KỸ THUẬT ĐA ANTEN

Bi 7

TS. Nguyn Phm Anh Dng

Chng 10K THUT A ANTEN

10.1. GII THIU CHUNG

10.1.1. Cc ch c trnh by trong chng

Cc ch c trnh by trong chng ny bao gm:

Cc cu hnh a anten

Cc li ch ca s dng cc k thut a anten

a an ten thu

a anten pht

Ghp knh khng gian

10.1.2. Hng dn

Hc k cc t liu c trnh by trong chng

Tham kho thm [17],[18],[19]10.1.3. Mc ch chng

Mc ch ca chng ny nhm cung cp cho sinh vin cc kin thc tng quan cc k thut a anten c s dng pht trin 3G v 4G c bit l cho HSPA v LTE.

10.2. CC CU HNH A ANTEN

Cc k thut a anten c s dng ci thin hiu nng h thng bao gm: ci thin dung lng h thng (nhiu ngi s dng hn trn mt ) v vng ph ( ln hn) cng nh cung cp dch v tt hn nh tc s liu trn mt ngi s dng cao hn. Cc k thut ny bao gm vic s dng nhiu anten ti my pht v my thu kt hp vi x l s tin tinMt c trng quan trng ca mi cu hnh a anten l khong cch gia cc phn t anten do quan h gia khong cch anten v tng quan tng h gia phainh knh v tuyn ti cc anten khc nhau.

Cc anten trong cu hnh a anten c th c t kh cch xa nhau t c tng quan lng h thp. Tuy nhin trong cc cu hnh khc, cc anten c th c t kh gn nhau tng tng quan tng h, khi ny tn hiu cc anten khc nhau s b pha inh tc thi gn ging nhau. Cc cu hnh a anten ny c phn loi thnh phn tp, to bp v ghp knh khng gian s c xt cc phn di y.

Khong cch anten thc t cn thit t c tng quan thp hoc cao ph thuc vo bc sng hay tn s sng mang s dng cho thng tin v tuyn. Ngoi ra n cng ph thuc vo kch bn trin khai.

Trong trng hp cc anten trm gc trong cc mi trng thng gp l cc v m (cc kh ln, v tr t anten kh cao ), khong cch gia cc anten thng l vi chc bc sng m bo tng quan phainh tng h thp. Nhng i vi cc u cui di ng trong mi trng tng t, khong cch gia cc anten ch cn na bc sng (0,5() l m bo tng quan tng h kh thp. L do khong cch anten khc nhau gia trm gc v u cui di ng trong mi trng cc v m l cc phn x a ng gy ra phainh ch yu xy ra vng gn u cui di ng. V th nhn t u cui di ng, cc ng truyn khc nhau thng n vi gc rng v iu ny c ngha l tng quan phainh thp ngay c khi khong cch gia cc anten nh.

Tri li, trong cc kch bn trin khai khc, nh trin khai vi m vi cc anten trm gc c t thp hn mi nh hay trin khai trong nh, mi trng nhn t pha trm gc rt ging vi mi trng nhn t pha u cui di ng. Trong cc kch bn ny khong cch anten trm gc nh hn cng m bo tng quan phainh thp.

Cc phn tch trn tng ng vi gi thit rng cc anten c cng phn cc. Mt gii php khc t c tng quan phainh tng h thp l s dng phn cc khc nhau cho cc anten khc nhau. Khi ny c th t cch anten rt gn nhau nhn c mt kt cu anten nh gn m vn m bo tng quan gia chng thp.

Ph thuc vo mc s dng ca a anten ti my pht v (hoc) my thu, cc k thut a anten c th c phn lai thnh mt u vo nhiu u ra (SIMO: Single- Input Multi-Output), nhiu u vo mt u ra (MISO: Multi-Input Single Output) hay nhiu u vo nhiu u ra (MIMO: Multi-Input Multi-Output). Nh vy trong kch bn trm gc c nhiu anten thng tin vi UE chj c mt anten, ng ln c gi l SIMO v ng xung c gi MISO. Khi mt u cui di ng nhiu anten c s dng ng truyn tr thnh MIMO y . Mc d vy i khi thut ng MIMO c s dng theo ngha rng bao hm c SIMO v MISO nh l cc trng hp c bit. Trong khi mt lin kt MIMO im n im gia trm gc v MS c gi l MIMO n ngi s dng (SU-MIMO: Single User MIMO), th MIMO a ngi s dng (MU-MIMO: Multi-User MIMO) cho php nhiu MS thng tin ng thi vi mt trm gc chung trn c s s dng cng mt ti nguyn trong min tn s v min thi gian. M rng ra, nu xt mt ng cnh nhiu , khi cc trm gc ln cn chia s cc anten ca mnh theo cch MIMO o thng tin vi cng mt tp UE trong cc khc nhau ta c thut ng MIMO a ngi s dng a . Tng quan qu trnh pht trin khi nim MIMO t bc phn tp n gin nht n MIMO a ngi s dng a tng lai c minh ha trn hnh 10.1.

Hnh 10.1. Qu trnh pht trin cng ngh a anten t thng tin n anten (SISO: Single-Input Single- Output) n MU-MIMO v MU-MIMO a tng lai.

10.3. CC LI CH CA VIC S DNG CC K THUT A ANTEN V CC VN THC TIN CA CC S MIMO

10.3.1. Cc li ch ca vic s dng cc k thut a anten

Cc k thut a anten ti my pht v ti my thu c th c s dng theo cc cch khc nhau t c cc mc ch khc nhau:

Nhiu anten ti my pht v (hoc) ti my thu c th c s dng m bo phn tp chng phainh trn knh v tuyn. Trong trng hp ny, cc knh truyn do cc anten ny to ra phi c tng quan phainh tng h thp, hay ni mt cch khc cn c khong cch gia cc anten ln (phn tp khng gian), hoc s dng cc anten c phn cc khc nhau (phn tp phn cc)

Nhiu anten ti my pht v (hoc) ti my thu c th c s dng to dng bp anten tng hp (bp pht v bp thu) chng hn t c tng ch cc i trong trong phng n my pht hoc n my thu hoc trit cc tn hiu nhiu chnh. Qu trnh to bp ny c th c thc hin da trn tng quan phainh tng h cao hoc thp gia cc anten

S c mt ng thi nhiu anten ti my pht v my thu c th c s dng to ra nhiu knh thng tin song song trn giao din v tuyn. iu ny m bo kh nng s dng bng thng cao m khng gy gim hiu sut s dng cng sut hay ni mi cch khc cho php tc truyn dn cao m khng gy nh hng ln n ph sng. Gii php ny c gi l ghp knh khng gian.

Mc d MIMO rt a dng v i khi phc tp, cc k thut SU-MIMO v MU-MIMO u da trn mt s cc nguyn l cn bn vi mc ch tng cng mt s cc thuc tnh knh truyn sng v tuyn a anten quan trng. Tn ti ba u im lin n cc knh ny (so vi SISO) l:

li phn tp

li mng li ghp knh khng gian

li phn tp lin quan n vic lai b nh hng ca pha inh a ng bng cch bng cch pht hoc thu trn nhiu anten m ti pha inh khng tng quan vi nhau. Thng n c din t bng thut ng bc ni v s lng cc nhnh phn tp c lp hiu dng hoc dc ca ng cong t l li bit dng ph thuc vo t s tn hiu trn tp m (SNR) (hay li SNR trong qu ng truyn h thng).

Trong khi li phn tp lin quan n ci thin cc thng s thng k ca SNR tc thi trong knh pha inh th li mng v li ghp knh li mang tnh cht hon ton khc. Cc li ny lin quan n hnh hc v l thuyt cc khng gian vect. li dn th hin li cng sut ca MIMO so vi SISO nh s dng to bp, cn li ghp knh c coi l kh nng pht nhiu lung s liu song song v phn tch chng trn c s ch k khng gian ca chng.

10.3.2. Cc vn thc tin ca cc s MIMO

Trc ht, ta s lit k cc hn ch thc tin quan trng nh hng ln hiu nng thc t ca cc h thng MIMO l thuyt v cc hn ch ny thng mang tnh quyt nh khi la chn mt chin lc truyn dn c th trong mt mi trng truyn sng cho trc v trong qu trnh thit lp h thng.

Cc li ch y ca MIMO ( li mng, li phn tp v li ghp knh) ch t c vi gi thit cc anten phi khng tng quan v cc ma trn MIMO phi l ma trn c hng y . Lin quan n iu ny, mi trng truyn sng v thit k anten (khong cch gia cc phn t) ng vai tr quan trng. Trong trng hp n ngi s dng, cc anten c pht trm gc ln MS thng c phn cch trong khong t na bc sng n vi bc sng l nhiu nht. Khong cch ny rt nh so vi khong cch t trm gc n MS. Trong tnh trng LOS (Line of Sight: trc x), iu ny s dn n tng quan mnh gia cc ch k khng gian (hay li knh) v lm hn ch cc s ghp knh khng gian. C th to ra mt ngoi l cho bng cch s dng cc anten c thit k sao cho chng m bo cc thuc tnh trc giao ngay trong cc tnh trng LOS. Chng hn s dng hai anten (ti c pha pht v pha thu) lm vic ti cc phn cc trc giao (phn cc ng v phn cc ngang) hay tt hn l cc phn cc c gi l +450 v -450. Cc anten ny s cho kh nng ghp knh gp i ngay c khi LOS. Tuy nhin vic s dng cc anten phn cc trc giao c th dn n cc mu pht x c hng trong mt ngang, v th khng phi bao gi cng nn dng chng. Nu khng xt cc ngoi l ni trn, th trong SU-MIMO iu kin c lp ca cc ch k khng gian ch c th c tho mn nu truyn sng giu a ng ngu nhin. Trong cc s nh hng phn tp, ta khng cn tnh o ln ca ma trn knh, hn na cc phn t ca ma trn knh phi khng tng quan thng k vi nhau. Mc d t s nng lng LOS v nng lng khng LOS ln hn c xu th lm cc h s phainh trn cc anten tng quan, nhng nh hng ny s c b tr bi vic gim pha inh nh phn t LOS.

Mt l do dn n s khc nhau gia cc li MIMO l thuyt v li t c thc t l kh nng my thu c tnh chnh xc cc h s knh mi khi my pht cn. Ti my thu, c tnh knh thng c thc hin trn mt mu hu hn ca cc tn hiu tham chun (RS: Reference Signal). Trong trng hp to bp pht v tin m ha MIMO da trn SVD, my pht phi nhn c thng tin knh ny (hay trc tip thng tin v b tin m ha) t my thu thng qua phn hi c hn. V ng thng tin phn hi c hn s l nguyn nhn gim cp hn na CSIT (Channel State Information at Transmitter: thng tin trng thi knh ti my pht).

10.4. M HNH MIMO TNG QUT

Hnh 10.2 cho thy m hnh knh MIMO tng qut gm Nt anten pht v Nr anten thu.

Hnh 10.2. M hnh knh MIMO vi Nt anten pht v Nr anten thu

Ma trn knh cho m hnh NtxNr MIMO trn hnh 10.2 l mt ma trn knh NrxNt c biu din nh sau:

(10.1)

trong hm,n l li knh gia anten pht th n v anten thu th m.

Gi s l vect cc s liu pht t Nt anten pht v l s liu thu t Nr anten thu v l tp m Gauss trng phc ca Nr my thu, trong (.)T k hiu php ton chuyn v. Quan h gia tn hiu u vo knh v tn hiu u ra knh c xc nh nh sau:

(10.2)

Ta c th vit li quan h vo ra knh ma trn NrxNt trong phng trnh (10.2) nh sau:

(10.3)

trong :

(10.4)

(10.5)

(10.6)

, cng sut tp m. Ta k hiu cho bin ngu nhin ca (i nh sau: ..

10.5. M HNH H THNG MIMO TI U

10.5.1. M hnh knh SVD MIMO

Ta xt mt h thng truyn dn v tuyn bao gm Nt anten pht v Nr anten thu nh trn hnh 10.2. tin phn tch ta vit li phng trnh (10.3):

(10.7)

Trong l vect AWGN phc vi v ,H l ma trn knh Nr(Nt; khi khong cch gia cc anten >(/2 v mi trng nhiu tn x ta c th coi H c cc hng v cc ct c lp vi nhau. Khi ny phn chia gi tr n (SVD: Singular Value Decomposition) cho ta:

H=UDVH

(10.8)

trong Trong U v VH l cc ma trn nht phn (unitary) c kch thc Nr(Nr v Nt(Nt, (.)H l chuyn v Hermitian (ma trn chuyn v lin hp phc); i vi cc ma trn nht phn ta c: UUH=INr v VVH=INt. D l ma trn Nr(Nt gm

(10.9)

cc gi tr n khng m c k hiu l trn ng cho chnh ca n, trong (i vi i=0,2,, NA-1 l cc gi tr eigen ca ma trn HHH. Cc gi tr eigen ca HHH c xc nh nh sau:

(10.10)

trong Q l ma trn Wirshart c xc nh nh sau:

`

(10.11)

Cc ct ca ma trn U l vect eigen ca HHH cn cc ct ca ma trn V l vect eigen ca ma trn HHH.

S cc gi tr eigen khc khng ca ma trn HHH chnh bng hng ca ma trn ny.

Nu Nt=Nr th D l mt ma trn ng cho. Nu Nt>Nr th gm mt ma trn ng cho Nr(Nr v sau l Nt-Nr ct bng khng. Nu NtNr), U s l ma trn Nr( Nr v V s l ma trn Nt( Nt v D s c to ra t ma trn vung bc Nr tip sau l Nt-Nr ct bng khng nh sau:

(10.12)

trong trng hp ny ma trn V ch c Nr hng s dng c, cn Nt-Nr hng cn li khng s dng dc. Khi ny Nr phn t u ca ma trn c s dng v Nt-Nr phn t cn li ca n c t vo khng. Trng hp c bit ta c Nt anten pht nhng ch c mt anten thu (Nr=1). Khi ny ma trn U c kch thc 1(1 v ch s dng c mt hng ca ma trn V.

Trng hp th hai tng ng vi khi s anten thu nhiu hn s anten pht (Nt1 v Nr=1. Nu c c mt s hiu bit v cc knh ng xung ca cc anten pht khc nhau v nht l mt s hiu bit v pha knh tng i ti pha pht, cc anten pht ngoi phn tp cn c th m bo to bp, ngha l to dng cho ton b bp anten theo phng n my thu ch. Ni chung to bp c th tng cng tn hiu ti anten thu ln n tha s Nt, ngha l t l vi s anten pht. Khi ni v cc s truyn dn da trn nhiu anten pht cung cp to bp, ta cn phn bit gia cc trng hp tng quan anten tng h cao v thp.

Tng quan anten tng h cao lin quan n cu hnh anten vi khong cch gia anten nh nh trn hnh 10.6a. Trong trng hp ny, cc knh gia cc anten khc nhau v my thu c th hu nh ging nhau k c phainh knh v tuyn ngoi tr s khc nhau v pha ph thuc vo phng. Khi ny c th li bp truyn dn tng n cc phng khc nhau bng cch s dng cc dch pha khc nhau i vi cc tn hiu pht trn cc anten khc nhau nh trn hnh 10.6b.

Hnh 10.6. To bp kinh in vi tng quan anten tng h cao: (a) cu hnh anten v (b) cu trc bp

Phng php to bp pha pht bng cch s dng cc dch pha khc nhau cho cc anten c tng quan cao i khi c gi l to bp kinh in. Do khong cch gia cc anten nh, bp pht tng kh rng v cc iu chnh phng bp sng (trong thc t l iu chnh cc dch pha anten) thng thng c thc hin kh chm. iu chnh c th c thc hin trn c s nh gi phng n u cui di ng ch c rt ra t o c trn ng ln. Ngoi ra do gi thit tng quan cao gia cc anten pht khc nhau, to bp kinh in khng th m bo phn tp chng phainh knh v tuyn ngoi vic ch tng cng tn hiu thu.

Tng quan anten tng h thp lin quan n khong cch gia cc anten ln (hnh 10.7) hay cc phng phn cc anten khc nhau. Vi tng quan anten tng h thp, nguyn l to bp c s cng ging nh s trn hnh 10.6. Tuy nhin khc vi to bp kinh in, cc trng s anten phi c gi tr phc ngha l phi c th iu chnh c pha v bin ca tn hiu pht trn cc anten khc nhau. iu ny phn nh thc t l do tng quan anten tng h thp, c pha v li tc thi ca cc knh anten c th khc nhau.

Hnh 10.7. To bp da trn b tin m ha trong trng hp tng quan anten tng h thp

Trong to bp pht Nt>1, Nr=1. Mi ln pht, mt k hiu c a n cc anten. Khi s dng cc trng s phc khc nhau cho cc tn hiu cn pht trn cc anten khc nhau, ta c th biu din vect tn hiu cho cc anten pht nh sau:

(10.33)

trong Wn (n=0,1,, Nt-1) l trng s cho anten th n, W= [W0,,WNt-1]T l vect trng s anten hay ma trn tin m ha, v vect k hiu cn pht, xn (n=0,1,, Nt-1) v =[x0, x1,..., xNt-1]T l k hiu pht v vect cc k hiu pht sau khi nh trng s, v x l k hiu cn pht. W c tnh ton da trn hiu bit v knh (thng l thng tin phn hi t pha thu)

Cn lu rng to bp kinh in trn hnh 10.6 cng c th c m t theo phng trnh (10.33), c ngha rng tin m ha anten vi hn ch li bng mt v ch m bo cc dch pha cho cc anten pht khc nhau.

Gi thit rng cc tn hiu pht i t cc anten khc nhau ch b pha inh phng (phainh khng chn lc tn s) v tp m trng, ngha l khng c tn thi v my pht bit r knh truyn, t c cng sut tn hiu thu cc i, cn chn cc trng s tin m ha nh sau:

(10.34)

Ngha l trng s phi bng lin hip phc ca li knh hn c chun ha m bo tng cng sut pht c nh. Hay ni mt cch khc vect ma trn tin m ha W phi l chuyn v Hermitian ca vect li knh H: W=. Vect tin m ha phi m bo:

Quay pha tn hiu pht b tr pha tc thi ca knh v m bo cc tn hiu thu c ng b pha

n nh cng sut cho cc anten khc nhau theo nguyn tc cng sut ln hn c n nh cho cc anten c iu kin knh tc thi tt hn ( li knh |hi| cao)

m bo tng cng sut pht bng mt (hay mt hng s bt k)

im khc nhau cn bn gia to bp kinh in trn hnh 10.6 vi gi thit tng quan tng h gia cc anten cao v to bp trn hnh 10.7 vi gi thit tng quan tng h gia cc anten thp l ch trong trng hp th hai h thng cn bit knh chi tit nh gi phainh tc thi ca knh. Cc gi tr mi ca vect tin m ha phi c tnh trong mt khong thi gian ngn bt kp cc thay i ca phainh. Do cc iu chnh cho cc trng s b tin khuch i c xt n phainh tc thi (bao gm li knh tc thi), nn to bp trn hnh 10.7 cng cung cp phn tp chng phainh knh v tuyn.

Ngoi ra trong trng hp thng tin da trn ghp song cng phn chia theo tn s (FDD: Frequency Division Duplex) trong truyn dn ng ln v ng xung s dng cc bng tn khc nhau, nn thng thng phaing gia ng ln v ng xung khng tng quan vi nhau. V th trong trng hp FDD ch u cui di ng l c th nh gi c phadinh knh ng xung. Nn u cui di ng phi bo co c tnh knh ng xung cho trm gc thng qua bo hiu ng ln. Mt cch khc, u cui di ng c th t mnh la chn vect tin m ha t mt tp hu hn cc vect tin m ha c th c (c gi l bng m tin m ha) v bo co vect ny cho trm gc.

Mt khc trong trng hp ghp song cng phn chia theo thi gian (TDD: Time Division Duplex), trong truyn dn ng ln v ng xung s dng chung mt bng tn nhng trong cc khe thi gian cch bit nhau, nn thng thng tn ti tng quan phainh cao gia ng ln v ng xung. Trong trng hp ny trm gc (t nht l v mt l thuyt) c th xc nh phainh ng xung t o c trn ng ln nh vy trnh c vic phn hi thng tin. Tuy nhin iu ny ch m bo khi coi rng u cui di ng thng xuyn pht trn ng ln.

Cc phn tch trn y c a ra vi gi thit l knh khng thay i trong min tn s. Trong trng hp knh chn lc tn s, r rng rng khng th ch c mt h s knh trn mt anten c tnh ton theo (10.34). Tuy nhin trong trng hp OFDM, mi sng mang con thng tri qua mt knh khng chn lc tn s. V th trong trng hp truyn dn OFDM, tin m ha trn hnh 10.8 c th c thc hin trn c s tng sng mang con nh trn hnh 10.8, trong cc trng s ca b tin m ha ca tng sng mang con c chn theo (10.34).

Wi,n (i=0,1,,N-1 v n=0,1) k hiu cho trng s i vi sng mang con th i (i=0,1,.., N=1) v anten th n (n=0,1)

Hnh 10.8. Tin m ha cho tng sng mang con trong trng hp OFDM (hai anten pht)

Cn lu rng trong trng hp truyn dn n sng mang nh trong 3G WCDMA, phng php mt trng s trn mt anten nh trn hnh 10.7 c th c m rng xt n c knh tn thi/tn tn.

10.6.2. To bp pha thu

Trong trng hp to bp pha thu, Nt=1 v Nr>1 (gi thit ch c mt lung). Nguyn l ca to bp pha thu c th hin trn hnh 10.10. Trong trng hp ny ti mt thi im ch mt k hiu c pht. Vec t tn hiu thu c xc nh nh sau:

(10.35)

My thu kt hp cc tn hiu my t Nr anten trn c s s dng cc trng s W= [W0,,WNr-1]T. Tn hiu thu c sau kt hp s l:

(10.36)

Sau khi my thu nhn c nh gi knh, n c th t vect to bp W vo gi tr ti u t c SNR thu cc i. t c iu ny cn ng b vect to bp vi knh ca MS thng qu vic kt hp t l cc i (MRC): W=HH (c th coi y l mt phin bn khng gian ca b lc phi hp). Lu rng cng c th t c loi b tn hiu nhiu bng cch chn vect to bp trc giao vi knh ca ngun nhin n.

Hnh 10.9. Khi nim to bp v xa khng nhiu

B kt hp t l cc i cung cp tha s ci thin Nr trong SNR thu so vi trng hp SISO, ngha l li mng 10lg10(Nr) dB trong qu ng truyn.

Tng qut s hnh 10.9 c th p dng cho nhiu kiu s MIMO v MU-MIMO quan trng (tr s nh hng phn tp) nh sau:

B to bt N anten c th khuch i mt ngun pht (khng nhiu) vi h s N ti gi tr SNR trung bnh: To bp

B to bp N anten c th ly ra mt ngun pht v loi b N-1 cc ngun khc: trit nhiu

To bp pht thc hin cng li nh to bp phia thu nu c CSIT: to bp pht v xa khng nhiu

N ngun pht c th c ng thi ly ra (vi gi thit coi rng N-1 ngun cn li l nhiu) bi b to bp: ghp knh khng gian

N ngun c th c n nh cho N ngi s dng khc nhau: MU-MIMO, SDMA

Mt s trong s N ngun c th thuc cc khc a cng tc

10.7. GHP KNH KHNG GIAN

T cc nghin cu trong cc phn trc ta hiu rng vic s dng nhiu anten ti my pht v my thu cho php ci thin t s tn hiu trn tp m t l vi s lng anten khi p dng to bp ti c my thu v my pht. Trng hp tng qut vi Nt anten pht v Nr anten thu, h thng a anten c th tng t s tn hiu trn tp m NtxNr ln. Vic tng t s tn hiu trn tp m ny trong my thu cho php tng tc s liu trong trng hp tc s liu b gii hn bi cng sut ch khng b gii hn bi bng thng. Tuy nhin mt khi t n vng hot ng b gii hn bi bng thng cc tc s liu s bt u bo ha nu khng th tng bng thng

S dng nhiu anten ti c u pht v u thu cn cho php ghp knh khng gian t c hiu sut s dng ph tn cao hn trong trng hp t s tn hiu trn tp m/nhiu cao v tc s liu cao trn giao din v tuyn. Vn ny phn no c cp trong phn 10.3, trong phn ny ta s phn tch n c th hn.

10.7.1. Nguyn l c s

Nh thy t phng trnh (10.24), khi SNR thp, dung lng t l vi s anten thu, C= (Nrlog2e.

Ta cng c th biu din dung lng nh sau:

(10.37)

trong (tol l tng SNR _ca h thng MIMO.

V th

(tol = (Nr

(10.38)

Nh vy trong trng hp ny h thng MIMO cho li cng sut (hay cn gi l li mng) bng 10lgNr dB nh to bp hay phn tp anten.

Tuy nhin khi SNR cao t phng trnh (10.25)) dung lng t l vi NA=min(Nt,Nr):

C

Trong trng hp ny bng cch s dng nhiu Nt v Nr, ta to nn NA knh song song nh vy tng dung lng. Phng php ny c gi l ghp knh khng gian. Thut ng x l anten MIMO cng thng c s dng, tuy nhin thut ng chnh xc cho tt c cc trng hp l a anten pht v a anten thu bao hm c trng hp phn tp pht thu kt hp.

Gi s (i nh nhau cho tt cc cc knh v cng sut pht c phn u cho cc knh, ta c th vit dung lng dng n gin sau:

(10.39)10.7.2. Ghp knh khng gian khi my pht khng c thng tin v knh

hiu c nguyn l c s v cch to lp cc knh song song trong trng hp nhiu anten ti my pht v nhiu anten ti my thu, ta xt cu hnh anten 2x2, ngha l c hai anten pht v hai anten thu nh trn hnh 10.10. Ngoi ra ta gi thit rng cc tn hiu pht ch b phainh khng chn lc tn s (pha inh phng) v tp m trng, ngha l khng xy ra tn thi knh.

Hnh 10.10. Cu hnh anten 2x2

T hnh 10.10 ta c th biu din cc tn hiu thu nh sau:

(10.40)

Trong H l ma trn knh 2x2, hm,n l li knh t anten pht n (n=0,1) n anten thu m (m=0,1). C th coi biu thc ny l n gin ha biu thc (10.2) c vit cho nhiu anten pht vi nhiu tn hiu khc nhau c pht trn cc anten pht khc nhau.

Nu coi rng ma trn H l kh o, th my thu c th hon ton khi phc c vect v c hai tn hiu x1 v x2 m khng cn nhiu gia chng bng cch nhn vect thu vi ma trn W=HH =H-1 (xem hnh 10.11):

(10.41)

Hnh 10.11. Thu tuyn tnh/gii iu ch cc tn hiu ghp knh khng gian

Mc d c th khi phc hon ho vect trong trng hp khng c tp m chng no ma trn knh H cn kh o, biu thc (10.38) cng cho thy rng cc thuc tnh ca H s quyt nh phm vi tng mc tp m sau gii iu ch kt hp hai tn hiu. c bit, ma trn knh cng gn vi ma trn n nht (singular matrix, ma trn n nht l ma trn khng kh o), mc tng tp m s cng ln hn.

gii thch ma trn W, ta cn hiu rng cc tn hiu pht t hai anten pht l cc tn hiu gy nhiu cho nhau. Khi ny cc anten thu c s dng thc hin IRC Interference Rejection Combination: kt hp loi b nhiu), m thc cht l trit b hon ton nhiu t tn hiu pht trn anten th hai khi tch tn hiu pht t anten th nht v ngc li. Cc hng ca ma trn W ch n thun thc hin IRC.

Trong trng hp tng qut, mt cu hnh a anten s gm Nt anten pht v Nr anten thu. Nh ni trn, trong trng hp ny, s lng cc tn hiu song song c th c ghp khng gian trong thc t b gii bi bin trn NA=min{Nt,Nr}. Gi s Nr(Nt. Khi ny NA=Nt v ta c th coi rng c Nt lung pht s dng Nt anten khc nhau. V my pht khng bit ma trn H, nn khng th ci thin s ghp knh khng gian bng cch s dng b tin m ha ph thuc knh. Nh vy b tin m ha n thun ch l mt ma trn n v c nhim v sp xp Nt lung ln Nt anten tng ng. Trong trng hp ny ta c th biu din hm sp xp k hiu ln tn hiu pht nh sau:

(10.42)

Trong ma trn P l ma trn ng cho n nh cng sut kch thc NtxNt vi (n=0,1,..,Nt-1) l phn t ng cho th n, trong pn l cng sut c n nh cho anten th n vi gii hn ca tng cng sut pht chun ha Pt. Trong trng hp ny n nh cng sut nh nhau cho cc lung nn ta c th b qua ma trn P.

Ti pha thu, cc k thut tch sng tuyn tnh v phi tuyn khc nhau c s dng khi phc li . Gii php t phc tp nht l s dng k thut tch sng tuyn tnh, trong my thu a ra Nt b to bp w0, w1, , wNt-1. S dng w( ((=0,1, , Nt-1) ta c tch sng lung ( [x(,0, x(,1, , x(,Ns-1] nh sau:

(10.43)

Tiu chun tch sng l phi m bo dung ha gia to bp n lung v loi b nhiu (t N-1 lung khc). Nhiu gia cc lung hon ton c loi b khi la chn my thu cng bc v khng (ZFBF: Zero Force Beamforming) nh sau:

(10.44)

Trong

(10.45)Trong Trace(m) l php ton cng tt c cc phn t trn ng cho chnh ca ma trn m.

Tuy nhin i vi hiu sut ti u, s ph v cn bng gia ng b vi h( v tnh trc giao i vi tt c cc ch k hk khc (k((). C th t c s cn bng ny bng my thu sai s qun phng cc tiu (MMSE-BF Minimum Mean-Squared Error-Beamforming) sau y:

(10.46)

Trong ( dc chn sao cho

(10.47)

Ngoi cc cu trc tch sng tuyn tnh kinh in nh cc my thu ZF v MMSE, nhiu b tch sng tin tin nhng phi tuyn khc c th c s dng m bo t l li tt hn khi chn im cng tc SNR nhng vi tr gi bng phc tp cao. Th d v cc b tch sng ny bao gm b tch sng lai b nhiu ln lt (SIC: Successive Interference Cancellation) v b tch sng kh ging nht (MLD: Maximum Likelihood Detector).

Phn tch ghp knh khng gian cho thy:

Khng th pht nhiu hn Nt tn hiu, ngha l cc i ch c th ghp khng gian Nt tn hiu

Vi Nr anten thu, cc i ch c th trit Nr-1 tn hiu gy nhiu, ngha l cc i ch c th ghp khng gian Nr tn hiu

Tuy nhin trong nhiu trng hp, s lng cc tn hiu c ghp khng gian, hay bc ghp knh khng gian s t hn NA ni trn:

Trong trng hp iu kin knh rt km (t s tn hiu trn tp m thp) s khng cn li ghp knh khng gian v dung lng knh l mt hm ph thuc gn nh tuyn tnh vo t s tn hiu trn tp m. Trong cc trng hp ny, cu hnh a anten pht v thu phi l to bp ci thin t s tn hiu trn tp m ch khng phi ghp knh khng gian (chn knh khng gian tt nht v loi b cc knh khng gian cht lng km).

Trong trng hp tng qut hn, bc ca ghp knh khng gian phi c xc nh da trn cc thuc tnh ca ma trn knh NrxNt. Khi ny cc anten b sung cn c s dng m bo to bp. T hp to bp v ghp knh khng gian c th t c bng ghp knh khng gian da trn b tin m ha s c xt di y.

10.7.3. Ghp knh khng gian da trn b tin m ha khi my pht c thng tin knh

Tin m ho tuyn tnh trong trng hp ghp knh khng gian c p dng ti pha pht x l tuyn tnh bng mt ma trn tin m ha V kch thc NAxNt (hnh 10.12). Nh ni trn, trong trng hp tng qut, NA bng hoc nh hn Nt, ngha l NA tn hiu c ghp khng gian v c pht i bng cch s dng Nt anten.

Hnh 10.12. Ghp knh khng gian da trn b tin m ha

Lu rng nh ni trn, tng qu ha c th coi ghp knh khng gian nh to bp da trn b tin m ha c vect tin m ha kch thc 1xNt c thay bng ma trn tin m ha c kch thc NAxNt.

B tin m ha trn hnh 10.12 thc hin hai mc ch:

Trong trng hp khi s lng cc tn hiu c ghp khng gian bng vi s lng anten pht (NA=Nt), b tin m ha c s dng trc giao ha cc truyn dn song song nhm ci thin s cch ly tn hiu ti pha thu.

Trong trng hp khi s lng cc tn hiu c ghp song song nh hn s lng anten pht (NA1GHz) d hn ti cc bng tn thp (