CÔNG NGHỆ TRUYỀN THÔNG VÔ TUYẾN

7/31/2019 CNG NGH TRUYN THNG V TUYN

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Chng 1. TNG QUAN V THNG TIN V TUYN

1.1. Gii thiu

1.2. Knh thng tin v tuyn

1.2.1. Knh tp m AWGN

1.2.2. Knh pha-inh a ng

1.2.3. Knh pha-inh Rayleigh

1.3. Cc k thut phn tp

TI LIU THAM KHO

[1] J. K. Cavers, Mobile Channel Chracteristics. Kluwer Academic Publishers,

2000.


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Chng 1. TNG QUAN V THNG TIN V TUYN

1.1. Gii thiu

Cc mng v tuyn di ng truyn thng c xy dng trn c s khi nim t bo(cellular), bng cch ny c th p ng tt v kin trc. Trong cc thit b di ngthng tin vi cc im truy nhp ging (nh cc trm c s) c ni vi mng c nh.Mng v tuyn c phn loi theo 3 kiu tng ng theo vng ph sng l: (1) mngv tuyn ni b (WLAN, Wi-Fi/IEEE 802.11), mng ny c th cung cp truy nhpinternet tc cao nhng b gii hn v v tr cng nh khong cch; (2) cc mng vtuyn t bo, c th cho php truy nhp phm vi ton cu nhng b gii hn v tc ; (3)cc mng v tuyn khu vc thnh ph (nh WiMAX/ IEEE 802.16) v mng v tuyn din

rng nh mng th h th 3.S pht trin ca cng ngh truyn thng v tuyn trong nhng nm gn y l rt

nhanh, tp trung vo vic ci tin cc giao thc v mi trng truyn dn, bi vy ngidng c p ng yu cu dch v mi lc, mi ni.

Hnh 1.1. Phn lp mng v tuyn


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Mng v tuyn cc b (WiFi)

Thut ng WiFi m ch s hp nht cc tiu chun 802.11, 802.11a,b,g trn

thit b truyn thng v tuyn hnh thnh mng WMAN. C ngha l cho phpcc nh sn xut khc nhau ch to thit b truyn thng IEEE 802.11 WLANtng tc c vi nhau.

Bng 1: Cc tiu chun IEEE 802.11

Tiu chun 802.16

Xu hng pht trin mng di ng t bo


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Hnh 1.2

Cng ngh truyn thng v tuyn khng ngng pht trin v cng tc nhng thnh tu ng k. Ngy nay, cc mng WLANs (Wireless Local Area

Networks) t c tc 10 Mbit/s n 100 Mbit/s. Tuy nhin, vi tc trnvn c th khng p ng c khi i mt vi vic i hi tc truy cp d liungy cng cao do ni dung truyn thng tr nn a dng. c bit l s cnh tranhvi nhng mng LAN (hu tuyn) vi cng ngh xDSL (ng dy thu bao s)v mng cp quang. Cc tin b khoa hc gn y minh chng rng nng cao

cht lng truyn thng v tuyn th khng ch tn dng cc ti nguyn v thigian (phn tp thi gian), ti nguyn v tn s (phn tp tn s) m cn c th sdng ngun ti nguyn ln l khng gian.

1.2. Knh thng tin v tuyn

1.2.1. Knh tp m AWGN

Thut ng tp m (noise) m t cc tn hiu in khng mong mun xut hintrong h thng. S xut hin ca tp m lm gim kh nng tch chnh xc ca cc

tn hiu pht v v vy, lm gim tc truyn dn thng tin. Tp m c to rat nhiu ngun khc nhau, nhng c th phn loi thnh hai loi chnh l nhn tov t nhin. Ngun tp m nhn to xut hin t cc ngun nh la, chuyn mchhay cc pht x in t. Tp m t nhin gm tp m xut hin trong cc mch haylinh kin in t, xo ng kh quyn hay cc ngun thin h. Thit k cc mchin, thit b hay h thng cho php loi b hoc gim nh ng k nh hng cacc tp m bng cch ni t, chn v tr t thit b hay s dng cc phng phplc. Tuy nhin, c mt ngun tp m t nhin khng th loi b l tp m nhit.

Tp m nhit xut hin do chuyn ng ca cc in t trong tt c cc linh kinin t nh in tr, dy dn hay cc phn t dn in khc. S chuyn ng ngunhin v c lp ca v hn cc in t to nn cc c tnh thng k Gauss theonh l gii hn trung tm. V vy, tp m nhit c th m t nh mt qu trnhngu nhin Gauss c gi tr trung bnh bng khng. V d v tp m Gauss vi gitr trung bnh 0 v phng sai 2 1 = c miu t hnh 1.2.


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Hnh 1.2: M t tp m Gauss.

Hm mt xc sut (PDF: Probability Density Function) ca mt qu trnh

ngu nhin Gauss n(t) c biu din nh sau [1]:

( )2

2

1exp

22n

xp x

=

(1.1)

Hnh v 1.3 bu din hm PDF Gauss vi gi tr trung bnh bng khng

( )0= v lch chun (standard deviation).

Hnh 1.3: Hm mt xc sut Gauss vi 2 1 = .

Tp m trng: Mt c tnh quan trng ca tp m nhit l mt ph tn sca n nh nhau ti mi tn s. Tc l, n l ngun tp m pht ra mt lng cngsut nh nhau trn mt n v bng tn ti tt c cc tn s bng:


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( ) [ ]W/Hz2

on

NG f = (1.2)

nh m t hnh 1.4(a) di y. H s trong cng thc trn ch th rng ( )nG f l

mt hm mt ph cng sut 2 pha cnoN

th c gi l mt ph cng suttp m. Tp m vi cng sut c mt ph u nh vy c gi l tp m trng(white noise).

Hnh 1.4: Mt ph cng sut v hm t tng quan ca tp m trng.

Hm t tng quan ca tp m trng l php bin i Fourier ngc ca mt ph cng sut tp m cho bi:

( ) ( ){ } ( ) ( )

( ) ( )

1 2 1.3

1.42

j f

n n n

o

R G f G f e df

N

= =

=

Nh vy, hm t tng quan ca tp m trng l mt hm xung delta ti 0=

c nhn vi trng s 2oN . rng ( ) 0nR = vi mi 0 nn bt k hai mu

khc nhau no ca tp m trng u khng tng quan vi nhau bt k chng gnnhau n mc no. Do tp m nhit c cng vi tn hiu nn n cn c gi ltp m cng (additive noise). Tng hp cc c tnh ca tp m nhit trn chngta c th tm tt li rng tp m nhit trong cc h thng thng tin l tp m Gauss

trng cng (AWGN: Additive White Gaussian Noise).

1.2.2. Knh pha-inh a ng

Hnh v 1.5 m t mt ng lin lc gia anten trm gc (BS: Base Station)v anten trm di ng (MS: Mobile Station). Xung quanh MS c nhiu vt phn xnh nh, cy, i ni, ... trong khi xung quanh BS li c rt t hoc khng c cc


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vt phn x do anten trm BS c t trn cao. Cc vt phn x ny c gichung l vt tn x. Lin lc gia BS v MS thng qua nhiu ng (path), ming chu mt hay nhiu phn x, v tn hiu n my thu l tn hiu tng hp ttt c cc ng ny. Do cc ng c bin , pha v tr khc nhau, nn tn

hiu truyn qua cc ng c th kt hp vi nhau mt cch c li hoc khng cli, to nn mt sng ng ngu nhin. Hin tng ny c gi l truyn sng

pha-inh a ng. Knh truyn sng kiu ny c gi l knh pha-inh ang.

Hnh 1.5: M hnh truyn sng a ng.1.2.2.1. M hnh ton hc ca pha-inh

Tn hiu v tuyn lun l tn hiu bng thng (bandpass) v c bng tn hp

(narrowband). Tn hiu bng thng pht i ( )s t ti tn s sng mang cf vi ng

bao phc c biu din nh sau:

( ) ( ) 2 cj f ts t s t e = % (1.5)

Trong [ ] biu din php ton ly phn thc.

t di ca ng l l lx v k hiu c l tc nh sng th thi gian

truyn sng t BS ti MS l lx c . Gi s suy hao ca ng l l la , th tn hiu

thu c ti MS khng tnh n tp m l:


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( ) lll

xr t a s t

c

=

(1.6)

Th (1.5) vo (1.6) chng ta c:

( ) ( )

( )

2

2

1.7

1.8

lc

lc

xj f tcl

l

l

xj f t

cll

l

xr t a s t e

c

xa s t e

c

=

=

%

%

Vit li ( )r t di dng:

( ) ( )

( ) ( )

22

2

1.9

1.10

lc

c

c

xj f

j f tl cl

l

j f t

x

r t a s t e ec

r t e

=

=

%%

Trong thnh phn ng bao tn hiu thu:

( ) ( ) ( )2 1.11c lj fl ll

r t a s t e = % %

c gi l tn hiu bng tn gc tng ng ca ( )r t , cnl

l

x

c = l thigian tr ca ng th l.

1.2.2.2. nh hng do chuyn ng ca MS

Khi MS chuyn ng vi tc v , di ng truyn sng th l thay i.

Nu gc ti ca tia th l so vi hng chuyn ng l l th di ca tuyn thay

i nh l mt hm ca tc v v thi gian t l:

( )loslx v c t (1.14)

Nh vy, ng bao phc ca tn hiu thu l:


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( )

( ) ( )

2

cos2 2 cos

l lc

llc c

x xj f

l lcl

l

v txj f j f

llc cl

l

x xr t a e s t

c

v txa e e s t

c c

+

+ =

= +

% %

% (1.15)

Do s thay i tr tn hiu ( )cos lv t c l rt nh so vi thang thi gian

ca tn hiu iu ch ( )s t% nn chng ta c th b qua chng. Nh vy, nu t:

( )2 1.16c lj fl la e =

chng ta c th vit li ( )r t% dng rt gn sau:

( ) ( ) ( )2 cos

l

vj t

l l

l

r t e s t

= % % (1.17)

Vi l bc sng ca sng mang. Hay:

( ) ( ) ( )2 cosD lj f t

l l

l

r t e s t

= % % (1.19)

T cng thc ny chng ta c th thy rng vt tn x th l dch tn hiu

pht i l v thi gian v ( )cosD lf v tn s.

dch Doppler cc i l:

cD

f vf v

c = =

Biu thc (1.19) miu t mt knh v tuyn in hnh vi tn x ri rc.

1.2.2.3. Hu qu ca truyn sng pha-inh a ng

Hu qu ca truyn sng pha-inh a ng l:

* Pha-inh chn lc theo thi gian gn vi tri Doppler, c to ra dochuyn ng ca MS.

* Tri tr (delay spread) gn vi pha-inh chn lc theo tn s.

Doppler spread: Nu MS chuyn ng qua cc vng ngu nhin, n chu nhhng thay i v cng v pha tn hiu vi tc thay i tu thuc vo vn


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tc chuyn ng ca MS. Gi s bng tn tn hiu l rt nh (ng vi pha-inh

phng), b rng bng thng nh sao cho thi gian tr l khng nh hng ti tn

hiu, v ta c ( ) ( )ls t s t % % . Nh vy, cng thc (1.19) c th vit li thnh:

( ) ( ) ( ) ( ) ( )2 cosD lj f t

l

l

r t s t e g t s t = =% % % (1.20)

, li phc ( )g t ca knh thay i theo thi gian bi v gc pha

( )2 cosD lf t thay i theo thi gian. T y chng ta c th thy rng chuyn ng

ca MS (hay dch Doppler) lm cho li ng truyn bin i. Do ( )g t thay

i nhanh hn ( )s t% nn tn hiu thu b tri trn thang tn s. c bit, m tn gc

s tri trn mt vi thnh phn trong bng [ ],D Df f cho mi trng hp tn x

(scatterer). Chnh v vy, hin tng ny cn c gi l Doppler spread.

Delay spread: Xt trng hp tn s Doppler rt nh tng ng vi MS ngyn, chng ta c th coi pha ca cc vt th tn x l khng i. Nh vy cngthc (1.20) c th c vit li nh sau:

( ) ( )l ll

r t g s t = % % (1.21)

vi ljl lg e = v l l pha ngu nhin. Knh truyn:

( ) ( ), l ll

g t g t = (1.22)

lc ny ng vai tr l mt b lc c p ng xung hu hn (FIR: Finite ImpulseRespose filter). Hnh (1.6) di y m t p ng xung ca mt b lc FIR ti

thi im quan st ot no . Di D c gi l tri tr (delay spread). Chng ta c

th thy do di cc ng truyn sng khc nhau nn lm cho tn hiu bdch chuyn tr.


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Hnh 1.6: p ng xung ca b lc FIR.

Thc hin bin i Fourier ln p ng xung ny, chng ta c p ng tn s:

( ) ( )2 1.23lj fll

G f g e =

T y chng ta thy rng tri tr D lm bin i p ng tn s ( )G f , tc l

ti cc tn s khc nhau th bin ( )G f c gi tr khc nhau. Knh truyn dn

trong trng hp ny c gi l knh chn lc tn s (frequency selective).

1.2.3. Knh pha-inh Rayleigh

Hm truyn t ca knh thc cht l mt qu trnh xc sut ph thuc cthi gian v tn s. Bin hm truyn t ca knh ti mt tn s nht nh stun theo phn b Rayleigh nu mi trng truyn dn tha mn cc iu kin sau:

+ Mi trng truyn dn khng c tuyn trong tm nhn thng, c ngha lkhng c tuyn c cng sut tn hiu vt tri.

+ Tn hiu my thu nhn c t v s cc hng phn x v nhiu khcnhau.

Trong trng hp mi trng truyn dn c tuyn truyn dn trong tm nhnthng th cng sut tn hiu t tuyn ny vt tri so vi cc tuyn khc. Xc sutca bin hm truyn t ca knh s tun theo phn b Rice.

1.2.3.1. M hnh knh pha-inh Rayleigh

T cng thc (1.20), chng ta vit li c li knh trong trng hp MSchuyn ng nh sau:


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( ) ( )2 cosD lj f t

l

l

g t e

= (1.24)

Trong trng hp khng tn ti tia trc tip gia BS v MS th h s c th

coi l cc s ngu nhin c gi tr trung bnh bng 0. Nh vy, ( )g t l tng ca ccbin s ngu nhin phc c gi tr trung bnh bng 0, v theo lut s ln, th khi s

lng cc tia l ln, ( )g t c th c coi l mt qu trnh Gauss phc [4]. Tc l,

nu chng ta biu din ( )g t dng s phc:

( ) ( ) ( ) ( )1.25I Qg t g t jg t= +

vi ( )Ig t v ( )Qg t u l cc s thc, th ( )Ig t v ( )Qg t s u l cc qu trnh xc

sut c lp, vi gi tr trung bnh bng khng v phng sai nh nhau

( )2 2 2I Qg g g

= @ .

S dng cng thc v phn b Gauss chng ta c [4]:

( ) ( )

( ) ( )

2

2

2

2

1exp 1.26

22

1exp 1.27

22

II

g

Q

Q

g

gp g

gp g

=

=

Do ( )Ig t v ( )Qg t l cc qu trnh c lp nn chng ta c phn b:

( ) ( ) ( ) ( )2

2 2

1exp 1.28

2 2I Q

g

gp g p g p g

= =

vi

( ) ( ) ( ) ( )2 2 2

1.29I Qg t g t g t= +

Chuyn ( )g t sang h to cc ( ) ( ) ( )j tg t r t e = chng ta c pdf kt hp:

( ) ( ) ( ) ( )2

2 2

1, exp 1.30

2 2g

r rp r p r p

= =

Hay:


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( ) ( )

( ) ( )

2

2 2exp , 0. 1.31

2

1, . 1.32

2

g

r rp r r

p

=

=

Tc l, pdf ca bin ( )r t l phn b Rayleigh, v pha-inh kiu ny c

gi l pha-inh Rayleigh. Hnh 1.6 m t phn b Rayleigh vi 1.=

Hnh 1.7: Phn b Rayleigh.

1.3. Cc phng php phn tp

Trong thng tin v tuyn, cc phng php phn tp c s dng hn ch nhhng ca pha-inh v nng cao cht lng truyn thng tin. K thut kt hp phn tp(diversity combining) c th nghim trong thng tin v tuyn ln u tin vo u nm1927 v c nghin cu rng ri vo nhng nm cui ca thp nin 60, u 70 [4].Phng php phn tp i hi s tn ti ca mt s ng truyn c cc tham s thng kc lp, nhng truyn ti cng mt thng tin ging nhau. Bn cht ca phng php phntp l tn hiu c truyn trn cc ng truyn c lp s chu nh hng ca hiu ngpha-inh khc nhau. Tc l, trong s cc tn hiu thu c s c tn hiu thu c vi cht

lng tt v c tn hiu thu c vi cht lng xu. Do , nu kt hp cc tn hiu nymt cch thch hp, chng ta c th thu c mt tn hiu tng hp chu nh hng capha-inh t hn. Kt qu ny ng ngha vi vic tn hiu c truyn i vi tin cy caohn.


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Ty theo min (domain) ng dng, cc phng phn tp s dng trong thng tin vtuyn c th c phn loi thnh:phn tp thi gian, phn tp tn s, phn tp phn ccv phn tp khng gian.

1.3.1. Phn tp thi gian

Do tnh cht ngu nhin ca pha-inh, bin ca mt tn hiu chu nh hng pha-inh ngu nhin ti cc thi im ly mu cch xa nhau ln v thi gian s khng tngquan vi nhau. V vy, truyn mt tn hiu ti cc thi im cch bit ln tng ngvi vic truyn mt tn hiu trn nhiu ng truyn c lp, to nn s phn tp v thigian. Khong thi gian cn thit m bo thu c cc tn hiu pha-inh khng tngquan ti my thu ti thiu l thi gian ng b (coherence time) ca knh truyn. i vi

thng tin di ng, khong thi gian ng b ny l 2c cT c f= , trong 83 10 /c m s= l

tc nh sng,

l tc di chuyn ca my di ng, v cf

l tn s sng mang. Vicc my di ng lm vic tn s 800 MhZ v di chuyn vi tc 50 Km/h, thi gian

ng b 13,5cT ms= . to ra dM nhnh phn tp, tn hiu cn c truyn i ti dM

khe thi gian. V vy, khong thi gian gi chm cn thit truyn tn hiu trn dM

nhnh phn tp l 2d cM c f . i vi truyn dn tn hiu thoi, tc ly mu cn thit t

nht l 8 kHz. ng thi, m bo rng xung truyn nm trong bng tn truyn dn,

chng ta ch c th s dng ti a 50dM = nhnh phn tp. Do thi gian cch bit t l

nghch vi tc di chuyn nn, khc vi cc phng php phn tp khc, phng php

phn tp thi gian khng c ngha trong trng hp my di ng ng yn.

Gn y, trong cc h thng thng tin di ng hin i, m sa li c s dng kthp vi phng php xen k tn hiu (interleaving) to nn mt phng php phn tpthi gian mi. Do thi gian xen k di s gy nn gi chm gii m ln, nn phngphp ny ch thch hp i vi cc knh pha-inh bin ng nhanh.

Nhc im chnh ca phng php phn tp thi gian l lm suy gim hiu sutbng tn do c s d tha trong min thi gian.

1.3.2. Phn tp tn s

Tng t nh phng php phn tp thi gian, chng ta c th s dng mt tp hpcc s tn s truyn i cng mt tn hiu, to nn s phn tp tn s. Khong cch giacc tn s cn phi ln, vo khong vi ln bng tn ng b (coherence bandwidth), m bo pha-inh ng vi cc tn s s dng khng tng quan vi nhau. i vi thng


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tin di ng, bng tn ng b o c vo khong 500 kHz, v vy khong cch cn thitgia cc nhnh phn tp tn s t nht l 1-2 MHz.

Trong thng tin di ng hin i, phn tp tn s cn c th nhn c thng quavic s dng cc k thut iu ch a sng mang (multicarier modulation) hay s dngphng php nhy tn (frequency hopping).

Nhc im ca phng php phn tp tn s l s tiu tn ph tn s. Ngoi ra, docc thnh phn phn tp c tn s khc nhau nn mi nhnh cn s dng mt my thu phtcao tn ring.

1.3.3. Phn tp phn cc

Nghin cu cho thy tn hiu truyn i trn 2 phn cc trc giao trong mi trngthng tin di ng c cc tham s thng k c lp. V vy, hai phn cc ny c th ccoi l c s ca hai nhnh phn tp phn cc. Do ch tn ti hai phn cc sng trc giaonn s lng ti a cc nhnh phn tp c th to c ch l hai. Ngoi ra, do s hn chca cng sut my pht nn cng sut tn hiu pht cn phi chia u cho hai nhnh, v vvy cht lng tn hiu thu cng b suy gim i 2 hay 3 dB.

1.3.4. Phn tp khng gian

Phn tp khng gian l phng php c s dng rng ri trong thng tin vtuyn. Phng php ny s dng nhiu anten my thu, my pht hoc c pha my thu

v my pht to nn cc nhnh phn tp khng gin khc nhau. Khong cch cn thitgia cc anten ti thiu l mt na bc sng ( )2 . Khi s dng nhiu anten my pht,

chng ta c h thng phn tp khng gian pht, v tng t chng ta c phn tp khnggian thu nu s dng nhiu anten thu. Trng hp s dng nhiu anten ti c my pht vmy thu chng ta c mt tp hp knh truyn vi nhiu u vo v nhiu u ra. Cc hthng phn tp thu pht khng gian kiu ny thng c gi l cc h thng a u vo-a u ra (MIMO: Multiple Input-Multiple Output).

Cng ging nh phng php phn tp phn cc, so hn ch v cng sut pht, nu

dM nhnh phn tp pht c s dng th cht lng tn hiu b suy gim i 1 dM ln sovi phn tp thu khng gian cng bc. Tuy nhin, s dng phn tp pht rt c ngha dohai l do sau: (i) tit kim, do ch cn thc hin trm gc ch khng phi tt c ccmy di ng, (ii) d thc hin, do yu cu v s nh gn ca my di ng nn rt kh cth t c hai hay nhiu anten thu cch xa nhau hn na bc sng (khong 19 cm tn


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s 800 MHz). V vy, s dng phn tp pht ti trm gc s thay th cho phn tp ti mythu.

u im ca phng php phn tp khng gian l khng lm suy gim hiu sutbng tn, khng tiu tn ph tn s, d s dng, v trn l thuyt khng c s hn ch vs lng cc nhnh phn tp. Do c cc u im ni trn, phng php phn tp khnggian c nghin cu rng ri t nm 1927 n tn ngy nay. Cc nghin cu v phntp khng gian tp trung ch yu vo cc k thut kt hp tn hiu phn tp.

1.4. K thut kt hp phn tp khng gian thu

1.4.1. M hnh tn hiu

Gi s tn hiu ( )s t c truyn qua mi trng pha-inh Rayleigh ti my thu s

dng phn tp khng gian vi M nhnh phn tp. Sau khi tch sng cao tn, tn hiu thu

c tch nhnh phn tp (anten) th { }, 1, 2, ,m m M L , c biu din nh sau:

( ) ( ) ( ) ( ) ( )my 1.36m mt h t s t z t = +

Trong ( )mh t , l mt s phc Gauss c gi tr trung bnh bng khng, biu din

ng truyn pha-inh t anten pht ti anten thu th m ; ( )mz t l tp m nhnh phn

tp m .

T M tn hiu nhnh trn, chng ta mong mun s dng mt phng php kt hpthch hp sao cho tn hiu u ra b kt hp c cht lng tt hn. Trong cc phn tiptheo, chng ta s tm hiu ba phng php kt hp phn tp khng gian c s dng phbin my thu. C th l cc phng php kt hp chn lc (selection combining), kt hpt l ti a (maximal-ration combining) v kt hp ng li (equal-gain combining).

1.4.2. Kt hp chn lc (Selection Combining)

Cu hnh ca b kt hp chn lc c minh ha hnh di. Ti mt thi im t,mch chn lc logic thc hin vic o lng v tnh ton t s tn hiu trn tp m SNR

ca tng nhnh phn tp v chn ra tn hiu nhnh c t s SNR ln nht. Trong thc t,vic o lng t s SNR rt kh thc hin v, v vy, tn hiu trn nhnh phn tp c tngcng sut tn hiu v tp m ln nht s c chn.


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Hnh 1.7: Phng php kt hp chn lc.1.4.3. Kt hp t l ti a (Maximal Ratio Combining)

Phng php kt hp t l ti a c Kahn xut nm 1954. S dng phngphp ny, tn hiu ca M nhnh phn tp c nhn trng s (weighted) cn xng theo tl SNR ca cc nhnh, sau c iu chnh ng pha ri kt hp (cng) vi nhau. S cu hnh mt b kt hp t l ti a c trnh by nh hnh di y:

Hnh 1.10:Phng php kt hp t l ti a.

T s tn hiu trn tp m (SNR) trung bnh: Tn hiu thu c ti mt nhnhphn tp, sau khi c chuyn v bng tn gc (baseband) c biu din nh sau:

( ) ( ) ( ) ( ) ( )1.51m m my t h t s t z t= +


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Trong , ( )mh t l mt s phc Gauss c gi tr trung bnh bng khng, biu din

ng truyn pha-inh t anten pht ti anten thu th m; ( )mz t l tp m nhnh phn tp

m. Sau khi c ng pha v nhn trng s, ng bao ca tn hiu kt hp l

( ) ( ) ( )1

1.52M

m m

m

y t w y t=

=

Vi mw l mt s phc, thng c gi l trng s kt hp (combining weight). Thay

( )my t trong biu thc trn ta c:

( ) ( ) ( ) ( )

( ) ( ) ( ) ( )

1

1 1 1.53

M

m m m

m

M M

m m m mm m

y t w h t s t z t

s t w h t w z t

=

= =

= +

= +

1.4.4. Kt hp ng li (Equal Gain Combining)

Nh ch ra phn trc, MRC l phng php kt hp ti u cho li phn tp lnnht trong tt c cc phng php kt hp phn tp thu. Tuy nhin, phng php MRC yucu cn phi bit chnh xc c cc trng s kt hp, do tng i phc tp. Hn na, li thu c ca phng php MRC khng ln hn nhiu so vi phng php kt hpchn lc. iu ny c ngha l phn ln li phn tp thu c t nhnh phn tp ccng sut ln nht v nu mt phng php kt hp c th thu c li t nhnh phn

tp th tng li thu c hu nh khng thay i. Quan st ny dn n mt phngphp phn tp mi, k thut kt hp phn tp ng li (EGC: Equal Gain Combining),n gin hn phng php MRC. S dng phng php kt hp EGC, tn hiu ti ccnhnh c ng pha (co-phasing) ging nh trng hp MRC, nhng sau c nhnvi cc trng s c cng ln, ri kt hp vi nhau. Trng hp n gin nht l t li ca cc trng s bng hng s n v. Nh vy, phng php kt hp EGC ch l mttrng hp c bit ca phng php MRC.

1.4.5. Kt hp phn tp thu v tch sng MLD

Trong phn trc, chng ta bit phng php kt hp phn tp MRC l phngphp kt hp ti u. V vy, phng php phn tp ny c s dng ph bin nht trongthc t. Trong phn ny chng ta s nghin cu s thc hin phng php MRC kthp vi phng php tch sng hp l ti a (MLD: Maximum Likehood Detection). Mhnh ca mt h thng v tuyn trong my thu s dng 2 nhnh phn tp MRC c chra nh trong hnh v di y.


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Hnh 1.12: My thu vi 2 nhnh phn tp MRC v mt b tch MLD.Tn hiu u ra b kt hp MRC: gi s ti mt thi im k, mt tn hiu ks c

truyn t my pht. li knh truyn gia anten my pht ti cc anten my thu th nht

v th hai ti thi im k c k hiu l 1,kh v 2, kh . Gi s knh truyn chu nh hng

ca pha-inh bin i chm. Nh ta c th b qua ch s thi gian k, tc l, 1, 1kh h= v

2, 2kh h= . Cc knh ny c biu din nh sau:

( )

( )

1

2

1 1

2 2

1.75

1.76

j

j

h e

h e

=

=

Trong mmv (m = 1, 2) tng ng l bin phc v pha ca knh.

Cc tn hiu thu ti hai nhnh phn tp l

( )

( )

1, 1 1,

2, 2 2,

1.77

1.78

k k k

k k k

y h s z

y h s z

= +

= +

Trong ,m kz l cc mu (samples) tp m Gauss.

Nh ch ra phn trc, ti a ha SNR u ra, cc tn hiu thu trn hai nhnh phn

tp c nhn vi cc h s trng s mw ri kt hp li vi nhau. Theo trn m mw h= , tc l


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( )

( )

1 1

2 2

1.79

1.80

w h

w h

=

=

V vy, s dng cc b c lng knh truyn my thu, ta c th bit c cc knh

truyn 1h v 2h , v nh tnh c cc trng s kt hp 1w v 2w . Gi s cc knhtruyn c lng c 1h v 2h l chnh xc, tc l, 1 1h h=% v 2 2h h=% . S dng phng

php MRC, tn hiu u ra b kt hp c cho bi

( )1, 1 2, 2 1.81k k ks y w y w= +

Thay cc biu thc ca 1w v 2w , cng thc trn tr thnh:

( ) ( )

( )

1, 1 2, 2

1 1, 1 2 2, 2

2 2

1 1, 1 2 2, 2

1.82

k k k

k k k k

k n n

s y h y h

h s z h h s z h

h s z h h z h

= +

= + + +

= + + +

c biu din rt gn thnh

( ) ( )2 21 2 1, 1 2, 2 1.83k k k k s h h s z h z h = + + +

Tch sng ti u: gi s rng tn hiu pht l k cs , trong c l biu phn

phi im tn hiu (signal constellation). Do cc mu tp m ,m kz c phn ph Gauss, lut

quyt nh xc sut ti a (maximum likelihood decision rule) ti my thu l

( ) ( )

22

,

1

22

,

1

arg min

arg min , 1.84

k c

k c

M

k m k m k s

m

M

m k m k s

m

s y h s

d y h s

=

=

=

=

=

=

Trong

( ) ( ) ( ) ( )22

, , , ,, 1.85m k m k m k m k m k m k m k m k d y h s y h s y h s y h s

= @

L bnh phng khong cch Euclid gia cc tn hiu thu c ,m ky v cc tn hiu khng

cha tp m m kh s .


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Khai trin i s ca cng thc trn ta c

( ) ( ) ( ) ( )

( ) ( )

2 2

1, 1 2, 2

1, 1 1, 1 2, 2 2, 2

1, 1, 1 1, 1, 1 1 1 1.86

MRC k k k k

k k k k k k k k

k k k k k k k k

A y h s r h s

y h s y h s y h s y h s

y y h s y y h s h s h s

= +

= +

= +

Do2

1, 1, 1,k k ky y y= v

2

2, 2, 2,k k ky y y= l cc thnh phn chung v khng nh hng ti

quyt nh ks , chng ta c th b qua cc thnh phn ny v thu c

( ) ( )

( ) ( )

1 1, 1, 1 1 1 2 2, 2, 2 2 2

2 2 2 2

1 1, 1, 1 1 2 2, 2, 2 2

2 2 2 2

1 2 1 1, 2 2, 1, 1 2, 2

2

1

kk

MRC k k k k k k k k k k k k

k k k k k k k k k k

k k k k k k k k

ss

A h s y y h s h s h s h s y r h s h s h s

h y s y h s h s h y s y h s h s

h s h s h y h y s y h y h s

h

= + + +

= + +

= + + +

=

%%

1 44 2 4 43 1 4 4 2 4 43

( ) ( )2 2

2 1.87k k k k k h s s s s s + % %

rng

( )2 2 1.88m mj jm m m m m mh h h e e

= = =

Ta c

( )

( ) ( )

22 2

1 2

2 2 2 22 2

1 2. 1.89

MRC k k k k k

k k k k k

A s s s s s

s s s s s

= +

= + +

% %

% %

tip rng2

ks% l thnh phn chung v thc t khng nh hng ti lut quyt nh, ta

c th b qua thnh phn ny v thu c

( )

( ) ( )

2 2 22 2

1 2

2 22 2

1 2 1 . 1.90

MRC k k k k

k k k

A s s s s

s s s

= +

= + +

%

%

V vy, lut quyt nh ML (1.84) cho phng php MRC lc gin thnh

( ){ } ( )2 22 21 2arg min 1 . 1.91k c

k k k k s

s s s s

= + + %


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i vi trng hp tn hiu PSK (Phase shift keying), nng lng tn hiu,2

s kE s= , l

nh nhau cho tt c cc im phn phi tn hiu. Do , lut quyt nh ML tip tc clc gin thnh

{ } ( )2arg min . 1.92

k ck k k

ss s s

= %


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Chng 2. H THNG MIMO-OFDM

CÔNG NGHỆ TRUYỀN THÔNG VÔ TUYẾN

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