TRNG I HC BCH KHOA H NIKHOA IN T - VIN THNG NTT NGHIP I HC ti:ng
b trong h thng OFDM v OFDMA.Xy dng chng trnh C truyn d liu gia hai
my tnh bng k thut OFDMSinh vin thc hin:NGUYN NG QUANGLp T5 K50Ging
vin hng dn:THS. NGUYN QUC KHNGH Ni, 5-2010B GIO DC V O TO CNG HA X
HI CH NGHA VIT NAMTRNG I HC BCH KHOA H
NI--------------------------------------------------c lp - T do -
Hnh phc---------------------------------NHIM V N TT NGHIPH v tn
sinh vin:..... S hiu sinh vin: Kho:.Khoa: in t - Vin thngNgnh:
.........1. u n:.......2. Cc s liu v d liuban u:............3. Ni
dung cc phn thuyt minh v tnh ton:.........4. Cc bn v, th ( ghi r cc
loi v kch thc bn v ):.......5. H tn ging vinhng dn: ..6. Ngy giao
nhim v n: .7. Ngy hon thnh n: ..Ngythng nmCh nhim B mn Ging vin hng
dnSinh vin hon thnh v np n tt nghip ngythng nmCn b phn binB GIO DC
V O TOTRNG I HC BCH KHOA H
NI---------------------------------------------------BN NHN XT N TT
NGHIPH v tn sinh
vin:.......................................................................
S hiu sinh
vin:...........................Ngnh:..................................................................................................Kho:....................................................Ging
vin hng
dn:..............................................................................................................................................Cn
b phn
bin:.......................................................................................................................................1.
Ni dung thit k tt
nghip:........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................2.
Nhn xt ca cn b phn
bin:........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................Ngy
thng nmCn b phn bin(K, ghi r h v tn )LI NI UNgy nay, nhu cu truyn
thng khng dy ngy cng tng, c bit l h thng thng tin di ng do tnh linh
hot, mm do, di ng v tin li ca n. Cc h thng thng tin v tuyn hin ti v
tng lai ngy cng i hi c dung lng cao hn, tin cy tt hn, s dng bng
thng hiu qu hn, kh nng khng nhiu tt hn. H thng thng tin truyn thng
v cc phng thc ghp knh c khng cn kh nng p ng c cc yu cu ca h thng
tng lai. Ph tn l mt ti nguyn v cng quan trng trong thng tin v tuyn.
S dng trit ph tn l vn cp thit. Mt gii php c a ra l vic s dng k thut
ghp knh a sng mang trc giao OFDM cng k thut a truy nhp cc sng mang
trc giao OFDMA vo truyn thng v tuyn, gp phn to nn h thng thng tin v
tuyn hon thin hn. OFDM l gii php cng ngh khc phc nhc im v v hiu qu
s dng ph tn thp ca cc h thng thng tin di ng trc y. OFDM s dng k
thut to ra cc sng mang con trc giao truyn d liu, gip cho vic s dng
bng tn knh ti u.Trong n ny, chng ta s tm hiu v k thut OFDM, k thut
a truy nhp OFDMA v ng dng cc k thut cho vic to ra h thng thng tin v
tuyn c nhiu u im hn so vi cc h thng c.Vi kin thc c bn tip thu c
trong qu trnh hc tp ti trng i hc Bch KhoaHNi cngvi snhhng,
gipcathygioThs. NguynQuc Khng, em chn ti: Nghin cu k thut iu ch a
sng mang trc giao OFDM v k thut a truy nhp OFDMA - vn ng b trong
hai k thut ny.Em xin chn thnh cm n cc thy c gio trng i hc Bch Khoa
H Ni dy d, ch bo em trong qu trnh hc tp trng. c bit em xin chn thnh
cm n thy gio Ths. Nguyn Quc Khng tn tnh gip v ch bo trong qu trnh
thc hin n ny.Vi thi gian v kin thc cn hn hp nn n khng trnh khi tn
ti nhiu thiu st. Em mong s nhn c s ch bo, gp ca thy c v cc bn. Mong
rng ti ny s c hon thin hn na.4Nguyn ng Quang _ T5-K50TM TT NTrong n
ny, chng ta s tm hiu v h thng thng tin v tuyn ni chung v h thng s
dng k thut iu ch a sng mang trc giao OFDM, k thut a truy nhp s dng
a sng mang trc giao l OFDMA ni ring. Ta s tm hiu v m hnh h thng
OFDM, to v thu tn hiu OFDM, phn tch nh hng ca nhiu ln h thng v
tuyn, hiu qu ca vic s dng ph tn trong h thng OFDM, c bit l vn ng b
trong h thng Qua xy dng m hnh truyn OFDMA qua card m thanh ca my
tnh. C th n c chia lm 5 chng nh sau: Chng 1: Nhng vn chung.Chng 2:
Nguyn l c bn k thut OFDM.Chng 3: K thut ng b trong OFDM.Chng 4: K
thut ng b trong OFDMA.Chng 5: Xy dng m hnh truyn OFDM qua card m
thanh ca my tnh v kt qu.AbstractIn this thesis, we will consider
about wireless information system and wireless
systemwhichusesOrthogonal Frequency Division Multiplexing (OFDM)
and Orthogonal Frequency Division Multiplexing Access (OFDMA) in
particular. We will gointodetailsforOFDMsystemmodel,
creatingandreceivingOFDMsignal for transmit and receiver it,
analyzing the effect of noise to wireless system, the efficiency of
spectrumusinginOFDMsystem,especiallythe synchronizationproblemin
OFDM, OFDMAThereby, well buildamodel of OFDMAsystemwiththe
computer's sound card. Project is divided into 5 chapters as
follows: Chapter 1: The general knowledge.Chapter 2: The basic
characteristic of OFDM.Chapter 3: The synchronization in
OFDM.Chapter 4: The synchronization in OFDMA.Chapter 5: Building a
model of transmission and receiving data through the computers
sound card with OFDM and results.5Nguyn ng Quang _ T5-K50MC LCLI NI
U...............................................................................................................4TM
TT
N........................................................................................................5MC
LC.....................................................................................................................6Chng
1. Nhng vn
chung..................................................................................141.1.Gii
thiu v mng thng tin v tuyn
..............................................................151.2.Cc
h thng thng tin v
tuyn.........................................................................171.3.Cc
hin tng in hnh trong lnh vc truyn thng v
tuyn........................171.4.Suy hao ng truyn v
tuyn..........................................................................201.4.1.Suy
hao truyn dn
.....................................................................................201.4.2.Hin
tng tr a ng (Multipath
fading)...............................................22Chng 2.
Nguyn l c bn k thut
OFDM.............................................................262.1.Tng
quan v
OFDM.........................................................................................262.1.1.K
thut iu ch n sng
mang...............................................................262.1.2.Phng
php iu ch a sng mang
FDM.................................................272.1.3.Phng
php iu ch a sng mang trc giao
OFDM...............................282.2.S trc giao trong OFDM
(ORTHOGONAL)
..................................................312.2.1.S trc
giao trong min thi gian ca tn hiu
OFDM................................322.2.2.S trc giao trong min tn
s ca tn hiu OFDM.....................................342.3.M hnh h
thng OFDM
..................................................................................382.4.Cc
nhn t nh hng ln h thng v cch khc
phc....................................402.4.1.Nhiu ISI v cch khc
phc.......................................................................402.4.2.Nhiu
ICI v cch khc
phc......................................................................432.4.3.Ci
thin hiu nng h thng trn c s s dng m
Gray..........................442.4.4.Nng cao hiu qu s dng ph
tn.............................................................466Nguyn
ng Quang _ T5-K50Chng 3. K thut ng b trong
OFDM..................................................................513.1.M
u
..............................................................................................................513.2.Tng
quan v ng b trong
OFDM..................................................................523.2.1.Nhn
bit
khung..........................................................................................533.2.2.c
lng v b khong dch tn s
FOE..................................................563.2.3.Bm ui
li thng d
................................................................................583.3.ng
b k t
....................................................................................................593.3.1.Li
thi gian v thc hin ng
b..............................................................603.4.ng
b tn
s...................................................................................................703.4.1.ng
b tn s ly
mu...............................................................................723.4.2.ng
b tn s sng
mang..........................................................................733.5.nh
hng ca sai li ng b ti ch tiu cht lng h
thng........................813.6.Kt
lun..............................................................................................................83Chng
4. K thut ng b trong
OFDMA...............................................................844.1.M
u
..............................................................................................................844.2.C
bn v k thut
OFDMA..............................................................................844.2.1.Cc
giao thc
OFDMA...............................................................................854.2.2.Cu
trc k hiu OFDMA v phn knh
con...............................................874.2.3.c
im.....................................................................................................904.2.4.Phng
php ghp
(Duplexing)..................................................................934.3.Phn
tch khung Downlink v phng thc ng b
OFDMA..........................94Chng 5. Xy dng m hnh truyn OFDM
qua card m thanh ca my tnh v kt
qu...............................................................................................................................995.1.M
u...............................................................................................................99Xy
dng m
hnh..................................................................................................1005.2.Chng
trnh truyn OFDM v
OFDMA.........................................................1017Nguyn
ng Quang _ T5-K505.3.Hng pht trin h
thng................................................................................106Kt
qu t
c....................................................................................................1095.4.Kt
lun............................................................................................................110KT
LUN
CHUNG................................................................................................111TI
LIU THAM
KHO..........................................................................................1128Nguyn
ng Quang _ T5-K50DANH SCH HNH VDANH SCH HNH
V...............................................................................................9Hnh
1.1 M hnh h thng thng
tin...........................................................................15Hnh
1.2 S khi vng kha pha
PLL.....................................................................19
....................................................................................................................................21Hnh
1.2 Hin tng a ng trong thng tin v
tuyn.............................................22Hnh 1.3 p ng ca
knh fading la chn tn
s......................................................25Hnh 1.4 nh
hng ca knh fading la chn tn
s.................................................26Hnh 2.1 Ph tn
hiu OFDM v
FDM.........................................................................28Hnh
2.3 S trc giao tn hiu trong min tn
s..........................................................37Hnh
2.4 M hnh h thng
OFDM..............................................................................39Hnh
2.5 Hin tng a ng gy nn nhiu
ISI........................................................41Hinh
2.6 Chn khong bo v cho mi k hiu
OFDM................................................42Hnh 2.7 S chm
sao
16-QAM..............................................................................45Hnh
2.9 c tuyn b lc dng ca s Kaiser=3.4 ???
..................................................50Hnh 3.2 Nhn bit
khung
truyn.................................................................................54Hnh
3.3 Tng quan theo chui
PN............................................................................55Hnh
3.4 Cu trc khung OFDM thc hin ng
b....................................................60Hnh 3.5 c im
lung d liu
................................................................................61Hnh
3.6 Tn hiu nhn tng
quan.............................................................................63Hnh
3.7 Hnh dng d liu thc
t..............................................................................65Hnh
3.8 Cu trc khung OFDM s dng khung ng b
FSC...................................66Hnh 3.9 ng b khung k t dng
FSC.....................................................................68Hnh
3.10 Quan h gia ngng ti u Th1 v
SNR...................................................69Hnh 3.11
Sai lch tn s tn hiu gy mt ng
b.....................................................719Nguyn ng
Quang _ T5-K50Hnh 3.12 S sai lch tn s sng mang gy ra s mt ng
b...................................72Hnh 3.13 S sai lch tn s sng
mang gy nn s mt ng b................................74Hnh 3.14 S
khi ng b sng mang s dng b dao ng VCO........................76Hnh
3.15 V tr tin t lp
CP.....................................................................................78Hnh
3.16 Khung
OFDM.............................................................................................80Hnh
4.1
OFDM...........................................................................................................85Hnh
4.2
OFDMA........................................................................................................85Hnh
4.3 Cu trc sng mang con
OFDMA.................................................................87Hnh
4.4 Knh con phn tp tn s
DL........................................................................88Hinh
4.5 mo hinh
AMC...............................................................................................89Hnh
4.6 Cu trc tile cho UL
PUSC...........................................................................89Hnh
4.7 Th d v
OFDMA........................................................................................90Hnh
4.8 ODFM v
OFDMA.......................................................................................91Hnh
4.9 M t v FDD v
TDD..................................................................................93Hnh
5.1 M hnh truyn dn
OFDM.........................................................................100Hnh
5.2 Giao din chng trnh pha
truyn.............................................................101Hnh
5.3 Giao din chng trnh pha
nhn...............................................................103Hnh
5.4 Giao din pha truyn khi chy
tht.............................................................104Hnh
5.5 Giao din chng trnh pha nhn khi chy
tht..........................................105Hnh 5.6 iu ch thch
nghi
AOFDM......................................................................107Hnh
5.7 nh file text truyn th
nghim..................................................................109Hnh
5.8 nh file text thu khi thc
hin.....................................................................110DANH
SCH BNG10Nguyn ng Quang _ T5-K50DANH SCH
BNG..................................................................................................10Bng
1.1 Gi tr tri tr ca mt s mi trng tiu
biu........................................24Bng 2.1. Bng chuyn i m
Gray...........................................................................44Bng
3.1 Suy hao SNR theo li ng
b......................................................................81Bng
5.1 Cc thng s ca m
hnh...........................................................................101CC
T VIT TT11Nguyn ng Quang _ T5-K50AAOFDM Adaptive Orthogonal
Frequency Division MultiplexingGhp knh a sng mang trc giao thch
nghiAMC Adaptive Modulation and Codding M ha v iu ch thch ngAWGN
Additive White Gaussian Noise Nhiu trng (nhiu cng)A/D
Analog/Digital Chuyn i Tng t/SBBER Bit Error Ratio T l bit liBLER
Block Error Ratio T l li theo khiBTSBaseTransceiver Station Trm thu
pht gcCCDMA Code Division Multiple Access a truy nhp phn chia theo
mCP Cyclic Prefix Tin t lpDDL Downlink ng xungDAB Digital Audio
Broadcasting Pht thanh sDPLL Digital Phase Losked Loop Vng kha pha
sDSP Digital Signaling Process B x l tn hiu sDVB-T Digital Video
Broadcasting TerrestrialTruyn hnh s mt tD/A Digital/Analog B chyn i
S/Tng tFFCH Frame control header Khung tiu iu khinFDD Frequency
Division Deplex Song cng phn chia theo tn sFDM Frequency Division
Multiplexing Ghp knh phn chia theo tn sFFT Fast Fourier Transform
Chuyn i Furier nhanhIIEEEInstitute of Electrical and Electronic
EngineersHc vin ca cc k s in v in tISIInter-Symbol Interface Nhiu
lin k tICI Inter-Channel Interface Nhiu lin knhIQ
Inphase-QuaderatureMMAI Multiple Access Interference Nhiu a truy
nhpMIMO Multiple Input Multiple Output H thng nhiu u vo nhiu u
raMMSE Minimum Mean Square Error B lc trung bnh bnh phng ti
thiuOOFDM Orthoganal Frequency Division MultiplexingGhp knh phn
chia theo tn s trc giaoOFDMAOrthoganal Frequency Division Multiple
Accessa truy nhp phn chia theo tn s trc giao12Nguyn ng Quang _
T5-K50PPC Personal Computer My tnh c nhnPLL Phase Losked Loop Vng
kha phaQQAM Quaderate Amplitude Modulation iu ch bin vung gcRRF
Radio Frequency Tn s v tuynSSNIR Signal Noise Interference Ratio T
l tn hiu trn tp m + nhiuSNR Signal Noise Ratio T l tn hiu trn tp
mTTDD Time Division Duplex Song cng phn chia theo thi gianTDMA Time
Division Multiple Access Ghp knh phn chia theo thi gianWWiMax
Worldwide Interoperability for Microwave AccessKh nng khai thc lin
mng trn ton cu i vi truy cp vi baZZF Zero Forcing B lc p
khng13Nguyn ng Quang _ T5-K50Chng 1. Nhng vn chung14Nguyn ng Quang
_ T5-K501.1. Gii thiu v mng thng tin v tuyn 15Nguyn ng Quang _
T5-K50(M hnh knh)Ngun tinM ha ngun(source coding)M ha knh(channel
coding)iu ch(modulation)Knh v tuyn (channel)Gii iu
ch(Demodulation)Gii m knh(Channel Decoding)Gii m ngun(source
decoding)Tn hiu ch (Destination)Hnh 1.1 M hnh h thng thng tinHnh
1.1 th hin m hnh n gin ca mt h thng thng tin v tuyn. Ngun tin trc
ht c m ha ngun gim cc thng tin d tha, sau c m ha knh chng li do knh
truyn gy ra. Tn hiu sau khi qua m knh c iu ch c th truyn i c 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 bn pht thu
c tn hiu gc. Cht lng tn hiu thu ph thuc vo cht lng knh truyn, cc
phng php iu ch v m ha khc nhau. Chng ta s tm hiu nhng khi nim c bn
trong thng tin v tuyn. Knh truynKnh truyn l mi trng truyn dn cho
php truyn lan sng v tuyn. Mi trng truyn dn c th l trong nh, ngoi
tri hoc phn x trn cc tng in ly. Ty thuc vo mi trng truyn dn m knh
truyn c cc tnh cht khc nhau. 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 cao tn 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. Sng mangSng mang l sng c tn s cao,
c nhn vi tn hiu c ch trc khi gi ra anten pht. Sng mang bn thn n
khng mang tn hiu c ch. Tuy nhin, nh sng mang c tn s cao nn khi
truyn trong mi trng v tuyn th tn hiu c ch c iu ch vo s t b suy hao
v c th truyn c i xa. bn thu c th khi phc li tn hiu c ch bng vic tch
ra t sng mang . Ty thuc vo mi trng truyn dn v bng tn cho php m ngi
ta la chn gi tr tn s sng mang. Thng thng th sng mang l sng trung tm
ca gii bng tn cho php ca h thng thng tin. Qun l ti nguyn v tuynTi
nguyn v tuyn l b rng ph cho php truyn tin. B rng ph cho php l c gii
hn. Trong khi , bt k h thng truyn dn no u cn c mt cht lng ti thiu v
nhu cu v tc ngy cng cao p ng cc dch v phc tp.16Nguyn ng Quang _
T5-K50Vn qun l ti nguyn v tuyn l lm sao vi mt di bng tn c nh cho
trc h thng hot ng vi mt cht lng tt nht v vi tc truyn d liu cao nht.
Vi cht lng cng cao v tc truyn tin tc cao, ngi ta ni h thng c hiu
sut s dng ph cao. Nhim v ca qun l ti nguyn v tuyn cn l phn chia b
rng ph sn c cho cc h thng thng tin khc nhau sao cho cc h thng c hiu
sut s dng ph cao nht. i vi cc h thng nhiu ngi s dng th qun l ti
nguyn v tuyn l s phn chia b rng bng tn v iu khin a truy nhp sao cho
h thng c ti u v cht lng v ph tn hiu.1.2. Cc h thng thng tin v
tuynCc h thng thng tin v tuyn c th phn loi theo s cung cp dch v: h
thng pht thanh v truyn hnh. Dch v ca 2 h thng l thoi v hnh nh.C th
phn loi h thng thng tin v tuyn theo phng thc truyn dn nh h thng
truyn song cng (di ng) hay bn song cng (b m).C th phn loi theo mi
trng truyn dn nh thng tin viba (yu cu truyn dn trong tm nhn thng) v
thng tin mng my tnh khng dy (phn x a ng v khong cch ngn).1.3. Cc
hin tng in hnh trong lnh vc truyn thng v tuyn Hiu ng DopplerHiu ng
Doppler l mt hiu ng vt l, trong tn s v bc sng ca cc sng m, sng in t
hay cc sng ni chung b thay i khi m ngun pht sng chuyn ng tng i vi
ngi quan st.i vi sng chuyn ng trong mt mi trng, nh sng m, ngun sng
v ngi quanst ucthchuynngtngi sovi mi trng. Hiung Doppler lc l s tng
hp ca hai hiu ng ring r gy ra bi hai chuyn ng ny.C th, nu ngun di
ng trong mi trng pht ra sng vi tn s ti ngun l f0, mt ngi quan st ng
yn trong mi trng s nhn c tn s f:011 /f fv c| ` +. ,17Nguyn ng Quang
_ T5-K50Victc lan truyn ca sng trong mi trng,vl thnh phn vn tc
chuyn ng ca ngun so vi mi trng theo phng ch n ngi quan st (m nu i v
pha ngi quan st, dng nu ngc li).Tng t, khi ngun ng im cn ngi quan
st chuyn ng:01vf fc| ` + . ,Tc, khi ngun sng v my thu chuyn ng vi
nhau, tn s thu c s khc tn s pht ra. C bn v nhiu trng AWGNNhiu trng
AWGN (Additive white Gaussian noise) l mt tn hiu ngu nhin c mt phn
b cng sut phng ngha l tn hiu nhiu c cng sut bng nhau trong ton
khong bng thng v c phn b Gauss. Tn hiu ny c tn l nhiu trng v n c
tnh cht tng t vi nh sng trng. Tc l nhiu ny c mc nh hng nh nhau i vi
mi tn s.i vi mi h thng thng tin v tuyn th u phi chu nh hng ca nhiu
trng. C bn v vng kha pha s PLLVng kha pha PLL l mt h thng vng kn hi
tip, trong tn hiu hi tip dng kha tn s v pha ca tn hiu ra theo tn s
v pha tn hiu vo.Tn hiu vo c th c dng sin tng t hoc dng s.K thut PLL
c s dng rng ri trong cc mch lc, tng hp tn s, iu ch v gii iu ch, iu
khin t ng Nguyn tc hot ng: hot ng theo nguyn tc vng iu khin m i lng
vo v i lng ra l tn s v chng so snh c vi nhau v pha. Vng iu khin pha
c nhim v pht hin v iu chnh nhng sai s nh v tn s gia tn hiu vo v tn
hiu ra. Ngha l PLL lm cho tn s f0 ca tn hiu ra VCO bm theo tn s fo
ca tn hiu vo.18Nguyn ng Quang _ T5-K50Khi khng c tn hiu vi ng vo,
in p ng ra b khuch i vdc(t)=0, b dao ng VCO hot ng tn s t nhin
fN.Khi c in p vo, cc khi s hot ng theo chc nng v cho php ta iu khin
tn s VCO bm theo tn s tn hiu vo. n khi tn s fVCO bng vi tn s tn hiu
vo fi, ta ni b VCO bt kp tn hiu vo. Lc ny s sai lch gia 2 tn hiu ch
l s sai lch v pha, b tch sng pha s tip tc so snh pha gia 2 tn hiu
iu khin cho VCO hot ng sao cho s lch pha gia chng t n gi tr nh nht.
C bn v chui gi ngu nhin Pseudo Noise PNC nhiu loi chui PN, c to ra
t cc thanh ghi dch c hi tip. Loi PN hay c s dng c th l chui c di
(chu k chui) cc i hay cn gi l m-19Nguyn ng Quang _ T5-K50vdcvif0Tch
sng phaLc thng thpKhuch i mt chiuVCOHnh 1.2 S khi vng kha pha
PLLsequence (m-dy hay chui m). Chui m c to ra bng mt b ghi dch hi
tip c cu hnh Fibonacci hay Galois (hai cu hnh ny v mt ton hc th tng
ng nhau), c hnh thnh theo mt a thc sinh quy nh cc u ra cc khu nh ca
thanh ghi tham gia hay khng vo mch hi tip. a thc sinh l mt a thc
nguyn thy (primitive polynomial), khi chui chip li ra s l mt chui
PN c chu k lp li ln nht, l N = 2^m - 1, trong m l s khu nh ca thanh
ghi. Mt c im c bn ca chui m l c hm t tng quan dng thumb-nail, mt
chu k ca n c hai phn chnh: a) Tr ca hm t tng quan chun ha gim tuyn
tnh t 1 xung cn -1/N khi bin tau (tau tr truyn dn trn cc tuyn)tng t
0 ti Tc (Tc l rng mt chip); b) Tr ca hm t tng quan l -1/N (khng i)
khi bin tau tng t Tc ti (N-1).Tc. Nu N rt ln (nh s dng thanh ghi
dch c m ln, trong cc h thng CDMA nh IS-95 th m = 42 vi chui m di
dng phn bit ngi s dng v bng 15 i vi m ngn dng phn bit trm BS) th
trong on sau, hm t tng quan c th xem l bng 0.1.4. Suy hao ng truyn
v tuynChng ta s tm hiu nhng vn chnh ca truyn sng v tuyn v nhng kh
khn m chng gy ra trong h thng truyn dn thng tin s. Cc tn hiu khi
truyn qua knh v tuyn di ng s b phn x, khc x, nhiu x, tn x v do gy
ra hin tng a ng. Nhng nh hng ca truyn sng v tuyn nh suy hao
ngtruyn, fadingphng, fadingchnlctns, hiungDoppler, tri tra ng, u lm
gii hn hiu qu ca truyn thng v tuyn.1.4.1. Suy hao truyn dn Suy hao
truyn dn trung bnh xy ra do cc hin tng nh: s m rng v mi hng ca tn
hiu, s hp thu tn hiu bi nc, l cy v do phn x t mt t. Suy hao truyn
dn trung bnh ph thuc vo khong cch v bin i rt chm ngay c i vi cc thu
bao di chuyn vi tc cao. Ti anten pht, cc sng v tuyn s c truyn i
theo mi hng (ngha l sng c m rng theo hnh cu). 20Nguyn ng Quang _
T5-K50Ngay c khi chng ta dng anten nh hng truyn tn hiu, sng cng c m
rng di dng hnh cu nhng mt nng lng khi s c tp trung vo mt vng no do
ta thit k. V th, mt cng sut ca sng gim t l vi din tch mt cu. Hay ni
cch khc l cng sng gim t l vi bnh phng khong cch. Phng trnhtnh cng
sut thu c sau khi truyn qua mt khong cch R4R T T RP P G GR _ RP:
Cng sut tn hiu thu c (W)_ TP: Cng sut pht (W)_ RG: li anten thu
(anten ng hng)_ TG: li anten pht _ : bc sng ca sng mang _ R: bn knh
truyn sng v tuynGi Lpt l h s suy hao do vic truyn dn trong khng
gian t do:( ) ( ) ( ) 10lg( ) 10lg( ) 20lg( ) 20lg( ) 47, 6( )pt T
R T RL db P db P db G G f R db + + Ni chung chng ta c th xy dng c
mt m hnh kh chnh xc cho cc tuyn thng tin v tinh v cc tuyn lin lc
trc tip (khng vt cn ) nh cc tuyn lin lc vi ba im ni im trong phm vi
ngn. Tuy nhin do hu ht cc tuyn thng tin trn mt t nh thng tin di ng,
mng LAN khng dy, mi trng truyn dn phc tp hn nhiu do vic to ra cc m
hnh cng kh khn hn. V d i vi nhng knh truyn dn v tuyn di ng UHF, khi
iu kin v khng gian t do khng c tho mn, chng ta c cng thc suy hao ng
truyn nh sau:( ) 10lg( ) 10lg( ) 20lg( ) 20lg( ) 40lg( )pt T R BS
MSL db G G h h R 21Nguyn ng Quang _ T5-K50Vi _BSh, _MSh
