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

2

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.

3

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

4

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

5

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

6

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

7

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

8

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:

9

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

10

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

11

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.

12

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.

13

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

14

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:

15

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)

16

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

17

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

18

(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.

19

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

20

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

21

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.