De Cuong Thong Tin Vo Tuyen

7/24/2019 De Cuong Thong Tin Vo Tuyen

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cng bi ging Thng tin v tuyn

Nguyn Th Huyn Linh 1

MC LC

MC LC ...................................................................................................................... 1

CHNG 1. TNG QUAN V CC H THNG THNG TIN V TUYN ......... 4

1.1. Cc khi nim v thng tin v tuyn.................................................................... 41.1.1. Khi nim v thng tin v tuyn ................................................................... 41.1.2. Lch s pht trin ca thng tin v tuyn...................................................... 41.1.3. Khi nim knh truyn .................................................................................. 51.1.4. Khi nim v sng mang ............................................................................... 51.1.5. Khi nim v truyn dn bng tn c s v truyn dn bng thng ....... 61.1.6. Phn loi cc h thng thng tin v tuyn..................................................... 61.1.7. Khi nim v chun v tuyn ........................................................................ 6

1.2. Phn chia di tn s v tuyn v ng dng cho cc mc ch thng tin.............. 6

CHNG 2. L THUYT V KNH V TUYN.................................................... 9

2.1. Khi nim v knh truyn dn phn tp a ng............................................... 9

2.2. p ng xung v hm truyn t ca knh.......................................................... 92.2.1. p ng xung ca knh khng ph thuc thi gian...................................... 92.2.2. Hm truyn t ca knh khng ph thuc thi gian.................................. 10

2.3. B rng n nh v tn s ca knh .............................................................. 10

2.4. Hiu ng Doppler............................................................................................... 11

2.5. Knh ph thuc thi gian ................................................................................... 12

2.6. B rng n nh v thi gian ca knh ......................................................... 122.7. Khi nim v sng v tuyn............................................................................... 13

2.7.1. Sng b mt ................................................................................................. 132.7.2. Sng khng gian .......................................................................................... 13

2.8. ng truyn lan sng v tuyn ....................................................................... 142.8.1. S lan truyn ca bng tn s thp .............................................................. 152.8.2. S truyn lan ca bng tn s cao ............................................................... 15

2.9. Cc phng thc truyn lan sng in t .......................................................... 152.9.1. S truyn lan sng t ................................................................................. 16

2.9.2. S truyn lan sng khng gian .................................................................... 162.9.3. S truyn lan sng tri ................................................................................ 17

2.10. Mt s thut ng v nh ngha truyn sng.................................................... 182.10.1. Tn s ti hn v Gc ti hn.................................................................... 182.10.2. cao o ................................................................................................... 19

2.11. Cc nhn t nh hng n s truyn lan sng v tuyn ............................... 202.11.1. Suy hao khi truyn lan trong khng gian t do ......................................... 202.11.2. nh hng ca pha ing v ma.............................................................. 202.11.3. S can nhiu ca sng v tuyn ............................................................... 21

2.12. c im mt s di sng v tuyn ................................................................. 212.12.1. Sng cc di v sng di ........................................................................... 21


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2.12.2. Sng trung ................................................................................................. 212.12.3. Sng ngn (SN) ......................................................................................... 222.12.4. Cc sng cc ngn (SCN) ......................................................................... 23

CHNG 3. NHIU TRONG THNG TIN V TUYN......................................... 25

3.1. Cc loi nhiu trong thng tin v tuyn ............................................................. 25

3.1.1. Khi nim v nhiu trng (White Gaussian Noise)..................................... 253.1.2. Tp m nhit trng chun cng tnh (Additive White Gaussian Noise)...... 263.1.3. Khi nim v nhiu xuyn k t ISI (Inter symbol interference) ............... 273.1.4. Khi nim v nhiu xuyn knh ICI (Inter Channel Interference).............. 293.1.5. Khi nim v nhiu ng knh CCI (Co- Channel Interference)................ 293.1.6. Khi nim v nhiu a truy nhp MAI (Multiple Access Interference)...... 31

3.2. Cc phng php gim nhiu trong thng tin v tuyn..................................... 313.2.1. Tm tt v l thuyt dung lng knh ca Shannon................................... 313.2.2. nh l gii hn bng thng Nyquist. ......................................................... 32

3.2.3. Gim nhiu xuyn k hiu ISI s dng cc phng php lc ..................... 33CHNG 4. KIN TRC H THNG THNG TIN V TUYN ......................... 36

4.1. Cc vn c bn trong thit k cc h thng v tuyn.................................... 364.1.1. Nhit tp m h thng v h s tp m................................................... 364.1.2. nhy thu ................................................................................................. 384.1.3. Cc hin tng phi tuyn............................................................................. 39

4.2. Cc chun v tuyn ............................................................................................ 394.2.1. Cc h thng thng tin di ng th h th nht (1G- 1Generation )........... 394.2.2. Cc h thng thng tin di ng th h th hai (2G) .................................... 41

4.2.3. Cc h thng thng tin di ng th h 2.5G................................................ 444.2.4. Cc h thng thng tin di ng th h th ba (3G) ..................................... 45

4.3. Kin trc cc h thng v tuyn......................................................................... 464.3.1. H thng pht v tuyn................................................................................ 464.3.2. Kin trc my thu ........................................................................................ 47

CHNG 5. CC K THUT IU CH-GII IU CH GIAO DIN VTUYN ......................................................................................................................... 54

5.1. K thut iu ch OFDM ................................................................................... 545.1.1. M u......................................................................................................... 54

5.1.2. Cc u v nhc im................................................................................. 555.1.3. S ng dng ca k thut OFDM Vit Nam............................................ 55

5.2. T iu ch n sng mang n iu ch trc giao OFDM.............................. 565.2.1. Phng php iu ch n sng mang........................................................ 565.2.2. Phng php iu ch a sng mang FDM ................................................ 575.2.3. Phng php iu ch a sng mang trc giao OFDM .............................. 58

5.3. L thuyt v iu ch OFDM............................................................................. 595.3.1. Khi nim v s trc giao ca hai tn hiu.................................................. 595.3.2. B iu ch OFDM ..................................................................................... 60

5.3.3. Chui bo v trong h thng OFDM........................................................... 625.3.4 Php nhn vi xung c bn (Basic Impulse)................................................ 63


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5.3.5. Thc hin iu ch OFDM bng thut ton IFFT....................................... 64

5.4. L thuyt v gii iu ch OFDM...................................................................... 655.4.1. Khi nim v knh truyn dn phn tp a ng...................................... 655.4.2. B gii iu ch OFDM .............................................................................. 655.4.3. Thc hin b gii iu ch thng qua php bin i nhanh FFT................ 68

5.5. Ph tn hiu OFDM............................................................................................ 695.5.1. Biu din ton hc ca ph tn hiu OFDM................................................ 695.5.2. Hiu sut ph tn hiu ca h thng OFDM................................................ 70

CHNG 6. CP PHT KNH V TUYN V QUN L TI NGUYN VTUYN ......................................................................................................................... 73

6.1. H thng thng tin v tuyn t bo - c s thit k h thng ............................ 736.1.1. T bo v vic phn b tn s ..................................................................... 736.1.2. Nhiu ng knh v dung lng h thng v tuyn t bo......................... 73

6.2. Cc phng php a truy nhp mng................................................................. 746.2.1. a truy nhp phn chia theo tn s ............................................................. 746.2.2. a truy nhp phn chia theo thi gian......................................................... 776.2.3. a truy nhp phn chia theo m .................................................................. 806.2.4. a truy nhp phn chia theo khng gian..................................................... 82

6.3. Cc phng thc truyn dn .............................................................................. 836.3.1. Truyn dn c dy ....................................................................................... 836.3.2. Truyn dn khng dy ................................................................................. 85


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Ngun tin M ngun(source coding)

M knh(Channel coding)

iu ch(Modulation)

Knh v tuyn(Channel)

Tn hiu ch(Destination)

Gii m ngun(source decoding)

Gii m knh(Channel Decoding)

Gii iu ch(Demodulation)

Hnh 1.1 M hnh h thng thng tin

M hnh knh(Discrete Channel)

CHNG 1. TNG QUAN V CC H THNG THNG TIN V TUYN

1.1. Cc khi nim v thng tin v tuyn

1.1.1. Khi nim v thng tin v tuynHnh 1.1 th hin mt m hnh n gin ca mt h thng thng tin v tuyn.

Ngun tin trc ht qua m ngun gim cc thng tin d tha, sau c

m knh chng cc li do knh truyn gy ra. Tn hiu sau khi qua m knh c

iu ch c th truyn ti c i xa. Cc mc iu ch phi ph hp vi iu

kin ca knh truyn. Sau khi tn hiu c pht i my pht, tn hiu thu c

my thu s tri qua cc bc ngc li so vi my pht. Kt qu tn hiu c gii m

v thu li c my thu. Cht lng tn hiu ph thuc vo cht lng knh truyn,

cc phng php iu ch v m ha khc nhau.

1.1.2. Lch s pht trin ca thng tin v tuynVo u th k 20 Marconi thnh cng trong vic lin lc v tuyn qua i Ty

dng, Kenelly v Heaviside pht hin mt yu t l tng in ly hin din tng phatrn ca kh quyn c th dng lm vt phn x sng in t. Nhng yu t mra mt k nguyn thng tin v tuyn cao tn i quy m. Gn 40 nm sau Marconi,thng tin v tuyn cao tn l phng thc thng tin v tuyn duy nht s dng phn xca tng i lu, nhng n hu nh khng p ng ni nhu cu thng tin ngy cnggia tng.

Chin tranh Th gii ln th hai l mt bc ngot trong thng tin v tuyn.Thng tin tm nhn thng - lnh vc thng tin s dng bng tn s cc cao (VHF) v c nghin cu lin tc sau chin tranh th gii - tr thnh hin thc nh s

pht trin cc linh kin in t dng cho HF v UHF, ch yu l pht trin ngnh


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Raa. Vi s gia tng khng ngng ca lu lng truyn thng, tn s ca thng tinv tuyn vn ti cc bng tn siu cao (SHF) v cc cao (EHF). Vo nhng nm1960, phng php chuyn tip qua v tinh c thc hin v phng php chuyntip bng tn x qua tng i lu ca kh quyn xut hin. Do nhng c tnh uvit ca mnh, chng hn nh dung lng ln, phm vi thu rng, hiu qu kinh t cao,

thng tin v tuyn c s dng rt rng ri trong pht thanh truyn hnh qung b, vtuyn o hng, hng khng, qun s, quan st kh tng, lin lc sng ngn nghipd, thng tin v tinh - v tr v.v... Tuy nhin, can nhiu vi lnh vc thng tin khc liu khng trnh khi, bi v thng tin v tuyn s dng chung phn khng gian lmmi trng truyn dn.

i ph vi vn ny, mt lot cc cuc Hi ngh v tuyn Quc t

c t chc t nm 1906. Tn s v tuyn hin nay c n nh theo "Quy ch

thng tin v tuyn (RR) ti Hi ngh ITU Geneva nm 1959. Sau ln lt l Hi

ngh v phn b li di tn s sng ngn s dng vo nm 1967, Hi ngh v b

sung quy ch tn s v tuyn cho thng tin v tr vo nm 1971, v Hi ngh v phn

b li tn s v tuyn ca thng tin di ng hng hi cho mc ch kinh doanh vo

nm 1974. Ti Hi ngh ca ITU nm 1979, di tn s v tuyn phn b c m

rng ti 9kHz - 400 Ghz v xem xt li v b sung cho Quy ch thng tin v tuyn

in (RR). gim bt can nhiu ca thng tin v tuyn, ITU tip tc nghin cu

nhng vn sau y b sung vo s sp xp chnh xc khong cch gia cc sng

mang trong Quy ch thng tin v tuyn:

- Dng cch che chn thch hp trong khi la chn trm.

- Ci thin hng tnh ca anten

- Nhn dng bng sng phn cc cho.

- Tng cng ghp knh.

- Chp nhn s dng phng php iu ch chng li can nhiu.

1.1.3. Khi nim knh truynKnh truyn l mi trng truyn dn cho php truyn lan sng v tuyn.

Thng tin v tuyn s dng khong khng gian lm mi trng truyn dn. Ty thuc

vo mi trng truyn dn m knh truyn dn c cc tnh cht khc nhau.

1.1.4. Khi nim v sng mangSng mang l sng c nhn vi tn hiu c ch trc khi gi ra ngten pht.

Sng mang bn thn n khng mang tn hiu c ch. Ty thuc vo mi trng truyn dn


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100 1022

Hm

mthanh

102 104

Sium

AMradio

1020106

TV,FM

Viba,vtinh,raa

108 1010

Hngngoi

Tianhnthy

1012 1014 1016 1018

Cctm

TiaX

Tiagamma

Tia vtr

Tn s (Hz)

Di tn s radio Di si quang

Hnh 1.2 Ph tn s v tuyn v ng dng

v di tn hiu cho php m ngi ta la chn gi tr tn s sng mang. Thng thng thsng mang l sng trung tm ca di bng tn cho php ca h thng thng tin.

1.1.5. Khi nim v truyn dn bng tn c s v truyn dn bng thngTruyn dn v tuyn thng thng c thc hin bng thng , ngha l tn

hiu phi c iu ch bng mt sng mang no trc khi pht i. Truyn dn

bng tn c s l vic truyn dn khng qua sng mang. Tn hiu khng qua sng

mang khng c kh nng truyn c i xa do suy hao ln.

1.1.6. Phn loi cc h thng thng tin v tuynCc h thng thng tin v tuyn c th c phn loi theo s cung cp dch

v. V d h thng pht thanh v truyn hnh, dch v ca hai h thng l thoi v hnh

nh. C th phn loi h thng thng tin v tuyn theo phng thc truyn dn nh hthng truyn bn song cng (b m) hay song cng (h thng thng tin di ng).

Cng c th phn loi h thng theo mi trng truyn dn.

1.1.7. Khi nim v chun v tuyn xy dng mt h thng v tuyn trong phm vi a quc gia hay ton cu

ngi ta a ra cc chun c th cho cc h thng c th. Cc chun ny quy nh cu

trc my pht, my thu, cu trc ton b h thng thng tin mt cch thng nht. Cc

hng sn xut thit b thng tin s theo cc quy chun ny thit k h thng. Trn

th gii c hai t chc ln thc hin cc vn ny l Hip hi in t Th gii

IEEE v t chc quy chun ca Chu u ETSI.

1.2. Phn chia di tn s v tuyn v ng dng cho cc mc ch thng tinThng tin v tuyn m bo vic pht thng tin i xa nh cc sng in t . Mi

trng truyn sng (kh quyn trn mt t, v tr, nc, ...) l chung cho nhiu knh

thng tin v tuyn. Vic phn knh ch yu da vo tiu chun tn s. Ph tn tng

cng v min p dng cho chng c ch ra trn hnh 1.2.


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Ph tn ko di t cc tn s di m thanh (vi Hz) n cc tia v tr (1022 Hz)v c chia tip thnh cc on nh gi l cc bng tn. Ton b di tn s v tuyn(RF) li c chia thnh cc bng nh hn, c tn v k hiu theo y ban t vn vthng tin v tuyn quc t CCIR.- Cc tn s cc k thp (ELF) c gi tr nm trong phm vi 30Hz n 300Hz, cha c

tn s mng in AC v cc tn hiu o lng t xa tn thp.

- Cc tn s ting ni (VF), c gi tr nm trong phm vi 300Hz n 3kHz, cha cc

tn s knh thoi tiu chun.

- Cc tn s rt thp (VLF), c gi tr nm trong phm vi 3kHz n 30kHz, cha phn

trn ca di nghe c ca ting ni. Dng cho cc h thng an ninh, qun s v

chuyn dng ca chnh ph nh l thng tin di nc (gia cc tu ngm).

- Cc tn s thp (LF) c gi tr nm trong phm vi 30kHz n 300kHz (thng gi lsng di), ch yu dng cho dn ng hng hi v hng khng.

- Cc tn s trung bnh (MF) c gi tr nm trong phm vi 300kHz n 3MHz (thng

c gi l sng trung), ch yu dng cho pht thanh thng mi sng trung. Ngoi ra

cng s dng cho ng dn hng hi v hng khng.

- Cc tn s cao (HF) c gi tr nm trong phm vi 3MHz n 30MHz (thng gi l

sng ngn). Phn ln cc thng tin v tuyn hai chiu s dng di ny vi mc ch

thng tin c ly xa xuyn lc a, lin lc hng hi, hng khng, pht thanh qung b,...- Cc tn s rt cao (VHF) c gi tr nm trong phm vi 30MHz n 300MHz (cn gi

l sng mt), thng dng cho v tuyn di ng, thng tin hng hi v hng khng,

pht thanh FM thng mi, truyn hnh thng mi.

- Cc tn s cc cao (UHF) c gi tr nm trong phm vi 300MHz n 3GHz (cn gi

l sng decimet), dng cho cc knh truyn hnh thng mi, cc dch v thng tin di

ng mt t, cc h thng in thoi t bo, mt s h thng raa v dn ng, cc

h thng vi ba v thng tin v tinh.

- Cc tn s siu cao (SHF) c gi tr nm trong phm vi 3GHz n 30GHz (cn gi l

sng centimet), ch yu dng cho vi ba v thng tin v tinh.

- Cc tn s cc k cao (EHF) c gi tr nm trong phm vi 30GHz n 300GHz (cn

gi l sng milimet), t s dng cho thng tin v tuyn.

- Cc tn s hng ngoi c gi tr nm trong phm vi 0,3THz n 300THz, ni chung

khng gi l sng v tuyn. S dng trong cc h thng dn ng tm nhit, chp nhin t v thin vn hc.


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- Cc nh sng nhn thy c gi tr nm trong phm vi 0,3PHz n 3PHz, dng trong

h thng si quang.

- Cc tia cc tm, tia X, tia gamma v tia v tr rt t s dng cho thng tin.

Ch thch: 1 THz (terahertz) = 1012Hz

1 PHz (petahertz) = 1015Hz

1 EHz (exahertz) = 1018Hz


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Vtphn

x

My thu

ng ten

Tuyn 1 (1)

Tuyn 2 (2)

Hnh 2.1 M hnh phn x trong truyn dn phn tp a ng

CHNG 2. L THUYT V KNH V TUYN

2.1. Khi nim v knh truyn dn phn tp a ng

Tn hiu t ng ten pht c truyn n my thu thng qua nhiu hng phnx hoc tn x khc nhau. hnh trn tn hiu gi s nhn c bng hai lung tnhiu. Mt lung l tn hiu truyn thng c tr truyn dn tng ng l 1, tuyn thhai phn x t vt cn c tr truyn dn l 2. Tn hiu my thu l tng ca tn hiunhn c t 2 tuyn truyn dn, tn hiu thu c lung 1b suy gim mt mc khc so vi suy gim lung 2. Do vy cng tn hiu thu c lung 1khc vi tn hiu lung 2 cho d my pht pht i hai tn hiu c cng bin . Hintng ny c gi l hin tng fading min tn s (fading c hiu l hin tngsuy hao ca tn hiu pht nhn c ti my thu do knh truyn dn gy ra). Knhtruyn dn phn tp a ng gy nn hiu ng fading min tn s gi l knh ph

thuc tn s. Thc cht ca hin tng knh ph thuc tn s l hm truyn t caknh ph thuc vo gi tr ca tn s ca tn hiu pht.

M hnh tng qut ca truyn dn phn tp a ng khng phi ch l haituyn truyn dn m c th l v s cc tuyn truyn dn khc nhau. Tn hiu pht cth l phn x, tn x hoc khc x theo nhiu hng khc nhau ri mi n my thu.Trong trng hp my pht v my thu t trong tm nhn thng th s c mt lungtn hiu trong tm nhn thng. Tn hiu ny thng c cng ln hn hn v chtlng tn hiu thu tt hn nhiu.

2.2. p ng xung v hm truyn t ca knh

2.2.1. p ng xung ca knh khng ph thuc thi giana) Khi nim v knh khng ph thuc thi gian

Knh khng ph thuc thi gian l knh truyn dn trong trng hp khng cs chuyn ng tng i gia my pht v my thu.Bn cht ca hin tng ny l c p ng xung v hm truyn t ca knh khng

ph thuc thi gian.b) nh ngha ca xung Dirac

Xung (t) c nh ngha l xung Dirac nu n tha mn hai iu kin sau :


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h()

Hnh 2.2 p ng xung ca knh

0)( t nu t 0 v

1)(t

c) Khi nim v p ng xung ca knhp ng xung ca knh l mt dy xung thu c my thu khi my pht pht

i mt xung cc ngn gi l xung Dirac (t).p ng xung ca knh khng ph thuc thi gian v mt ton hc c biu

din nh sau :

pN

kkkah

1

)(.)(

Trong : k l ch s ca tuyn truyn dn, h() l p ng xung ca knh, lbin tr truyn dn, k l tr truyn dn tng ng vi bin k, ak l h s suy hao v Npl s tuyn truyn dn.

Hnh 2.2 m t p ng xung ca knh.

2.2.2. Hm truyn t ca knh khng ph thuc thi gianChuyn i Fourier ca p ng xung h() ta c hm truyn t H(j) :

h() FT

H(j)Hm truyn t ca knh l :

k

p

j

N

k

k

j eadehjH

1

)()(

Da vo hm truyn t ca knh ta c th nhn bit c min tn s no tnhiu b suy hao hay tng ng vi fading ln, hoc min tn s no tn hiu t b

suy hao.

2.3. B rng n nh v tn s ca knhB rng n nh v tn s ca knh c nh ngha :

max

1)(

cf

Trong , (f)c l b rng n nh tn s ca knh cn max l tr truyn dnln nht ca knh. Ty thuc vo b rng bng tn ca h thng so vi b rng nnh tn s ca knh m knh c nh ngha l knh ph thuc tn s hay khng.


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Vtphn

x

My thu

ng ten

Tuyn 1: 1(t)

Tuyn 2: 2(t)

Hnh 2.3 Hm truyn t ca knh

1

Nu b rng n nh tn s ca knh ln hn nhiu so vi b rng bng tnca h thng:

(f)c >>Bth knh c nh ngha l khng ph thuc vo tn s. Trong trng hp ngc li :

(f)c


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A

ffDmax+f0fDmax-f0 f0

(fyy

Hnh 2.4 Mt ph ca tn hiu thu

Mt ph ca tn hiu thu b nh hng bi hiu ng Doppler do Jake tm ranm 1974, nn cn c gi l ph Jake. ngha ca ph tn hiu:

Gi thit tn hiu pht i tn s sng mang f0, khi tn hiu thu s khngnhn c chnh xc trn tn s sng mang f0 m b dch i c v hai pha vi dchl fDmax. S dch tn s ny nh hng n s ng b ca nhiu h thng. Hiu ngDoppler cn gy ra s ph thuc thi gian ca knh v tuyn.

2.5. Knh ph thuc thi gianS dch chuyn tng i gia my pht v my thu gy ra hiu ng Doppler

v hin tng ph thuc vo thi gian ca knh. S ph thuc thi gian ca p ngxung knh v tuyn c biu din phng trnh sau :

))((),( )2(

teath ktfj

kkkD

Thi gian tr truyn dn lin quan n di ca tuyn truyn dn v vn tcnh sng, cn thi gian tuyt i t lin quan n im thi gian quan st knh. imthi gian quan st knh c th l ban ngy hay ban m, mang ngha tuyt i, cntr truyn dn c tnh l hiu s gia thi im nhn c tn hiu v thi im phttn hiu.p ng xung ca knh l php biu din ton hc ca knh min thi gian. Bin

i Fourier ca p ng xung ca knh cho ta hm truyn t ca knh, v vy hmtruyn t ca knh l php biu din ton hc ca knh min tn s. Hm truyn tca knh c biu din nh sau :

p

kkkD

N

k

tjtfj

k

j eeadethtjH1

)()2(),(),(

2.6. B rng n nh v thi gian ca knh nh gi s ph thuc vo thi gian ca knh, Proakis a ra nh ngha v

b rng n nh v thi gian ca knh nh sau :


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max.2

1)(

D

cf

t

Ty thuc vo s so snh gia b rng n nh v thi gian ca knh vi chuk ly mu tn hiu s cho ta kt qu liu knh v tuyn c gi l knh ph thucthi gian hay khng.

Nu b rng s n nh v thi gian ca knh ln hn nhiu so vi di mtmu tn hiu ca h thng : (t)c >> Tsth knh truyn dn ca h thng c coi l khng ph thuc thi gian.

Trong trng hp ngc li, c ngha l : (t)c


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+ Tng in ly: l tng kh quyn cao nm t cao (80 - 800)km, min ny hp thnhiu tia t ngoi c nng lng ln, cc tia ny c tc dng phn ly cc phn t khtr thnh cc ion t do, tng ny mt phn t kh gim thp. Khi tia sng c

pht ln gn tng in ly th cng b phn x b cong v quay tr li mt t do vy rtthch hp cho vic truyn sng ngn.

+ Sng trc tipL sng truyn trc tip t anten pht n anten thu khng b phn x trn

ng truyn. Trong iu kin truyn lan bnh thng, n c bin ln nht so vicc sng khc n my thu.+ Sng phn x t mt t

Sng ny n an ten thu sau khi phn x mt vi ln t mt t hoc t cc vtth xung quanh. Sng phn x ti anten thu c bin v pha khc vi bin v phaca sng trc tip, lm tn hiu thu khng n nh. Nu hiu khong cch ngtruyn ca tia phn x v tia trc tip bng s l ln na bc sng th anten thusng phn x lch pha vi sng trc tip mt gc 1800 v kt qu lm suy gim tnhiu sng trc tip, n mt mc no ph thuc vo bin ca sng phn x.+ Sng phn x tng i lu

Do thay i ch s khc x ca khng kh theo cao so vi mt t, nn sngc th b phn x, tu theo gc sng ti c th xy ra phn x ton phn t tng ilu. Trong trng hp ny xut hin mt bin gii c tc dng ging nh mt b mt

phn x, gi sng tr li mt t. Mt s tia ny s n an ten thu, c th lm suy gimsng trc tip do s thay i pha v bin gy ra. Sng truyn theo tng i lu c

th lan rng n 10 dm (khong 15km).

2.8. ng truyn lan sng v tuynSng v tuyn khng truyn lan theo dng l tng khi chng trong khng

gian do nh hng ca mt t v tng i lu. Hnh 2.5(a) m t ng truyn snggia cc u pht T v u thu R v ch cho thy cn c sng phn x t b mt t t ti trm thu, ngoi sng trc tip theo ng thng.

Hnh 2.5 ng i ca Sng v tuynKhi khong cch gia trm pht v trm thu xa nhau hn, thng tin bng sng

i thng tr nn khng th c do cong ca b mt tri t nh trnh by tronghnh 2.5(b) nhng vn c th c sng v tuyn truyn lan xung mt t do c sng bmt, sng phn x v sng tri (hnh 2.6). Ni chung, sng b mt, sng trc tip v

sng phn x, tr sng tri, u c gi l sng t. Sng tri l sng in t b thayi hnh trnh ca mnh ti tng in ly v quay tr v tri t; tng in ly l ni hi


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t ca v s in tch, nh hnh ti cao 80-800 Km. Ngoi sng b mt v sng tricn c sng tn x - l phn x do nhng s bin i mnh lit ca tng i lu vin ly hoc do sng in t va chm vi cc vt cht, chng hn nh cc sao bng, v

b tn x ri t ti u thu. Sng tn x c s dng trong phng php chuyntip qua tn x i lu.

Hnh 2.6 Hnh trnh ca sng v tuyn i qua ng chn tri

2.8.1. S lan truyn ca bng tn s thpS lan truyn ca bng tn s thp l nh vo sng t. Do nhiu x sng in

t, nhiu x t l nghch vi bc sng cho nn tn s s dng cng cao, sng tcng yu ( truyn lan tn s thp). Hin tng nhiu x c mi tng quan cht chvi dn in v hng s in mi ca t trong ng lan truyn. V c ly truynsng trn mt bin di hn so vi mt t cho nn tn s thp c s dng rng ri

trong thng tin v tuyn o hng. Trong trng hp tn s cc thp, bc sng lnhn nhiu so vi chiu cao t b mt tri t ln ti tng in ly. Cho nn, mt t vtng in ly ng vai tr nh hai bc tng. N c gi l ch ng dn sng mtt - in ly m nh n, c th thng tin ti ton th gii. Bng tn s cc thp cs dng ch yu cho thng tin hng hi v thng tin o hng.

2.8.2. S truyn lan ca bng tn s caoThng tin c ly xa bng bng tn s cao c thc hin nh s phn x ca

sng tri trn tng in ly. Trong phng thc thng tin ny, mt thu sng tri ph

thuc vo tn s v tuyn v trng thi ca tng in ly, trng thi ny thay i theothi gian, theo ngy, theo ma v theo iu kin thi tit. Cho nn vic d bo trngthi ca tng in ly l v cng quan trng i vi thng tin lin lc s dng sng tri.

2.9. Cc phng thc truyn lan sng in tCc sng bc x t im pht c th n c cc im thu theo nhng ng

khc nhau. Cc sng truyn lan dc theo b mt qu t gi l sng t hay sng b mt;cc sng i ti cc lp ring bit ca tng ion v phn x li gi l sng in ly hay sngtri; sng trc tip v sng phn x t mt t gi l sng khng gian (hnh 2.7).


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2.9.1. S truyn lan sng tSng tl sng truyn lan dc theo b mt tri t, do cn c gi lsng

b mt. Sng t l sng phn cc ng bi v in trng trong sng phn cc ngangs song song vi b mt tri t, v cc sng nh th s b ngn mch bi s dn inca t.

Thnh phn in trng bin i ca sng t s cm ng in p trong b mttri t, to ra dng in chy. B mt tri t cng c in tr v cc tn hao inmi, gy nn s suy hao sng t khi lan truyn. Sng t lan truyn tt nht trn bmt l cht dn in tt nh nc mui, v truyn km trn vng sa mc kh cn. Tnhao sng t tng nhanh theo tn s, v th sng t ni chung hn ch cc tn s

thp hn 2 MHz. Sng t c dng rng ri cho lin lc tu thy - tu thy v tuthy - b. Sng t c dng ti cc tn s thp n 15 kHz.Cc nhc im ca truyn lan sng t l:

- Sng t yu cu cng sut pht kh cao.- Sng t yu cu anten kch thc ln.- Tn hao thay i ng k theo loi t.

Cc u im l:- Vi cng sut pht ln, sng t c th dng lin lc gia 2 im bt k

trn th gii.

- Sng t t b nh hng bi s thay i iu kin kh quyn.

2.9.2. S truyn lan sng khng gianGm sng trc tip v sng phn x t mt t, truyn trong vi kilomet tng

di ca kh quyn. Sng trc tip lan truyn theo ng thng gia cc anten pht vthu, cn gi sng nhn thng (LOS:Line-Of-Sight). V th, sng khng gian b hn ch

bi cong ca tri t. Sng phn x t t l sng phn x t b mt tri t khi lantruyn gia anten pht v thu. cong ca tri t to nn chn tri i vi s truynlan sng khng gian, thng gi l chn tri v tuyn. C th ko di chn tri v

tuyn bng cch nng cao anten pht hoc anten thu (hay c hai) bng thp hoc t

Sng nhn thng

Sng b mtB mt tri t

Anten pht Anten thuSng phn x t t

Kh quyn tri t

Hnh 2.7 Cc phng thc truyn sng


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Tia nhn thng LOS

Hnh 2.8 Sng khng gian v chn tri v tuyn

Anten pht Anten thu

hthr

dt dr

d

trn nh ni (ta nh). Hnh 2.8 ch ra nh hng ca cao anten n chn tri vtuyn.

Chn tri v tuyn nhn thng i vi mt anten bng: hd 2

trong : d= khong cch n chn tri v tuyn (dm, 1 dm = 1,5km), h = caoanten so vi mc nc bin (feet, 1 feet = 0,304767m). Do , khong cch gia anten

pht v anten thu l:

rtrt hhddd 22 trong : d l tng khong cch (dm hoc km), ,dt, dr l chn tri v tuyn i vianten pht v anten thu (dm hoc kilomet), ht, hr cao anten pht v anten thu (phthoc mt). Nh vy, khong cch truyn sng khng gian c th tng bng cch tng cao anten pht, anten thu hoc c hai.

Do cc iu kin tng di kh quyn hay thay i nn mc khc x thayi theo thi gian. Trng hp c bit gi l truyn lan trong ng sngxy ra khi mt t mc sao cho cc sng in t b by gia tng ny v b mt tri t. Cc lp

kh quyn hot ng nh ng dn sng v cc sng in t c th lan truyn rt xavng theo cong tri t v trong ng.

2.9.3. S truyn lan sng triCc sng in t c hng bc x cao hn ng chn tri (to thnh gc kh

ln so vi mt t) c gi lsng tri. Sng tri c phn x hoc khc x v trit t tng in ly, v th cn gi l sng in ly. Tng in ly hp th mt s lngln nng lng ca tia cc tm v tia X bc x ca mt tri, lm ion ha cc phn tkhng kh v to ra electron t do. Khi sng in t i vo tng in ly, in trng

ca sng tc ng lc ln cc electron t do, lm cho chng dao ng. Khi sngchuyn ng xa tri t, s ion ha tng, song li c t hn phn t kh ion ha. Do, phn trn ca kh quyn c s phn trm phn t ion ha cao hn phn di. Mt ion cng cao, khc x cng ln. Ni chung, tng in ly c phn chia thnh 3lp: lp D, E, v F theo cao ca n; lp F li c phn chia thnh lp F1, F2 (hnh2.9). cao v mt ion ha ca 3 lp thay i theo gi, ma v theo chu k vt enca mt tri (11 nm). Tng in ly m c nht vo ban ngy v ma h.


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0 1 2 3 4 5 6 7 8 9 10 12 13 1511 14 17 18 2016 19 22 2421 23 Gi trong ngy

km

50

100

137200

300

400

Lp D (ch c ban ngy)Lp ELp F

Lp F1

Lp F2

Hnh 2.9 Tng in ly v s thay i ca chng theo thi gian trong

F1+F2F1+F2

Lp D: l lp thp nht, c cao 50 100 km v nm xa mt tri nht, do cion ha t nht. Nh vy lp D t c nh hng n hng truyn lan sng v tuyn. Song

cc ion lp ny c th hp th ng k nng lng sng in t. Lp D bin mt vm. Lp ny phn x sng VLF v LF, hp th cc sng MF v HF.

Lp E: c cao 100 140 km, cn gi l lp Kennelly - Heaviside theo tnca hai nh bc hc khm ph ra n. Lp E c mt cc i ti cao 70 dm vo22 gia tra khi mt tri im cao nht. Lp E hu nh bin mt v m, h tr slan truyn sng b mt MF v phn x sng HF mt cht v ban ngy. Phn trn calp E i khi c xt ring v gi l lp E tht thng. Lp ny gy bi hin tngnht hoa v hot ng ca vt en mt tri. y l lp mng c mt ion ho rt

cao, cho php ci thin khng ng c ly lin lc.Lp F: gm 2 lp F1 v F2. Lp F1 c cao 140 250 km vo ban ngy. LpF2 c cao 140 300 km v ma ng v 250 350 km v ma h. V m, 2lp ny hp li vi nhau to thnh mt lp. Lp F1 hp th v suy hao mt s sngHF, cho qua phn ln cc sng n F2 , ri khc x ngc v tri t.

2.10. Mt s thut ng v nh ngha truyn sng

2.10.1. Tn s ti hn v Gc ti hnCc tn s cao hn di UHF thc t khng b nh hng bi tng in ly v

bc sng ca chng cc k ngn. Ti cc tn s ny, khong cch gia cc ion l khln, do cc sng in t i qua chng hu nh khng b nh hng.

Nh vy phi tn ti gii hn tn s trn i vi s truyn lan sng tri. Tn sti hn l tn s cao nht m sng in t vn cn c th tr v tri t bi tng inly. Tn s ti hn ph thuc vo mt ion v thay i theo gi v ma. Nu gc bcx ng gim, cc tn s ln hn hoc bng tn s ti hn cn c th khc x tr li bmt tri t, v chng s i khong cch di hn trong tng ion v c nhiu thi gianhn khc x. V th, tn s ti hn ch c dng lm im chun so snh. Mi tn

s c gc ng cc i ti n c th lan truyn v cn khc x li bi tng ion. Gcny c gi lgc ti hn (hnh 2.10). Nh vy, gc ti hn lin quan n mt ion


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Anten pht

Gc ti hn

B mt tri t

Khc x v tri t

Phn x lin tip

Xuyn qua tng ion v bin mt

Hnh 2.10 Gc ti hn

ca tng in ly v tn s ca tn hiu. Gc ti hn cao cho tn s thp v thp cho tns cao.

Mt k thut o lng gi l thm d tng in ly bng m thanh ( ionosphericsounding) i khi c s dng xc nh tn s ti hn. Tn hiu c truyn thngt b mt t v tng dn tn s. Ti cc tn s thp hn, tn hiu s b hp th honton bi kh quyn. Khi tn s tng dn, mt phn s tr li tri t. Ti tn s no , tnhiu s xuyn qua kh quyn, i vo khng gian xa xi v khng tr li tri t. Tn scao nht s cn tr li tri t theo hng thng ng chnh l tn s ti hn.

2.10.2. cao o cao o l cao ti sng khc x cng chnh l sng phn x d kin(hnh 2.11).

Hnh 2.11 cao o v cao thc tSng bc x c khc x li tri t theo ng A. cao cc i thc t m

sng n c l ha. ng B l ng d kin sao cho sng phn x s i qua v cntr v tri t ti cng v tr. cao cc i m sng phn x d kin ny s ti cchnh l cao o hv.


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2.11. Cc nhn t nh hng n s truyn lan sng v tuyn

2.11.1. Suy hao khi truyn lan trong khng gian t doKhong khng m trong cc sng truyn lan b suy hao c gi l khng

gian t do. Mc suy hao ca sng v tuyn c pht i t anten pht n anten thu

trong khng gian t do t l thun vi khong cch gia hai anten v t l nghch vi di bc sng. Suy hao ny gi l suy hao truyn lan trong khng gian t do, ctnh nh sau:

2244

c

fddLp

Trong : Lp l suy hao ng truyn khng gian t do (khng c th nguyn).D l khong cch (tnh theo mt)f l tn s (Hz) l bc sng (mt)

c l tc nh sng.Chuyn sang dB ta c:

dfc

dBc

fddBLp lg20lg20

4lg20

4lg20][

Nu tn s tnh theo MHz v khong cch tnh theo km, ta c:

)()(

)()(8

36

lg20lg2032,4

lg20lg2010.3

)10)(10(4lg20][

kmMHz

kmMHzp

df

dfdBL

Nu tn s tnh theo GHz v khong cch tnh theo km, ta c:)()( lg20lg2092,4][ kmGHzp dfdBL

2.11.2. nh hng ca pha ing v maPha ing c nh ngha l s thay i cng tn hiu sng mang cao tn

thu c do s thay i kh quyn v phn x t, nc trong ng truyn sng.Thc t cho thy nh hng do ma v pha ing nhiu tia l nhng nh hng lantruyn ch yu i vi cc tuyn v tuyn tm nhn thng trn mt t lm vic trongdi tn GHz. V chng quyt nh cc tn hao truyn dn v do quyt nh khong

cch lp cng vi ton b gi thnh ca mt h v tuyn chuyn tip. Pha ing nhiutia tng khi di ca tuyn tng tuy nhin n khng ph thuc nhiu vo tn s. Cntiu hao do ma tng ln khi tn s tng. Chng hn, i vi cc tuyn s dng tn strn 35GHz thng suy hao do ma ln do m bo cht lng tn hiu truyndn th cc khong cch lp thng chn di 20km, ngoi ra vic gim di ngtruyn s lm gim cc nh hng ca pha ing nhiu tia. Vy i vi cc ngtruyn di v c tn s hot ng thp th pha ing nhiu tia l nh hng chnh. Cni vi cc tuyn ngn v c tn s hot ng cao hn th tiu hao do ma l nh

hng ch yu.


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Suy hao dB/km6GHz 10GHz 20GHz 40GHz

Ma va 0,25mm/h 0 0 0,013 0,07

Ma ln 5mm/h 0,012 0,08 0,45 1,5Bo 50mm/h 0,22 1,2 5,5 13

Bo ln 150mm/h 1,2 5,5 18 27Bng 2.1 Kt qu thc nghim v suy hao do hi nc - kh hu theo tn s sng v

tuyn ca Alcatel

2.11.3. S can nhiu ca sng v tuynThng thng nhiu xy ra khi c thnh phn can nhiu bn ngoi trn ln vo

sng thng tin. Sng can nhiu c th trng hoc khng trng tn s vi sng thngtin. Chng hn h thng Vi ba s ang s dng b nh hng bi s can nhiu t cch thng vi ba s ln cn nm trong cng khu vc, c tn s sng v tuyn trng hocgn bng tn s ca h thng ny, ngoi ra n cn b nh hng bi cc trm mt tca cc h thng thng tin v tinh ln cn.

2.12. c im mt s di sng v tuyn

2.12.1. Sng cc di v sng diC th lan truyn nh sng t v sng khng gian. Vic sng t bng tn

ny lan truyn i hng trm, thm ch hng nghn kilomet l do cng trng cacc sng ny gim theo khong cch kh chm, ni cch khc l nng lng ca chng

b mt t hoc mt nc hp th t. Bt u t khong cch 300 400 Km so vi mypht xut hin sng in ly phn x t lp di ca tng in ly (t lp D cao 60 80Km vo ban ngy hoc lp E cao 100 130 Km). Do bc sng ln hn nhiu so vichiu cao t b mt tri t ln ti tng in ly, cho nn mt t v tng in ly ngvai tr nh hai bc tng. N c gi l ch ng dn sng mt t - in ly mnh n lin lc ton cu cc sng ny thc hin bng cc sng truyn lan trong ngsng hnh cu to bi mt t v tng in ly. Cc sng ny c th xuyn su vo ncv truyn lan trong mt s loi t. Bc x hiu qu sng di v cc di ch t cvi cc anten rt cng knh, c kch thc xp x bc sng, bi vy ngi ta thngtng cng sut pht n hng trm KW hoc hn b li.

- Nhc im c bn ca sng di v cc di: Dung lng tn s nh.- Min ng dng thc t: Thng tin vi cc i tng di nc, thng tin theo

cc ng trc ton cu v thng tin di mt t.

2.12.2. Sng trung Tnh cht truyn lan: B hp th nhiu hn sng di v cc di khi truyn lan theomt t, cn cc sng n c tng in ly b hp th mnh bi lp D (khi D tn ti)

v phn x tt bi lp E.


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C ly lin lc: V ban ngy ma h th b hn ch (ch bng sng t), v ban mma ng v ma h th c ly thng tin tng r rt. Dung lng tn s: Cao hn nhiu sng di v cc di song do c nhiu i phtthanh cng sut ln nn gp nhiu kh khn trong vic tn dng dung lng. Min ng dng thc t: Nhiu nht l trong cc vng bc cc lm d phng khi

thng tin sng ngn b tn hao ln do nhiu t nhiu kh quyn. Anten: Kh hiu qu v c kch thc chp nhn c.

2.12.3. Sng ngn (SN) Tnh cht truyn lan: Bao gm c sng t v sng in ly C ly lin lc: Vi sng t v cng sut khng ln lm, c ly lin lc khng vtqu vi chc kilomet v sng t b hp th mnh trong t (tng theo tn s). Sngin ly do phn x mt hoc nhiu ln t tng in ly c th lan truyn xa tu . Sngny b hp th yu bi cc lp D v E, phn x tt bi cc lp trn (ch yu l F2).

Hin nhin iu kin tt nht cho cc tn s gia ngy v ti s khc nhau, chu kvng quanh lc mt tri mc v lc mt tri ln l ht sc khng n nh, vi iukin tt nht cho cc tn s lin lc tng nhanh t gi lc mt tri mc v gim t gilc mt tri ln. i vi nguyn nhn ny, mt s lin lc nh k trong thi gian ny

bao gm mt s knh kim tra rng tn s khc nhau. Vic lin lc tng tgim dn bt buc thay i mt vi MHz cao hn hoc thp hn. Dung lng tn s: Ln hn nhiu cc sng trn do m bo s lm vic ngthi ca s lng ln cc my thu pht v tuyn. Anten: Vi kch thc nh vn c hiu qu kh cao v hon ton p dng c chocc i tng c ng.V sng ngn chim v tr c bit trong thng tin v tuyn nn ta xt k hn:- Thng tin v tuyn bng cc sng in ly c th thc hin c nu cc tn s sdng nm thp hn cc gi tr cc i xc nh bi mc ion ho ca cc lp phnx i vi mi c ly lin lc. Ngoi ra thng tin ch c th c nu cng sut my phtvi h s khuch i ca cc anten c s dng bo m cng trng cn thitti im thu vi s hp th nng lng cho trong tng ion. iu kin u hn chgii hn trn ca tn s s dng, iu kin sau - gii hn di. V vy thng tin SN

bng sng in ly ch c trong mt khong tn s nht nh. B rng ca khong nyph thuc vo tng in ly, ngha l ph thuc vo thi gian ca mt ngy m, voma, vo chu trnh hot ng ca mt tri. Cho nn vic d bo trng thi ca tngin ly l v cng quan trng i vi thng tin lin lc s dng sng tri.- Hin tng pha inh trong thng tin SN: Hin tng ny lm gim nhiu cht lngthng tin SN bng sng in ly. Pha inh xut hin do s thay i cu trc ca cc lp

phn x tng ion, do s nhiu lon ca tng ion v do s truyn sng theo nhiu tia.Bn cht ca pha inh v c bn l s giao thoa ca mt vi tia ti im thu vi phathay i lin tc do s thay i trng thi ca tng in ly.

Pha inh cn c th xy ra do s thng ging phn cc cc sng khi phn x t tngin ly, dn ti s thay i mi quan h gia cc thnh phn ng v ngang ca


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trng in t ti ni thu. Pha inh phn cc xy ra him hn nhiu pha inh giao thoa(10 15% tng s).- Cc bo t v bo ion c th nh hng ng k n trng thi thng tin SN. y lcc nhiu lon ca tng in ly v ca t trng qu t di tc ng ca cc dnght in tch do mt tri phun ra. Cc dng ht ny thng ph hu lp phn x c bn

F2. Cc nhiu lon tng ion xy ra c chu k v lin quan n thi gian mt tri quayquanh trc ca mnh (ngy m).- Cc v n ht nhn do con ngi thc hin trong kh quyn c th gy ra s ion honhn to tng kh quyn v ko theo s nh hng tnh trng thng tin SN.

2.12.4. Cc sng cc ngn (SCN)Bao gm mt lot cc on tn s c dung lng ln. Tnh cht truyn lan: Nng lng SCN b hp th mnh bi mt t (ni chung l tl vi bnh phng ca tn s), v vy sng t b suy gim rt nhanh. SCN khng c

s phn x u n t tng ion, do thng tin SCN ch yu da trn sng t vsng trc tip. C ly thng tin: C ly thng tin bng sng t ph thuc nhiu vo bc sng. C lyln nht l cc sng mt gn vi di SN (lu rng vi s tng tn s hiu qu cah thng anten tng ln nh b li tn hao nng lng trong t). SCN c nghaln i vi thng tin trong khng gian t do, tc l trong vng tn hao nng lng rtnh. C ly thng tin gia cc thit b bay trang b my thu pht cng sut nh c tht ti vi trm kilomet. Min ng dng thc t: Thng tin v tr, thng tin trong mng c s c bit l giacc i tng c ng.Sng cc ngn dng nhiu trong thng tin iu hnh nn ta xt chi tit hn mt s cim ca chng:- Cc sng mt (SM) c tnh cht nhiu x, ngha l c th un cong theo a hnh.Hin tng khc x tng i lu lm tng c ly thng tin SM. S thay i h s khcx theo cao ca kh quyn gy nh hng n sng khng gian. Kh quyn tiuchun l mt kh quyn l tng c mt t l bin i h s khc x theo cao mtcch u n, bi v n c mt h s thay i c nh ca p sut kh quyn theo

cao, nhit v m. Khi trng thi ca tng i lu bnh thng (gim u nnhit vi chiu cao), tia sng ni cc my thu pht ang lin lc b un cong v

pha trn do bo m c thng tin trn cc tuyn b che khut (i pht khngnhn thy i thu). V c s bin i h s khc x mt cch lin tc, cho nn ng ithc t ca sng khng gian l khc vi ng trc tip (thng). b li s khcnhau ny, c ly thng tin cc i thc t c tnh ton theo ng trc tip da trnquy nh bn knh hiu qu ca tri t KR (K = 4/3 trong kh quyn tiu chun). Hs khc x tng i lu thay i theo thi gian do cc thay i cc iu kin kh tng,iu ny dn ti pha inh tn hiu (khc vi pha inh SN, chng xy ra rt chm v

khng su)- SM nhiu khi lan truyn rt xa. l do:


24/86



+ S hnh thnh cc m my ion ho tht thng cao vi chc kilomet di lpD (lp tht thng FS) FS c th xut hin vo bt k thi gian no. c im ca ccm my ny l nng ion ho rt cao, i khi phn x cc sng ca c diSCN. Lc ny c ly gia pht v thu c th l 2000 2500 km. Cng tn hiu

phn x FS rt ln ngay c khi cng sut pht nh.

+ S tn ti thng xuyn ca lp FS trong nhng nm mt tri hot ng mnh nht.C ly lin lc c th t ti phm vi ton cu.+ Cc v n ht nhn trn cao: lc ny xut hin mt min pha trn ( mc lp FS)SM xuyn qua min di, b hp th mt t, phn x t min trn v quay tr li mtt. Khong cch bao trm nm trong phm vi 100 2500 km. Cc tn s thp nht bhp th mnh nht cc min ion ho pha di, cc tn s cao nht b phn x khngton phn t min trn. Nh vy, vic lan truyn ca sng v tuyn nh hiu ng tnx i lu ca khu vc kh quyn rng ln trong tng i lu c dng cho trnVHF. Phng php ny c nhiu u im ca thng tin bng rng v ghp knh cngnh thng tin ng thi cho mt khu vc rng. Mt khc n cng i hi cng sut

pht ln v my thu c nhy cao.


25/86



Hnh 3.1 Hm phn b Gauss

h()H(j)

x(t)

Tn hiu pht Tn hiu thu

n(t)

y(t)

Hnh 3.2 Mi trng truyn dn vi s c mt ca nhiu trng

CHNG 3. NHIU TRONG THNG TIN V TUYN

3.1. Cc loi nhiu trong thng tin v tuynNhiu l mt vn rt quan trng trong thng tin v tuyn, nh hng n cht

lng ca tn hiu, khi x l tn hiu v khi truyn tn hiu lm gy mo tn hiu hocxut hin cc tp m trong cc thit b ti to li tn hiu.

V vy ta phi gim st c chng v tm bin php khc phc tn hiu nhiun mc ti a tng cht lng ca ca tn hiu.

3.1.1. Khi nim v nhiu trng (White Gaussian Noise)Nhiu trng l qu trnh xc xut c mt ph cng sut phng (khng i) trn

ton b di tn.Nhiu trng l mt loi nhiu c hm mt xc sut tun theo phn b Gauss.V mt ton hc,ngun nhiu trng n(t) c th m hnh bng mt bin xc sut

Gauss vi gi tr k vng =0 v lch chun l 2

0 xE

22 )( xEPhn b Gauss nh trn hnh 3.1:

Nhiu trng c th do nhiu ngun khc nhau gy ra :- Nhiu sinh ra do s chuyn ng nhit ca cc in t trong cc linh kin bn dn.- Nhng m thanh nh ting gi, ting nc cng l nhng ngun nhiu trng- Cc vn nh thi tit, con ngi

Tn hiu thu do vy c vit li nh sau:y(t)=x(t)*h()+n(t)

M hnh knh nh trn hnh 3.2.


26/86



t

I0

in(t)

i(t)

Hnh 3.3 b

RE i(t)

Hnh 3.3 a

I0

in(t)

3.1.2. Tp m nhit trng chun cng tnh (Additive White Gaussian Noise)a) Nguyn nhn tp m

Do dng in ca my thu ngoi chu tc ng ca sc in ng (to intrng in t chuyn ng) cn chu tc ng ca chuyn ng nhit (chuynng hn lon ca cc in t).

Tc l ngoi dng in do in p tn hiu gy nn cn chu tc ng ca dngin do chuyn ng nhit gy nn, l tc ng khng mong mun tng ngvi tp m (hnh 3.3 a).

ng lin nt tng ng dng in t chuyn ng c hng (dng in camy thu).

ng t nt tng ng vi tc ng nhit.

Khi RE

I

Dng I(t) c minh ha trn th hnh 3.3 b.

Trong REI 0 , trn th ta thy i(t) khng bng phng m nhp nh, ta c th coi:

i(t) = I0 + in(t).Ngoi tc ng ca dng in cn c tc ng ca dng l do chnh h thng gy

nn (do chuyn ng nhit bn trong mch).b) Tnh cht ca tp m nhit

- Tnh cng vo tn hiu : i(t) = I0 + in(t)- Do va chm ca cc in t vi tn sut 109 1011 (nn tng ng vic nhn

mt chui xung c tn s f = 109 1011Hz), v th b rng ph ca tp m xp x 109

1011Hz.Bng thng ca tn hiu thng thng khong vi chc MHz. Khi iu ch, tn

hiu c a ln min tn s cao, bin cn 1 na v rng bng thng tng gpi nhng cng ch khong my chc MHz. ng thi tn hiu c trn v iu chn u th tp m cng c trn ln n vi b rng ph rt rng. V nguyn tc,fc i n u cng c nhiu, v ph tp m tri di nn trong on bng thng coi nh

ph tp m l thng (hnh 3.4).


27/86



fc f

)(fS

Tpm

Vi chc MHz Hnh 3.4 Ph tp m

iu ch/trn

thun tin tnh ton, ta c th m hnh ha tp m c quy ra u vo nhmt ngun tp trng tc c hm mt ph cng sut t 0 n ( thc cht khngtin n nhng v tn hiu i n u cng gp tp m nn coi nh ). Do , tnhcht ca tp m ny l tp m m hnh ha (gi nh) c ph phng t - n +

ging ph nh sng trng nn gi l tp m trng.- Nhiu l tng ca 109 1011 xung in khc phng khc chiu, ta c :

N

i

in iN

i1

1l dng in trung bnh ca tng cc dng in gy ra bi cc xung con.

Vi mi ii cc b, ng gp phn rt yu trong ii, c phng chiu hon ton ngunhin, thi gian tn ti khc nhau.

nh l gii hn trung tm pht biu nh sau : nu x l tng ca mt s rt lncc bin xi c lp thng k vi nhau v mi x i c tc ng rt yu th bin x c phn

b chun . Theo nh l gii hn trung tm th in l bin ngu nhin phn b chun.

Tp m nhit in(t) l tp m c gi tr bin ti bt k thi im quan trc nocng l mt bin ngu nhin phn b chun. Tnh cht ny cho thy tp m nhit l tpm c phn b chun.

T ba tnh cht trn nn tp m c tn gi l tp m nhit trng chun cng tnhAWGN (Additive White Gaussian Noise).

3.1.3. Khi nim v nhiu xuyn k t ISI (Inter symbol interference)Nhiu xuyn k t, vit tt l ISI, c coi nh h qu tt yu ca h thng thng

tin v tuyn ln hu tuyn. ISI c ch n ngay t thi k u ca truyn dn

thng tin.i vi tn hiu ti u thu chng c khuynh hng tri rng ra (spreading) v

mt thi gian v suy gim (smearing) v mt bin nh minh ha trn hnh 3.5


28/86



Bin

Thi gian

1

2

3 4

5

6

Hnh 3.6 Hnh dng xung ca chui bit 101101 c truyn

Bin

Thi gian

1

2

3 4

5

6

Hnh 3.7 Hin tng tri thi gian v suy gim bin

Nhiu t bit 1 sang bit 2 v 3

Hin tng ny l do tnh cht ca mi trng thng tin v khong cch truyn

dn. Mt gii php n gin khc phc hin tng ny l s dng cc trm trunggian (cc trm lp) gip hn ch s bin thin rng xung v bin tn hiu.

c th hnh dung v ISI, ta kho st trng hp nhiu gia cc bit k cn:chui bit c truyn l 101101, s dng xung vung (Thc t dng ca nhng xungvung ny c biu th bng ng t nt) nh trn hnh 3.6

Tn hiu nhn c ti u thu s c dng nh hnh 3.7

T hnh 3.7 ta thy bit 1 nh hng ti bit 2 v 3, bit 2 nh hng ti bit 3 v4, Nh vy 1 bit b nh hng bi 2 bit trc . Do , mc chnh xc ca tnhiu sau khi khi phc ph thuc vo mc ti cc thi im ly mu tng ng viu pht.

Nhiu xuyn k t chnh l hin tng tri ra v mt thi gian v suy gim v

mt bin - ni cch khc l s bin i rng v bin - t mt k t sang cck t ln cn, gy kh khn trong vic khi phc tin tc ti u thu. Hin tng ISI c

Bin

Thi gianThi gian k hiuBin

Thi gianTri tr

a)

b)

Hnh 3.5 a) Tn hiu pht, b) Tn hiu thu


29/86



Hnh 3.8 Nhiu knh xuyn knh

th do nhiu nguyn nhn, trong c th k n nh: lc sai, pha inh (c bit lpha inh la chn tn s), hoc do cc thit b phi tuyn, ISI khng xy ra i vimt vi h thng, nh hng nhiu i vi thng tin v tuyn

3.1.4. Khi nim v nhiu xuyn knh ICI (Inter Channel Interference)

Nhiu xuyn knh do cc thit b pht trn cc knh lin nhau gy ra.

Nhiu xuyn knh thng xy ra do tn hiu truyn trn knh v tuyn b dch tngy can nhiu sang cc knh k n.

khc phc nhiu lin knh c hai bin php :- B tr cc knh lin lc cch nhau mt ng sng.V d, vi GSM th cc sng mang ca h thng cch nhau 0,2MHz. Nu hai

sng mang lin nhau th nhiu l rt ln v th 2 sng mang trong cng 1 t bo btbuc cch nhau 0,4MHz tc cch nhau mt ng sng.

- Phi lc tht cht nhng vic ny lm gim cng sut pht dn n gim cngph sng.

3.1.5. Khi nim v nhiu ng knh CCI (Co- Channel Interference)

Nhiu ng knh CCI gy bi cc ngun nhiu t cc h thng khc c tn sbng hoc gn bng vi tn s ang xt.V d v CCI :

1) Cc h thng vi ba s di siu cao tn (sng cc ngn):UHF: f 300MHz, bc sng di dmSHF: f 3GHz, bc sng di cmEHF: f 30GHz, bc sng di mmSng cng ngn th cng d b chn v vt chn sng phi c kch thc so snh

c vi bc sng, v vy truyn c sng siu cao tn th phi c cc trm tip

sng.Vi ba lm vic trong tm nhn thng v c phn b tn s nh sau trn hnh 3.9

Ngi ta cho php s dng cng lc hai loi phn cc trn mt tn s c th

truyn hai lung tin khc nhau, k hiu H l phn cc ngang (in trng di chuynsong song vi b mt tri t) v V l phn cc ng (in trng di chuyn vung

f

Hnh 3.9 Phn b tn s ca Vibaf


30/86



f

H(V)

V(H) Nhiu CCI

1 f1

2 f23 f1

gc vi b mt tri t). Nu trn l phn cc ngang th di l phn cc ng vngc li. Khi ph tn s c dng nh trn hnh 3.10.

Do trn thc t khng to c 2 phn cc hon ton vung gc vi nhau vphn cc ngang chu tn hao nhiu hn phn cc ng nn hnh chiu ca phn ccny ln phn cc kia chnh l nh hng gia hai lung tin v c goi l nhiu ngknh CCI.

CCI gy bi cc knh cng tn s v khc phn cc, nhng can nhiu ny l rt

yu. Gi s cng sut pht l 1 th nhiu ch l 1/10000, tn hao cng sut l 40dB.Khi trn cng mt tn s c hai knh lin lc, tc c nhiu CCI th u thu phi

c b trit xuyn nhiu (XPIC) phn cc cho. tc ng ca nhiu CCI nh th chiu cao ca anten thp, gc ngng ca anten

ln v khong cch truyn ln.2) Vi thng tin di ng : do ph tn nh nn s tn s sng mang s dng rt b.

ph sng, ngi ta chia vng a l thnh cc cell (t bo), mi cell c mt trmgc nh trn hnh 3.11.

Do s tn s sng mang qu t khng cho mi t bo (mi trm) s dng mttn s nn phi s dng li tn s, nh vy s gy nhiu nu gn nhau. Do vy, cc tbo ln cn khng s dng tn s trng nhau. Tuy nhin, gi s trm 1 v 3 s dngcng tn s f1, nu trm 1 c ct anten cao v c bit gia 1 v 3 c mt nc gy

phn x mnh th 3 vn nhn c sng t 1 chnh l nhiu CCI.Nh vy, vi thng tin di ng, CCI gy bi nhiu vt chng do cc t bo xa

c cng tn s gy nn.Nh vy, nhiu ng knh xy ra khi c hai my pht trn cng mt tn s hoc

trn cng mt knh. My thu iu chnh knh ny s thu c c hai tn hiu vi

cng ph thuc vo v tr ca my thu so vi hai my pht.Nhiu CCI khng th trit c nn ngi ta gim nhiu bng cch:

Hnh 3.10 Ph tn s ca Viba khi s dng phn cc

Hnh 3.11 Cc t bo sng mang


31/86



- Pht vi cng sut va .- iu chnh chiu cao ngten v gc ngng ca ngten m bo nhiu CCI nh.

3.1.6. Khi nim v nhiu a truy nhp MAI (Multiple Access Interference)Nhiu a truy nhp l nhiu do cc tn hiu ca cc ngi s dng giao thoa vi

nhau, l yu t nh hng trc tip n dung lng ca h thng.

Trong cc h thng a truy nhp:- TDMA: a truy nhp phn chia theo thi gian

Trong TDMA l s giao thoa ca cc tn hiu khe thi gian ny vi khe thi gian

khc do s khng hon ton ng b gy ra.Ngi ta phi c khong bo v (guard time) gim xc sut ngi dng b

giao thoa nhng cng ng thi lm gim hiu sut s dng ph.- FDMA: a truy nhp phn chia theo tn sCc hiu ng Doppler lm dch ph tn s dn n c s giao thoa gia cc di

tn con.Di tn bo v gim xc xut giao thoa gia cc knh k nhau dn n gim

hiu sut s dng ph.- CDMA: a truy nhp phn chia theo mTrong CDMA ngi ta s dng tnh trc giao ca m nn hu nh khng c

nhiu gia cc ngi s dng. kh MAI ngi ta thng dng cc phng php trc giao nhng trn thc t

khng th c s trc giao hon ton. Do , MAI vn tn ti trong cc h thng atruy nhp

3.2. Cc phng php gim nhiu trong thng tin v tuyn

3.2.1. Tm tt v l thuyt dung lng knh ca Shannon

Cc m sa li c th gip gim bt mt s li, nhng khng th pht hin ra ttc li trong h thng truyn k thut s. L thuyt ca Shannon xc nh rng victruyn khng li l c th xy ra min l dung lng ca thit b truyn khng vtqu dung lng ca knh truyn.

Shannon ch ra rng dung lng ca mt knh truyn c nhiu c cho bicng thc sau:

C=B.log2(1+S/N)Trong :C l dung lng ca knh truyn (Kbps)

B l bng thngS l bin ca tn hiu thit b pht

N l bin ca nhiu nhn ch


32/86



Vout(t)

tT0 2T0-T0-2T0

V d 3.2.1.1:Knh in thoi vi bng thng B=3 kHz v S/N=1000 dung lng ca knh

truyn theo Shannon s l:C = 3000 . log 2 (1 + 1000) = 30 kbps

3.2.2. nh l gii hn bng thng Nyquist.

gim nh hng ca ISI, knh truyn dn cn phi c di thng rng, tuynhin khi tp m s tng ln do nhit, giao thoa tn s v tuyn. Do di thngca thit b cn phi c thit k tt v rng, dc chng cc can nhiu, tuynhin phi t mt gi tr ti u khng to ra ISI do gii hn di lm xung daong ko di v dc tng to ra tr nhm ln.

Nu ta a mt xung Dirc (t) vo b lc thng thp l tng th p ng nhnc ti u ra c dng nh trn hnh 3.12

Ngoi tr gi tr nh ti trung tm, cc gi tr khc u bng khng mi thiim nT0, trong n l s nguyn dng khc khng . Khong cch T0 c gi lkhong cch Nyquist.

p ng ny c nh ngha l tn hiu khng c ISI. K hiu tn hiu khngc ISI l VN(t) v thi im ly mu bng nT0 (trong n l s nguyn v T0 l chu kly mu) khi nh ngha c biu din di dng ton hc l:

)().0()().( 0 tVnTttV Nn

N

c tnh trit tiu cc xung ca xung khng c ISI c m t trn hnh 3.13.Cc tn hiu nh vy s trit tiu mi xung ca hm ly mu ngoi tr ti mt im.

Hnh 3.12 p ng xung


33/86



Vout(t)

tT0 2T0-T0-2T0Hnh 3.14

(t-nT0)

t0 T0 2T0-T0-2T0VN(t)

tT0 2T0-T0-2T0VN(0).(t)

t0 T0 2T0-T0-2T0

=

Nu ta pht i mt dy xung irc (t) gm cc xung kim cch nhau mtkhong Nyquist, th c th trnh c nhiu giao thoa gia cc k hiu, tn hiu nhnc nh trn hnh 3.14.

Nu khong cch gia cc xung irc T nh hn khong cch Nyquist T0, ths chng ln ca cc xung ny lm ta khng th phn bit c chng. Nimt cch khc rng bng tn cn thit phn bit cc xung (cc k hiu)c tc k hiu Rs= 1/T vi 2f0 = 1/T th :

20

sRf

y chnh l nh l gii hn bng thng Nyquist.

3.2.3. Gim nhiu xuyn k hiu ISI s dng cc phng php lcTrong cc h thng n sng mang, ISI l mt vn kh nan gii. L do l

rng bng tn t l nghch vi khong thi gian k hiu, do vy, nu mun tng tc truyn d liu trong cc h thng ny, tc l gim khong k hiu, v hnh chung lm tng mc tri tr tng i. Lc ny h thng rt nhy vi tri tr. V vic thmkhong bo v kh trit tiu ht ISI.

gim nhiu xuyn k hiu ngi ta phi lm th no hn ch di thng m vn

khng gy ra ISI. Khi di thng b gii hn, xung s c nh trn thay v nh phng.

Hnh 3.13


34/86



-fN +fN f

)(0 fH

0

f0

RO(f)

f0

)(fH

fHnh 3.15 Minh ha nh l i xng ca Nyquist

+

=

nh l Nyquist 1 nh ngha iu kin i xng trn H(f) n cn phi c thamn hiu r p ng xung di gc khng c ISI.

nh l Nyquist 1:Nu mt b lc c modul hm truyn l tng modul hm truyn b lc l tng

v mt hm un i xng tm ti fN

th p ng xung ca b lc s tha mn tiuchun Nyquist.

Minh ha nh l ny trn hnh 3.15. nh l ny c th khi qut ha cho cc blc c p ng xung thng thp khng c ISI.

)()()( 0 fROfHfH

Trong : )(0 fH l modul hm truyn ca b lc thng thp l tng.

RO(f) l hm un i xng tm ti fN.f gi l lng m rng ph.

Nf

f gi l lng m rng ph tng i.

Hm un c th l cung trn hay ng cong bt k, d tnh ton ngi ta coil mt on hm cos. Trong thc t k thut, ngi ta thng tnh ton vi sn ca

b lc l mt ng cos c nng ln. Khi b lc c gi l b lc cosin nng.p ng bin ca b lc cosin nng c dng sau:

)(f0

)()(f2

cos12

1

)(f1

)( N0

ff

fffffff

ff

fH

N

N

N

N


35/86



V l thuyt 1,0 nhng trn thc t th 75.0,2.0

cng ln th b lc cng d ch to nhng tn bng thng, nh tit kim cbng thng nhng kh ch to.

Khi =1 b lc c gi l cosin nng y v trong trng hp ny p ng bin s tr nn n gin hn:

N

NN

N

NN

f

ff

f

f

ff

f

fH

2f0

2f4

cos

2f0

2f2

cos12

1

)(2

Bin i Fourier ngc ca fH ta c p ng xung ca b lc cosin nng

y:

241.22cos.2sin

2)(tftf

tftffth

NN

NNN

Khi ny rng bng tn cn thit phn bit cc xung hay rng bng tnNyquist c xc nh theo cng thc sau:BN = fN(1+ ) = Rs(1+ )/2

Trong Rs l tc truyn dn cc k hiu.


36/86



BPF

Mixer

LO

BPF

PA

My pht

Knh v tuy n

BPF BPF

LNA

BPF

Mixer

LO1

LPF

Mixer

ADC

Amp

LPF ADC

Amp

900LO2

DSP

Knh v tuyn

Hnh 4.1 S khi ca mt h thng v tuyn

CHNG 4. KIN TRC H THNG THNG TIN V TUYN

4.1. Cc vn c bn trong thit k cc h thng v tuynKhi phn tch v thit k h thng my thu-pht v tuyn (transceiver), cc tham

s sau cn c phn tch v tnh ton:- Cc tham s tuyn tnh- Cc tham s v tp m- Cc tham s v cng sut- Cc tham s phi tuyn

T hp cc tham s ny cho ta bit s nh hng ca tn hiu v tp m i vitnh nng ca h thng.

Hnh 4.1 trnh by s khi ca mt h thng v tuyn.

Chi tit chc nng v nguyn l hot ng ca cc khi trong my pht v mythu s c trnh by trong phn sau. Trong phn ny, gi thit my pht nh mt

ngun pht tn hiu l tng. Hn ch ch yu khi tn hiu thu yu l mc tp mnhit thu bi ngten v to ra chnh trong my thu. Ba thng s c trng cho hintng ny l : h s tp m ca my thu, nhit tp m h thng v nhy my thu.Trong trng hp tn hiu thu v nhiu mnh hn ch l do my thu c cc phn t phituyn nh cc b khuch i, cc b trn tn, b chuyn i tng t s. B trn tntrong my thu s to ra cc tn s mi c th hoc trong bng tn s dng v do slm suy gim t s tn hiu trn tp m v do gim nhy my thu.

4.1.1. Nhit tp m h thng v h s tp m

Tp m y c coi nh l tp m trng Gaussian c c trng bi qutrnh ngu nhin Gaussian c tr s trung bnh bng khng v c mt ph cng sut


37/86



G,rxN

P , BoutN

P

Ngun tp m Ta

Z0My thu

Hnh 4.2 Ngun tp m anten v my thu

khng i bng N0/2 (W/Hz). Gi s cc tp m (tp m kh quyn, mt tri, thin h)thu bi anten l tp m nhit , thm ch mt ph cng sut ca chng khng phi lmt hng s khng i. Tp m ny nh hng n tn hiu theo cch cng (AWGN).Lin quan n anten ta c:

22

0 akTN

Vi k = 1,38.10-23J/K l hng s BoltzmannTa l nhit tp m anten (K : Kelvin)Trong trng hp anten v my thu c phi hp tr khng trong mt bng

thng hiu dng ca tp m B. Cng sut ti my thu s l :PN = N0.B = k.Ta.B

Tp m nn c nh ngha bi cng sut ny. i vi mt ngun tp m nhit chun 290K, cng sut tp m nn u vo my thu biu din theo n vdBm s l :

PN(dBm) = -174 + 10lg10B.Trong mt h thng gm mt anten v mt my thu c khuch i cng sut

G, tp m ca my thurxN

P v bng thng hiu dng tp m B nh trn hnh 4.2.

Cng sut tp m nn u ra my thu s l :

rxout NaN PBTkGP ...

seaN

aN TBkGTTBkGBkGPTBkGP rxout ...)(....

..

Trong :GKB

PT rxNe c gi l nhit tp m hiu dng h thng.

Ta + Te = Ts c gi l nhit tp m ca h thng chun ha u vo my thu.Mt tham s khc xc nh mc tp m gy ra trong mt bn cc (mi tng

trong my thu c th coi l mt bn cc) l h s tp m F. H s tp m F c xcnh l t s gia cng sut tp m nn u ra v cng sut tp m u ra ca bncc phn hi khng nhiu vi gi thit rng ngun tp m u vo c nhit tp m


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G1,1e

T , F1

Ta

Z0Tng 1

Hnh 4.3 H s tp m ca my thu gm n tng mc ni tip

G2,2e

T , F2

Tng 2

Gn,ne

T , Fn

Tng n

.

u vo chun ha

chun T0 (thng ly 290K). H s tp m F cng c th c xem nh l s suy gimt s tn hiu-tp m (S/N) gia u vo v u ra ca bn cc.

00

0 1TT

BGkT

PBGkTF eNrx

Hay Te = T0(F-1)

Trong trng hp mt my thu gm n tng (mi tng coi nh mt bn cc)mc ni tip nh trn hnh 4.3.

H s tp m ca h thng c tnh theo cng thc sau :

1211

21

....

1...

1

n

n

GGG

F

G

FFF

Trong : Gi l h s khuch i ca tng th iTei l nhit tp m hiu dng ca tng th i.

Fi h s tp m ca tng th i.Do nhit tp m hiu dng c th vit di dng :

1211 .......2

1

n

ee

eeGGG

T

G

TTT n

Ngoi ra, h s tp m ca mt bn cc suy gim bng chnh h s suy gim L :F = L

4.1.2. nhy thu

nhy ca my thu l cng sut ti thiu u vo my thu cho php t mtmc t s sng mang tp m (C/N) nht nh u vo b gii iu ch hoc t stn hiu tp m (S/N hay SNR) u ra vi mt mc t s li bit BER khi phngthc iu ch c la chn. Hay ni cch khc nhy ca my thu c xc nh lmc tn hiu thu nh nht m h thng c th thu c vi mt t s tn hiu nhiuchp nhn c.

tnh ton nhy thu, theo nh ngha h s tp m ta c :

out

RSsig

out

in

SNR

PP

SNRSNR

F/

Trong : SNRin l t s tn hiu tp m u vo.SNRout l t s tn hiu tp m u ra.


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Psig l cng sut tn hiu vo trn 1 n v bng thng (1 Hz)PRS l cng sut tp m gy ra bi in tr ngun trn 1 Hz

nhit phng, PRS c tnh nh sau :PRS(dBm) = k.T0 = -174dBm/Hz

Do : Psig = PRS.F.SNRout

Khi xt trn ton b bng thng B, ta c :Psig.total = PRS.F.SNRout.B

Phng trnh trn d on nhy thu l mc tn hiu ti thiu a vo mythu m vn m bo u ra mt t s tn hiu tp m nht nh.

Biu din bng n v dBm, ta c :BdBSNRdBFHzdBmPdBmP RSin lg10)()()/()( minmin

y,minin

P l cng sut tn hiu u vo ti thiu t c t s SNRmin v

B l bng thng (Hz).

Vy : minlg10174min SNRBFPin Ba thnh phn u tin v phi ca thnh phn trn l tng ca tp m tch

hp ca h thng v thng c gi l tp m nn. Ngoi ra ta cn thyminin

P khng

ph thuc vo khuch i ca h thng.Khi nhit ca ngun tp m khng phi l T0, nhy thu c trn nhit

tp m ca h thng Ts.

4.1.3. Cc hin tng phi tuynMt h thng l tuyn tnh nu u ra ca n c th c biu din l mt s

kt hp tuyn tnh ca cc kch thch t u vo.Gi s,

21, inin VV l hai u vo, 21 , outout VV l hai u ra tng ng.

Khi , nu u vo l )()(21

tbVtaV inin th u ra tng ng s l

)()(21

tbVtaV outout . Trong a, b l cc hng s.

Bt k h thng no khng tha mn iu kin ny l h thng phi tuyn.My pht v my thu v tuyn c cha cc phn t tuyn tnh v phi tuyn, cc

phn t phi tuyn s gy ra cc hin tng lm suy gim cc tnh nng ca h thng

hoc s lm thay i cc tham s no ca h thng hoc to ra cc thnh phn tnhiu k sinh nh cc hi, Hin tng ny xy ra trong cc phn t tch cc nhtransistor, diode, b chuyn mch, b trn tn, b khuch i, Ngoi ra, hin tng

phi tuyn cng c th gp phi trong cc thit b tuyn tnh do vic gii hn bng tnlm cho cc tn hiu b mo.

4.2. Cc chun v tuyn

4.2.1. Cc h thng thng tin di ng th h th nht (1G- 1Generation )Cc h thng thng tin di ng th h u tin trn th gii gm :

- H thng AMPS (Avanced Mobile Phone Service) : H thng dch v inthoi di ng tin tin.


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- H thng TACS (Total Access Communication System) : H thng thng tintruy cp tng th.

- H thng NMT (Nordic Mobile Telephone) : H thng thng tin di ng Bcu

Cc thng s k thut chnh ca cc h thng ny nh sau :

- H thng AMPS :Di tn : Di tn thu : 869 894 MHz

Di tn pht : 824 849 MHz.Phng php truy nhp knh : FDMAPhng php iu ch : iu tn FM. rng knh v tuyn : 30kHz.S lng knh : 832Phng php ghp knh : FDD.

- H thng TACS :H thng ny c hai loi : ETACS v NTACSDi tn : Di tn thu : 916-949 MHz v 860-870 MHz

Di tn pht : 871-904 MHz v 915-925 MHz.Phng php truy nhp knh : FDMAPhng php iu ch : iu tn FM. rng knh v tuyn : 25kHz v 12,5kHz.S lng knh : 1000 v 400.Phng php ghp knh : FDD.

- H thng NMT :Trong c chun : NMT 400 v NMT 900Di tn : Di tn thu : 463-486 MHz v 935-960 MHzDi tn pht : 453-458 MHz v 890-915 MHz.Phng php truy nhp knh : FDMAPhng php iu ch : iu tn FM. rng knh v tuyn : 25kHz v 12,5kHz.S lng knh : 200 v 1999.Phng php ghp knh : FDD.

Cc h thng thng tin di ng th h th nht khng cn ng dng trn thtrng do nhiu hn ch ca chng nh gi cc thit b u cui ca h thng cao vhn ch v dung lng ph. Hn ch chnh l do s dng phng php iu ch tngt FM trong h thng hn ch ng k s lng ngi s dng vi cc h thngdng phng php iu ch s.

Hu ht cc h thng u l h thng analog v yu cu chuyn d liu ch yul m thanh. Vi h thng ny, cuc gi c th b nghe trm bi bn th ba. Nhngim yu ca th h 1G l dung lng thp, xc sut rt cuc gi cao, kh nngchuyn cuc gi khng tin cy, cht lng m thanh km, khng c ch bomtdo vy h thng 1G khng th p ng c nhu cu s dng .


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4.2.2. Cc h thng thng tin di ng th h th hai (2G)Cc h thng thng tin di ng th h th hai c nhiu u im vt tri so vi

cc h thng thng tin di ng tng t 1G. Cc u im vt tri l vic ng dngcc cng ngh s cho php ci thin cht lng thng tin v tng ng k s lngngi s dng.

- H thng hay chun GSM.Nm 1992, h thng thng tin di ng t bo th h th 2 xut hin Chu u

l h thng GSM (Global System for Mobile Communication) hay cn gi l thngtin di ng ton cu. y l h thng thng tin Chu u u tin s dng cc kthut iu ch s. y ch gii thiu cc thng s k thut c trng c bn cachun GSM.

Di tn: Di tn thu: 935-960 MHzDi tn pht: 890-915 MHz

Phng php truy nhp knh: TDMA v FDMAPhng php iu ch: GMSK 1bit/symbol (1bit/1 k hiu). (kha dch pha cc tiu) rng knh v tuyn: 200kHz.S lng knh: 124, mi knh cho 8 ngi s dng.Phng php ghp knh: FDD.M ha thoi: 13kbps.Tc bit: 270833Kbps.B lc s dng : B lc Gauss h s 0,3.Vi s tng trng v s lng ngi s dng v c bit l s pht trin ca cc

dch v i hi tc cao nh internet tc cao, in thoi hnhtrong khi yucu kt ni khng ch kt ni gia cc thu bao di ng vi nhau m cn kt ni t ccthu bao n cc nh cung cp dch v internet, cc cng tytrong ln nht l ccnh cung cp dch v internet, cc mng internet s i hi mt mng thng tin ding tc cao. Ngoi ra, vi d liu, mng phi p ng c tc ty theo yucu ca ngi s dng v vic thanh ton cc ph phi c tnh theo s lng thngtin. Tuy nhin, mng GSM da trn chuyn mch knh nguyn thy kt ni khngth p ng cc dch v i hi tc cao nh trn. tng tc bit, nhiu gii php c a ra cho mng thng tin di ng GSM nh h thng dch v v tuyn gi

chung GPRS (General Packet Radio Services) trn nn tng ca GSM hoc h thngEDGE (Enhanced Data Rate for GSM Evolution). Cc h thng ny cn c gi lh thng 2G+ hoc 2,5G.- H thng DCS :

H thng thng tin di ng t bo th h th hai, h thng DCS (DigitalCommunication System : H thng thng tin s), xut hin Chu u nm 1993. Hthng ny c cc thng s k thut chnh ging nh h thng GSM nhng tn s1800 MHz. Do mt s nc Chu u ngi ta cn gi chun ny l GSM 1800.Cc thng s k thut c trng nh sau:

Di tn : Di tn thu: 1805-1880 MHz


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Di tn pht: 1710-1785 MHzPhng php truy nhp knh: TDMA v FDMAPhng php iu ch: GMSK rng knh v tuyn: 200kHz.S lng knh: 1600, mi knh cho 3 ngi s dng.

Phng php ghp knh: FDD.M ha thoi: 13kbps.Tc bit c s: 270833Kbps.B lc s dng: B lc Gauss h s 0,3.

- H thng DECT:Nm 1993, cng ti Chu u xut hin mt h thng in thoi khng dy th h

th hai, h thng DECT (Digital European Cordless Telephone : H thng in thoikhng dy s Chu u). H thng ny s dng phng php iu ch tn s GFSK(Gaussian Frequency Shift Keying), b lc s dng l b lc Gaussian c h s 0,5.Cc thng s k thut c trng c tm tt nh sau :

Di tn : 1880-1900 KHzPhng php truy nhp knh : TDMA v FDMAPhng php iu ch : GMSK rng knh v tuyn : 1,728 kHz.S lng knh : 100, mi knh cho 24 ngi s dng.Phng php ghp knh : TDD.M ha thoi : 32kbps.

Tc bit c s: 1152Mbps.B lc s dng : B lc Gauss h s 0,3.- H thng IS-54 :

Cng vo thi im 1992 Bc M, h thng in thoi t bo th h th 2 (IS-54 hoc NADC North American Digital Cellular : H thng in thoi v tuyn t bos Bc M) xut hin. Cng ging nh GSM Chu u, cc bc tin trin tip theoca h thng IS-54 cng c nghin cu v pht trin. H thng NADC s dng

phng thc iu ch pha4

DQPSK, b lc s dng l b lc cosin nng vi h s

truy cp bng h s roll-off bng 0.35. Cc thng s k thut chnh ca chun IS-54 :Di tn : Di tn thu : 869-894 MHz

Di tn pht : 824-849 MHzPhng php truy nhp knh : TDMA v FDMA

Phng php iu ch :4

DQPSK 2bit/1 symbol

rng knh v tuyn : 30kHz.S lng knh : 832.Phng php ghp knh : FDD.M ha thoi : 8kbps.


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Tc bit c s: 48,6Kbps.B lc s dng : B lc cosin nng.

- H thng IS-95 hay CDMA One :Cng nm 1992 Chu M, mt h thng in thoi t bo khc xut hin, l

h thng IS-95 hay CDMA one. Cc thng s k thut chnh ca IS-95.

Di tn : Di tn thu : 869-894 MHzDi tn pht : 824-849 MHz

Phng php truy nhp knh : CDMA v FDMAPhng php iu ch : QPSK 2bit/1 symbol rng knh v tuyn : 1250kHz.S lng knh : 20.Phng php ghp knh : FDD.M ha thoi : < 9,6kbps.B lc s dng : B lc cosin nng.

- H thng IS-136 :H thng IS-136 hay cn gi l h thng in thoi di ng s tin tin (Digital

Advanced Mobile Phone System) l s pht trin ca h thng tng t AMPS c ths dng bng tn 800 MHz ca chun IS-54 v bng tn 1900 MHz ca h thng PCS.Cc thng s k thut chnh ca IS-136 :

Di tn : Di tn thu : 869-894 MHzDi tn pht : 824-849 MHz

Phng php truy nhp knh : TDMA v FDMA

Phng php iu ch : 4 DQPSK

rng knh v tuyn : 30kHz.S lng knh : 832.Phng php ghp knh : FDD.M ha thoi : 8kbps.Tc bit c s : 48,6 kbps.B lc s dng : B lc cosin nng.

- H thng PHS :

y l mt chun khng dy th h th hai ca Nht Bn xut hin nm 1993.H thng PHS (Personal Handy Phone System : h thng in thoi v tuyn cm tay)

s dng iu ch4

DQPSK, b lc s dng l b lc cosin nng vi h s truy cp

bng l 0,5. Cc thng s k thut chnh ca PHS :Di tn : 1895-1918 MHz.Phng php truy nhp knh : TDMA v FDMA

Phng php iu ch :4

DQPSK

rng knh v tuyn : 30kHz.S lng knh : 300.


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Phng php ghp knh : TDD.M ha thoi : 32kbps.Tc bit c s : 384 kbps.B lc s dng : B lc cosin nng.

4.2.3. Cc h thng thng tin di ng th h 2.5GDo s tng trng v s lng ngi s dng v s pht trin mnh cc dch v

a phng tin nh Internet, in thoi hi ngh, i hi cc h thng thng tin ding c tc truyn dn cao. Tuy nhin, h thng thng tin di ng th h th 2khng cung cp tc bit ln p ng cc yu cu cao ny, do yu cu phinng cp v pht trin mng GSM hin hu. Ngi ta xut nng cp h thngGSM Chu u ln thnh cc dch v v tuyn gi chung GPRS (General Packet RadioServices) v h thng EDGE (Enhanced Data Rate for GSM Evolution). Cc h thng2,5G ny nh l s st nhp h thng 2G ln h thng thng tin di ng th h th 3

(3G). u im ca cc h thng ny l tn dng c c s vt cht sn c ca mngGSM, ch yu cu thay i rt t c th khai thc c.- H thng GPRS

Mt s pht trin u tin ca h thng GSM t c tc bit thch hp vicc ng dng yu cu tc bit trung bnh l vic da trn nn GSM sn c v thmmt lp trong th tc thng tin cho php vn chuyn cc d liu dng gi vi tc

bit khong 115kbit/s v gi nguyn kiu chuyn mch cho cc tc bit thp khongchc kbit/s. H thng ny c tn l GPRS. Bng vic s dng li cc tn s, khungtruyn dn v cc c s vt cht c sn ca mng GSM, ch c duy nht c thay i v

phn mm trong vic ci t h thng.Chun GPRS s dng khung truyn dn TDMA ca chun GSM. Ty theo s

lng bit thng tin cn truyn i, c th mt khe thi gian hoc nhiu khe thi gianc s dng chuyn cc gi tin. Do , kiu chuyn mch knh c s dng truyn thng tin c tc bit thp nh ting ni v kiu chuyn mch gi truynthng tin tc bit cao hn nh truyn cc gi d liu. GPRS kt hp cc khe thigian, to ra tc bit cao hn so vi GSM. GPRS cho php tng tc truyn d liuln 8 ln nu s dng c 8 khe thi gian ca tn s sng mang.

- H thng EDGEMt gii php k thut khc t c tc bt tng thch vi cc ng dng

truyn thng a phng tin tc cao l gii php EDGE (Enhanced EnhancedData Rate for GSM Evolution).Gii php ny s dng li cc c trng ph ca chunGSM cho php tc bit t ti 384kbit/s. Chun EDGE cho php t c tc bitny bng vic kt hp s dng phng thc iu ch pha 8PSK v s dng hay kthp nhiu khe thi gian trong qu trnh truyn dn nh trong mng GPRS thay v chs dng khe thi gian nh trong mng GSM. Bng vic c th s dng li hu ht ccc c vt cht ca mng GSM, gii php ny cho php gim gi thnh u t. Phng

thc iu ch 8PSK, c ngha l trong kt hp 3 bit cho mt k hiu s tng tc bit ln 3 ln so vi iu ch GMSK ca chun GSM vi cng mt di ph chim dng.


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Tc bit c s chuyn t 270kbps ln 810kbps hoc tc truyn s liu 384kbpscho chun EDGE thay v 9,6kbps cho chun GSM. Tuy nhin hiu sut cng sut ca

phng thc iu ch 8PSK trong mng EDGE ch bng 4/9 ca phng thc iu chGMSK trong mng GSM. Do cng sut ca my pht EDGE phi ln gp i sovi GSM, iu ny s nh hng ti vic ch to thit b u cui v cc trm thu pht

cng sut nh nh Micro BTS, pico BTS. khc phc kh khn v cng sut camy u cui ngi ta a ra gii php trong my u cui ng ln s phttn hiu s dng phng thc iu ch GMSK cn trm thu pht v tuyn gc BTS s

pht ng xung tn hiu iu ch 8PSK vi l do phn ln cc ng dng tc caou nm ng xung (nh truy cp internet, ng dng dch v a phng tin), gii

php ny nhm hn ch tnh phc tp cho my u cui.H thng EDGE tuy nhin vn da vo chuyn mch knh v chuyn mch gi

v hn ch tc 384kbps nn s kh khn cho vic ng dng cc dch v i hi vicchuyn mch linh ng hn v tc truyn d liu ln hn (khong 2Mbps), khi

buc phi s dng h thng thng tin di ng th ba hoc bn

4.2.4. Cc h thng thng tin di ng th h th ba (3G)Cc dch v v tuyn di ng th h th ba khng tng ng vi mt giao din

thng tin duy nht v ton cu, m tng ng vi s kt hp cc h thng c nhiugiao din vi nhau. S kt hp ny c gi l IMT2000 v n kt hp h thngUMTS (Universal Mobile Telephone System), h thng UWC-136 v CDMA 2000. M, pht trin h thng CDMA 2000 tng ng vi s tin trin ca h thng IS-95 v h thng UWC-136 l mt tin trin ca IS-136. Chu u, cc h thng c pht trin tng ng vi chun UMTS. Chun UMTS v CDMA 2000 s dng

phng php truy cp theo m CDMA, trong khi chun UWC-136 da trn phngphp truy cp theo thi gian TDMA.

Mc ch ca cc chun ny l cho php cc dch v truyn thng tin a phngtin tc cao qua mng internet v m ra mt mng thng tin chung trn ton thgii. Cc chun 3G phi tng thch vi cc h thng 2G v 2,5G. Do , k thutthng tin s dng bng tn kp hoc a bng c s dng.- H thng UMTS

UMTS l mt h thng c la chn cho cc h thng thng tin di ng th hth ba. UMTS c qun l bi lin on vin thng th gii ITU trong khun kh dn IMT-2000 ton cu. Nm 1998, d n UMTS m rng n cc i tc, cc tchc M, Nht v Hn Quc. D n ny c tn l GPP (Third Generation GroupProject Partnership). Vin cc tiu chun Vin thng Chu u ETSI (EuropeanTelecommunication Standards Institute) l c quan t chc chun ha cc giao din vtuyn ca h thng UMTS, cng vic thc hin bi nhm 3GPP. Mc ch caUMTS l pht trin vng ph sng tt nht, tng s lng thu bao, ci tin s hit gia in thoi c nh v di ng. UTRA (UMTS Terestrial Radio Access) l giao

din v tuyn ca UTMS. UTRA c hai kiu hay phng thc a truy cp : truy cptheo m bng rng W-CDMA (Wide Band Code Division Multiple Access) dnh cho


46/86



DAC LPF

DSP

DAC LPF

BPF BPF

PA

Match900

PLL

Hnh 4.4 Kin trc my pht trc tip

mode FDD v phng php phn b theo m trong min thi gian TD-CDMA (TimeDivision Code Division Multiple Access) cho mode TDD. Phng thc iu ch cs dng cho mi knh l QPSK. Ty theo nhu cu s dng ph v tc bit truyn, hs tri ph s thay i cho ph hp. K thut iu ch v tri ph c kt hp gi lHPSK. Cc thng s k thut chnh ca chun UMTS nh sau :

Di tn : Di tn thu : 1920-1980MHz v 1850 1910MHzDi tn pht : 2110-2170 MHz v 1930-1990MHz

Phng php truy nhp knh : WCDMA v TD-CDMAPhng php iu ch : QPSK rng knh v tuyn : 5MHz, 10MHZ v 20MHz.Phng php ghp knh : FDD.Tc bit c s : 144 kbps, 384kbps v 2048kbps.B lc s dng : B lc cosin nng.Tc bit d liu thay i ty theo vn tc di chuyn v h thng UMTS hot

ng theo phng thc truyn dn khng ng b. Tc d liu thay i v h thngcho php gim cc nh hng fading trong knh v tuyn.

4.3. Kin trc cc h thng v tuyn

4.3.1. H thng pht v tuynC hai kiu kin trc my pht l :- My pht trc tip hay cn gi l my pht khng qua trung tn- My pht hai tng hay cn gi l my pht c trung tn

a) My pht trc tip khng qua trung tnHnh 4.4 gii thiu s khi chc nng ca mt my pht trc tip.

Pha pht, c th l ting ni, d liu,c m ha v sp xp gim thiugiao thoa gia cc k hiu v hn ch b rng bng thng bi mt khi x l tn hius DSP (Digital Signal Processing). Sau khi qua b chuyn i s tng t DAC(Digital to Analog Converter) v b lc thng thp LPF (Low pass filter), cc tn hiu

bng tn c bn I (tn hiu ng pha) v Q (tn hiu lch pha 900) a n u vo cahai b trn tn cho php thc hin iu ch I/Q. Hai nhnh c nhn vi hai tn hiutrc giao n t b giao ng ni ( vng kha pha PLL : Phase Locked Loop). Sau

iu ch tn hiu c lc bi b lc thng di BPF (Band Pass Filter) v a n bkhuch i cng sut PA sau c lc li loi b cc hi bc cao gy ra do s


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DAC LPF

DSP

DAC LPF

BPF1 BPF2

PA

Match900

PLL1

Hnh 4.5 S khi chc nng my pht 2 tng

Sin1t

cos1t

PLL2

I

Q

phi tuyn ca b khuch i cng sut trc khi a ti h thng phi hp tr khngMatch (Matching) a ra ngten pht i.

Trong cu trc ny, tn s ca tn hiu cao tn pht bng vi tn s ca dao ngni trong iu ch, do my pht ny c tn l my pht trc tip.b) My pht c trung tn (my pht hai tng)

Vi my pht khng qua trung tn th c nhc im l nhiu gia b khuch icng sut v b to dao ng ni, v th my pht c trung tn thc hin trn tn hiu

bng tn c bn theo hai tng cho ph ca tn hiu u ra b PA khc xa vi cctn s ca b dao ng ni.

Hnh 4.5 gii thiu s khi chc nng ca my pht hai tng.

B iu ch I-Q chuyn tn hiu bng tn c bn (BF : Base Frequency) ln tns trung gian hay cn gi l trung tn (IF : Intermedia Frequency) f1, sau qua b lcthng di th nht BPF1 v c a n khi nng tn th hai, f2. Tn hiu iu chsau khi nng tn ny v sau khi qua b lc thng di th 2 BPF2 c tn s l f1+f2. B

lc thng di th nht BPF1 loi b cc hi ca tn hiu tn s trung tn, trong khib lc thng di th hai loi b tn hiu tn s f1-f2.Tng u tin thc hin vic iu ch v nng tn ln tn s trung tn IF. Tn s

trung tm thp hn tn s cao tn pht, do my pht hai tng c nhng u im sovi my pht trc tip nh sau :

- S mt cn bng v bin v pha gia hai tn hiu I v Q nh hn nhiu sovi my pht trc tip, do giao thoa gia hai tn hiu I v Q nh.

- B lc knh c th s dng trung tn thay v cao tn hn ch tp m vcc tn hiu k sinh pht trong cc knh ln cn.

Kh khn gp phi trong kiu my pht ny l vic thit k b lc thng di thhai. B lc ny phi loi b cc bng tn khng mang mun vi mt h s lc cao.

4.3.2. Kin trc my thua) Gii thiu chung

Vai tr ca my thu l nhn t ngten mt tn hiu iu ch cao tn sau htn v lc trc khi a vo b iu ch tn hiu. Hnh 4.6 gii thiu s khi chcnng ca mt my thu s.


48/86



ADC

DSP

ADC

BPF BPF

LNA

900

PLL2

Hnh 4.6 S khi chc nng ca my thu s

PLL1

BPF

BPF

BPF

Amp

Amp

LPF

Amp

LPFAmp

RF

IF

Tn hiu iu ch cao tn thu bi ngten c a qua b lc thng di BPF, bkhuch i tp m thp LNA (Low Noise Amplifier), sau li c lc v c htn ln th nht xung trung tn thng qua b trn tn v b to dao ng ni th nhtPLL1. Tn hiu trung tn c tch ra hai nhnh v a vo b gii iu ch I/Q. Tnhiu bng tn c bn sau b gii iu ch I/Q c khuch i v chuyn i ttng t sang s nh b chuyn i tng t - s ADC. Sau vic lc, khuch i v

khi phc li tn hiu pht ban u tn s c bn c thc hin thng qua b x ltn hiu s.

My thu khng ch thu tn hiu hu ch m cn thu c nhiu gy ra bi chnh hthng hoc bi cc h thng ln cn. Do my thu phi lc tt cc tn hiu ny cho b gii iu ch c th duy tr tnh nng yu cu. Bt k my thu no, b lc utin hay cn gi l b lc chn la trc c vai tr hn ch bng tn thu u vo mythu nm trong bng tn thu ca h thng. V d trong h thng GSM l t 925MHzn 960MHz. Cht lng ca b lc ny s quyt nh mc lc cc nhiu ngoi

bng. Cc b lc khc s c tnh ton v thit k theo chc nng lc cc tn hiu huch v theo mc cc nhiu trong bng tn thu. Khi my thu hot ng, bn thn ncng c th gy ra cc nhiu, cc nhiu ny s c a ti ngten.

Trong tt c cc chun v tuyn, cc tn hiu pht k sinh c kim sot nkhng gy nhiu cc h thng khc. Ngun chnh gy ra nhiu cc nhiu k sinh nyn t b dao ng ni. Cc nhiu k sinh khc l cc tn hiu c tn s l bi lnca xung ng h s dng. Cc nhiu s lm gim nhy ca my thu. Do mythu phi lc c knh mong mun v loi b cc nhiu ln cn. Tuy nhin nhiu khithc hin khng n gin do yu cu cc b lc c h s phm cht rt cao trong khi

li khng mun suy hao ln. Thng ta phi dung ha gia hai tiu ch l vic loi bcc nhiu ngoi bng bng cch tng h s phm cht ca b lc v suy hao trong

bng. Ngoi ra cn phn bit gia bng tn v knh. Bng tn bao gm ton b di tnm mt chun hay h thng no c cp php. V d my thu GSM c bng tn t935MHz n 960MHz. Trong khi knh cp n bng thng ca tn hiu ca duynht mt ngi s dng trong h thng. V d trong GSM, knh c b rng 200kHz.Do cn ch phn bit la chn bng tn v la chn knh. La chn bng tn lthc hin vic loi b cc nhiu ngoi bng v la chn knh l loi b cc nhiungoi knh. Thng b lc thng di u tin dng lc bng tn cn la chn knhs do cc khi tip theo trong my thu thc hin.


49/86



ADC

DSP

ADC

BPF BPF

LNA

Hnh 4.7 Kin trc my thu Heterodyne

LO1

BPF

LPF

LPF

I/QDemod

LO2

I

Q

fRF f

fIF = fLO1 - fRFfIF = fRF fLO1

FBF = fLO2 fIFFBF = fIF fLO2

ffIF fRF ffBF fIF

Mt im quan trng khc khi thit k my thu pht v tuyn l di ng catn hiu. Cng vi fading a ng v suy gim ng truyn, di ng yu cu catn hiu thu thng ln hn 100dB.

Cc kiu kin trc my thu vn ang c nghin cu v thit k nhm tng tnhnng cng nh cc thng s k thut, cc ch tiu kinh t khc,C th phn loi cc

kin trc my thu nh sau :- My thu to phch hay my thu heterodyne (heterodyne receiver)- My thu loi b tn s nh (Image-reject receiver)- My thu gii iu ch tn hiu trc tip hay my thu homodyne (homodyne

receiver)- My thu trung tn s (Digital IF receiver)

b) My thu HeterodyneTn hiu thu t anten thu qua b lc thng di v khuch i tp m thp LNA

c chuyn xung tn s trung tn lc sau tip tc c h xung tn s c bnv tch thnh hai tn hiu trc giao I v Q. Cc tn hiu I v Q c a qua b lcthng thp loi b thnh phn tn s cao, sau c ly mu chuyn i t tnhiu tng t sang tn hiu s nh hai b chuyn i tng t - s ADC. Cc tn hiul cc mu ri rc c a ti khi x l tn hiu s thc hin cc chc nng x lmong mun nh khi phc d liu, lc,

Kin trc my thu ny c s dng nhiu v n c nhiu u im v mt kthut. Tuy nhin kin trc my thu ny gp phi vn v vic loi b tn s nh. Thtvy tn s nh c mt u vo ca b trn tn th nht c trn vi tn s ca bdao ng ni th nht cng cho ra mt tn hiu c cng vi tn s trung tn. Do cn

phi loi b tn s nh ny trc khi i qua b trn tn. Nh trong hnh 4.3.2.2, mt blc thng di la chn trc v mt b lc tn s nh thng c s dng trc v

sau b khuch i tp m thp.


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f

BPF

LNA

Hnh 4.8 Kin trc my thu Hartley

LPF

LPF

LORF input

900

SinLO(t)

CosLO(t)

900

C

ffLO fRFfimage IfIF

Q

Q

fIF

QA

IB

Thng thng cc b lc ny l cc b lc sng m b mt c kch thc ln, githnh cao v kh tch hp, do s tng kch thc cng nh gi thnh ca h thng.

Ngoi ra, cc b lc ny thng khng c cng tr khng vi b khuch i tp mthp, do cn thm cc phn t th ng thc hin vic phi hp tr khng. Dovy, ngi ta cn tm cc gii php loi b tn s nh m khng s dng cc b lc

ny.c) My thu loi b tn s nh Kin trc my thu Hartley

Nguyn l ca my thu Hartley da trn vic gii iu ch tn hiu I v Q. Nhtrn hnh 4.8, tn hiu cao tn RF u vo c trn vi hai tn hiu cu phng ca bdao ng ni LO. Mi nhnh sau c lc bi b lc thng thp, sau mt tronghai nhnh c di pha 900 trc khi chng c a qua b cng.

Gi thit tn hiu thu c dng : )cos(.)( tAtr RFRF

u ra ca b tn tn : ).cos(.)..cos(. tAtAI LOLORFRF

).sin(.)..cos(. tAtAQ LOLORFRF

Sau lc loi b thnh phn tn s fRF+fLO ta c :

).cos(.2

.t

AAI IF

LORF v ).sin(.2

.t

AAQ IF

LORF

Nu nh u vo b trn tn, ta c mt tn hiu tn s nh c dng :)cos(.)( tAtr imageimageimage

Vi tn s nh fimage = fLO fIF, u ra ca b trn ta

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