thu r . Nu s dng 2 anten

pht, hai anten thu th ta c p ng knh nh sau:


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Anten thu 1 Anten thu 2

Anten pht 1 h11 h12

Anten pht 2 h21 h22

hiu cc tn hiu thu ti anten thu:

Anten thu 1 Anten thu 2

he thi gian th nht he thi gian th hai

Biu thc cho cc tn hiu thu nh sau:

= h1,1x1 + h2,1x2 += - h1,1 + h2,1+ = h1,2x1 + h2,2x2 + = -h1,2 + h2,2+

: H s nhiu tc ng ln anten th nht ti khe thi gian th nht.


9/22

: H s nhiu tc ng ln anten th nht ti khe thi gian th hai.: H s nhiu tc ng ln anten th hai ti khe thi gian th nht.: H s nhiu tc ng ln anten th hai ti khe thi gian th hai.

Vector tn hiu thu nhn c bng cch sp xp cc tn hiu thu nh sau:

Y =

[

]

Kt hp khe thi gian th nht v th 2 ta c :

1 111 211 1

* *1 * 1 *21 11 12 2

2 212 22 21 1

* *2 * 2 *22 122 2

( ) ( )

( ) ( )

h hy nh h xy n

h h xy n

h hy n

Trong :

11 21

* *

21 11

12 22

* *

22 12

h h

h hH

h h

h h

V:

1

1

1 *

2

2

1

2 *

2

( )

( )

n

nN

n

n

Ta nhn thy cc ct ca ma trn H l trc giao vi nhau, v y l cch thit k hiu

qu ca m khi lamouti.My thu xy dng 2 i lng thng k quyt nh da trn s

kt hp tuyn tnh ca cc tn hiu nhn c. Cc i lng thng k quyt nh 1d v

2d nh sau:


10/22

*

*

* 1 1 * 2 2 *

1 11 1 2 21 12 1 22 2

* 1 1 * 2 2 *

2 21 1 2 11 22 1 12 2

( ) ( )

( ) ( )

d h y y h h y h y

d h y y h h y h y

(trang 18 sgk)

Vector quyt nh d c to ra bng cch lc ph hp vector tn hiu thu i vi

knh c dng nh sau:

d = HHy

d = HH(Hx+n)

d = HHHx + HHn

d = x + vTrong :

.

11 21

* *

21 11

12 22

* *

22 12

h h

h h

h h

h h

t Eh =

2 2 2 2

11 21 12 22| | | | | | | |h h h h

Vecto d gm : d =

d1 = Ehx1 + v1

d2 = Ehx2 + v2

b.

Gi s tn hiu pht l x c pht trn knh c p ng knh l h,vi mu tp m ln, tn hiu thu c l y.

Th ta c y = hx + n.

Theo lut quyt nh xc sut ti a ti my thu l :

= arg min{ |yhx|2}


11/22

= arg min{d2(y,hx)}

Trong d2(y,hx) =| yhx|2 = (y hx)(y-hx)* l bnh phng khong cch Euclid

gia cc tn hiu thu c y v cc tn hiu khng cha tp m hx.

ut quyt nh M (Maximum likelihood) cho c x1 v x2 l:1 2 1 * * 2

1 2 1 11 1 21 2 2 11 2 21 1

2 2 2 * * 2

1 12 1 22 2 2 12 2 22 1

{ , } arg min{| ( ) | | ( ) |

| ( ) | | ( ) | }

x x y h x h x y h x h x

y h x h x y h x h x

(*)

hai trin i s ca (*) ta c:

*1 1

1 11 1 21 2 1 11 1 21 2

*1 * * 1 * *

2 11 2 21 1 2 11 2 21 1

*2 2

1 12 1 22 2 1 12 1 22 2

*2 * * 2 * *

2 12 2 22 1 2 12 2 22 1

1

1

| ( ) | ( )

( ) ( )

( ) ( )

( ) ( )

| |

STBCA y h x h x y h x h x

y h x h x y h x h x

y h x h x y h x h x

y h x h x y h x h x

y

2 1 * * 1 * * 1 * 2 2 * * 1 * * * 2 21 11 1 1 21 2 11 1 1 11 1 11 21 1 2 21 1 2 21 2 11 1 21 21 2 1 * 1 * * 1 * 2 2 * * * 1 * * * 2 2

2 2 11 2 2 21 1 11 2 2 11 2 11 2 21 1 21 1 2 21 1 21 2 21 1

( ) ( ) ( ) ( ) | | | |

( ) ( ) ( ) ( ) ( ) ( ) ( )

y h x y h x h y x h x h h x x h y x h x h x h x

y y h x y h x h x y h x h x h x h x y h x h x h x

1 2 2 * * 2 * * 1 * 2 2 * * 2 * * * 2 2

2 1 12 1 1 22 2 12 1 1 12 1 12 1 22 2 22 2 1 22 2 12 1 22 2

2 2 2 * 2 * 2 * * * 2 * * * 2 * * *

2 2 12 2 2 22 1 12 2 2 12 2 12 2 22 1 22 1 2 22 1 12 2 22

( ) ( ) ( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) ( ) ( )

y y h x y h x h x y h x h x h x h x y h x h x h x

y y h x y h x h y x h x h x h x h x y h x h x h

2 21( )x 2 2 2 2 2 2 2 2 2 2 1 2 1 2 2 2 2 2

11 21 12 22 1 21 11 22 12 2 1 2 1 2

1 * 2 * 1 * 2 * *

1 11 1 12 21 2 22 2 1

1 * 1 * 2 * 2 *

11 1 2 21 12 1 2 22 1

2 * 2 * 1 * 1

1 22 12 2 11 2 1

( )( ) ( )( ) ( ) ( ) ( ) ( )

( ) ( )

( ) ( )

( ) ( )

h h h h x h h h h x y y y y

y h y h h y h y x

h y y h h y y h x

y h h y h y y h

* *21 21 * 1 * 2 * 2 *

21 1 2 11 22 1 2 12 2( ) ( )

x

h y y h h y y h x

rng 1 2 1 2 2 2 2 21 2 1 2( ) ( ) ( ) ( )y y y y l cc thnh phn chung v khng lm thay i

lut quyt nh, v vy chng ta c th b qua chng cho n gin. V vy ta c:2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

11 21 12 22 1 1 1 1 1 21 11 22 12 2 2 2 2 2( )( ) | | | | | | ( )( ) | | | | | |

STBCA h h h h x d x d x h h h h x d x d x

Mt im th v c th nhn thy rng lut quyt nh M ng thi cho c x 1 v x2

c chia thnh cc lut quyt nh c lp cho x1 v x2. Do cc lut quyt nh cho x1


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v x2 l nh nhau, sau khi b qua thnh phn chung2

1| |d chng ta thu c lut quyt

nh chung cho lamouti STBC:

2 2 2 2 2 2

1 11 21 12 22 1 1

2 2 2 2 2 2

2 11 21 12 22 2 2

arg min{( ) | | | | }

arg min{( ) | | | | }

x

x

x h h h h x d x

x h h h h x d x

i vi tn hiu PS, do nng lng tn hiu nh nhau i vi tt c cc thnh phn

tn hiu, lut quyt nh M c lc gin thnh:

2

1 1

2

2 2

arg min | |

arg min | |

x

x

x d x

x d x

1.3.4 h i c Auti TC vi MC

Vi lamouti, thng tin c truyn t hai anten pht. Do truyn ti nng

lng lamouti c s dng gp hai ln so vi MC. Do cn bng chng ta cn

lm cho nng lng truyn t2 anten trong trng hp STBC mnh ln bng vi nng

lng truyn t 1 anten trong MRC.Vi quy m ny chng ta c th thy rng hiu sut

BER ca trng hp 2Tx, 1x lamouti STBC km hn 3dB so vi vi trng hp

1Tx, 2Rx so MRC.

T l li bit cho iu ch BPSK trong knh Rayleigh vi trng hp 1 anten pht, 2anten thu l:

(1.14)

Trong :

1/2

0

1 1 11

2 2 /MRC

b

pE N

(1.15)hi 2 anten pht v 1 anten thu nhn c :

(1.16)

T l li bit:


13/22

2, 1 2(1 )e STBC STBC STBC P p p (1.17)

Tng t, t hnh v chng ta c th thy rng h thng Alamouti vi 2 anten thu s

t c khnng phn tp nh mt h thng phn tp thu MC 4 nhnh nhng c tnh

li km hn 3 dB. Thng thng, h thng Alamouti vi 2 anten pht v nR anten thu c

cng li phn tp vi h thng phn tp thu MRC c 1 anten pht v 2nR anten thu.

c tnh ca h thng lamouti th km hn 3 dB, sthua km l do nng lng pht

x t mi anten pht trong h thng Alamouti ch bng phn na so vi anten n trong

h thng phn tp thu MRC do 2 h thng ny c cng cng sut pht tng. Nu mi

anten pht trong h thng Alamouti pht x cng mc nng lng vi anten pht n

trong h thng phn tp thu MRC th h thng Alamouti s tng ng vi h thng

phn tp thu MRC.

Phm cht BER ca cc h thng MISO 2 X 1 v MIMO STBC 2 X 2 s dng iu

chBPS c so snh vi phm cht ca cc h thng SIMO MRC 1X 2 v SIMO

MRC 1 X 4. Do tng cng sut pht cc h thng STBC c chun ha thnh n v

(tc l, cng sut pht t tng anten l mt na), cc ng cong BER ca cc h thng

STBC c cng dc cc ng cong BER ca cc h thng MC systems tng ngnhng dch sang bn tri 3 dB. iu ny chng t rng cc h thng STBC v MRC c

cng cp phn tp.


14/22

Hn c tnh BER ca h thng BPSK Alamouti vi 2 v 1 anten thu trn knh fading

Rayleigh chm

Vi trng hp s dng mt ng-ten pht v mt ng-ten thu cht lng km hn so

vi trng hp 2 ng-ten pht, 1 ng-ten thu v 2 ng-ten pht, 2 ng-ten thu s dng m

ha khng gian thi gian STBC.

u im-

t nhiu ng-ten pha pht, nn li dng c vic khng b gii hn v cngsut Base Station.

- Trnh nhiu- Tch tn hiu chnh xc- Thit k v gii m n gin

Nhc im- h khn trong vic c lng ma trn knh truyn- STBC phi dng 2 slot thi gian truyn, c thtng tc d liu bng cch

chuyn sang ghp d liu trn min tn s: y l k thut SFBC, lc ny thay v

dng ti nguyn thi gian th ta dng ti nguyn tn s.


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1.4. d c Alamouti STBC

1.4.1. Wifi 802.11n

Mt trong nhng iu mong i nht ca ngi dng thit bu cui Wi-Fi khng

g khc ngoi tc v tm phsng. Theo c t k thut, chun 802.11n c tc lthuyt ln n 600Mbps (cao hn 10 ln chun 802.11g) v vng ph sng rng khong

250m (cao hn chun 802.11g gn 2 ln, 140m). Hai c im then cht ny gip vic s

dng cc ng dng trong mi trng mng Wi-Fi c ci tin ng k, phc v tt cho

nhu cu gii tr a phng tin, nhiu ngi dng c th xem phim cht lng cao (HD,

Full HD, Full HD 3D...), gi in thoi qua mng Internet (VoIP), ti tp tin dung lng

ln ng thi... m cht lng dch vv tin cy vn lun t mc cao.

Hnh 1: Logo chng nhn sn phm t chun 802.11n

Bn cnh , chun 802.11n vn m bo khnng tng thch ngc vi cc sn

phm trc , chng hn, nu sn phm Wi-Fi chun n s dng ng thi hai tn s2,4GHz v 5GHz th stng thch ngc vi cc sn phm chun 802.11a/b/g.

Chun 802.11n c IEEE (Institute of Electrical and Electronics Engineers) ph

duyt a vo s dng chnh thc v cng c Hip hi Wi-Fi (Wi-Fi Alliance) kim

nh v cp chng nhn cho cc sn phm t chun. Chng nhn chun Wi-Fi 802.11n


16/22

l bc cp nht thm mt stnh nng ty chn cho 802.11n d tho 2.0 (draft 2.0, xem

thm bi vit ID: 0905_100) c Wi-Fi Alliance bt u t hi thng 6/2007; cc yu

cu c bn (bng tng, tc , MIMO, cc nh dng khung, khnng tng thch ngc)

khng thay i. y l tin vui cho nhng ai ang shu thit bt chng nhn 802.11n

draft 2.0. Chng nhn Wi-Fi n vn m bo cho hn 700 sn phm c cp chng nhndraft 2.0 trc y (gm my tnh, thit bin ttiu dng nh tivi, my cha phng

tin (media server) v cc thit b mng). Tt c thit bc cp chng nhn dng draft

n c iu kin s dng logo "Wi-Fi CERTIFIED n" m khng cn phi kim tra

li.(Hnh 1).

Vy u l nhng cng ngh quan trng ca chun 802.11n? Mt cng ngh mi

lun gn lin vi cc ci tin nng cao cht lng dch v v Wi-Fi 802.11n cng vy.

Mc tiu chnh ca cng nghny l tng tc v tm ph sng cho cc thit b bngcch kt hp cc cng nghvt tri v tin tin nht (xem hnh 2).

MIMO trthnh bt buc

Vi c t k thut c ph chun, MIMO (tham kho ID: A0905_100) l cng

ngh bt buc phi c trong cc sn phm Wi-Fi 802.11n. MIMO c thlm tng tc

ln nhiu ln thng qua k thut a phn chia theo khng gian (spatial multiplexing) -

chia mt chui d liu thnh nhiu chui d liu nhhn v pht nhiu chui nh song

song ng thi trong cng mt knh - tng t cc ln xe trn xa l. Ngoi ra, MIMO


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cn gip ci thin phm vi phsng v tin cy (gim t l li) ca thit b thng qua

mt k thut c gi l phn tp khng gian (spatial diversity). Kt hp vi cng ngh

MIMO l 2 k thut (ty chn): M ha d liu STBC (Space Time Block Coding) gip

ci thin vic thu/pht tn hiu trn nhiu anten; v ch HT Duplicate (MCS 32) - cho

php gi thm gi tin tng t cng lc ln mi knh 20MHz khi thit b hot ng ch 40MHzgip tng tin cy cho thit b pht (xem hnh 3).

Nng cao knh tn s

Ngoi nhng li ch t c t MIMO, cng ngh 802.11n cn s dng mt s k

thut khc nhm tng tc d liu nhanh hn bng cch s dng knh (channelization)

rng hn. Thay v ch s dng knh 20MHz nh cc chun 802.11a/b/g trc y, chun

802.11n s dng c hai knh 20MHz v 40MHz. Cc knh 40MHz gip tc truyn d

liu tng gp i, ln n 150Mbps/mt chui d liu khng gian (spatial stream), xembng tc ca chun 802.11n (xem hnh 4 v bng tc ).

Nm 2007, IEEE cho ra i chun 802.11n cho mng Wifi c tc l thuyt ln

n 600Mbps v vng ph sng rng khong 250m. Hin nay IEEE 802.11n vn cn

ang trong giai on th nghim nhng hu ht mi thit b trn thtrng iu c chun

ny.

u im ca 802.11n l tc nhanh nht, vng ph sng tt nht; trkhng ln

hn chng nhiu tcc tc ng ca mi trng. Nhc im ca 802.11n l chac ph chun cui cng; gi cao hn 802.11g; s dng nhiu lung tn hiu c th gy

nhiu vi cc thit b 802.11b/g k cn.

MIMO l cng ngh bt buc phi c trong cc sn phm Wi-Fi 802.11n. MIMO c

thlm tng tc ln nhiu ln thng qua k thut a phn chia theo khng gian (spatial

multiplexing) - chia mt chui d liu thnh nhiu chui d liu nhhn v pht nhiu

chui nh song song ng thi trong cng mt knh - tng t cc ln xe trn xa l.

Ngoi ra, MIMO cn gip ci thin phm vi phsng v tin cy (gim t l li) ca

thit b thng qua mt k thut c gi l phn tp khng gian (spatial diversity). Kthp vi cng ngh MIMO l k thut: M ha d liu STBC (Space Time Block Coding)

gip ci thin vic thu/pht tn hiu trn nhiu anten.

Mi trng nh


18/22

Vi phm vi v sng tng ln v, 802.11n cho php video cht lng cao v m

thanh hnh nh (AV) cc ng dng a phng tin truyn trong mi trng gia nh.

Bng thng cao v cht lng dch v ca 802.11n gip m bo mt kt ni internet

c thxc nh c mt cch chc chn c chia s bi s lng ngy cng tng v

loi hnh Wi-Fi cho php cc thit b trong nh m khng c s xung cp ca dch v.

Tc d liu cao hn ca 802.11n lm tng thong lng qua cc mng Wi-Fi chng

cho.

Phm vi tng ln ca 802.11n cung cp vng ph sng ca ton bngi nh, t xa

hn so vi cc cng ngh c v gim thiu "im cht" hoc cc khu vc c tc thp

trong nh. Ngay c mt ng-ten thit bdi ng, chng hn nh in thoi Wi-Fi, sc

hng nhng li ch ca vic tng phm vi v thng lng ca 802.11n Wi-Fi

CERTIFIED nhkhnng truyn ti a dng, chng hn nh M ha khi khng gianthi gian (STBC),.

Hu ht cc mng li giao dch, bao gm c dch v thoi v d liu, sc hng

li ng k s kt hp t cc cng ngh khung 802.11n. In n cc tp tin tmy tnh n

my in, chuyn cc tp tin gia cc my tnh v a mng v cc file chia s gia cc

my tnh, my tnh xch tay v cc thit b khc trn mng tr nn hiu qu hn vi

802.11n

Mi trng doanh nghip802.11n l doanh nghip cp cng ngh m s cung cp cho qun l IT vi cng mt

dch vng tin cy m h mong i t cc mng Ethernet ca h. Nhim v quan

trng ng dng doanh nghip, chng hn nh qun l quan h khch hng (CRM) v

Enterprise Resource Planning (ERP) truy cp, cng c hp tc, hi ngh truyn thoi v

video, sc hng li tthng lng v phm vi ca 802.11n.

Hiu qu v ci tin ca 802.11n trn a ch MAC v lp vt l (PHY), kt hp vi

khnng WMM cht lng dch v, ci thin cht lng ca VoIP tng slng

cc cuc gi ng thi trn cc Airlink. Tng vt l truyn ti ca 802.11n, chng hn

nh M ha khi khng gian thi gian (STBC),nng cao nhn ngay c cho wifi s dng

mt anten nhn nhtruyn nhiu bn sao ca mt dng d liu thng qua nhiu ng-ten.

Nhiu phin bn ca tn hiu n in thoi nhn c v x l vi k thut gii m

d tr, tip nhn tn hiu v ti u ha.


19/22

M trng hc v thMng trng hc v ththng hot ng trong mi trng khc nghit phm vi

l vn ln nht. 802.11n c s dng ci thin phm vi hot ng ngay ci vi

thit b cm tay s dng mt ng-ten trong mng ngoi tri nh vy. Tng phm vi ca

cc thit b cm tay t c thng qua AP truyn v nhn.

1.4.2. WiMax chu 806 s d hi h i thi i (TC)

Ngy ny Internet ngy cng trthnh ph bin v l mt cng c khng th thiu

trong cuc sng ca con ngi. Theo s pht trin ca Internet, nhu cu truy cp Internet

vi tc cao ngy cng ln dn ti s pht trin ca cng ngh truy cp bng rng.

Hin nay chun truy cp v tuyn bng rng 802.16 Wimax ng dng trong m hnh

mng khng dy thWMN ang c nghin cu, trin khai trn phm vi ton

th gii v c Vit Nam. Wimax ra i p ng c nhng nhu cu cao v s

dng Internet nh tc truy cp cao, bn knh vng ph rng, cht lng dch v

tt.ha hn mt s pht trin nhanh chng, thay th nhng dch v hin ti nh DS

(Digital Subcriber ine: ng dy thu bao s), wifi trong tng lai. Truyn tin v

tuyn chu nh hng ca mi trng, a hnh, gy nn suy hao ln. Mt trong nhng

cng ngh gip cho Wimax khc phc nhng nhc im trn l s dng m hnh a u

voa u ra MIMO vi nhng k thut phn tp gip ci thin hiu sut ph, tng

dung lng h thng. C nhiu k thut phn tp c s dng trong h thng MIMO

nh m khi khng gian thi gian(STBC),

WiMAX l 1 k thut mng truyn dn da trn 2 tiu chun l IEEE 802.16 v

ETSI HiperMAN. Nhm pht trin WiMAX gi l WiMX forum (ra i thang 6/2001)

hp tc vi nhm IEEE 802.16 xy dng nn mt mng giao tip v tuyn hot ng

lp PHY v MAC. Phin bn u tin, WiMAX 802.16a.2001 xy dng 1 h thng

cung cp dch vbng thng rng (BWA) cho cc user cnh bng k thut truyn dn

v tuyn nh l 1 phng thc trin thay th cho mng cable bng thng rng (DSL).

Cc phin bn sau 802.16d.2004 (h trcho fixed v nomadic user) v 802.16e.2005 (htr nomadic v mobile user). n nm 2007 th WiMX c cng nhn nh l mt

mng di ng th h 3G (IMT2000). V s l 4G vi WiMAX 802.16m.

Nh vy WiMX ra i khi m UMTS (3GPP) v 1xEVDO(3GPP2) bt u trin

khai, WiMAX gn nh xy dng t con s 0 vi mc ch ban u khc khng phc v


20/22

di ng, nn vic WiMAX ko cn phi tng thch vi h thng thit b ca 2G, 2.5G l

iu hin nhin. Ngay c cc thit b theo tiu chn 802.16e cng ko tng thch ngc

vi 802.16d. Nhng 802.16m stng thch ngc vi cc chun c

Hin WIMAX c cc chun sau 802.16a,b,c,d,e v m nhng nhn chung 2 chun ph

bin v c ni n nhiu nht l d v e . Chun 802.16d l chun dnh cho WIMAX c

nh. Chun 802.16e l chun dnh cho WIMX di ng. Chun m ang trnh ln

ITU-T ph duyt l chun WIMX t chun 4G ( cnh l 1 Ghz v di ng l 100

Mhz).

Cng ngh WIMAX l cng ngh v tuyn tc cao DL v UL bt i xng. S

dng ph tn t2Ghz n 11Ghz. Vit Nam ph tn c cp php l 2.3 --> 3.5 Ghz.

H trc FDD v TDD. Khnng m xuyn ca Wimax vt xa so vi Wifi nhs

dng anten thng minh nh hng.Tnh nng u tin phi kn trong WiMax l khnng bo mt. Nh chng ta

bit, kt ni WiFi tn ti rt nhiu vn , c bit l khi truy cp Internet ti cc im

truy cp cng cng. Vi nhng ngi tng s dng WiFi, khng c l do g khin h

ngh rng nhng s ctng t s khng xy n vi WiMax. Ngoi ra, hu ht cc s

c bo mt ca WiFi u bt ngun t thc t rng ngi dng cng l qun tr vin

mng v thng thng h khng phi l nhng ngi bo mt mng c kinh nghim.

WiMax khng nm trong tm kim sot ca ngi dng, n l mt gii php ca nh

cung cp dch v vin thng. iu ny c ngha l nh cung cp dch v ny s tin hnh

qun tr mng v cc nhn vin bo mt mng c kinh nghim s bo mt cho mng.


21/22

Lin quan ti s suy gim thc thi m nhiu ngi dng gp phi, y khng phi l

mt vn vi WiMax. Trc tin, vn ny c khc phc trit trong WiMax

vi MIMO (multiple-input and multiple-outputnhiu u vo v nhiu du ra). Tng

tMIMO c tch hp trong chun WiMax.

Mt im khc ca WiMax gip hn ch s suy gim ca kh nng thc thi (suy

gim tc cng nh nhiu) l WiMax s dng quang ph. iu ny c ngha l

WiMax s dng nhng tn schi ph cao. Do , in thoi vi sng hay khng dy s

khng b nhiu sng vi kt ni WiMax v cc nh cung cp dch v c th phn bbng

thng rng ph hp cho mi ngi dng gim thiu s suy gim tc .

Hin WIMX c mt khong 70 quc gia trn ton th gii, pht trin mnh

M,Chu u, Hn Quc, n v.v.v. Cc hng vin thng ln trn th gii nh T&T ,

Centel, Samcomtel, Intel,Motorola v.v.v v ang trin khai WIMX.Tnh n nay, B

TT&TT cp php trin khai th nghim cng ngh WiMAX cho 10 doanh nghip

Vit Nam, trong c VNPT, Viettel, VTC, FPT Telecom, CMC, EVN Telecom, ng

Dng Telecom. VNPT l doanh nghip u tin c cp php th nghim WiMAX v

cng l doanh nghip ang trin khai th nghim mnh m nht cng ngh ny ti Vit


22/22

Nam. Tuy nhin, n nay vn cha c doanh nghip no chnh thc cung cp dch v

WiMAX.

Trin khai th nghim WIMAX cnh cc min ni pha bc ca VDC ti Lo

Cai hay cc cng ty khc gp mt skh khn nh Modem thu WIMX b che khut v

khng gian trng khng c cc ng NLOS nn gp mt s vn . Nm ngoi vo dp

Festival Hu VNPT( VDC) v lcatel trin khai th nghim thnh cng ti mt s

im thnh ph Hu, H Long th tnh hnh tt hn nhiu khi cc bn i du lch H

Long th trn thuyn kt ni Wimax d dng v tc ti mt trang web trung bnh

khong 2s, ni chung l th nghim thnh cng. Ngi dng u cui ch cn mua mt

thit bIndoor Wimax (kch thc bng mt modem ADSL), ri cm dy mng l c th

dng c Internet tc cao. Ngoi ra, Wimax c nh cng c th thay th ng

truyn leased-line ca cc DN.Thit bu cui ca dch vWimax di ng c th l cccard PCMCIA, USB, hoc c tch hp sn vo trong con chip my tnh (kiu nh

cng ngh Centrino ca Intel). Gi cc thit b cng ngy cng gim PCMCIA ca

Alcatel khong 75$.Nhiu ng dng cng ngh cao nht c p dng nh: truy cp

Internet tc cao, video streaming, video on Demand, dch v thoi VoIP

bao cao(sua)

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