Ssanh Ty So Cong Suat Dinh Trung Binh

7/27/2019 Ssanh Ty So Cong Suat Dinh Trung Binh

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Tuyn tp Bo co Hi ngh Sinh vin Nghin cu Khoa hc ln th 6 i hc Nng - 2008

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SO SNH T S CNG SUT NH TRUNG BNH CAH THNG FOURIER OFDM V WAVELET OFDMPEAK-TO AVERAGE POWER RATIO COMPARISON BETWEEN FOURIER

OFDM SYSTEM AND WAVELET OFDM SYSTEM

SVTH:NGUYN THY TRINHNGUYN THNH THO

03DT2, KhoaTVT, Trng i hc Bch Khoa, HNGVHD: ThS. NGUYN DUY NHT VINKhoa in t Vin thng, Trng i hc Bch Khoa

TM TTT s cng sut nh trung bnh (PAPR) ln l nhc im chnh ca k thut ghp knh phnchia theo tn s trc giao (OFDM). Mc ch chnh ca bi bo ny l nghin cu so snhPAPR ca h thng OFDM v h thng OFDM s dng php bin i Wavelet. Php bin iWavelet l mt php bin i trc giao c nhiu u im mi, n s c thay th cho phpbin i Fourier trong h thng OFDM truyn thng v c th gi h thng mi ny l WaveletOFDM. T kt qu m phng, chng ta c th chn la m hnh ng dng ca Wavelet voh thng OFDM c gi tr PAPR ph hp nht.

ABSTRACT

High Peak-to-Average Power Ratio (PAPR) is a major drawback of Orthogonal FrequencyDivision Multiplexing (OFDM) technique. Objective of this paper is the study of PAPRcomparison between OFDM system and Wavelet base in OFDM system. The wavelettransform which offers many of new advantage is an orthogonal transform. The conventionalOFDM system is replaced Fourier transform with Wavelet transform. Afterward this newsystem is called Wavelet OFDM system. Basing on all results given in this paper, we couldselect the most suitable PAPR model of Wavelet OFDM system.

1.t vn K thut OFDM l mt trng hp c bit ca iu ch a sng mang, ph hp cho

vic thit k mt h thng c tc truyn dn cao, loi b c nhiu ISI, ICI Bn cnhnhng u im ni bt, n vn tn ti nhng nhc im nht nh. Nhc im chnh caOFDM l t s cng sut nh trung bnh PAPR kh ln.

PAPR ln do OFDM s dng nhiu sng mang truyn thng tin, gi tr cc i cak t trn mt sng mang c th vt xa mc trung bnh trn ton b sng mang. V vy, khng lm mo tn hiu pht, b khuch i cng sut phi t ch d tr ln nn hiusut s dng khng cao. khc phc nhng hn ch ny, ngi ta a ra nhiu phng

php. Tuy nhin, khng phng php no c coi l u th hn nhng phng php khc.V hngnghin cu thay i php bin i FFT trong b iu ch OFDM bng php bini Wavelet l mt hng hon ton mi.

2.T s cng sut nh trung bnh PAPR

PAPR c nh ngha l t s cng sut nh tc thi trn cng sut trung bnh. cbiu din bi cng thc ton hc sau:

2

0

2

0

)(

)(max

tsmean

tsPAPR

s

s

Tt

Tt

(1)


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82

Vi s(t) l k t a sng mang trong khong thi gian 0


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83

(a) b

hiu trong bng gc ( min thi gian) v c chn khong bo v trc khi pht i. Tngt nh vy, bn thu s thc hin cc qu trnh ngc li.3.2.H thng Wavelet OFDM

Hng nghin cu vi mc ch thay i php bin i FFT trong b iu ch OFDMbng php bin i Wavelet (DWT) nhm ci thin s nhy cm ca h thng i vi hiu

ng dch tn do mt ng b gy ra v gim ti thiu chiu di chui bo v, c bit gimPAPR trong h thng OFDM l hng nghin cu rt mi v ang c quan tm trongnhng nm gn y.

Hnh 5: (a) Cu trc b lctng hp bn pha pht

(b)S tng ngca mt php bin i IDWT

H thng Wavelet OFDM s dng hai bng lc: bng lc tng hp SFB (synthesisfilter bank) bn pha pht v bng lc phn tch AFB (analysis filter bank) bn pha thu. KhiSFB c chc nng ging nh khi IFFT bn pha pht v khi AFB c chc nng nh khiFFT bn pha thu trong h thng OFDM. So vi h thng da trn FFT, chng ta c th iuchnh cc bng con hay sng mang ph chng ph ln nhau trong cu trc SFB-AFB bngcch iu chnh bng lc wavelet trong mi bng con. Tnh linh hot ny chnh l mt trongnhng u im ca h thng Wavelet OFDM. V c bit h thng Wavelet OFDM khngcn s dng khong bo v, v vy khng phi tiu tn khong 25% bng thng cho vic chnkhong bo v nh trong h thng OFDM truyn thng.

Trong hnh 5 th hin cu trc bng lc tng hp bn pha pht, vi 2 n tn hiu uvo, n l s bng, Hi l b lc thng cao (HPF), Hol b lc thng thp (LPF). Trc khi qua

Rx bitstream

Tx bit

stream M

O

D

S/PSFB AFB P/S

D

E

M

Supersymbol

(=OFDM symbol)

Hnh 4: S khi b thu pht h thng OFDM da trn php bin i Wavelet

Ho

Ho

Ho

Ho

Ho

Ho

HoHi

Hi

Hi

HiHi

Hi

Hi

Tx Signals

+

+

+

+

+

+

+

2

2

2

2

2

2

2

2

2

2

2

2

2

2

a 3

a 1

a 2

a4

a

a 5

a 6

a 2n

a 1

a 2 2

2 Hi

Ho

+ Out put


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b lc tn hiu c c tng s mu ln 2 ln (upsampling), tng ng vimt php bini IDWT. Bn pha thu s c thc hin ngc li v s dng php DWT tng ng.

4.Kt qu m phng S dng ngn ng Matlab m phng, vi phin bn Matlab 7.0 c h tr thm nhiu

hm v tin ch mi h tr tt hn cho vic m phng. Mphng tn hiu qua h thng OFDM 64 knh nh s khi hnh 4. Tnh PAPR trn mi

tn hiu OFDM theo hm CCDF. M phng cng tn hiu u vo ca h thng OFDM i vi h thng Wavelet OFDM, thay

php bin i IFFT bng php bin i IDWT v s dng hm CCDF v PAPR trn cng th so snh.

Kt qu ca php bin i Wavelet thay i khi ta chn h Wavelet khc nhau. Ba php bini Wavelet c s dng trong bi m phng so snh vi php bin i Fourier l:Haar Wavelet (Haar), Daubechies Wavelet (Db), Symlets Wavelet (Sym).

4.1.So snh hm CCDF ca PAPR Wavelet 2 bng vi OFDM 64 knh

i vi h thng Wavelet OFDM s dng m hnh 2 bng lc tng hp bn pht(tng ng 1 php bin i IDWT) v 2 bng lc phn tch bn thu (tng ng 1 php

bin i DWT).Nhn Xt: Trn hnh 6, ta thy rng hm CCDF ca PAPR Wavelet OFDM cho kt

qu tt hn nhiu Fourier OFDM. Haar Wavelet cho kt qu tt nht, Papr ln nht ch khong3db. Cn Sym v Db bng nhau nhng ln hn Haar gn gp i v mc 6db l ln nht.Fourier OFDM cho papr khong 9-10db.4.2.So snh hm CCDF ca PAPR Wavelet 4 bng vi OFDM 64 knh


bin i DWT).Nhn Xt: Trn hnh 7, ta thy rng: xc sut 10-3, Papr ca OFDM l 10db. Haar

cho kt qu gim hn 4db, cn Db v Sym th ch gim c 3db. Tuy nhin Papr ca Wavelet4 bng cho kt qu xu hn 2 bng.4.3.So snh hm CCDF ca PAPR Wavelet 8 bng vi OFDM 64 knh

Hnh 7: Hm CCDF ca PAPR Wavelet4 bng so vi OFDM 64 knh



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bin i DWT).Nhn Xt: Trn hnh 8, ta thy

rng: xc sut 10-3

, Papr ca OFDM l10db. Wavelet 8 bng cho kt qu Haar,Db v Sym gn bng nhau v ch gimc 2db. Tuy nhin Papr ca caWavelet 8 bng cho kt qu xu hn 4

bng v 2 bng.

5.Kt lunBi bo a ra cc m hnh

ng dng c th php bin i Wavelet

vo h thng OFDM vi s bng lc l:2, 4, 8 bng. V so snh nh gi hmCCDF ca papr gia cc bng lc vi hthng OFDM 64 knh. Ty theo yu cu v ng dng c th m chn m hnh bng l c chokt qu ph hp nht.

TI LIU THAM KHO

[1] Khoirul Anwar Peak-to-Average Power Ratio Reduction of OFDMSignals ,February 3, 2005

[2] Seema Khalid and Syed Ismail Shah PAPR Reduction by Using Discrete Wavelet.Transformin IEEE-ICET 2006 2nd International Conference on EmergingTechnologies,Peshawar, Pakistan, 13-14 November 2006

[3] B.G. Negash and H.Nikookar Wavelet Based OFDM for Wireless Channelsin IEEE,2001

[4] P.Foomooljareon and W.A.C.Fernand PAPR Reduction in OFDM SystemsThammasaItn t. J. Sc.T ech.,V ol.7, No.3, September-December 2002

[5] Haixia Zhang, Dongfeng Yuan, Matthias Ptzold Novel study on PAPRs reduction inwavelet-based multicarrier modulation systems, 5 September 2006


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Ssanh Ty So Cong Suat Dinh Trung Binh