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I HC QUC GIA H NI TRNG I HC CNG NGH
NGUYN TH N
M KHI KHNG THI GIAN TRONG H THNG
TIN BNG RNG
Chuyn ngnh: K thut in t M s: 60 52 70
TM TT LUN VN THC S
H Ni 2011
1
M U Trong nhng nm gn y, nhu cu thng tin lin lc
ca con ngi ngy cng gia tng v c nhng bc tin vt bc. S pht trin nhanh chng ca cc loi hnh dch v nh Video, thoi v thng tin d liu trn Internet, in thoi di ng, cng nh nhu cu v truyn thng a phng tin ngy cng mt pht trin. S hot ng ca cc h thng v tuyn tin tin ny ph thuc rt nhiu vo c tnh ca knh thng tin v tuyn nh: Pha inh, gii hn v bng thng, iu kin ng truyn thay i mt cch nhanh chng v tc ng qua li ca cc tn hiu. V vy vn khm ph ti nguyn, s dng hiu qu ti nguyn nhng vn m bo cht lng truyn lun l nhng ch c quan tm nghin cu, trin khai.
Hai k thut OFDM v MIMO c coi l k thut ch o cho cc h thng v tuyn th h sau nh WIMAX v 4G: T quan im lp lun ny, vn c tnh cht then cht l xy dng cc gii thut khai thc v x l trit c tnh ca knh truyn v tuyn. H thng MIMO ra i l mt gii php hiu qu ci thin dung lng ca h thng thng tin v tuyn. H MIMO c u im l hiu sut s dng ph v phn tp cao. Mt cch tng phm cht, gim (BER) ca MIMO l thc hin m ha khng thi gian, mt k thut m ha c thit k vi nhiu anten pht, thu. Vic s dng h thng n sng mang truyn thng cho nhng dch v ny dn n cu trc ca h thng thu pht c phc tp cao hn rt nhiu so vi vic s dng h thng a sng mang, ghp knh phn chia theo tn s trc giao (OFDM) l mt trong nhng
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gii php ang c quan tm gii quyt vn ny. Trong ti ny i su vo tm hiu mt s k thut v m ha, c bit l m khi trc giao v nh hng ca n i vi k thut OFDM. Vi c s l thuyt ny, em m phng trc quan chng minh s nh hng ca m khi i vi h thng truyn thng OFDM.
Lun vn gm c 5 chng :
Chng 1: Trnh by v knh fading trong mi trng v tuyn. Bao gm nhng khi nim c bn v h truyn thng v tuyn v nhng nh hng ca fading i vi mi trng v tuyn.
Chng 2: Trnh by v m khng gian thi gian, bao gm mt s loi m khng gian thi gian v tp trung ch yu vo m khi khng gian thi gian.
Chng 3: Trnh by v h OFDM, bao gm s khi v nguyn l hot ng ca h OFDM cng nh nhng u nhc im ca h OFDM.
Chng 4: Trnh by v m khi khng gian thi gian trong h OFDM, ni dung ca chng tp trung nh gi vn hiu nng ca h thng OFDM c s dng m ha khng gian thi gian.
Chng 5: y l chng m phng nhm nh gi mt s kt qu ca mt s vn trnh by trong cc chng trc. Bao gm m phng h truyn thng STBC-OFDM v so snh hiu nng ca h OFDM c s dng m ha v khng s dng m ha.
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CHNG 1: KNH FADING TRONG MI TRNG V TUYN
1.1. Cc c tnh ca knh truyn v tuyn
Cc knh v tuyn l cc knh mang tnh ngu nhin, n c th thay i t cc ng truyn thng n cc ng b che chn i vi cc v tr khc nhau. Do , xem xt cc c tnh ca knh truyn v tuyn ta phi xem xt chng mt s c tnh nh khng gian, tn s, thi gian.
1.1.1. Min khng gian
i vi cc m hnh truyn sng truyn thng vic nh gi cng sut thu trung bnh ti mt khong cch cho trc so vi my pht c gi l nh gi tn hao ng truyn. Khi khong cch thay i trong phm vi mt bc sng th knh th hin cc c tnh ngu nhin rt r rt. iu ny c gi l tnh chn lc khng gian.
1.1.2. Min tn s
1.1.2.1. Thay i tn s:
Thay i tn s do hiu ng Doppler gy ra khi c s chuyn ng tng i gia my thu v my pht dn n thay i tn s mt cch ngu nhin. Dch tn s trong tn s thu do chuyn ng tng i ny c gi l dch tn s Doppler
1.1.2.2. Chn lc tn s
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Trong khun kh ni dung ca lun vn ny s phn tch vn chn lc tn s cng vi mt thng s khc trong min tn s l bng thng kt hp. Bng thng kt hp (coherence bandwidth) l mt s o thng k ca di tn s trn mt knh pha inh c coi l knh pha inh "phng" (l knh trong tt c cc thnh phn ph c truyn qua vi khuych i nh nhau v tuyn tnh v pha).
1.1.3. Min thi gian
1.1.3.1. Tri tr trung bnh qun phng, RDS
Thng s thi gian quan trng ca tn thi l tri tr trung bnh cn qun phng (RDS: Root Mean Squared Delay Spread). RDS l mt s o thch hp cho tri a ng ca knh.
1.1.3.2. Tr tri cc i
Tr tri cc i (XdB) ca h s v tr cng sut c nh ngha l tr thi gian m nng lng a ng gim XdB so vi nng lng cc i.
1.1.3.3. Thi gian kt hp
Mt thng s khc trong min thi gian l thi gian kt hp (coherence time). Thi gian kt hp xc nh tnh "dng" ca knh. Thi gian kt hp l thi gian m knh tng quan rt mnh vi bin ca tn hiu thu, k hiu Tc.
1.1.4. Quan h gia cc min khc nhau
1.1.4.1. Bng thng kt hp v tri tr trung bnh qun phng
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Ta c th trnh by knh trong min tn s bng cch s dng cc c tnh p ng tn s ca n. Tng t nh cc thng s tri tr trong min thi gian, ta c th s dng bng thng kt hp cho c trng knh trong min tn s.
1.1.4.2. Thi gian kt hp v tri Doppler
Thi gian kt hp chu nh hng trc tip ca dch Doppler, n l thng s knh trong min thi gian i ngu vi tri Doppler
1.2. Vn pha inh trong mi trng v tuyn
1.2.1. Pha inh l g?
Pha inh l qu trnh m t s dao ng ca tn hiu thu khi tn hiu n c anten thu.
1.2.2. Nguyn nhn gy nn pha inh
Pha inh c th c miu t do nhng nguyn nhn c bn l a ng hoc Doppler.
1.2.3. Phn loi pha inh
V c bn c th phn pha inh thnh cc loi sau: pha inh phng, pha inh chn lc tn s, pha inh nhanh, pha inh chm
1.2.3.1. Pha inh a ng
Pha inh a ng xy ra do s tn ti ca nhiu ng tn hiu t my pht ti my thu.
1.2.3.2. Pha inh phng v pha inh chn lc tn s
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Nu tt c cc thnh phn ph ca tn hiu truyn b nh hng mt cch nh nhau, pha inh c gi l khng la chn tn s hoc pha inh phng.
Nu cc thnh phn ph ca tn hiu truyn b nh hng bi s thay i ca bin v pha, th pha inh c gi l pha inh chn lc tn s.
1.2.3.3. Pha inh chm v pha inh nhanh
Khi nim ny c lin quan n thi gian kt hp Tc ca knh, pha inh c cho l chm nu ko di ca k hiu Ts nh hn thi gian kt hp Tc ca knh, ngc li c coi l nhanh.
1.2.4. M hnh pha inh theo quan im thng k
1.2.4.1. Pha inh Rayleigh
C th coi phn b pha inh Rayleigh l phn b ng bao ca tng hai tn hiu phn b Gauss vung gc
1.2.4.2. Pha inh Rice
Pha inh Rice khng ch l h qu ca hin tng a ng l thm vo mt s ng ngu nhin ca tn hiu, n cn th hin s truyn sng LOS gia my pht v my thu
CHNG 2: M KHI KHNG GIAN THI GIAN
2.1. Gii thiu chng
Lm th no c th ti a ha tc truyn bng cch s dng m n gin, thut ton gii m n gin cng nh l
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xc sut li bit c gim thiu? M khi khng thi gian l mt nguyn l p ng c cc yu cu ny, v y cng l ni dung s c cp ch yu trong chng ny.
2.2. Phn tp
2.2.1. Cc k thut phn tp
Phn tp thi gian:
Nu cng mt tn hiu c pht i ti cc khe thi gian khc nhau, th cc tn hiu thu c s l cc tn hiu khng tng quan.
Phn tp tn s:
Phn tp tn s truyn mt tn hiu trn nhiu tn s sng mang khc nhau.
Phn tp khng gian:
Phn tp khng gian, cn gi l phn tp anten l mt trong cc loi phn tp ph bin dng trong h thng khng dy.
2.2.2. Phn tp thu pht
Trong cc h thng thng tin di ng t ong hin nay, nhiu anten thu c s dng cho cc trm thu pht c s vi mc ch l kh nhiu ng knh v gim thiu nh hng ca pha inh
2.3. H thng m khng gian thi gian Chng ta xem xt mt h thng thng tin s dng m
ha khng thi gian trn bng gc vi nT anten pht v nR anten
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thu nh sau. Cc d liu pht i c m ha bi b m ha khng thi gian.
2.3.1. Xt v mt hiu nng Trong phn tch hiu nng chng ta gi s rng chiu
di khung d liu pht l L k hiu cho mi anten. Chng ta nh ngha mt ma trn t m khng thi gian nT x L, bng cch sp xp chui pht trong mt mng nh sau.
2.3.2. Xt v mt cu trc M ha khng thi gian c th thc hin phn tp pht
v tng ch cng sut so vi h thng khng m ha m khng lm thit hi bng tn.
Mt cch tng qut, m khi khng thi gian c th c xem nh l s nh xa mt chui ca nN k hiu {s1,s2,,sN} vo ma trn S kch thc nt x N:
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Ma trn m khi khng thi gian S c th c m t di dng sau y:
2.4. M Alamouti
Nguyn l Alamouti l m khi khng thi gian u tin a ra t c phn tp y mi tc d liu i vi h hai anten pht.
2.4.1. M ha khng thi gian Alamouti. S khi ca b m ha khng thi gian Alamouti
c m t nh hnh sau.
2.4.2. Ma trn knh o tng ng ca m Alamouti
Ma trn knh o tng ng ca m Alamouti ph thuc vo cu trc ca m v h s knh. Khi nim n gin v cc EVCM l phn tch pht STBC. S tn ti ca EVCM l mt trong nhng c im quan trng ca STBCs
2.4.3. T hp v gii m hp l nht
Nguyn l pht Alamouti l mt nguyn l pht phn tp n gin gip ci thin cht lng tn hiu u thu bng cch s dng mt thut ton x l tn hiu n gin (STC) my pht. S phn tp thu c thc hin bng cch p dng t
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hp t s ti a (MRC) vi mt anten ti my pht v hai anten ti ni tip nhn cc tn hiu.
2.4.4. Nguyn l Alamouti vi nhiu anten thu
Nguyn l Alamouti c th c ng dng i vi mt h thng vi hai anten pht v nR anten thu. Vic m ha v pht i vi m hnh ny ging vi trng hp mt anten thu
2.4.5. M khi khng gian thi gian vi cc chui tn hiu thc
Da vo kiu ca cc chui tn hiu, m khi khng thi gian c th c chia thnh m khi khng thi gian vi cc tn hiu thc v m khi khng thi gian vi cc tn hiu phc
2.4.6. M khi khng thi gian vi cc chui tn hiu phc
Nguyn l Alamouti c th c coi nh mt m khi khng thi gian vi cc tn hiu phc i vi hai anten pht
2.5. Kt lun chng
C th, trong chng trnh by:
- Mt s khi qut chung v h thng m khng thi gian
- Phn tch hiu nng ca h thng m khng thi gian
- Tm hiu m Alamouti, mt loi m khi khng thi gian, trong s dng tnh trc giao ca dy tn hiu trong m n gin tnh ton, gip cho vic gii m d dng.
CHNG 3: H OFDM
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3.1. Gii thiu chng
Hin nay, OFDM c s dng trong nhiu h thng nh ADSL, cc h thng khng dy nh IEEE802.11 (Wi-Fi) v IEEE 802.16(WiMAX), pht qung b m thanh s (DAB), v pht qung b truyn hnh s mt t cht lng cao (HDTV). hiu d v OFDM ta s ln lt xt:
3.2. Tnh trc giao
3.3. M hnh h thng truyn dn OFDM
3.3.1. M t ton hc tn hiu OFDM
3.3.2. S h thng truyn dn OFDM
My pht: Chuyn lung d liu s pht thnh pha v bin sng mang con. Cc sng mang con c ly mu trong min tn s, ph ca chng l cc im ri rc. Sau s dng bin i Fourier ri rc ngc (IDFT) chuyn ph ca cc sng mang con mang d liu vo min thi gian.
My thu: Thc hin hot ng ngc li ca pha pht.
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3.3.2.1. Tng chuyn i ni tip sang song song
Tng chuyn i ni tip sang song song chuyn lung bit u vo thnh d liu pht trong mi k hiu OFDM.
3.3.2.2. Tng iu ch sng mang con
Tng iu ch sng mang con lm nhim v phn phi cc bit d liu ngi dng ln cc sng mang con.
3.3.2.3. Tng chuyn i t min tn s sang min thi gian
Php bin i Fourier ngc nhanh (IFFT) s chuyn tn hiu OFDM trong min tn s sang min thi gian.
3.3.2.4. Tng iu ch sng mang RF
u ra ca b iu ch OFDM l mt tn hiu bng tn c s, tn hiu ny c trn nng tn ln tn s truyn dn v tuyn.
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3.4. Cc thng s c trng v dung lng h thng truyn dn OFDM
3.4.1. Cu trc tn hiu OFDM
Cu trc ca cc k hiu OFDM trong min thi gian.
l thi gian truyn d liu hiu qu, l thi gian
bo v. Cng thy cc thng s khc, l thi gian ca s. Thy r quan h gia cc thng s l.
Ca s c a vo nhm lm mn bin chuyn v khng ti cc ranh gii k hiu, v gim tnh nhy cm ca dch tn s.
3.4.2. Cc thng s trong min thi gian TD
Thng s ny nh gi hiu qu ti nguyn c dng trong min thi gian v c th c dng tnh ton thng lng (throughput).
3.4.3. Cc thng s trong min tn s FD
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Ton b rng bng tn cho tt c cc sng mang con B, rng bng tn sng mang con f, v s sng mang con
. Quan h gia chng l
. 3.4.4. Quan h gia cc thng s trong min thi gian v min tn s.
Thng s min thi gian v thng s min tn s f c quan h vi nhau, ngha l chng l t l nghch ca nhau. V vy, vic t gi tr cho mt thng s l thit k h thng.
3.4.5. Dung lng ca h thng OFDM
Dung lng knh theo Shannon.
Dung lng knh ph thuc vo t s tn hiu trn tp m (SNR) v rng bng thng ca tn hiu c xc nh bng cng sau:
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Dung lng knh cho cc h thng OFDM.
Tc bit tng ca h thng OFDM bng:
3.5. Cc nhn t nh hng ca knh pha inh ln hiu nng h thng OFDM v cch khc phc
3.5.1. nh hng ca ISI
- Nguyn nhn do tnh chn lc ca knh pha inh trong min thi gian, tnh ph thuc thi gian ca knh pha inh, tnh bt n nh ca knh gy ra giao thoa gia cc k hiu ISI truyn qua n.
- Hu qu ISI: lm cho my thu quyt nh k hiu sai, kh khn trong vic khi phc nh thi
3.5.2. nh hng ca ICI
- ICI xy ra do tnh chn lc tn s ca knh pha inh (knh pha inh chn lc tn s), nguyn nhn chnh l hin tng dch Doppler do tnh di ng ca my thu.
- Hu qu l s khng phn bit c ranh gii gia cc k hiu truyn trn cc sng mang con, dn n pha thu s quyt nh sai k hiu mt tnh trc giao.
khc phc cc nh hng ny, c nhiu cch khc nhau, nh ni l dng phn bo v (vi ISI) v tin t vng (ICI)
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3.6. Kt lun
Trong chng ny ta nghin cu cc vn sau:
Nguyn l hot ng c bn ca OFDM
S khi v hot ng ca cc khi: bao gm phn tch cc phn t ca m hnh v cc thng s c trng cng dung lng h thng OFDM
u im:
- Phn tch hiu qu ph tn vt tri ca OFDM.
- Loi b nh hng ca ICI v ISI gy ra do pha inh a ng, chn lc tn s do dng CP.
- Dng IFFT v FFT n gin phn cng cho pht v thu.
Nhc im:
- PAPR ln
- Nhy vi nguyn nhn gy mt ng b.
CHNG 4: STBC-OFDM
Vic thit k v phn tch hiu nng ca m khng gian thi gian c tho lun cho cc h thng khng dy bng hp, c c trng bi knh pha inh phng khng la chn tn s. Gn y, c s quan tm ngy cng tng trong vic cung cp cc dch v d liu tc cao nh hi ngh truyn hnh, a phng tin, v my tnh di ng trn cc knh bng rng khng dy. Trong truyn thng khng dy bng rng, chu
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k k hiu t lin quan hn so vi tri tr ca knh, do , mong mun ca cc tn hiu truyn l cc knh pha inh la chn tn s. Cng ngh m ha khng thi gian c th c s dng t c tc d liu rt cao trong cc h thng bng rng. V vy, nghin cu s nh hng ca pha inh chn lc tn s trn hiu nng ca m ha khng thi gian l vn thit yu.
Ni dung ca chng ny s trnh by vn hiu nng ca m ha khng gian thi gian trn cc knh bng rng khng dy vi pha inh chn lc tn s, phn tch v nh gi hiu nng ca m khng gian thi gian i vi h thng bng rng. M khng gian thi gian c th c ng dng trong mt s h thng bng rng nh OFDM v CDMA nhng trong gii hn lun vn ny ch trnh by cc vn lin quan n h OFDM
CHNG 5: M PHNG
hiu hn mt s vn c trnh by trong nhng chng trc. Trong chng cui ny s trnh by chng trnh m phng nh hng ca m khi khng thi gian trong h OFDM. Lun vn chn ngn ng Matlab m phng v l do sau: Matlab l ngn ng lp trnh bc cao, ngn ng k thut, ngn ng chuyn gia c th vin ton hc cc mnh, giao din ha phong ph, kh nng tng thch cc ngn ng khc, cho php to giao din ngi dng tin li. Matlab c u vit ni tri l kh nng m phng h thng ng, cho php th hin tn hiu v h thng trong nhiu min xt
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(min thi gian, min tn s, min Z). C l v th m hu ht cc trng i hc, cc vin ln u chn Matlab phc v cng tc o to, nghin cu pht trin.
Theo mc ch chnh ca chng trnh ny l m phng hai trng hp sau:
- Th nht: m phng h thng truyn thng STBC - OFDM (MIMO OFDM c dng m). - Th hai m phng nh gi hiu nng ca OFDM c s dng m khi khng thi gian so vi h thng OFDM khng s dng m khi khng thi gian. Qua nhm lm r nh hng ca m khi khng thi
gian i vi h OFDM thng qua cc tc ng ca knh Rayleigh v nh hng n gi tr BER.
M hnh m phng h thng STBC OFDM c cho hnh sau. D liu ca ngi dng l dy ngu nhin vi cc bit 0,1. D liu nh phn ny c a qua b iu ch (BPSK, QPSK, 16QAM), tip theo d liu sau iu ch c a n khi STBC (alamouti 2x2) v sau a ti b iu ch OFDM. D liu sau iu ch OFDM c pht qua knh v tuyn n my thu. My thu s thc hin ngc cc cng vic vi pha pht. D liu nh phn s c khi phc v so snh vi d liu ban u.
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Chng ny gii thiu tnh nng ca chng trnh
m phng h thng truyn dn STBC OFDM. Phn tch phng php m phng STBC - OFDM, phn tch cc thng s h thng truyn dn STBC - OFDM. So snh hiu qu ca h thng OFDM s dng m STBC v khng s dng m STBC, tiu ch so snh c th hin thng qua nh gi t s bit li
qua 2 h thng ny. Chnh kt qu rt quan trng ny gip nh gi hiu nng BER ca h thng c s dng STBC i vi mi loi iu ch u tt hn h thng khng dng STBC.
KT LUN
Qua qu trnh thc hin n, em tm hiu v thu
np thm nhiu kin thc v cng ngh mng di ng cng nh
nhng k thut tin tin nh MIMO v OFDM. n cng
thc hin m phng thnh cng h thng OFDM c s dng
m khi khng thi gian qua nh gi c hiu nng ca
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h thng OFDM c s dng m khi khng thi gian so vi
trng hp khng s dng m. n thc hin c cc
cng vic sau:
- Tng quan v h thng tin v tuyn nhiu anten, m
hnh h thng MIMO.
- i su vo tm hiu k thut phn tp anten, y l
k thut rt quan trng trong h MIMO nng cao n nh
trong qu trnh truyn d liu.
- Tm hiu nguyn l Alamouti, m khi khng thi
gian. M ha khng thi gian l mt k thut nhm nng cao
dung lng truyn d liu, gii quyt nhu cu ngy cng cao
ca ngi dng v tc ng truyn.
Tuy vy, n mi ch dng mc m phng m khi
Alamouti 2x2.
M khng thi gian Alamouti trong k thut MIMO
hin ang l k thut c k vng trong h thng truyn
thng v tuyn hin i. N c ng dng vo cuc sng thc t
l rt rng ri. Kt hp MIMO vo cng ngh khc nh MIMO
- OFDM, hay ng dng vo cng ngh WIMAX- mt cng
ngh bng rng khng dy ang pht trin rt mnh hin nay.
Kha lun tuy ch cp ti kha cnh nh v cn nhiu thiu
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st nhng n gi ln kh nng pht trin mi- cng ngh
MIMO - OFDM.
Trong thi gian ti, mc ch pht trin ca n s l thc hin m phng ton b thut ton bng gc. Cc thut ton nng cao hiu nng ca h thng nh ghp xen, m ha knh, xa trnCc la chn khc v MIMO nh ghp knh theo khng gian SM hay m li khng gian thi gian STTC cng c th c la chn thay cho m STBC Alamouti. Cc cu hnh MIMO mi nh 4x4, 8x8 phc tp hn cng s c cp n.