1.nh ngha iu ch QAM.2.iu ch QAM.3.B iu ch QAM.4.B gii iu ch
QAM.5.ng dng.
6.Kt lun
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Phng php iuch M-QAM l phng phpnng cao hiu qu ca mt knh truyn
mkhng cntng cng sut pht hay tngrng
bng thng.Vic iu ch hai thnh phn ngpha v pha vung gc mt cch clpvi
nhaucho ta mt s iuch mi gi l iu chbin vung gc (hay cu phng) M
trngthi (QAM, Quadrature Amplitude Modulation).Nhvy, trong siuch ny
sng mang biuchcv bin ln pha.
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Dng tng qut ca iu ch QAM, 14 mc (m-QAM) c xc nh nh sau:
Trong :
E0: nng lng ca tn hiu c bin thp nhtai , bi : cp s nguyn c lp c
chn ty theo v tr bntin. i=1,2L.
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TttfbT
t c.i 0)2sin(2
)(1
TttfaT
t c.i 0)2sin(2
)(2
C th phn tch Si(t)thnh cp hm c s:
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1,1...3,31,1
....
3,1...3,33,1
1,1...1,31,1
,
LLLLLL
LLLLLL
LLLLLL
baii
3,33,13,13,3
1,31,11,11,3
1,31,11,11,3
3,33,13,13,3
,ii
ba
Ta cc im bn tin l v vi :
i vi 16-QAM ta c L=4
0Eai 0Ebi
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1,1...3,31,1
..
..
3,1...3,33,1
1,1...1,31,1
,
LLLLLL
LLLLLL
LLLLLL
baii
3,33,13,13,3
1,31,11,11,3
1,31,11,11,3
3,33,13,13,3
,ii
ba
Ta cc im bn tin l v vi :
i vi 16-QAM ta c L=4
0Eai 0Ebi
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Thnh phnng pha v vung pha trong 16-QAM
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Chm tn hiu ca 16-QAM
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Chm tn hiu M-QAM
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Hot ng ca b iu chB phn lung (demux) chuyn i lung nh phn b(t) tc
bit Rb=1/Tb u vothnh bn lung c lp, trong hai bit l c a n b chuyn i
mc nhnh trn cn hai bit chn c a n b chuyn i mc nhnh di. Tc
k hiu trong trng hp ny s bng Rs=Rb/4.Cc b bin i mc chuyn i 2 mc
vo L mc () to ra cc tn hiu L mctng ng vi cc u vo ng pha v pha vung
gc.Sau khi nhn hai tn hiu L mc vi hai sng mang c pha vung gc c to
tb dao ng ni pht TLO (Transmitter Local Oscillator) ri cng li ta c
tnhiu M-QAM.
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Hot ng ca b gii iu chTn hiu thu c a ln 2 nhnh ng pha v vung pha,
sau c nhn vi 2 hm
trc giao ging pha pht c to ra t b dao ng ni thu RLO (Receiver
LocalOscillator). Nh tnh cht trc giao m ta tch c 2 thnh phn tn
hiu.Tn hiu sau c a qua b tng quan ly mu, nh gi ngng (so snh vi
L-1ngng) thu c k hiu.Sau cng hai chui s nh phn c tch ra ni trn s kt
hp vi nhau b bin isong song vo n tip khi phc li chui nh phn pha pht
(c tnh chui pht ).
