BÁO CÁO LUẬN VĂN 2011

I HC QUC GIA TP. H CH MINH TRNG I HC BCH KHOA KHOA IN IN T B MN VIN THNG ------

LUN VN TT NGHIP I HC H CHNH QUY

LC NHIU TN HIU THOI DNG PHNG PHP EMD

GVHD: SVTH:

PGS. TS. HONG NH CHIN 1. TRN HIU TRUNG MSSV: 40602711 2. DNG MINH TIN MSSV: 40602471

TP. H CH MINH, THNG 1 NM 2011

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I HC QUC GIA TP. H CH MINH TRNG I HC BCH KHOA

CNG HA X HI CH NGHA VIT NAM

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c lp T do Hnh phc. ----- ----Khoa: in in t B Mn: Vin Thng

NHIM V LUN VN TT NGHIPH V TN: NGNH: 1. TRN HIU TRUNG 2. DNG MINH TIN VIN THNG MSSV: 40602711 MSSV: 40602471 LP: DD06DV03

1. ti: LC NHIU TN HIU THOI DNG PHNG PHP EMD 2. Nhim v (Yu cu v ni dung v s liu ban u): .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. 3. Ngy giao nhim v lun vn: ........................................................................................... 4. Ngy hon thnh nhim v: ............................................................................................... 5. H v tn ngi hng dn: Phn hng dn 1. ............................................................ ................................................... 2. ............................................................ ................................................... Ni dung v yu cu LVTN c thng qua B Mn. Tp. HCM, ngy ..thng..nm 2011 CH NHIM B MN NGI HNG DN CHNH

PHN DNH CHO KHOA, B MN: Ngi duyt (chm s b): .......................................... n v:........................................................................ Ngy bo v: ............................................................... im tng kt: ............................................................ Ni lu tr lun vn: ..................................................

TRNG I HC BCH KHOA KHOA IN IN T ----- -----

CNG HA X HI CH NGHA VIT NAM c lp T do Hnh phc ----- -----Ngy .. thng .. nm 2011

PHIU CHM BO V LUN VN(Dnh cho ngi hng dn) 1. H V TN: TRN HIU TRUNG DNG MINH TIN NGNH: VIN THNG 2. ti: LC NHIU TN HIU THOI DNG PHNG PHP EMD 3. H tn ngi hng dn: PGS. TS. HONG NH CHIN 4. Tng qut v bn thuyt minh: S trang Bng s liu S ti liu tham kho S chng . S hnh v . Phn mm tnh ton . -MSSV: 40602711 -MSSV:40602471

5. Nhng u im chnh ca LVTN .. .. 6. Nhng thiu st ca LVTN ngh: c bo v Khng c bo v B sung thm bo v

7. Cu hi sinh vin tr li trc Hi ng a. b. c. d. e. 8. NH GI CHUNG (bng ch: GII, KH, TB)im: .. . K tn (ghi r h tn)

TRNG I HC BCH KHOA KHOA IN IN T

CNG HA X HI CH NGHA VIT NAM

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c lp T do Hnh phc ----- -----Ngy .. thng .. nm 2011

PHIU CHM BO V LUN VN(Dnh cho ngi phn bin) 1. H V TN: TRN HIU TRUNG DNG MINH TIN NGNH: VIN THNG 2. ti: LC NHIU TN HIU THOI DNG PHNG PHP EMD 3. H tn ngi hng dn: PGS. TS. HONG NH CHIN 4. Tng qut v bn thuyt minh: S trang Bng s liu S ti liu tham kho S chng . S hnh v . Phn mm tnh ton . -MSSV: 40602711 -MSSV:40602471

5. Nhng u im chnh ca LVTN . . .. 6. Nhng thiu st ca LVTN ngh: c bo v Khng c bo v B sung thm bo v

7. Cu hi sinh vin tr li trc Hi ng a. b. c. d. e. 8. NH GI CHUNG (bng ch: GII, KH, TB)im: K tn (ghi r h tn)

LI CM N -----Trc tin, chng ti xin gi li cm n chn thnh n PGS.TS. Hong nh Chin v cho chng ti c hi c lm vic trong lnh vc x l thoi, vi nhng hng dn tn tnh v y kinh nghim ca thy, cng nh nhng li ng vin, khuyn khch ti trong sut qu trnh lm lun vn. S tn tnh ca thy va l ngun ng lc va to cho chng ti s ham thch nghin cu khoa hc.Chng ti lun trn trng nhng hng dn v gi rt chuyn mn, s kin nhn v c bit s thn thin ca thy. K n, chng ti cng xin c tri n qu thy c khoa inin t, c bit l B mn Vin thng vung p cho chng ti nn tng kin thc c th thc hin lun vn ny. Sau na, chng ti cng gi li cm n n Anh V Tun Nam l ngi trc tip ch bo, hng dn tn tnh cho chng ti khi chng ti gp kh khn.V cng xin cm n hai bn H Hi Triu v Mai Vn nh, vn l nhng ngi cng lm v ti x l tn hiu thoi, v nhng trao i ca cc bn l ngun ng lc cho chng ti trong qu trnh nghin cu. Cui cng, chng ti xin cm n gia nh v bn b gip chng ti trong sut thi gian qua.

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TM TT

Nhiu nn l mt trong nhng vn gy kh khn nht trong h thng vin thng v cc h thng thoi v lm suy gim cht lng cng nh ni dung ca tn hiu thoi. Do , cn c mt phng php lc nhiu ci thin cht lng ca tn hiu thoi trong h thng. Mc ch ca vic lc nhiu tn hiu thoi l ci thin cm nhn ca ngi nghe hoc nng cao cht lng ca tn hiu thoi dng trong cc h thng nhn dng v tng hp ting ni ng dng trong lnh vc robot, thit b t ng c th giao tip vi con ngi bng ting niTrong sut hai mi nm gn y, vic nghin cu v pht trin rng ri ca h thng thng tin s m ra xu hng nghin cu vcc phng php lc nhiu tn hiu thoi trong lnh vc x l tn hiu. Vi mc ch , ni dung ca lun vn ny s trnh by mt s phng php lc nhiu tn hiu thoi da trn vic p dng mt s thut ton ly ngng trong min EMD (Empirical Mode Decomposition - M hnh thc nghim phn r). Trong t nhin, tn hiu thoi l phi tuyn(non-linear) v khng dng (non-stationary), v vy m hiu qu ca vic ci thin cht lng tn hiu thoi ph thuc rt ln vo phng php phn tch.Mc d bin i Fourier v phn tch Wavelet c nhng ng gp rt ln trong x l tn hiu, nhng cc phng php ny vn gp phi mt s hn ch vi tn hiu phi tuyn v khng dng.Gn y, phng php EMD c Huang [16], [17]a ra nh mt phng php phn tch mi m v hiu qu cho tn hiu phi tuyn v khng dng, m ra mt hng mi trong nghin cu vic ci thin cht lng thoi.V c bn, EMD l mt phng php phn tch dliu thch nghi cho cc dng d liu phc tp phn r chng thnh cc thnh phn dao ng c trung bnh khng (zero mean), gi l Hm dao ng ni ti IMF(Intrinsic mode function).Nhng nghin cu gn y pht hin rng chng ta c th xc nh thnh cng cc thnh phn nhiu t cc IMF ca tn hiu b can nhiu. Chnh v l do ny m vic ci thin tn hiu thoi da trn phng php EMD ngy cng c ch hn. Lc nhiu tn hiu thoi s dng phng php EMD c thc hin da vo thut ton ly ngng.y l mt thut ton lc nhiu c s dng rng ri trong rt nhiu cciii

phng php lc nhiu. tng ca thut ton ly ngng l xc nh mt mc ngng cho tng on ri tr vo thnh phn tn hiu c cho l nhiu.Tuy nhin, vic xc nh v loi tr cc thnh phn nhiu m vn khng lm suy gim thnh phn thoi ban u l mt vn khng h d dng. Do nhc im ln nht ca loi thut ton ny l lm suy gim tn hiu thoi gc, c bit l i vi tn hiu c t s tn hiu trn nhiu (SNR) cao. hn ch s suy gim ny, ngi ta p dng thut ton ly ngng mm p dng cho tng khong tn hiu.Cc IMF ca tn hiu thoi b can nhiu c lc nhiu bng cch p dng gii thut ly ngng mm i vi tng h s ca mi IMF. Vi gii thut ny th hu ht cc thnh phn nhiu b loi b thnh cng trong khi cc thnh phn thoi vn c bo ton.Tuy nhin tr ngi ln nht trong gii thut ly ngng mm ny l lm sao xc nh chnh xc mc ngng tng ng trong tng IMF. m vn m lun vn ny ang vp phi. Do h thng l thuyt ca phng php EMD cng nh thut ton lc nhiu trong min EMD cha hon chnh, vn ang trong qu trnh nghin cu nn lun vn ny trnh by mt s phng php ly ngng khc nhau trong min EMD xem xt hiu qu ca tng phng php.

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MC LC

Ni dungTrang Ph ba ..................................................................................................................................... i Li cm n ......................................................................................................................................... ii Tm tt .............................................................................................................................................. iii Mc lc .............................................................................................................................................. v Danh sch cc t vit tt ................................................................................................................... vii Danh sch cc bng.......................................................................................................................... viii Danh sch cc hnh ............................................................................................................................. x Chng 1:TNG QUAN ................................................................................................................. 1 1.1Gii thiu chung........................................................................................................................ 1 1.2 Mt s ng dng ca vic lc nhiu tn hiu thoi..................................................................... 2 1.3 Tnh cp thit ca ti nghin cu .......................................................................................... 2 1.4 Cc phng php lc nhiu tn hiu thoi ................................................................................. 3 1.4.1 Phn loi ........................................................................................................................... 3 1.4.2 Tr ph ............................................................................................................................. 4 1.4.3 Ly ngng Wavelet ......................................................................................................... 6 1.4.4 Ly ngng DCT............................................................................................................... 8 1.4.5 Hng nghin cu ca ti.............................................................................................. 9 1.5Mc tiu nghin cu .................................................................................................................10 1.5.1 Mc tiu tng qut ............................................................................................................10 1.5.2 Mc tiu c th .................................................................................................................10 1.6 Tnh mi v nhng ng gp ca ti ...................................................................................10 1.7i tng v phm vi nghin cu .............................................................................................10 1.8 Phng php nghin cu .........................................................................................................11 1.9 Pht biu bi ton ....................................................................................................................11 Chng 2 GII THIU EMD ........................................................................................................13 2.1 Gii thiu ................................................................................................................................13 2.2 Khi nim c bn ca EMD.....................................................................................................13 2.2.1 Hm ch ni ti IMF ....................................................................................................13 2.2.2 Qu trnh chn lc ............................................................................................................15 2.3 Nng lng nhiu ca cc IMF ................................................................................................18

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2.4 EMD cho tn hiu thoi ...........................................................................................................20 Chng 3 LC NHIU TRONG MIN EMD ..............................................................................22 3.1 Gii thiu ................................................................................................................................22 3.2 Ly ngng EMD da trn c tnh trong khong gia ca hai im v khng ........................24 3.2.1 Ly ngng EMD IT .....................................................................................................24 3.2.2 Ly ngng EMD - SIT ....................................................................................................28 3.2.3 Ly ngng EMD - IIT.....................................................................................................32 3.2.4 EMD - CIIT ......................................................................................................................34 3.3 Ly ngng EMD - SST..........................................................................................................36 3.4 Ly ngng mm kt hp DCT - EMD....................................................................................40 3.4.1 Ly ngng mm DCT tng th nht .............................................................................42 3.4.2 Ly ngng mm EMD tng th hai ..............................................................................42 Chng 4 THC HIN M PHNG ...........................................................................................45 4.1 Cc thng s m phng ...........................................................................................................45 4.1.1 Khung tn hiu ..................................................................................................................45 4.1.2 Phng sai nhiu ca cc IMF ..........................................................................................46 4.1.3 Vector ly ngng ............................................................................................................46 4.2 Kt qu m phng ...................................................................................................................47 4.2.1 Cc phng php ly ngng trn tng khong v khng. ................................................47 4.2.2 Phng php ly ngng EMD - SST ...............................................................................51 4.2.3 Phng php ly ngng kt hp DCT EMD .................................................................52 4.3 Kho st cc thng s nh hng n cht lng tn hiu.........................................................53 4.3.1 nh hng ca vector ly ngng ....................................................................................54 4.3.2. nh hng ca thng s M1, M2.....................................................................................57 4.3.3. nh hng ca h s lp k ...............................................................................................59 4.3.4. nh hng ca cch chuyn i ngu nhin v tr cc mu ca IMF u tin trong phng php lp ....................................................................................................................................61 Chng 5 KT LUN V HNG PHT TRIN .....................................................................63 5.1 Kt lun ..................................................................................................................................63 5.2 Hng pht trin .....................................................................................................................65 TI LIU THAM KHO .............................................................................................................67

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DANH SCH CC T VIT TT

CIIT DCT DFT DWT EMD IIT IMF iSNR IT MAD oSNR SCAD SIT SST

Clear Iterative Interval- Thresholding Discrete Cosine Transform Discrete Fourier Transform Discrete Wavelet Transform Empirical Mode Decomposition Iterative Interval- Thresholding Intrisic Mode Function Signal to Noise Ratio input Interval Thresholding Median Absolute Deviation Signal to Noise Ratio output Smoothly Clipped Absolute Deviation Soft Interval Thresholding Subframe - Soft Thresholding

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DANH SCH CC BNGChng 4 Bng 4.1: So snh kt qu SNR u ra ca 3 phng php ly ngng trn tng khong v 0 ............................................................................................................... 48 Bng 4.2: So snh kt qu SNR u ra ca phng php EMD CIIT da trn 3 thut ton ly ngng l IT, SIT, v SCAD. ....................................................................... 50 Bng 4.3: So snh kt qu SNR u ra ca phng php ly ngng EMD SST v ly ngng kt hp DCT - EMD............................................................................... 53 Bng 4.4:nh hng ca thng s C i vi phng php ly ngng cng trn tng khongvkhng (EMD - IT)....................................................................................... 54 Bng 4.5: nh hng ca thng s C i vi phng php ly ngng cng trn tng khongvkhng (EMD SIT) .................................................................................... 54 Bng 4.6: nh hng ca thng s C i vi phng php ly ngng EMD SCAD .................................................................................................................................. 55 Bng 4.7: nh hng ca thng s i vi phng php ly ngng EMD SST .. 55 Bng 4.8: nh hng ca thng s i vi phng php ly ngng kt hp DCT EMD .......................................................................................................................... 56 Bng 4.9: nh hngca M2 i vi phng php ly ngng cng trn tng khongvkhng (EMD - IT)....................................................................................... 57 Bng 4.10: nh hngca M2 i vi phng php ly ngng mm trn tng khongv khng EMD - SIT) ..................................................................................... 58 Bng 4.11: nhhngca M2 i vi phng php ly ngng mm SCAD trntngkhongvkhng (EMD - SCAD) ................................................................... 58 Bng 4.12: nhhngthngs Ki vi phng php ly ngng cng .................... 59 Bng 4.13: nhhngthngs Ki vi phng php ly ngng mm ................... 60viii

Bng 4.14: nhhngthngs Ki vi phng php ly ngng mm SCAD ......... 60 Bng 4.15: nhhngcchhonimui vi phng php ly ngng cng .......... 62

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DANH SCH CC HNHChng 2 Hnh 2.1: Mt loi IMF vi s cc tr v s zero crossing bng nhau, v cc bin trn v bin di i xng nhau c trung bnh bng 0....................................................... 14 Hnh 2.2: Qu trnh chn lc... ................................................................................... 15 Hnh 2.3: EMD ca tn hiu thoi b can nhiu c SNR l 10dB v 8 IMF u tin thm phn d hng s. ............................................................................................... 17 Hnh 2.4: Tn s tc thi ca cc IMF ....................................................................... 18 Hnh 2.5: ng cong din t mi quan h gia nng lng ca cc IMF tng ng vi qu trnh EMD s dng s ln chn lc t 1 n 15. ng cong mu c th hin xut ca Flandrin v cng s. ........................................................................ 19 Hnh 2.6: So snh v nng lng gia m hnh cc IMF ch c nhiu v IMF thc t .................................................................................................................................. 20

Chng 3 Hnh 3.1: Thut ton ly ngng da vo c tnh ca khong gia hai im v 0 a) trn mt IMF; b) trn mt on ca IMF trc v sau khi ly ngng. ...................... 26 Hnh 3.2: Phng php EMD IT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu so vi tn hiu sch ban u ............................ 27 Hnh 3.3: Phng php EMD IT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu ............................ 28 Hnh 3.4: Phng php EMD SIT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu so vi tn hiu sch ban u ............................ 29 Hnh 3.5: Phng php EM SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu ............................ 29 Hnh 3.6: Phng php EMD SCAD trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu so vi tn hiu sch ban u ............................ 31

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Hnh 3.7: Phng php EMD SCAD trn tn hiu thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu. ........................... 31 Hnh 3.8: Phng php EMD IIT ly ngng da vo EMD SIT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; v tn hiu c lc nhiu so vi tn hiu sch ban u lp vi b) K=3; c) K=20. ............................................................... 33 Hnh 3.9: Phng php EMD IIT ly ngng da vo EMD SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu vi K=10. .......................................................................................... 34 Hnh 3.10: Phng php EMD - CIIT: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu so vi tn hiu sch ban u lp vi K=15. ................................. 35 Hnh 3.11: Phng php EMD CIIT ly ngng da vo EMD SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu vi K=10. ................................................................................... 36 Hnh 3.12: S khi ca phng php ly ngng EMD SST.............................. 37 Hnh 3.13: Phng php ly ngng EMD - SST: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu. ...................................... 39 Hnh 3.14: S khi ca phng php ly ngng mm EMD DCT .................... 41 Hnh 3.15: Phng php kt hp DCT EMD vi tn hiu thoi thc t: a) Tn hiu sch; b) Tn hiu b can nhiu; Tn hiu c lc nhiu bng phng php: c) DCT; d) DCT EMD ................................................................................................................. 43

Chng 4 Hnh 4.1: Kt qu lc nhiu trn tng khong v khng: a) tn hiu sch; b) tn hiu b can nhiu; Tn hiu sau khi lc nhiu bng phng php: c) EMD - IT; d) EMD SIT; e) EMD SCAD........................................................................................................ 47 Hnh 4.2: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD - IT: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=10. .......................... 49 Hnh 4.3: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD - SIT: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=10. .......................... 49

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Hnh 4.4: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD - SCAD: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=10 .................... 50 Hnh 4.5: Kt qu lc nhiu bng phng php ly ngng EMD - SST: a) tn hiu sch; b) tn hiu b can nhiu c SNR=0dB (I) v SNR=10dB (II); Tn hiu sau khi lc nhiu. ......................................................................................................................... 51 Hnh 4.6: Kt qu lc nhiu bng phng php ly ngng kt hp DCT - EMD: a) tn hiu sch; b) tn hiu b can nhiu c SNR=0dB (I) v SNR=10dB (II); Tn hiu sau khi lc nhiu. ............................................................................................................. 52

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Chng 1: Tng quan

GVHD: PGS. TS. Hong nh Chin

Chng 1

TNG QUAN

1.1 Gii thiu chung Trong nhiu h thng, a phn tn hiu thoi u b tc ng rt ln t cc ngun can nhiu. Nhng ngun can nhiu ny lm suy gim cht lng cng nh ni dung ca tn hiu thoi gc, do lm suy gim thm ch l mt i kh nng thc thi cc ng dng ca chng trong h thng. Cc ngun can nhiu ny c th l nhiu bng rng vi hnh thc nhiu trng hoc nhiu mu, hoc cc tn hiu nh ting vo ve, ting n trong mi trng ng ngi, ting ng c, ting ma hay mt dng ca nhiu fading. Mt tn hiu thoi c th b tc ngng thi mt hoc nhiu ngun can nhiu khc nhau. Trong , dng nhiu ph bin nht v kh loi tr nht l nhiu trng. l l do m trong lun vn ny chng ti tp trung vo loi nhiu ny. Nh ni, s nh hng ca nhiu nn ln tn hiu thoi lm suy gim cht lng cng nh tnh kh thi ca h thng thoi.Do vy m vic lc nhiu c t ra l cn thit ci thin cht lng tn hiu thoi. Vic ci thin cht lng thoi phi gip nng cao cht lng thnh gic v bo ton ni dung ca tn hiu thoi m thit b hay ngi nghe c th hiu c trong mi trng nhiu, ch yu thng qua cc gii thut lc nhiu. Cc loi gii thut nh vy c th c ng dng cho cc h thng thng tin di ng, h thng nhn dng v tng hp ting ni ng dng trong lnh vc robot, thit b t ng c th giao tip vi con ngi bng ting ni c trng ca mt h thng c nh gi qua hai tiu ch l cht lng v tnh hiu c (ni dung) ca thng tin.Cht lng ca tn hiu thoi sau khi ci thin lin quan ti trong ca tn hiu, mc d nhiu ca tn hiu Hu ht cc phng php ci thin cht lng thoi u nng cao c cht lng ca tn hiu tuy nhin li lm gim i tnh hiu c ca n, ngha l mc hay s t m ngi nghe nhn ra c khi nghe.Thm ch ch bng cch lng nghe k hn ngi nghe c th nhn c nhiu SVTH: Trn Hiu Trung Dng Minh Tin 1

Chng 1: Tng quan


thng tin t tn hiu b can nhiu hn l tn hiu c ci thin. Chnh v vy, chng ta cn tm ra thut ton m bo y hai c trng ny nng cao cht lng ca h thng thng tin thoi. 1.2 Mt s ng dng ca vic lc nhiu tn hiu thoi Trong k thut thng tin ngy nay, c nhiu lnh vc m ci thin cht lng thoi c s dng nng cao cht lng ca h thng H thng nhn v tng hp ting ni: h thng lm gim nh hng ca nhiu nn cng nh cc ngun can nhiu. H thng p dng trong vic giao tip gia ngi v my, h thng li xe t ng... y l lnh vc s pht trin mnh m v ph bin trong tng lai, khi , gia ngi v my mc, robot s giao tip trc tip bng ting ni. Vin thng: vic ci thin cht lng thoi tp trung vo vic loi b cc thnh phn nhiu nn trong h thng. Ngoi ra, h thng vin thng cn c mt vn khc nm nhiu knh. Bng cch ci thin cht lng thoi trc khi a n vo knh truyn, h thng c th gim c nh hng ca nhiu knh. Cc h thng tr thnh s: Ngi nghe thng kh hiu c thng tin thoi trong mi trng nhiu ln nh ngoi ng, trong nh my, trn tu thuyn Do , h thng ny l phn rt quan trng nng cao s cm nhn thng tin thoi trong mi trng nhiu cho ngi nghe. 1.3 Tnh cp thit ca ti nghin cu Nh ni trn, yu cu xy dng mt phng php lc nhiu mnh m, thch hp cho tn hiu phi tuyn v khng dng nh tn hiu thoi ang ngy cng tr nn cp thit.Cho n nay, c rt nhiu phng php c dng ci thin cht lng tn hiu nh cc phng php ly ngng trong min Fourier, Wavelet, v bin i cosine ri rc -DCT (Discrete Cosine Transform).Mc d cc phng php ny c nhng ng gp rt ln trong x l tn hiu, nhng chng vn gp phi mt s hn ch vi tn hiu phi tuyn v khng dng nh tn hiu thoi.Gn y, phng php lc nhiu da trn thut ton ly ngng trong min EMD c nh gi nh l mt phng php SVTH: Trn Hiu Trung Dng Minh Tin

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Chng 1: Tng quan


phn tch mi v mnh m cho tn hiu phi tuyn v khng dng, m ra mt hng mi trong nghin cu lnh vc ci thin cht lng thoi. V c bn, EMD c u im l thch nghi hon ton vi d liu phn tch chng thnh cc thnh phn dao ng c trung bnh khng (zero mean), gi l Hm dao ng ni ti (IMF - Intrinsic mode function).Nhng nghin cu gn y pht hin rng chng ta c th xc nh thnh cng cc thnh phn nhiu t cc IMF ca tn hiu b can nhiu vi phng php EMD. C th vi trng hp nhiu trng, cc thnh phn nhiu thng ch tp trung 3 IMF u tin.Chnh v l do ny m vic lc nhiu da trn phng php ly ngng trong min EMD ngy cng c nghin cu rng ri hn. 1.4 Cc phng php lc nhiu tn hiu thoi 1.4.1 Phn loi Cho ti nay, c rt nhiu phng php c a ra cho ci thin cht lng thoi.Cc thut ton c a ra c th phn thnh 2 lp chnh l cc phng php c tham s (parametric) v khng c tham s (non-parametric). Phng php c tham s gi nh mt m hnh cho qu trnh to ra tn hiu. M hnh ny m t cc cu trc c tin on trc v cc mu c th quan st c trong qu trnh hnh thnh tn hiu. Vic gim nhiu c thc hin ph thuc vo thng tin theo cch din dch ny. Do vic lc nhiu da trn m hnh tham s nn vic la chn m hnh l quan trng nht trong nhng thut ton ny. Phng php khng c tham s th ch cn c lng nhiu, thng thng l phng sai nhiu. Ph nhiu ny c th c c lng t khong ngng m ngi ni im lng (n knh) hoc mt ngun tham chiu (a knh). Tuy nhin, vic phn loi thnh lc nhiu n knh v lc nhiu a knh li c s dng ph bin hn.Phn bit c bn gia lc nhiu thoi n knh v a knh l da trn s lng knh v loi tn hiu c th quan st c.

SVTH: Trn Hiu Trung Dng Minh Tin

3

Chng 1: Tng quan


Lc nhiu n knh ch dng mt ngun m (thanh) v do ch c mt ngun b can nhiu a ra cc thng tin ph. Ph nhiu ny phi c c lng t khong ngng v nhiu ny c xem nh n nh v khng gin on. iu ny lm cho s ci thin cht lng thoi n knh gp nhiu thch thc. l l do ti sao cht lng ca k thut n knh b hn ch. Lc nhiu a knh dng ng thi nhiu ngun m hot ng trong mi trng, dn ti c nhiu ngun b can nhiu. Nhiu ngun b can nhiu ny a ra u im ca vic a vo thng tin c min khng gian m min ph. Tuy nhin, v nhiu ngun m (thanh) s tc ng v gi thnh. Trong lun vn ny, chng ti ch trnh by cc phng php lc nhiu n knh v khng c tham s.Do , phn tip theo chng ti xin gii thiu tng qut v mt s phng php lc nhiu trong nhm ny. 1.4.2 Tr ph Tr ph l mt trong nhng phng php c in nht v cng l k thut ph bin nht c a ra nhm gim thiu nh hng ca nhiu nn.K thut tr ph sm nht c a ra vo nm 1974 bi ng Weiss.Nh ni trn, tr ph l mt phng php khng c tham s nn ch i hi mt thut ton c lng v ph nhiu.Trong trng hp n knh, s c lng ny c thc hin trong cc khong thi gian m ngi ni im lng, c xem nh cc khong ngng. tng chnh ca phng php ny l c lng ph ca tn hiu nhiu v loi tr n khi tn hiu b can nhiu thu c tn hiu sch. Do ph nhiu c c lng t cc khong ngng v c s dng cho ton b d liu nn k thut tr ph ch thch hp vi cc ngun nhiu n nh hoc thay i rt chm s thay i trong ph cng sut nhiu c cp nht. Gi s rng tn hiu thoi sch ngun b can nhiu: (1.1) Dng bin i Fourier thu c: (1.2) SVTH: Trn Hiu Trung Dng Minh Tin b can nhiu bi ngun nhiu to ra mt

4

Chng 1: Tng quan


Nhn hai v cho lin hip phc ca chng ta c: (1.3) Trong l sai pha gia tn hiu thoi v nhiu (1.4) Ly gi tr k vng ca hai v ta c:

(1.5) Cng thc trn cn hai gi thuyt: 1. Gi tr ca ph bin ca nhiu v thoi l c lp vi nhau. 2. Pha ca nhiu v thoi l c lp vi nhau v c lp vi bin ca chng. C kh nhiu phng php da trn l thuyt tr ph, chng c th c chia thnh 2 lp chnh c trnh by sau y. 1.4.2.1 Tr ph cng sut Vi phng php tr ph cng sut, chng ta gi s rng (1.5) bng 0, ta s c: (1.6) Mt ph cng sut c c lng t khong lng ca ting ni v gi s rng s bin thin mt ph nhiu nm trong gii hn cho php, n s c tr ra khi mt ph tn hiu thoi b can nhiu thu c tn hiu thoi c ci thin (1.7) 1.4.2.2 Tr ph bin Vi phng php tr ph bin , gi s rng , do : trong cng thc


5

Chng 1: Tng quan


Suy ra: (1.8) Mt ph bin nhiu c ly trung bnh trong sut khong lng ca ting ni v vi gi s rng nhiu l c nh, th mt ph bin ca tn hiu thoi s c c lng bng cch tr i mt ph trung bnh ca nhiu nn t mt ph bin ca tn hiu b can nhiu. (1.9) 1.4.3 Ly ngng Wavelet Bin i wavelet c s dng rng ri trong nhiu lnh vc x l tn hiu.Do nhng u im ca n trong vic s dng cc ca s thi gian c th thay i kch thc cho mi bng tn khc nhau, bin i Wavelet tr thnh mt cng c mnh m cho dng tn hiu khng dng nh tn hiu thoi. Hn na, trong trng hp n knh, nhn chung, vic s dng qu trnh x l bng con (sub-band) cng t kt qu cht lng tt hn. Do , php bin i wavelet c th cung cp mt m hnh xp x tn hiu thoi cho vic lc nhiu. Vi tn hiu thoi b can nhiu, mc nng lng ca thoi sch thng tp trung mt wavelet c kch thc (ca s) nh. Mc khc, nng lng nhiu li tri khp mt lng ln cc h s. l l do ti sao cc h s ca khung c kch thc nh c em so snh mt cch tng i vi cc h s ca khung khc. V vy, bng cch xc nh mc ngng v gn cc h s nh hn v 0, chng ta c th lc nhiu gn nh ti u trong khi vn duy tr c thng tin quan trng ca tn hiu thoi. Gi s rng tn hiu thoi sch c c: b can nhiu bi nhiu trng c phng sai

ging nh cng thc (1.1). Bng cch p dng phn tch Wavelet ri rc, chng ta


6

Chng 1: Tng quan


(1.10) Trong biu din php bin i wavelet.Cc h s wavelet ca tn hiu b can

nhiu c ly ngng thu c tn hiu sch ban u. Vic ly ngng c th c thc hin bng nhiu cch v c nhiu k thut khc nhau c a ra p ng mc ch ny.Tuy nhin, hai thut ton ph bin nht l ly ngng cng v ly ngng mm. 1.4.3.1 Ly ngng cng Xt l h s ca khung wavelet thu c t tn hiu b can nhiu v l mc

ngng ca thut ton lc nhiu. Thut ton ly ngng cng gn bt k h s no c gi tr tuyt i nh hn hoc bng mc ngng v 0: (1.11) Trong l h s c ly ngng.Gi tr ca ph thuc vo lch chun

c c lng t tn hiu nhiu v c th thay i ph thuc vo cc thut ton c a ra.Theo Donoho [3], [4], gi tr ca c a ra nh sau: (1.12) Vi l kch thc ca khung wavelet.

1.4.3.2 Ly ngng mm Thut ton ly ngng mm gn cc h s c gi tr tuyt i nh hn hoc bng mc ngng v 0, v cc h s ln hn mc ngng b tr i gi tr ngng. (1.13) Thut ton ly ngng mm ny c s dng rng ri v cng thch hp cho phng php tr ph, v phn tch Fourier.C th thy rng, thut ton ly ngng mm cth loi b cc thnh phn nhiu tt hn thut ton ly ngng cng.Tuy nhin thut ton ny cng lm suy hao thnh phn tn hiu cao hn.


7

Chng 1: Tng quan


1.4.4 Ly ngng DCT Trong hai thp k gn y, phng php bin i cosine ri rc (DCT) ang ni ln nh mt phng php phn tch ph bin trong nhiu h thng x l nh, m thanh v c video.V bn cht DCT l php bin i Fourier ri rc (DFT) nhng ch s dng cc s thc. Gi s rng tn hiu thoi sch hiu vi php bin i DCT, chng ta c c: b can nhiu bi nhiu trng to ra tn

, ging nh cng thc (1.1). Bng cch p dng

Vi

v (1.15)

Cng nh wavelet, php bin i DCT cng da vo hai phng php ly ngng chnh l ly ngng cng v ly ngng mm. 1.4.4.1Ly ngng cng Thut ton ly ngng trong min DCT c thc hin tng t nh trong wavelet.Thut ton ny cng gn cc h s DCT nh hn mc ngng v khng theo cng thc (1.11) vi c th thay i ty theo thut ton xut.

1.4.4.2 Ly ngng mm Thut ton ly ngng mm tng t nh cng thc (1.13).Tuy nhin, trong phn ny chng ti s trnh by mt gii thut khc.Thut ton ny da vo vic phn tn hiu thnh cc on trong khong thi gian ngn v p dng php bin i DCT trn tng khung thay v trn ton tn hiu.Cc h s DCT ca mi khung c phn thnh 8 khung con, mi khung gm c L h s DCT. xc ly ngng thch hp, tng khung con c phn thnh khung c tn hiu tri hoc c nhiu tri ph thuc vo mt


8

Chng 1: Tng quan


nng lng thoi v nhiu.Nu mt khung tha iu kin (1.16) th khunng c gi l khung c tn hiu tri v ngc li gi l khung c nhiu tri.

Vi L l s h s DCT trong mt khung tn s, con,

l h s DCT th Ktrong khung

l phng sai nhiu.

Khi khung tn s c phn loi, vic ly ngng s c p dng khc nhau. y, khung c nhiu tri c ly ngng theovector tuyn tnh thay cho gi tr hng s hn ch mt mc thng tin thoi trong khung. Cn vi cc khung c tn hiu tri, vic ly ngng c th gy tc ng ngc.V vy m i vi khung tn hiu tri ny tt nht l gi nguyn, khng ly ngng. Trong trng hp khung c nhiu tri, u tin cc gi tr tuyt i ca cc h s DCT c sp xp theo th t tng dn v p dng ly ngng tuyn tnh nh sau: (1.17) Trong , ca l hm ngng tuyn tnh v l h s sp xp . Gi tr c lng

c th c c lng nh sau: [5], [6]

Vi

, v

l gi tr hng s c c lng cho ph hp vi mc SNR ca

tn hiu u vo. 1.4.5 Hng nghin cu ca ti Nh ni trn, mc d phng php lc nhiu a knh cho hiu qu cao hn lc nhiu n knh, nhng trong thc t n li t c s dng hn v cc ngun thoi a knh i hi chi ph cao v khng phi lun lun c th lp t c mi mi trng. Do , lun vn ny i theo hng gii quyt bi ton lc nhiu thoi n SVTH: Trn Hiu Trung Dng Minh Tin

9

Chng 1: Tng quan


knh. Bn cnh , EMD l mt phng php phn tch d liu cn kh mi; vi mc ch n gin, lun vn chn phng php khng tham s lm c s.Tm li, lun vn ny gii quyt bi ton lc nhiu thoi n knh v khng tham s bng thut ton ly ngng trong min EMD. 1.5 Mc tiu nghin cu 1.5.1 Mc tiu tng qut Mc ch ca ti Lc nhiu tn hiu thoi dng phng php EMDl xut thut ton ly ngnghiu qu trong min EMDi vi tn hiu thoi b can nhiu v qua chng minh kh nng mnh m ca EMD trong vic phn tch cc tn hiu phc tp nh thoi. Tn hiu quan st c l mt knh thoi b can nhiu bi ngun nhiu c phn b Gausse, tng tc cng v khng tng quan vi thoi, trong tp trung vo trng hp nhiu trng. Trn c s nghin cu thut ton ly ngng trong min EMD v m phng, ngi thc hin xut mt s gii php ci thincht lng thoi vi ngun thoi c SNR thay i trong mt khong rng. 1.5.2 Mc tiu c th - Gp phn h thng ha c s l thuyt v thc tin v ng dng EMD vo lnh vc lc nhiu thoi. - xut mt s phng php lc nhiu thoi tm ra phng php c hiu qu cao. - nh gi kh nng ng dng thut ton nghin cu vo thc tin. 1.6 Tnh mi v nhng ng gp ca ti Vic lc nhiu thoi trong min EMD hin vn cn l gii php mi, ang trong qu trnh nghin cu v cha tht s hon thin v h thng l thuyt cng nh gii php thc hin.Vi ti ny, chng ti hy vng xut c mt gii php c hiu qu cao, bn vng v ng dng c trong thc tin.

1.7 i tng v phm vi nghin cu


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Chng 1: Tng quan


Khch th nghin cu l ting ni con ngi c s ha v c cng vi nhiu Gausse. Lun vn tm hiu v cc c trng ca ting ni v nhiu t tm ra cch thc tch nhiu nn ra khi thoi thng qua cng c EMD. i tng nghin cu l vn kh nhiu c phn b Gausse, tng tc cng v khng tng quan ra khi tn hiu hn hp vi gi nh l tn hiu c th quan st c ch l mt knh thoi b can nhiu. 1.8 Phng php nghin cu C s tch nhiu ra khi thoi l s khc bit trong phn b nng lng ca thoi v nhiu trong min tn s: thoi c phn b nng lng tp trung trong vng tn s thp v trung bnh; trong khi , nhiu trng c phn b nng lng u trn ton b vng tn s. Lun vn t gi thuyt l da trn kh nng nhn dng cc thnh phn tn s ca EMD, chng ta c th c lng c nhiu t tn hiu thoi b can nhiu t c th lc nhiu mt cch c hiu qu bng thut ton ly ngng trong min EMD. Lun vn c thc hin vi gi thit rng d liu quan st c l mt knh thoi b can nhiu trong nhiu c phn b Gausse, tng tc cng v khng tng quan vi thoi gc. Nghin cu l thuyt v cc phng php lc nhiu thoi, trong tp trung vo trng hp lc nhiu n knh v khng tham s.Tip , tc gi hiu chnh cc thut ton lc nhiu ny cho ph hp vi cc c trng ca EMD.Cui cng, cc kt qu nghin cu c thc nghim v kim chng thng qua m phng vi c s d liu NOIZUES v cc file do ngi Vit ghi m. Qu trnh nghin cu l ngi thc hin bt u tin hnh nghin cu c s l thuyt hn lm da trn cc ti liu thu thp c, sau tin hnh m phng kim chng l thuyt trn my vi tnh qua cng c MATLAB. 1.9 Pht biu bi ton Vi tn hiu b can nhiu, chng ta c th kh cc thnh phn nhiu nm ngoi bng thoi mt cc d dng. Tuy nhin, ti cc tn s trong bng thoi s tn ti c thoi v nhiu, nn rt kh xc nh v loi b cc thnh phn nhiu m khng lm suy bin tn hiu thoi.Do , chng ta kh m xc nh v lc nhiu trong min ph vi cc thnh SVTH: Trn Hiu Trung Dng Minh Tin

11

Chng 1: Tng quan


phn tn s nm trong bng tn thoi ti mt thi im tc thi. Chnh v iu ny m ti ny s tm hiu mt phng php phn tch c th tch c nhiu v thoi trong min thi gian. EMD (Empirical Mode Decomposition) c xem l phng php phn tch d liu mi v mnh m cho cc tn hiu phi tuyn v khng dng, m ra mt hng mi v c hiu qu cho vic lc nhiu thoi.V c bn, EMD l phng php phn tch d liu thch nghi vn phn tch mt tp d liu bt k thnh cc thnh phn dao ng c trung bnh bng khng, gi l cc IMF (Intrinsic Mode Funcion). Cc IMF ny gip xc nh tn s tc thi.Cc IMF c th c tn s chng ln ln nhau, nhng ti mt thi im tc thi, tn s tc thi vn c biu din qua IMF s khc nhau.Do EMD khng ging nh b lc thng di, m l php phn tch tn hiu phi tuyn v khng dngtheo cc c tnh tn s cc b ca chng. Nh tnh cht ny, trong trng hp tn hiu b can nhiu, EMD gip chng ta c th tch c cc thnh phn nhiu n cha trong thoi sch. Mi IMF vn s cha c cc thnh phn thoi v nhiu nhng mt v phn b ca chng theo thi gian li khc nhau. Do , chng ta c th nhn dng mt cch c hiu qu cc thnh phn nhiu. Trong ti ny, chng ti xut thut ton ly ngng trong min EMD cho ng dng lc nhiu thoi. Lun vn ny c trnh by theo b cc nh sau: Chng 2 - Gii thiu EMD, vn trnh by cc vn c bn trong phng php phn tch EMD; Chng 3 Ly ngng trong min EMD, trnh by cc thut ton ly ngng khc nhau trong min EMD; Chng 4 - M phng, trnh by cch thc hin m phng v cc kt qu m phng; v Chng 5 - Kt lun, a ra kt lun v hng pht trin ca ti.


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Chng 2: Gii thiu EMD


Chng 2

GII THIU EMD

2.1 Gii thiu Nh chng ta bit, phn ln tn hiu thoi u l phi tuyn v khng dng nn vic phn tch cng nh x l s phc tp hn so vi tn hiu tuyn tnh v n nh.Hin ti c kh nhiu phng php c p dng cho vic ci thin tn hiu thoi.Nh ni chng 1, cc phng php ny c phn thnh cc phng php c tham s hoc khng c tham s, v n knh hoc a knh.Lun vn ny ch gii hn phng php khng tham s v n knh. Trong nhm ny, c rt nhiu phng php c a ra t rt sm nh ni trn...V gn y, m hnh thc nghim phn r EMD (Empirical Mode Decomposition) c Huang a ra ln u tin vo nm 1996 phn tch tn hiu phi tuyn v khng dng. EMD l mt k thut phn r tn hiu thnh cc m hnh c c tnh tn s khc nhau, c gi l Hm ch ni ti IMF (Intrinsic Mode Function). Chng ny s a ra cc vn trong EMD phn tch cc tn hiu phi tuyn v khng dng mt cch hiu qu. 2.2 Khi nim c bn ca EMD M hnh phn r thc nghim EMD gn y c pht trin phn r tn hiu khng dng v phi tuyn thnh cc thnh phn dao ng tun theo cc c tnh c bn c gi l Hm ch ni ti IMF 2.2.1 Hm ch ni ti IMF Nguyn l ca k thut EMD da trn vic phn r mt tn hiu mn hai iu kin c bn: bt k thnh mt

tp hp cc thnh phn (hm) dao ng trung bnh khng, gi l IMF.Mi IMF tha


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(1) Trong ton b d liu, s cc tr v s im v khng (zero crossing) phi bng nhau hoc ch khc nhau nhiu nht mt im. (2) ti mt im bt k, gi tr trung bnh ca ng bao c xc nh theo cc cc i v ng bao c xc nh bi cc tiu bng khng. iu kin u tincng tng t nh yu cu bng hp cho mt tin trnh Gaussian n nh. iu kin th hai l mt yu cu cc b c to ra t mt yu cu ton cc, v cn thit m bo rng cc tn s tc thi s khng c cc bin ng vt l khng mong mun gy ra bi dng sng bt i xng. Tn ca IMF - hm ch ni ti, c t ra l do n th hin m hnh dao ng c nhng vo d liu. Vi nh ngha ny th IMF c xc nh bi cc im zero, n ch bao gm ch dao ng m khng phi l cc dng sng li phc tp. IMF khng b gii hn vi tn hiu bng hp; n c th c iu chnh theo c bin v tn s, v thc t th IMF l mt hm khng dng. Hnh 2.1 th hin mt loi IMF.

Hnh 2.1: Mt loi IMF vi s cc tr v s zero crossing bng nhau, v cc bin trn v bin di i xng nhau c trung bnh bng 0. tng tm kim cc IMF da trn vic tr i cc thnh phn dao ng ln nht t d liu vi qu trnh tng bc c gi l qu trnh chn lc (Sifting process).


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2.2.2 Qu trnh chn lc Mc d mt m hnh ton hc vn cha c pht trin, nhng c nhiu phng php khc cho vic tnh ton EMD c xut.Thut ton sm nht c gi l qu

Hnh 2.2: Qu trnh chn lc, a) d liu gc; b) ng bin trn c v bng nt chm gch, v ng bin trung bnh lin m; (c) , l s khc nhau gia d liu gc v IMF, khi c cc cc i m v cc tiu dng.

v bin di c v bng nt

. y vn cha l mt


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trnh chn lc, c a ra tm cc IMF cad liu. Qu trnh chn lc rt n gin v kho lo. N bao gm cc bc sau: 1) Xc nh cc tr (c cc i v cc tiu) ca 2) To ra bin trn v bin di ( 3) Xc nh trung bnh cc b 4) Do IMF cn phi c trung bnh cc b zero, tnh 5) Kim tra xem 6) Nu l mt IMF hay khng (da vo c trng ca IMF) v ) bng cch ni cc im cc i v cc tiu bi ni suy ng trn bc 3 (Cubic spline interpolation)

khng phi l IMF, s dng n nh l d liu mi v lp li bc 1

n bc 6 cho n khi nhn mt IMF. Khi mt IMF u tin l chu k nh nht trong c suy ra, n c nh ngha l ,

. tnh ton cc IMF cn li, ta tnh tn hiu cn d

. By gi phn d cha cc thng tin v cc thnh phn ca khong thi gian ln hn.Qu trnh chn lc s c tip tc cho n khi phn d cui cng l mt hng s, mt hm n iu, hoc mt hm ch c mt cc i v cc tiu m khng c mt IMF no c th c suy ra.Cc IMF tip theo v cc phn d c tnh nh: (2.1) Khi kt thc phn r, cc d liu s(t) s c biu din nh l mt tng ca n tn hiu IMF cng vi mt tn hiu phn d, m ni chung l mt hng s hay mt xu hng n iu: (2.2) Mt tn hiu thoi b can nhiu v mt vi thnh phn IMF c v trong hnh 2.3. Ta c th thy rng cc IMF c s th t ln hn th cha thnh phn dao ng tn s thp hn cc IMF th t nh hn. iu ny l hp l v qu trnh chn lc da vo


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tng t vic tr i thnh phn c chu k ln nht t d liu n khi thu c mt IMF.

Hnh 2.3: EMD ca tn hiu thoi b can nhiu c SNR l 10dB v 8 IMF u tin vi phn d hng s. Do , IMF u tin s cha thnh phn dao ng cao ln nht, l cc thnh phn c tn s cao nht. V kt qu l, cc IMF c th t cao hn th s cha cc thnh phn tn s thp hn.Tuy cc IMF c th b chng ln tn s nhng mt thi im bt k SVTH: Trn Hiu Trung Dng Minh Tin 17



th tn s tc thi c th hin bi mi IMF l khc nhau.Hin tng ny c th hin qua tn s tc thi ca 6 IMF u tin trong hnh 2.4.

Hnh 2.4: Tn s tc thi ca cc IMF 2.3 Nng lng nhiu ca cc IMF n gin, ta xt m hnh cc IMF cha 100% l nhiu. Do , nng lng ca cc IMF ny c [7], [8], [9] ngh nn c xem nh l gim tuyn tnh trong th logarit. iu ny chng t IMF u tin mang nng lng ln nht.Ta gi s nhiu trong tn hiu l nhiu trng (nhiu Gauss), khi nng lng ca cc IMF c nhiu c xc nh nh sau:

Trong

l nng lng ca IMF u tin v thng s

ph thuc vo s ln m bo

chn lc trong qu trnh thc hin EMD to ra cc IMF.Theo Flandrin v cng s [8], khi thc hin t 1 - 15 ln chn lc ngh


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tnh tuyn tnh ca nng lng cc IMF trong min logarit v ti u s ln chn lc.Khi :

Hnh 2.5: ng cong din t mi quan h gia nng lng ca cc IMF tng ng vi qu trnh EMD s dng s ln chn lc t 1 n 15. ng cong mu c th hin xut ca Flandrin v cng s. Ta thy rng khi s ln chn lc cng tng th cc ng cng tim cn vi nhau, c bit ch gy khc cng tim cn r rt. Do , thng s tnh tng i. By gi, khi chng ta xem xt m hnh cc IMF thc t (t tn hiu b can nhiu bi ngun nhiu trng) so snh vi m hnh cc IMF ch c nhiu. Nng lng ca cc IMF cng c tnh nh trn c a ra ch mang


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Hnh 2.6: So snh v nng lng gia m hnh cc IMF ch c nhiu v IMF thc t. Hnh 2.6 cho thy rng nng lng ca 5 IMF u tin l nh nhau vi c hai m hnh. Tuy nhin, k t IMF th 6 tr v sau th chng c s khc bit ngy cng ln.iu ny chng t rng IMF c th t cng ln th cha cng t thnh phn nhiu.Do trong qu trnh ly ngng ta thng xt nhiu tp trung ch yu cc IMF u tin. 2.4 EMD cho tn hiu thoi Do phng php EMD c hiu qu trong vic phn r tn hiu khng dng thnh cc thnh phn dao ng c trung bnh khng vi tn s tc thi c x l tt, EMD thch hp vi hu ht cc loi phn tch d liu, v lun t hiu sut cao. Do , EMD tr thnh mt cch thc mi v hiu qu cho nhiu lnh vc nghin cu x l tn hiu. X l tn hiu thoi l mt trong nhng lnh vc m EMD c p dng rt thnh cng. Nh c trnh by trong mc 2.2 trn, tng tm IMF da trn vic tr i cc thnh phn dao ng ln nht t d liu c gi l qu trnh chn lc. Do cc IMF c c tnh tn s khc nhau, bin trn cha cc IMF tn s cao. Vi nhng c tnh mnh m ny, cc nghin cu gn y cho thy rng n c th xc nh v loi b thnh cng phn ln cc thnh phn nhiu t cc IMF ca tn hiu thoi b can nhiu. Mc d tt c cc IMF cha nng lng ca c phn thoi v nhiu, nhng tng mt nng lng l khc nhau.V thoi tp trung dy tn s thp v trung nn cc thnh phn nhiu cao tn ch yu cc IMF u tin.V d, trong trng hp nhiu trng, hu ht cc thnh phn nhiu tp trung 3 IMF u tin, trong khi phn thoi li SVTH: Trn Hiu Trung Dng Minh Tin 20



chim u th t cc IMF th 3 n th 6, nh trong hnh 2.3. Do , EMD c th tch bit r rng nhiu cao tn khi thnh phn thoi chnh. Trong bo co n ny, bng cch p dng thut ton ly ngng, EMD c th loi b thnh cng cc thnh phn nhu t cc IMF. V chng ta khng mun lm suy gim tn hiu thoi ban u trong khi loi b hiu qu cc thnh phn nhiu, mt khung tn s da trn k thut nh ngng mm c xut cho cc IMF vi mt s cc tiu chun c iu chnh.


21

Chng 3: Lc nhiu trong min EMD


Chng 3

LC NHIU TRONG MIN EMD

3.1 Gii thiu Ly ngng l mt k thut c dng ph bin kh cc thnh phn nhiu bng cch tr i mt gi tr ngng ra khi cc h s ca tn hiu b can nhiu trong mt min khng gian bin i trc giao. Vi ginh nhiu trng, cng, tun theo phn b Gausse v khng tng quan vi tn hiu, m hnh tn hiu b can nhiu c dng nh phng trnh (1.1). Qu trnh ly ngng c thc hin qua ba bc: Bin i d liu b can nhiu thnh cc thnh phn IMF trong min EMD. p dng ly ngng cho cc h s ca IMF trong min EMD. Bin i ngc tr li min khng gian d liu ban u. Trong thut ton ny th vic xc nh mc ngng l mt khu quan trng nht lc nhiu ti u m khng lm suy bin tn hiu gc.Mc ngng ny c c lng theo phng sai nhiu ca cc IMF. Mi phng php c xut s c mt cch xc nh mc ngng khc nhau da vo phng sai nhiu.Nh ni chng 2, cc IMF u tin hu nh ch cha thnh phn nhiu.V vy, trong IMF ny, thnh phn thoi chim rt t v ng vai tr l cc gi tr bt thng.Ta c th c lng lch ca IMF ny thng qua b c lng lch trung v tuyt i (median absolute deviation - MAD) bi v y l php c lng ny t b nh hng vi cc gi tr bt thng.(3.1)

Gi s nhiu trong IMF u tin c phn b Gauss, khi lch chun c tnh nh sau:


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lch nhiu ca cc IMF cn li c tnh thng qua

. T cng thc (2.4):

Vi mc ngng c xc nh, thut ton ly ngng c chia thnh hai dng l ly ngng cng v ly ngng mm.Thut ton ly ngng cng xc nh mt mc ngng lm mc v gn nhng mu c gi tr nh hn hoc bng mc ngng ny bng khng, v gi nguyn cc mu c gi tr ln hn mc ngng ny. (3.4) Vi v l gi tr cc mu trong IMF th trc v sau khi ly ngng; ph thuc vo lch chun l mc c

ngng ca IMF th . Gi tr ca

c lng t tn hiu nhiu v c th thay i ph thuc vo cc thut ton c xut.Vi cc din gii ny, thut ton ly ngng cng s khng th lc ht c thnh phn nhiu c v nhng mu c gi tr ln hn mc ngng vn c th cha nhiu trn ln vi thoi.Thut ton ly ngng mm trong min EMD c a ra khc phc hn ch ny.Ngha l, nhng mu c gi tr ln hn mc ngng c tr i mt tham s no loi thnh phn nhiu cng vi thnh phn thoi. y, chng ti chn tham s ny bng chnh mc ngng ca n. (3.5) Chng ta c th thy rng thut ton ly ngng mm kh c nhiu thnh phn nhiu hn so vi thut ton ly ngng cng.Tuy nhin, trong ly ngng mm, tn hiu cng b suy bin nhiu hn. Do , gii thut ly ngng nn c chn ty thuc vo yu t mong mun ch quan v khch quan. Mc khc, chng ta cng khng nn ly ngng tt c cc IMF bi v cc thnh phn nhiu ch tp trung ch yu mt vi IMF u tin cn cc IMF cng v sau cng c rt t thnh phn nhiu. Do , SVTH: Trn Hiu Trung Dng Minh Tin

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thut ton ch ly ngng trn mt s IMF u tin v sau tn hiu c ti to nh sau:

Trong , l tn hiu u ra sau khi c lc nhiu bng cch tng hp (cng) li cc thnh phn IMF ly ngng v khng ly ngng, v l cc h s ty chn , v thch hp vi trng hp vi tng mc SNR ca tn hiu. Thng thng, chc chn rng IMF u tin l IMF c cha nhiu thnh phn nhiu nht. 3.2 Ly ngng EMD da trn c tnh trong khong gia ca hai im v khng Thut ton ly ngng ny c thc hin da trn tng ly ngng Wavelet, nhng c thc hin trong tng khong v khng ca cc mu IMF v thay i hm ly ngng ph hp vi c tnh ca chng. Gia phng php Wavelet thresholding v EMD thresholding c 2 s khc bit ln: Trong phng php Wavelet, vic ly ngng c p dng trc trip trn cc thnh phn wavelet. Trong khi EMD th ngc li, vic ly ngng c p dng ln N mu ca mi IMF, m cc mu ny v mt c bn l 1 phn cu thnh tn hiu, c cha ng trong mi bng con thch nghi. Cc IMF khng tun theo lut phn b Gauss, khng c phng sai bng vi phng sai nhiu nh cc thnh phn wavelet. Thc t nhiu trong IMF l nhiu mu v c nng lng khc nhau trong tng IMF khc nhau. Do , trong phng php lc nhiu trong min EMD, cc vector ngng s ph thuc vo tng IMF tng ng. Tip theo, chng ti s trnh by cc phng php xc nh gi tr ngng theo cc thut ton khc nhau. 3.2.1 Ly ngng EMDIT Gii thut ly ngngcng trong tng khong EMD - IT (Interval Threshoding) l thut ton ly ngng trong min EMD pht trin da trn thut ton ly ngng SVTH: Trn Hiu Trung Dng Minh Tin 24 v thc hin ly



cngdo Yannis K. [21], [22] xut. Tuy nhin, phng php ny c thc hin trn tng khong gia hai im v khng (zero crossing) thay v trn tng h s ca IMF trnh b nh hng ng k n tnh lin tc ca tn hiu sau khi c ti to.C th l thng thng cc IMF ging nh nhng dng sng hnh sin thu c t cc iu ch AM hoc FM, v c tr trung bnh bng 0. Do , trong khong gia 2 im v khng =[ ], gi tr tuyt i ca IMF th s rt xung di mc ca bt k v .

gi tr ngng khc khng no trong vng ln cn ca 2 im v khng l

Ni cch khc, da trn gi tr bin ca cc mu b c lp trong IMF, ta khng th suy ra c nhng mu l nhiu hay l tn hiu. Tuy nhin, ta c th phng on c trong khong cc tiu) th tn hiu tri hay nhiu tri da vo cc tr (cc i hoc ny. Khi , nu gi tr tuyt i ca cc tr ny nh

trong khong

hn mc ngng th trong khong ny khng tn ti hoc tn ti rt t thnh phn tn hiu.Ngc li, nu khong ny cha a phn l thoi th gi tr tuyt i ca cc tr ny s ln hn mc ngng. Da trn c tnh ny, phng php ly ngng EMDIT thc hin ly ngng trong tng khong gia hai im v khng nh sau:

Trong

l l gi tr ca cc mu trong khong

v

ca IMF th

v

l gi tr ly ngng ca chng. Thut ton ny c Yannis [21], [22] xut mc ngng nh sau: (3.8) vi l nng lng ca IMF th , v C l hng s c th thay i c th ph hp

vi cc tn hiu khc nhau.


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Hnh 3.1: Thut ton ly ngng da vo c tnh ca khong gia hai im v 0 a) trn mt IMF; b) trn mt on ca IMF trc ()v sau ( ) khi ly ngng.

Hnh 3.1a cho ta thy c IMF trc v sau khi ly ngng. Hnh b1biu din mt on IMF trc khi ly ngng v hnh b2 l IMF sau khi ly ngng cng trc tip c biu din di dng ng t nt, hnh b3 l IMF c ly ngng cng trn tng khong v khng. Ta thy 1 phn ca on IMF gm nhng gi tr khc khng sau khi ly ngng. Vi cch ly ngng trc tip, on IMF sau khi ly ngng b mt tnh lin tc kh ln bi v ch nhng gi tr ln hn mc ngng c gi li. Trong khi ,vi cch ly ngng theo khong v khng,nu cc tr trong khong c tr tuyt i ln hn mc ngng th ton b gi tr cc mu trong khong u c gi li, do s bt b mt tnh lin tc. iu ny chc chn s nh hng n cht lng tn hiu.


26



Hnh 3.2: Phng php EMDIT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu ( ) so vi tn hiu sch ban u ( ).

Hnh 3.2 th hin kt qu lc nhiu vi tn hiu Dopler c SNR=5dB, b can nhiu bi ngun nhiu trng. Thut ton ny th hin s thch nghi vi tn hiu c dng sng sine nh ni trn.Tn hiu c kh nhiu gn ging vi tn hiu sch c 1 s im m gi tr ca mu vt qu gi tr ca mu tng ng trong trong tn hiu sch , nguyn nhn ny do nhiu gy ra. Sau y, hnh 3.3 biu din kt qu ly ngng ca mt tn hiu thoi ting Vit thc t mc 5dB.


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Hnh 3.3: Phng php EMD IT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu. 3.2.2Ly ngng EMD-SIT Trong thut ton ly ngng mm trong tng khong EMD SIT (Soft Interval Thresholding) do Yannis K. [21], [22] xut, nhng im cc tr trong khong [ nu c gi tr ln hn gi tr ca vector ngng s b tr i 1 khong l T so s t v 0.

vi gi tr gc ca IMF, cn nhng im cn li trong khong [

Vi

c tnh theo cng thc (3.8) trn.


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Hnh 3.4: Phng php EMD SIT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu ( ) so vi tn hiu sch ban u ( ).

Hnh 3.5: Phng php EM SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu.


29



Trong hnh 3.4, tn hiu c lc nhiu kh lin tc v gn vi tn hiu sch. Ta c th thy,tt c cc mu ca tn hiu kh nhiu c tr tuyt i nh hn vi cc mu tng ng ca tn hiu sch,c bit l cc gi tr cc tr c 1 khong dch chuyn, nguyn nhn ny do vector ngng gy ra. Thut ton EMD - SIT m bo c khi c bt k gi tr cc tr no c din t qua biu thc sau: (3.10) Vi thut ton ly ngng ny, kt qu ly ngng t tn hiu Dopler, v tn hiu thoi thc t 5dB s c th hin bng hnh 3.4 v hnh 3.5 trn. 3.2.3. Ly ngng EMD-SCAD Phng php ly ngng EMD-IT c hn ch l khng lin tc, cn phng php ly ngng EMD-SITli lm dch chuyn gi tr ca mu tn hiu bi 1 gi tr T ngay c khi nhng mu tn hiu c bin rt cao so vi mc ngng, vic ny to ra nhng dch chuyn khng cn thit. Tc khi gi tr ca mu cao hn rt nhiu so vi mc ngng th khng cn thit phi ly ngng. khc phc nhng hn ch trn, mt cch ly ngng mi cA.Antoniadis v J.Fan [1], xut da trn lch tuyt i c ct xn mt cch trn tru gi l SCAD (Smoothly Clipped Absolute Deviation).p dng cch ly ngng SCAD trong min EMD, ta c phng php EMD-SCAD. m c gi

tr vt qu mc ngng th qu trnh ly ngng s lm co li gi tr cc tr ,

Trong ,

l mt hng s c c lng theo thc nghim. Theo xut ca s t c kt ly ngng qu tt nht.

A.Antoniadis v J.Fan [1], vic chn


30



Hnh 3.6: Phng php EMD SCAD trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu ( ) so vi tn hiu sch ban u ( ). Trong hnh 3.6, tn hiu c lc nhiu gn ging vi tn hiu sch.Tn hiu khng c nhng khong dch chuyn gi tr ln ca cc mu v khng c nhiu mu vt l.

Hnh 3.7: Phng php EMD SCAD trn tn hiu thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu. SVTH: Trn Hiu Trung Dng Minh Tin

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3.2.4Ly ngng EMD - IIT Gii thut lp trong phn r v ly ngng trong tng khong EMD IIT (Iterative EMD Interval Thresholding) c Yannis K. xut [22] thc hin quy trnh nh sau: 1) Thc hin bin i EMD trn tn hiu gc , ta thu c IMF ,

2) Thc thin ti to li mt phn tn hiu t c

IMF cui cng, ta thu

3) Chuyn i ngu nhin v tr cc mu trong IMF u tin

4) To mt phin bn nhiu mi t tn hiu ban u 5) Thc hin bin i EMD trn tn hiu hoc EMD-SIT, hoc nhiu t ln t bc 3-6 thu c phin bn kh nhiu ca thu c cc IMF mi

6) Thc hin vic ly ngng trn cc IMF mi ny, dng phng php EMD-IT, EMD - SCAD thu c 1 tn hiu c kh

7) Lp li thm l

8) Ly trung bnh cng t K phin bn, ta c tn hiu c kh nhiu mong mun


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Hnh 3.8: Phng php EMD IIT ly ngng da vo EMD SIT trn tn hiu Dopler: a) Tn hiu b can nhiu c SNR=5dB; v tn hiu c lc nhiu ( vi tn hiu sch ban u ( Trong (nh phng php ny, ) lp vi b) K=3; c) K=20. bn tn hiu c kh nhiu ) so

nhng phin

) t tn hiu gc ban u (nh

) c ti to t nhng

phin bn IMF khc nhau sau khi kh nhiu. Phng php ny da trn c tnh ca bin i EMD, c th l IMF u tin. Chng ta bit rng trong trng hp nhiu trng Gauss th IMF u tin ch yu l nhiu lng nhiu trong IMF u tin chim phn ln trong tn hiu v p o so vi cc IMF cn li.Bng vic thay i ngu nhin v tr cc mu trong IMF u tin, ta to c cc phin bn tn hiu khc nhau (nh hiu gc. SVTH: Trn Hiu Trung Dng Minh Tin 33 t tn hiu ). Cng v trong IMF u tin ton b l nhiu nn tng phng sainhiu ca phin bn l ging vi tn



Trong bc 3, khi thay i ngu nhin v tr cc mu trong IMF u tin, ta c 2 cch thc hin: Cch 1: cc mu c dch chuyn ngu nhin theo vng trn (ramdom circulation) Cch 2: cc mu c thay i v tr ngu nhin (ramdom permutation) Cch chuyn v mu trong IMF u tin v s lng cc phin bn ca tn hiu c nh hng n cht lng tn hiu. Chng ta s kho st nhng nh hng ny chng 4.

Hnh 3.9: Phng php EMD IIT ly ngng da vo EMD SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu vi K=10. 3.2.4 EMD-CIIT Phng php lp ci tin trong phn r vly ngng tng khongEMD-CIIT (Clear Iterative EMD Interval Thresholding) c pht trin t EMD IIT ng dng cho tn hiu c SNR cao.Khi tn hiu c SNR cao th IMF u tin cng c cha 1 lng thng tin ng k nn khi thc hin chuyn i ngu nhin v tr cc mu th lng SVTH: Trn Hiu Trung Dng Minh Tin 34



thng tin c ch trong IMF u tin s tri ra v lm nh hng xu n tn hiu. ci thin nhc im ca EMD-IIT th nhng thng tin c ch s c ly ra khi trc khi cc mu IMF u tin b thay i v tr.Ta c th dng cc phng php ly ngng cp trc y (nh EMD - IT, EMD SIT, EMD -SCAD) rt nhng thng tin c ch, vic ny rt quan trng v nhng thng tin ny s c ti to vi IMF cn li.

Hnh 3.10: Phng php EMD - CIIT: a) Tn hiu b can nhiu c SNR=5dB; b) tn hiu c lc nhiu ( ) so vi tn hiu sch ban u ( )lp vi K=15.

Cc bc thc hin tng t nh trong phng php EMD-IIT nhng ta ch ly ngng cho IMF u tin.C th nh sau: 1) Thc hin bin i EMD trn tn hiu gc 2) Thc hin ly ngng IMF u tin IMF c lc nhiu . , ta thu c n IMF. ca tn hiu gc , ta thu c

3) Suy ra tn hiu ton nhiu tht s ca IMF u tin


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4) Thc thin ti to li mt phn tn hiu t thoi), ta thu c

IMF cui cng cng vi

IMF u tin c ly ngng (c th xem nh l IMF ch c thnh phn = +

5) Chuyn i ngu nhin v tr ca cc mut tn hiuton nhiu ca IMF u tin = ALTER

Cc bc tip theo thc hin tng t nh thut ton ly ngng EMD IIT trn.

Hnh 3.11: Phng php EMD CIIT ly ngng da vo EMD SIT trn tn hiu thoi thc t: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu vi K=10. 3.3 Ly ngng EMD - SST Khng ging nh thut ton ly ngng da vo c tnh trong khong gia hai im v khng, thut ton ly ngng mm trn tng khung con EMD - SST(Sub - frame Soft Thresholding) thc hin chia cc IMF thnh mt s (thng chn l 8) phn khung (sub frame). Cc khung con ny c c lng nhiu ring bit, sau c SVTH: Trn Hiu Trung Dng Minh Tin

36



Hnh 3.12:S khi ca phng php ly ngng EMD - SST phn loi thnh hoc l khung con c nhiu tri hoc l khung con c tn hiu tri. V vic ly ngng c p dng cho khung con c xc nh l khung con nhiu tri. S gii thut ca phng php c th hin trong hnh 3.12.Da vo s khi ny, u tin tn hiu b can nhiu c bin i EMD to ra n IMF. Thut ton chn mt vi IMF u tin (thng thng t 4 n 6 IMF u tin) ly ngng.Cc IMF c chn ly ngng ny s c chia thnh cc khung con.Vic phn loi cc khung con l mt trong nhng im chnh ca thut ton ly ngng mm ny. Mc ch chnh ca vic phn loi ny l loi b tn hiu tp m m khng lm gim cc thnh phn thoi ban u. iu ny lm cho cc thut ton ly ngng mm c SVTH: Trn Hiu Trung Dng Minh Tin 37



p dng cho mt lot cc gi tr SNR. Tuy nhin, vic ngdng trc tip thut ton ny cho cc IMF ca tn hiu thoi c nhiu s tht bi v hai l do. Trc tin, cc IMF s phn b nhiu v nng lng tn hiu khc nhau, m cho thy rng mi IMF s c phng sai nhiu v thoi khc nhau.Th hai, do s phn rEMD, phng sai ca cc khung con IMF s c nhiu bin ng hn so vi khung con tn hiu thoi b can nhiu. V vy, phng sai nhiu ca mi IMF cn c quy nh mt cch ring bit v mc ngng gii hn cho vic phn loi cc khung con nn c gi tr ln hn mc ngng gii hn trong phng php ly ngng mm DCT, m bo rng tt c cc khung con nhiu c ly ngng tt nht. Mc ngng gii hn ny da trn tng rng mt khung con c th c nh ngha nh l mt khung con nhiu tri, nu cng sut nhiu cao hn cng sut thoi. Do , mc ngng ny nn c thit lp cho trng hp m phng sai nhiu c: (3.12) do , trong (3.13) l phng sai nhiu ca khung con v Cov l hm hip phng sai. Do ta v thoi l nh nhau. i vi 1 khung con bt k, ta

gi thuyt rng nhiu cng v khng tng quan vi tn hiu thoi nn phng sai nhiu v phng sai thoi l c lp vi nhau. V vy, hm hip phng sai bng khng, khi chng ta c: (3.14) Khi cng sut nhiu v thoi bng nhau, chng ta nhn c;

V vy, trong trng hp cng sut nhiu v thoi nh nhau, phng sai ca mt khung con bng . Do , nu cho khung con th tho iu kin:


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th khung con ny c nh ngha nh l khung con tn hiu tri, ngc li l mt khung con nhiu tri. Sau khi phn loi khung con, vic ly ngng c thc hin trn cc khung con c nhiu tri theo cng thc tng t (1.17) v (1.18): (3.17) y tnh. l nhng mu c nh ngng v tch l hm nh ngng tuyn

Vi

, v

l gi tr hng s c c lng cho ph hp vi mc SNR ca

tn hiu u vo.

Hnh 3.13: Phng php ly ngng EMD - SST: a) Tn hiu sch ban u; b) Tn hiu b can nhiu c SNR=5dB; b) Tn hiu c lc nhiu.


39



Theo Deger [5], [6], gi tr thc nghim ca tr ti u theo SNR ng vo l:

nm trong khong [0.2 - 0.8] v c gi

3.4 Ly ngng mm kt hpDCT - EMD Phng php c ngh c da trn vic ng dng gii thut ly ngng mm trong 2 tng: tng u tin, gii thut DCT ly ngng mm c dng nh l mt b tin x l (pre-process) loi b nhng thnh phn nhiu cho di tn rng. Tng th 2, chng ta p dng gii thut EMD ly ngng mm xc nh v loi b thnh phn nhiu cn st li t tn hiu c ci thin c c t tng u. Tuy nhin, cc IMF nn c xc nh cn thn c ly ngng ph hp, bi v tn hiu va c nh ngng 1 ln tng u tin v cc IMF cng khc nhau v thnh phn nhiu v thoi. xc nh nhng im ny, nhng nghin cu v nhng phn tch thc nghim gn y cho ta 1 s kt lun sau: Trong trng hp tn hiu thoi c nhiu (nhiu trng) th IMF u tin bao gm ch yu cc thnh phn nhiu. Tuy nhin, IMF ny cng c mt lng ng k thng tin thoi c ch.V vy, IMF ny cn c ly ngng vi mt vector ngng gi nhng thnh phn tn hiu. IMF th 2 vn ch yu l nhiu, nhng thnh phn thoi nhiu hn so vi IMF u tin. V th vector ngng ny nn nh hn so vi vector ngng u tin. Tuy mt lng ln thnh phn nhiu va c loi b, nhng vn cn nhng thnh phn nhiu ng k IMF th 3, th 4. V vy, nhng IMF ny cn c nh ngng vi mc ngng nh hn so vi 2 IMF u tin. Bi v tn hiu c nhiu ban u c ci thin cht lng nh gii thut DCTly ngng mm va c ng dng trong tng u tin v hu ht nhiu tp trung ch yu trong 4 IMF u tin, nn cc IMF bc cao hn (t IMF


40



th 5tr i) th ch yu l tn hiu thoi. Do , nhng IMF ny khng cn ly ngng v vic ly ngng s lm suy gim thnh phn thoi.

Hnh 3.14: S khi ca phng php ly ngng mm EMD - DCT Da vo s trn, u tin cn chia tn hiu thnh cc khung trong min thi gian v khng cn chng ln nhau.Vic chia khungnynhm y nhanh tc tnh ton v nu x l ton b vng d liu th s va chim nhiu b nh va khin tc x l chm.Lu l cch chia khung ny c ngha khc vi chia khung trong phng php c in nh tr ph v EMD khng yu cu tnh cht dng ca tn hiu. SVTH: Trn Hiu Trung Dng Minh Tin 41



3.4.1 Ly ngng mm DCT tng th nht Vic ly ngng DCT tng th nht c thc hin ging nh l thuyt ly ngng mm DCT c trnh by phn 1.4.4.Cc h s cng c chia thnh cc khung con, sau phn thnh hai nhm l khung c tn hiu tri v c nhiu tritheo cng thc (1.16). Khung c tn hiu tri khng c ly ngng nhm gim i mt mc cc thnh phn thoi. Khung c nhiu tri c th c sp xp theo th t tng dn ri ly ngng theo cng thc (1.17) v (1.18). y, chng ti xin trnh by mt cch ly ngng mm khc.Thut ton ny khng sp xp cc h s, m thc hin ly ngng trn tng khung c nhiu tri:

Vi

ln lt l phng sai nhiu v cng sut ca khung con ang ly ngng. c xc nh bng thc nghim, theo [1] v .

Cc hng s

Sau khi ly ngng, cc khung con c tn hiu tri v cc khung con c nhiu tri c ly ngng s c tng hp li tr v tn hiu c ly ngng trong min tn s.Tn hiu ny s tip tc c lc nhiu mt ln na tng th hai. 3.4.2 Ly ngng mm EMD tng th hai Cc bc thc hin tng th hai ny hon ton ging nh thut ton ly ngng mm trong khung con ca IMF c trnh by mc 3.3. u tin,tn hiu u ra ca tng th nht s c p dng phn tch EMD thu c cc IMF. Sau , cc IMF ny c chia thnh cc khung con, do mi khung c Lmu. Cc khung con ny c tip tc phn loi thnh khung con tn hiu tri hoc nhiu tri. Cc khung c tn hiu tri khng ly ngng hn ch vic mt mc tn hiu.Cc khung c nhiu tri c p dng thut ton ly ngng mm nh cng thc (3.18) v (3.19) nh trn.Tuy nhin, do tn hiu u ra ca tng th nht c lc nhiu mt phn nn mc ngng cn c c lng thp hn.


42



Sau khi ly ngng cc khung con ny c ghp li thnh mt IMF c ly ngng. Tt c cc IMF c ly ngng v khng ly ngng sau c tng hp li thnh tn hiu c ly ngng trong min thi gian. Qu trnh ly ngng kt thc vi hai ln ly ngng khc nhau, ly ngng ln th nht trong min tn s v ly ngng ln th hai trong min thi gian.

Hnh 3.15: Phng php kt hp DCT EMD vi tn hiu thoi thc t: a) Tn hiu sch; b) Tn hiu b can nhiu; Tn hiu c lc nhiu bng phng php: c) DCT; d) DCT EMD Hnh 3.15 th hin kt qu lc nhiu ca phng php ly ngng DCT so vi phng php kt hp ly ngng DCT EMD.Tuy s khc bit cha tht s r rt, nhng y cng l mt hng nghin cu ng c tip tc pht trin v s kt hp lc nhiu SVTH: Trn Hiu Trung Dng Minh Tin

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trong c min thi gian v min tn s.S kt hp ny c th gip gim bt hn ch ca thut ton ly ngng trong mi min.


44

Chng 4: Thc hin m phng


Chng 4

THC HIN M PHNG

4.1 Cc thng s m phng Thut ton xut trong chng 3 c m phng bng phn mm MATLAB.Nh ni trn, ti ny tp trung vo phng php lc nhiu thoi theo m hnh khng tham s v n knh.Trong thut ton ly ngng ny, hu ht cc tham s ly ngng u l tham s thc nghim, ngha l c c lng thng qua qu trnh thc nghim.y cng l im hn ch ca phng php bi v vic c lng thc nghim thng khng cho tin cy v tnh bn vng cao ca thut ton vi nhiu loi tn hiu khc nhau.V vy m lun vn ny tp trung m phng nhiu ln cc thut ton ly ngng khc nhau vi s thay i lin tc cc tham s ly ngng a ra c mt ci nhn khi qut v s nh hng ca cc tham s ny. Sau y, chng ti s trnh by mt s thng s quan trng nh hng n kt qu ca thut ton c xut. 4.1.1 Khung tn hiu Chia khung y nhanh tc tnh ton v nu x l ton b vng d liu th s va chim nhiu b nh va mt nhiu thi gian.Mt khung c chia nn c di nm trong khong 20 - 30ms v khng chng ln nhau. Lu l cch chia khung ny c ngha khc vi chia khung trong phng php c in (tr ph chng hn).Tr ph l thut ton da trn bin i Fourier, nn yu cu d liu phi c tnh cht dng. Trong khi , trong khong thi gian 20 - 30ms, c quan pht m ca con ngi (mi, li, vm mi...) khng thay i hnh dng, dn n tn hiu thoi tng ng c tnh cht dng. Ngoi ra, phng php c in yu cu chia khung chng ln nhau v hnh dng ca c quan pht m thay i lin tc ch


45



khng phi ri rc; v bin i Fourier i hi phi ca s ha khung (v d nh ca s Hamming).V vy m cc khung cn chng ln ln nhau trnh gy sai s. phng php EMD vn khng yu cu tn hiu phi c tnh cht dng nhng vn chn khung 20 - 30 ms ch l mt s k tha, ci tin phng php c in, do ta c th chn khung di hn (nhng c th x l cng chm hn). 4.1.2Phng sai nhiu ca cc IMF Ta p dng m hnh IMF u tin ch cha nhiu v cc nng lng cc IMF suy gim tuyn tnh trong ming logarit. lchchun nhiu ca IMF u tin l:

Hm median l hm c bn trong MATLAB, c php median(). Nh chng mnh chng 3, lch chun nhiu ca IMF th tr ca phng sai nhiu ca IMF u tin nh sau: c c lng theo gi

Phng sai nhiu ca cc IMF c tnh l bnh phng ca lch chun. 4.1.3 Vector ly ngng Trong thut ton ly ngng, vic xc nh vector ly ngng l khu quan trng nht, nh hng trc tip n hiu qu lc nhiu ca phng php.Chnh v vy m mc ngng ny c a ra cn c tin cy v tnh chnh xc cao. Trong phng php lc nhiu n knh, vector ly ngng c c lng thng qua phng sai nhiu hay thng qua nng lng ca IMF ngng ny cng c c lng tng ng. Trong lun vn ny, thut ton ly ngng s dng hai vector ly ngng khc nhau do Deger E. [5], [6] v Yannis K. [19] [24] a ra nh trnh by chng 3. Ngoi ra, cn rt nhiu thng s nh hng n hiu qu ca thut ton nh s IMf cn ly ngng, s ln lp v cch chuyn v ngu nhin trong thut ton lp IIT, SVTH: Trn Hiu Trung Dng Minh Tin 46 . V bi v phng sai nhiu ca cc IMF c c lng ph thuc vo phng sai nhiu u tin nn vector



CIIT, h s

trong phng php DCT - EMD. Cc thng s ny s c kho st

trong phn sau ca lun vn ny.

4.2 Kt qu m phng 4.2.1 Cc phng php ly ngng trn tng khong v khng.

Hnh 4.1: Kt qu lc nhiu trn tng khong v khng: a) tn hiu sch; b) tn hiu b can nhiu; Tn hiu sau khi lc nhiu bng phng php: c) EMD - IT; d) EMD SIT; e) EMD SCAD Hnh (I) th hin kt qu thc hin lc nhiu trn tn hiu b can nhiu c SNR=0dB. Hnh (II) th hin kt qu thc hin lc nhiu trn tn hiu b can nhiu c SNR=10dB. Ta thy, mt mc no , phng php ly ngng trn tng khong v khng khng thc s c hiu qu cao. c bit l vi tn hiu u vo c SNR cng ln th hiu qu ca cc phng php ny cng hn ch.Bng 1 sau y thng k hiu qu ca cc phng php trn da vo SNR u ra tng ng vi SNR u vo. Ta thy vi SVTH: Trn Hiu Trung Dng Minh Tin

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tn hiu u vo c SNR nh th hiu qu ca cc phng php ny t ra c hiu qu tng i tt. Tuy nhin, khi tn hiu u c SNR cng cao th hiu qu ci thin cht lng thoi cng gim. Bng 4.1: So snh kt qu SNR u ra ca 3 phng php ly ngng trn tng khong v 0 iSNR (dB) -5 0 5 10 15 EMD - IT 0.48 3.88 7.38 11.05 15.07 oSNR(dB) EMD - SIT 1.70 4.76 8.08 11.84 15.60 EMD - SCAD 1.68 4.92 8.39 12.10 15.68

gip nng cao SNR u ra, phng php ly ngng tng khong v khng lp li EMD CIIT c pht trin da trn l thuyt lp EMD IIT c p dng.Tuy nhin, v mt l thuyt, phng php ny cng da vo thut ton ly ngng ca cc phng php trn nn b hn ch vi tn hiu u vo c SNR cao.


48



1

0

-1 1

0

0.5

1

1.5 a

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 b

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 c

2

2.5

3

3.5 x 104

0

-1

0

0.5

1

1.5 d

2

2.5

3

3.5 x 104

Hnh 4.2: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD - IT: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=101

0

-1 1

0

0.5

1

1.5 a

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 b

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 c

2

2.5

3

3.5 x 104

0

-1

0

0.5

1

1.5 d

2

2.5

3

3.5 x 104

Hnh 4.3: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD -SIT: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=10 SVTH: Trn Hiu Trung Dng Minh Tin 49



1

0

-1 1

0

0.5

1

1.5 a

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 b

2

2.5

3

3.5 x 104

0

-1 1

0

0.5

1

1.5 c

2

2.5

3

3.5 x 104

0

-1

0

0.5

1

1.5 d

2

2.5

3

3.5 x 104

Hnh 4.4: Kt qu lc nhiu bng phng php EMD CIIT da trn thut ton ly ngng EMD - SCAD: a) tn hiu sch; b) tn hiu b can nhiu c SNR= 0dB; Tn hiu sau khi lc nhiu da trn thut ton ly ngng: c) K=1; d) K=10 Cc hnh 4.2, 4.3, 4.4 th hin kt qu so snh gia cc thut ton EMD- CIIT da trn 3 thut ton ly ngng l IT, SIT v SCAD vi s ln lp K=1 v K=10. Vi phng php ny, s ln lp ln s cho kt qu tt hn. Tuy nhin, s ln lp cng ln th ko theo thi gian x l cng chm. c ci nhn khch quan hn, chng ta cng xt bng so snh kt qu SNR u ra ca cc thut ton ny. Bng 4.2: So snh kt qu SNR u ra ca phng php EMD CIIT da trn 3 thut ton ly ngng l IT, SIT, v SCAD. oSNR(dB) S ln lp IT iSNR=0dB K=01 K=05 K=10 SVTH: Trn Hiu Trung Dng Minh Tin 3.88 4.71 4.83 SIT 4.76 4.91 4.96 SCAD 4.92 5.11 5.16

50



Bng 4.2 cho thy kt qu lc nhiu ca phng php CIIT da trn c 3 thut ton c nh gi qua SNR ca tn hiu u ra. Quan st bng ny, ta nhn thy, phng php ly ngng CIIT da trn thut ton SCAD cho kt qu tt hn so vi hai thut ton cn li. Tuy nhin kt s khc bit ny khng qu ln v nhn chung l khng c s vt tri hn so vi cc phng php cn li. 4.2.2 Phng php ly ngng mm EMD - SST

Hnh 4.5: Kt qu lc nhiu bng phng php ly ngng EMD - SST: a) tn hiu sch; b) tn hiu b can nhiu c SNR=0dB (I) v SNR=10dB (II); Tn hiu sau khi lc nhiu Vi thut ton ly ngng mm EMD SST ny, tn hiu ng ra khng thc s ci thin r rt. L do y l do vector ly ngng tuyn tnh khng thch nghi vi SNR ca tn hiu u vo. Tuy nhin, trong lun vn ny, thut ton ly ngng EMD


51



SST cng ch c trnh by nh mt thut ton ly ngng nn tng cho phng php ly ngng kt hp DCT EMD s c trnh by ngay sau y. 4.2.3 Phng php ly ngng kt hp DCT EMD

Hnh 4.6: Kt qu lc nhiu bng phng php ly ngng kt hp DCT - EMD: a) tn hiu sch; b) tn hiu b can nhiu c SNR=0dB (I) v SNR=10dB (II); Tn hiu sau khi lc nhiu Phng php ly ngng mm kt hp DCT EMD cng thch hp vi tn hiu u vo c SNR nh.V vic ly ngng c thc hin hai tng nn rt d lm mt mc i thnh phn thoi i vi tn hiu c SNR cao. Bng 4.3 trn cho ta thy c kt qu ci thin ng k ca phng php ly ngng kt hp DCT EMD so vi ch dng EMD SST.Tuy nhin, do p dng thut ton ly ngng kt hp, nn phng php ny cng b nh hng bi nhc im ca thut ton ly ngng DCT l sinh ra nhiu nhc.y cng l vn trong hu ht cc thut ton ly ngng vp phi. SVTH: Trn Hiu Trung Dng Minh Tin 52



Bng 4.3:So snh kt qu SNR u ra ca phng php ly ngng EMD SST v ly ngng kt hp DCT - EMD iSNR (dB) 0 5 10 oSNR(dB) EMD - SST 3.04 6.91 11.0 DCT - EMD6.17 9.35 12.25

4.3 Kho st cc thng s nh hng n cht lng tn hiu Trong cc phng php ly ngng trong min EMD,c 4 thng s cn phi chn la hp l ph hp vi tng mc SNR ca tn hiu b can nhiu cng nh p dng vi cc thut ton ly ngngkhc nhau. Bn thng s ny gm c: Vector ly ngng tm ra gi tr ti u ca v . H s M1, M2 quy nh s lng IMF cn ly ngng. H s Ktrong phng php ly ngng EMD CIIT. Cch chuyn i v tr mu trong IMF u tin trong phng php lp.

Ta xt cc tn hiu ban u cSNR ln lt l -5dB, 0dB,5dB v 10dB.


53



4.3.1 nh hng ca vector ly ngng Bng 4.4: nh hng ca thng s C i vi phng php ly ngng cng trn tng khong v khng (EMD - IT)iSNR (dB) oSNR (dB) C=0.2 C=0.4 C=0.5 C=0.6 C=0.7 C=0.8 C=0.9 -4.5 0.4 5.35 10.36 -1.98 2.67 7.02 10.98 -0.49 3.62 7.34 10.51 0.3 3.86 7.04 9.67 0.43 3.60 6.52 0.34 3.00 5.85 0.12 2.43 5.08 C=1 -0.05 1.86 4.44 C=1.1 -0.13 1.46 4.40 C=1.2 -0.23 1.11 4.30 -

-5 0 5 10

Bng 4.5: nh hng ca thng s C i vi phng php ly ngng mm trn tng khong v khng (EMD - SIT)iSNR (dB) -5 0 5 10 oSNR (dB) C=0.2 C=0.3 C=0.4 C=0.5 C=0.6 C=0.7 C=0.8 C=0.9 -1.03 4.35 8.08 11.38 1.39 4.71 7.47 9.84 1.43 4.08 6.34 1.03 3.25 5.27 0.60 2.51 4.38 0.27 1.90 0.05 1.43 0.05 1.08 C=1 -0.17 0.82 C=1.1 -0.22 0.62 -


54



Bng 4.6: nh hng ca thng s C i vi phng php ly ngngEMDSCADiSNR (dB) C=0.2 C=0.3 C=0.4 C=0.5 C=0.6 C=0.7 C=0.8 C=0.9 -5 0 5 10 -0.66 3.82 8.1 12.1 1.25 4.88 8.16 11.08 1.42 4.32 7.03 9.46 1.03 3.43 5.76 0.6 2.60 4.70 0.27 1.95 0.05 1.45 -0.10 1.08 C=1 -0.17 0.82 C=1.1 -0.22 0.62 oSNR (dB)

Bng 4.7: nh hng ca thng s

i vi phng php ly ngng EMD -SSToSNR (dB)

iSNR (dB) -5 0 5 10 =0.2 -3.6250 1.4165 6.2451 10.9946 =0.3 -2.9529 2.0714 6.6775 10.9801 =0.4 -1.9844 2.6275 6.9032 10.6423

=0.5 -1.3968 3.0349 6.9069 10.0965

=0.6 -0.9753 3.2589 6.7168 9.4835

=0.7 -0.7534 3.3040 6.3890 8.8615

=0.8 -0.6743 3.2226 6.0011 8.2715


55



Bng 4.8: nh hng ca thng s

i vi phng php ly ngng kt hp DCT EMD

iSNR (dB) =0.2 1.8072 6.1661 9.3486 =0.3 2.2005 6.1349 8.7251 =0.4 2.4552 5.8877 7.9915

oSNR (dB) =0.5 2.7337 5.4329 7.2364 =0.6 2.8584 4.9028 6.5029 =0.7 2.8016 4.3701 5.8231 =0.8 2.344 3.9554 5.2148

-5 0 5

T s liu cc bng trn, ta nhn thy rng ng vi cc tn hiu u vo c mc SNR khc nhau th s c cc gi tr C v ti u tng ng.Vi cng 1 mc SNR ca tn hiu c nhiu th gi tr SNR ng ra thay i rt nhiu khi h s C thay i trong cng phng php v vi nhng phng php khc nhau. Cc tn hiu ng vo c mc SNR cng ln th gi trti u ca C v cng

nh.Nguyn nhn l: khi SNR ng vo cng ln th thnh phn tn hiu thoi nhiu hn thnh phn nhiu v bin ca thoi cao hn so vi nhiu. Do , gi tr C v cng ln th tn hiu c x l thm ch c mc SNR thp hngi tr SNR ng vo bi v ngoi nhng thnh phn nhiu th thnh phn thoi cng b ly ngng. Vi cng 1 mc SNR ca tn hiu c nhiu th ga tr C v phng php ny cngrt khc nhau. Lu l gi tr C v ti u ca cc ti u ca cc

phng php thuc nhm ly ngng trn khong v khng c bin thay i ln hn nhm ly ngng trn khung con, v ly ngng kt hp Trong cc phng php c trnh by, phng pho ly ngng kt hp DCT EMD cho kt qu tt nht, v ni tri hn so vi cc phng php cn li. Tuy nhin, nhn chung, tt c cc phng php ny u t ra hn ch vi tn hiu c SNR cao. SVTH: Trn Hiu Trung Dng Minh Tin

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4.3.2. nh hng ca thng s M1,M2 H s M1, M2 quyt nh s lng s lng cc IMF u tin c ly ngng. Ta bit rng khi p dng EMD phn r tn hiu ra thnh cc IMF th IMF u tin c gi s l cha nhiu nhiu nht. Cc IMF cn li cha nhiu t hn v bc ca IMF cng ln th thnh phn nhiu trong IMF cng nh.Ni cch khc, IMF c bc cng cao th cng cha nhiu cc thnh phn thoi.Do , ta mc nh M1=1 v ch xt s thay i ca M2 trong khong t 2 n 6. Trong phn ny, ta s dng li cc vector ly ngng ti u i vi tng mc SNR ca tn hiu c nhiu ( c tm trong phn 4.3.1) v K=1. Tn hiu sau khi c x l c ti to li:

Bng 4.9:nhhng ca M2 i vi phng php ly ngng cng trn tng khong v khng (EMD - IT) iSNR (dB) -5 0 5 10 M2=2 -0.28 3.63 7.09 10.98 3 0.3 3.88 7.38 11.05 4 0.43 3.86 7.34 10.98 oSNR (dB) 5 0.42 3.74 7.31 10.95 6 0.48 3.77 7.29 10.93 C C=0.7 C=0.6 C=0.5 C=0.4


57



Bng 4.10: nhhng ca M2 i vi phng php ly ngng mmtrn tng khong v khng (EMD - SIT) iSNR (dB) -5 0 5 10 M2=2 0.02 4.14 7.55 11.67 3 1.01 4.59 8.02 11.84 4 1.43 4.71 8.08 11.83 oSNR(dB) 5 1.54 4.68 8.06 11.80 6 1.70 4.76 8.08 11.81 C C=0.4 C=0.3 C=0.2 C=0.15

Bng 4.11:nhhng ca M2 i vi phng php ly ngng mmSCAD trn tng khong v khng (EMD - SCAD) iSNR (dB) -5 0 5 10 M2=2 0.03 4.20 7.78 11.87 3 1.02 4.73 8.33 12.10 4 1.42 4.88 8.38 12.10 oSNR(dB) 5 1.53 4.84 8.38 12.10 6 1.68 4.92 8.39 12.09 C C=0.40 C=0.30 C=0.25 C=0.20

y, chng ti khng xt nh hng ca cc h s ny i vi cc phng php ly ngng EMD SST v ly ngng kt hp DCT EMD.T s liu bng 4.9 n 4.11, ta nhn thy: Tn hiu ban u c mc nhiu thp (-5dB, 0dB) th gi tr M2 =6 l gi tr tt nht bi v khi khng nhng 4IMF u tin cha nhiu m cc IMF bc cao


58



cng c cha 1 lng nhiu ng k. Do , s thay i ca M2nh hng nhiu n SNR ng ra. Ngc li, tn hiu ban u c mc nhiu cao (SNR=5dB, 10dB) th M2 nh hng khng ng k n SNR ng ra.Tuy nhin, do ta gi thuyt cc IMF c bc thp cha nhiu nhiu hn, nn ta nn chn M2=3, hoc 4. Trong 3 phng php trn th phng php EMD - SCAD c tn hiu ng ra l tt nhtbi v m bo s lin tc ca tn hiu v EMD - IT c mc SNR thp nht v c s thay i t ngt gia mu thng tin thoi v mu nhiu. 4.3.3. nh hng ca h s lp K Trong phn ny, cc gi tr C, M1,M2 c chn trc v tn hiu t gi tr tt nht ti nhng gi tr nytng ng vi K=1.Ta nh gi cht lng tn hiu c x l thng qua vic o mc SNRtng ng vi 3 ga tr K=1, 5 v 10.

Bng 4.12:nh hng thng s Ki vi phng phply ngng EMD - IT oSNR(dB) iSNR (dB) oSNR K=1 IIT -5 0 5 10 0.48 3.88 7.38 11.05 1.21 3.69 5.22 6.22 K=5 CIIT 1.44 4.71 7.88 11.02 IIT 1.32 3.69 5.03 5.45 K=10 M2 v C CIIT 1.63 4.83 7.90 10.9 M2=6, C=0.7 M2=4, C=0.6 M2=3, C=0.5 M2=3, C=0.4


59



Bng 4.13: nh hng thng s Ki vi phng phply ngng EMD - SIT oSNR(dB) iSNR (dB) SNR K=1 IIT -5 0 5 10 1.70 4.76 8.08 11.84 1.82 3.82 5,29 5.68 K=5 CIIT 1.86 4.91 7.69 10.61 IIT 1.87 3.80 5.02 5.28 K=10 M2 v C CIIT 1.87 4.96 7.64 10.46 M2=6, C=0.4 M2= 6, C=0.23 M2=4, C=0.2 M2=3,C=0.15

Bng 4.14: nh hng thng s Ki vi phng phply ngng mm SCAD iSNR (dB) SNR K=1 K=5 IIT -5 0 5 10 1.68 4.92 8.39 12.11 1.68 4.09 5.37 5.53 CIIT 1.92 5.11 8.22 11.17 IIT 1.68 3.88 5.13 5.45 K=10 CIIT 1.93 5.16 8.18 11.06 M2=6, C=0.40 M2=6,C=0.30 M2=6, C=0.25 M2=5, C=0.20 M2 v C oSNR(dB)

T bng 4.12 n 4.14 ta nhn thy rng phng php ly ngng mm SCAD x l tn hiu c mc SNR thp tt hn hn so vi 2 phng php cn li.Nu xt gia hai nhm thut ton ly ngng khng lp v c lp th ta li thy: i vi tn hiu c mc SNRnh (nh hn 5dB) c mc nhiu ln th phng php EMD CIIT v EMD-IITt ra vt tri hn so vi phng php khng lp.C thkhi SVTH: Trn Hiu Trung Dng Minh Tin 60



tn hiu gc l -5dB, 0dB th gi tr Kcng ln th mc SNR ng ra cng cao.Tuy nhin, thi gian x l cng tng ln rt nhiu. Mt khc, EMD - CIIT ci thin cht lng tt hn EMD - IIT bi v thc t cc IMF u tin khng phi lun lun c 100% l nhiu m vn tn ti mt vi mu thng tin thoi. Ngc li, vi nhng tn hiu c mc SNR ln th vic p dng phng php EMD IIT v EMD - CIIT khng hiu qu v lc ny IMF u tin khng phi ch c nhiu.Ngoi ra,khi tn hiu gc c SNR cao (ln hn 10dB) phng php EMD - IIT thm ch cn lm gim cht lng tn hiu.Nguyn nhn: trong IMF u tin c cha 1 lng thng tin c ch,nhng khi tin hnh to cc phin bn ca tn hiu th gii thutEMD - IIT khng c gi li nhng lng thng tin ny.Trong khi ,vi gii thut EMD - CIIT th mc tn hiu c x l cao hn tn hiu gc v trong gii thut ta c gi li nhng thnh thoi c trong IMF u tin. Tuy nhin, mc SNR ca tn hiu sau khi x l,vi thng s Kcng ln th khng c s ci thin bi v cu trc ph v s phn b nng lng trong IMF u tin b thay i. 4.3.4. nh hng ca cch chuyn i ngu nhin v tr cc muca IMF u tin trong phng php lp Trong gii thut EMD - IIT v EMD - CIIT khithay i ngu nhin v tr cc mu trong IMF u tin to racc phin bn ca, ta c 2 cch thc hin: Cch 1: cc mu c hon v ngu nhin theo vng trnhay cn gi l hon v vng (ramdom circulation) Cch 2: cc mu c thay i v tr ngu nhin (ramdom permutation) Trong phn m phng ny, ta ch kho st s nh hng ca cch chuyn v mu trong phng php EMD - CIIT EMD - CIIT(c): chuyn v mu theo vng hay cn gi l hon v vng EMD - CIIT(p): chuyn v mu ngu nhin


61



Bng 4.15:nh hng cch hon i mu i vi phng phply ngng cng iSNR (dB) SNR K=1 K=5 CIIT(p) -5 0 5 10 0.48 3.88 7.38 11.05 1.54 4.61 7.69 10.86 CIIT(c) 1.44 4.71 7.88 11.02 oSNR(dB) K=10 CIIT(p) 1.76 4.77 7.82 10.8 CIIT(c) 1.63 4.83 7.90 10.9 M2=6, C=0.7 M2=4, C=0.6 M2=3, C=0.5 M2=3, C=0.4 M2 v C

Chng ta quan st thy rng trong tt c cc trng hp hon v vng ci thin cht lng tn hiu tt hn so vi chuyn v ngu nhin. IMF u tin c tp trungtrong nabng tn trn ca ph tn hiu v do quy trnh chuyn vmu s thc thi tt nht trong vic ci thin cht lng thoikhi nhng IMF c thay i mu cng chim gi nhng tn s ging nh IMF gc. Tuy nhin, vic ny ch ng i vi trng hp hon v vng. Ngc li, phng php thay i mu theo kiuchuyn v ngu nhin chc chn dn ns ti phn bca nng lng IMF trn tonbng .Do , khi thc hin chuyn v ngu nhinth vn kh nhiuphi c xem xt kht khe hn theo hngphn nhiunm trong ccphin bn tn hiukhng cn l nhiu trng.


62

Chng 5: Kt lun v hng pht trin


Chng 5

KT LUN V HNG PHT TRIN

5.1 Kt lun Trong lun vn ny,chng ti p dng m hnh thc nghim EMD phn r tn hiu v ly ngngqua cc thut ton.Nghin cu ca ti ny tp trung vo bi ton nng cao cht lng thoi b nh hng bi nhiu trng, c tnh cht cng v khng tng quan vi thoi gc.Trong phn kt lun ny, chng ti xin a ra nhn xt v kt lunring cho tng chng. Chng 1:Lc nhiu tn hiu thoi nhm mc ch ci thin kh nng nhn dng v hiu c thng tin thoi thng qua vic lc b cc thnh phn nhiu. Lc nhiu tn hiu thoi c th c ng dng cho h thng thng tin di ng, h thng nhn dng ting ning dng cho robot, tu binBi v tm quan trng ca vic ci thin cht lng tnhiu thoi, nn c rt nhiu thut ton, phng php v ang c pht trin cho mc ch ny. Cc phng php, thut ton c nghin cu c th chia thnh 2 nhm: c tham s v khng c tham s. Ngoi ra chng cn c chia theo phng php n knh v a knh. Trong lun vn ny, ccgii thut c cp thuc nhm khng c tham s v n knh. Chng 2: M hnh thc nghim phn r (EMD) [16], [17]c xem nh l mt phng php phn tch rt hu hiu vi tn hiu phi tuyn v khng dng. EMD phn r tn hiu thnh cc hm dao ng c tr trung bnh bng khng gi l hm dao ng ni ti IMF.Cc IMF cho php nhn dng hu hiu v thch hp vi nhng tn s tc thi.iu ny lm cho EMD c hiu qu rt cao v vt tri hn sovi bin i Fourier v wavelettrong vic phn tchnhng tn hiu khng dng. Mt trong nhng ng dng thnh cng ca EMD l ci thin cht lng ting ni. Bi v quy trnh chn lcto ra cc IMF da vo vic tr nhng thnh phn dao ng cao nht theo tng bc, nn trong IMF u tin cha ch yu l cc thnh phn dao ng SVTH: Trn Hiu Trung Dng Minh Tin 63

Chng 5: Kt lun v hng pht trin


c tn s cao. V vy,nhng IMF c bc cng thp th cha nhng thnh phn dao ng c tn s cng cao.Tuy cc IMF c th b chng ln tn s nhng mt thi im bt k th tn s tc thi c th hin bi mi IMF l khc nhau. Do , mc d cc IMF l nm trong min thi gian nhng chng vn c s khc nhau v ph ti mt thi im bt k.Ngoi ra, cng v c tnh tn s ca cc IMF nn phng sai ca thnh phn nhiu v thoi l khc nhau trong mi IMF.Thut ton ly ngng c th c p dng trn cc IMF loi b nhng thnh phn nhiu. p dng m hnh IMF u tin ch cnhiu v s dng lch tuyti trung v (MAD)c lng phng sai nhiu vn c vai tr quan trng trong vic xc nh ngng lc nhiu. Chng 3:Chng ny trnh by mt s thut ton ly ngng trong min EMD lc nhiu thoi. Cc thut ton ny c chia thnh ba nhm: Thut ton ly ngng EMD trn khong v khng; trong khung con IMF v thut ton ly ngng mm kt hp DCT - EMD. Ccphng php ly ngng trong minEMD trong khong v khng bao gm: Phng php ly ngng trn tng khong (IT, SIT, SCAD) v phng php lp (EMD - IIT v EMD -CIIT). Cc phng php ny

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BÁO CÁO LUẬN VĂN 2011