Máy phát viba

7/31/2019 My pht viba

1/14

My pht viba

1. Chuyn tip sng v tuyn.ng chuyn tip sng vi ba c ng dng trong ngnh truyn hnh (xem hnh 1), thng thng gmthit b khung c nh v chia lm 2 loi sau:

ng truyn phn tn, trong truyn tn hiu hnh nh v m thanh t cc phng sn xut chng trnh nmy pht, hoc gia cc my pht vi nhau. Trong trng hp tn hiu b mo cc phn khc nhau ca chui tnhiu t camera n ngi xem, on phn tn chng trnh c cho php rt t, do gim thiu suy gim l rtquan trng. Trong mt s trng hp s phn phi chng trnh n mt s hay tt c my pht hnh chnh trongmng s c pht i thng qua PTO (t chc iu hnh vin thng cng cng) cung cp mch thng trc, mtrong cp quang chim u th hay bng v tinh. Trong mt s trng hp cc ng truyn v tuyn s cthu bao gia tng vng dch v m cc mch c nh s rt kh tip cn hay chi ph qu cao cung cp.Trong mt s trng hp c bit, cc thit b c th i hi nhiu ng truyn tn hiu khc nhau, s dnglin kt v tuyn nh ng truyn th 2 s l gii php kinh t hn so vi l xy dng ng cp th 2.ng truyn tng hp, c s dng truyn chng trnh th t cc phng dng bn ngoi hay ENG (thuthp tin tc in t) hay im tip nhn OB n phng sn xut chng trnh. Trong nhiu trng hp cc ngtruyn ny s i theo cng mt tuyn vi ng truyn phn phi nhng theo hng ngc li. Trong qu kh chc s khc bit duy nht gia ng truyn phn phi v ng truyn tng hp l yu cu v tnh kh dng,nhng vi cc phng dng s, nh dng tn hiu truyn i c th khc nhau.

2. Lin kt v tuyn s v tng t.

Mi cho n nm 1980, tt c lin kt v tuyn u dng k thut tng t, iu ch tn s bng rng c sdng trong truyn hnh v a hp phn chia tn s nhiu knh trong in thoi. Ngy nay, hu ht cc thit bc dng trong in thoi c thay th bng cc ng truyn s mang d liu v lu lng ting ni. Loiiu ch ny s dng trong khong t pha hai cc hay kha dch pha cho dung lng knh thp, thit b tn scc cao, n kha dch pha vung gc, iu ch bin vung gc 16, 64,128 mc, iu ch m Trellis 256 hay512 mc dng cho cho h thng c dung lng cc cao mang 1,2 v 4 knh ca STM1, lu lng155Mbit.Tt c thit b nu trn c thit k c hot ng trong dy tn s c chun quc t trn cc knh phi hp

trnh vic can nhiu ln cc knh khc. Mt s thit b th tng thch vi lin kt c nh dng nhng di tnv k thut dnh cho mng cc b ca my vi tnh. Cc thit b ny thng c kim sot chc ch khi chnghot ng trn cc knh chia s vi cc dch v khc.Ngoi ra, ph thuc vo cc ng dng truyn hnh nh dng ca cc tn hiu c truyn i s nh hng loithit b dng trong lin kt v tuyn.

2.1. Truyn hnh tng t.

Cc tn hiu truyn thng ny c pht i bi cc i pht hnh v cc t chc iu hnh vin thng cng cng(PTOs) bng lin kt v tuyn da trn k thut tng t, nhng hin nay hu ht cc chng trnh c gisang thit b PTO v truyn dn chng trnh gia i pht a s s dng k thut s. M ha tn hiu hnh nh

mc 140Mbit s dng gim nn c dng trong cc h thng truyn dn cn em n hnh nh r rng chongi s dng.


2/14

Mc khc, 34Mbit, hay c th thp hn trong mt s trng hp, c th dng cho cc mc ch phn phi. Vinhng trng hp ny vic ct gim tc bit h thng ca bt k teletext cn phi c tri ra v truyn i trncc knh d liu ring bit. Nhiu knh c th c ghp li vi nhau v gi trn cc ng truyn dung lngcao hn, khng cn dng n k thut PDH, ch cn 3 dng truyn 34Mbit thng thng va cho dng truynd liu SDH 155M bit. ng truyn tng hp c th l 8, 34, 140Mbit hay tng t, ph thuc vo c tnh yucu, cht lng v kh nng tip nhn ca trm chuyn tip.Trong nhiu trng hp yu cu lin kt v tuyn, thit b tng t vn t ra thch hp nht:

Tn hiu nhn c t cc trm pht li truyn hnh. Trong hu ht trng hp cc tn hiu ny khng phhp vi vic cc gim tc bit m ha s m khng km theo cc chng trnh x l thch hp.

Thm vo vic ct gim on m ha s tc bit trong vic thit lp mng s to ra s phc tp khi

truyn cc tn hiu lin tc t cc trm v kim tra cht lng. V vy, khng c s thay i trong mng phn phi, nhiu ng truyn s cn phi mang tn hiu truyn

hnh tc 140Mbit, dung lng ng truyn ny c th chu c nhiu trn ng truyn v tnhiu thu tt. iu dn n vic cn cc bc nhy (truyn v tuyn t im ny n im khc trn trit thng qua tng in ly m khng cn phn x b mt tri t) v xy dng cc trm ti lp, ngay cnu c th lm c, chi ph vn rt mc v gia tng chi ph bo dng.

2.2. Truyn hnh s mt t.

nc Anh, tn hiu truyn hnh s c phn phi bi cc ng truyn mt t do PTO cung cp v dch vv tinh t nhn.Trong c 2 trng hp tn hiu a hp thch ng mc chun PDH c tc 34 hay 140Mbit v c truyn dn

trong cc mch truyn thng thng thng. Cc ng truyn mt t thng s dng ng quang. nc M, phn quan trng ca tn hiu DTT c phn phi bi lin kt v tuyn. Loi thit b chuyn bit scho php truyn lin tc cc tn hiu tng t v s trn cng truyn song hnh trn knh RF.


3/14

2.3. Di tn.

S phn phi cc ng truyn v tuyn c nh trong khong 1 n 57Ghz. Mc ch s dng v tnh cht cacc di tn khc nhau c ch ra y:

1-3Ghz di ph c dng cho in thoi di ng v dch v d liu. N c u im l ng truyn di

v qu trnh truyn dn khng b nh hng ca thi tit. Anten dng trong nhng h thng dung lngthp thng c kch thc nh v gi thnh hp l. 3-10Ghz di ph ny thch hp cho a knh s c dung lng trung bnh v cao hay cc ng truyn

tng t, cung cp dch v c tin cy cao vi cc khong cch trn 70km. Cc tn hiu ny khng bnh hng ca ma dc theo ng truyn, nhng nhiu lan khng kh c th to ra fading (s thay it t cng tn hiu thu) v mo.

10-22Ghz di ph ny thch hp cho cc ng truyn tng t v s c dung lng trung bnh v cao,khi tng tn s hot ng tc ng ca ma trn ng truyn v tuyn cng tng ln. Do , kh nngthu ca ng truyn hot ng trong di bng ny b nh hng bi lng ma nhiu hn l s nhiulon khng kh. di ng truyn chun c th t 40km i vi ng truyn lu lng thp tn s10Ghz cho n 3 hay 4km cho ng truyn dung lng cao tn s 22Ghz, nhng c 2 ng truyntrn l c lp, ty thuc vo yu cu v mt kinh t.

22-57Ghz Di ph ny c s dng rng ri trong cc khong cch ngn, ng truyn s n knh c

dung lng thp, trung bnh, cao. di ca ng truyn c th xc nh da vo suy hao lng mav suy hao do mi kh quyn.

Hin nay, nhiu dy tn ch dnh ring cho ng truyn s. Mc d c s ch nh di tn trn 23Ghz l dnh chocc ng truyn thoi tng t, nhng c rt t thit b pht hot ng tn s ny. iu l do nhu cu thpv gi thnh phn cng thch hp cc c tnh k thut tn s cao ny l rt mc.2. H thng ng truyn v tuyn.

2.1. B tr h thng.H thng chun bao gm cc thnh phn sau:Trang thit b v tuyn:

My pht v My thu. Nhnh ng trc v ng dn sng. ng dn sng hay cp ng trc v cc ph kin gia cc thit b pht v anten. Khung thp ni gia anten v thp hay ta nh. Thit b gim st hot ng. Khi ngun (pin mt tri, my pht in chy bng du diesel, b sc v pin)

Thit b ph:

H thng anten (thp anten hay xy dng gi treo trn tng hay mi). Phng t thit b v tuyn.

H thng iu ha.

2.2. Cu hnh h thng.

Cu hnh c s dng nhiu nht l cc dng n hng, song hng, khng sao lu v sao lu (b d phngnng, ng dn song hnh v N+1). Cc cu hnh th h c ch c bit nh l c bo v v khng c bov.Trong cc ng dng pht sng, s truyn dn c th xem nh theo 1 hng v do vic thit lp cc ngtruyn cho mc ch ny l n hng. iu ny s gip ta tit kim c gi thnh thit b v ph ng k chocc knh truyn. Tuy nhin, nu chng trnh c pht i trn cc ng truyn s, thng thng hot ngtheo chun song hng, phn ln thit b loi ny c cc khi pht v thu thng nht thnh 1 khi; v vic iukhin h thng v hin th c th hot ng sai nu loi b ch n hng.

2.2.1. Cu hnh n hng khng c bn sao (unduplicated).My pht v thu c ni trc tip n cng anten thng qua ng dn sng v b lc knh ng trc, c ch ra hnh 2(a). Thit b n c tnh kinh t nhng khi thit b pht hay thu b li chng trnh chc chn s b mt.


4/14

2.2.2. Cu hnh n hng, a knh khng sao lu .

Vi my pht hay my thu c kt hp pht chung trn h thng anten iu c ngha l phn nhnh RF (hayghp knh), v iu i hi phi kt hp cc b lc v b chuyn mch vng, ch ra hnh 2(b), Tn hiu tmy pht A c ni trc tip n b lc knh v b chuyn mch vng. Ng ra ca my pht B, hot ng tn skhc my pht A, c ni trc tip b chuyn mch vng v n b lc knh ca my pht A, v t s c

phn x theo hng b chuyn mch vng. C 2 tn hiu s c kt hp trn cng 1 cng ca b chuyn mchvng.Cc tiu tn s gia cc my pht ph thuc vo bng thng v c tnh phn x ca b lc, v chun thng l49 hay 56 Mhz cho truyn hnh v cc ng truyn dn s c dung lng cao v trung bnh. Theo cch nhngha ny ta c th kt hp nhiu hn 2 my pht.

2.2.3. Cu hnh song hng.

Nhnh RF c th kt ni n my pht v my thu sau kt ni n h thng anten chung. Cu hnh ny l songhng. S kt ni ny gim thiu s chuyn mch vng. Trong trng hp kt ni nhiu knh cn quantm n qu trnh xuyn iu ch.

2.2.4. Ch sao lu (bo v).

Thit b c th nhn i trnh vic mt tn hiu truyn dn khi xut hin li. Vic s dng 2 my pht v 2 b thucng lc, khi xut hin li th b chuyn mch t ng s chn la thit b hot ng.

C nhiu cch khc nhau thit k h thng my pht v thu; v iu dn n s b tr thit b c m tdi y:


5/14

2.2.4.1. D phng nng.

Nh c ch ra hnh 6.6.2(d), tn hiu iu ch c chia ra 2 my pht hot ng cng tn s v ng ra RFc kt ni vi cng tc hay b ghp (coupler) c th chn la my pht c kt ni n h thng anten.Nu s dng h thng cng tc, c 2 my pht u phi chy ng cng sut thit k, nu s dng coupler thmy khng c chn s c tt.ng chuyn tip ng trc v cng tc chuyn ng dn sng c suy hao chn thp (


6/14

B to phch (hnh 3).Tn hiu vo c iu ch bi b dao ng trung tn, sau ng ra s c nng tn s (1 hoc 2 ln) v clc ri khuych i ln mc cng sut chun. Tn s trung tn u tin thng dng nht l 70Mhz, nhng hin nay

di tn thng c s dng l 200-400Mhz. Trung tn th 2 thng nm trong khang 800Mhz-2Ghz.

2.3.1. My pht tng t.

Di bng gc tn hiu hnh truyn trong my pht xp x khong 10Mhz, trong khi tn hiu PAL 625 dng ch chim5.5Mhz. Di ph cn li dng truyn ting hay tn hiu d liu dung lng thp trn cc sng mang ph. Tchc ITU-R ngh tn s sng mang ph l 7.02,7.5,8.065 v 8.59Mhz, nhng phi quan tm n xuyn iuch chng c th to ra cc sng mang ph trong khong 5.8 -10 Mhz. Tn hiu hnh vo c khuych i v lcloi b cc nhiu v tng mu sai, trc khi c cng vi sng mang ph.Tn s tn hiu tng hp ny iu bin bi b dao ng th kim, trong trng hp iu ch trc tip tn s ngra l 70Mhz trong h thng b to phch. Sau qu trnh iu ch tn hiu s c nng tn s (bao gm b dao

ng ni to tn s c nh, b trn, b lc), mc ch dch chuyn tn s tn hiu sang trung tn cao hn. Tn hiuny tip tc c nng tn ln 2, bao gm b dao ng ni tng hp lp trnh c, v b trn dch chuyn tnhiu iu ch sang tn s u ra. Ba thnh phn tn s quan trng nht ca ng ra b trn:

Dao ng ni .

Tn s trn thp. -IF

Tn s trn cao. +IF

Trong trng hp 2 ln bin i, tn hiu trung tn nm trong khang 800Mhz v 2 Ghz, b lc thng di (c rng chun 500Mhz) c th chn la tn s trn cao. B trn cn bng cng c th dng loi b tn s dao ngni. Tn hiu sau khi lc s c cho qua b khuych i bng rng, trong trng hp ny hot ng b khuychi hot ng ch bo ha.


7/14

2.3.2. My pht s.

Phn ln ng dng trong my pht tng t cng c dng trong my pht s. Tin trnh x l hnh u vo sc thay th bng d liu u vo v cc byte FEC (kim tra li truyn), trc khi c a vo b iu ch s.Phn ln b iu ch s c kt cu nh ch ra hnh 4.i vi my pht s, yu t cn thit l cc tng iu ch tn hiu, bao gm c khi khuych i cng sut ng ra,chng phi hot ng tuyn tnh; nu khng tn hiu ng ra s b mo v sinh ra cc bit li.

2.3.3. My thu.

Cc my thu vi ba trong cc trm pht li truyn thng thng dng bin i tn s 1 ln, nhng vi s ra i bdao ng ni vi ba nhiu thp lp trnh c cho php thit k cc thit b c kh nng iu chnh trong phm vi

500Mhz, ngy nay h thng to phch bin i tn s 2 ln chim u th.Tn hiu i vo h thng thng qua b lc thng di, thng l 1 phn ca nhnh r RF. Tn hiu sau c cho qua b khuych i nhiu thp (LNA) v 1 b lc thng di loi b nhiu sinh ra ti LNA, ti b thu

tn s nh trc khi i vo b trn SHF vi tn s dao ng ni . Kt qu l trung tn th nht

tn sTrong my thu tng t tn hiu s c ct ngn loi b bt c iu ch bin no sn sinh bi nhiu vmo trong tng IF v RF. Sau tng gii iu ch, tn hiu tng hp (bao gm hnh nh v sng mang ph) s cchia lm 2 ng. ng th 1, c dnh cho cc tn hiu hnh nh, s c cho qua lc thng thp (ct rathnh phn 5.5Mhz i vi h thng PAL 625 dng) loi b nhiu tn s cao v sng mang con, tn hiu sau

c gii tin nhn v khuych i ln mc 1 . ng th 2 dnh cho vic gii iu ch cc sng mang

con.Trong my thu s c nhiu k thut gii iu ch c s dng, sp xp mch vng kha pha chuyn i tnhiu trung tn sang dng d liu my tnh v x l bng phn mm. Hu ht cc u thu s u c b p ngtn s v cn bng min thi gian gim tc ng ca mo a ng.

2.4. Thit b truyn dn.

My pht v my thu c kt ni thng qua cp ng trc hay ng cng, ng dn sng hnh ch nht. Ng ra vtuyn, thng c gi l cng anten, thng bao gm u ni ng trc cho thit b tn s trn 3Ghz v lp gdn sng cho tn s cao hn. Cp ng trc hay ng dn sng hnh elip c dng kt ni ng ra audio my phthay thu n anten. tn s thp hn 3Ghz, thng s dng cable c lp bng hay khng kh cch in. ng knh cp nm trong

khong inch cho n 1 5/8 inhc (6.35mm-4,13 cm), suy hao chn t l nghch vi ng knh cp. Cp cch inbng khng kh c ng knh tng t v suy hao t hn, nhng loi cp ny cn thit b iu p (thit b top sut n nh gia li cp v v bc).ng dn sng dng elip thng c dng tn s trn 3 Ghz. iu l do cp c suy hao thp v c th


8/14

un cong c c th linh hot s dng ti cc ch un cong khi lp t feeder (cp dn sng RF). Cc uni cui mi phn chuyn i kt ni ng dn sng cho php gn kt ng dn sng hnh ch nht c dngtrong cc nhnh r hay dy feeder n anten. Kch thc khc nhau ca ng dn sng tng ng cho tng ditn hot ng khc nhau. Cc ch c bit over-mode thch hp cho di tn trn 10 Ghz, c u im l suyhao thp. Nu cn suy thp hn, th cn s dng ng dn sng hnh trn, nhng ng dn sng loi ny kh lpt khi ng truyn thng tuyt i. Ngoi ra vic lp t ng dn sng ny ti cc ct anten cao hay cc thptruyn hnh cng rt kh khn do h thng khung thp v cp gy tr ngi cho vic lp t.

Cc ph kin cho h thng feeder chun bao gm:

u ni ti cc im cui. Ming m ti cc l thng cp ti cc vch tng bo m tnh ngn np cng nh tc ng ca thi

tit vo phng t my. Mc tip t bo m feeder v anten c tip t tt. Cc co ni cong dng cho cc ng cng, cp hay ng dn sng c ng knh ln ti nhng im m cc

loi cp ny khng th kt ni trc tip vo thit b thu/ pht hay anten. Mng treo v gi bo v feeder khi dn i trong ta nh hay a ra thp pht sng. Trong qu trnh lp t, cc dng c cui cp hay u g gi cp theo chiu thng ng, b ti ko.

Thit b iu p cn thit cho cc cp cch in bng khng kh v h thng dn sng bao gm:

B kh nc t ng v bm to p sut n nh mc 1-5psi(0.070-0.35kgf/cm2). ng gp(nu c 2 hoc nhiu hn cp). Ca p sut. Kha p sut c th bo li h thng t xa.

2.5. Anten.

Anten dng parabol hay dng trn dng trong h thng tip pht vi ba. Chng c tnh kinh t, li cao, kh nngtrit mi bn (bp sng khng mong mun) tt v t s ti/ di cao. S phn x c xoay trn trn tm aluminiumc, hoc kt cu dng li, chng sng hn v t b sc cn ca gi, thng c s dng tn s di2.5Ghz. Vic s dng phn x dng li khng c u im hn dng phn x c ngoi vic chng gim tcng ca gi v ti nhng vng bng tuyt.Anten chun th phi c t s sng ng thp (VSWR), v tnh nng cao v phn thu sng c th n hay a cc(ng, ngang hay c hai). Phn cc trn t khi s dng trong h thng ng truyn c nh. Kch thc ngknh t 0.5m cho n 3.7m, mc d hin nay ngi ta thng s dng loi c ng knh t 0.3 -2.4m.Hu ht anten c s dng ti nhng vng ni a t khi phi chu ng sc gi mnh (>200km/h). Loi gia clc, s dng 4 im g v thm mi che chu lc, thch hp s dng ti cc khu vc c go mnh. Khi lp t thitb chng li s n mn ca khng kh, chng hn nh gn cc ng khi hay hi nc bin, anten cn phi thmlp bo v bn ngoi.Anten, feeder v thit b t ti cc vng b ng tuyt cn phi bo v cn thn trnh tc hi tuyt l khi nhit gia tng.

3. Thit k h thng.ng truyn vi ba tnh thng dng trong cng nghip truyn hnh truyn dn cc chng trnh gia

studio v my pht hnh v gia trung tm thuc a phng.

V tr cc trm my pht hnh v studio thng c la chn theo cc chc nng c bn ca n ch khng phitheo ng truyn vi ba. V vy k s thit k cn phi la chn ng truyn c tnh kinh t v phi ph hp yucu c s h tng hin c.

i vi ng truyn ngn vi cc anten pht c v tr cao, ng truyn line-of-sight (anten pht v thu phi nhnthy nhau) c th thc hin ch vi 1 hop. Tuy nhin, s cch bit gia cc trm tng ln, hoc c chng ngi vtgia ng truyn line-of-sight, 2 hoc nhiu hop cn phi c s dng kt ni tun t. Cc trm tc thic gi l repeaters (trm tip pht).

Mt s cng vic lin quan vic thit k ng truyn viba:

Ln k hoch s lc ng truyn gia 2 a im v xc nh cao anten v xc nh trng tritrn ng truyn. Nu c chng ngi vt trn ng truyn trc tip gia studio v my pht, xembc tip theo.


9/14

Chn la trm pht li t bn chi tit, hoc xem xt k lng thc a, tm kim v tr c gc ngng phhp, m bo di ng truyn. Nu c th th t trm tip pht ti cc ngn i trnh xa ng x,khi tnh ton ga thnh quan tm n s tin li trong qa trnh xy dng v vn hnh.

Chun b phc tho s lc ng truyn gia cc trm tip pht v xc nh chiu cao anten tc tiu chun thng thong. V tr v bn kho st ng truyn phi xc nhn thng tin chnh xc tbn , nh cy ci, cc ta nh hoc cc cu trc nhn to cn tr ng truyn. Thit b thu GPS rthu dng trong qu trnh kho st khu vc xc nh cc v tr c th b khut tm nhn.

Chn la ng truyn dn s kt hp tt v di hop v chiu cao anten. Tnh ton cng sut my pht v kch thc anten c c fade margin ph hp. Mt s iu kin c bit ca thp hay ta nh.

3.1. Xc nh ng truyn.

Nhim v u tin l tnh ton ng truyn ph hp xc nh khong thng thong gia chm sng vi ba vcao cc vt cn trn ng truyn. ng truyn vi ba l ng thng ni lin 2 anten nh ch ra hnh v 5.Ngoi ra s phn x khng kh c th un cong chm sng v tuyn tn s vi ba, thng thng ta gi s bnknh hiu dng tri t k=4/3 c th ln hn thc t. Trong thc tin, iu ny c ngha l chm sng vi ba s b bcong xung di mt t do s thay i dn tnh cht khc x ca khng kh theo cao.

S thay i khc x khng kh lm cho h s k gim xung cn 2/3. Do vy to thun li hn trong vic v cao

mt t trn cc giy v biu dng thng tuyn tnh (hnh 4) v thm vo earth bugle 2 gi tr ca k l 4/3 v2/3.

EB c th c tnh bng cng thc.

Trong , d1, d2 l khong cch t im c bit trn ng truyn n tng u cui, tnh bng km.

3.2. Vng Fresnel.

Bc k tip l tnh ton bn knh vng fresnel utin, trong xc nh a im ca tt cc cc im bao quanh chm tia m t cc chm phn x c di ng truyn bng bc sng hoc cao hn chmtia chnh.


10/14

Bn knh vng Fresnel th nht c tnh theo cng thc (n v m).

Trong d1, d2l khong cch t vtcn n tng u cui ca ng truyn theo km, v F l tn s tnh theo Ghz

Hnh 7 ch ra khong khng gian m ng truyn mti 60% ca bn knh vng fresnel u tin.

3.3. Mc tn hiu thu.

Cng sut tn hiu ti ng vo u thu vi ba ph thuc cc yu t sau:

S mt do r nhnh u cui ng truyn. S mt ti ng dn sng hay cp dn sng c 2 u cui ng truyn. Suy hao ng truyn trong khng gian. li anten.

Suy hao truyn dn theo thang dB di iu kin thng thng c tnh theo cng thc , trong d l khongcch ng truyn v l bc sng( c 2 tnh theo m).Ta c th khai trin theo cng thc , trong f l tn s(Hz) v c l vn tc nh sng ( ).

V vy mc tn hiu thu c (theo dBW) s bng cng sut my pht tr cho suy hao r nhnh tr suy hao do

cp dn tr suy hao truyn trong khng gian cng cho li anten.


11/14

3.4. Fade margin.

Thng s quan trng nht trong ng truyn v tuyn l fade margin. Thng s ny nh ngha s khc bit giatn hiu thng thng v mc tn hiu nhn khng b m v mc tn hiu thp nht thu c m b thu c th giim c.

i vi ng truyn tng t mc tn hiu thu c thp nht c th s dng c tng ng vi mc nhiutrn tn hiu vo khong 34 dB khi o c trn bng thng 10Khz n 5Mhz. Vi u thu c h s nhiu 4 dB vtn hiu c bng thng 8Mhz vi lch nh 8 Mhz, cn bng tn hiu ng vo u thu l -75dBm (-105dBW).

i vi ng truyn s mc tn hiu thu c thp nht c th s dng c tng ng vi vic gii iu chc tc sai s bit l . iu ny yu cu chun l -76dBm cho QPSK (kha dch pha vung gc) 34Mbit v-68dBm cho TCM 128 mc tc 155Mbit.

3.5. Kh nng thu.

suy hao ng pht sng c tnh ton theo bng trn t cc suy hao c nh. Tuy nhin, cng sut tn hiun u thu trong h thng v tuyn thc t c th khc bit ng k v s b fade (thay i cng tn hiu)di cc gi tr cho trc ty theo iu kin thi tit.

3.6. Multi-path Fading.

Multi-path Fading (s thay i cng tn hiu do nhiu ng v tuyn cng n im thu) trn ng truynvi ba ph thuc tn hiu n u thu anten t nhiu ng khc nhau, chng hn:

ng truyn trc tip gia cc anten. Sng phn x t mt t hay cc ta nh. Phn x trong kh quyn.


12/14

Thnh thong cc tn hiu c cng pha v tn hiu tng s mnh hn mc bnh thng.Vo thi im khc tnhiu s kh nhau v tn hiu tng s yu i.

Trong mt s thi im, hu ht cc tn hiu u kh nhau, mc tn hiu s xung thp hn ngng thu v ngtruyn s b rt. May thay s thay i cng tn hiu ch xy ra trong thi gian ngn.

Multi-path Fading ph thuc cc nhn t sau:

g gh ca mt t (b mt phng d phn x hn). Hon cnh thi tit (thay i h s khc x ca kh quyn, to ra nhiu chm tia khc x khc nhau n

u thu). di ng truyn (ng truyn cng di th cng d xy ra cc hin tng trn). Tn s.

Mt s cng thc rt kt t thc tin c th d an tc ng ca multi-path Fading trn kh nng thu ca ngtruyn vi ba. Phng php Vigants tin on kh nng thu theo cng thc sau:

Trong :d = di ng truyn(km)A= h s xc nh g gh ca ng truyn.B= h s iu kin kh huf= tn s(Ghz)F= fade margin(dB).

H s A=4 i vi vng c b mt phng (mt nc), 1 vng t thng thng, 0.125 i vi vng ni hay ccvng t rt kh cn.

H s B=0.5 cho vng kh hu nng, m t,0.25 cho vng ng bng rng ln, 0.125 cho cc vng kh cn.

3.7. Ma. tn s trn 10Ghz ma c th lm suy hao tn hiu; tn s 22 Ghz hay trn na, l phn quan trng xcnh fade.. ti nhiu vng trn th gii.

V d, suy hao do ma gy ra mc 50mm/h (bo c sm st) tn s 10Ghz l 1.5dB cho mi km, trong khi tn s 23Ghz n ln n 8dB. Do tt c ng truyn dung lng thp, di ng truyn ch gii hntrong khong 10km.

3.8. Hi nc.

Suy hao do hi nc gy ra l khng ng k trong cc trm tip pht sng v tuyn. suy hao (trn km) thphn 0.01dB tn s 10 Ghz v 0.3bB tn s 23 Ghz v n ch tht s ng quan tm tn s trn 55Ghz, tn


13/14

s cc dao ng cng hng ca phn t xy lm suy hao tn hiu khong 14dB.

4. Tng cng kh nng thu.

Kh nng thu tn hiu c th gia tng khi s dng phn tp (diversity) khng gian v phn tp tn s ti u thu

4.1. Phn tp khng gian.

Phn tp (phng php trch tn hiu nh rt ra tn hiu kt qu ti u t mt t hp nhiu ng truyn)khng gian cn phi c 2 anten i vi mi u thu. Nh ch ra hnh 9, tn hiu c th c cng trc khi avo b gii iu ch trnh xi kha chuyn. Khong khng gian l tng gia 2 anten c xc nh theo vithng s sau: cao anten, di ng truyn, tn s, iu kin kh hu v a cht ti v tr t trm thu.

4.1.1. Phn x t mt nc.

Khi tn hiu viba truyn trn mt nc, vo thi im m chm tia trc tip v phn x kh nhau ti u thu. Nut thm 1 anten th 2 ti v tr trn hay di anten gc sao cho khong cch ng truyn s tng hay gim 1na bc sng, chm tia s c tng cng thay v trit tiu.

Mch kim tra ti u thu s d s thay i ti u thu v chuyn t u thu ny n u thu khc khi tn hiu gia

tng hay suy gim.4.2. Phn tp tn s.

S bt tin ca phng php phn tp khng gian l vic tng gp i gi thnh ca anten v h thng feeder ticc trm thu v phi xy dng thp chu lc cao.

Phn tp tn s c cu hnh tng t dy song hnh c cp phn trc, nhng c 2 knh mang thng tinging nhau v tn s hot ng c th c phn chia rng hn c th l xp x 5 n 10%. Hin nay hu ht ccnc di ph c cung cp kh ngn, vic thm vo cc tn s cho vic phn tp l kh c th thc hin.


14/14

Documents

Máy phát viba