Mo Hinh Kenh Va Uoc Luong Kenh

7/29/2019 Mo Hinh Kenh Va Uoc Luong Kenh

1/19

-38-

HVTH: L Vn Hng

Chng 6 - M HNH KNH TRUYNV C LNG K NH

6.1 Nhng c tnh ca k nh truynv cc loi suy hao

nh gi kh nng hot ng ca mt h thng thng tin v m tchnh xc k nh truyn khng dy th phi xc nh c c tnh mi trng

truyn. Cu trc ca mt h thng thng tin v tuyn s cho thy mt quy tc

quan tr ng trong m hnh ha knh truyn.Sau y l nhng c tnhquan tr ngca k nh truyn trong h thng

truyn thng khng dy[18], [4], [5].Suy hao ng truyn(Path Loss)

Path Loss b nh hng bi nhiuyu t bao gmkhong cch gia bn phtv bn nhn, mi trng truyn, chiu cao v v tr ca anten, rng bngthng c s dngvvMt m hnh n gin cho suy hao ng truyn L l:

21r

t

P L K

P f d a = = vi r P l cng sut trung bnh ca tn hiu nhn,t P l cng

sut truyn, d l khong cch gia bn nhn v bn pht, f l tn s v k l hng

s truyn.H s suy hao ng truyn 2a = cho ng truyn trong khng

gian t do, thng thng trong cc mi trng tiu biu a nm trong khong2 4a

Gii hn ca suy hao ng truyn.T s cng sut tn hiu tr n nhiu c

cho bi biu thc:

0

1.t r n

KPPSNR

P d N Ba

= = vi N0 l mt ph cng sut ca nhiu v B l bng

thng ca tn hiu.Suy hao do che chn (Shadow Fading )

Shadow Fading l mt hin tng suy hao do bin v pha dao ng xung

quanh mt gi tr hng s trong thikhong truyn mt k hiu thng tin. N csinh ra do s cn tr ng truyn tn hiu gia bn nhn v bn pht.Suy hao do che chn bin i cng vi mi trng nitn hiu ang truyn dohnh d ng ca ahnh, cy ci v s hin din ca cc ta nh, i niv.v


2/19

-39-

HVTH: L Vn Hng

Cng sut tn hiu nhnPr (dB) thay i xung quanh gi tr trung bnh ca n

Pr (dB)= ( )r sP dB G+ vi 2(0, ), 4 10s s sG N dBs s l hm phn b Gauss[41].

Suy hao a ng(Multi-path Fading )

Suy hao a ng l do s phn tn mt cch t nhin ca mi trngtruyn.Truyn khngtheo ng thng trc tip (Non line-of-sight) trong knh truynkhng dy lm cho cng mt tn hiu truyn theo nhiu ng khc nhau vn u nhn nhng thi imkhc nhau vi pha v bin b thay i. u

nhn xem k nh truyn nh l mt k nh b tr hon m y k hiu c

truyn n tr hn so vi ng truyn thng(hnh 6.3). Pht tn tr hon lmt tham s c s dng n gin ha s nh hng ca a ng truyn

v n ph thuc vo a hnh, khong cch, hng caanten.v.vM t knh a ng thng c biu din di dng h s k nh truyn h l

mt hm theo thi gian t, ( ) ( )i ji ii

h t a e t t q d = -

Dch chuynDoppler(Doppler shift )Dch chuynDoppler l mt yu t khc nh hng n qu tr nh truyn do sdi chuyn tng i gia bn nhn hoc bn pht trong khi truyn.

Hnh 6.1: nh hng Doppler Gi s rng mt my thu di chuyn vi vn tcv nh hnh 6.1.Khong thay i ng truyn trong khi nhn l cosl d q = vi d v t = D d n n

s thay i v pha 2 lp f l

D = lc s dch chuyn Doppler c cho bi biu

thc:


3/19

-40-

HVTH: L Vn Hng

1 . .cos2d

v f

t f

q p l

D= =D

Trong biu thc tr n ta thy rng nu my thu di chuyn hng v trm pht thsinh ra d ch chuyn dng ngc li l sinh ra d ch chuyn m.Cc loi suy hao thnggp trong k nh truyn

Frequency Selective Fading (Suy hao tn s chn lc) Nu bng thng ca k nh truyn nh hn bng thng ca tn hiuth knhtruyn to nn hin tng suy hao theo tn s chn lc. p ng tn s ca tn

hiu v tuyn s to nn mt ch trng suy hao do hin tng a ng.

r ng bng thng k nh truyn s gii hn bng thng ca tn hiu v d n n s

chng lp ca cc k hiu lin tip nhau trong min thi gian. Ton b thigian truyn tn hiu tr nn di hn thi gian truyn tht s. Hin tng nyc gi l can nhiu gia cc k hiu ISI.

Flat fading(Suy hao nh phng) Tn hiu s b suy hao nh phng nu rng bng thng k nh truyn ln hnso vi rng bng thng ca tn hiu.Hnh d ng ph tn ca tn hiu vn duytr nhng thay i v li. Tn hiu bng hp s ri vo trong loi suy haony v bng thng ca chng nh hn so vi bng thng ca k nh truyn. K nhsuy hao nh phng s gy nn nhiu thch thc nh s thay i v li v

ph tn s. Fast fading(suy hao nhanh)

Bng thng ca k nh truync th thay i do k nh truyn.Suy hao nhanh xyra nu thi gian tn ti k nh nh hn thi gian pht tn hiu. K nh truyn tr

nn thay i theo thi gian v p ng xung ca k nh xy ra mt cch nhanhchnh so vi thi khongca tn hiu. Nu tc ca tn hiu thp th d bnh hng ca hin tng suy hao nhanh.

Slow Fading(suy hao chm) Nu bng thng ca tn hiu ln hn nhiu so vi s tr hon Doppler th d ch

chuyn do hin tng Doppler tr nn khng nghim tr ng. y c gi lsuy hao chm v p ng xung ca knh thay i chm so vi tc thng tin

c truyn.


4/19

-41-

HVTH: L Vn Hng

Rayleigh Fading (Suy hao Rayleigh)c tnh hnh thnh v tiu hy ca cc thnh phn a ng trong k nh suyhao nh phng c th c xp x bi s phn b Rayleigh. Nu khng c

ng truyn trc tip c ngha l khi khng c ng truyn khng trctip gia bn pht v bn nhn. Phn b Rayleigh v c bn c bin l tng

ca hai bin ngu nhin Gaussian c quan h c lp v tr c giao. Hm mt

xc sut c cho bi: 2

222( ) 0

r r p r e r s

s

-=

(6.1)

trong 2s l cng sut trung bnh theo thi gian ca tn hiu nhn Lun vn ny s s dng m hnh knh Rayleigh m phng k nh

truyn trong phn cn bng k nh cho h thng OFDM s dng DFE-PIC vtin hnh kho st tr n h thng k nh truyn DVB-T m y vic truyn vnhn vn duy tr tr ng thi khng thay i. L do l mi trng ca mng n

tn lm vic trong nhiu ngi pht v pht cng loi d liu cng mt lc t

nhng v tr tng ng ca chng. u nhn s hiu iu ny nh mt tnhhung m ch c mt my pht pht i tn hiu v sao chp nhiu ln nu nhn, mi tn hiu i theo mt ng truyn ring v tng ng vi s tr hon ca chng.

C nhiu cch m hnh v c lng k nh cho mt h thng thng tinkhng dy. Nhng ta phim hnh mt k nh truyn sao cho ph hp vi vicc lng cng nh cnbng knh. iu quan trng l phi to ra mt m

hnh tng ng thuc bnggc vi cc h s tr hon theo cu trcgingnh mt lc FIR (nhhnh 6.2). Trong phn ny ta tm hiu c hai m hnh lm hnh bt bin thi gian v m hnh thay i theo thi gian. C nhiu phng php khc nhau c lng mt k nh truyn bao gm c lng

d a vo c s hun luyn, c lng m, c lng na m hoc c lngd a vo cc sng pht d n ng(sng pilot).

Trong cch c lng hun luyn, mt chui thng tin khng bit tiu thu c truyn theomt m hnh vi cu trcthu nhn. Trong cch c


5/19

-42-

HVTH: L Vn Hng

lng m, khng c mt chui thng tinno c sn c truyn i m thay vo knh c c lng da tr n duy nht tn hiu nhn c ti u thu cs

tc ng ca nhiu v cc tnh cht khc thuc v bn cht ca tn hiu. c

lng na m s dng kt hp cch c lng da tr n phng php clng chun luyn v phng php c lng m. Cn cch c lng da

vo cc sng pilot n s dng mtchuihun luyn tun hon, chui ny c

cng chng chp vo lung thng tin nhng c cng sut thp. Bin casng pilot thng ln hn so vi bin ca lung thng tin c pht.

Hnh 6.2 Biu din knh theo p ng xung di dngtn schn lc

v thi gianchnlctheo cu trc cc ng nhnh tr hon (lc FIR).Truyn tn hiu qua mt k nh truyn khng dy(trong nh hoc ngoi

tr i) tn hiu thu c ti u nhn n t nhiu ng truyn khc nhau. Nhng ng truyn ny sinh ra do s phn x, khc x, hoc do s nhiu xtrong knh truyn. Hin tng a ng ny s thu c u nhn l s chng

chp ca nhiu tn hiu l phin bn ca tn hiu ban u n t cc ng

khc nhau theo mt t l tr hon no . y chnh l nguyn nhn gy nn hin tng suy hao c chn lc trong min tn s. Ngc li, nu mi trngxung quanh u pht v u nhn c th thay i theo thi gian dn n s thayi trong p ng ca k nh truyn theo thi gian v iu ny gy nn hintng suy hao mt cch cchn lc trong min thi gian. Mt k nh truyn cth c mt ng truyn c u th (ng truyn trctip khng b pht tn

phn x) v nhiu ng truyn ph khc, cng c th c k nh truyn ngunhin khng c ng truyn u th.

Hin tng a ng gy nn nhiu gia cc k hiu ti u thu, hintng n

y c th dn n mt t l li cao trong vc tch k hiu. B cn bng

thng c thit k b cho mo d ng do nhng k nh truyn nh trn gy


6/19

-43-

HVTH: L Vn Hng

nn. Ta c th thit k mt b cn bng trc tipti u nhn hoc c th clng k nh truyn ri sau thit k mt b cn bng da tr n nhng thng tinknh c c lng. Sau khi qua vi qu trnh x l (chng hn nh dng

lc tng khp) tn hiu nhn cl lin tc theo thi gian sau c lymu theo tc tn hiu hoc cao hn trc khi x l c lng cng nh cn

bng k nh. Nguyn nhn l v ta mun xy dng mt m hnh knh r i rc thi

gian thuc bng tn c s.

Hnh 6.3 Knh a ng Thng thng, mt chui hun luyn (c bit ti u nhn) c

truyn trong sut thi gian khi lp(trong ch thu nhn). Trong giai on

hot ng, thit b nhn chuyn sang ch quyt nh trc tip, trong ch ny k hiu c tch v c cn bng trc c s dng nh l mt

chui hun luyn (gi) cng vi d liu nhn c s cp nhn thng tin k nhv h s cn bng.6.2 M hnh knh6.2.1 M hnh knh thayi theo thi gian

Xt mt k nh truyn thay i theo thi gian vi tn hiu nhn l x(t), tnhiu ny l tn hiu phc lin tc theothi gian thuc bng tn c s , tn hu

pht l s(t) c hai u c chu k tn hiu l Ts. Tn hiu nhn x(t) c cho bi

biu thc (6.2)[33].


7/19

-44-

HVTH: L Vn Hng

( ) ( ; ) ( ) ( ) x t h t s t d wt t t t +

-

= - + (6.2)

Trong ( ; )h t t l p ng xung theo thi gian ca k nh truyn, n biu dinp ng xung ca k nh ti thi im t v tn ti trongkhong thi giant tothnh hnh d ng ca mtxung n v v w(t) l nhiu Gauss. Mt hm pht tntr hon Doppler ( ; ) H f t c nh ngha nh l bin i Fourier ca hm

( ; )h t t :

2( ; ) ( ; ) exp j ft H f h t dt p t t + -

-=

(6.3)

Nu ( ; ) 0 H f t vi d t t > th d t c gi l s pht tn tr hon ca k nh.

Nu ( ; ) 0 H f t vi d f f > th d f c gi l pht tn Doppler ca k nh

truyn. phng tr nh 6.2 l cng thc ph bin nht cho k nh truyn di ng. khi qut c tnh phc tp ca tnhcht tc dng ca k nh truynvi h

s k nh thc th ( ; )h t t c m hnh mt cch c th nh l mt qu tr nh xl ngu nhin ca mt hm hai bin c tr trung bnh bng 0. Trong chng ny ta m hnh ( ; )h t t nh l mt hm ngu nhin cho d dng

nm bt m n c th m t c cc c tnh nh nu tr n.6.2.2 M hnh knh ri rc thi gian

Ta xt m hnh mt k nh truyn ri rc thi gianvi tn hiu digc l bin phc. Nu s dng c chch iu ch tuyn tnhth tn hiu bng gc c th c biu din nh phng tr nh (6.4).

( ) [ ] ( )T si

s t I k g t kT

=-

= -

(6.4)

Trong {I[k]} l chui thng tin v ( )T g t l lc thng thp. V th tn hin

bng gc u nhn c cho bi phng tr nh (6.5).


8/19

-45-

HVTH: L Vn Hng

( ) [ ] ( ; ) ( ) ( )T si

x t I k h t g t kT d w t a a a

-=-

= - - +

(6.5)

Sau khi s dng lc u thu vi p ng xung( ) Rg t tn hiu bng gcnhnc tr thnh:

( ) [ ] ( ) ( ; ) ( ) ( ) R T sk

y t I k g t h g kT d d v t b b a b a a b

- -=-

= - - - +

(6.6)

vi ( ) ( ) ( ) Rv t g w t d t t t = - . Nu y(t) l tn hiu lin tc trong min thi gian

c ly mu mt ln trong mi thi gian Ts th ta thu c chui ri rc thigian c m t theo phng trnh (6.7).

[ ] : ( ) | [ ] [ ; ] [ ]

[ ; ] [ ] [ ]

S t n T k

k

y n y t I k h n n k v n

h n k I n k v n

== -

= -

= = - +

= - +

(6.7)

trong h[n;k] l p ng k nh ti thi im n ko di theo xung lc n vtrong khongthi gian l n-k v

[ ; ]: ( ) ( ; ) ( ) R S T S h n n k g nT h g kT d d b b a b a a b

- -- = - - - (6.8)

Ch r ng {v[n]} trong phng tr nh (6.7) khng cn thit phil nhiu trng,

n c th c trng ha bng lc tuyn tnh bt bin thi gian. Nn t y tr

i ta gi s rng lun c mt lc trng ha p dng cho y[n]. Vi h thng nhnqu, h[n;k]=0 vi kL. Trong trng hp ny ta thay i phng trnh (6.7) nh phng tr nh(6.9).

0[ ] [ ; ] [ ] [ ]

L

k

y n h n k I n k w n=

= - +

(6.9)


9/19

-46-

HVTH: L Vn Hng

Phng tr nh (5.7) biudinknh tuyn tnh theo tn s v thi gian chn lc.

Vi h thng thay i chm theo thi gian ta c th xem n nh bt bin theo

thi gian v phng tr nh (6.9) c th biu din li nh phng tr nh (6.10).

0[ ] [ ] [ ] [ ]

L

k

y n h k I n k w n=

= - +

(6.10)

Trong h[k]=h[0;k] l p ng k nh bt bintheo thi gian. Phng tr nh(6.10) biudinmt k nh truyn tuyn tnh theo tn s chn lc v khng theothi gian chn lc.

Gi s rng h[n;k]=h[n]( ,0)k d vi ( ,0)k d l xung denta Knonecker ti thi

im 0 ( ,0)k d =1 khi k=0 v ( ,0)k d =0 kho k 0. i vi k nh truyn chn lcthi gian v khng chn lc tn s l:

[ ] [ ] [ ] [ ] y n h n I n w n= +

(6.11)

Cui cng mt k nh truyn khng theo tn s chn lc v khng theo thi gianchn lc l:

[ ] [ ] [ ] y n hI n w n= + (6.12)

vi h l bin ngu nhin.6.2.3 Knh bt bin thi gian

Tn hiu lin tc thi gian c ly mu tc baud (tc k hiu)hoc cao hn trc khi thc hin c lng k nh hoc cn bng knh. l ldo thun tin thao tc tr n tn hiu c s ri rc thi gian di s tc ng

ca nhiu trng. Vi h thng c ly mu theo tc k hiu th m hnhknh tng ng thuc bng gc c cho bi phng tr nh:

0[ ] [ ] [ ] [ ]

L

k

y n f k I n k w k =

= - +

(6.13)

Trong {w[k]} l nhiu Gauss trng c phng sai l2s ,{I[k]} l chui

thng tin (k hiu) c lp c tr trung bnh bng khng v c phn b u. N

c th l tn hiu phc v c pht i t mt tp d liu xc nh, {f[k]} l


10/19

-47-

HVTH: L Vn Hng

lc tuyn tnh c p ng xung hu hn (c cc h s c th phc), {y[k]} ltp tn hiu nhn c u nhn. Cu trc v cc h s tr hon cho m hnhny c minh ha nh hnh 6.4.

Hnh 6.4 M hnh knh theo tn schn lc nhng khng theo thi gian chn

lc Phng tr nh (6.13) m t m hnh knh tng ng cho h thng

phc ri rc thi gian thuc bng gcSISO. Chui u ra {y[n]} trong phng

trnh (6.13) cng l chui ri rc thi gian, nu tc ly mu di tc Nyqist th s dn n hin tng chng lp ph trong min tn s hay cn gi

l hin tng bit danh. Mt b cn bng tuyn tnh c thit k da tr n c

s ca mt k nh c ly mu theo tc baud (tc k hiu) s rt nhy vili nh thi k hiu ti u nhn. Ban u trong trng hp c hun luyn,

chui ly mu ri rc s uc kim tra bi b cn bng v li nh thi. Gi s rng ta ly mu tc P ln tc baud vikhongcch mu l Ts/Pgiy vi Ts l chu k ca tn hiu th biu din chui ri rc thi gian cho tnhiu ly mu c cho bi phng tr nh (6.14).

0[ ] [ ] [ ] [ ]; ( 1,2,..., )

L

i ik

y n f k I n k w n i P=

= - + = (6.14)

By gi ta c P mu tr n mi chu k tn hiu v c ch s mi mu l i. Ch r ng, chui thng tin I[n] vn c ly mt mu tr n mt k hiu. Chui nhiutrong phng tr nh (6.14) l k t quca phn on tc ly mu. V thchui nhiu l tr ng tc ly mu nhng li c lin quan n tc phnon.6.3 c lng v cn bng k nh


11/19

-48-

HVTH: L Vn Hng

6.3.1 Gii thiuKnh suy hao theo tn s chn lc v thay i theothi gian l mt

thch thc i vi ngi thit kh thng truyn thng khng dy.Vic nhn

mt tn hiu a sng mang nh OFDM phi thc hin mt vai tr k p lc lng k nh v cn bng k nh. C nhiucch thit k mt u nhn ph

thuc vo m hnh knh cs dngv tnh phc tp trong mi nhim v

thc thica h thng. c tnh ca a sng mang c th c th mnh trong kthut c lng cng nh cn bng.6.3.2c lng k nh

Trong lin k t a sng mang, cc bit iu ch c phn b trong sutthi gian tr uyn thng qua s tc ng ca k nh truyn. Knh truyn sinh ra sd ch chuyn bin v pha ca tn hiu iu ch do c tnh thay itheo thi

gian v tn s chn lc ca k nh truyn v tuyn[5].

Hnh 6.5 Tn hiu b mo dng do k nh truyn cc my thu u nhn thu c cc bt nguyn thy cn phi tnh

n cc thay i cha bit xy ra trong lc truyn. Hnh 6.5 m t mt hthng

truyn a sng mang OFDM tiu biu cs nh hng ca k nh truyn. u

nhn s dng b tch sng hockhng lin k t hoc lin k t khi phc licc bit ban u. Tch sng kiu lin k t s dng cc gi tr tham chiu c

truyn cng vi cc bt d liu. u nhn c th iu chnh vo cc gi tr


12/19

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HVTH: L Vn Hng

v c lng k nh ch cc v tr m cc gi tr tham chiu c xc nh.

Ton b k nh truyn c th c c lng bng cch s dng nhiu k thut

ni suy. i vi k thut tch khng lin k t khng s dng btc gi tr tham

chiu no m s dng cch iu ch vi sai, k thut ny truyn sai bit cahai k hiu lin tip. My thu s dng hai k hiuk cntrong min thi gian

hoc hai sng mangk cntrong min tn s so snh mt k hiu vi mt

k hiu khc c truyn. Trong phn ny ta s kho st mt vi k thut c lng knh. t cc

pilot cch nhau mt khong thi gian t p N v cch nhau mt khong f p N nh

biu thc(6.15)

1 1t f p p

d symb

N N B T f t s

D

(6.15)

vi f D l rng bng thng ca sngmang, Tsymb l thi gian truyn k hiu.

C nhiu loi sp xp cc sng pilot khc nhau ph thuc vo iu kin

knh truyn. Cc loi ny uc phn thnh hai cp cao hn. Mt l khi sp

xp pilot th im vi k nh truyn suy hao chm. Tt c sng mang c sd ng cng mt lc v knh c lng c sd ng cho tch k hiu ti tiptheo. iu ny s cho kt qu ngv s thay i ca k nh truyn r t chm vknh truyn c tng quan n chui d liu lin tip thu c. Cch th hail sp xpcc pilot th im theo hnh rng lc. iu ny c thc hin bngcch chn cc piot theo thi gian v tn s. c lng kiu hnh rng lc sd ng trong nhiu k thut ni suy c lng ton b k nh truyn. K nh suyhao theo nh phng c th c c lng bng cch sp xp cc pilot theo

hnh rng lc v s ln pilot tng nu k nh truyn l knh suy hao theo tn s

chn lc.H thng OFDM da trn c lng k nh pilot c m t nh hnh

6.6. Lung thng tin nh phn u tin c nhm li v nh x thnh cc khiu theo mt cch thc iuch no . Sau khi chn cc pilot mt cch u

n vo sng mang hoc chn vo lung d liu, khi IDFT c s dng


13/19

-50-

HVTH: L Vn Hng

chuyn lung d liuc chiu di N {X[k]} t min tn s sang min thigian{x[n]} theo ph ng trnh (6.16).

21

0

( ) { ( )} 0,1, 2,.., 1

( )kn N j

N

k

x n IDFT X k n N

X k ep -

=

= = -

= (6.16)

Hnh 6.6 H thng OFDM c bn Vi N l chiu di DFT, theo khi IDFT,khong thi gian bo v c chn

sao cho ln hnkhongthi gian tr hon ca k nh truyn mong mun v cchn vo chng li hin tng can nhiu gia cc k hiu..Khongthi gian

bo v ny l mt phn chu k m rng ca k hiu OFDM loi b can

nhiu gia cc sng mang. K t qu ca mt tn hiu OFDM c cho bi phng tr nh (6.17).

( ), , 1,..., 1( )

( ), 0,1,..., 1g g

f

x N n n N N x n

x n n N

+ = - - + -=

= -

(6.17)

Trong Ng l chiu di ca khong thi gian bo v. Tn hiu c truyn lxf (n) s truyn qua k nh truyn suy hao theo tn s chn lc v thay i theothi gian vi s tc ng ca nhiu. Tn hiu nhn c cho bi phng tr nh

(6.18)( ) ( ) ( ) ( ) f f y n x n h n w n= +

(6.18)

y w(n) l nhiu Gausse trng v h(n) l p ng xung ca k nh. p ng

xung ca k nh c th c m t bi phng tr nh (6.19).


14/19

-51-

HVTH: L Vn Hng

21

0( ) ( ) 0 1

Dir j f Tn

N i i

i

h n h e n N p

t d s t -

=

= - - (6.19)

r l tng tt c cc ng truyn, hi l p ng xung phc ca k nh truyn thi f Di l dch chuyn tn s Doppler ca k nh truyn th i, t s l ch s tr

hon. T l chu k mu v it l tr hon knh truyn th i c chun ha bi

thi gian ly mu. Ti u nhn sau khi qua b chuyn i ri rc thi gian

A/D v lc thng th p, khongbo v uc loi b

( ) 1

( ) ( ) 0,1,.., 1

f g

f g

y n N n N

y n y n N n N

- -

= + = - (6.20)

Sau y(n) c gi n khi DFT cho cc thao tc tip theo.

21

0

( ) { ( )} 0,1, 2, ... 1

1 ( ).kn N j

N

n

Y k D FT y n k N

y n e N

p - -

=

= = -

=

(6.21)Gi s rng khng c hin tng ISI th ti liu [23] chng t rng c mi

lin h gia Y(k) v H(k)=DFT{h(n)}, (I(k) l ICI do hin tng Doppler) vW(k)= DFT{w[n]} c cho bi phng tr nh (6.22).

( ) ( ) ( ) ( ) ( ) 0,1... 1Y k X k H k I k W k k N = + + = - (6.22)

Trong 21

0

sin( )( )i

Di

r j k j f T Di N i

i Di

f T H k h e e

f T

p t p p

p

- -

=

= (6.23)

22 ( )1 1

2 ( )0 0,#

( ) 1( )1

i Di

Di

j f T k K r N j K i N

j f T k K i K N K k

h X K e I k e

N e

pt p

p

- +- - -

- += =

-=+

(6.24)


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HVTH: L Vn Hng

Sau khi DFT tn hiu pilot c tch ra v c lng k nh He(k) c s

d ng cho vic c lng k hiutip theo. c lng k hiu c cho bi phng tr nh (5.25)

( ) 0,1,... 1( )e e

Y k X k N H k

= = -

(6.25)

Sau cng tn hiu c lng c gii iu ch tr thnh lung thng tin nh phn nh ban u. 6.3.3 Sp xp ccnhm pilotc lng k nh d a vo cc khi pilot c pht trin d i gi thit l knhtruyn suy hao chm, c lng k nh d a vo cc pilot sp xp nh vy c thd a tr n thut ton bnh phng ti thiu (LS) hoc cn bc hai ca tr trung

bnh nh nht MMSE. i vi MMSE tn hiu nhn s l:2{ } H H YY hh N R E YY XFR F X I s = = +

(6.26)

R YY l ma tr n phng sai ca Y, R hh l ma tr n phng sai ca h v 2s l

phng sai ca nhiu E{|W(k)|2}Tri li c lng LS l cc tiu ha sai bit gia gi trnhn v gi tr c

c lng. V th LS s cc tiugi tr ( ) ( ) H Y X F h Y X F h- - Knh c c lng trong LS c cho bibiu thc (6.27).

1 , LS H X Y -=

(6.27)

6.3.4c lng k nh c quyt nh hi tip Khi knh truyn l knh suy hao chm, th c lng k nh bn trong cc

khi c cp nht bng cch s dng b cn bng c quyt nh hi tip chomi sng mang. B cn bng c quyt nh hi tip DFE ca sng mangth k c th c m t nh sau [21].

p ng k nh cho sng mang th k c lng t k hiu trc {He(k)}c s dng tm uc lng cho k hiu pht tip theo {Xe(k)}.


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HVTH: L Vn Hng

( )( ) 0,1,... 1( )e e

Y k X k k N

H k = = -

(6.28)

{Xe(k)} c gii nh x thnh d liu nh phn thng qua bgii nhx tn hiu v sau nh x tr li thnh {X pure(k)} thng qua b nh x

tn hiu. Knh c lng H e(k) ccp nht theo biu thc(6.29).

( )( ) 0,1,... 1( )e pure

Y k H k k N

X k = = -

(6.29)Gi s rng b cn bng quyt nh hi tip thc hin quyt nh ng,

knh truyn suy hao nhanhcng c th gy nn hin tng mt cc tham s

knh c c lng. V th khi suy hao k nh truyn tr nn thay i nhanhhn th c mt s nh igia li c lngdo ni suy v li do mt du vt

ca k nh truyn. i vi k nh truyn suy hao nhanh loi c lng da vo pilot hnh lc thc hin c lng tt hn so vi cc cch sp xp khc ca pilot.

6.3.5 Sp xp pilot theo h nh rng lc c lng k nh d a tr n pilot hnh rng lc c xut s dng

cn bngknh khi s thay i ca k nh xy ra trong mt k hiu ca tn hiua sng mang OFDM.Loi c lng ny bao gm cc thut ton c lngknh thng qua tn s ca chui pilot v ni suy k nh.c l ng knh cc tn s pilot da tr n sng pilot hnh lc c th s dng

thut ton LS, MMSE hoc LMS. Mt s ti liu chng t rng MMSE thc

hin tt hn so vi LS, tnh phc tp ca MMSE c gim bt bng cch s

d ng php s dng c lng hng thp ti u v phn tch tr ring trong i

s tuyn tnh. Php ni suy trong c lng k nh pilot hnh lc c th ph thuc vo ni

suy tuyn tnh, ni suy bc hai, ni suythng thp hoc ni suy trong min


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HVTH: L Vn Hng

thi gian. Ti liu [55] cho thy ni suy bc hai tt hn so vi ni suy tuyn

tnh.

Hnh 6.7 Pilot c sp xp mt cch u n theo hnh rng lc

Trong c lng k nh d a tr n pilot hnh lc tn hiu pilot NP c chn mtcch u n vo lung d liu X(k) theo phng tr nh (6.30)

( ) ( )( ), 0

Data 1,... 1 p

X k X mL l

x m l

l L

= +=

= = -

(6.30)

Vi L =(s sng mang /N p) v x p(m) l gi tr pilot th m. Ta nh ngha{Hp(k), k=0,1,..,N p} l p ng tn s ca k nh ti cc v tr ca sng mang pilot. c lng k nh d a trn sng mang pilot c cho bi biu thc(6.31).

0,1,... 1 pe p p

Y H k N

X = = -

(6.31)

Vi Y p(k) v X p(k) l u ra v u vo ca sng mang pilot th k V c lng LS d b mc phi nhiu v ICI, do MMSE c xut s

d ng vi phc tp cao hn. V phng php MMSE phi tnh ton nghcho ma trn mi ln lp nn c lng MMSE tuyn tnh n gin ha c xut s dng. Trong phin bn n gin ha, nghch o ma trn ch tnh

mt ln. Mt s ti liu chothy tnh phc tp c th gim xung khi p dngxp x hng thp (low- rank) bng cch s dng php phn tch tr ring.

6.3.6 K thut ni suy


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HVTH: L Vn Hng

c lng da tr n pilot hnh lc mt k thut ni suy hiu qu cn

thit c lng k nh trong min thi gian hoc tn s. Cc sng mang pilot

c th c chn vo trong mt k hiu hoc gia cc k hiu. Gi tr pilot c

lng c lin k t li theo mt trt t ly gi tr c lng cho gi tr cad liu. Cck thut sau y thc hin ni suy trong mt k hiu nhng cng

c th p dng gia cc k hiu. 6.3.7 Ni suythng thp

Ni suythng thp thc hin bng cch chn 0 vo trong lung d liunguyn thy ri sau p dng lcthng thp c p ng hu hn FIR, lc nycho php lung d liu nguyn thy truyn qua nhng khng thay i. Sau ni suy gia chngtheo cch ging nh trung bnh cn bc hai ca li gia ccim ni suy v gi tr l tng ca chng c cc tiu. 6.3.8 Ni suy trong min thi gian

Ni suytrong min thi gian l gii php ni suy vi phn gii cao

d a trn phng php chn 0 v bin i IDFT/FFT. Sau khi thu c mt clng k nh{Hp(k), k=0,1,,Np-1}, u tin n c chuyn sang min thi

gian bng php bin i IDFT:

21

0( ) , 0,1,... 1

p

p

kn N j N

p pk

G n H e n N p -

=

= = - (6.32)

Sau bng cch s dng cc tnh cht x l a tc tn hiu c ni suy

bng cch chuyn N p imv N im vi phng php nh biu thc(5.33):

12, 0 2

0,2

( 2 1), 1

p

p

p N

p

N M

G n M

N G N M

G n N M M n N

= + -

= -

- + - - - < -

(6.33)

c lng k nh tt c min tn s t c:


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21

0( ) ( ) , 0 1

N j nk N

N n

H k G n e k N p - -

=

= -

(6.34)

Cc tn hiu pilot khng nht thit phi tun hon, chng c sp xp theoiu kin knh thay i. S pilot v v tr ca chng c th thay i mt cch

bin ng.

Mo Hinh Kenh Va Uoc Luong Kenh

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