kenh truyen

7/28/2019 kenh truyen

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CHNG 1: KNH TRUYN V TUYN1.1 GII THIU CHNG

Cc phng tin thng tin ni chung c chia thnh hai phng

php thng tin c bn, l thng tin v tuyn v thng tin hu

tuyn.

Trong mng thng tin v tuyn ngoi ngun tin v nhn tin th

knh truyn l mt trong ba khu quan trng nht, v c cu trc

tng i phc tp. N l mi trng truyn thng tin t my

pht n my thu. V th chng ny tm hiu cc thng tin v knh

truyn.

1.2 NIM V H THNG THNG TIN V TUYN

1.3 KNH V TUYN

1.3.1 Gii thiu

Cht lng ca cc h thng thng tin ph thuc nhiu vo knh

truyn, ni m tn hiu c truyn t my pht n my thu. Khng

ging nh knh truyn hu tuyn l n nh v c th d on c,

knh truyn v tuyn l hon ton ngu nhin v khng h d dng

trong vic phn tch. Tn hiu c pht i, qua knh truyn vtuyn, b cn tr bi cc to nh, ni non, cy ci , b phn x, tn

x, nhiu x, cc hin tng ny c gi chung l fading. V kt

qu l my thu, ta thu c rt nhiu phin bn khc nhau ca tn

hiu pht. iu ny nh hng n cht lng ca h thng thng tin

v tuyn.

Trang : 1

Ngun tin Knh tin Nhn tin

Hnh 1.1: S khi chc nng h thng truyntin


2/24

Hin tng fading trong mt h thng thng tin c th

c phn thnh hai loi:

Fading tm rng (large-scale fading) v fading tm hp

(small-scale fading).

C ba c ch chnh nh hng n s lan truyn ca tn hiu

trong h thng di ng:

Phn x xy ra kh sng in t va chm vo mt mt

bng phng vi kch thc rt ln so vi bc sng tn

hiu RF.

Nhiu x xy ra khi ng truyn sng gia pha pht v

thu b cn tr bi mt nhm vt cn c mt cao v kch

thc ln so vi bc sng.

Tn x xy ra khi sng in t va chm vo mt mt

phng ln, g gh lm cho nng lng b tri ra (tn x )

hoc l phn x ra tt c cc hng.

1.3.2 Cc hin tng nh hng n cht lng knh truyn1.3.2.1 Hin tng a ng (Multipath)

Trong mt h thng thng tin v tuyn, cc sng bc x in t

thng khng bao gi c truyn trc tip n anten thu. iu ny

xy ra l do gia ni pht v ni thu lun tn ti cc vt th cn tr

s truyn sng trc tip. Do vy, sng nhn c chnh l s chng

chp ca cc sng n t hng khc nhau bi s phn x, khc x,tn x t cc to nh, cy ci v cc vt th khc. Hin tng ny

c gi l s truyn sng a ng (Multipath propagation).

1.3.3.2 Hiu ng Doppler

Hiu ng Doppler gy ra do s chuyn ng tng i gia my

pht v my thu nh trnh by hnh 1.4. Bn cht ca hin tng

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ny l ph ca tn hiu thu c b x lch i so vi tn s trung tm

mt khong gi l tn s Doppler.

1.3.2.3 Suy hao trn ng truyn

M t s suy gim cng sut trung bnh ca tn hiu khi truyn

t my pht n my thu. S gim cng sut do hin tng che chn

v suy hao c th khc phc bng cc phng php iu khin cng

sut.

1.3.2.4 Hiu ng bng rm (Shadowing)

Do nh hng ca cc vt cn tr trn ng truyn, v d nh

cc to nh cao tng,cc ngn ni, i, lm cho bin tn hiu b

suy gim.

1.3.3 Cc dng knh truyn

Knh truyn chn lc tn s v knh truyn khng chn lc

tn s

Knh truyn chn lc thi gian v Knh truyn khng chn

lc thi gian

1.3.4 Cc m hnh knh c bn

Khi nghincu cc knh v tuyn di ng, thng cc phn b

Trang : 3

Vtphnx

Tuyn 1

v

1

Tuyn 2

)(2 t

)(1 t

Trm pht

Hnh 1.4: Hm truyn t ca knh


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Rayleigh v Rice c s dng m t tnh cht thng k thay i

theo thi gian ca tn hiu pha inh phng. Sau y, chng ta s xt

cc phn b ny v a ra cc c tnh c bn ca chng.

1.3.4.1 Knh theo phn b Rayleigh

C th coi phn b pha inh Rayleigh l phn b ng bao ca

tng hai tn hiu phn b Gauss vung gc.

1.3.4.2 Phn b Pha inh Rice

Khi tn hiu thu c thnh phn n nh (khng b pha inh) vt

tri, ng truyn trc tip (LOS), phn b ng bao pha inh

phm vi hp c dng Rice. Trong phn b Rice, cc thnh phn a

ng ngu nhin n my thu theo cc gc khc nhau v xp chng

ln tn hiu vt tri ny.

Trang : 4


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CHNG 2: K THUT CDMA2.1 GII THIU CHNG

Trong bt k h thng thng tin v tuyn no, di bng tnc cho php s dng l lun b gii hn. V vy, vic chia s

knh truyn nhiu ngi c th s dng ng thi l mt nhu

cu cp thit. K thut CDMA ra i, da trn nguyn l tri ph

v c s dng rng ri cho thng tin v tuyn trn ton th gii.

2.2 NGUN GC CDMA

L thuyt v CDMA c xy dng t nhng nm 1980 v

c p dng trong thng tin qun s t nm 1960 cng vi s pht

trin ca cng ngh bn dn v l thuyt thng tin vo nm 1980

CDMA c thng mi ho t phng php thu GRS v ommi -

Tracs, phng php ny c xut trong h thng t ong ca

Qual Comm - M vo nm 1990.

2.3 NGUYN L CA K THUT A TRUY NHP PHN

CHIA THEO M ( CDMA )Trong h thng CDMA, mi ngi dng c gn cho mt

chui m xc nh, v tt c cc ngi dng c th s dng chung

khong bng tn trong cng mt khong thi gian. Do CDMA da

trn nguyn l tri ph, do mi trm pht s s dng mt chui

tri ph gi ngu nhin tc ng vo tn hiu tin tc. Khi my thu

nhn c tn hiu t nhiu trm pht khc nhau, n s ly tn hiumong mun bng cch gii m tn hiu bng chui m ring ca

chnh tn hiu . Ta c c kt qu ny l do tnh t tng quan v

tng quan cho ca cc chui m tri ph.

2.4 CC C TNH CA CDMA

2.4.1 Tnh a dng ca phn tp

Trang : 5

Hnh 2.5: Phn b xc sut Rice vi cc gi tr K khc nhau


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Phn tp l 1 hnh thc tt lm gim pha inh : C 3 loi phn

tp :

Phn tp theo thi gian

Phn tp theo tn s - tn hiu bng rng 1,25 Mhz

Phn tp theo khong cch

2.4.2 iu khin cng sut CDMA

H thng CDMA cung cp chc nng iu khin cng sut

2chiu (T BTS n my di ng v ngc li). Mc ch iu khin

cng sut pht ca my di ng cho tn hiu pht ca tt c cc

my di ng trong mt vng phc v c th thu c vi nhy

trung bnh ti b thu ca BS. B thu CDMA ca BS chuyn tn hiu

thu t my di ng tng ng thnh thng tin s bng hp cn tn

hiu ca cc my di ng khc ch cn l tn hiu tp m ca bng

rng.

2.4.3 B m - Gii m thoi v tc s liu bin i

Do s tch bit tn s 45 MHz gia knh hng i v hng vvt qu di thng lin kt ca cc knh nn my di ng c gi

thit l cc gi tr ca tn hao hai a ng dn ging nhau, do

khng c kh nng o suy hao ng dn hng v mc d c

1chnh lch gia suy hao a ng trn knh hng i o my

di ng v suy hao a ng ca knh hng v. Phng php o

ny cung cp gi tr trung bnh chnh xc cho cng sut pht nhng 1phng php ph c cn ti tnh ton giao thoa Rayleigh ch ra

cc tnh cht khc nhau trn 2 knh.

2.4.4 iu khin cng sut mch vng kn trn knh hng v

ca CDMA

Cc tn hiu cng sut pht c iu khin bi thng tin iu

Trang : 6


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khin pht i t trm gc. Khi xy ra s suy gim ng k trn knh

hng i trong mt vng nht nh v nu cng sut pht khng tng

ln th cht lng thoi ca cc cuc gi qua knh ny gim xung

di tiu chun cht lng. iu khin cng sut trn knh hng

i trm gc gim theo chu k cng sut pht ra ti my di ng,

gim cng sut ra nh vy duy tr cho n khi cc my di ng yu

cu cng sut ra b xung nh d thy tng t l li khung v trm

gc tng cng sut pht ra vi s gia 0,5 db. S tng gim cng sut

ra c thc hin mt ln cho mi khung mi ting ni tc l c

trung bnh 12 n 20 ms chm hn tc tng hay gim cng sut ra

trn knh hng vng ng tng hay gim cng sut ra c th iu

khin ti +6,-6 db xung quanh cng sut ra trung bnh.

2.4.5 iu khin cng sut trn knh hng i ca CDMA

2.4.6 Bo mt cuc gi

H thng CDMA cung cp chc nng bo mt cuc gi cao v

v c bn to ra xuyn m, vic s dng my thu tn kim v sdng bt hp php knh RF l kh khn i vi h thng t ong s

CDMA v tn hiu c trn ( Seranbling ). V c bn th cng

ngh CDMA cung cp kh nng bo mt cuc gi v cc kh nng

bo v khc, tiu chun xut gm kh nng xc nhn v bo mt

cuc gi c nh r trong CIA/TIA/SI-54-B. C th m ho knh

thoi s mt cch d dng nh s dng DES hoc cc cng ngh mtiu chun khc.

2.4.7 Chuyn vng mm

2.4.8 Dung lng, dung lng mm

2.5 K THUT TRI PH

2.5.1 Gii thiu

Trang : 7


8/24

K thut tri ph ra i t nhu cu bo mt thng tin trong qun

s. Mc ch ca k thut tri ph l lm cho tn hiu c pht

ging nh tp m i vi cc my thu khng mong mun, lm cho

cc my thu ny kh khn trong vic tch v ly ra c bn tin.

Mt h thng thng tin c xem l tri ph khi tha 2 iu kin:

Bng thng tn hiu tri ph ln hn rt nhiu so vi bng

thng tn hiu thng tin.

M dng tri ph c lp vi tn hiu thng tin.

u im ca k thut thng tin tri ph

Kh nng a truy cp

Tnh bo mt thng tin cao

Bo v chng nhiu a ng

2.5.2 Cc k thut tri ph c bn2.5.2.1 K thut tri ph bng cch phn tn ph trc tip (DS

SS: Direct Sequence Spread Spectrum)

Tn hiu truyn i c biu din di dng lng cc, sau

nhn trc tip vi chui gi ngu nhin. my thu, tn hiu thu c

nhn vi chui tri ph ln na to li tn hiu tin tc.

2.5.2.2 K thut tri ph bng phng php nhy tn s ( FH

SS: Frequency Hopping Spread Spectrum)

K thut FH SS pht trin da trn iu ch BFSK. Trong ,tn s sng mang c thay i lin tc theo mt quy lut gi ngu

nhin (da trn chui m ngu nhin s dng), nh vy m ph ca

tn hiu FH SS c tri rng trn trc tn s.

2.5.2.3 K thut tri ph bng phng php nhy thi gian (TH

SS: Time Hopping Spread Spectrum )

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Trc thi gian c chia thnh cc khung (frame). Mi khung li

c chia thnh k khe thi gian (slot). Trong mt khung, ty theo m

ca tng user m n s s dng mt trong k khe thi gian ca khung.

Tn hiu c truyn trong mi khe c tc gp k ln so vi tn

hiu truyn trong ton b khung nhng tn s cn thit truyn

tng gp k ln.

2.5.3 Cc chui tri ph c bn

2.5.3.1 Chui tn hiu nh phn gi ngu nhin

Chui tn hiu nh phn gi ngu nhin l chui tn hiu nh phn

tun hon nhng c chu k lp li rt ln, do nu khng c bit

trc quy lut ca n, ngi quan st kh nhn bit c quy lut.

Ta gi l chui gi ngu nhin (PRBS: Pseudo Random Binary

Sequence). Chui PRBS c to ra t mch chui gm ND-

FlipFlop ghp lin tip nhau nh hnh 2.11.

Hnh 2.11: S mch to chui gi ngu nhin

2.5.3.2 Chui Hadamarh Walsh

Cc hm Walsh c to ra t cc ma trn vung c bit NN

Trang : 9


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gi l cc ma trn Hadamard. Cc ma trn ny cha mt hng ton

s 0 v cc hng cn li c s s 1 v s s 0 bng nhau. Hm Walsh

c cu trc cho di khi N=2j trong j l mt s nguyn

dng. Cc t hp m cc hng ca ma trn l cc hm trc

giao c xc nh nh theo ma trn Hadamard nh sau: [ ],01 =H

,10

002

=H ,

0110

1100

1010

0000

4

=H

=NN

NN

N

HH

HHH

2

Trong l o c s hai ca HN

2.6 HIU NNG CA CC H THNG DS/SS

2.6.1 nh hng ca tp m trng

2.6.2 nh hng ca nhiu giao thoa

2.6.3 Truyn a tia

2.7 H THNG DS CDMA ( Direct Spread Code DivisionMultiple Access )

H thng DS CDMA da trn k thut tri ph bng cch phn

tn ph trc tip v cc l thuyt c bn v k thut CDMA. M

hnh n gin ca mt h thng tri ph gm K ngi s dng chung

mt bng tn vi cng mt tn s sng mang fc v iu ch BPSK

Trang : 10


11/24

CHNG 3: K THUT OFDM3.1 GII THIU CHNG

OFDM l k thut ghp knh phn chia theo tn s trc giao,

chia ton b bng tn ra thnh nhiu sng mang nhnh m cc sng

mang ny phi trc giao.

3.2 NGUYN TC C BN CA OFDM

Trong OFDM chui d liu u vo ni tip c tc cao (R)

c chia thnh N chui con song song (t chui d liu 1 n chui

d liu N) c tc thp hn (R/N). N chui con ny c iu ch

bi N sng mang ph trc giao, sau cc sng mang ny c

cng vi nhau v c pht ln knh truyn ng thi, c m t

nh hnh 3.1. pha qu trnh thu tin th ngc li.

Trang : 11

Dliu

Tc R

D liu 1

D liuN

D liu 2

D liu

tng

Biuch

Bphnchuyninitip/songsong

Chnchu

ibov

Tin

Tc R/N

Hnh 3.1: S qu trnh pht tin


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3.3. M HNH TRUYN DN OFDM

3.3.1. M t ton hc tn hiu OFDM

3.3.2 S h thng truyn dn OFDM

My pht: Chuyn lung d liu s pht thnh pha v bin

sng mang con. Cc sng mang con c ly mu trong

min tn s, ph ca chng l cc im ri rc. Sau s

dng bin i Fourier ri rc ngc (IDFT) chuyn ph ca

cc sng mang con mang d liu vo min thi gian. Tuy

nhin cc h thng trong thc t dng bin i Fourier

ngc nhanh (IFFT) v n tnh hiu ca n. Tn hiu OFDM

trong min thi gian c trn nng tn ln tn s truyn

dn v tuyn.

My thu: Thc hin hot ng ngc li ca pha pht.

3.4 CC VN K THUT TRONG OFDM c lng tham s knh.

ng b sng mang

Gim t s cng sut tng i cc i PAPR ( Peak to

Average Power Ratio)

3.5 TNH TRC GIAO

3.5.1 tng tng OFDM l truyn dn song song (ng thi) nhiu bng

con chng ln nhau trn cng mt rng bng tn cp pht ca h

thng.

3.5.2 nh ngha

3.5.3 Minh ho

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OFDM t tnh trc giao trong min tn s bng cch phn phi

mi tn hiu thng tin ring bit vo cc sng mang con khc nhau.

Cc tn hiu OFDM c to ra t tng ca cc hm sin tng ng

vi mi sng mang. Tn s bng tn c s ca mi sng mang con

c chn l mt s nguyn ln ca tc k hiu, kt qu l ton

b cc sng mang con s 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 3.5

Trang : 13


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3.6 NHIU GIAO THOA K T V NHIU GIAO THOA

SNG MANG

3.6.1 Khi nim

Trong mi trng a ng, k t pht n u vo my thu vi

cc khong thi gian khc nhau thng qua nhiu ng khc nhau.

S m rng ca chu k k t gy ra s chng ln gia k t hin

thi vi k t trc v kt qu l c nhiu lin k t (ISI). Trong

OFDM, ISI thng cp n nhiu ca mt k t OFDM vi k t

trc .

3.6.2 Phng php chng nhiu lin k hiu

3.7 CI THIN HIU NNG H THNG TRUYN DN

TRN C S KT HP M HO GRAY

3.7.1 Cc nh hng

Tp m tn ti trong ton b h thng truyn thng. Ngun tp

m chnh l tp m nhit nn, tp m in trong b khuch i pha

thu. Ngoi ra tp m c to ra trong ni b h thng nh ISI, ICIv IMD. Chng lm gim SNR v lm gim hiu qu ph tn ca h

thng. V th cn phi nghin cu nh hng ca tp m i vi t l

li truyn thng v ho hp gia mc tp m v hiu qu ph tn.

2.7.2 Gii php khc phc

M ho Gray: l mt phng php m cc im IQ cnh nhau

trong chm sao s ch khc nhau mt bit. M ho Gray cho php tiu t s li bit v gim xc sut li nhiu bit xut hin trong mt k

hiu n. Thng tin hnh m ho Gray khi iu ch M-QAM hay

M-PSK

3.8 U IM V KHUYT IM CA OFDM

3.8.1 u im

Tng hiu qu s dng bng thng.

Trang : 14

Hnh 3.5: Dng sng ca mt tn hiu OFDM trong minthi gian v tn s, [sim_ofdm_time_domain.m]


15/24

Bn vng vi fading chn lc tn s do cc k hiu c

bng thng hp nn mi sng mang ph ch chu fading

phng.

Chng c nhiu lin k hiu ISI do chu k k hiu di

hn cng vi vic chn thm khong bo v cho mi k hiu

OFDM.

S phc tp ca my pht v my thu gim ng k nh

s dng FFT v IFFT.

C th truyn d liu tc cao.

3.8.2 Khuyt im

Nhy vi offset tn s

+ Ch cn mt sai lch nh cng c th lm mt tnh trc

giao ca cc sng mang ph. V vy OFDM rt nhy vi

hiu ng dch tn Dopler.

+ Cc sng mang ph ch tht s trc giao khi my pht

v my thu s dng cng tp tn s. V vy, my thu phic lng v hiu chnh offset tn s sng mang ca tn

hiu thu c.

Ti my thu, s rt kh khn trong vic quyt nh v tr

nh thi ti u gim nh hng ca ICI v ISI.

T s cng sut nh trn cng sut trung bnh PAPR (Peak

to Average Power Ratio) l ln v tn hiu OFDM l tngca N thnh phn c iu ch bi cc tn s khc nhau.

Khi cc thnh phn ny ng pha, chng to ra ng ra mt

tn hiu c bin rt ln. Ngc li, khi chng ngc pha,

chng li trit tiu nhau lm ng ra bng 0. Chnh v vy,

PAPR trong h thng OFDM l rt ln.

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CHNG 4 : K THUT MC-CDMA

4.1 GII THIU CHNG

Bn cnh vic cung cp cc dch v v ng dng mi, thnh cngca 4G s ph thuc nhiu vo s la chn cc khi nim v s i

mi cng ngh trong kin trc, phn phi ph, s dng v khai thc

ph. V vy cc k thut a truy nhp mi rt cn thit cung cp

tc d liu cao vi s phn phi di tn linh hot. Nm 1993,

tng v s kt hp gia CDMA v OFDM dn n vic ra i ca

m hnh a truy cp mi. Mt trong nhng m hnh h thng ny l

MC-CDMA (MultiCarrier CDMA).

4.2 NGUYN L CHUNG CA MC CDMA

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Hnh 4.1: Nguyn tc chung ca MC-CDMA v MC-DS-CDMA

MC-CDMA da vo s kt hp cht ch gia tri ph chui trc

tip v k thut iu ch a sng mang. Lung d liu tri ph DS

tc cao c li x l PG (Processing Gain) c iu ch a

sng mang theo cch sau: Cc chip ca cc k hiu d liu tri ph

c truyn song song v cc k hiu ny c truyn ng thi trnmi sng mang con (Xem hnh 4.1).

4.3 H THNG MC-CDMA

Trang : 17

Hnh 4.3: S khi m hnh h thng MC-CDMA

D liuvo Chuyn

i S/P Tri phIDFT

&OFDM

Chuyni P/S

Chnkhong

d phng

D liura Chuyn

i P/S Nn phDFT

&OFDM

Chuyni S/P

Khkhong d

phng

KnhtruynAWGN


18/24

4.3.1 My pht

My pht MC-CDMA tri tn hiu bng gc trong min tn s

bng mt m tri cho truc. Ngoi ra, mi phn ca k t tng ng

vi mt chip ca m tri c iu ch bng mt sng mang ph

khc nhau. i vi truyn a sng mang, chng ta cn t c

fading khng chn lc tn s trn mi sng mang. ( hnh 4.4 )

4.3.2 My thu MCCDMAB thu l b OFDM thm vo mt cng vic kt hp tch d

liu c pht i vi mi ngi s dng mong mun. (hnh 4.5)

Hnh 4.4: My pht MCCDMA

Hnh 4.5:My thu

MC-CDMA

4.3.3

Knh

truyn

Trong

h thng MC-CDMA bng rng knh truyn fading Rayleigh chn

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19/24

tn s bin i chm l knh truyn in hnh. Knh truyn ca h

thng c bng thng rng c chia thnh N knh bng hp m mi

knh nh vy ch chu tc ng ca fading phng, ngha l ch c

mt h s li trn mi knh ph. V mi knh truyn ph c

li khc nhau nn khi xt n knh truyn ca h thng th n l

knh truyn c tnh chn lc tn s.

4.3 NHIU MAI V NHIU ICI

4.4.1 Nhiu MAI

L loi nhiu a truy nhp do tn hiu ca cc thu bao khc

ang cng tham gia hot ng trong h thng tc ng ln tn hiu

ca thu bao ang xt. Nhiu ny sinh ra khi thc hin tng quan

cho gia m tri ph ang xt v cc m khc th gi tr ny khng

trit tiu. C hai phng php gim nhiu MAI:

Tri ph bng cc m trc giao c tng quan cho bng 0.

Dng cc b trit nhiu MAI. Cc b trit nhiu rt hiu qu

cho ng ln.

4.4.2 Nhiu ICI

L mt trong cc loi nhiu chnh ca h thng MC-CDMA. Khi

cc sng mang ph khng hon ton trc giao th thnh phn tn hiu

trn knh con ny s gy nhiu ln thnh phn tn hiu trn knh con

khc hay xut hin nhiu ICI.

4.5 K THUT TCH SNG N THU BAO

4.5.1 Tch sng kt hp khi phc tnh trc giao ORC

4.5.2 Tch sng kt hp li bng nhau EGC

4.5.3 Tch sng kt hp t s cc i MRC

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4.5.4 Tch sng kt hp sai s trung bnh bnh phng ti thiu

MMSEC

4.6 K THUT TCH SNG A THU BAO

4.6.1 Tch sng a thu bao SIC

Ngc li vi b tch sng SIC, k thut PIC thc hin

tch sng kt hp trit nhiu MAI cho ng thi tt c cc thu

bao. S ln tch sng-trit nhiu trong h thng ph thuc vo

phc tp cho php ca h thng.

4.6.2 Tch sng a thu bao PIC

Mt vn cn quan tm lin quan n cht lng h thng MC-

CDMA l dch tn s sng mang. Bi n lm suy gim bin ca

tn hiu mong mun v lm mt tnh trc giao gia cc sng mang

ph dn n nhiu lin sng mang ICI. M ta bit nguyn nhn chnh

gy ra dch tn s l do: Tri Doppler do thit b di ng tc cao.

Sai lch gia b to dao ng cho cc sng mang pha my

pht v pha my thu.

V th ta cn s dng k thut c lng dch tn s v

sau tm cch ng b tn s sng mang, ng thi kt hp mt s

k thut khc. Mc ch l tm cch ci thin cht lng h thng.

4.8 MY THU RAKE

4.8.1 M u

Thng thng cc h thng CDMA c thit k c 1 s

ng phn gii trong tr a ng ( 10 ln RDS ). ng thi tri tr

c chn nh hn rng Tb ca 1 bit. Nu tri tr c chn ln

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hn thi gian ca 1 bit th tc s li cao hn rng bng tn

nht qun v iu ny dn n nhiu gia cc k hiu, tc s liu

phi c chn thp hn rng bng tng nht qun.

My thu Rake cho bi hinh 4.7 bao gm tp hp cc b

tng quan, mi b c s dng tch sng 1 trong s phn

t ng mnh nht. V bn cht my thu ny l phn tp

trn c s l cc phn t a ng trong 1 h thng CDMA

khng tng quan nu tr tng i ln hn thi gian ca 1

chp.

V cc tn hiu nhn c my thu b dch theo thi gian,

nn trong my thu trc khi a ln cc b phn tng quan

cc tn hiu thu c.

4.8.2 My thu RAKE

Ta xt K = 1ngi s dng knh. gi thit rng tri rng

ca tr a ng l giy. Kai ny ta c L = [ /Tc] + 1 tn

hiu a ng phn gii c my thu. Tng ng trong s

L ng phn gii s c suy hao ngu nhin c lp vi Rij v

pha ngu nhin ij , trong i = 1,2,3,,L. Gi thit rn

phadinh chm c th nh gi c cc thng s Rij vij

, chng hn bng cch s dng thng tin nhn c t ccon bt trc.

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ck(t)

k1

B tngquan

B tngquan

B tngquan

Mch quyt

nh

r (t)

ck(t)

ck(t)

kL

k2 Y

k

bk


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Hnh 4.7: My thu Rake vi L nhnh

4.9 U IM V KHUYT IM CA MC-CDMA

4.9.1 u im

MC CDMA tha hng tt c nhng u im ca CDMA v

OFDM

Do mi sng mang ph ch chu nh hng ca fading phng

nn h thng bn vng vi fading chn lc tn s v c th gim

phc tp ca cc b cn bng my thu.

Do chu k k hiu di hn nn h thng chng c nhiu

lin k hiu ISI v hn na l vic gi ng b tr nn d dng hn.

S phc tp ca my pht v my thu gim ng k nh s

dng thut ton FFT v IFFT.4.9.2 Khuyt im

Rt nhy vi offset tn s

Offset tn s xy ra do hiu ng Dopler hay do s sai lch

trong vic to dao ng cho cc sng mang my pht v

my thu. N lm cho cc sng mang mt tnh trc giao v do

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23/24

nhiu lin sng mang ICI v nhiu a truy cp MAI xut

hin, nh hng nghim trng n cht lng knh truyn.

T s cng sut nh trn cng sut trung bnh PAPR ln.

KT LUN V HNG PHT TRIN

Kt lun:

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n trnh by tng quan v knh truyn v tuyn, u

im ca h thng MC-CDMA. T nhng u im ca MC-

CDMA em li m ta chn n ci thin cht lng knhtruyn. Xy dng chng trnh m phng nh gi, so snh t

l li bit BER ca h CDMA v MC_CDMA

Hng pht trin:

Xy dng h thng thng tin kt hp gia k thut MC-

CDMA v anten thng minh ci thin cht lng knhtruyn tt hn

Trong phn m phng th xy dng h thng hon chnh

hn (nh c khi m ho), cho nhiu loi nhiu tc ng n

h thng hn, trong h thng DS-CDMA s dng thm my

thu Rake cn h thng MC-CDMA th s dng cc k thuttch sng(ORC, EGC, MRC v MMSEC).

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