Digital Modulation (Dich)

7/21/2019 Digital Modulation (Dich)

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Note:1- Ti liu c dch t ebook SATELLITE COMMUNICATIONS SYSTEMSchap 4.2.

2- y l ti liu tdch khng phi ti liu chnh thc hc thut blabla nn c thc sai st nhng lm v thy cn v theo sthch .NN KHNG NHN GP GCH KIN C DI MI HNH THC =)))))

3- K hiu hiu sut phiu ch thay bng k hiu G.Thc mc vui lng xem li 2 trn =)))Facebook:Fb.com/neo.steam.79Gmail:[email protected]

IU CHSTRONG THNG TIN VTINHHnh 4.5 biu din nguyn l ca biu ch.N bao gm-Bto k t-Bm ha hoc mapper (bnh x)-Bpht tn hiu(sng mang)

Hnh 4.5 Nguyn l ca biu chcho truyn dn sBto k tto ra cc k tvi M trng thi.Trong M= 2m. Tm bit lin tip ca dng

bit u vo.Bm ha thit lp mt stng ng gia M trng thi ca cc k tv Mtrng thi ca sng mang c truyn.Hai kiu thit lp thng gp-nh xtrc tip direct mapping Mt trng thi ca k txc dnh bi mt trng thi casng mang-encoding of transitions (differential encoding) M ha chuyn tip (m ha vi sai)- Mttrng thi ca k txc nh bi mt schuyene tip gia hai trng thi lin tip nhau casng mang

i vi mt tc bit Rc (bit/giy) ti u vo biu ch,tc tn hiu Rs ti u raca biu ch(slng thay i trng thi ca sng mang trn giy) c cho bi cngthc:

Rs =Rc / m = Rc / log2 M (baud)

iu chpha,hay kthut iu chkha dch pha -PSK(phase shift keying) c bit phhp cho cc lin kt vtinh.Trong thc tn c li thl ng bao sng mang l hngsv so vi k thut iu chkha dch tn FSK th PSK c hiu sut ph tt hn(S


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bit/giy c truyn trong mt n vbng thng di tn sv tuyn).Phthuc vo smca cc bit trn k t.Cc kthut iu chkha dch pha PSK M mc thng dng l:

-Hnh thc n gin nht l iu chhai trng thi c bn (M=2), c gi l kha dchpha nhphn ' (BPSK-'binary phase shift keying) vi tiu chun nh xtrc tip. Khi m

ha vi sai c s dng n c gi l 'kha dch pha nh phn m ha vi sai ' (DE-BPSK). N quan tm bi v n c khnng gii iu chn gin (Gii iu chvi sai,xem phn 4.2.6.1).-Nu hai bit lin tip c nhm li xc nh k t, th c xc nh l iu chbntrng thi (M=4), c gi l kha dch pha cu phng' (QPSK-quadrature phase shiftkeying) vi cch nh x trc tip. m ha vi sai QPSK (DE-QPSK) c thc to ra,nhng n khng c s dng trong thc t (ngoi tr cho trng hp c th ca /4QPSK, xem phn 4.2.3.2) v gii iu ch vi sai c hiu sut thp hn ng kkhi so vigii iu chlin tc khi M l ln hn 2.-iu chmc cao (M=8,8-PSK; M=16,16-PSK; vv.) thu c vi m=3,4, vv bit /k t.

khi mc iu chtng ln , hiu sut phtng tlvi sbit / k t. Mt khc, iu chmc cao i hi nng lng nhiu hn cho mi bit (Eb) c c tlli bit tng t(BER) ti u ra ca bgii iu ch(xem phn 4.2.6).

Vi iu chmc cao (M bng hoc ln hn 16), hiu qut c tt hnbng cch kthp kha dch pha v bin (APSK -Amplitude Phase Shift Keying kha dch pha bin). Cc trng thi ca sng mang tng vi gi trca pha v bin sng mang (haigi trcho 16APSK, ba gi trcho 32APSK).

iu chhai trng thi BPSK v DE-BPSK

Hnh 4.6 biu din cu trc ca biu chpha hai trng thi. Khng c bto k tbiv cc k tnhphn l cc bit u vo. Nu coi bkl gi tr logic ca mt bit u v biu ch

Hnh 4.6 Biu chpha hai trng thi (BPSK).


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trong khong thi gian [kTc, (k+ 1)Tc]. Bm ha bin i bit u vo bk thnh mt bitiu chvi gi trlogic mk nh sau:- i vi nh x trc tip (BPSK): mk= bk.- i vi m ha vi sai (DE-BPSK): mk= bk mk-1 Trong du l php ton logicHOC

B to tn hiu knh c iu khin bi cc bit mk v n c i din trong khongthi gian [kTc, (k + 1) Tc] bi mt in p v(kTc)= V. Tn ssng mang fc = / 2cthc biu thtrong khong thi gian ny:

C(t) = (V) (4.2)Ti C l nng lng sng mang v A bin sng mang tham chiu, Ti v l gi trb logic nu =0 nu nu . Trongkhong thi gian ny, sng mang c trng cho trng thi pha pha , hoc l 0 hoc .Phn cuiphng trnh (4,2) cho thy rng iu chpha c thc coi l iu chbinsng mang vi hai trng thi bin V (Ch rng ng bao vn gikhng i).

Phng php iu chny n gin, nh minh hotrong hnh 4.6, bng cch nhn sngmang tham chiu vi in p v(t) . Bng 4.1 cho thy mi quan hgia bkv pha sngmang cho hai kiu m ha.

iu chbn trng thi QPSKHnh 4.7 biu din cu hnh ca mt biu chpha bn trng thi. Bto k t l bchuyn i ni tip-song song ti to ra hai lung nhphnAkvBk, vi mi lung ctc bit Rc / 2,Bng 4.1 Mi quan hgia bit v pha sng mang trong BPSK(a)nh xtrc tipbk Pha0 1 0

(b) M ha vi saiTrng thi trc Trng thi hin ti

bk mk-1 Pha mk Pha0

0 0 Khng thay i

pha1 0 1 0

1 0 1 0 Thay i pha1 0 0


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Hnh 4.7 Biu chpha bn trng thi QPSKTdng u vo tc bit Rc. K tAk Bk l mt 'dibit-nhm hai bit' trong chimkhong thi gian [kTs, (k+ 1)Ts] bng Ts= 2Tchoc thi gian ca hai bit. Bnh x,hoc m ha, bin i dibit AkBk vo dibitIkQk.Nh tho lun trn, chc nh xtrc tip thng c sdng:

Ik=Ak Qk=BkBto tn hiu chng hai sng mang vung gc 90 . Hai sng mang c iu chbin(vi sng mang bchn) do cc bit Ikv Qkc i din trong thi gian khong [kTs,(k + 1)Ts] bng hiu in thvI(kTs)= V v VQ(kTs)= V. Sng mang trong khong thigian [kTs, (k + 1)Ts] c biu din bng cng thc :

C(t) = (kTs) cos(ct)-(kTs)

sin(ct)=AVcos(ct+ ) (V)

Trong = 45, 135,225, hoc 315theo cc gi trca in p vI(kTs)v VQ(kTs).Trong hnh 4.8, c ththy c sng mang c thly mt trong bn pha, mi pha linkt vi mt gi trca k tIkQk. Ni chung, hai pha lch pha nhau 90 lin kt vi nhmhai bitIkQkkhc nhau bi mt bit n (m Gray). V thmt li ti pha nhn trong qutrnh nhn pha gia hai pha lin kdn n mt li trong mtbit n. Bng 4.2 cho thy

stng ng gia nhm hai bit AkBkv pha sng mang.

Cc dng khc ca QPSK

Trong iu chQPSK, cc in p m iu chhai sng mang vung gc thay i cngmt lc v sng mang c thc i pha 180.


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Hnh 4.8 Gin pha QPSK .Bng 4.2 Mi quan hgia cc cp bitAkBkv pha sng mang trong QPSK (m ha trc

tip)AkBk Pha

00 5/401 /410 /411 /4

Trong mt lin kt v tinh m bao gm cc b lc, cc khong dch pha ln l nguynnhn gy ra iu bin ca sng mang. Cc mo phi tuyn ca knh chuyn i nhng

bin bin vo trong cc binpha (xem Chng 9) lm gim hiu sut ca bgii iuch. Mt sbin thca iu chQPSK c xut hn chbin ca dch pha.Hn na, to hnh xung ca bng gc c gii thiu l hn chrng phca qutrnh iu chdng sng bng gc bng cch sdng mt blc lm mt cc xungin hnh chnht m NRZ (nonreturn to zero) kt hp vi cc bit trong hnh 4.7.Cc bin thphbin nht ca QPSK l:- offsetQPSK (OQPSK);- 4 QPSK;- Kha dch ti thiu (MSK).

Offset QPSK

Vi offset QPSK (OQPSK), cng c gi l staggeredQPSK (SQPSK), cc dng bitiu chIkv Qkc b bng thi gian na k t, tc l Ts / 2 = Tc, thi gian ca mt

bit.Pha ca sng mang thay i mi ch kbit nhng ch 90 hoc 0 . iu ny trnhsdch pha 180 lin quan n sthay i ng thi trong hai bit trong nhm nhm 2 bitiu chvi QPSK . N l kt qugim bin ng bao sng mang iu chc lc.


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Cc dch vhng khng Inmarsat sdng thit b Aviation QPSK (A-QPSK). N tngng vi OQPSK, nhng thay thbng gcNRZ iu chin p ca bit Ik v Qk tronghnh 4.7 bng cch png vi mt xung ca bin V ca mt blc xung cosin nngvi hm chuyn H (f) c cho bi [PRO-01, trang. 546]:

y lhsmch lc .Hsmch lc ca A-QPSK l =1

S iu chny l cch tip cn khc trnh cc on dch pha 180. N sdng mha vi sai. Cc dliu iu chIkv Qkti thi gian k c xc nh tnhm 2 btAkBkn v trc nhm 2 bt iu chIk-1Qk-1,chuyn theo cc bc bn di:

( ) ( ) (

)

Hnh 4.9 iu chQPSK: (a) chm sao 1, (b) chm sao 2, (c) pha c th chuyn tip vok tktip.

Ti y=/4, 3/ 4, - 3/ 4, -/ 4, ty thuc vo nhm hai bit n AkBk(11, 01, 00,10). V d, k-1, pha sng mang l mt trong bn pha ca chm sao 1 (hnh 4.9a). Ti k, sng mang ly mt trong cc gi trpha c thc ca chm sao 2 (Hnh 4.9b). Gingnhchm sao c dch lch / 4, cc pha c ththay i l / 4or 3/ 4, cminh ha bng cc ng k trong hnh 4.9c cho thy s chuyn pha c thbt u t

mt pha cthti k-1Kha dch ti thiu MSK

iu chMSK l trng hp c bit ca OQPSK, thay th xung in hnh ch nhtNRZ gn lin vi bit Ik v Qk trong hnh 4.7 bng cch png vi mt xungbin Vca mt blc xung vi p ng xung h(t) c cho nh sau[GRO-76]:-Vi lung Ik


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h(t)={

- Vi lung Qk

h(t)={

Pha sng mang tng trong khong thi gian bit Tc vi , nh thhintrong hnh 4.10. Cc pha thay i tuyn tnh vi thi gian trong tng chu k bit Tc v

bng bi s ca / 2 cui mi k bit. Bin pha ny c chuyn thnh mt tn skhng i trong tng chu kbit, m slm cho qu trnh iu chny tng ng viiu chkha dch tn (FSK) vi hai tn s, v ty thuc vo bitn , y fc l tn ssng mang tham kho.trnh nhng thay i t ngt gc pha vo cui mi khong thi gian bit trong hnh4.10, Lc Gaussian MSK (GMSK) sdng lc ti iu chbng gc vi mt tn sp

ng hnh Gaussian . iu ny dn n sng mang tri qua ccbc pha / 2 gim rng phca n. Kthut ny c sdng tit kim bng thng . Sbt tin l nhiuxuyn k tc to ra can thip vo mi bit dliu nh hng n pha sng mang trongmt khong thi gian vt qu chu kbit (thng 3).

Hnh 4.10 Pha ca sng mang c iu chMSK

PSK mc cao v APSKKhi kt hp ba bit lin tip xc nh k t,th c nh ngha l iu ch8PSK (M =8) (hnh 4.11b). iu chPSK mc cao c thc xem xt. Tuy nhin, nh mc caqu trnh iu chtng , cc pha khc nhau gia cc trng thi sng mang gim v n lcn thit tng bin ca sng mang duy tr cng mt khong cch gia cc sng


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mang c c cng t l li bit (BER) ti u ra ca b gii iu ch (xem Phn4.2.6.3).Da vo cu hnh ca biu chpha cu phng bpht tn hiu tn sv tuynchng hai sng mang vung pha, c thhnh dung qu trnh iu ch bin ca misng mang sdng hai tn hiu in p tch cc + v hai tn hiu in p tiu cc - (4 k

tbin ). iu ny scho kt qutrong iu chbin cuphng 16 '(16QAM), tiy cc min sng mang c biu din theo hnh vung (Hnh 4.11c). Phng php iuchny, khng hin thng bao khng i khi c ba gi trc thc cabin sngmang lm cho phng php iu ch nhy cm vi s khng tuyn tnh ca knhtruyn vtinh.Gicng mt slng cc trng thi sng mang (M = 16), n c thlm gim slngcc gi trbin xung hai (cng gim cc nhiu phi tuyn tnh) bng cch phn phicc sng mang trn hai vng trn ng tm. iu chny c gi l 16-amplitude and

phase shift keying '(16APSK, xem hnh 4.12a). Nu knh gn nh-tuyn tnh c sn (nhvo vic sdng bng tuyn tnh c sn .Xem chng 9) sng mang kha bin 3 mc

c thc xem xt . Kt hp vi cc gi tr pha khc nhau, cho kt qutrongphngphp iu ch32APSK (hnh 4.12b).

Hnh 4.11 Mt sphng php iu ch (a) QPSK (b) 8-PSK (c) 16-QAM iuchpha v bin y vI(kTs ) v vQ(kTs ) nhn cc gi tr{1,3}


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Hnh 4.12 iu ch(a) 16APSK v (b) 32APSK

Ph ca cc sng mang c iu chkhng qua blcHnh 4,13 biu din hnh dng ca mt phnng lng (W/Hz) ca cc sng mang

c iu ch theo cc phng php iu ch, nh l mt hm ca tn s chun ha.Trc dc hin thmc tng i trong thang decibels (dB) ca mt nng lng ivi gi tr ti a tn ssng mang khng qua iu ch fc. Tn s chun ha trn trcngang c xc nh l schnh lch gia tn sxem xt f v tn ssng mang cha quaiu chfc i vi tc bit sng mang iu ch, Rc.Phc hiu thtng ng vi cc sng mang c iu chcha qua lc. C hai ch: Chiu rng ca bp sng chnh ca phcc sng mang c iu ch,yu cu v

bng thng; v phphn r ca cc bp sng bn vi tn s, scan thip vo cc sngmang lin k. Trong phn ny, QPSK (M =4, m = 2) hn so BPSK (M = 2, m = 1) vrng ph. Nguyn tc chung l , chiu rng ca bp sng chnh gim theo hm gi trcam bit/k t, ni cch khc l hiu sut phcao hn. MSK cho thy bp sng bn phn rnhanh hn, khi so snh vi QPSK c cng rng bp sng QPSK.Trong thc t, qu trnh lc c thc hin ti pha thu v nhn, hn chccs can thip cc sng mang ngoi di tn. Tc dng ca qu trnh lc c tho luntrong phn 4.2.7.


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Gii iu chVai tr ca bgii iu chl xc nh cc pha (hoc dch pha) ca sng mang nhnc v suy ra t gi trca cc bit ca dng nhphn c truyn i. Gii iu chcthl:

- Coherent(Lin tc): Cc bgii iu ch, sdng mt tn hiu tham chiu hnh sin ccng tn sv pha nh cc sng c iu chmy pht. Cc bgii iu chbin dch

pha ca sng mang nhn c bng cch so snh n vi cc pha ca tn hiu thamchiu. Gii iu ch ln tc cho php ti to cc dng nhphn cho chai trng hpm ha trc tip truyn (BPSK v QPSK) v vi sai (DE-BPSK v DE-QPSK).

Hnh 4.13 Phca cc sng mang


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-Vi sai: Bgii iu chso snh pha ca sng mang nhn c trong sut thi giantruyn dn ca mt k tv pha ca n trong thi gian ca k t trc. Bgii iu chs pht hin sthay i pha. Thng tin c truyn c thc phc hi chkhi n cha

sthay i vpha ; Gii iu chvi sai l lun lun lin quan n qu trnh truyn v mha vi sai. Kiu iu chv gii iu chny c gi l gii iu chvi sai (D-BPSK).Cu trc ca cc bgii iu ch BPSK v QPSK c kim tra trong cc phn sau vsau so snh hiu sut ca cc kiu iu chv gii iu ch.

Cc bgii iu chvi sai v lin tc

(a )Gii iu ch lin tc ca iu chBPSK (Hnh 4.14a): Sng mang c iu chnhn c l t l thun vi hm . N c nhn vi sng mang thamchiu

nhn c ti u ra ca mch phc hi sng mang. Kt qul tlthun

vi hm .Cc b lc thng thp gip loi bcc thnh phn tn s2fc=2v u ra mtin p tlthun vi l tch cc hay tiu cc ty thuc vo vic = hay 0.

Hnh 4.14 Bgii iu chlin tc cho(a) BPSK v (b) DE-BPSK


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Gi trhin ti ca bit c quyt nh bng cch so snh in p v ngng khng cabtch sng (detector)ti nh thi phc hi bit bng mch hi phc nh thi.

(b) Gii iu chvi sai bng m ha vi sai BPSK (hnh 4.14b): sng mangDE-BPSK nhn c c a vo mt ng tr(vi trbng thi gian mt bit) v

c nhn vi u ra b lm tr ca dng. Kt qu ca php nhn, , c lc bi blc thng thp, u ra l1/2 gi trcamk c suy ra tdu ca biu thc (c) Gii iu chlin tc ca QPSK (Hnh 4.15): Bgii iu chl mt phn mrngca gii iu chlin tc song mang BPSK ti pha v vung gc vi cc knh

Hnh 4.15 Cu trc ca bgii iu chlin tc cho iu chpha 4 trng thi

Xc sut li bit v k tPha sng mang (hoc dch pha) thay i di nh hng ca nhiu dn n li trong vicxc nh cc k tnhn c, ri n cc bit nhn c. Xc sut li k tl xc sut camt k tc pht hin li; xc sut li bit l xc sut mt ch bt bpht hin li.i vi iu chhai trng thi, cc k t l bit. Do xc sut li k t(SEP) i dincho xc sut li bit (BEP):

BEP=SEP

i vi iu chbn trng thi , y skt hp ca k t IkQkvi cc trng thi phatheo m Gray , xc sut li bit c cho bi:

BEP =SEP/2Tng qut hn:

BEP =SEP/ log2 M Vi


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Bng 4.3 cung cp cho cc cng thc cho xc sut li bit cho cc Bcc bgii iu chtrc y[PRO-01]. Hnh 4.16 thhin ng cong xc sut li bit tng ng. Hm erfcl hm li bsung c xc nh bi:

Gi trerfc xp xl khi (= 6dB)

Tlpht sinh trong biu thc cho xc sut li, ti Ec l nng lng cho mibit knh. y l cc kt quca nng lng ca sng mang nhn c thi gian mtbit, cthl Ec= CTc=C/Rc. Do :

==i vi mt lin kt vtinh minh bch, gi trca c sdng l hiu sutlinkt tng (C/N0)T nh trong Chng 5. i vi mt lin kt vtinh ti ch, gi trca C/N0 c sdng l ca mt trong hai ng ln hoc xung.

Bng 4.3 Biu thc cho xc sut li bitLoi iu chv gii iu ch Xc sut li bit

Gii iu chlin tcM ha trc tip

BPSK QPSK

M ha vi saiDE-BPSK

DE-QPSK Gii iu chvi sai (Chc m ha vi sai)

D-BPSK


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Hnh 4.16 L thuyt xc sut li bit (BEP)


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Tlli bit (BER)Tlli bit (BER) o lng hiu sut ca bgii iu chbng cch m sbit li, n,trong mt dng N bit nhn c t:

BER = n/N

BER cu thnh nh gi xc sut li bit (BEP). Mc tin cy c lin quan ti nh ginh sau:BEP= BER

Mc 63% ca tin cy thu c khi k = 1 v mc tin cy 95% ca sttin cho k =2.V d, nu n = 100 li c quan st thy khi chy N =105bit, BEP l vitin cy 63%.Bng 4.4 Cc gi trl thuyt ca Ec/N0t c xc sut li bit (Ec= nng lng chomi bit knh, N0 = mt phnhiu). = sai phn trong biu thc Ec/N0 so snh giaB-PSK v Q-PSKBEP BPSK

QPSKDE-BPSK ()DE-QPSK

D-BPSK () D-QPSK ()103 6.8 dB 7.4 dB (0.6 dB) 7.9 dB (1.1 dB) 9.2 dB (2.4 dB)104 8.4 dB 8.8 dB (0.4 dB) 9.3 dB (0.9 dB) 10.7 dB (2.3 dB)105 9.6 dB 9.9 dB (0.3 dB) 10.3 dB (0.7 dB) 11.9 dB (2.3 dB)106 10.5 dB 10.8 dB (0.3 dB) 11.2 dB (0.7 dB) 12.8 dB (2.3 dB)107 11.3 dB 11.5 dB (0.2 dB) 11.9 dB (0.6 dB) 13.6 dB (2.3 dB)108 12.0 dB 12.2 dB (0.2 dB) 12.5 dB (0.5 dB) 14.3 dB (2.3 dB)

109 12.6 dB 12.8 dB (0.2 dB) 13.0 dB (0.4 dB) 14.9 dB (2.3 dB)

Mc tiu l quy nh tlli bit nht nh (mc 3.2), t cc gi trcn thit caEc /N0 c xc nh. Bng 4.4 chra cc gi tr l thuyt ca Ec /N0 cn thit tc mt xc sut li bit BEP a ra cho mi loi iu chv gii iu ch. Nhng constrong ngoc chra skhc bit gia gi trca Ec/ N0 cho kiu iu ch/ gii iu chc xem xt v cc gi trthu c vi BPSK hoc QPSK.C ththy rng iu chm ha vi sai i hi mt gi trE / N0cao hn,ni cch khcl yu cu C /N0cao hn t c mt xc sut li bit . Tuy nhin,cng nh cc thngtin c chuyn ti trong dch pha gia hai tn hiu lin tip, khng cn phc hi chnhxc pha tham chiu ca sng mang. Vi gii iu chlin tc, iu ny trnh c phigii quyt cc pha sng mang tham chiu khng r rng c a ra bi mch phc hisng mang theo kt quca vic ly tn sti u ra ca thit bbnhphng trong hnh4.14a v chia n vi 2. Vi m ha trc tip, sm hc gii quyt bng cch chn

phn u ca mt chui bit bit (phn mo u) pha truyn v pht hin phn mou bn nhn, n slm phc tp s truyn dn. Gii iu chvi sai trnh vic phikhi phc sng mang tham chiu,th s lm cho b gii iu ch n gin nhng lmgim hiu sut ca n vi gii iu chlin tc .


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Nh kt quca vic thc hin gii iu ch, gi trca BER l cao hn so vi BEP, nhcc gi tr c a ra bi trong Bng 4.4. c c gi trBER cn thit, ngi ta

phi tng i lng ca gi tr Ec /N0, thu c t cc biu thc trong Bng 4.4. ilng ny l thc hin gim cp (implementation degradation)biu ch. Ty thucvo cng ngh v xem xt gi trBER, s gim cp dao ng t 0,5 dB cho iu ch

BPSK n gin ti mt vi dB cho iu chmc (M = 16 hoc 32)phng php m haybli ng bv knh truyn phi tuyn tnh .

Hiu sut phiu chHiu sut ph iu ch c th c nh ngha l t l tc truyn bit Rc trn bngthng chim dng ca sng mang. Bng thng chim dng bi sng mang phthuc vo

phca sng mang c iu chv qu trnh lc n tri qua.

Phhin thtrong hnh 4.13 tng ng cc sng mang c iu chkhng lc . Trongthc t,qu trnh lc c thc hin ti cpha truyn v nhn, hn chscan thip

ca cc sng mang ngoi di tn. Tuy nhin, qu trnh lc nhiu xuyn k t(ISI)[PRO-01, trang. 536-537], slm gim hiu sut BER so vi kt qutrn l thuytXung hnh ch nht , c sdng trong BPSK v QPSK, Nhiu xuyn k t -ISI lantruyn tdo c thc loi bc bng cch sdng mt blc chnht ( blcbrick wall') tng ng vi bng thngcn thit ti thiu ca sng mang. Bng thng ny(bng thng Nyquist) l bng 1/Ts nghch o ca thi gian tn hiu hoc k tTs. iuny khng thc hin trong thc t, nh l mt blc vi cc min tn skhng c thc.Mc d phn r chm trong p ng min thi gian ,mch phc hi xung clock bin dch

bt kli nh thi no vo quyt nh li trong qu trnh tch ca my thu v iu nynh hng ti hiu sut BER.

Nhiu xuyn k tlan truyn tdo c thloi bc vi blc cthtrong min tns, chng hn nh cc blc c sin nng c gii thiu trong phn (4.5). Bng thngln hn c yu cu, ty thuc vo gi trhsmch lc rolloff factor . i vimt blc c sin nng vi hsmch lc ,bng thng B ca sng mang sl:

B=(1+s (Hz)Do hiu sut phcho mt chng trnh iu chM trng thi l:

G =RcTs/(1+)= 2M/(1+) (bit s-1 Hz-1)y m=log2M l sbit trn k tVi hsmch lc Bng thng yu cu l 1,35/Ts ,v hiu sut phG =0,7 bit s-1 Hz-1 cho BPSSK, v G =1.5 bit s-1 Hz-1 cho QPSK ,G=2.2 cho 8PSKQu trnh lc c thc hin chai u ca lin kt. Nu knh tuyn tnh, l thuyt chra rng qu trnh lc tng nn c chia u gia cc blc pha thu v pha pht. Mtknh vtinh thng phi tuyn tnh, l kt quca cc c tnh phi tuyn tnh ca cc bkhuch nng lng ti cc trm tri t v ca vtinh. Phng php lc trn khng cnlan truyn tdo nhiu xuyn k t. Hn na, knh phi tuyn tnh qung b phlan truynca sng mang c iu chlc, lm tng scan thip knh k (ACI).Lng nghe


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backking offim lm vic bkhuch i l mt cch gim phlan truyn lm gimchi ph ca nng lng sng mang.

Nng lng nhiu c qung b vo pha thu theo cng thc N=N0BN, y BNbngthng nhiu-(noise bandwidth)pha thu. i vi mt blc Nyquist, bng thng nhiu

bng 1/ Ts, c lp vi hsmch lc

. Trong thc t, bng thng nhiu ca cc blc

phthuc vo hsmch lc. cho thun tin ta coi bng thng nhiu ca pha thultng ng vibng thng sng mang, tc l Bn=B, nh c a ra bi phng trnh(4.11a).

Digital Modulation (Dich)

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