TRNG I HC BCH KHOA H NI

VIN IN T - VIN THNG

N

TT NGHIP I HC ti:

PHN TCH V THIT K ANTEN A U VO A U RA BNG THNG SIU RNGSinh vin thc hin : HONG TH VIT Lp T9 K51 Ging vin hng dn: Th. S. NGUYN KHC KIM PGS. TS. O NGC CHIN

H Ni - thng 6 nm 2011

B GIO DC V O TO TRNG I HC BCH KHOA H NI --------------------------------------------------

CNG HA X HI CH NGHA VIT NAM c lp - T do - Hnh phc ---------------------------------

NHIM V N TT NGHIP

H v tn sinh vin: ..... S hiu sinh vin: Kho:.Vin: in t - Vin thng Ngnh: ......... 1. u n: .. 2. Cc s liu v d liu ban u: ...... ..... 3. Ni dung cc phn thuyt minh v tnh ton: ... ... Cc bn v, th ( ghi r cc loi v kch thc bn v ): ... ... 4. H tn ging vin hng dn :.. 5. Ngy giao nhim v n: . 6. Ngy hon thnh n: Ngy thng nm Ch nhim B mn Ging vin hng dn

Sinh vin hon thnh v np n tt nghip ngy

thng

nm

Cn b phn bin

B GIO DC V O TO TRNG I HC BCH KHOA H NI ---------------------------------------------------

BN NHN XT N TT NGHIPH v tn sinh vin: ....................................................................... S hiu sinh vin: ........................... Ngnh: .................................................................................................. Kho: .................................................... Ging vin hng dn:.............................................................................................................................................. Cn b phn bin: ....................................................................................................................................... 1. Ni dung thit k tt nghip:........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... .................................................................................................................................................................................................

2. Nhn xt ca cn b phn bin:........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ........................................................................................................................................................................................................................... ...........................................................................................................................................................................................................................

Ngy

thng Cn b phn bin

nm

( K, ghi r h v tn )

Li ni uTrong truyn thng ni chung th c hai vn cn phi quan tm l: tc d liu v tin cy truyn tin. Vi truyn thng khng dy th hai vn ny l quan trng hn c v mi thit k u phi da trn hai thng s ny lm sao cho tc d liu ngy cng tng v tin cy ngy cng cao. Trong truyn thng khng dy th c hai hin tng gy tr ngi cho h thng ca chng ta l: Fading v giao thoa gia cc k hiu. Do , nng cao tc truyn d liu th cn phi c bng thng ln nhng iu ny b hn ch v di tn s l mt ti nguyn khan him. ng thi, mun cht lng tn hiu c ci thin v gim nh hng ca phading th my pht phi t c cng sut ln hoc tng kch thc anten duy tr hiu sut bc x; tuy nhin, i vi nhng thit b di ng cm tay nh in thoi di ng, my tnh xch tay c kch thc nh gn th khng th p dng phng php ny c. Hin nay, h thng anten s dng nhiu phn t bc x c pha pht v pha thu hay cn gi l k thut a u vo a u ra (MIMO) c ng dng trong k thut anten. N em li nhiu u th v cht lng truyn tn hiu cng nh tc truyn ti d liu. K thut MIMO ra i nhm mc ch khc phc nhng nhc im trn trong h thng thng tin v tuyn. Bng thng rng khng cn l mt iu mi m trong thng tin di ng na, m n tr thnh mt c im thit yu trong rt nhiu ng dng khc nhau, v d nh: truyn hnh cht lng cao, truyn hnh di ng, Internet bng thng rng, game trc tuyn, gii tr a phng tin hay giao tip gia cc thit b trong khong cch ngn. Mt gii php c a ra l s dng cc thit b hot ng mt di tn siu rng t 3.1Ghz n 10.6Ghz, gi l di tn siu rng (UWB). y l di tn khng phi ng k v c cc tiu chun xy dng mt h thng UWB hon chnh. Tuy nhin. nhu cu con ngi khng ngng gia tng v trong tng lai khng xa khi bng tn ca h thng UWB khng cn ph hp na th cng ngh mi phi ra i. Khi , cng ngh Extremely Wide-band (EWB) s l hng i 1

mi ph hp vi tin trnh pht trin ca thi i. Cng ngh ny vi di tn hot ng cc k rng t vi GHz n vi chc GHz s c p dng cho nhng thit b u cui v tuyn a bng tn. Vi di tn bao trm ton b bng tn ca cc h thng nh: WLAN, WiMAX, UWB th anten EWB hon ton c th c s dng hot ng trong nhiu bng tn vi nhiu h thng khc nhau. Cng vi vic thit k anten EWB cho h thng MIMO s gp nhiu thch thc v tr ngi. Vn u tin t ra l kch thc ca anten phi nh gn t c yu cu ca cc nh sn xut thit b di ng khi tch hp vo sn phm ca h. Hn na, trong h thng nhiu phn t bc x, nh hng tng h gia chng l ng k, hin tng ny cn phi c gim thiu nng cao n nh v hiu sut bc x ca h thng. Anten vi di l mt loi anten c nhiu u im tha mn c cc yu cu t ra trn: nh gn, c th tch hp c trn nhiu b mt khc nhau, d ch to, r tin. V vy, nhim v t ra l thit k anten vi di ng dng trong h thng a u vo a u ra bng thng siu rng. Trong qu trnh thc hin n, ti nhn c rt nhiu cc s gip t cc thy c trong vin in t Vin Thng, c bit phi k n s tn tm, nhit tnh ca Th. S. Nguyn Khc Kim, B mn H Thng Vin Thng, vin in T Vin Thng, trng i hc Bch Khoa H Ni, ging vin trc tip chu trch nhim hng dn n tt nghip. Ti xin gi li cm n n thy v cc thy c trong vin in t Vin Thng. Qua y, ti xin cm n thy gio PGS. TS. o Ngc Chin, B mn H Thng Vin Thng, vin in T Vin Thng, ngi gip ti trong vic nh hng v cc c s vt cht hon thnh n. Ti cng xin cm n tt c cc bn trong phng th nghim nghin cu v pht trin truyn thng CRD, tng 6 th vin T Quang Bu i hc Bch Khoa H Ni, gp v chia s kinh nghim vi ti trong hn mt nm qua. H Ni, ngy 30 thng 05 nm 2011

Hong Th Vit

2

Tm tt nKhi k thut a u vo a u ra (MIMO) c ng dng cho anten bng thng siu rng, n em li cho h thng kh nng truyn ti mt khi lng d liu ln hn mc d khng c s m rng v bng thng. Tuy nhin, vic thit k h thng ny gp phi nhiu kh khn trong vic gim nh hng tng h gia cc phn t bc x m tc ng ca n ti cht lng tn hiu l ng k. Mc ch ca n ny l nghin cu v thit k m hnh anten vi di lm vic di tn siu rng t xp x 2.7Ghz n 20Ghz. ng thi kt hp vi k thut MIMO, tnh ton, nh gi v tm cch gim thiu tc dng tng h gia cc anten t c th ch to th nghim. Trn thc t, ti tin hnh thit k thnh cng m hnh cho mc tng h gia cc anten thnh phn tha mn yu cu t ra, trong , cc thng s truyn t, th bc x phng hng v c bit l kch thc h thng c ti u. Trong m hnh ti xut, h hai anten nm trn cng mt bn dn v cng mt mt. Vi hai phin bc x kim loi c cu to bi nhng ng gp khc hot ng nh mt cu trc bin i u c tc dng lm tng bng thng m bng thng ca anten MIMO hot ng di tn s rt rng t 2,7 GHz n 20 GHz. gim tc ng tng h gia cc phn t anten n th cu trc hai stub c thm vo gia hai phin bc x. Cui cng, nh vo kch thc nh, anten c xut c th tch hp vo cc thit b di ng cm tay.

3

AbstractOnce the Miltiple-input Multipe-output technology is applied in the field of Ultra Wideband antena technique, this will offer the ability of data transmitting with a huge amount of capacity though the bandwidth of the channel is limited. However, in the process of designing this system, we could suffer the problem in reducing the mutual coupling proportion between antenna elements so that the signals in the operating system cant be interfered. The aim of this project is a method of designing and simulating an antenna model which operates at the extremely wide band of frequency from 2.7 Ghz to 20 Ghz after finishing the process. In addition, by futher development with Multi input Multi output, the antenna system with much higher effeciency will be offered. The caculation, analysis, estimation of the system can be difficult because of the effect of mutual coupling between antenna elements. After reseaching deeper of antenna field, I did success of producing a model which has a low mutual coupling, low return loss, omnidirectional radiation partern and the optimum dimensions of the system. In the proposed system, MIMO model is constructed by placing two single antennas side by side. With the radiation patchs is structured by broken lines for producing a gradually variant structure to increase bandwidth, the designed antenna satisfies the voltage standing wave ratio (VSWR) requirement of less than 2.0 in an extremely wide frequency band ranging from 2.7 GHz to 20 GHz. To reduce mutual coupling to be used in diversity antenna environment, two stubs are added between two patches. And then, this antenna can be intergrated in lots of handheld devices because of its compact dimensions.

4

Mc lc

Li ni u .................................................................................................................... 1 Tm tt n ................................................................................................................ 3 Abstract ......................................................................................................................... 4 Danh sch cc hnh v .................................................................................................. 8 Danh sch cc bng biu............................................................................................ 10 Bng danh sch cc t vit tt v i chiu thut ng Anh Vit ........................... 11 Phn m u ............................................................................................................... 14 Chng 1 Gii thiu................................................................................................... 16 1.1 1.2 1.3 1.4 Hin trng ca bi ton ................................................................................... 16 ng lc pht trin ......................................................................................... 18 Hng i mi trong tng lai ......................................................................... 19 Kt qu mong i ........................................................................................... 19

Chng 2 Cng ngh bng thng siu rng v h thng a u vo a u ra .. 21 2.1 Cng ngh bng thng siu rng Ultra Wideband ......................................... 21 Lch s cng ngh bng thng siu rng ................................................. 21 c im chnh ca cng ngh bng thng siu rng ............................. 22 So snh cc cng ngh truyn thng wireless c ................................ 25 Cc ng dng ca cng ngh UWB ........................................................ 27 ng dng v radar............................................................................. 27 ng dng v truyn thng tc cao ............................................... 28 Mt s anten c ng dng trn thc t thit kt anten

2.1.1 2.1.2 2.1.3 2.1.4

2.1.4.1 2.1.4.2 2.1.4.3

UWB.. .......................................................................................................... 28 2.1.5 Kt lun .................................................................................................... 31

5

2.2

Khi nim v m hnh h thng MIMO ......................................................... 31 nh ngha v c im ........................................................................... 31 Nhng hn ch ca knh truyn khng dy............................................. 32 u im ca h thng MIMO ................................................................. 34 H thng a nten v nh hng tng h .............................................. 35 Gii thiu h thng a anten ............................................................. 35 Cc nhn t nh hng n h anten MIMO .................................... 37

2.2.1 2.2.2 2.2.3 2.2.4

2.2.4.1 2.2.4.2 2.2.5

K thut phn tp anten ............................................................................ 39 Gii thiu v k thut phn tp anten ................................................ 39 Cc phng php phn tp ................................................................ 39

2.2.5.1 2.2.5.2 2.2.6

Kt lun .................................................................................................... 41

Chng 3 Thit k anten vi phn mm HFSS....................................................... 42 3.1 L thuyt c bn trng in t ..................................................................... 42 Phng trnh Maxwell v cc iu kin bin .......................................... 42 Cc phng trnh th ............................................................................... 43 Cc iu kin bin.................................................................................... 48

3.1.1 3.1.2 3.1.3 3.2

Phng php phn t hu hn (FEM) ............................................................ 50 Khi nim ................................................................................................. 50 Ni dung .................................................................................................. 50 ng dng ................................................................................................. 51

3.2.1 3.2.2 3.2.3 3.3

Phn mm m phng Ansoft HFSS................................................................ 51 Gii thiu ................................................................................................. 51 M phng ................................................................................................. 52 Phng php phn t hu hn .......................................................... 52 Tnh ton ........................................................................................... 53 S khi thc hin m phng ........................................................ 56

3.3.1 3.3.2 a. b. c.

Chng 4 Thit k anten vi di bng thng siu rng p dng k thut MIMO . 57 6

4.1

Qu trnh thit k v phn tch ....................................................................... 57 Thit k anten n ................................................................................... 61 M hnh anten ................................................................................... 61 Kt qu m phng v o c............................................................. 63 Phn tch m hnh anten n............................................................. 67 M hnh ch to ca cc anten .......................................................... 72

4.1.1

4.1.1.1 4.1.1.2 4.1.1.3 4.1.1.4 4.1.2

H thng anten MIMO............................................................................. 73 M hnh anten ................................................................................... 73 Kt qu .............................................................................................. 73 Nhn xt v nh gi ......................................................................... 76

4.1.2.1 4.1.2.2 4.1.2.3 4.2

Kt lun........................................................................................................... 83 Kt lun chung .................................................................................................. 84

Ti liu tham kho ..................................................................................................... 85

7

Danh sch cc hnh vHnh 1. 1 Cc ng dng ca cng ngh UWB ..........................................................17 Hnh 2. 1 Tng quan cng ngh bng thng siu rng .............................................23 Hnh 2. 2 Mt n ph cng sut cho UWB trong nh ...............................................24 Hnh 2. 3 Mt n ph cng sut cho UWB ngoi tri ..............................................24 Hnh 2. 4 Phn chia di tn trong UWB ...................................................................24 Hnh 2. 5 So snh v khong cch tng tc v tc truyn d liu .....................25 Hnh 2. 6 So snh v rng bng tn v mt ph cng sut .............................26 Hnh 2. 7 Cc ng dng point to multipoint ca UWB ............................................29 Hnh 2. 8 H thng MIMO ........................................................................................32 Hnh 2. 9 M hnh MMA(a) v MEA (b) ...............................................................35 Hnh 2. 10 Cc nguyn nhn gy tng h. .............................................................36 Hnh 2. 11 Cc qu trnh gy nhiu gia lin chn t...............................................37 Hnh 2. 12 Hm tng quan gia cc anten thnh phn nh l hm ca khong cch theo bc sng ..........................................................................................................38 Hnh 3. 1 iu kin bin ca E v B .........................................................................48 Hnh 3. 2 Cch chia phn t hu hn trong HFSS: (a) thnh cc tam gic trn b mt, (b) thnh cc t din trong khng gian ba chiu ...............................................53 Hnh 3. 3 M hnh m phng ....................................................................................56 Hnh 4. 1 Kt cu bin i t t ................................................................................59 Hnh 4. 2 Cu to ca ng dn sng ng phng......................................................60 Hnh 4. 3 Mode sng truyn trong CPW ..................................................................60 Hnh 4. 4 Kch thc anten EWB n ......................................................................62 Hnh 4. 5 Cng c tnh ton tr khng CPW ............................................................63 Hnh 4. 6 T s sng ng ca anten (VSWR) .........................................................64 Hnh 4. 7 th phng hng bc x ta cc ti (a) 3GHz, (b) 10 GHz, (c) 7.5GHz v (d) 15GHz ...............................................................................................65 Hnh 4. 8 th phng hng bc x trong khng gian ba chiu ti (a): 7.5GHz v (b): 10GHz ................................................................................................................65 8

Hnh 4. 9 H s tng ch ca anten n ....................................................................66 Hnh 4. 10 Mt dng b mt: a) 10 Ghz, b) 5Ghz, c)15GHz v d)20GHz ..........66 Hnh 4. 11 S thay i h s tn hao ngc theo gc m ca kt cu chuyn tip .67 Hnh 4. 12 th biu din s thay i h s tn hao ngc theo chiu di ca ng dn ng phng .........................................................................................................68 Hnh 4. 13 Anten vi cu trc khe ..........................................................................69 Hnh 4. 14 T s sng ng vi cu trc khe dng loi b bng tn cho (a) WLAN v (b) WiMAX ..........................................................................................................70 Hnh 4. 15 th bc x ca anten vi cu trc khe loi b bng tn WLAN ti (a) 3GHz, (b) 10GHz, (c) 7.5GHz v (d) 15GHz ...........................................................71 Hnh 4. 16 H s tng ch nh ca hai m hnh anten khng c cu trc khe (a) v c cu trc khe (b) .....................................................................................................72 Hnh 4. 17 Sn phm anten EWB .............................................................................72 Hnh 4. 18 M hnh anten MIMO .............................................................................73 Hnh 4. 19 T s sng ng ca cc anten ................................................................74 Hnh 4. 20 Tham s S12&S21 ca hai m hnh MIMO ...........................................75 Hnh 4. 21 th bc x phng hng tn s 10GHz ti (a) Anten n, (b) Anten MIMO, (c) Anten MIMO loi b bng tn WLAN .......................................75 Hnh 4. 22 th phng hng bc x trong khng gian ba chiu ca m hnh MIMO loi b bng tn WLAN ti (a) 7.5GHz v (b) 15GHz .................................76 Hnh 4. 23 M hnh anten ghp ni tip ...................................................................77 Hnh 4. 24 S11 v S21 ca anten ghp ni tip cch nhau .......................................77 Hnh 4. 25 M hnh anten ghp ni tip s dng Stub .............................................78 Hnh 4. 26 S11 v S21 ca anten ghp ni tip vi cu trc Stub ............................79 Hnh 4. 27 Mt dng in ti 10 GHz ca m hnh MIMO c Stub ...................79 Hnh 4. 28 Mt dng in ca (a) m hnh MIMO khng c Stub v (b) m hnh MIMO c Stub ..........................................................................................................80 Hnh 4. 29 H s tn hao ngc khi thay i chiu di khe ch V ngc ...............81 Hnh 4. 30 H s tn hao trong m hnh MIMO vi cu trc khe ch V ngc ......81 Hnh 4. 31 th bc x phng hng ca anten MIMO vi cu trc khe ti (a) 5GHz, (b) 6.2 GHz ....................................................................................................82

9

Hnh 4. 32 Mt dng b mt: a) 5.9 Ghz b) 10Ghz .............................................82 Hnh 4. 33 H s tng ch ca (a) anten MIMO v (b) anten MIMO loi b bng tn ...................................................................................................................................83

Danh sch cc bng biuBNG 2. 1 BNG SO SNH THNG S ............................................................. 26 BNG 2. 2 CC C IM YU CU I VI ANTEN UWB CA THIT B DI NG ........................................................................................................... 29 BNG 4. 1 KCH THC CC KHE DNG LOI B BNG TN CA H THNG WLAN V WiMAX .................................................................... 70

10

Bng danh sch cc t vit tt v i chiu thut ng Anh VitCDMA COFDM DFT DSSS Code Division Multiple Access Coded OFDM Discrete Fourier Transform Direct Sequence Spreading Spectrum FCC Federal Communications Commission GPS GSM IEEE Global Positioning System Global System for Mobile Communication Institute of Electrical and Electronics Engineers LAN MBOFDM MB-UWB NB OFDM Multi Band-UWB Narrow Band Orthogonal Frequency Division Multiplexing PSD UWB WLAN WPAN MIMO Power Spectrum Density Ultra Wide Band Wireless Local Area Network Wireless Personnal Area Network Multi-input multi-output UWB a bng Bng hp Ghp knh phn chia theo tn s trc giao Mt ph cng sut Bng thng siu rng Mng cc b khng dy Mng khng dy c nhn H thng nhiu u vo nhiu u ra Local Area Network Multi Band-OFDM Mng cc b OFDM a bng Hi ng truyn thng lin bang H thng nh v ton cu H thng thng tin di ng ton cu Vin k s in v in t a truy nhp phn chia theo m OFDM m ha Bin i Fourier ri rc Tri ph chui trc tip

11

SNR LOS CCI ISI TDMA

Signal to Noise Ratio Light of sight Co-channel interference Inter-Symbol Interference Time division multiple access

T l tn hiu trn tp m Tm nhn thng Nhiu ng knh Nhiu xuyn tn hiu a truy nhp phn chia theo thi gian a truy nhp phn chia theo tn s a truy nhp phn chia theo khng gian a truy nhp phn chia theo m n u vo n u ra M khng gian thi gian M khi khng gian thi gian M li khng gian thi gian M khng gian thi gian theo lp

FDMA

Frequency division multiple access

SDMA

Spatial division multiple access

CDMA SISO STC STBC STTC LSTC

Code division multiple access Single input Single Output Space time codes Space-time Block codes Space-time trellis codes Layered Space- time codes

ML MRC PSK BLAST

Maximum likelyhood Decoding Maximum Ratio Combiner Phase shift keying Bell labs layered space time

B gii m ging nhau ln nht. B t hp t s ln nht Kha dch pha M khng gian theo lp c xut ca Bell

H-Blast V- Blast SC EGC AOA DOA FDTC

Horizontal blast Vertical blast Scanning and Selection combiner Equal Gain Combiner Angles of Arival Angles of Departure Finite Difference Time Domain

M blast ngang M Blast dc B t hp qut v la chn B t hp cng li Gc n Gc ti Phng php min thi gian vi sai hu hn Bin i furie ri rc

DFT

Discrete Fourier Transform

12

FFT EBG

Fast Fourier Transform Electromagnetic band gap

Bin i Furie nhanh

13

Phn m uTrong thp k qua, chng ta chng kin s pht trin vt bc v khng ngng tin ti ca cng ngh truyn thng. Cc thit b ngy cng c ci tin nng cp tin dn ti s hon thin v thit k, tnh nng cng nh tc x l tn hiu. Do vic giao tip gia cc thit b l mt iu kin khng th thiu trong mt h thng i hi tnh ng dng cao v mm do trong nhiu hon cnh khc nhau. Hy hnh dung rng chic my tnh bn ca bn hon ton khng dy, tt c cc thnh phn ca n lin lc vi nhau bng cng ngh khng giy tc cao hay bn ch cn mt vi giy ti mt b phim ln n vi Gbs xung thit b cm tay nh in thoi di ng Cng ngh UWB ra i nhm tha mn nhng yu cu nh vy. Thm vo , vi s kt hp ca nn tng MIMO, h thng MIMO UWB em li cho cng ngh truyn thng nhng ng dng mang tnh t ph v tc . Nhng trong tng lai khng xa khi m cc ng dng khng dy i hi tc truyn ln hn, bng thng cng i hi cao hn th hin nhin yu cu mi v cng ngh s c t ra v cng ngh Extremely Wide-band (EWB) vi di tn hot ng ln hn rt nhiu s p ng c nhng nhu cu trn. Theo , anten l mt yu t v cng quan trng trong mt h thng hon chnh, n l thit b chung gian chuyn i tn hiu in thnh sng in t mang thng tin lan truyn trong khng gian v ngc li. Vy, bi ton t ra l thit k mt h thng anten MIMO EWB tha mn c nhng yu cu k thut theo cc tiu chun chung cng vi vic gim kch thc h thng ny c th c tch hp trn cc thit b di ng nh gn. Trong phm vi n ny, ti trnh by lm 4 chng: chng I s gii thiu chung v hin trng cng ngh hin nay, ng lc pht trin, hng i mi trong tng lai v kt qu mong mun t c trong thit k anten cho cng ngh bng thng siu rng.

14

n chng II, nhng kin thc tng quan v cng ngh bng thng siu rng v h thng a u vo a u ra (MIMO) s c trnh by. Nhng l thyt, kin thc v phng trnh Maxwell, iu kin bin v phn mm m phng HFSS c cp n chng III. Cui cng, m hnh cc anten c xut phn tch v thit k, cng nh cc kt qu m phng, o c s c trnh by chi tit trong chng IV.

15

Chng 1 Gii thiu

1.1

Hin trng ca bi tonTrc khi i su nghin cu thit k mt m hnh h thng anten MIMO EWB

hon chnh, ta hy im qua mt s cc s kin trn th gii v ng dng ca cng ngh bng thng siu rng (UWB), thy c rng, cng ngh bng thng siu rng c ngha to ln th no i vi cng ngh truyn thng. Vi tc ph v k lc so vi tt c cc cng ngh truyn dn khng dy khc, UWB c gn cho mt bit danh tht n tng Gigabit over the Air. V ngy nay, ta thy rng, cng ngh HD v 3D cho ra i nhng sn phm gii tr tuyt vi, nhng nhc im ca chng l dung lng lu tr qu ln. c th truyn ti c khi d liu ny gia cc thit b trong mi trng wireless vi khong cch gn th cn phi c mt gii php cn bng gia tc , n nh cng nh tnh tin cy. Cu tr li chnh l cng ngh UWB, chc chn gii php ny s m ra mt tng li ha hn nh vic ti b phim c phn gii cao ch trong tch tc cho cc giao din kt ni a phng tin phm vi hp. Vic ng dng n trn cc thit b di ng c tnh nng gii tr ang ngy cng tr nn ph bin. cng ngh UWB c ph bin v ng dng trong cc thit b in t cng nghip, cc nh nghin cu khoa hc, hip hi, t chc in t, tin hc trn th gii t chc rt nhiu cc cuc hi tho, hi ngh a ra chun quc t cho cng ngh ny. Mi th c khi ng bi tuyn b ca FCC vo ngy 14 thng 2 nm 2002 ln u tin a ra tiu chun cho cng ngh UWB mc d trc n c ng dng nhng ch yu l trong lnh vc qun s. y ban chu u cui cng cng a ra chi tit cc thng tin cp php cho vic thit k mng UWB chu u, truyn dn d liu bng thng siu rng c thit 16

lp v s dng thay th cho mng cp cho php cc file ln c truyn trong khong cch ngn vi tc chun l 480 Mbps. Tip vo nm thng 9 nm 2005, UWB c a vo bn bo co v ngh ca hip hi vin thng quc t ITU-R sau nhng cn nhc k cng. Vi nhng s kin ny cc nh pht trin hi vng s sm a UWB vo ng dng thc t trn ton th gii khng ch dng chn ti chu u. Thng 8 nm 2007, t chc y quyn cnh trnh v qun l vin thng c lp Ofcom Anh cng c nhng tuyn b tng t.

Hnh 1. 1 Cc ng dng ca cng ngh UWB

c rt nhiu cc thit b in t ng dng cng ngh UWB theo chun ca FCC, a s l radar, h thng xc nh v tr v hnh nh. bt kp vi cng ngh, cc nh sn xut v cung cp cc thit b in t nh Ericsson, IBM, Nokia, Toshiba a cng ngh ny vo sn phm ca h hoc kt hp vi cng ngh khng dy khc (Bluetooth) to nn gii php y mnh tc tip th cho cc thit b bng thng siu rng m khng phi tn tin u t pht trin cc sn phm c cng ngh c. C th thy c rng, cng ngh bng thng siu rng ang c c s quan tm rt ln t nn cng nghip truyn dn. Tuy vy, vic trin khai v p dng n phi c xem xt k lng bi mt s vn sau y: phm vi ph sng rt hn ch, trong khong cch di 10m, tc rt n tng nhng khi vt qu khong

17

cch ny th tn hiu truyn n thit b gim st ng k. Chnh v vy, UWB c ng dng ch yu trong mng c nhn (Personal Arial Network), h thng gii tr gia nh (Home Multimedia). UWB c th thay th c cng ngh bluetooth hay hng ngoi nhng khng th thay th c Wifi v Wimax. ng thi, cng chnh v nhng mng truyn dn khng dy trc ra i v qu ph bin, cc cng ngh bng hp ny trng ph vi cng ngh bng thng siu rng nn vic thit k h thng v c bit l anten thu pht sng UWB phi m bo khi hot ng khng gy can nhiu ln tn s lm vic ca cc thit b khc. Cui cng m bo cht lng tn hiu n nh, do tn hiu UWB khng c iu ch m trc tip bc x qua anten, nn cn c nhng cng ngh khc cng c n nh lm vic ca h thng. Trong , MIMO l mt trong nhng k thut anten c kt hp vi cng ngh UWB to nn mt h thng hon chnh a mng truyn dn khng dy c nhn ln mt nh cao mi. Ta c th t tin u t, nghin cu pht trin v h thng anten ny do n c nhng ng lc pht trin mnh m.

1.2

ng lc pht trin- Cc ch tiu k thut, m hnh h thng c cc t chc trn th gii c

kt, ci tin a ra mt cch y cc tiu chun. Chng ta s cp n cc tiu chun ny trong nhng chng sau ca n. - Cc giao din, thit b v ang c h tr bi cc nh sn xut cng nghip in t. - Vic nghin cu su hn c th m ra nhiu hng pht trin cng nh thch thc mi, trong , kh nng kt hp cc cng ngh vi nhau c cc nh nghin cu quan tm nht. S kt hp gia MIMO v UWB l mt trong nhng hng i em li mt bc tin ang k. - Ch ph u t khng ln nhng hiu qu em li rt ng k. C th ly mt trong nhng hng pht trin ca cng ngh ny lm v d: h thng anten thit k

18

cho cc thit b di ng cm tay rt nh gn v c ch to bi nhng vt liu r tin nhng c nhiu s ci tin vt bc v mt tc so vi cc cng ngh khc nh bluetooth hay hng ngoi, vic thay th cho cc cng ngh c ny ch l vn thi gian .

1.3

Hng i mi trong tng laiNgy nay, khi nhu cu s dng nhng ng dng truyn thng khng dy tc

cao, bng thng rng khng ngng ra tng th c th khng nh rng trong tng lai nhng cng ngh hin nay s khng p ng. Gii php t ra l: khi cng ngh khng cn p ng c nhng nhu cu trn ca con ngi th cng ngh bng thng rng hn phi c hnh thnh. V cng ngh EWB s l hng i cho tng lai khi m di tn ca cc rng ca n (c th ln n hng chc GHz) hon ton bao trm di tn hot ng ca cc h thng khc nh: WLAN, WiMAX trong c c h thng UWB th nhiu ng dng khng dy tc cao u c th c p ng. Cng ngh ny s c ng dng vi cc thit b u cui a bng tn. Tuy nhin, vi tng ng dng c th khng gy nhiu gia cc h thng vi nhau khi chng cng hot ng th yu cu thit k anten trong cng ngh ny l phi loi b mt s bng tn khng mong mun ca h thng khc khi c ng dng trong cc h thng khc nhau.

1.4

Kt qu mong i n ny hng ti pht trin, nghin cu, thit k mt h thng anten hon

chnh ng dng cho cng ngh MIMO EWB. Khi ch tiu thit k s l: thit k anten MIMO hot ng tt di tn siu rng, ng thi phi gim tc ng tng h gia cc phn t ring l ca anten. Qu trnh thit k s gp nhiu tr ngi trc tin l vic la chn loi anten ph hp nht p ng c nhng yu cu nh gi thnh r, d ch to, bng thng rng, tnh tch hp cao cho cc thit b di ng. Sau l giai on p dng mt s phng php k thut iu chnh cc thng s anten: h s tn hao ngc, bc x ng hng, band rejectingv cui cng l ghp cc anten tr thnh mt h

19

thng hon chnh trong khi vn m bo cc thng s anten thay i trong iu kin cho php. Sau y chng ta s i su tm hiu chi tit hn v mt l thuyt ca cng ngh EWB v MIMO, t to nn hng thit k cho h thng anten.

20

Chng 2 Cng ngh bng thng siu rng v h thng a u vo a u ra

2.1

Cng ngh bng thng siu rng Ultra WidebandUltra Wideband hay cn c gi l cng ngh bng thng siu rng bao gm

nhiu tiu chun xy dng nn mt h thng giao tip khng dy hot ng bng tn siu rng khng cn ng k 3.1Ghz 10.6Ghz.

2.1.1 Lch s cng ngh bng thng siu rngNgay t nhng nm su mi ca th k trc, cng ngh bng thng siu rng c p dng cho cc Rada qun s d tm v nh v. Mt thi gian di sau , cng ngh ny vn c nghin cu nhng cc kt qu nghin cu khng c cng b v ch yu cc nghin cu ny p dng trong qun s. Nm 1994 d n nghin cu chnh thc u tin v cng ngh bng thng siu rng (UWB) c trin khai nhng cng khng c cng b. Thng 2 nm 2002, Hi ng truyn thng lin bang M (FCC Federal Communications Commission ) cp php cho cng ngh bng thng siu rng, chnh thc ra i UWB technique. Thng 11 nm 2002, IEEE (Intitute of Electrical and Electronics Engineers) a ra chun 802.15.3a v xut cc b chuyn i vt l cho 802.15.3 da trn cng ngh bng thng siu rng. Nhng nm u th k 21, tn ti c lp nhiu k thut x l tn hiu bng thng rng nh : tri ph chui trc tip (Direct sequency-DR), x l tn hiu a bng (multiband - MB), ghp knh phn chia theo tn s trc giao (OFDM), . 21

Thng 7 nm 2003, IEEE 802.15.3a xut hp nht 2 k thut : DS-UWB v MB OFDM, nhm a ra mt k thut x l tn hiu duy nht cho chun 802.15.3 k tha cc u im ca cc k thut c. Thng 5, 2004 : IEEE a ra chun 802.15.4a cho cc ng dng tc thp tiu tn cc t nng lng cng da trn cng ngh bng thng cc rng. Thng 8 nm 2004 : FCC chp thun cho cc h thng s dng cng ngh bng thng siu rng ng c lp, khng ph thuc vo cc cng ngh khc cng s dng bng thng siu rng. T y, cng ngh bng thng siu rng tr thnh mt cng ngh mi ha hn nhiu tim nng cho cc ng dng khng dy v n cn pht trin theo nhiu hng khc nhau.

2.1.2 c im chnh ca cng ngh bng thng siu rngBng thng siu rng v cn bn khc so vi cc cng ngh khng dy bng hp v tri ph nh cng ngh Bluetooth v 802.11a/b/g/n. UWB s dng mt bng cc k rng ca ph tn s truyn d liu. Do , UWB c kh nng truyn ti nhiu d liu hn i vi mt chu k thi gian so vi cc cng ngh truyn thng. Tc truyn d liu qua ng truyn v tuyn l t l thun vi bng thng ca knh v ph thuc theo hm loga vi t s tn hiu trn nhiu (nh lun Shannon). Nhng rng bng thng ca knh v tuyn c n nh trc, cc nh thit k kh c th thay i thng s ny trong cc h thng v tuyn v cc ng dng. Cng ngh Bluetooth, Wi-Fi, in thoi khng dy, mt s thit b khc s dng cc bng tn khng ng k 900MHz, 2,4GHz v 5,1 GHz. Mi knh v tuyn bt buc chim gi ch mt di tn hp so vi cng ngh UWB. UWB l cch s dng ph tn s mi duy nht. SS spread Spectrum : tri ph NB Narrowband : Bng hp UWB Ultra WideBand : bng thng cc rng

22

Hnh 2. 1 Tng quan cng ngh bng thng siu rng

Di tn s s dng rng 7,5 GHz, t 3,1GHz n 10,6GHz. Mi knh chim rng bng tn hn 500 MHz, rng bng tn ca knh ph thuc vo tn s trung tm. cho php bng tn tn hiu rng, FCC yu cu rt kht khe cho vic hn ch cng sut pht. Bi th, cc thit b cng ngh bng thng siu rng c th lm vic di tn cc rng trong khi nng lng pht ra khng ln nh cc thit b s dng cng ngh bng hp gn nh 802.11a/b/g/n. y l s chia s ph tn s cho php thit b t c tc truyn d liu rt cao nhng chng phi t gn nhau. S gii hn nghim ngt v cng sut c ngha l bn thn chng phi c cng sut tiu th thp. V i hi cng sut thp, l li th trin khai cng ngh UWB vi chi ph thp. Vi cc c tnh cng sut thp, gi r, tc d liu cao khong cch gii hn, cng ngh UWB c th trin khai cho cc h thng tc cao WPAN. Cng ngh bng thng siu rng cho php s dng li tn s. Cc nhm thit b tng t nhau c th s dung chung knh v tuyn. Do hn ch v khong cch, cc v tr khc nhau c th dng chung tn bng tn. UWB vt tri so vi cc cng ngh khc chnh nh s u vit trong x l tn hiu. UWB s dng k thut iu ch xung nhiu bng v k thut iu ch a sng mang nhiu bng . Tn hiu xung trong UWB c chu k nh, c ns. UWB chn ln cc u im ca cc k thut x l tn hiu bng rng trc n nh : MB-OFDM, DS, to thnh cc k thut MB_UWB, TH-UWB, MB-OFDM,

23

Hnh 2. 2 Mt n ph cng sut cho UWB trong nh

Hnh 2. 3 Mt n ph cng sut cho UWB ngoi tri

Hnh 2. 4 Phn chia di tn trong UWB

24

2.1.3 So snh cc cng ngh truyn thng wireless c

Hnh 2. 5 So snh v khong cch tng tc v tc truyn d liu

Cng ngh bng thng siu rng l mt cng ngh khng dy mi, c nhiu u im so vi cc cng ngh khng dy ang tn ti. Trong phn ny, ta ch a ra so snh nhng thng s c bn nht ca cc cng ngh khng dy : khong cch tng tc, nng lng bc x ng hng, tc truyn d liu. So snh v khong cch tng tc v tc truyn d liu : so snh vi cc cng ngh khng dy cng khong cch tng tc, cng ngh bng thng siu rng vt tri v v tc truyn d liu. Nh Blucetooth l cng ngh khng dy bng hp c khong cch tng tc ngn, hot ng di tn 2.4GHz n 2.483 GHz, nhng tc truyn d liu ch t 1Mb/s l ln nht. V th cng ngh UWB ra i s l th h tip theo ca Bluetooth. Mt s cng ngh khc ang tn ti trong cc chun cng ngh cao 802.11a,b,g c khong cch tng tc ln nhng li hn ch v tc truyn d liu. Bng 2.1 biu din cc tham s ca cng ngh UWB tng ng vi cc chun truyn thng quc t khc vi chun truyn thng UWB.

25

Hnh 2. 6 So snh v rng bng tn v mt ph cng sut

BNG 2. 1 BNG SO SNH THNG S IEEE 802.11b IEEE 802.11g+

Thng s

BlueTooth

IEEE 802.11a

UWB

Tc truyn (max) Khong cch

1 Mb/s

11Mb/s

54Mb/s

24Mb/s

480Mb/s

10m

100m

100m

50m

3m 3.110.6GHz

Bng tn

2.4GHz

2.4GHz

2.4GHz

5.5GHz

rng bng tn

1MHz

25MHz

25MHz

20MHz

500MHz

COFDM iu ch GFSK QPSK OFDM QPSK, 16 QAM Cng sut pht (max)

10mW

100mW

1W

1W

< 0.56mW

26

Chi ph

~1

~4

~4

~6

~1

2.1.4 Cc ng dng ca cng ngh UWBDa vo c im ca cng ngh UWB ngui ta c th a ra nhiu ng dng cho h thng UWB. Tuy nhin, ni bt nht bao gm c hai loi ng dng c bn. 2.1.4.1 ng dng v radar

Nh ph tn rt rng, cc tn hiu UWB c th d dng xuyn qua cc vt cn khc nhau. Vi tnh nng ca radar mi c to ra bi cng thm UWB, th trng radar s pht trin t ngt v radar s c s dng trong nhng vng m hin ti khng th tng tng ni. Mt vi ng dng chnh ca radar UWB l c th c tc ng mnh ca cc cm bin t ng, cm bin trnh xung t, ti kh thng minh, sng kin thng minh trn ng cao tc, cm bin an ninh c nhn, o c chnh xc v kh nng nhn xuyn tng. Ngy nay, kh nng d chuyn ng xuyn tng l c thc. Cc thit b gi hng triu xung UWB trn mt giy, to ra mt tn hiu m trong bt c trng hp no u c th m xuyn qua hu ht cc vt th trong ta nh, bao gm tng b tng, khi b tng, thch cao, gch, g, nha, lt v si thy tinh. Kt qu l mang li rt nhiu chc nng d tm vi nhiu s hu ch mi. Thit b radar ny nhm ti mc tiu qun s v h tr cc i lm nhim v b mt. c tnh ny gip cho cc radar xuyn lng t da trn UWB l thit b rt hu ch cho cc i cu nn pht hin nhng ngi sng st b chn vi trong cc ng nt khi thm ha xy ra. Trong lnh vc thng mi, cc h thng radar nh vy c th c s dng trong xy dng nh v ng dn, inh vt v c dy in. Cng ngh tng t di nhng quy tc khc c th c s dng cho cc mc ch x l hnh nh trong y t nh h thng gim st tim t xa. Thm na, radar UWB c th c s dng trong cng nghip t ng ha cho h thng trnh va chm. Hn th na, vi cng sut truyn dn thp ca cc xung UWB n tr thnh s la chn l tng cho truyn thng trong qun s (cn phi che du). Cc xung UWB rt kh b pht hin

27

cng nh b bin dch v th i phng s khng th truy cp vo cc thng tin qun s ti quan trng. Mt khc, cc thit b UWB c mch b thu pht n gin hn b thu pht bng hp, chng c th c ch to vi kch thc nh v gi r hn ca cc h thng bng hp. 2.1.4.2 ng dng v truyn thng tc cao

Vi bng tn rng hot ng, cc ng dng s tip tc pht trin v chim ht bng tn vi nhiu nhu cu ny sinh. Cc ng dng u tin ca UWB s xoay quanh nhu cu ca th trng hin ti cho ng truyn d liu tc cao. Cc gii php UWB s p ng hon ton cc nhu cu v ng dng bi bng tn rng sn c ca UWB. c bit, cc ng dng a truyn thng phn tp cao, nh l ng dng streaming trong cc im bn hng tr tin ngay nh sn bay, trung tm mua sm hoc khu vc ng dn c, s yu cu bng tn m hin nay khng c kh nng nh l cc cng ngh bng hp sng lin tc. Vic ti xung cc b phim, clip l mt v d v ng dng tc d liu rt cao c th c thc hin bi UWB. Cc cng ngh truyn thng UWB khong cch ngn cng dn xa b cng ngh truyn dn bluetooth. Cc my tnh c th kt ni vi in thoi nh c ng truyn UWB thay v bluetooth, hng ngoi. Cc USB UWB cng hon ton p o v nhng c im vt tri hn so vi USB bluetooth. Cc ng dng UWB c th trnh by nh trn hnh v s 2.7. 2.1.4.3 UWB Trn thc t, c rt nhiu nhng nghin cu v thit k anten dng trong h thng thong tin bng thng siu rng. c nhiu loi anten c dng thit k cho h thng UWB, sau y l mt vi loi anten UWB c ng dng nhiu trong thc t(tham kho ti liu: UWB theory and application ca tc gi IanOppermann, Matti Hamalainen and Jari Iinatti):

Mt s anten c ng dng trn thc t thit kt anten

28

Hnh 2. 7 Cc ng dng point to multipoint ca UWB

Anten n cc v anten lng cc (Dipole and monopole) Anten khung (Loop antenna) Anten dy (Array antenna). Anten loa b mt gp (Folder horn antenna) Anten pht x xung (Impulse Radiating Antenna) Anten vi di (Microstrip antenna). Trong n ny, ti s dng anten vi di thit k h anten MIMO cho h thng bi v anten vi di c nhng c im sau: BNG 2. 2 CC C IM YU CU I VI ANTEN UWB CA THIT B DI NG rng bng tn Hiu sut bc x Pha th bc x H s nh hng 3.1Ghz 10.6Ghz Cao > 70% Gn tuyn tnh, tr nhm khng i ng hng Thp

29

H s tng ch rng bp sng na cng sut Hnh dng

Thp Rng > 60o Nh, gn, ng phng

Mt s u im c bn ca anten vi di so vi cc loi anten siu cao tn khc l: - Trng lng thp, kch thc nh, hnh dng phng mng nn c th ch to d dng v gn trn cc thit b di ng mng, nh. - Gi thnh sn xut thp, d dng tin hnh sn xut hng lot. - C th c lm mng, thch hp cho cc thit b di ng. - Cc anten vi di ny c th c trang b d dng trn tn la, v tinh m khng cn nhng s iu chnh nhiu. - Cc anten vi di ny c vng tn x nh. - Cc phn cc thng v phn cc trn c th t bng mt thay i n gin v tr feed. - Cc anten hai bng tn c th c ch to d dng bng cch iu chnh kch thc ming patch bc x. - Cc thit b nh b dao ng, b khuch i, b suy hao thay i, chuyn mch, b iu bin, b trn, b dch pha c th c thm vo trc tip trn b mt lp ca anten. - ng feed line v mng phi hp c sn xut ng thi vi cu trc ca anten. Bn cnh nhng u im nh trn th cc anten vi di cng c mt vi nhc im nh sau: - Bng thng hp.

30

- Suy hao, do lm cho s khuch i thp hn mt cht. - Tt c cc anten vi di u bc x trn mt na mt phng. - Cc gii hn thc t ca s khuch i ln nht. - Hiu xut bc x endfire thp. - S cch ly gia cc phn t bc x v feed km. - Kh nng ca s kch thch ca cc sng b mt. - Cng sut thp

2.1.5 Kt lun chng ny, ta trnh by s lc v lch s pht trin, cc c im c bn, nhng ng dng ca cng ngh bng thng siu rng v so snh cng ngh UWB vi cc cng ngh khng dy hin ti. cui chng, nhng yu cu ca anten cho cng ngh UWB cng c nhc n. thit k mt anten vi di p ng yu cu cho cng ngh UWB, ta phi nm r v anten vi di v cc k thut thit k anten bng rng. Chng sau s gip ta hiu r hn v anten vi di, ng dng ca n, cc nguyn l v cc k thut m rng bng tn cho anten.

2.2

Khi nim v m hnh h thng MIMO

2.2.1 nh ngha v c imTrong thng tin v thuyn, MIMO l vic s dng nhiu anten c b pht v b thu nng cao cht lng truyn tin. y l mt trong nhng dng ca cng ngh anten thng minh. H thng MIMO l mt h thng m my pht v my thu u s dng nhiu anten. Anten my pht v anten my thu u cch nhau mt khong cch nht nh. Trong m hnh MIMO, ta gi s rng: - Cc symbol thi gian cho trc. 31 c truyn t anten t nhng khong

Hnh 2. 8 H thng MIMO

- My thu thu cc khe thi gian cc tn hiu c truyn

l s tng hp ca v thnh phn AWGN

l cc h s knh truyn gia anten pht th v anten thu th . Mi quan h u ra v u vo ca h thng vit nh sau: anten c th c

=

+

(2.1)

hay (2.2) vi gi l ma trn knh.

2.2.2 Nhng hn ch ca knh truyn khng dyVic truyn tn hiu RF gia hai anten s chu s tn tht nng lng trong khng gian. iu ny lm gim ng k n s hot ng ca ng truyn. S tn tht nng lng gia my pht v my thu l kt qu ca ba hin tng khc nhau:

32

-

S suy gim ph thuc vo khong cch gi l tn hao ng truyn hay tn hao khng gian t do. S hp th ca cc phn t kh quyn tri t. S suy gim do hin tng phaing gy ra. S hp th ca kh quyn l do cc electron, phn t hi nc khng ngng v phn t ca cc loi kh. Tn hao ng truyn l s suy gim v mt l thuyt. S tn hao ny xy ra di iu kin LOS t do v tn hao ny tng theo my pht v my thu.

-

-

Phaing l s suy gim bin thin gia max v min mt cch khng u n. Khi thit b u cui di chuyn qua mt khu vc no c nhiu chng ngi vt, c kch thc khc nhau, v d: i, ni, to nh, hm Nhng chng ngi vt ny s che ph hay ct hon ton tn hiu. Mc d kt qu ca hiu ng ny ph thuc vo kch c ca vt chn v khong cch n n. Do vy, cng ca tn hiu thu c bin thin mt cch tt yu. Loi phaing ny gi l shadow phaing. Cch khc phc l t cc trm BS cao v gn nhau th ta c th trnh c cc vt cn trong khi truyn dn. Ngoi ra cn c cc loi phading khc nh: Rayleigh phaing, multipath phaing, shortterm phaing. y cng l cc loi khc ca hin tng phaing gy ra s suy gim ca cng tn hiu thu c. Rayleigh phaing l kt qu ca vic thu vi tn hiu ca my thu. Cc tn hiu ny c phn x t nhiu vt v nhiu hng khc nhau trong mt khu vc. Do khong cch khc nhau nn cc tn hiu thu c khc nhau v pha nn chng c th lm tng thm hay lm trit tiu tn hiu tng hp. S di chuyn ca cc thit b u cui cng gy ra s bin thin khng th d on c ca pha, tn hiu theo thi gian lm cho s suy gim bin thin mnh. Rayleigh phaing thng c trong khu vc th. Hin tng phaing su thng xy ra cc vng tn s cao v khi cc thit b di chuyn nhanh. trnh hin tng phaing su th gi tr trung bnh ca cc tn hiu thu c phi cao hn vi dB so vi nhy my thu.

33

Nhiu xuyn Symbol (ISI): v di thng ca knh ni chung b hn ch v khi mt xung c truyn qua knh th n s gy ra s mo m tn hiu ang truyn trong min tn s. Tng t, l s tn sc ca xung theo thi gian v xung ca mi symbol s trn sang khong thi gian ca mi symol k tip. Loi nhiu ny c bit n l nhiu xuyn symbol (ISI). iu ny lm gia tng sc sut li my thu trong vic d tm symbol. R rng rng xung di thng hn ch c chn truyn dn nhm trnh s mo trong min tn s do knh truyn c gii thng hn ch. Tuy nhin, s ct xn di thng ca tn hiu c truyn li lm gim xung trong min thi gian. iu nay s gy ra s chng cho ca cc symbol. Nhiu ng knh (CCI): ngoi nhiu gy ra bi knh truyn, mt loi nhiu khc cng lm hn ch hiu qu hot ng ca h thng v cng sut ca h thng l nhiu ng knh (CCI). CCI tn ti trong bt k mt h thng ang truy nhp no. Trong TDMA, SDMA, FDMA tn s c ti s dng ngha l c nhiu ngi s dng cng chia s mt bng tn cng mt thi im v do vy nhng ngi s dng cng knh chc chn to ra CCI ln nhau. Do vy cn c s cn bng gia hiu sut ph v hiu qu hot ng ca h thng. Tp m: v d nh tp m nhit lm gii hn t s SNR.

2.2.3 u im ca h thng MIMOCng sut cao v vi bng thng t v him, s lng cc trm gc

b hn ch, nh c vic s dng nhiu anten pht v nhiu anten thu lm tng tc truyn d liu nh vo k thut phn knh khng gian trong khi khng cn m rng bng thng. Cht lng truyn dn tt hn hay ni cch khc t l li bit gim do s

dng nhiu anten bn thu gip chng c phaing. V vi vic s dng k thut dy anten c th gip gim nhiu. li phn knh khng gian: li cng sut thu c t vic s dng nhiu anten c hai pha ca knh truyn v tuyn m khng cn tng cng sut my pht hay m rng bng tn.

34

li phn tp: Nng cao tin cy knh truyn bng cch pht trng d liu trn nhng nhnh phaing c lp. Nh vy u ra gp ln u vo.

2.2.4 H thng a nten v nh hng tng h2.2.4.1 Gii thiu h thng a anten

H a anten(Multiantenna) l h m cc ngun c kt ni vi nhng phn t pht x c lp nhau, hoc cng chung mt phn t pht x nhng s dng cc thuc tnh vt l khc nhau ( khc nhau v tnh phn cc, khc nhau v th bc x), cn gi l h anten a cng ( Multiport antenna _ MPA).

Hnh 2. 9 M hnh MMA(a) v MEA (b)

Chng ta c th phn loi h a anten lm hai loi: loi s dng chung phn t pht x v loi s dng cc phn t pht x c lp. Loi s dng chung phn t bc x, c nhiu ngun tip in, s dng chung mt phn t bc x, tuy nhin, mi ngun vo s dng cc thuc tnh bc x khc nhau: nh l tnh phn cc khc nhau, tn s khc nhau hay ch khc nhau ( Multimode antenna, Multipolarized antenna). Loi th 2, s dng cc phn t bc x c lp nhau, nh hnh v. Mc ch l gim nh hng gia cc cng (port). nh hng tng h, thng thng l nhng nh hng khng mong mun. Trong h a anten s dng n phn t s c nh hng tng h gia cc cng (port) l rt ln, v t c s dng d c u im l nh gn hn. H a anten s 35

dng a phn t pht x vn tn ti nh hng tng h, v chng tn ti trong cng mt h thng nn khong cch gia cc phn t pht x nhn chung khng qu xa nhau. c bit, khi h anten nay c tch hp trn mt thit b di ng, th m kch thc l hn ch, th khong cch gia cc anten l rt nh so vi na bc sng, nh hng tng h tr ln nghim trng. Xt m hnh nh hng tng h gia cc anten trn cng mt ming in mi nh hnh v sau: Hnh v ch ra hai nguyn nhn c bn ca hin tng tng h: tn x trng khu gn v do sng b mt. Tn x trng khu gn l s bc x t anten nguyn t ny sang ci cn li qua trng khng gian t do. Do cc anten t trong khong cch ngn, nh hng tng h l ng k. Nguyn nhn th 2, nh hng tng h gy ra bi sng b mt. V cc anten nguyn t nm chung trn cng ming , sng b mt gia hai phn t l rt ln.

Hnh 2. 10 Cc nguyn nhn gy tng h.

nh hng tng h gy ra nhiu gia cc anten thnh phn. hiu r c ch gy nhiu, ta xt qu trnh sau. Ta c th chia lm 4 qu trnh nh nh sau: nh hng tng h gy ra nhiu gia cc anten thnh phn. hiu r c ch gy nhiu, ta xt qu trnh sau. Ta c th chia lm 4 qu trnh nh nh sau: 1. T mt phn t pht x ta pht tn hiu nh mong mun. 2. Do hiu ng dng in b mt, v do tn x trng khu gn, dng in ca phn t pht x mong mun s gy ra dng in cm ng (nh hng tng h) n cc phn t pht x khc(trc tip hay gin tip).

36

3. Dng in trn cc phn t cm ng gy suy hao. 4. Dng in cm ng trn cc phn t cm ng gy nh hng ngc tr li phn t pht x lm phn t pht x b sai khc. Dn trc tip s sai khc ca c tnh phng hng. nh hng tng h (Mutual coupling) mang tnh cht ni ti trong anten, khng ph thuc vo mi trng truyn dn (knh truyn) . Tuy nhin, nh hng ca n, ph thuc rt nhiu vo cc phn t lin kt vi anten

Hnh 2. 11 Cc qu trnh gy nhiu gia lin chn t

2.2.4.2

Cc nhn t nh hng n h anten MIMO Khong cch gia cc chn t:

Ni chung, khong cch gia cc chn t nh hng ln n nh hng tng h gia cc anten thnh phn. Nguyn nhn l do mt ph cng sut tn hiu t l vi hm m ca khong cch. Gia cc anten thu, ngi ta thng cho khong cch gia cc chn t l ln, qua hn ch ng k nh hng tng h. Khong cch gia anten trong cc trm pht sng c nh thng hng vi bc sng, th d 10 . i vi thit b di ng MS, khong cch thng thng xc nh khong cch gia cc chn t l xa hn mc /2. Chng ta hy quan st hnh v 2.12, biu th mi quan h gia khong cch gia cc anten thnh phn v hm tng quan tn hiu gia cc anten thnh phn.

37

Ti khong cch bng khng, th tng quan gia cc thnh phn anten l ln nht.

Hnh 2. 12 Hm tng quan gia cc anten thnh phn nh l hm ca khong cch theo bc sng

Bi v khong cch bng 0 ng vi s tip xc gia cc anten, cc anten c cng tn hiu. Tn hiu trn anten 1 v anten 2 l hon ton ging nhau, ng ngha vi tng quan bng 1. Cng trn hnh v biu th, tng quan nh nht ng vi khong cch nh hn na bc sng, 0.383 ln bc sng. S ph thuc tng quan tn hiu trn anten vo khong cch cn ph thuc vo cu hnh anten. Tuy nhin, nhn chung, khong cch cng xa s cho tng quan cng thp. - M hnh kin trc dy cc anten thnh phn: Dy anten nn l dy c chiu di trong mt phng nm ngang v khi c th khai thc c cc tia a ng. Nhng thnh phn a ng ny rt khc trong mt phng phng v Aziminth AOA. Dy hai chiu (trong mt mt phng thng ng) cng c th gii quyt thnh phn a ng khc nhau theo chiu cao. Tuy nhin trong nhiu mi trng c bit nh outdoor th s tri ca AOA theo chiu cao l nh. Do vy khong cch gia cc phn t anten theo chiu ng phi ln to ra cc tn hiu khng tng quan. iu ny c ngha l i vi mt khu vc cho trc th tt hn ht l phn b chng theo mt phng ngang. Trong vi trng hp khi m s tri AOA trong mt phng phng v l nh v hng tng i ca n so vi dy l khng bit th c th s dng dy trong mt phng ngang. - Hng ca cc anten thnh phn : 38

Hng ca cc anten thnh phn nh hng n mt ph cng sut ti v tr cc anten ln cn. Do tt c cc anten vn d chng bao gi ng hng l tng c. - Tnh cht phn cc ca anten thnh phn: Tnh cht phn cc quyt nh n s cm ng in t trn anten. Cc anten c ghp ni ng tnh cht phn cc s c tng quan tn hiu l nh nht.

2.2.5 K thut phn tp anten2.2.5.1 Gii thiu v k thut phn tp anten

K thut phn tp l mt trong nhng phng php dng hn ch nh hng ca phaing. tng cho vic phn tp l to ra cch knh c lp vi nhau v phaing cc knh khng xy ra ng thi. Trong h thng thng tin di ng, k thut phn tp c s dng hn ch nh hng ca phaing a tia, tng tin cy ca vic truyn tin m khng phi tng cng sut hay bng thng. Phn tp c th p dng cho c bn pht v bn thu. Phn tp bn pht l mt k thut lin quan n m khng gian-thi gian cn phn tp bn thu cho php thu c nhiu bn sao ca cng mt tn hiu truyn. Cc bn sao ny cha cng mt lng thng tin nh nhau nhng c t s tng quan v phaing. Tn hiu thu c bao gm mt s kt hp hp l ca cc phin bn tn hiu khc nhau s chu nh hng phaing t nghim trng hn so vi tng phin bn ring l. Cc phng php phn tp thng gp l phn tp tn s, phn tp thi gian, phn tp khng gian (phn tp anten). Trong k thut phn tp anten hin ang rt c quan tm v ng dng vo h thng MIMO nh kh nng khai thc hiu qu thnh phn khng gian trong nng cao cht lng v dung lng h thng, gim nh hng ca phaing, ng thi trnh lng ph bng thng tn s - mt yu t rt c quan tm trong hon cnh ti nguyn tn s ngy cng khan him. 2.2.5.2 Cc phng php phn tp

Phn tp tn s:

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Mt phng thc to ra s c lp knh truyn l to ra nhiu sng mang tn s khc nhau khi truyn t c mc phn tp ln th cc sng mang phi phn bit nhau m cc knh khng tng quan. Phn tp thi gian : Tc l pht d liu cc khong thi gian khc nhau. Nu cc khong thi gian di hn thi gian gn kt ca knh truyn th sc xut m cc tn hiu c pht i m cng b phading su l rt thp. Cng ging nh a dng v tn s s rt tt nu s dng m m c kh nng sa li khi c hin tng phading su. Phn tp khng gian: L mt k thut ph bin . V phng thc ny cc my thu s t cc i im khc nhau. iu ny dn n c cc ng truyn khc nhau n my thu. Chng ta c th s dng nhiu anten pht t c s phn tp php. C th hiu rng s tng quan gia cc knh gim khi khong cch gia cc anten tng. Khong cch yu cu c c cc knh m khng tng quan vi nhau c quy nh bi bc song ca song mang. S tng quan ny nh mt hm ca khong cch v bc song ca song mang. Cn c vo mc 2.2.4.2 (: cc nhn t nh hng trong h a anten) th khong cch anten trong khong 0.383 ln bc sng s cho tn hiu tng quan l nh nht. Phn tp gc: Mt tn hiu phaing nhiu ng bao gm rt nhiu thnh phn tn hiu b tn x, n anten thng qua nhiu ng. Nhng thnh phn ny n t cc gc khc nhau v theo cc ng truyn khc nhau. Do vy cc thnh phn tn hiu n t cc hng khc nhau s l c lp. Mt anten m c tnh n hng n DOA c th t c phn tp t nhng tn hiu khng tng quan ny. Phng php ny c p dng tn s snmang khong 10 GHz. Ti tn s nh vy th tn hiu c truyn b tn x cao trong khng gian. Khi my thu s dng hai anten nh hng cao c trong cc hng khc nhau. iu ny cho php my thu thu c hai mu ca cng mt tn hiu . Hai mu ny c lp vi nhau. Phn tp phn cc : 40

Khi hai tn hiu c phn cc ngang v phn cc ng c truyn bi hai anten c phn cc khc nhau. V c thu tng ng hai anten thu c phn cc khc nhau m bo khng c s tng quan gia hai dng d liu m chng quan tm n khong cch gia hai anten. Khi thit k anten MIMO th ta thng s dng kt hp phn tp khng gian vi phn tp gc, phn tp phn cc. V h thng MIMO pht trn cng mt di tn nn khng s dng phn tp tn s.Tuy nhin ta cng khng th s dng tt c cc k thut phn tp cho anten.

2.2.6 Kt lunH thng a anten l mt h thng ngy cng c s dng ph thng. Nh v h thng a anten, m ngi ta c th tng dung lng thu pht tn hiu. Tuy nhin, trong h thng MIMO, cc anten c th b nh hng tng h rt ln, do khong cch gia cc anten khng ln hn kch thc h thng. c bit trong xu hng s dng cc thit b di ng kch thc b hin nay, nh hng tng h cng tr nn trm trng, gy nhiu ln. Trong chng 3, chng ta tm hiu h thng anten trong h thng MIMO, phn tch qu trnh gy nhiu ng thi a ra phng php phn tp anten gim nh hng tng h. Tip theo, chng ta s tm hiu c bn l thuyt phng php FEM v phn mm HFSS s dng phng php ny.

41

Chng 3 Thit k anten vi phn mm HFSS

3.1

L thuyt c bn trng in t

3.1.1 Phng trnh Maxwell v cc iu kin binTrong khng gian t do h phng trnh Maxwell v cc phng trnh lin quan [1] c biu din nh sau: (3.1)

(3.2) (3.3) (3.4) (3.5) (3.6) Trong cc phng trnh ny: l vc-t in trng l vc-t t trng l vc-t mt thng lng t l mt thng lng in l mt dng in dn l mt in tch 42

l h s in mi trong khng gian t do l h s t thm trong khng gian t do i vi cc vt liu dn in, nh lut bo ton in tch c biu din bi quan h:

(3.7) Mt dng v cng in trng lin h vi nhau bi nh lut Ohm: (3.8) l h s ph thuc tnh dn in ca mi trng Nu vt dn chuyn ng vi vn tc cng phi bao gm thm thnh phn trong t trng th in trng tng

c sinh ra do hiu ng chuyn ng: (3.9)

3.1.2 Cc phng trnh thTrng ph thuc thi gian bin i nhanh Khi trng ph thuc vo thi gian bin i nhanh th in trng v t trng nh hng tng h ln nhau. Trng phn b ph thuc c vo thi gian v v tr, (r,t), (r,t) . T trng thay i theo thi gian sinh ra in trng xoy v in trng thay i theo thi gian sinh ra t trng xoy. Nh vy in trng v t trng sinh ra l cc i lng ng. Trong mi trng khng suy hao v min ngun khng gian t do rt d dng nhn thy rng (3.2) ta c: v tho mn phng trnh sng. i vi , t phng trnh

(3.10)

43

y l phng trnh sng ca . Tng t, ta thu c phng trnh sng ca t phng trnh (3.1):

(3.11) Khi gii phng trnh Maxwell thun tin ta nh ngha hm v hng hm vector th tho mn:

v

(3.12)

(3.13) Thay (3.12) v (3.13) vo (3.1) v (3.2) ta c:

(3.14)

(3.15)

Do

v

l cc hm tu nn ta c th chn chng sao cho:

(3.16)

Trong mi trng suy hao th phng trnh sng s dng dng sau:

(3.17) Cui cng phng trnh sng thu c c dng:

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(3.18)

(3.19) Phng trnh (3.18) v (3.19) c dng tnh ton sng bc x ca anten, trng tn x ca vt liu v s truyn sng trong ng dn sng hay cc thit b in t khc. Trng cn bng Khi bi ton c xt trong iu kin trng bin i theo thi gian rt chm th trng thi cn bng xp x c s dng.Tiu chun c gi l chm nu n tho mn iu kin sau: (3.20) l tn s gc ca tn hiu hnh sin. Tiu chun ny c ngha rng dng dn chim u th v dng dch c th c b qua. Do , t trng xoy sinh ra bi in trng khng tn ti. Khng c mi lin h gia s thay i v tr v bin i theo thi gian ca trng. V vy khng c s truyn sng. Thng thng, trong cc bi ton trng cn bng i lng (r,t) v (r,t), (r,t),

l hm iu ho theo thi gian. Do trng phn b ch ph thuc

vo v tr v s tr pha ti tng v tr trong khng gian. Trong trng hp ny cc phng trnh Maxwell c rt gn thnh: (3.21) (3.22) (3.23)

45

(3.24) Khi l hng s th v tun theo phng trnh truyn parabol:

(3.25)

Trong trng hp nh vy thun tin ta gi thit s tn ti ca vc-t t th v vc-t in th . Vic xc nh Maxwell v : v xut pht trc tip t h phng trnh

(3.26)

Theo nh lut Amper th mi quan h gia cng t trng in th l:

v vc-t

(3.27) Trong l th t v hng. Phng trnh (3.27) c suy ra t phng .

trnh (3.21), (3.26) v phng trnh

Phng trnh vi phn ca 2 vc-t th c th thu c bng cch thay phng trnh (3.12), (3.13) v (3.27) vo h phng trnh Maxwell, sau mt s bin i n gin ta c hai phng trnh sau: (3.28)

(3.29)

Trong l mt dng mt.

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ng dng quan trng nht ca trng thi xp x cn bng l xc nh s phn b ca dng xoy trong vng dn v trong li kim loi. Tu thuc vo hng s vt liu, s xp x c th c gi tr n khong tn s ca tia X. Trng tnh v gn tnh: Cc i lng trng tnh l c lp vi thi gian, v d v

trng phn b ch l hm ca v tr. Nu tn s nh th in trng xoy sinh ra bi t trng ca dng dch l rt nh. Trng phn b trong trng hp ny thc t gi l phn b tnh hay gn nh l tnh. Tiu chun ca trng gn tnh l trong l bc sng, L l kch thc vng trng.

Trong trng hp trng tnh v gn tnh h phng trnh Maxwell c rt gn thnh:

(3.30)

(3.31) Da vo phng trnh v vc-t t th c biu din di dng: , c in th, t th v hng

(3.32)

T phng trnh (3.30), (3.31), v (3.32) Ta thu c h phng trnh Poisson v Laplace:

(3.33)

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Thng thng trong trng tnh v gn tnh th tiu chun Coulomb c tho mn nh sau: (3.34)

3.1.3 Cc iu kin binTi b mt ca cc vt liu khc nhau dng tch phn ca h phng trnh Maxwell c rt gn li thnh:

(3.35)

Trong n l vc-t php tuyn n v ca b mt trong hnh 4.1, v l ca trng 2 pha ca b mt, ng thi K

v l mt ca dng mt v th mt.

Hnh 3. 1 iu kin bin ca E v B

Nu vc-t th in v hng c coi nh l mt bin th iu kin bin gia 2 mt l:

48

(3.36)

Do s dch chuyn i xng i vi t trng, iu kin bin b mt l:

(3.37)

Nu cc bi ton t trng c xt n trong khng gian 3 chiu th vc-t t th c phn tch lm 3 thnh phn: (3.38) Trong l 3 thnh phn ca , v l 2 vc-t n v trc giao tho mn: (3.39) Tnh lin tc ca thnh phn tip tuyn ca cng t trng din bng biu thc sau: c biu

vi vc-t chun hng . Vi tiu chun Coulomb thnh phn chun

(3.40) Phng trnh trn c th c phn tch thnh 2 phng trnh:

(3.41)

(3.42) Phng trnh (3.41) v (3.42) ch ra rng cc iu kin bin cho t trng 3 chiu l phc tp hn so vi trng v hng. Do s la chn m hnh ton hc

49

xp x i vi bin cha bit v tiu chun bin l phng php chnh gii bi ton trng in t trong khng gian 3 chiu.

3.2

Phng php phn t hu hn (FEM)

3.2.1 Khi nimPhng php phn t hu hn (Finite Element Method - FEM) l mt phng php gn ng gii mt s lp bi ton bin. Theo phng php phn t hu hn, trong c hc, vt th c chia thnh nhng phn t nh c kch thc hu hn, lin kt vi nhau ti mt s hu hn cc im trn bin (gi l cc im nt). Cc i lng cn tm nt s l n s ca bi ton (gi l cc n s nt). Trong mi phn t, i lng cn tm c xp x bng nhng biu thc n gin v c th biu din hon ton qua cc n s nt. Tp hp cc phn t theo iu kin lin tc s nhn c h phng trnh i s i vi cc n s nt ca ton vt th.

3.2.2 Ni dungPhng php phn t hu hn c ni dung nh sau: gii mt bi ton bin trong min W, bng php tam gic phn, ta chia thnh mt s hu hn cc min con Wj (j = 1,..., n) sao cho hai min con bt k khng giao nhau v ch c th chung nhau nh hoc cc cnh. Mi min con Wj c gi l mt phn t hu hn (phn t hu hn). Ngi ta tm nghim xp x ca bi ton bin ban u trong mt khng gian hu hn chiu cc hm s tho mn iu kin kh vi nht nh trn ton min W v hn ch ca chng trn tng phn t hu hn Wj l cc a thc. C th chn c s ca khng gian ny gm cc hm s 1(x),..., n(x) c gi tr trong mt s hu hn phn t hu hn Wj gn nhau. Nghim xp x ca bi ton ban u c tm di dng: c11(x) + ... + cnn(x) (4.43)

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Trong cc ck l cc s cn tm. Thng thng ngi ta a vic tm cc ck v vic gii mt phng trnh i s vi ma trn tha (ch c cc phn t trn ng cho chnh v trn mt s ng song song st vi ng cho chnh l khc khng) nn d gii. C th ly cnh ca cc phn t hu hn l ng thng hoc ng cong xp x cc min c dng hnh hc phc tp. phng php phn t hu hn c th dng gii gn ng cc bi ton bin tuyn tnh, phi tuyn v cc bt phng trnh.

3.2.3 ng dngVi s h tr ca my tnh in t, phng php phn t hu hn ang c s dng rng ri v c hiu qu trong nhiu lnh vc nh tnh ton trng in t, l thuyt n hi v do, c hc cht lng, c hc vt rn, c hc thin th, kh tng thu vn, v.v

3.3

Phn mm m phng Ansoft HFSS

3.3.1 Gii thiuAnsoft HFSS (Ansoft High Frequency Structure Simulator) l mt phn mm m phng dng gii trng in t da trn phng php phn t hu hn (FEM) ton sng cho cu trc ba chiu bt k. HFSS s dng phng php phn t hu hn ton sng ba chiu tnh ton cc c trng in hc ca cc linh kin tn s cao v tc cao. Vi HFSS, cc k s c th tch cc tham s k sinh (S, Y, Z), hnh dung trng in t ba chiu (trng khu gn v trng khu xa), to ra cc mu chng trnh m phng chuyn dng cho mch in (SPICE - Simulation Program with Integrated Circuit Emphasis), v thc hin thit k ti u. HFSS m t chnh xc hot ng in ca cc linh kin v nh gi hiu qu cht lng tn hiu, bao gm tn hao ng truyn, tn hao phn x do khng phi hp tr khng, i ngu k sinh, v pht x. HFSS c th m phng cc trng in t, dng in v pht x trong mt cu trc ba chiu bt k bao gm kim loi, in

51

mi, vt liu t v.v da trn phng php phn t hu hn ba chiu. HFSS c s dng rng ri trong cng nghip cho tn s v tuyn RF, anten, v thit k mch. HFSS l phn mm chun cng nghip cho vic tch tham s S v chng trnh m phng chuyn dng cho mch in ton sng (full wave SPICE) v cho m phng in t ca cc linh kin tn s cao vi tc cao. HFSS c s dng rng ri cho vic thit k ca cc phn t th ng nhng trn chip, cc u ni mch in, anten, cc linh kin RF/vi ba, v cc gi IC tn s cao. HFSS pht trin cc sn phm khoa hc, gim thi gian pht trin v khng nh r hn thnh cng ca thit k. Phin bn mi nht ca HFSS a ra nhng pht trin sn phm ti cc k s RF/vi ba v m rng vic phi hp thit k in t ti cc nhnh khc ca k s lm vic trong cc khu vc thit k IC RF/analog v cc thit k multi-gigabit cng nh EMI/EMC. HFSS c dng m phng cc u ni, cc ng dn sng, cc linh kin trn chip, cc anten, v.v v dng cho vic kho st cc tham s, ti u cu trc, v.v HFSS l mt phn ca h sn phm ca Ansoft.

3.3.2 M phnga. Phng php phn t hu hn HFSS c da trn phng php phn t hu hn ba chiu. l mt phng php min tn s (cc li gii c tnh cho tng tn s ring bit). Phng php phn t hu hn chia c khng gian thnh cc phn t hu hn (tam gic hoc t din) v m t trng trong mi min con bng mt hm cc b. Vi th tch tng cng ca min con, trng tn x c th c m rng thnh mt lot cc hm

c bn bit vi cc h s m rng cha bit. Gi tr ca s lng trng vc-t cc im trong mi t din c ni suy s hc t cc gi tr cc nh v trung im cc cnh. B m phng bin i cc phng trnh Maxwell thnh cc phng trnh ma trn, c th c gii s dng cc phng php s hc. Thi gian m

52

phng t l vi

(khi c gii theo phng php lp) hay

(nu ma trn h

thng c o trc tip).

(a)

(b)

Hnh 3. 2 Cch chia phn t hu hn trong HFSS: (a) thnh cc tam gic trn b mt, (b) thnh cc t din trong khng gian ba chiu

b. Tnh tonQu trnh phn tch thch ng

Mt phn tch thch ng l mt qu trnh gii trong li c lp tinh chnh trong nhng vng c li cao, lm tng chnh xc ca li gii. Ngi s dng t cc tiu chun iu khin cho vic tinh chnh li trong li gii trng thch ng. Nhiu vn c th c gii quyt ch bng vic s dng tinh chnh thch ng. 1. HFSS to ra mt li khi to. 2. S dng li khi to, HFSS tnh ton cc trng in t tn ti bn trong cu trc khi n c kch thch tn s nghim. (Nu bn ang chy mt tn s qut, mt nghim thch nghi c thc hin ch tn s nghim c th) 3. c da trn li gii phn t hu hn hin ti, HFSS xc nh cc vng cha vn m vic tnh nghim gp li. T din trong nhng vng ny c tinh chnh. 4. HFSS to ra mt li gii khc s dng li tinh chnh.

53

5. HFSS tnh li li, v qu trnh lp ny (gii phn tch li tinh chnh) lp li cho n khi cc tiu chun hi t c tha mn hoc hon tt s ln thng qua thch ng. 6. Nu tn s qut c thc hin, HFSS s gii vn cc im tn s khc m khng cn tinh chnh thm na Cc b qut tn s Thc hin mt b qut tn s khi ngi s dng mun to ra mt li gii qua mt di cc tn s. Ta c th la chn mt trong nhng loi b qut sau: 1. Nhanh: To ra mt li gii trng y duy nht cho mi php chia trong mt khong tn s. Tt nht cho cc mu t ngt cng hng hay thay i hot ng trong bng tn s. Mt b qut nhanh s cha mt s m t chnh xc cc c im gn cng hng. 2. Ri rc: To ra cc li gii trng cc im tn s c th trong mt di tn. Tt nht khi ch c vi im tn s cn thit phi m t chnh xc cc kt qu trong mt di tn. 3. Ni suy: nh gi mt li gii cho ton b di tn. Tt nht khi di tn rng v p ng tn s l bng phng, hay nu s yu cu b nh ca b qut nhanh vt qu ti nguyn ca bn. Kch thch Thit lp cc kch thch cho mt thit k HFSS cho php bn c th ha cc ngun trng in t v cc in tch, cc dng in hay cc in p trn vt hay trn b mt: Wave port; Lumped port; Sng ti; Ngun in p; Ngun dng in; Ngun phn cc t. Cc ng bin Cc iu kin bin c th ha c tnh trng trn cc b mt ca vng bi ton v cc giao din vt th. Khu vc ny ca nhng ch k thut bao gm thng tin v cc loi ng bin: Perfect E; Perfect H; Tr khng; Pht x; PML; Cht dn in hu hn; i xng; Ch - t; Lumped RLC; Tr khng phn lp; Cc mt phng t v hn. 54

Cc vt liu Cc tnh cht ca vt liu tuyn tnh: H s t thm tng i H s in mi tng i in dn khi Tn hao in mi tip tuyn Tn hao t tip tuyn

Cc tnh cht ca vt liu fert:

-

ng bo ha t H s Lande G Delta H

Thng tin cng bao gm nhng iu sau: Cc vt liu khng ng hng Cc tnh cht ca vt liu ph thuc tn s

Loi li gii 1. Li gii Driven Modal La chn loi li gii Driven Modal khi bn mun HFSS tnh cc h s S da trn cch thc ca linh kin th ng, cc cu trc tn s cao nh vi di, cc ng dn sng v cc ng truyn dn. Cc li gii ma trn S s c biu din thnh cch s hng ca cc nng lng ti v phn x ca cc mt ng dn sng. 2. Li gii Driven Terminal La chn loi li gii Driven Terminal khi bn mun HFSS tnh ton cc tham s S da trn u cui ca cc cng truyn dn a cht dn. Cc li gii ma trn S s c biu din thnh cc s hng ca cc in p u cui v cc dng in u cui. 3. Li gii mt ring La chn loi li gii mt ring tnh cc mt ring, hay cc cng hng ca mt cu trc. B gii mt ring tm cc tn s cng hng ca cu trc v cc trng cc tn s cng hng. 55

c. S khi thc hin m phng

Hnh 3. 3 M hnh m phng

56

Chng 4Thit k anten vi di bng thng siu rng p dng k thut MIMO

T cc nguyn l c bn v cc k thut to bng thng siu rng cho anten vi di, s dng phng php FEM tnh ton v dng phn mm Ansoft HSFF m phng. Trong chng ny ti xin a ra tng bc thit k v kt qu ca qu trnh tnh ton v m phng. Xuyn sut qu trnh thit k, hai thng s rt quan trng, c ngha nht i vi anten vi di bng thng siu rng l h s tn hao ngc (Returrn Loss S11) v th bc x phng hng (Radiation Partern). H s tn hao ngc phi nh hn hoc bng mc -10dB v anten bc x ng hng. i vi h s tng ch G, anten bng thng siu rng khng yu cu qu cao do khong cch truyn pht tn hiu ngn, thng thng l nh hn 4 dBi. Sau khi hon thnh anten EWB n l n giai on ghp cc anten thnh h thng MIMO EWB. Trong qu trnh ny, phi c bit quan tm n h s truyn t gia 2 anten (S12&S21) m bo h s tn hao ngc v th bc x phng hng thay i trong phm vi cho php. Sau y ta s i su vo qu trnh thit k h thng v a ra mt s phn tch nh hng ca cc thng s.

4.1

Qu trnh thit k v phn tchTon b qu trnh thit k v phn tch c thc hin theo nhng bc sau

y: Xem xt cc yu cu k thut ca anten la chn anten thch hp

57

M hnh anten vi di c a ra c th c tch hp cho cc thit b di ng s dng cng ngh bng thng siu rng. M hnh anten vi di thit k l mt anten monopole ng phng bc x ng hng vi kch thc nh, cu trc n gin v c sn xut bng cng ngh mch in nn gi thnh r. Vi cu trc phng, anten c th c dng trong cc thit b ca h thng my tnh c nhn hoc cc thit b cm tay Chn nguyn l to anten bng rng thch hp thit k V mt l thuyt, c 3 nguyn l c bn to bng rng cho anten: nguyn l bin i t t, nguyn l tng t, nguyn l t b. Trn thc t, cc nh thit k c th s dng rt nhiu k thut khc nhau to bng rng cho anten. Trong m hnh thit k anten vi di EWB ny, nguyn l bin i t t c chn s dng to bng rng. Bng cch ch to phin kim loi bao gm cc ng gy khc s to nn cu trc bin i t t t ng tip in ng dn sng ng phng ra ming bc x ca anten. -Nguyn l bin i t t: Anten c xem nh l mt thit b chuyn tip gia h thng fide tip in v mi trng truyn sng (khng gian t do), bin i sng in t rng buc trn fide thnh sng t do trong khng gian. V vy gim nh s ph thuc ca tr khng vo anten vi tn s, bn thn anten cn c dng kt cu chuyn tip ( y kt cu chuyn tip l cc ng gy khc m u ra t ng dn sng ng phng 1 gc bng 60o), ngha l kch thc ca n cn c bin i mt cch t t. Khi tr khng sng ca on chuyn tip s khng i v s khng xut hin sng phn x. Sng in t truyn trong fide t dng sng phng c chuyn dn thnh sng cu, trn ra khng gian bn ngoi thnh sng bc x. Chn phng php tip in thch hp Trong qu trnh thit k anten, la chn phng php tip in cho anten l mt bc quan trng. i vi anten vi di, c 3 cch tip in thng dng l: tip in bng

58

Hnh 4. 1 Kt cu bin i t t

cp ng trc, tip in bng ng truyn vi di, tip in bng ng dn sng ng phng. La chn phng php tip in ph hp c th d dng phi hp tr khng gia feeder v anten. Ngoi ra, cch tip in cho anten cng gp phn ng k quyt nh n kch thc v thuc tnh ca anten. Trong m hnh anten c thit k, cu trc tip in bng ng dn sng ng phng c s dng. ng dn sng ng phng (CPW) c cu trc mng, gp phn ng k thu nh kch thc anten. Cc thit k anten gn y cn cho thy vic s dng ng dn sng ng phng c th cho php m rng bng thng. -Phn tch v tnh ton tr khng c tnh ca CPW: Phn tch: ng dn sng ng phng l mt loi ng truyn vi di m ng dn kim loi v mt phng t (GND) cng nm trn mt mt ca lp in mi. s c hai mt phng GND chy song song vi ng dn kim loi gia v khong cch t ng dn n hai mt phng GND l bng nhau. (Hnh 4.2) Hnh 4.3 th hin thy phn b trng in t ca ng dn sng ng phng. i vi CPW th trng in t phn b tp trung khng gian gia ng vi di v hai mt phng GND. Ty thuc vo kch thc ca cc phn t cu to nn CPW m mode sng c truyn i ti mt tn s no l mode chn (Even mode) hay mode l (Odd mode).

59

b a

i vi thit k dng trong m hnh anten monopole th tip in cho anten hot ng chnh xc th cc kch thc phn t to nn CPW phi cho ra trng mode chn.

Tnh ton tr khng c tnh ca CPW: Cc thnh phn quyt nh n tr khng ca CPW cng nh mode sng truyn trong CPW l: b dy lp h, b dy lp kim loi t, b rng ng dn din gia a, khong cch gia hai im gn nht ca hai mt phng GND b, hng s in mi r. Mt phng GND nn ln hn 5b, v ngn nhng mode sng c bc cao hn (mode sng khng mong mun) c truyn th b nn nh hn /2. Khi , cng thc tnh tr khng c tnh ca ng truyn CPW nh sau:

t hr

Hnh 4. 2 Cu to ca ng dn sng ng phng

Mode chn

Mode l

Hnh 4. 3 Mode sng truyn trong CPW

(4.1)

(4.2)

60

Vi : (4.3) (4.4) (4.5) , (4.6) (4.7) (4.8) Tnh ton thit k v s dng phn mm m phng T cc nguyn l c bn v cc k thut to bng thng siu rng cho anten vi di, gii php a ra l kt hp s dng phng php phn t hu hn FEM (Finite Element Method) tnh ton thit k anten v s dng phn mm Ansoft HSFF m phng kt qu.

, ,

4.1.1 Thit k anten n4.1.1.1 M hnh anten M hnh anten l anten monopole hnh ch nht, tip in bng ng dn sng ng phng, c in trn mt lp bng in mi FR-4 c kch thc kh nh gn 40 x 35 x 1.6 mm. Do EWB c di tn hot ng t 2.7 GHz - 20 GHz nn khi thit k anten s chn tn s cng hng trung tm vo khong 10 GHz. Khi , kch thc ca ming bc x (monopole) s ph thuc vo tn s trung tm, rng ca bng tn v hng s in mi hiu dng ca lp . Kch thc tnh ton c th ca anten nh hnh 4.4. T cng thc tnh ton tr khng ca ng dn sng ng phng th mt s trang Web da trn cng thc ny a ra cng c tnh ton hiu qu. Sau y l mt cng c trn trang: http://www1.sphere.ne.jp/i-lab/ilab/index_e.htm (Hnh 4.5) c ti dng tnh ton tr khng c tnh ca CPW dng cho m hnh. 61

W=24.2 M1 = 16 M4=2M3=7.5 L=16 N=1 M2 = 3 t=0.035

s=1Hnh 4. 4 Kch thc anten EWB n

Kch thc ca ng dn in gia, kch thc ca khe hp v hng s in mi tng i ca lp s quyt nh tr khng c tnh ca ng dn sng ng phng. Vi kch thc ghi trn hnh 4.4, khi chn lp lm bng FR4 c = 4.4, th tr khng c tnh thu c s t xp x l 50 Ohm. T cc yu cu ca anten cho cng ngh bng thng siu rng c th chn cc thng s t s in p sng ng VSWR (Voltage Standing Wave Ratio), th bc x v h s tng ch nh gi anten. i vi cc anten thit k, yu cu phi t VSWR < 2 trong di tn s hot ng. (iu ny tng ng vi vic trong nhiu thit k anten s dng tiu ch nh gi h s tn hao ngc (RL-Return Loss) nh hn -10dB trong di tn s hot ng). Sau khi hon tt qu trnh v m hnh anten, cc thng s nh vt liu ming , vt liu ca GND, tn s cn quan st, cc thng s port, thng s qut bin, thit lp farfieldc nh ngha.

60

M5=3.2

62

60

M6=0.3

g=0.2

h=1.6

Hnh 4. 5 Cng c tnh ton tr khng CPW

Ngoi ra, khc vi cc phn mm m phng khc, HFSS s dng phng php tnh ton trng in t FEM (Phng php phn t hu hn) nn phi b sung AirBox chng trnh c th tnh ton, phn tch trong phm vi bn trong hnh khi ny. Nu khng bo li th sau mt thi gian nht nh d cho phn mm x l v a ra kt qu, ta c th quan st c rt nhiu cc thng s ca anten: h s tn hao ngc, tr khng, gain, th bc x phng hng hay phn b dng b mt 4.1.1.2 Kt qu m phng v o c

Sau khi thit lp mi thng s ca anten trong phn mm HFSS, nu khng c li th kt qu s c tnh ton sau mt khong thi gian ty theo kch thc v c tnh nh ngha cho anten. Di y l th t s song ng ca anten theo tn s t 2 n 20 Ghz. Da trn hnh 4.6 ta c th thy c trn bng tn EWB t 2.7 GHz n 20 GHz, h s ny nh hn 2 tha mn iu kin anten c th lm vic c.

63

Mt trong nhng yu cu na ca anten ng dng EWB l th phnghng bc x phi c dng ng hng. Di y l th phng hng bc x di h ta cc c ly cc tn s 3, 7.5, 10 v 15 Ghz (Hnh 4.7).5 M phng o c

4

VSWR

3

2.7Ghz

2

1

2

4

6

8

10 12 14 Tn s [GHz]

16

18

20

Hnh 4. 6 T s sng ng ca anten (VSWR)

T th ta thy rng bc x ca anten ti cc tn s c trng ca di tn EWB gn nh l ng hng. Ti tn s cao th th bc x xut hin nhng gn sng nhng s thay i l khng ln. Hnh 4.8 l th phng hng bc x ca anten trong khng gian ba chiu ti tn s 7.5GHz v 10GHz t c bc x ng hng trong mt phng vung gc vi mt phng ca anten ( v y trong chng trnh m phng anten c t trn mt phng xOy). H s tng ch ca anten bng thng siu rng thng khng ln do anten ch hot ng trong phm vi hp. y, h s tng ch ca anten t gi tr ln nht l gn 10dB ti tn s 18GHz (Hnh 4.9).

64

0 10 0 -10 -20 330 30

0 10 0 330 30

300

60

-10 -20

300

60

-30

-30-40 -30 -20 -10 0 10 210 180 150 240 120 270 90

-40 -30 -20 -10 0 10

270

90

Mt phng E Mt phng H

240

120

Mt phng E210 180 150

Mt phng H

(a)

(b)

0 10 0 -10 -20 -30 -40 -30 -20 -10 0 10 210 180 150 240 120 270 90 330 30

0 10 0 330 30

300

60

-10 -20 -30 -40 -30 -20 -10 270

300

60

90

240

120

Mt phng E Mt phng H

0 10 210 180 150

Mt phng E Mt phng H

(c)

(d)

Hnh 4. 7 th phng hng bc x ta cc ti (a) 3GHz, (b) 10 GHz, (c) 7.5GHz v (d) 15GHz

(a)

(b)

Hnh 4. 8 th phng hng bc x trong khng gian ba chiu ti (a): 7.5GHz v (b): 10GHz

65

12

10

H s tng ch nh

H s tng ch nh [dB]

8

6

4

2

0 2 4 6 8 10 12 14 16 18 20

Tn s [GHz]

Hnh 4. 9 H s tng ch ca anten n

Mt dng b mt ca anten tn s trung tm 10 Ghz v tn s anten c th lm vic c 5 Ghz, 15GHz, 20GHz c biu din trn hnh 4.10.

(a)

(b)

(c)

(d)

Hnh 4. 10 Mt dng b mt: a) 10 Ghz, b) 5Ghz, c)15GHz v d)20GHz

66

nhng im c cng dng ln th c mu m, v nhng im c cng dng in nh khng ng k c mu xanh. C th d dng nhn thy khi anten lm vic cc tn s khc nhau th mt dng b mt phn b tp trung nhng phn khc nhau ca anten. iu ny cho thy mi tn s khc nhau th anten u to ra c nhng im cng hng khc nhau, do vy nn bng thng ca anten s t c di rt rng. 4.1.1.3 Phn tch m hnh anten n

Anten c cu to bi nhng ng gp khc hot ng nh cu trc bin i dn dn. M hnh thit k s c phn tch ti nhng gc m khc nhau ca nhng ng gp khc thy r nguyn l ca cu trc bin i ny. Vi cc gc m: 300, 600 v 750 th di tn hot ng ca anten s khc nhau (Hnh 4.11). iu ny cho thy vi cu trc chuyn tip cng t bin t ngt t ng truyn tip in n anten th khi bng thng s tng ln do s phn x ca sng gim i. di ca ng dn sng ng phng cng nh hng n di tn lm vic ca anten. Vi di cng ngn th h s tn hao ngc ca anten cng nh, nh hnh 4.12 th ng lin nt t c h s tn hao ngc thp nht vi chiu di ca ng dn sng trong trng hp ny l ngn nht trong ba trng hp.0 -5 -10 -15

S11 [dB]

-20 -25 -30 -35 -40 -45 -50 2 4 6 8 10 12 14 16 18 20

S11 ti gc m 750 S11 ti gc m 300 S11 ti gc m 600

Tn s [GHz]

Hnh 4. 11 S thay i h s tn hao ngc theo gc m ca kt cu chuyn tip

67

Anten EWB hot ng mt di tn s rt rng bao trm rt nhiu bng tn ca cc h thng khc nh: WLAN, WiMAX, UWB v ng xung ca v tinh Chnh v vy, n s c s dng cho cc h thng khc nhau vi bng tn khc nhau. Tuy nhin, khi cc h thng cng lm vic trong cng mt mi trng truyn dn th s gy nhiu giao thoa vi nhau. Mong mun t ra l lm sao gii quyt c vn trn v phng php loi b di tn s khng mong mun ca h thng khc khi anten ang c s dng trong mt h thng no s l mt phng php ti u. C nhiu cch lm c iu trn nh: s dng b lc, s dng cu trc khe v cu trc EBG (Electronic BandGap); y, ti s dng cu trc khe nh l phng n thit k ti u, v khng nhng t c hiu qu cao trong vic loi b tn s m cn l phng php n gin nht v t phc tp nht.

0 -5 -10 -15

S11 ti d=1mm S11 ti d=3mm S11 ti d=5mm

S11 [dB]

-20 -25 -30 -35 -40 -45 2 4 6 8 10 12 14 16 18 20

Tn s [GHz]

Hnh 4. 12 th biu din s thay i h s tn hao ngc theo chiu di ca ng dn ng phng

Cu trc khe c xut trong thit k l khe hnh ch U vi cc tham s c a cho bi cng thc:

68

L slot

0.45c f notchr eff _ slot

(4.9)

eff _ slot

1 2

(4.10)

Vi:

r l hng s in mi ca lp , Lslot l chiu di khe, c=3.10^8 l vn

tc nh sng. T cng thc trn vi: r bit v tn s trung tm ca di tn cn loi b ta s d dng tnh ra c chiu di ca khe. Cng vi qu trnh ti u cc tham s khc nh: chiu rng, chiu di on gp khc trong qu trnh m phng th di tn cn loi b s c tm ra. y, ti thc hin loi b hai bng tn ca h thng WLAN v WiMAX khi anten c lm vic trong h thng UWB. Hnh dng v kch thc ca tng khe c th hin trong hnh 4.13 v bng s liu 4.1:

W=24.2 M1 = 16 M4=2 Ws1

M2 = 3

Lh

M3=7.5M5=3.2

L=16 N=1

Hnh 4. 13 Anten vi cu trc khe

LsWs6960

g=0.2 s=1

Ws2 t=0.05M6=0.3 h=1.6

BNG 4. 1KCH THC CC KHE DNG LOI B BNG TN CA H THNG WLAN V WiMAXLslot WLAN 2.65 WiMAX 6.5 0.7 0.2 8.8 11.7 0.3 0.3 8.54 11.6 Ws1 Ws2 Lh Ws

Di y l kt qu m phng v o c ca m hnh anten vi khe hnh ch U c khot trn phin kim loi:6 M phng 5 (2) o c (1): 2.58 GHzVSWR

8 7 6 (1) 5 (2) 4 3 (3) M phng o c (1): 2.28 GHz (2): 2.91 GHz (3): 3.62 GHz

VSWR

4 (1) 3 (3)

(2): 4.7 GHz (3): 6.01 GHz

2

2 1 2 4 6 8 10 12 14 Tn s [GHz] 16 18 20

1

2

4

6

8

10 12 14 Tn s [GHz]

16

18

20

(a)

(b)

Hnh 4. 14 T s sng ng vi cu trc khe dng loi b bng tn cho (a) WLAN v (b) WiMAX

T th 4.14 ta thy vi cu trc khe hnh ch U v kch thc cho bng 1 th bng tn ca hai h thng WLAN v WiMAX c loi b. th bc x ca anten vi cu trc khe dng loi b bng tn h thng WLAN c th hin trong hnh 4.15, khi so snh vi th bc x ca anten khi khng c khot khe th gn nh khng thay i ng k. iu ny cho thy vi cu trc khe c thm vo th c tnh phng hng ca anten khng thay i.

70

0 10 0 -10 -20 -30 -40 -30 -20 -10 0 10 210 180 150 240 120 270 90 330 30

0 10 0 330 30

300

60

-10 -20 -30 -40 -30 -20 -10 270

300

60

90

240

120

Mt phng E Mt phng H

0 10 210 180 150

Mt phng E Mt phng H

(a)

(b)

0 10 0 -10 -20 -30 -40 -30 270 90 330 30

0 10 0 330 30

300

60

-10 -20 -30 -40 -30 270

300

60

90

-20 -10 0 10 210 180 150 240 120

-20 -10 0 10 210 180 150 240 120

Mt phng E Mt phng H

Mt phng E Mt phng H

(c)

(d)

Hnh 4. 15 th bc x ca anten vi cu trc khe loi b bng tn WLAN ti (a) 3GHz, (b) 10GHz, (c) 7.5GHz v (d) 15GHz

H s tng ch nh ca hai m hnh: m hnh khng c cu trc khe v m hnh c cu trc khe c biu din trong hnh 4.16. Vi m hnh c cu trc khe th ti di tn ca bng tn b loi b t 5 n 6 GHz, h s tng ch ca anten b suy gim i so vi di tn s xung quanh. V anten l bc x ng hng nn h s tng ch thng khng cao, gi tr h s tng ch nh ca c hai m hnh trung bnh t xp x 4dB. Ring trng hp khi c cu trc khe th do nh hng bc x ca khe nn mt s im tn s t c gi tr cao hn.

71

10

148

H s tng ch nh [dB]

H s tng ch nh

12 10 8 6 4

H s tng ch nh

H s tng ch nh [dB]

6

4

2

20 2 4 6 8 10 12 14 16 18 20

0

Tn s [GHz]

2

4

6

8

10

12

14

16

18

20

Tn s [GHz]

(a)

(b)

Hnh 4. 16 H s tng ch nh ca hai m hnh anten khng c cu trc khe (a) v c cu trc khe (b)

4.1.1.4

M hnh ch to ca cc anten

Hnh 4. 17 Sn phm anten EWB

M hnh c ch to vi chun ca lp in mi hin nay l FR-4 vi hng s in mi l 4.4 v dy lp kim loi l 0.035mm. Nhn vo hnh nh ta c th thy rng kch thc anten rt nh gn vi chiu di anten l 24.2mm.

72

4.1.2 H thng anten MIMO4.1.2.1 M hnh anten

M hnh anten MIMO bao gm 2 anten EWB c ghp li vi nhau vi cc mt bc x nm trn cng mt mt ca in mi. Kch thc ca ming kh nh gn (44,4mm 16mm 1.6mm) d dng tch hp c vo cc thit b di ng cm tay. Khong cch ngn nht gia hai phin kim loi l: 4.2 mm. Vi hai cu trc stub hnh ch nht c kch thc (0.6mm 15mm) c ghp vo gia khng gian hai phn t anten n EWB v khong cch gia chng l 1mm th s tc ng tng h gia hai phin bc x c gim i ng k. Thm vo loi b c bng tn ca h thng WLAN th cu trc khe hnh ch V ngc c thay th cho cu trc hnh ch U. Kt qu m phng v cc nhn xt nh gi thit k bn di s cho thy r cc c tnh ca anten ny.44.4 mm1 mm 0.6 mm 16 mm2.1 mm 6.65 mm

phn t anten, v vy cn phi c mt thm thng s ngoi h s tn hao ngc (hoc t s sng ng) ca anten n. Trong ma trn tn x 2 phn t, thng s S21

16 mm

4.1.2.2

Trong h thng nhiu anten ny, ta phi phn tch nh hng tng h gia cc

0.8 mm

4.2 mm

11.4 mm 0.2 mm 1 mm

4.2 mm 18.8 mm

35um

1.6 mm

Hnh 4. 18 M hnh anten MIMO

Kt qu

73

l h s truyn t ca ca 1 n ca 2, tc l nh hng ca ca 2 n ca 1. H s ny c a ra nh gi s tc ng gia cc anten, S21 cng nh cng tt. Trc tin ta s cn kim tra xem vi cu trc MIMO th anten cn hot ng di tn s siu rng hay khng. V hnh 4.19 cho thy anten MIMO vn m bo c yu cu trn. C hai m hnh anten MIMO u c di tn hot ng nh trong m hnh anten n. c bit, m hnh anten MIMO vi cu trc hnh V ngc th bng tn cho h thng WLAN c loi b. m bo s nh hng tng h gia cc anten khng qu ln th h s truyn t gia cc anten khng c vt qu -15dB. Trn hnh 4.20 ta c th thy rng h s S12&S21 trn di tn t 2 n 20 Ghz u nh hn -15dB, tha mn yu cu ti thiu t ra anten hot ng bnh thng.5.0 4.5 4.0 3.5 3.0

VSWR ca anten n VSWR ca anten MIMO khng c cu trc khe VSWR ca anten MIMO c cu trc khe

(1)

VSWR

(2)

(1): 5 GHz (2): 6.2 GHz

2.5 2.0 1.5 1.0 0.5 0.0 2 4 6 8 10 12

14

16

18

20

Tn s [GHz]

Hnh 4. 19 T s sng ng ca cc anten

Chnh v nh hng tng h ca h anten l kh b nn cc tham s khc ( th bc x phng hng, h s sng ng) s b t bin i. T hnh trn ta c th thy c c rt nhiu s thay i ca th bc x phng hng so vi m hnh anten UWB n. Tuy cng trng bc x ca anten c s dao ng ln theo nhiu hng khc nhau nhng vn m bo c th bc x ng hng.

74

-10

-15

S12&S21 ca anten MIMO khng c cu trc khe S12&S21 ca anten c cu trc khe

-20

S12&S21 [dB]

-25

-30

-35

2

4

6

8

10

12

14

16

18

20

Tn s [GHz]

Hnh 4. 20 Tham s S12&S21 ca hai m hnh MIMO0 10 0 -10 -20 -30 -40 -30 -20 -10 0 10 210 180 150 240 120 270 90 330 30 10 0 -10 -20 -30 -40 -30 -20 -10 0 10 210 180 150 240 120 270 90 330 0 30

300

60

300

60

Mt phng E Mt phng H

Mt phng E Mt phng H

(a)0 10 0 -10 -20 -30 -40 -30 -20 -10 0 10 210 180 150 240 120 270 90 330 30

(b)

300

60

Mt phng E Mt phng H

(c) Hnh 4. 21 th bc x phng hng tn s 10GHz ti (a) Anten n, (b) Anten MIMO, (c) Anten MIMO loi b bng tn WLAN

75

th phng hng bc x trong khng gian ba chiu c th hin trong hnh 4.22 cho ta thy ti hai tn s khc nhau, tn s b 7.5GHz v tn s cao 15GHz, tuy s bc x c khc nhau nhng xt chung bc x anten vn m bo c yu cu trong mi trng truyn dn khng dy phm vi hp. 4.1.2.3 Nhn xt v nh gi

Cc phn t bc x c t ni tip nhau trn cng mt mt gip cho vic ch to c thun li hn rt nhiu. Tuy nhin, ta cng cn phi c bin php gim nh nht khong cch c th ca cc phn t bc x m nh hng tng h ca hai phn t l trong gii hn cho php. Khi hai phn t bc x c t cch nhau mt khong l D (dng) th m bo c s cch ly nhng khi kch thc anten s ln bng: 224.2+D (mm). Nhng nu cng di chuyn chng li gn vi nhau th tham s S12&S2 cng khng t yu cu. Vi mong mun va t c yu cu v gim nh hng tng h gia hai phn t bc x va thu gn c kch thc anten th sau mt s cc phng php c a ra nghin cu, ti tm ra c mt gii php thch hp. l s dng cu trc Stub chm vo gia hai phn t bc x. V kt qu theo nh hnh 4.19 v hnh 4.20 l t c yu cu, ng thi khi kch thc anten ch cn 44,4 mm.

(a)

(b)

Hnh 4. 22 th phng hng bc x trong khng gian ba chiu ca m hnh MIMO loi b bng tn WLAN ti (a) 7.5GHz v (b) 15GHz

Di y l m hnh 2 anten n ghp ni tip vi nhau cha s dng Stub vi khong cch hai phn t bc x l: 24.22+D (mm). y, D c th m hoc dng.

76

24.22+D (mm)16 mm

D (mm)

S12&S21 [dB]

- Xt nh hng ca khong cch D ti h s S12 v S21: Kt qu ca anten ghp ni tip kh tt khi D dng nhng khi D cng gim v chuyn sang gi tr m, tc hai phn t bc x cng tin gn n nhau hn, th h s S12&S21 tng dn ln v ti D= -4(mm) v t kt qu hnh 4.24 ta thy s cch ly ca hai phn t bc x khng cn t yu cu ra. Trong di tn s t 2.5Ghz n 8.5GHz th hai h s truyn t khng cn tha mn na. - Phng php s dng Stub:

16 mm11.4 mm 0.2 mm 1 mm-10 -15 -20 -25 -30 2 4 6 8

Hnh 4. 23 M hnh anten ghp ni tipS12&S21 ca anten MIMO khng c Stub

10

12

14

16

18

20

Tn s [dB]

Hnh 4. 24 S11 v S21 ca anten ghp ni tip cch nhau

77

Vi mt s cc phng php c th nghim, ti t c yu cu thit k ra vi hai cu trc Stub vi mi Stub l mt thanh