mo phong truyen dan ofdm

8/2/2019 Mo Phong Truyen Dan OFDM

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Mc lc

Chng 1 .......................................................................................................................1

Gii thiu chung ...........................................................................................................21.1. Nhng hn ch ca k thut hin hnh ...............................................................................31.2. Ghp knh phn chia theo tn s trc giao OFDM ..............................................................4

Chng 2 .......................................................................................................................5c tnh knh v tuyn di ng ...................................................................................5

2.1. M u ..................................................................................................................................52.2. Min khng gian ...................................................................................................................62.3. Min tn s ............................................................................................................................7

2.3.1. iu ch tn s .................................................................................................. 72.3.2. Chn lc tn s .................................................................................................. 7

2.4. Min thi gian .......................................................................................................................82.4.1. Tr tri trung bnh qun phng ........................................................................ 82.4.2. Tr tri cc i .................................................................................................. 92.4.3. Thi gian nht qun ........................................................................................... 9

2.5.Quan h gia cc thng s trong cc min khc nhau ..........................................................92.5.1. Bng thng nht qun v tri tr trung bnh qun phng ................................. 92.5.2. Thi gian nht qun v tri Doppler ................................................................ 10

2.6. Cc loi pha inh phm vi hp ...........................................................................................102.7. Phn b Rayleigh v Rice ...................................................................................................11

2.7.1. Phn b pha inh Rayleigh .............................................................................. 112.7.2. Phn b Pha inh Rice ..................................................................................... 13

2.8.1. M hnh knh trong min thi gian .................................................................. 142.8.2. M hnh knh trong min tn s ...................................................................... 16

2.9. nh hng ca tha s K knh Rice v tri tr ln cc thuc tnh knh trong min tn s....................................................................................................................................................172.10. Kt lun .............................................................................................................................20

Chng 3 ....................................................................................................................22Nguyn l hot ng ca OFDM ...............................................................................22

3.1. M u ................................................................................................................................223.2. Tnh trc giao ......................................................................................................................223.3. M hnh h thng truyn dn OFDM .................................................................................25

3.3.1. M t ton hc tn hiu OFDM ......................................................................................253.3.2. S h thng truyn dn OFDM ..................................................................................263.2.2.1. Tng chuyn i ni tip sang song song ...................................................... 283.3.2.1. Tng iu ch sng mang con ....................................................................... 283.3.2.3. Tng chuyn i t min tn s sang min thi gian .................................... 293.3.2.4. Tng iu ch sng mang RF ........................................................................ 29

3.4. Cc thng s c trng v dung lng h thng truyn dn OFDM ...............................313.4.1. Cu trc tn hiu OFDM ................................................................................. 313.4.2. Cc thng s trong min thi gian TD ............................................................. 323.4.4. Quan h gia cc thng s trong min thi gian v min tn s....................... 333.4.5. Dung lng ca h thng OFDM .................................................................... 33

3.5. Cc nhn t nh hng ca knh pha inh ln hiu nng h thng truyn dn OFDM vcc gii php khc phc ............................................................................................................35

3.5.1. ISI v gii khc phc ....................................................................................... 35


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3.5.2 nh hng ca ICI v gii php khc phc ...................................................... 383.5.3 Ci thin hiu nng h thng truyn dn trn c s kt hp m ho Gray ........ 413.5.4 Gii php nng cao hiu qu s dng ph tn ca h thng truyn dn OFDM 433.5.4.1 Phng php dng b lc bng thng ............................................................ 443.5.4.2 Phng php dng khong bo v cosin tng ................................................ 48

3.6 Kt lun ................................................................................................................................51

Chng 4 .....................................................................................................................52c tnh cht lng knh v cn bng knh ............................................................52

4.1 Gii thiu .............................................................................................................................524.2 c tnh knh bng PSAM .................................................................................................52

4.2.1 Ni suy Gauss ................................................................................................... 534.2.2 Ni suy FFT ...................................................................................................... 534.2.3 Ni suy Wienner ............................................................................................... 55

4.3 K thut cn bng p ng knh .........................................................................................554.3.2 B cn bng bnh phng li trung bnh tuyn tnh LMSE .............................. 57

4.4 Kt lun ................................................................................................................................59

Chng 5 .....................................................................................................................60iu ch OFDM thch ng .........................................................................................605.1 Gii thiu .............................................................................................................................605.2 M hnh h thng truyn dn iu ch thch ng ...............................................................61

5.2.1 Khi nim c bn v iu ch thch ng ........................................................... 615.2.2 Kin trc ca nhng h thng iu ch thch ng ............................................. 62

5.2.3 Nguyn tc xy dng gii thut iu ch thch ng .........................................................635.3 Xy dng gii thut thch ng cho h thng truyn dn OFDM thch ng .......................63

5.3.1 Thut ton thch ng theo SNR pht trn mi sng mang con .......................... 645.3.2 Thut ton thch ng da theo mc iu ch .................................................... 655.3.3 Thut ton thch ng da trn c ch chn lc sng mang ................................ 67

5.6 Kt lun ................................................................................................................................73Chng 6 ....................................................................................................................74Chng trnh m phng h thng truyn dn OFDM thch ng ...........................74

6.1 Gii thiu .............................................................................................................................746.2 M hnh m phng h thng truyn dn OFDM thch ng ...............................................74

6.2.1 M hnh m phng ........................................................................................... 746.2.2. Thit lp cc thng s m hnh m phng ....................................................... 77

6.3 Chng trnh m phng truyn dn OFDM thch ng .......................................................846.3.1 Giao din chng trnh m phng .................................................................... 84

6.3.2 Cc kt qu m phng v nh gi hiu nng ..................................................................87

6.3.2.1 Kt qu m phng khng dng c ch thch ng ........................................... 876.3.2.2 Kt qu m phng dng c ch thch ng mc iu ch ................................ 896.3.2.3 Kt qu m phng dng c ch thch ng chn lc sng mang ...................... 926.3.2.3 Kt qu m phng dng kt hp hai c ch thch ng chuyn mc iu ch vchn lc sng mang ................................................................................................... 95

6.4 nh gi hiu nng ca cc c ch thch ng thng qua kt qu m phng .................... 986.5 Kt lun ..............................................................................................................................106

Kt lun .....................................................................................................................107Ti liu tham kho ....................................................................................................108


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Danh mc bng

Bng 1.1. c tnh dch v ca UMTS..............................................................................1

Bng 1.2. Tham s c trng ca cc h thng s dng cng ngh OFDM.....................2Bng 2.1. Cc loi pha inh phm vi hp..........................................................................9

Bng 2.2. Cc c tnh knh trong ba min: khng gian, tn s v thi gian................. 20

Bng 3.1. Mi quan h gia cc tham s OFDM ............................................................32

Bng 3.2 M ho Gray cc bit nh phn.......................................................................... 41

Bng 3.3 Tham s khong bo v RC ca IEEE 802.11a............................................... 48

Bng 5.1 iu khin mc iu ch da trn cc mc SNR thu ......................................65

Bng 6.1Thng s h thng dng cho m phng tn hiu OFDM................................... 76

Bng 6.2 Thng s m phng h thng OFDM thch ng.............................................. 79

Bng 6.2 Tham s BER iu khin chuyn mc iu ch............................................... 89


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Danh mc hnh v

Hnh 2.1. Tnh cht knh trong min khng gian, min tn s v min thi gian.........................5

Hnh 2.3 Phn b xc sut Gauss trong khng gian.....................................................................11

Hnh 2.4 Phn b xc sut Rayleigh trong khng gian, 12 = ..................................................12

Hnh 2.5 Phn b xc sut Rice vi cc gi tr K khc nhau, 12 = .........................................13

Hnh 2.6. M hnh l lch tr cng sut trung bnh......................................................................15

Hnh 2.7 Ph tn hiu OFDM truyn qua m hnh knh pha inh Rice, vi s sng mang = 100,

kch thc FFT = 300...................................................................................................................17

Hnh 2.8. Ph thuc bin hm truyn t knh min tn s vo tn s v RDS.....................18

Hnh 2.9. Ph thuc bin hm truyn t knh vo K v tn s.............................................18

Hnh 2.10. Hm truyn t ca knh khi RDS=30ns vi cc gi tr K khc nhau.......................19

Hnh 3.1 Dng sng ca mt tn hiu OFDM trong min thi gian v tn s.............................23

Hnh 3.2 Hnh dng ph ca tn hiu OFDM bng tn c s 5 sng mang, hiu qu ph tn ca

OFDM so vi FDM.....................................................................................................................23

Hnh 3.3 Ph tng hp ca tn hiu OFDM trong bng tn c s vi 5 sng mang con.............24

Hnh 3.4 S khi h thng truyn dn OFDM........................................................................26

Hnh 3.5. Tn hiu pht 16-QAM s dng m ho Gray, v tn hiu 16-QAM truyn qua knh v

tuyn, SNR = 18 dB................................................................................................................27

Hnh 3.6. Tng IFFT, to tn hiu OFDM....................................................................................28

Hnh 3.7 iu ch cao tn tn hiu OFDM bng tn c s phc s dng k thut tngt..................................................................................................................................................28

Hnh 3.8 iu ch cao tn tn hiu OFDM bng tn c s phc s dng k thut s.................29

Hnh 3.9. Dng sng tn hiu OFDM trong min thi gian.........................................................29

Hnh 3.10 Tn hiu OFDM dch DC, W l bng tn tn hiu, fofftn s dch t DC, fc l tn s

trung tm......................................................................................................................................30

Hnh 3.11 Cu trc tn hiu OFDM.............................................................................................30

Hnh 3.12 rng bng tn h thng v rng bng tn sng mang con................................32

Hnh 3.13. Chn thi gian bo v cho mi k hiu OFDM.........................................................35Hnh 3.14. Cu trc tn hiu OFDM trong min thi gian...........................................................35

Hnh 3.15 Hiu qu ca khong bo v chng li ISI.................................................................36

Hnh 3.16 Hiu qu ca khong bo v loi b ISI................................................................37

Hnh 3.17 Nhiu nn do ICI i vi s sng mang con khc nhau............................................38

Hnh 3.18 nh hng ca ICI ti t s tn hiu trn nhiu........................................................38

Hnh 3.19 Cng sut ICI chun ho i vi tn hiu OFDM. N=102..........................................39

Hnh 3.20 Cng sut ICI chun ho cho sng mang con trung tm (fdT=0,2)............................40

Hnh 3.21 S IQ iu ch 16-QAM v 16-PSK s dng m ho Gray..................................41

Hnh 3.22 Biu IQ cho tn hiu 64-PSK v 128-PSK.............................................................42

Hnh 3.23 Biu IQ cho tn hiu 64 QAM v 1024-QAM.......................................................42


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Hnh 3.24 c tuyn b lc dng ca s Kaiser vi ft = 0.2 Hz, ft = 0.4 Hz, = 3.4,............... 44

Hnh 3.25 Cu trc ca ca s Kaiser vi 10= , v 50= ...................................................44Hnh 3.26 Ph ca tn hiu OFDM 52 sng mang (a) v 1536 sng mang con (b), khng dng b

lc...........................................................................................................................................45

Hnh 3.27 Ph tn hiu OFDM 20 sng mang khng dng b lc (a) v dng b lc vi ca s

Kaiser vi 10= (b)...................................................................................................................45

Hnh 3.28 Ph tn hiu OFDM 20 sng mang, dng b lc vi ca s Kaiser vi 50= ........46

Hnh 3.29 SNR ca mi sng mang con ca tn hiu OFDM khi s dng b lc.......................47

Hnh 3.30 Cu trc ca khong bo v RC..................................................................................47

Hnh 3.31 ng bao k hiu OFDM vi mt khong bo v phng v mt khong bo v RC

chng ln......................................................................................................................................48

Hnh 3.32 Cng sut ng bao bn ca tn hiu OFDM 20 sng mang con, vi chiu di khong

bo v RC thay i..........................................................................................................49

Hnh 3.33 Cng sut ng bao bn ca tn hiu OFDM 100 sng mang con, vi chiu di

khong bo v RC thay i..........................................................................................................49

Hnh 3.34 Cng sut ng bao bn ca tn hiu OFDM 4000 sng mang con, vi chiu di

khong bo v RC thay i..........................................................................................................49

Hnh 4.1 Khun dng mt khung truyn dn OFDM c gn cc k hiu hoa tiu.....................51

Hnh 4.2 Gii thut FFT...............................................................................................................52

Hnh 4.3 S h thng truyn dn.............................................................................................53

Hnh 4.4 S b cn bng trung bnh li bnh phng tuyn tnh............................................56

Hnh 5.1 Qu trnh pht trin ca cc cng ngh i u ch thch ng.........................................60Hnh 5.2 Lu thut ton iu ch thch ng............................................................................61

Hnh 5.3 Kin trc ca cc h thng iu ch thch ng..............................................................61

Hnh 5.4 Ngng SNR chuyn mc cho c ch thch ng theo s iu ch..........................65

Hnh 5.5 M hnh thut ton thch ng theo c ch chn lc sng mang cho h thng truyn dn

OFDM..........................................................................................................................................69

Hnh 5.6 Lu thut ton ca khi quyt nh..........................................................................70

Hnh 5.7 Lu thut ton ca khi iu khin chn.................................................................71

Hnh 6.1 M hnh m phng h thng truyn dn OFDM thch ng..........................................75Hnh 6.2 Tng thch gia tc d liu ngi dng v s sng mang.....................................77

Hnh 6.3 Tng thch gia kch thc FFT v s sng mang.....................................................78

Hnh 6.4 Sp xp cc mu tn s trong k hiu OFDM trc khi bin i IFFT........................79

Hnh 6.5 M phng tn hiu OFDM trong min thi gian...........................................................79

Hnh 6.6 p ng xung kim ca knh.........................................................................................82

Hnh 6.7 Hnh dng hm truyn t ca knh..............................................................................82


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Thut ng vit tt


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Chng 1

AC Alternating Current Dng xoay chiu (tn s khc 0)AM Adapting Multi-access scheam Thch ng lc a truy nhpAOFDM Adaptive Orthogonal Frequency

Division Multi-Accessa truy nhp phn chia theo tn strc giao thch ng

AWGN Additive White Gaussian Noise Tp m Gauss trng cngBER Bit Error Rate T s bit liBPS Bit per symbol S bit trn mt k hiuCCI Co-channel interference Nhiu ng knhCF Crest Factor Tham s CrestCINR Carrier to interference plus Noise

ratioT s sng mang trn nhiu v tpm

CIRChannel impulse response

p ng xung kim knhCOFDM Coding Orthogonal FrequencyDivistion Multiplex

M ho ghp knh phn chia theo tns trc giao

DAB Digital Audio Broadcast system H thng pht thanh sDAC Digital Analog Converter B chuyn i s sang tng tDC Direct Current Dng mt chiu (tn s bng 0)DFT Discreat Fourier Transformation Bin i Fourier ri rcDDS Direct Digital Synthesis ng b s trc tipDFE Decision Feed back Equalizer Phn hi quyt nhDMT Discete Multi-Tone a tn ri rc

DSP Digital Signal Process X l tn hiu sDS Delay Spread Tri trDVB Digital Video Broadcast Truyn hnh sFEC Forward Error Correction Sa li trcFFT Fast Fourier Transformation Bin i Fourier nhanhFIR Finite Impulse Response p ng xung kim hu hnHDTV Hight Difinition Television Truyn hnh nt caoHiperLAN2 High Performance Radio Local Area

Network, WLAN standard (Europe)based on OFDM, with maximum

data rate of 54 Mbps

Chun WLAN ca Chu u choOFDM vi tc d liu ti a l54 Mbps

ICI Inter-Carrier Interference Nhiu giao thoa gia cc sng mangIEEE802.11a WLAN standard (U.S) based on

OFDM, with a maximum data rateof 54 Mbps.

Tiu chun WLAN cho OFDM vitc d d liu ti a l 54 Mbps

IEEE802.11b WLAN standard (U.S) based onDSSS, with maximum data rate of

11 Mbps

Tiu chun WLAN da trn DSSSvi tc d liu ti a l 11 Mbps

IFFT Inverse Fast FourierTransformation

Bin i Fourier ngc nhanh

IMD Inter-Modulation Distortion Nhiu iu ch ni

IQInphase Quadrature

ng pha vung phaISI Inter-Symbol Interference Nhiu giao thoa gia cc k hiu

JPEG Joint Photographic Experts Group(Image compress standard)

nh dng file nh tnh ch nn

LM-MSE LinEariry-Mean Square error

Equalizer

B cn bng sai s bnh phng cc

tiu tuyn tnhLOS Line Of Sight ng nhn thngMIMO Multi-Input and Multi-Output H thng a ng vo a ng raMMSE Maximum Mean Square error

Estimationc tnh cc i trung bnh li bnh

phngMPEG Moving Picture Experts Group

(Video compress standard)nh dng file nh ng ch nn

M-PSK M-Phase Shift Keying Kho dch pha M trng thiMSE Mean Square Error Trung bnh li bnh phngOFDM Orthogonal Frequency Division

Multiplex

Ghp knh phn chia theo tn s trc

giaoPAPR Peak to Average Power Ratio T s cng sut nh trn cng sut

trung bnhPSD Power Spectrum Density Hm mt ph cng sutPSAM Pilot Symbol Assisted Modulation iu ch c h tr bi k hiu

hoa tiuQAM Quadrature Amplitude Modualtion iu ch bin cu phngQoS Quality of Service Cht lng dch vRC Rised Cosin Khong bo v cosin tngRF Radio Frequency Tn s v tuyn

RMS Root Mean Squared value Gi tr cn bc hai trung bnh qunphng

SF Spread Factor Tham s tri phSINR Signal to Interference Plus Noise

RatioT s tn hiu trn nhiu v tp m

SNR Signal to Noise Ratio T s tn hiu trn tp mUMTS Universal Mobile

Telecommunications SystemH thng vin thng di ng toncu

W-CDMA Wide Band Code DivisionMulti-Access

a truy nhp phn chia theo m bngtn rng

WLAN Wireless Local Area Network Mng khng dy ni vngZF Zero Forcing equalizer B cn bng cng bc khng


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Gii thiu chung

Do tnh di ng v tnh tin dng m cc h thng truyn thng v tuyn mang li hiuqu cao trong vic s dng, khai thc trao i thng tin cho ngi dng. V th nhu cu s

dng, chim dng ti nguyn v tuyn ngy cng gia tng nhanh chng, yu cu ngycng nhiu cc nh khai thc, cng nghip vin thng tp trung khai thc th mnh ny nhiu hnh thc khc nhau. Kt qu mang li ngun thu v kch thch thc y tngtrng kinh t c bit trong xu th hi nhp cnh tranh. Theo , ngy cng xut hinnhiu hnh thc dch v, tnh a dng ca cc cng ngh mi nhm khai thc trit tinguyn v i ph hiu qu nhng nh hng vn c ca mi trng v tuyn, v d nhmng khng dy ni ht (WLAN). Tuy nhin vi s tng trng theo hm m caInternet i hi nhng phng php mi c mng khng dy dung lng ln. Hthng di ng th h th ba, h thng truyn thng di ng ton cu (UMTS) vCDMA2000 [1] hin ang c trin khai ti nhiu quc gia trn th gii v bc u tc nhng thnh cng ng k. Bng 1.1 s lit k c tnh ca cc dch v m UMTSh tr:

Bng 1.1 c tnh dch v ca UMTSDch v Tc d liu yu cu Cht lng dch

v yu cuYu cu tnh

thi gian thcBn tin ngn(email, chat)

Thp (1-10 kbps) Cao Khng

Thoi Thp (4-20 kbps) Thp (BER < 10-3) CDuyt Web Kh bin (>10 kbps cho

n 100 kbps)Cao (BER < 10-9) Thng thng

l khngHi ngh truynhnh

Cao (100 kbps-1 Mbps) Trung bnh C

Camera theo di Trung bnh (50-300 kbps) Trung bnh KhngTing cht lngcao

Cao (100-300 kbps) Trung bnh C

Truy nhp c sd liu

Cao (> 30 kbps) Rt cao Khng

i vi nhng ng dng trong mi trng di ng , thy r trong tng lai gn mt shi t ca cng ngh in thoi di ng, my tnh, truy cp Internet, v nhiu ng dngtim nng khc nh video v audio cht lng cao, vi s thm vo kh nng gi v nhnd liu s dng my tnh sch tay v in thoi di ng. Khi ch vi mt chic inthoi nh b ngi dng c th xem truyn hnh theo yu cu (VOD), hi ngh truynhnh v nghe nhc, xem film cht lng cao trc tuyn, nhng tc d liu yu cu s

>30 Mbps. Vi tc cao nh vy th cc h thng di ng th h ba hin nay cha png c. V th yu cu c t ra l ci thin nhiu hn hiu qu ph tn v tc truyn d liu ca cc h thng di ng. Hin nay cc h thng WLAN, HiperLAN/2,IEEE 802.11a, IEEE 802.11b c trin khai thc t v cung cp tc truyn d liu


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rt cao. iu c bit l cc h thng trn u da trn c s cng ngh OFDM. Bng 1.2di y s lit k cc thng s c trng ca nhng h thng ny:

Bng 1.2 Tham s c trng ca cc h thng s dng cng ngh OFDM

Tham s h thng DAB DVB-T IEEE 802.11 HiperLAN/2

Tn s sng mang VHF VHF v UHF 5 GHz 5 GHzBng thng 1.54 MHz 7-8 MHz 20 MHz 20 MHzTc truyn d liuti a

1.7 Mbps 31.7 Mbps 54 Mbps 54 Mbps

S lng sng mangcon

192-1536 1705-6817 52 52

Kch thc FFT 256-2048 2048-8196 64 64

Ta thy u th ni bt ca cc h thng s dng cng ngh OFDM l thng lng ln,hiu qu s dng ph tn cao v i ph hiu qu nhng nhc im ca mi trng vtuyn (s c cp phn sau).

1.1. Nhng hn ch ca k thut hin hnh

K thut n sng mang

Cc k thut tri ph c s dng trong cc h thng thng tin di ng th h ba c khnng chng li pha inh v nhiu [2], song tn ti nhng yu cu khng thc hin cchng hn: nu ngi dng cn c tc 20 Mbps giao din v tuyn v h s tri ph

l 128 (gi tr in hnh hin nay), dn n phi x l tc 2,56 Gbps theo thi gianthc v th cn c rng bng tn ln khng thc t. Mt khc, thy r

Do ti nguyn ph tn hn hp, v vy cn phi s dng hiu qu.

Do nhng kh khn lin quan n hiu ng gn xa v c s tiu th cng sutln.

Ngoi ra, cc k thut n sng mang i ph km hiu qu i vi pha inh v truynlan a ng c bit trong trng hp tc bit rt cao. cc phng php iu chtruyn thng M-QAM, M-PSK, khi tc d liu truyn cao th ko theo rng khiu s gim, n mt gi tr m rng k hiu < tri tr cc i ca knh, khi knhs l knh la chn tn s v gy ISI cho tn hiu thu. y l mt nhc im chnh khincc h thng s dng cc phng php iu ch truyn thng khng th truyn d liu vitc cao, hoc gi thnh rt cao i vi nhng dch v yu cu tc d liu cao.


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K thut a sng mang trc giao OFDM

OFDM l mt cng ngh cho php tng rng k hiu truyn dn do dung sai ang ln hn rt nhiu so vi cc k thut s dng trc y, cho php khc phcnhng nhc im cn bn ca k thut n sng mang.

1.2. Ghp knh phn chia theo tn s trc giao OFDMGhp knh phn chia theo tn s trc giao (OFDM) l mt k thut iu ch c th thayth cho CDMA. OFDM c u im vt tri so vi nhng h thng CDMA v cung cp

phng php truy cp khng dy cho h thng 4G.

tng ca OFDM l chia ton b bng tn truyn dn thnh nhiu sng mang con trcgiao nhau truyn cc tn hiu trong cc sng mang con ny song song. Theo , lungd liu tc cao c chia thnh nhiu lung tc thp hn lm cho chu k k hiutng theo s sng mang con.

u im OFDM l gii php phn tp tn s. OFDM chia nh bng tn knh v tin hnh

truyn d liu c lp trn cc bng tn knh con ny.

OFDM t hiu qu s dng ph tn cao, do tnh trc giao ca cc thnh phnsng mang con.

OFDM l ng c vin ha hn cho truyn dn tc cao trong mi trng ding. S d OFDM lm c nh vy bi v, chu k k hiu tng cho nn dung sai tri trca h thng tng v hiu qu s dng ph tn cao ca cng ngh OFDM.

OFDM cho php gim c nh hng ca tr a ng v knh pha inh chnlc tn s chuyn thnh knh pha inh phng. V vy, OFDM l gii php i vi tnhchn lc tn s ca knh pha inh. Thun li ny ca OFDM cho php cn bng knh ddng.

Do tri rng pha inh tn s trn nhiu k hiu, nn lm ngu nhin ho li cm(do pha inh Rayleigh gy ra), nn thay v mt s k hiu cnh nhau b mo hon ton lmt s k hiu cnh nhau b mo.

Tnh kh thi ca OFDM cao do ng dng trit cng ngh x l tn hiu s vcng ngh vi mch VLSI.

Nhc im

OFDM nhy cm vi dch Doppler cng nh lch tn gia cc b dao ng nipht v thu. Do tnh trc giao ca cc sng mang con rt nhy cm vi knh truyn cdch Doppler ln.

Vn ng b thi gian. Ti my thu kh quyt nh thi im bt u ca khiu FFT.


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Chng 2

c tnh knh v tuyn di ng

2.1. M u

Trong thng tin v tuyn di ng, cc c tnh knh v tuyn di ng c tm quan trngrt ln, v chng nh hng trc tip ln cht lng truyn dn v dung lng. Trong cch thng v tuyn thng thng (khng phi cc h thng v tuyn thch ng), cc tnhcht thng k di hn ca knh c o v nh gi trc khi thit k h thng. Nhngtrong cc h thng iu ch thch ng, vn ny phc tp hn. m bo hot ngthch ng ng, cn phi lin tc nhn c thng tin v cc tnh cht thng k ngn hnthm ch tc thi ca knh.

Cc yu t chnh hn ch h thng thng tin di ng bt ngun t mi trng v tuyn.Cc yu t ny l:

Suy hao: cng trng gim theo khong cch. Thng thng suy hao nmtrong khong t 50 n 150 dB ty theo khong cch

Che ti: cc vt cn gia trm gc v my di ng lm suy gim thm tn hiu

Pha inh a ng v phn tn thi gian: phn x, nhiu x v tn x lm motn hiu thu bng cch tri rng chng theo thi gian. Ph thuc vo bng thng ca hthng, yu t ny dn n thay i nhanh cng tn hiu v gy ra nhiu giao thoa giacc k hiu (ISI: Inter Symbol Interference).

Nhiu: cc my pht khc s dng cng tn s hay cc tn s ln cn khc gynhiu cho tn hiu mong mun. i khi nhiu c coi l tp m b sung.

C th phn cc knh v tuyn thnh hai loi: "pha inh phm vi rng" v "pha inh phmvi hp". Cc m hnh truyn sng truyn thng nh gi cng sut trung bnh thu c ticc khong cch cho trc so vi my pht. i vi cc khong cch ln (vi km), ccm hnh truyn sng phm vi rng c s dng. Pha inh phm vi rng c biu th

bng tn hao do truyn sng khong cch xa. Pha inh phm vi hp m t s thng gingnhanh sng v tuyn theo bin , pha v tr a ng trong khong thi gian ngn haytrn c ly di chuyn ngn. Pha inh trong trng hp ny gy ra do truyn sng a ng.

Cc knh v tuyn l cc knh mang tnh ngu nhin, n c th thay i t cc ngtruyn thng n cc ng b che chn nghim trng i vi cc v tr khc nhau. Hnh2.1(a) cho thy rng trong min khng gian, mt knh c cc c trng khc nhau (bin chng hn) ti cc v tr khc nhau. Ta gi c tnh ny l tnh chn lc khng gian(hay phn tp khng gian) v pha inh tng ng vi n l pha inh chn lc khng gian.Hnh 2.1(b) cho thy trong min tn s, knh c cc c tnh khc nhau ti cc tn s khcnhau. Ta gi c tnh ny l tnh chn lc tn s (hay phn tp tn s) v pha inh tngng vi n l pha inh chn lc tn s. Hnh 2.1(c) cho thy rng trong min thi gian,knh c cc c tnh khc nhau ti cc thi im khc nhau. Ta gi c tnh ny l tnh

chn lc thi gian (hay phn tp thi gian) v pha inh do n gy ra l pha inh phn tpthi gian. Da trn cc c tnh trn c th phn chia pha inh knh thnh: pha inh chnlc khng gian (pha inh phn tp khng gian), pha inh chn lc tn s (pha inh phn


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tp tn s), pha inh chn lc thi gian (phn tp thi gian ). Chng ny s xt cc tnhcht knh trong min khng gian, thi gian v tn s.

Trong chng ny n s phn tch cc c tnh ca knh s dng chng trong ccgii thut iu ch thch ng ca mnh.

2.2. Min khng gian

Cc thuc tnh trong min khng gian gm: tn hao ng truyn v chn lc khng gian.

Tn hao ng truyn thuc loi pha inh phm vi rng cn chn lc khng gian thucloi pha inh phm vi hp. Cc m hnh truyn sng truyn thng nh gi cng sut thutrung bnh ti mt khong cch cho trc so vi my pht, c gi l nh gi tn haong truyn. Khi khong cch thay i trong phm vi mt bc sng, knh th hin rcc c tnh ngu nhin. iu ny c gi l tnh chn lc khng gian (hay phn tpkhng gian).

Tn hao ng truyn

M hnh tn hao ng truyn m t suy hao tn hiu gia anten pht v anten thu l mthm ph thuc vo khong cch v cc thng s khc. Mt s m hnh xt chi tit v a

hnh nh gi suy hao tn hiu, trong khi mt s ch xt tn s v khong cch.Chiu cao anten l mt thng s quan trng. Tn hao do khong cch truyn dn s tuntheo quy lut hm m.

PL d-n (2.1)

trong n l m tn hao (n=2 cho khng gian t do, n2 cho cc vng thnh ph ngoi tri), d l khong cch t my thu n my pht.

T l thuyt v cc kt qu o lng cho thy cng sut thu trung bnh gim so vikhong cch theo hm log i vi mi trng ngoi tri v trong nh. Hn na ti mikhong cch d, tn hao ng truyn PL(d) ti mt v tr nht nh l qu trnh ngu nhin

v c phn b log chun xung quanh mt gi tr trung bnh (ph thuc vo khong cch).Nu xt c s thay i theo v tr, c th biu din tn hao ng truyn PL(d) ti khongcch d nh sau:

Hnh 2.1. Tnh cht knh trong min khng gian, min tn s vmin thi gian

(a) (b) (c)


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( ) [ ] ( ) ( ) +

+=+= X

d

dlgn10dPLXdPLdBdPL

0

_________

0

________

(2.2)

Trong ( )_______

dPL l tn hao ng truyn trung bnh phm v rng i vi khong cch

pht thu d; X l bin ngu nhin phn b Gauss trung bnh khng (o bng dB) vi lch

chun (cng o bng dB), d0 l khong cch tham chun gia my pht v my thu, n lm tn hao ng truyn.

Khi cc i tng trong knh v tuyn khng chuyn ng trong mt khong thi giancho trc v knh c c trng bi pha inh phng i vi mt rng bng tn chotrc, cc thuc tnh knh ch khc nhau ti cc v tr khc nhau. Ni mt cch khc, phainh ch n thun l mt hin tng trong min thi gian (mang tnh chn lc thi gian).

T phng trnh 2.2 thy tn hao ng truyn ca knh c nh gi thng k phm virng cng vi nh hng ngu nhin. nh hng ngu nhin xy ra do pha inh phm vihp trong min thi gian v th hin cho tnh chn lc thi gian (phn tp thi gian). nh

hng ca chn lc khng gian c th c loi b bng cch s dng nhiu anten.MIMO (Multiple Input Multiple Output: Nhiu u vo nhiu u ra) l mt k thut chophp li dng tnh cht phn tp khng gian ny ci thin hiu nng v dung lng hthng.

2.3. Min tn s

Trong min tn s, knh b nh hng bi hai yu t: iu ch tn s v chn lc tn s.

2.3.1. iu ch tn s

iu ch tn s do hiu ng Doppler gy ra, khi c s chuyn ng tng i gia mythu v my pht dn n thay i tn s mt cch ngu nhin. Do chuyn ng tng igia BTS v MS, cc thnh phn sng a ng b dch tn s. Dch tn s trong tn sthu do chuyn ng tng i ny c gi l dch tn s Doppler, n t l vi tc chuyn ng, phng chuyn ng ca MS so vi phng sng ti ca thnh phn snga ng. Dch Doppler BD c th c biu din nh sau:

( ) ( )

=

= cosfc

cosB cD (2.3)

Trong l tc ca MS, l bc sng, l gc gia phng chuyn ng ca MS

v phng sng ti, c l tc nh sng v fc l tn s sng mang. T phng trnh chotrn thy nu MS di chuyn v pha sng ti dch Doppler l dng v tn s thu s tng,ngc li nu MS di chuyn ri xa sng ti th dch Doppler l m v tn s thu c sgim. V th cc tn hiu a ng n MS t cc phng khc nhau s lm tng rng

bng tn tn hiu. Khi v (hoc ) thay i dch Doppler thay i dn n tri Doppler.

2.3.2. Chn lc tn s

n s phn tch chn lc tn s cng vi mt thng s khc trong min tn s: bngthng nht qun. Bng thng nht qun 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 c khuch i nh nhau v pha tuyn tnh). Bng thng nht qun cho tadi tn trong cc thnh phn tn s c bin tng quan. Bng thng nht qun xcnh kiu pha inh xy ra trong knh v v th c ngha c s trong vic thch ng cc


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thng s iu ch. Bng thng nht qun t l nghch vi tri tr (xem phn 2.5). Pha inhchn lc tn s rt khc vi pha inh phng. Trong cng mt knh pha inh phng, tt ccc thnh phn tn s truyn qua bng thng knh u chu nh hng ca pha inh.

Ngc li pha inh chn lc tn s (cn gi l pha inh vi sai), mt s on ph ca tnhiu qua knh pha inh b nh hng nhiu hn cc phn khc, th hin r tnh chn lctn s ca knh ny. Nu bng thng nht qun knh nh hn rng bng tn ca tnhiu c truyn qua knh ny, th tn hiu ny chu nh hng ca pha inh chn lc( phn tp tn s). Pha inh ny s lm mo tn hiu.

2.4. Min thi gian

S khc bit gia cc knh hu tuyn v cc knh v tuyn l knh v tuyn thay i theothi gian, ngha l pha inh chn lc thi gian. C th m hnh ha knh v tuyn di ngnh l mt b lc tuyn tnh c p ng xung kim thay i theo thi gian. M hnh knhtruyn thng s dng m hnh p ng xung kim, y l mt m hnh trong min thigian. C th lin h qu trnh thay i tn hiu v tuyn phm vi hp trc tip vi p ngxung kim ca knh v tuyn di ng. Nu x(t) l tn hiu pht, y(t) l tn hiu thu v h(t, )l p ng xung kim ca knh v tuyn a ng ph thuc vo thi gian, th tn hiu thul tch chp ca tn hiu pht vi p ng xung kim ca knh nh sau:

( ) ( ) ( ) ( ) ( )==

,thtxd,thtxty (2.4)

Trong t l bin thi gian, l tr a ng ca knh i vi mt gi tr t c nh, * lk hiu tch chp.

nh hng a ng ca knh v tuyn thng c bit n dng phn tn thi gian

hay tri tr. Phn tn thi gian (tn thi), hay tri tr xy ra khi mt tn hiu c truynt anten pht n anten thu qua hai hay nhiu ng c cc di khc nhau. Mt mt tnhiu ny c truyn trc tip, mt khc n c truyn t cc ng phn x khc nhauc di khc nhau vi cc thi gian n my thu khc nhau. V vy tn hiu ti anten thuchu nh hng ca tn thi ny s b mo dng. Khi thit k v ti u ha cc h thngv tuyn s truyn s liu tc cao ta cn xt cc phn x ny.

Tn thi c th c c trng bi tr tri, tr tri trung bnh hay tr tri trung bnh qunphng.

2.4.1. Tr tri trung bnh qun phng

Thng s thi gian quan trng ca tn thi l tri tr trung bnh qun phng (RDS: RootMean Square Delay Spread): cn bc hai mment trung tm ca l lch tr cng sut. RDSnh gi cho tri a ng ca knh. V th c s dng nh gi nh hng canhiu giao thoa gia cc k hiu (ISI).

2_____2 =

(2.5)

( )

( )

=

kk

k

kk

P

P

(2.6)


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( )

( )

=

k

k

k

2

kk____2

P

P

(2.7)

trong P(k) l cng sut trung bnh a ng ti thi im k.

2.4.2. Tr tri cc iTr tri cc i (XdB) ca l lch tr cng sut c nh ngha l tr thi gian m nng lng a ng gim XdB so vi nng lng cc i.

2.4.3. Thi gian nht qun

Mt thng s khc trong min thi gian l thi gian nht qun. Thi gian nht qun xcnh c tnh "tnh" ca knh. Thi gian nht qun l thi gian m knh tng quanrt mnh vi bin ca tn hiu thu, c k hiu l Tc. Cc k hiu khc nhau truynqua knh trong khong thi gian nht qun chu nh hng pha inh nh nhau. V thnhn c mt knh pha inh kh chm. Cc k hiu khc nhau truyn qua knh bnngoi thi gian nht qun s b nh hng pha inh khc nhau. Khi ny knh pha inh khnhanh. Nh vy di tc ng ca pha inh nhanh, mt s phn ca k hiu tin s chutc ng pha inh ln hn cc phn khc. n s nghin cu thuc tnh ny phttrin gii thut iu ch thch ng ca mnh. Bng cch thit lp gi tr cho mt thng snht nh, s nhn c knh pha inh chm thay v knh pha inh nhanh v nh vy tc hiu nng tt hn.

2.5.Quan h gia cc thng s trong cc min khc nhau

n ch ra cc c tnh knh v cc thng s ca n trong cc min khng gian, tns v thi gian. Cc c tnh ny khng tn ti c lp nhau m c quan h mt thit giacc min xt. Mt s thng s trong min ny nh hng ln cc c tnh ca min khc.

2.5.1. Bng thng nht qun v tri tr trung bnh qun phng

Thy r, l lch tr cng sut v p ng tn s bin ca knh v tuyn di ng quan hvi nhau qua bin i Fourrier. V th, c th trnh by knh trong min tn s bng cchs dng cc c tnh p ng tn s ca n. Tng t nh cc thng s tri tr trong minthi gian, ta c th s dng bng thng nht qun c trng knh trong min tn s.Tuy tri tr trung bnh qun phng t l nghch vi bng thng nht qun v ngc li,song quan h chnh xc ca chng l mt hm ph thuc vo cu trc a ng. Nu khiu bng thng nht qun l BC v tri tr trung bnh qun phng l , th khi hmtng quan ng bao ln hn 90%, bng thng nht qun c quan h sau y vi tri trtrung bnh qun phng:

50

1BC (2.8)

Cho thy hai thng s trn lin quan cht ch vi nhau, nn ch cn xt mt thng s trongqu trnh thit k h thng.


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2.5.2. Thi gian nht qun v tri Doppler

Thi gian nht qun chu nh hng trc tip ca dch Doppler, l thng s knh trongmin thi gian v c tnh i ngu vi tri Doppler. Tri Doppler v thi gian nht qun lhai thng s t l nghch vi nhau. Ngha l:

DC B

1

T (2.9)

Khi thit k h thng ch cn xt mt trong hai thng s ni trn l .

2.6. Cc loi pha inh phm vi hp

Tu vo quan h gia cc thng s tn hiu ( rng bng tn, chu k k hiu,) v ccthng s knh (tri tr trung bnh qun phng, tri Doppler, ), m xc nh loi phainh phm vi hp da trn hai c tnh: Tri tr a ng v pha inh chn lc tn s.Tri tr a ng l mt thng s trong min thi gian, trong khi vic knh l pha inh

phng hay chn lc tn s li xt trong min tn s. V th thng s min thi gian, tritr a ng, nh hng ln c tnh knh trong min tn s. Tri Doppler dn n tntn v pha inh chn lc thi gian, v th da vo tri Doppler phn loi pha inh

phm vi hp thnh pha inh nhanh v pha inh chm. Tri Doppler l mt thng s trongmin tn s trong khi hin tng knh thay i nhanh hay chm li thuc min thigian. Vy trong trng hp ny, tri Doppler, thng s trong min tn s, nh hng lnc tnh knh trong min thi gian. Bit c cc quan h ny s tr gip trong qu trnhthit k h thng. Bng 2.1 lit k cc loi pha inh phm vi hp.

Bng 2.1. Cc loi pha inh phm vi hp

C s phn loi Loi Pha inh iu kinTri tr a ng Pha inh phng BS


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truyn dn, cn chn gi tr chu k k hiu trong gii thut iu ch thch ng t cknh pha inh phng thay v knh pha inh chn lc.

Da trn tri Doppler, phn loi knh thnh pha inh nhanh v pha inh chm. Nup ng xung kim knh (trong min thi gian) thay i nhanh trong chu k k hiu, nghal nu thi gian nht qun knh nh hn chu k k hiu ca tn hiu pht, knh s gy ra

pha inh nhanh i vi tn hiu thu. iu ny s dn n mo dng tn hiu. Nu p ngxung kim knh thay i vi tc chm hn nhiu so vi k hiu bng gc pht, knh sgy ra pha inh chm i vi tn hiu thu. Trong trng hp ny knh t ra tnh i vimt s chu k k hiu. Tt nhin ta mun c pha inh chm v n h tr cht lngtruyn dn n nh hn. Ta khng th xc dnh Doppler khi thit k h thng. V th, khicho trc tri Doppler, ta cn chn rng bng tn tn hiu (bng thng sng mangcon) trong gii thut iu ch thch ng nhn c knh pha inh chm thay v knh

pha inh nhanh. Nh vy ta s t c cht lng truyn dn tt hn.

2.7. Phn b Rayleigh v Rice

Khi nghin cu cc knh v tuyn di ng, thng cc phn b Rayleigh v Rice c sdng m t tnh cht thng k thay i theo thi gian ca tn hiu pha inh phng.Trong phn ny n s xt cc phn b ny v a ra cc c tnh c bn ca chng.

2.7.1. Phn b pha inh Rayleigh

C th coi phn b pha inh Rayleigh l phn b ng bao ca tng hai tn hiu phn bGauss vung gc. Hm mt xc sut (PDF) ca phn b pha inh Rayleigh c biudin nh sau:

( )


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Hnh 2.7 minh ho dng tn hiu OFDM trong min tn s, khi truyn qua mi trngknh pha ing Rice. Trn hnh v ta thy ti nhng im trng ca p ng knh th bin ph tn s ca tn hiu s b gim nhanh chng, v ti nhng im li ca p ngknh th bin ph ca tn hiu b ko ln theo ng p ng knh. Ti nhng imtrng hay li ca p ng knh u gy thu sai tn hiu sau b quyt nh ti pha thu.

Mc ch ca n l tm gii php i ph nhng v tr m p ng knh b thng gingbng cch s dng cc b cn bng, dng c ch thch ng chn lc sng mang v iuch M-QAM thch ng (AQAM) trong min tn s v thi gian c n cp trongchng 5.

Hnh 2.8 v hnh 2.9 cho thy cc thuc tnh knh trong min tn s ph thuc vo tri tr(RDS) v tha s K da trn cc kt qu m phng. C hai m hnh min tn s v minthi gian u c m phng. Trn hnh 2.8, ta gi thit rng K bng 0dB cn trn hnh2.9 ta gi thit rng RDS bng 42,1ns.

Hnh 2.7 Ph tn hiu OFDM truyn qua m hnh knh pha inh Rice, vis sng mang = 100, kch thc FFT = 256, [plot_ofdm_spectrum.m]


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Hnh 2.8 cho thy rng tri tr cao dn n thay i bin hm truyn t knh trongmin tn s nhanh hn. iu ny cho thy rng cn phi n nh nhiu sng mang conhn cho h thng OFDM khi tri ph ln hn. T hnh 2.9 cho thy, khi tha s K gim,

bin hm truyn t knh trong min tn s b pha inh nhanh hn. Khi tha s K ln,bin hm truyn t knh trong min tn s t b pha inh hn nhiu. Ni mt cchkhc khi tha s K ln, cho php n nh bng thng sng mang nh ngay c khi tri trln. Tuy nhin ta cn bit ti tha s K no nh hng tri tr i vi thit k bng thngsng mang con c th b qua. xc nh iu ny ta xt kt qu m phng trn hnh2.10.

Hnh 2.10 Biu th hm truyn t bin knh theo tn s i vi RDS bng 30ns vtha s K bng 0dB, 6dB v 15dB. Hnh ny cho thy rng tha s K nh dn n bin knh b pha inh nhanh hn trong min tn s. i vi K=0dB, pha inh bin c thln ti 12 dB ti mt tn s no , i vi K=10dB, bin pha inh nh hn 2,2dB trnton bng tn v i vi K=15dB, pha inh ch gii hn 1dB trn ton bng tn. Vy cth kt lun rng Khi K ln hn 10dB bin hm truyn t knh trong min tn skhng b pha inh nhiu v th khng cn t bng thng sng mang con theo tri tr mc

d bin ny pha inh nhanh hn khi tri tr ln.

Hnh 2.8. Ph thuc bin hm truyn t knh min tn s vo tn sv RDS. a) nhn t trn xung, b) nhn t bn.

Hnh 2.9. Ph thuc bin hm truyn t knh vo K v tn s


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T cc phn tch trn c th kt lun nh hng ca tha s K v tri tr ln cc thuctnh knh trong min tn s nh sau:

Tri tr nh hng ln tc thay i bin trong hm truyn t knh trong mintn s. Tri tr cng cao th tc thay i bin trong min tn s cng ln.

Tha s K xc nh ln ca thay i bin hm truyn t knh min tn s. Kcng ln th thay i bin cng nh.

Khi tha s K nh hn 10 dB, chng pha inh chn lc tn s, cn n nh bngthng sng mang con ln hn cho OFDM khi tri tr ln hn.

2.10. Kt lun

Chng ny xt cc c tnh knh. Theo truyn thng, cc knh c phn loi thnhcc knh pha inh phm vi rng v cc knh pha inh phm vi hp. Pha inh phm virng ch yu c biu th bng tn hao ng truyn gy ra bi truyn sng khong

cch xa (vi km). Pha inh phm vi hp biu th nh hng truyn dn a ng. V vy,khi xy dng thut ton cho iu ch thch ng, cn xt cc c tnh knh trong ba min:khng gian, tn s v thi gian nh cho bng 2.2. c tnh knh trong min khng gianlin quan n tn hao ng truyn phm vi rng v thng ging ngu nhin phm vi hpdo truyn a ng. Thng ging ngu nhin khi khong cch thay i t (vo khong

bc sng) dn n phn tp khng gian (pha inh chn lc khng gian). Vic pha inhchn lc khng gian mang tnh ngu nhin v kh lp m hnh dn n tnh trng khngr rng khi thit k h thng v kh tng cng cht lng h thng. Tuy nhin cngngh truyn dn MIMO (Multiple Input Multiple Output) cho php gii quyt vn ny.MIMO c th chuyn bt li ca truyn sng a ng thnh c li.

Hnh 2.10. Hm truyn t ca knh khi RDS=30ns vi cc gi tr K khc nhau


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Bng 2.2. Cc c tnh knh trong ba min: khng gian, tn s v thi gian

Min khng gian Min tn s Min thi gianThng s d;

Thng ging ngu nhin

BD;

cB

1

50

c

D

TB

1

Nhc im Chn lc khng gian Chn lc tn s Chn lc thi gianGii php MIMO OFDM Thch ngMc ch Li dng a ng Pha inh phng

(T )Pha inh chm(BS>>BD)

Ch thch d: khong cch thu pht; MIMO: Multile Input Multiple Output; BD:tri Doppler; BC: rng bng nht qun ca knh xt cho trnghp tng quan ln hn 90%; T: chu k k hiu; : tri tr trung

bnh qun phng; TC: thi gian nht qun ca knh; BS: rng

bng tn hiu pht

Cc thng s knh trong min tn s l tri Doppler v rng bng nht qun (xem bng2.2). Cc thng s knh min thi gian l thi gian nht qun v tri tr trung bnh qun

phng. Tri Doppler gy ra do chuyn ng tng i gia MS v BTS. Cc thng sny c th dn n pha inh chn lc thi gian (hay phn tp thi gian) trong min thigian v tri Doppler t l nghch vi thi gian nht qun ca ca knh.Tri tr xy ra dotr a ng. rng bng nht qun ca knh t l nghch vi tri tr trung bnh qun

phng. V th tri tr trung bnh qun phng c th dn n pha inh chn lc tn s(hay phn tp tn s) trong min tn s. OFDM a ra gii php cho pha inh chn lctn s v n c th chuyn pha inh chn lc tn s vo pha inh phng bng cch sdng chu k k hiu di hn tri tr trung bnh qun phng (xem chng 3). Ngoi rathch ng a ra gii php cho pha inh chn lc thi gian trong min thi gian, v n hunh lun lun lm cho rng bng tn hiu pht ln hn nhiu so vi tri Doppler bngcch thay i cc thng s ca h thng truyn dn theo cc thng s knh (xem chng5).


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Chng 3

Nguyn l hot ng ca OFDM

3.1. M u

Ghp knh theo tn s trc giao OFDM (Orthogonal Frequency Division Multiplexing) lmt phng php iu ch cho php gim thiu mo tuyn tnh do tnh phn tn ca knhtruyn dn v tuyn gy ra. Nguyn l ca OFDM l phn chia ton b bng thng cntruyn vo nhiu sng mang con v truyn ng thi trn cc sng mang ny. Theo ,lung s tc cao c chia thnh nhiu lung tc thp hn. V th c th gim nhhng ca tr a ng v chuyn i knh pha inh chn lc thnh knh pha inh

phng. Nh vy OFDM l mt gii php cho tnh chn lc ca cc knh pha inh trong

min tn s. Vic chia tng bng thng thnh nhiu bng con vi cc sng mang con dnn gim rng bng con trong min tn s ng ngha vi tng di k hiu. S sngmang con cng ln th di k hiu cng ln. iu ny c ngha l di k hiu lnhn so vi thi gian tri rng tr ca knh pha inh phn tn theo thi gian, hay rng

bng tn tn hiu nh hn rng bng tn nht qun ca knh.

Theo chng ny trc ht, n trnh by nguyn l hot ng ca mt h thngiu ch OFDM. Sau xt cc thng s hiu nng ca n. Cui cng xt nh hng cacc thng s knh truyn sng ln dung lng cng nh cht lng truyn dn ca hthng OFDM.

3.2. Tnh trc giao tng

tng OFDM l truyn dn song song (ng thi) nhiu bng con chng ln nhau trncng mt rng bng tn cp pht ca h thng. Vic xp chng ln cc bng tn contrn ton b bng tn c cp pht dn n ta khng nhng t c hiu qu s dng

ph tn c cp pht cao m cn c tc dng phn tn li cm khi truyn qua knh, nhtnh phn tn li m khi c kt hp vi cc k thut m ho knh kim sot li hiunng h thng c ci thin ng k. So vi h thng ghp knh phn chia theo tn s

FDM truyn thng, FDM cng truyn theo c ch song song nhng cc bng con khngnhng khng c php chng ln nhau m cn phi dnh khong bng tn bo v (gim thiu phc tp b lc thu).

Vy lm th no tch cc bng con t bng tng chng ln hay ni cch khc sau khic tch ra chng khng giao thoa vi nhau trong cc min tn s (ICI) v thi gian(ISI). Cu tr li v cng l vn mu cht ca truyn dn OFDM l nh tnh trc giaoca cc sng mang con. V vy ta kt lun rng nh m bo c tnh trc giao ca ccsng mang con cho php truyn dn ng thi nhiu bng tn con chng ln song pha thuvn tch chng ra c, c bit l tnh kh thi v kinh t cao do s dng x l tn hiu sv tn dng ti a u vit ca VLSI.


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Theo trc ht ta nh ngha tnh trc giao, sau ta p dng tnh trc giao ny vo hthng truyn dn OFDM hay ni cch khc s dng tnh trc giao vo qu trnh to v thutn hiu OFDM cng nh cc iu kin cn thit m bo tnh trc giao.

nh ngha

Nu k hiu cc sng mang con c dng trong h thng OFDM l )t(s&)t(s ji . mbo trc giao cho OFDM, cc hm sin ca sng mang con phi tho mn iu kin sau

( ) ( )

=

=+

ji,0

ji,1dtts.ts

T

1Tt

t

*

ji

s

s

(3.1)

Trong :

( )( )

=

=

,0

N,2,1k,ets

ftk2j

k

(3.2)

T1f = l khong cch tn s gia hai sng mang con, T l thi gian k hiu, N l s cc

sng mang con v N.f l bng thng truyn dn v ts l dch thi gian.

Minh ho

OFDM t tnh trc giao trong min tn s bng cch phn phi mi tn hiu thng tinring bit vo cc sng mang con khc nhau. Cc tn hiu OFDM c to ra t tng cacc hm sin tng ng vi mi sng mang. Tn s bng tn c s ca mi sng mang conc chn l mt s nguyn ln ca tc k hiu, kt qu l ton b cc sng mang cons c tn s l s nguyn ln ca tc k hiu. Do cc sng mang con l trc giao

vi nhau.Kin trc ca mt tn hiu OFDM vi 4 sng mang con c cho Hnh 3.1. Trong ,(3.1.1a), (3.1.2a), (3.1.3a) v (3.1.4a) th hin cc sng mang con ring l, vi tn stng ng 10, 20, 30, v 40 Hz. Pha ban u ca ton b cc sng mang con ny l 0.(3.1.5a) v (3.1.5b) th hin tn hiu OFDM tng hp ca 4 sng mang con trong minthi gian v min tn s.

Tnh trc giao trong min tn s ca tn hiu OFDM c th hin mt cch tng minh hnh 3.2. Thy r, trong min tn s mi sng mang con ca OFDM c mt p ng tns dng sinc (sin(x)/x). Dng sinc c ng bao chnh hp, vi nh suy gim chm khi

bin ca tn s cch xa trung tm. Tnh trc giao c th hin l nh ca mi sngmang con tng ng vi gi tr 0 ca ton b cc sng mang con khc. Hnh 3.2 cho tathy vi cng rng bng tn cp pht cho h thng th hiu qu s dng ph tn caOFDM ln gp hai ln so vi c ch FDM truyn thng.

p ng tng hp 5 sng mang con ca mt tn hiu OFDM c minh ho ng muen m trn hnh 3.3.


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Hnh 3.1 Dng sng ca mt tn hiu OFDM trong min thi gian v tns, [sim_ofdm_time_domain.m]

Hnh 3.2 Hnh dng ph ca tn hiu OFDM bng tn c s 5 sng mang,hiu qu ph tn ca OFDM so vi FDM, [sim_ofdm_mc.m]


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3.3. M hnh h thng truyn dn OFDM3.3.1. M t ton hc tn hiu OFDM

Tn hiu OFDM pht phc bng tn gc c xc nh nh sau:

( ) ( ),

==

kkTtsts k (3.3)

Trong sk(t-kT) l tn hiu OFDM pht phc bng gc th k c xc nh nh sau:

( )

( ) ( )

++

=

=

khcNu0,

TTkTtTTkT

kTtT

ij2expxkTtw

ktts winFFTGwin

12N

N/2i FFTki,

k (3.4)

Trong :

T l di k hiu OFDM

TFFT l thi gian FFT, phn hiu dng ca k hiu OFDM

TG l thi gian bo v, thi gian ca tin t chu trnh

Twin l thi gian m ca tin t v hu t to dng ph

f=1/TFFT l phn cch tn s gia hai sng mang

N l di FFT, s im FFT

k l ch s v k hiu c truyn

i l ch s v sng mang con, i{-N/2, -N/2+1, -1, 0, +1, ., -N/2}

xi,k l vect im chm tn hiu, l k hiu phc (s liu, hoa tiu,rng) c iu ch ln sng mang con i ca k hiu OFDM th k.

Hnh 3.3 Ph tng hp ca tn hiu OFDM trong bng tn c s vi5 sng mang con, [sim_ofdm_mc.m]


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w(t) xung to dng c biu din nh sau:

( )

( )[ ]

( )[ ]

+

+

=

winFFTFFTwinFFT

FFTG

GGwinwinG

TTtT,TTtcos121

TtT1,

TtTT,TTtcos12

1

tw (3.5)

Phn tch (3.4) ta thy biu thc ny ging nh biu thc ca dy Fourier sau: j nf

n

= 0u(t) = c(nf )e 02 (3.6)

trong cc h s Fourier phc th hin cc vect ca chm tn hiu phc cn nf0 thhin cc sng mang con i/TFFT. Trong h thng s, dng sng ny c th c to ra bng

bin i Fourier ngc nhanh (IFFT). Chm s liu xi,k l u vo IFFT v k hiu

OFDM min thi gian l u ra.Tn hiu u ra ca b iu ch v tuyn c xc nh nh sau:

k

=RF RF,ks (t)= s (t kT) (3.7)

trong sRF,k(t-kT) l tn hiu OFDM v tuyn th k c biu din nh sau:

( )

( ) ( )

++

+

=

=

khcNu,0

TTkTtTTkT

,kTtT

1f2jexpxkTtwRe

kTts winFF TGwin

12/N

2/Ni FFT

ck,i

k,RF (3.8)

Trong fc l tn s sng mang RF.

3.3.2. S h thng truyn dn OFDM

Hnh 3.4 trnh by s khi pht thu tn hiu OFDM in hnh. Theo , di y trnhby vn tt chc nng cc khi.

My pht: Chuyn lung d liu s pht thnh pha v bin sng mang con. Cc sngmang con c ly mu trong min tn s, ph ca chng l cc im ri rc. Sau sdng bin i Fourier ri rc ngc (IDFT) chuyn ph ca cc sng mang con mang dliu vo min thi gian. Tuy nhin cc h thng trong thc t dng bin i Fourier ngcnhanh (IFFT) v n tnh hiu ca n. Tn hiu OFDM trong min thi gian c trn nngtn ln tn s truyn dn v tuyn.

My thu: Thc hin hot ng ngc li ca pha pht. Theo trc ht, trn h tn tnhiu RF thnh tn hiu bng tn c s, sau s dng FFT phn tch tn hiu vo min tns. Cui cng thng tin dng bin v pha ca cc sng mang con c gii iu ch

thnh cc lung s v chuyn tr li thnh d liu s ban u.


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Chuyn iNi tip

thnh S ongsong

IFFT

Khuch i cngsut v gii iuch sng mang

cao tn

iu chsng

mang con

Chnkhongbo v

Ch n tng bkhung

iu chsng mang caotn v khuchi cng sut

Loi bkhongbo v

Tchkhung

FF TGii iuch sngmang con

Chuyn iSong songhnh Ni

tip

.

.

.

.

.

.

D l iu pht

ni tip

D liu thu

ni tip

Pha sngmang

Bin sng mang

I

Q

I

Q

I

Q

I

Q

I

Q

Sa li tn s

Tn hiu trong minthi gian

LO

Pha sngmang

Bin sngmang

My pht

My thu

Hnh3.4 S khi h thng truyn dn OFDM

ng bnh thi

Anten


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3.2.2.1. Tng chuyn i ni tip sang song song

Tng chuyn i ni tip sang song song chuyn lung bit u vo thnh d liu phttrong mi k hiu OFDM, thng mi k hiu pht gm 40-4000 bit. Vic phn b dliu pht vo mi mi k hiu ph thuc vo phng php iu ch c dng v slng sng mang con. V d, i vi iu ch sng mang ca16-QAM th mi sng mang

con mang 4 bit d liu, nu h thng truyn dn s dng 100 sng mang con th s lngbit trn mi k hiu s l 400. Ti pha thu qu trnh c thc hin ngc li, khi dliu t cc sng mang con c chuyn ngc tr li l lung d liu ni tip ban u.

Do tnh cht chn lc tn s ca knh pha inh (pha inh chn lc tn s) tc ng lnmt nhm cc sng mang con lm chng suy gim nhanh chng. Ti im p ng knhxp x 0, thng tin gi trn sng mang con gn im ny s b tn tht, hu qu l gycm li bit trong mi k hiu. Do c ch FEC l hiu qu cao nu cc li c phn tnrng (khng tp chung hay cm li), v vy ci thin hiu nng, a phn h thng dngngu nhin ho nh l mt phn ca chuyn i ni tip thnh song song. Vn ny

c thc hin bng cch ngu nhin ho vic phn b sng mang con ca mi mt bit dliu ni tip. Ngu nhin ho lm phn tn cc cm bit li trong k hiu OFDM do stng hiu nng sa li ca FEC.

3.3.2.1. Tng iu ch sng mang con

Tng iu ch sng mang con lm nhim v phn phi cc bit d liu ngi dng ln ccsng mang con, bng cch s dng mt s iu ch bin v pha. Vic xp xp iuch sng mang con i vi 16-QAM c cho hnh 3.5, mi k hiu 16-QAM s cha 4

bit d liu, mi t hp 4 bit d liu tng ng vi mt vector IQ duy nht.

nh hng ca tp m cng vo tn hiu pht 16-QAM (knh AWGN) c cho hnhhnh 3.5 (b) vi SNR thu = 18 dB.

Hnh 3.5. Tn hiu pht 16-QAM s dng m ho Gray, v tn hiu 16-QAMtruyn qua knh v tuyn, SNR = 18 dB, [sim_generate_qam.m]


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3.3.2.3. Tng chuyn i t min tn s sang min thi gian

Sau tng iu ch sng mang con, tn hiu OFDM c dng l cc mu tn s, tn hiuOFDM mun truyn trn knh phi c dng sng trong min thi gian. Php bin iFourier ngc nhanh (IFFT) s chuyn tn hiu OFDM trong min tn s sang min thigian. Tng ng vi mi mu ca tn hiu OFDM trong min thi gian (mi u ra caIFFT) cha tt c cc mu trong min tn s (u vo ca IFFT). Hu ht cc sng mangcon u mang d liu. Cc sng mang con vng ngoi khng mang d liu c t bng0.

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 nngtn ln tn s truyn dn v tuyn. C th s dng mt trong hai hai k thut iu chsng mang cao tn l: "tng t" c cho hnh 3.7 v "s" c cho hnh 3.8. Tuynhin hiu nng ca iu ch s s tt hn, do ng b pha chnh xc cho nn s ci thinqu trnh ghp cc knh I v Q.

Hnh 3.6. Tng IFFT, to tn hiu OFDM

Hnh 3.7 iu ch cao tn tn hiu OFDM bng tn c sphc s dng k thut tng t


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Hnh 3.9 m t dng sng trong min thi gian ca mt tn hiu OFDM. S lng sngmang = 500, kch thc FFT = 2000, khong thi gian bo v = 500. Sng mang iu chcao tn c tn s fc = 10 GHz.

Hu ht cc ng dng v tuyn, th tn hiu OFDM c to ra ti bng tn c s s dngcc mu phc, sau chuyn ph tn hiu bng tn c s ln ph RF bng cch dng mt

b iu ch IQ, nh c cho hnh 3.7 v hnh 3.8. B iu ch IQ s dch ph tn tnhiu OFDM t bng tn c s phc ln vng tn s v tuyn, v chuyn t tn hiu phcsang tn hiu thc (ly phn thc). Tn hiu RF pht lun l tn hiu thc v n ch bini gi tr cng trng.

Mt tn hiu thc s tng ng vi mt tn hiu bng tn c s phc c tn s trung

tm l 0 Hz trn vi tn s sng mang b iu ch IQ.

Hnh 3.8 iu ch cao tn tn hiu OFDM bng tn c s phcs dng k thut s

Hnh 3.9. Dng sng tn hiu OFDM trong min thi gian,[sim_ofdm_signal.m]


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offc f2

Wf += (3.9)

Trong cf l tn s sng mang dch tn hiu OFDM t bng tn c s phc ln tnhiu OFDM cao tn thc, W l rng bng tn tn hiu v offf l tn s dch t DC, xemhnh 3.10. Trong cc ng dng hu tuyn nh ADSL, hu ht cc sng mang con u c

tng dch DC thp hn rng bng tn tn hiu. iu ny c ngha rng c th trctip to tn hiu thc bng cch s dng tng IFFT thay v phi dng b iu ch IQ chuyn dch tn s.

to ra mt tn hiu OFDM thc ch cn mt na cc sng mang con s dng cho iuch d liu, mt khc na gm cc lt tn s cao ca IFFT s c gi tr bin l lin hp

phc ca na cn li gm cc lt c tn s thp hn.

3.4. Cc thng s c trng v dung lng h thng truyndn OFDM

3.4.1. Cu trc tn hiu OFDM

Hnh 3.11 cho thy cu trc ca cc k hiu OFDM trong min thi gian. FFTT l thi gian truyn d liu hiu qu, GT l thi gian bo v. Cng thy cc thng s khc, winT lthi gian ca s. Thy r quan h gia cc thng s l.

winGFFTsym TTTT ++= (3.10)

Hnh 3.10 Tn hiu OFDM dch DC, W l bng tn tn hiu,

foff

tn s dch t DC, fcl tn s trung tm (sng mang)

Hnh 3.11 Cu trc tn hiu OFDM


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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. Loi ca s c dng ph bin l loi ca scosine tng c nh ngha bi.

( )

( )

+

+

++

=

symsym

sym

sym

symsym

sym

sym

T1tT,T

Ttcos5.05.0

TtT,1

Tt0,

T

tcos5.05.0

tw

(3.11)

trong l h s dc ca cosin tng v khong thi gian k hiu SymT , n ngn hn tonb khong thi gian ca mt k hiu v ta cho php cc k hiu ln cn chng ln mt

phn trong vng dc (roll-off region).Mt k hiu OFDM bt u ti thi im symk kTtt == (bt u ca k hiu th k) cnh ngha bi cc phng trnh (3.12).

( )( )

( )[ ]

( )[ ]

++>

ngtbBER th phn t trong mng

Q tng ng vi thnh phn sng mang s c gn bng 1. Nu ngc li s

c gn bng 0. Gi tr 1 c ngha l khng truyn d liu trn sng mang ny, gi tr0 c ngha l vn s dng sng mang ny. Phn lp trnh cho khi tnh BER, BERngng v khi quyt nh c thc hin thng qua hm dem_loi c vit bng cngc MatLab. Lu thut ton cho khi quyt nh c m t nh sau:

BER ngngk = 1

k++

ng

tbBER>BER (k)

Bt u

Q (k) = 0

Q (k) = 1

Hnh 5.6 Lu thut ton ca khi quyt nh

ng

Sai

k = s sngmang

Kt thc

ng

Sai

ngtbBER

Khi iu khin chn: Khi ny ly thng tin chn t khi quyt nh, nu sng mangno c BER ngtbBER> khi phn t trong mng Q c ch s bng s th t ca sng

mang ny s c gi tr l 1 v ta phi tin hnh chn 0 ln sng mang ny, nu ngcli th s tin hnh truyn d liu bnh thng trn sng mang ny. Khi iu khin chns can thip th t ca k hiu pht trn mi sng mang m bo sao cho nu khng s

Dng Minh Khim, D2001-VT 71


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Chng 5: iu ch OFDM thch ng n tt nghip i hc

dng sng mang th s chn thm k hiu 0, v nu s dng th khng chn. Qu trnhsp xp ny rt phc tp, tuy nhin nu hiu k v cu trc ca k hiu OFDM, v cutrc k hiu em iu ch sng mang con th cng vic s d dng hn. Sau y l lu thut ton m t hot ng ca khi iu khin chn. Phn lp trnh cho khi iu khinchn c thc hin thng qua hm 'chen_song_mang (sig_in)' c vit bng cng c

MatLab.

Bt u

n = s lng k hiu phtk = 1, u = 1, v = 0data_out = 0data_in = tn hiu pht

v = phn d (u/s sngmang)

v = 0

Q(v) = 0

v = s sng mang

data_out(u) = data_in(k)k++u++

data_out (u) = 0u ++

k = n

Kt thc

ng

sai

ng

sai

ngsai

Hnh 5.7 Lu thut ton m t hot ng ca khi iu khin chn

Khi iu khin gii chn: Hot ng ca khi iu khin gii chn hon ton ngc livi khi iu khin chn. Khi ny ly thng tin chn t khi quyt nh, da trn thngtin v cc v tr chn khi ny s tin hnh gii chn tc l loi b nhng k hiu chn trnnhng sng mang con c chn v a nhng k hiu chn ny n b c tnh knh tin hnh tm ra p ng knh. Do tnh cht hot ng tng t nh khi iu khin chncho nn lu thut ton cng tng t nh khi iu khin chn, cho nn s khngc cp y. Phn lp trnh cho khi iu khin gii chn c thc hin thng quahm giai_chen_song_mang (sig_in) c vit bng cng c MatLab.



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Chng 5: iu ch OFDM thch ng n tt nghip i hc

5.6 Kt lun

Chng ny gii thch th no l iu ch thch ng, phn tch nguyn l thch ngchung cho cc h thng truyn dn, phn tch cc bc thc hin cho cc phng phpthch ng. ng thi xut cc phng n thch ng: Thch ng theo SNR pht, thchng theo mc iu ch, thch ng theo c ch chn lc sng mang. Phn tch u nhcim v tnh kh thi ca tng phng n thch ng, cui cng n chng minh tnh hiuqu v tnh kh thi ca ba phng n thch ng: thch ng theo mc iu ch, thch ngtheo c ch chn lc sng mang v phng n kt hp hai phng n thch ng ny.Chng ny cng xut m hnh h thng truyn dn OFDM s dng thut tonthch ng theo c ch chn lc sng mang, ng thi phn tch lu thut ton hotng ca cc module thch ng trong h thng.



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Chng 5: iu ch OFDM thch ng n TN i hc

Chng 6

Chng trnh m phng h thng truyn dn

OFDM thch ng6.1 Gii thiu

Cc chng trnh m phng c vit cho cc hm c trng ca qu trnh mphng h thng truyn dn OFDM thch ng. Mun vy, cn thut ton ho cc hm ctrng trn c s chn ngn ng lp trnh ph hp vit chng trnh m phng. n chn ngn ng lp trnh Matlab thc hin vit chng trnh v l do sau: Matlab lngn ng lp trnh bc cao, ngn ng k thut, ngn ng chuyn gia c th vin ton ccmnh, giao din ho phong ph, kh nng tng thch cc ngn ng khc (cho phpnhng cc chng trnh khc C, C++,... vo trong Matlab), cho php to giao din ngidng tin li. c bit cho php ng dng kt ni iu khin thit b phn cng bng lptrnh. Matlab c u vit ni tri l kh nng m phng h thng ng, cho php th hintn hiu v h thng trong nhiu min xt (min thi gian, min tn s, min Z.v.v..). Cl v th m hu ht cc trng i hc, cc vin ln u chn Matlab phc cng tco to, nghin cu pht trin, c bit thc thi ho kt qu cng trnh nghin cu vothc t. Vi ni dung, tnh cht c th cng nh phm vi xt ca ca n, tp trungkhai th vin ton ca Matlab hay ni cch khc s dng cc hm c bn c trong thvin, khai thc ti a kh nng phn lp hm ca Matlab vit chng trnh m phng.Theo chng trnh m phng h thng truyn dn OFDM thch ng c vit bngngn ng lp trnh Matlab, di dng cc file.m (mi module h thng l mt file).Chng trnh cho php ngi s dng da vo chng trnh Code nghin cu thutgii cng nh m rng, b xung tnh nng c bit ly d liu kt qu tnh ton t cccng thc theo mun ph hp vi ngi nghin cu, ngoi ra chng trnh cng xydng giao din tin ch thn thin khi ny ngi dng ch cn thc hin cc thao tc ngin trn giao din tin ch. Giao din c t chc thnh 4 cp: giao din cp 1, giaodin con cp 2, giao din con cp 3 v giao din cp 4. Ngi s dng c th t giao dinchnh chn i tng cn m phng, vo cc giao din con nghin cu chi tit nidung quan tm, t giao din con d dng tr li giao din chnh chn i tng nghincu khc. thit k h thng bng Matlab ngi lp trnh nu ch c cc kin thc vmy tnh l cha m ct li l phi c kin thc ca mt chuyn gia trong lnh vc mh nh vit chng trnh m phng.

6.2 M hnh m phng h thng truyn dn OFDM thchng

6.2.1 M hnh m phng

M hnh m phng h thng OFDM c cho hnh 6.1. D liu ca ngi dng c thl file text hoc file nh thuc cc nh dng: JPEG, BITMAP, D liu c c t filenh s c dng mt vector 3 chiu, vi cc phn t c dng thp phn 1-256. Nhng



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Chng 5: iu ch OFDM thch ng n TN i hc

khng th truyn trc tip mt vector 3 chiu c cho nn phi tin hnh chuyn ivector 3 chiu thnh vector 1 chiu, sau truyn qua h thng OFDM cn tin hnhchuyn i d liu thp phn sang t nh phn 8 bit. B tnh li s so snh s khc nhaugia d liu pht v thu v tnh s lng li, sau hin th li tnh hiu nng ca hthng. D liu nh phn sau c a ln iu ch M-QAM, tip theo d liu sau iu

ch M-QAM c a n b iu ch OFDM. D liu sau iu ch OFDM c phtqua knh v tuyn n my thu. My thu s thc hin ngc cc cng vic vi pha pht.D liu cui cng c dng vector 3 chiu vi cc phn t c dng thp phn s c khi

phc di dng file nh v hin th so snh vi nh ban u. Hot ng ca khi tnhBER, khi quyt nh, khi iu khin chn, khi iu khin gii chn hon ton tng tnh cc khi ny m hnh c ch thch ng chn lc sng mang c trnh by trongchng 5. Trong phng php thch ng theo c ch chn lc sng mang, nu ngngBER thit lp ban u cho cc thnh phn sng mang l nh th nh truyn qua hthng AOFDM s c cht lng cao, ngc li nu ngng BER c thit lp ln thcht lng nh s ti hn. Tuy nhin nu ngng BER c thit lp qu nh s dn n

tc truyn dn s thp khi iu kin knh qu ti. tng tc truyn dn trong iukin knh truyn nh vy hoc ta phi tng ngng BER hoc gim mc iu ch.

Hnh 6.1 ch trnh by m hnh h thng truyn dn OFDM thch ng dng c ch chnlc sng mang.M hnh h thng truyn dn OFDM thch ng dng c ch chuyn mciu ch s thay vai tr ca khi iu khin chn v gii chn sng mang con bng khiiu khin chuyn mc iu ch v s khng cn khi BER ngng. Khi quyt nh s

so snh gi tr BER hin thi ca h thng nhn t b tnh BER vi gi tr BER ngngchuyn mc, v s quyt nh mc iu ch hp l ng vi gi tr BER hin thi ny.Thng tin t khi quyt nh s a n khi iu khin chuyn mc iu ch, khi ny

s thc hin cng vic chuyn mc.



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c Filenh

Chuyn d liusang dng nh phn

(tng kh i8 b it)

M hoM-QAM

Tx KnhV tuyn

Rx Gii mM-QAM

Chuyn d liusang dng cckhi thp phn

Ghi F ilenh

To tp mGauss

So snh dliu ph t vd liu thu

Tnh li

Khi quyt nh BERngng

Chn sngmang

Gii chn sngmang

Khi iu khinchn, gii chn

Hnh6.1 M hnh m phng h thng truyn dn OFDM thch ng

Khi tnh BER



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6.2.2. Thit lp cc thng s m hnh m phng

Thit lp tnh tng thch cc thng s cho tn hiu OFDM

Trong phn trn ta nghin cu v cu trc mt tn hiu OFDM, tuy nhin m phngmt tn hiu OFDM s phc tp hn rt nhiu. Hai nhim v chnh cn thc hin trong

qu trnh m phng mt tn hiu OFDM l: Tng thch gia tc d liu ngi dng v s lng sng mang con s dng

truyn d liu.

Tng thch gia s lng sng mang con s dng truyn d liu v kch thcFFT.

Ngoi ra, cc cng vic khc nh: chuyn tn hiu t min tn s sang min thi gian,chn khong bo v, lc tn hiu OFDM dng b lc bng thng vi hnh dng b lc lca s Kaiser.

hiu r qu trnh m phng tn hiu OFDM, ta hy xt mt v d m phng mt tnhiu OFDM c th vi thi gian m phng 1 giy. Gi s cc tham s h thng c mt trong bng di y.

Bng 6.1 Tham s h thng dng cho m phng tn hiu OFDM

Tham s h thng Gi trTc d liu u vo: Rb 12 MbpsS lng sng mang: Nsub 90Kch thc FFT: NFFT 256Mc iu ch sng mang: M-QAM 16-QAM

Thi gian m phng: Tsim 1 sChu k c bn: T 1/ R b = 0,083sThi gian k hiu hu ch: TFFT Nsublog2(M) T = 30sThi gian khong bo v: TG 1/4 TFFT = 7,5sThi gian k hiu OFDM: Tsym TFFT + TG = 37,5 s

V tc d liu l 12 Mbps, nn trong khong thi gian 1 giy m phng s cbit1012 6 truyn. Nhng do phng php iu ch sng mang con l 16-QAM, do s

lng k hiu 16-QAM s l: 66

10341012 = k hiu.


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Qu trnh tng thch gia tc d liu ngi dng v s lng sng mang con

Mi sng mang con trong mt k hiu OFDM s mang mt k hiu d liu. Khi s lngk hiu khng bng bi s nguyn ln s lng sng mang con, m bo mi sngmang con u c d liu truyn ta phi tin hnh chn thm cc k hiu 0.

M mi k hiu OFDM s cha ton b chu k ca cc sng mang con, do mt khiu OFDM s cha ton b d liu truyn trn cc sng mang con. Trong trng hpny mi k hiu OFDM s cha d liu c trong 90 sng mang con. V c iu ny nn:

s lng k hiu OFDM = 3333490

103ceil

6

=

,

Trong : ( )[ ]

+

=

0x,Zx,x

0x,Zx,1]x[

Zx,x

xceil , vi [x] l phn nguyn cax.

S lng k hiu phi chn thm l: 90 - phn d 6090103 6 =

. Hnh 6.2 s minh ho

qu trnh ny:

Sau khi thc hin tng thch gia tc d liu u vo v s sng mang s dngtruyn d liu, cn tin hnh qu trnh tng thch tip theo gia kch thc FFT v ssng mang.

Qu trnh tng thch gia kch thc FFT v s sng mang con

Mt k hiu FFT c to nn t bin i IFFT cc mu tn s. Mu tn s l mu thuc do qu trnh ly mu cc tn s sng mang con. Do trong khong thi gian bin

i IFFT s cha ton b chu k ca cc sng mang con. Trong bin i Fourier dokhong tn s ly t );( + , nhng i vi phng php FFT th khong tn s b giihn do vy, sau bin i FFT s chu hiu ng bin, hiu ng bin lm sai lch cc gi tr

Hnh 6.2 Tng thch gia tc d liu ngi dng v s sng mang


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tn hiu ng vi cc tn s ngoi ra. V th gim hiu ng bin ta phi tng kch thcFFT so vi s sng mang, mun vy:

Kch thc FFT/2 > s sng mang con

Nhng cng v th m s lng cc mu tn s ti u vo FFT b thiu, lng thiu =Kch thc FFT/2 - s sng mang con. cho nn ta phi tin hnh chn thm cc tn s 0

trc khi bin i IFFT.Theo bng tham s dng m phng ta c: Kch thc FFT/2 = 128, m s sng mang =90. Do cn chn thm 128-90 = 38 sng mang 0. Cho nn s lng sng mang thct phi dng cho mi k hiu OFDM s l 90 + 38 = 128, tuy nhin 38 sng mang chnthm s khng mang d liu ngi dng v chng c t l 0. Hnh 6.3 s minh hoqu trnh ny:

90

Hnh 6.3 Tng thch gia kch thc FFT v s sng mang

30...90 3838

S sng mang

90 38 60 38

90

38

60

30S sng mang 0

chn thm

S k hiu 0chn thm

Phn k hiu d

S k hiu 16-QAM = 33334 128 = 4266752

Do s lng k hiu 0 phi chn thm l: 12667523000000-4266752 = k hiu. Tathy nu chn kch thc FFT qu ln so vi 2 ln s sng mang th s phi chn rtnhiu k hiu khng mang thng tin. iu ny s lm gim hiu qu truyn dn.

to dng ph thc cho tn hiu OFDM, th phn cn li ca kch thc FFT (c chiudi FFT/2 l phn c cc mu tn s c gi tr cao hn), s c lp y bng cch lylin hp phc phn u ca kch thc FFT (c chiu di FFT/2 gm cc mu tn s cgi tr thp). Qu trnh ny c m t bng hnh 6.4 di y:

90

Hnh 6.4 Sp xp cc mu tn s trong k hiu OFDM trc khi bin i IFFT

38... 90 90 389038 38

Ly lin hp phc

Kch thc FFT/2 Kch thc FFT/2

...

Phn tn s thp Phn tn s cao


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Sau khi sp xp cc mu tn s trong k hiu OFDM, s thc hin bin i IFFT ln khiu OFDM trong min tn s. Qu trnh ny s chuyn tn hiu OFDM t min tn ssang min thi gian.

Bc tip theo trong cng on m phng tn hiu OFDM l chn khong bo v, qutrnh ny c thc hin cho tng k hiu OFDM (xem li phn 3.5). Cui cng ta s

c mt tn hiu OFDM c dng:

Thit lp cc thng s khi to cho h thng truyn dn OFDM thch ngTrc tin hiu nguyn l hot ng ca chng trnh m phng ta hy xt cc thams vo ra ca h thng.

Bng 6.2 Thng s m phng h thng OFDM thch ng

Tham s Gi trTc d liu: Rb 2 MbpsMc iu ch sng mang M-QAM M = 2, 4, 16, 64Tn s ly mu: fs fs

S lng sng mang: Nsub 52 - 1024Kch thc FFT: NFFT NFFTChu k c bn: T 1/ R b = 0,5 sThi gian k hiu hu ch: TFFT TFFT = Nsub log2(M) TKhong thi gian bo v: TG TGChiu di k hiu OFDM : Tsym = TFFT + TGThi gian m phng: Tsim Tsim = 0,064 sCa s Kaiser, = 10M hnh knh Pha inh Rayleigh

Hnh 6.5 M phng tn hiu OFDM trong min thi gian,[sim_ofdm_signal.m]


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Khong thi gian bo v: UG Tn1

T = , n = 1/2, 1/4, 1/8, 1/64 ....,

Tn s ly mu:sym

s T

13f = , trong Tsym = TFFT + TG,

M l s trng thi iu ch QAM (M = 2 ng vi BPSK), v cc thng s c s lin quann nhau do ch cn khai bo cc thng s: Rb, M, Nsubcarr, NFFT, TG. T kch thc filenh hoc file text ta xc nh thi gian m phng:Tsim = s lng bit truyn/ Rb.

Trong chng trnh m phng, cc tham s khi to h thng c tp trung vo trongmt bng, gi tr ca cc tham s c thit lp khi khi to v c th thay i trong qutrnh thc hin chng trnh m phng nu chn chc nng iu ch thch ng. Tt nhincc tham s s gi nguyn gi tr nh khi khi to nu khng chn chc nng iu chthch ng. Giao din tham s khi to h thng c m ra c cu trc nh sau.

hiu hot ng ca chng trnh m phng AOFDM trc tin cn phi hiu r cctham s khi to h thng v nh hng ca n n hot ng ca h thng.

Kch thc FFT: Biu th bng nhn Kch thc FFT, tham s ny chnh l s imFFT dng trong mt k hiu OFDM hay chnh l di k hiu OFDM. m bo phOFDM l thc th kch thc FFT > 2 ln s lng sng mang (trong mc 3.3.2.4). Kchthc FFT nu chn qu ln s lng ph bng tn nhng b li s phn nng lng tnhiu u trn cc sng mang, iu ny c tc dng phn tn li. Tuy nhin nu kch thc


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FFT chn qu nh th mt nng lng trn mi sng mang con s ln v nu b li ths gy li cm. V vy nn chn gi tr tham s:

2:1< kch thc FFT: s sng mang 5:1 (6.1)

C th n thit lp l 256.

S lng sng mang: hin th bng nhn S sng mang, l s lng sng mang dng truyn d liu. Cn phi phn bit gia d liu v d liu ngi dng, dng thut ngd liu ngi dng c ngha l d liu i n b iu ch AOFDM, cn d liu l dliu ngi dng v phn n thm ( thch ng vi knh truyn), thng phn n thmny l cc k hiu 0. Tt nhin s lng sng mang cng phi tho mn phng trnh(6.1).S lng sng mang cng ln, th tr iu ch cng cao song c li v tnh phntp, do s phn tn li u trn bng tn ca knh. Mt khc v c ch thch ng m ns dng l chn lc sng mang tc l la chn cc sng mang c SNR cao (hay BERthp) truyn d liu do s lng sng mang ln c ngha c bit quan trng vkhi c ch chn lc s hot ng hiu qu hn, s la chn chnh xc cc vng tn s

tng i n nh trn knh. Tuy nhin thng chn s lng sng mang 500 v tnhcht m phng chy trn phn mm. Do tc x l khng cao nh tc x l phncng trong thc t. Trong n s lng sng mang s dng l 100.

Khong thi gian bo v: Hy tng tng mi k hiu u c mt khong bo v cgn trc mi k hiu v khong bo v ny c chn sao cho ln hn thi gian tr cci ca knh, th khi cc k hiu thu trc v sau s khng b chng ln ln nhau mch chng ln ln cc khong bo v. Do nng lng mi k hiu s khng b phn tntrn cc k hiu khc, tc l khng c hin tng ISI. V vy khong bo v trong k hiuOFDM c ngha c bit quan trng n gip tng chiu di k hiu, chng pha inh lachn tn s (xem chng 3), c bit chng li hin tng ISI gy thu sai tn hiu. Theo

chun DAB th khong bo v c chn = 1/4 kch thc FFT v khong bo v cgn pha trc k hiu. Tuy nhin khong bo v cng c chn la theo bt k t lno nh: 1, 1/2, 1/4, 1/8, . Tu theo kch thc FFT l ln hay nh v gi tr tri tr cci ca knh. n chn gi tr khong bo v = 1/4 kch thc FFT, trn giao din nhpd liu khong bo v c hin th bng nhn 'Khong bo v' (4 c ngha l khong bov = 1/4 kch thc FFT).

Ngng BER: Ngng BER l gi tr dng thit lp xem sng mang no s cdng truyn d liu ngi dng v sng mang no s khng s dng tc l phi tin hnhchn 0. Hm thc hin chc nng kim tra li cho tng sng mang con l hm

dem_loi. Hm ny s tnh ra gi tr BER ca tng sng mang con v kim tra xem nusng mang no c gi tr BER > BER ngng th s gn cho phn t ca mng Q tngng vi v tr sng mang ny gi tr l 1, tc l khng dng sng mang ny. Sau hmchen_song_mang s cn c vo gi tr ca mng Q m s quyt nh dng v khngdng sng mang no. Nu ngng BER c thit lp qu ln th s khng c tc dngchn lc sng mang v cc gi tr BER hin thi trn mi sng mang c th u nh hngi tr BER ngng. Tuy nhin nu thit lp gi tr BER ngng qu nh th s sngmang m bo gi tr BER nh hn gi tr BER ngng s rt t do tc truyn dns rt chm do ch tin hnh truyn d liu trn mt s t sng mang. Trong n gi trBER ngng c thit lp l 10-3.

Tn s Doppler: Do chuyn ng tng i gia my thu v my pht gy ra hiu ngDoppler. Doppler gy hiu ng iu tn ngu nhin i vi cc thnh phn sng mang do lm sai lch gi tr tn s ca cc sng mang con. iu ny dn n ph tn ca cc


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sng mang con chng ln qu mc ln nhau lm mt tnh trc giao ca cc sng mangcon. Khi khng cn gi tnh trc giao th nng lng tn hiu trn cc sng mang s chngln ln nhau iu ny s dn ti khng cn phn bit c ranh gii gia cc k hiu trncc sng mang con do gy li ti b quyt nh. Tt nhin tn s Doppler trong nch l gi tr c ngha m phng m bo tnh thc t ca m hnh knh, thngthng tn s Doppler < 100 Hz. n thit lp gi tr tn s Doppler l 50 Hz.

p ng xung kim knh: p ng xung kim ca knh chnh l s lng xung kim vgi tr bin suy gim ca cc xung kim cng vi thi gian tr.

1

0.32

0.28

0.13

Thi gian tr0 5 8 17

Hnh 6.6 p ng xung kim ca knh

Hnh 6.6 Cho thy hnh dng p ng ca knh xung kim, vi 3 ng tr. Tng ngcc gi tr bin trn hnh v l cc gi tr bin suy gim ca nhnh tr ca knh. Vthi gian tr ca tng nhnh l thi gian tr ca cc xung kim ca knh. Trn bng khito gi tr ban u cho h thng AOFDM th:

Thi gian tr ca xung th nht s l 5 tng ng vi nhn nhnh 1 trn bng. Thi giantr ca xung th hai l 5+3 = 8 tng ng vi nhn 'nhnh 2' trn bng. Thi gian tr ca

nhnh th ba l 5+3+9 = 17 tng ng vi nhn nhnh 3 trn bng. Cc gi tr bin suy gim ca nhnh tr s tng ng vi cc nhnh l cc nhn suy gim 1, suy gim2, suy gim 3 vi cc gi tr ln lt l 0.32, 0.28, 0.13. Tng ng vi cc gi tr nyth hnh dng hm truyn t ca knh s c dng:

Tn s ly mu k hiu pht: Mi k hiu sau khi iu ch k hiu OFDM u cly mu vi tn s ly mufs. m bo ly mu c y k hiu pht th tn sly mu cn tho mn nh l Nyquist: syms ff 2 tc l tn s ly mu ti thiu phi gphai ln tn s k hiu. Trn bng nhp cc gi tr khi u tn s ly mu c th hin

bng gi tr di nhn Tn s ly mu, trn bng gi tr ny l 3 tc l i vi mi k

Hnh 6.7 Hnh dng hm truyn t ca knh


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hiu OFDM sau iu ch chng ta s ly mu ti 3 v tr ca k hiu. Tt nhin k hiusau khi thu s l gi tr trung bnh ca cc mu ca cng tn hiu pht, do khi tn s lymu ln th gi tr k hiu thu s gn nh gi tr k hiu pht v th BER s gim. Tuynhin nu tn s ly mu qu ln s dn n gim tc bit thc truyn dn do pht lpli cng mt k hiu nhiu ln. Trong n gi tr tn s ly mu = 3 ln gi tr tn s khiu OFDM.

S trng thi iu ch: Trong n dng phng php iu ch ch yu l M-QAM,vi s trng thi iu ch thay i vi cc gi tr M = 2, 4, 16, 64. S trng thi iu ch

ban u c thit lp l gi tr nm di nhn 'Mc iu ch QAM' trong bng khi to.Tt nhin khi chn nguyn c ch thch ng chn lc sng mang hoc khng chn c chthch ng no th s trng thi iu ch c gi nguyn trong qu trnh m phng, nuchn c ch thch ng mc iu ch th s trng thi iu ch s c thay i tu theotnh trng knh hin thi hay gi tr BER tng cng sau mi ln pht k hiu OFDM.Trong n gi tr s trng thi iu ch ban u c thit lp l 2.

nh dng file truyn: linh hot trong vic to ra d liu ngi dng dng trong m

phng, n xy dng hm c th c cc loi file vn bn (text.txt) hoc file hnh nh(*.JPG, *.BMP,). Tt nhin mi loi file c mt nh dng khc nhau v dung lnghon ton khc nhau, do s cn c mt hm c file khc nhau. Trn bng khi tonh dng file c m t thng qua nhn Loi file. Nu

Loi file = 1 tc l s truyn file vn bn c tn Design.txt, kt qu m phng sau khitruyn qua mi trng knh s c lu trong file Result.txt.

Loi file = 2 tc l s truyn file ho c tn Fish.bmp, kt qu m phng sau khitruyn qua mi trng knh s c lu trong file anh_dep.jpg. Tt nhin kt qu sc hin thi thng qua giao din xem_anh. Nu dng file ho m phng th thi

gian c file v chuyn d liu sang dng nh phn, sau tin hnh iu ch chuyn mcri iu ch tn hiu OFDM s rt lu. ta c th quan st c qu trnh iu ch dliu v thu pht qua knh v tuyn th n thit lp giao din m phng OFDM_S.Trc tin d liu c t file c dng cc mng thp phn s c ct nh thnh tngkhi (thng thng mi khi gm 200 phn t). Sau s tin hnh chuyn tng khisang dng nh phn ri tin hnh iu ch OFDM d liu nh phn trong khi ny, tn hiusau iu ch OFDM s c pht qua m hnh knh v thu ti b thu OFDM. D liu thus c chuyn li thnh cc khi thp phn v c lu vo mt mng, sau khi kt thcm phng d liu thu di dng cc khi thp phn s c ghp li vi nhau theo nhdng file ban u ri tin hnh ghi ra file nh. Tip tc chng trnh s hin th kt qu

nh thu c so snh cht lng.

6.3 Chng trnh m phng truyn dn OFDM thch ng

6.3.1 Giao din chng trnh m phng

Chng trnh m phng c thit k bng ngn ng Matlab 6.5, v c t chc thnhtng module.

Tnh module cho php, ci tin, pht trin v nng cp sau ny.

Do c tnh module nn thun li cho vic kim tra, nh gi, kho st m hnh dng ri rc v lin kt.


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chy chng trnh m phng AOFDM, t ca s lnh ca MatLab ta g lnhAOFDM. Di y l phn giao din bt u ca chng trnh.

Phn giao din chnh ca chng trnh s c m khi nhn chut vo nt tip tc. Trckhi n vi giao din chnh, giao din minh ho hnh nh dng truyn s c m ra.Hnh bn tri vi tiu : nh dng pht s l hnh nh phc v cho sut qu trnhm phng. Trc to cn li bn phi s hin nh sau khi truyn qua AOFDM. Sau khihon thin m phng h thng AOFDM th nh ny s c m ra so snh gia cht

lng nh gc v nh sau khi truyn qua AOFDM. Tt nhin v l do thi gian truyn qulu khi dng nh mu 32 bit (do x l s bng phn mm c tc rt chm, trong cc hthng thc t tc tnhton cao do cc thao tc x ls c thc hin phn ln bng

phn cng ). Cho nn nhanh chng n ch m phng kt qu vi nh trng en.


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Khi kch hot vo nt tip tc giao din u tin th chng trnh s gi mt giao din m phng h thng truyn dn OFDM thch ng, giao din c tn hm l 'OFDM_S'.Di y s l phn giao din thc hin tng ca n, giao din c cu trc theotng module, mi module s thc hin mt nhim v v minh ha mt phn kt qu trongh thng truyn dn thch ng OFDM.

Phn khung cng c thc hin bao gm cc nt cng c:

Execute: s thc hin kch hot h thng AOFDM hot ng.

Input Data:s m ra mt giao din nhp cc thng s khi to h thng.

Close: s ng giao din m phng.

Back: s quay tr li giao din u tin AOFDM.

Phn khung cng c thch ng bao gm cc nt cng c:

Chn lc SM: l biu tng khi tch vo h thng s kch hot ch thch ng theo cch chn lc sng mang.

Mc iu ch: khi tch vo biu tng ny h thng s kch hot ch thch ng theomc iu ch (AQAM). Tt nhin ta c th kch hot ng thi hai c ch thch ng bngcch tch vo c hai biu tng trn.

Knh ng: l cng c m khi kch hot n s to ra mt knh v tuyn c hm truynt thay i theo thi gian. Nu khng kch hot cng c ny th knh m phng s l

knh tnh, cc tham s knh s gi nguyn trong qu trnh m phng.


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Phn hin th tn hiu M-QAM: s hin th chm sao M-QAM ti b thu sau khi tnhiu c gii iu ch OFDM. Khi s trng thi iu ch l M th phn giao din ny shin th chm sao M-QAM, trn phn giao din l chm sao 16-QAM.

Phn hin th tn hiu OFDM: s hin th dng sng trong min thi gian ca tn hiuOFDM pht v thu ng thi cng hin th hm truyn t ca knh v ph ca tn hiu

OFDM thu. Mc ch n xy dng giao din hin th ph ca tn hiu OFDM thu vhm truyn t ca knh d dng quan st c ch thch ng chn lc sng mang. ngvi nhng v tr m hm truyn t ca knh nhn cc gi tr khc 1 rt nhiu th ccsng mang con c gi tr tn s nm gn khong s khng c s dng, trn giao dinta thy ng vi cc gi tr ny th p ng tn s ca tn hiu thu s xp x 0. C chchn lc sng mang c coi l hot ng tt khi la chn c nhng khong m png ca knh l xp x 1.

Phn hin th kt qu: s cung cp cc gi tr BER hin thi ca tng sng mangcon v gi tr BER tng hp ca ton b h thng. Gi tr BER hin thi ca cc sngmang con c hin th di nhn Gi tr BER ca cc sng mang con. Gi tr BER ca

h thng c hin th di nhn BER. Ngoi ra tin theo di s lng cng vic hon thnh c bao nhiu phn trm, giao din cng a ra mt bng hin th thi gianthc hin v s phn trm cng vic thc hin.

Mt bng hin th rt quan trng theo di c ch thch ng chn lc sng mang l hinth s lng sng mang c dng truyn d liu ngi dng, l nhn S smtruyn. Trn phn giao din ta thy s sng mang thit lp ban u l 100 (ta thy chiudi ca trc honh ca nhn Gi tr BER ca cc sng mang con l 100), trong khi ssng mang truyn d liu ngi dng ch c 24. Tt nhin vic dng s lng t sngmang truyn dn s lm chm tc h thng nhng vi iu kin knh qu ti th nutruyn d liu trn cc sng mang cn li th s chu mt t l li qu ln vt qu gi trBER yu cu cho mi dch v v khi s khng m bo c QoS.

6.3.2 Cc kt qu m phng v nh gi hiu nng

n s nghin cu hot ng ca h thng truyn dn OFDM khi ch dng mt c chthch ng, khi dng ng thi cc c ch thch ng v khi khng dng thch ng. ngthi cng nghin cu hiu nng ca cc phng php thch ng trong cc iu kin knhkhc nhau: knh tnh, knh ng v cc tham s knh khi to khc nhau.

6.3.2.1 Kt qu m phng khng dng c ch thch ng

Trc tin d so snh gia cc phng php thch ng v so snh hiu nng ca c ch

thch ng vi khi khng thch ng, ta hy xem kt qu truyn d liu ngi dng l filenh vi phng php iu ch 4-QAM qua h thng OFDM m khng dng bt k c chthch ng no c.

Tham s ban u c thit lp nh giao din khi to pha trn. Giao din m phng snh sau:


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Ta thy rng nhng v tr p ng knh c gi tr khc 1 rt nhiu th gi tr BER cacc sng mang con ti s rt ln. Trn giao din m phng th nht gi tr BER cacc sng mang con ta thy c ba vng m BER rt ln so vi cc vng khc. Nhn ln


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phn giao din c tuyn tn s ca tn hiu OFDM thu, ta thy tng ng vi ba vngBER cao ny l ba vng m p ng knh b lm nghim trng. Do cc sng mang conti y bin b suy gim nhanh chng gy li ln sau b quyt nh. Kt qu trn giaodin m phng th hai cng tng t, ch c iu ch c hai vng BER cao.

Kt qu file nh truyn qua mi trng knh m khng s dng c ch thch ng no s

nh sau.

Ta thy trn nh thu c hnh thnh nhng ng li cm do cc sng mang ti nhngv tr knh bin ng ln s b li cm. c ch thch ng chn lc sng mang s giiquyt trit vn ny.

6.3.2.2 Kt qu m phng dng c ch thch ng mc iu ch

Gi tr BER thit lp trong n cho cc mc chuyn c cho trong bng 6.2.

C ch chuyn mc cng phi tun theo quy tc sau: Nu ang mc BPSK khng thtrc tip chuyn ln mc 16-QAM, 64-QAM c m phi qua bc trung gian l 4-QAM, v ang mc 4-QAM cng vy khng th chuyn trc tip ln 64-QAM. Nhngang mc 16-QAM, 64-QAM nu BER tng qu cao c th chuyn trc tip xungBPSK. V c c ch ny m h thng lm vic rt hiu qu, nguyn nhn l khi ang cc mc BPSK, 4-QAM nu km theo c ch chn lc sng mang th thng BER tng =0, nhng khng phi mc 16-QAM, 64-QAM th BER tng cng nh nh vy mthng BER tng s khng m bo mc ngng do m bo QoS th cn phichuyn qua mc trung gian.


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Bng 6.2 Tham s BER iu khin chuyn mc iu ch

Gi tr BER tng Ngng B

mo phong truyen dan ofdm

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