Tong quan vien thong

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  • 1. C S VIN THNG PHM VN TN

2. C s vin thng Phm Vn Tn Trang I.1 Chng I TIN TC V H THNG THNG TIN LCH S PHT TRIN CNG NGH VIN THNG IN T. PHN LOI CC NGUN TIN TC V CC H THNG THNG TIN. S NG XC NH V SNG NGU NHIN. S KHI MT H VIN THNG. S PHN CHIA CC VNG TN S (FREQUENCY ALLOCATIONS). S TRUYN SNG IN T. S O TIN TC. CC H THNG TIN L TNG. M HA (CODING). 3. C s vin thng Phm Vn Tn Trang I.2 LCH S PHT TRIN CNG NGH VIN THNG IN T. - T cui th k 18 u th k 19, cng ngh pht thanh v truyn thng bng in c pht trin. - Nm 1820, George Ohm a ra cng thc phng trnh ton hc gii thch cc tn hiu in chy qua mt dy dn rt thnh cng. - Nm 1830 Michall Faraday tm ra nh lut dn in t trng. - C th coi lch s thng tin d liu c bt u vo nm 1937 vi s pht minh in tn Samuel F. B.Morse. l h thng truyn cc xung in biu din cho cc du chm v vch (tng ng vi cc s nh phn 1, 0) trn cc ng dy ng nh cc my c in. Cc t hp khc nhau ca cc m ny thay cho cc ch, s, du,...c gi l m Morse. - Nm 1840, Morse ng k sng kin v in tn M. - Nm 1844 ng y in tn u tin c thit lp gia Baltimore v Washington DC. - Nm 1849, bn tin u tin c in ra nhng vi vn tc rt chm nhng n nm 1860 vn tc in t 15 bps. - Nm 1850, i s Boole ca George Boole to ra nn mng cho logic hc v pht trin r le in. Trong khong thi gian gian ny, cc ng cp u tin xuyn qua i ty dng lp t h thng in tn. - James Clerk Maxwell a ra hc thuyt in t trng bng cc cng thc ton hc vo nm 1980. Cn c vo cc hc thuyt ny Henrich Hertz truyn i v nhn c sng v tuyn thnh cng bng cch dng in trng ln u tin trong lch s. - Tng i in thoi u tin c thit lp vo nm 1876 (ngay sau khi Alexander Grhm Bell pht minh ra in thoi). Nm nm sau Bell bt u dch v gi ng di gia New York v Chocago. Cng khong thi gian , Guglieno Marconi ca Italia lp t mt trm pht sng v tuyn pht cc tn hiu in tn. - Nm 1900, Einstein, mt nh vt l ni ting v hc thuyt tng i vit rt nhiu ti liu quan trng v vt l cht rn, thng k hc, in t trng v c hc lng t. Vo khong thigian ny, phng th nghim Bell ca M pht minh v sng ch ra ng phng in cc cho cc knh thin vn xoay c. Tip theo , Le De Forest tr thnh ngu khi xng trong lnh vc vi mch in t thng qua pht minh ca ng v mt ng chn khng ba cc. Lc ny, h thng tng i tng t t ng c kh nng hot ng khng cn bng chuyn mch. - Nm 1910, Erwin Schrodinger thit lp nn tng cho c hc lng t thng qua cng b ca ng v cn bng sng gii thch cu to nguyn t v cc c im ca chng. Vo khng thi gian ny, pht thanh cng cng c bt du bng cch pht sng. - Nm 1920, Harold .S. Black ca phng th nghim Bell pht minh ra mt my khuch i phn hi m bn m ngy nay vn cn dng trong lnh vc vin thng v cng ng my in m. - V.K.Zworykin (M) pht minh ra n hnh cho v tuyn truyn hnh v cp ng trc (phng tin truyn dn hiu qu hn cc dy ng bnh thng). - Cui nhng nm 1940, phng th nghim Bell t ra nn mng cho cho cc cht bn dn c tch hp cao. Howard Aiken ca i hc Harward cng tc vi IBM thnh cng trong vic lp t mt my in ton u tin c kch thc 50 feets v 8 feets. V sau , J.Presper Ecker vi Jonh Mauchly ca i chc Pnnylvania pht trin my in ton ln mt bc gi l my in ton ENIAC. Von Neuman da vo y pht trin my in ton c lu gi chng trnh. - Vo nhng nm 1960, cc loi LSI (Large Scale Interated), cc my in ton mini, cp quang v my phn chia thi gian c pht trin v thng mi ho thnh cng. - Vo nhng nm 1970, truyn hnh nh qua v tinh, cc h thng tng i in t cng ln lt ra i. 4. C s vin thng Phm Vn Tn Trang I.3 Phn loi cc ngun tin tc v cc h thng thng tin. - Mt ngun tin digital ( digital information sourse ) to ra 1 tp hp hu hn cc bn tin ( Message ) c th. V d : My nh ch ; c mt s hu hn cc k t ( bn tin ) c pht ra t ngun ny. - Mt ngun tin tc analog to ra cc bn tin c xc nh lin tc. V d mt micro: in th ra din t tin tc v m thanh v n c phn b trn mt dy lin tc nhiu tr gi. - H thng thng tin digital chuyn tin tc t mt ngun digital n thit b thu ( Sink ). - H thng thng tin analog chuyn tin tc t mt ngun analog n Sink. Ni mt cch cht ch, sng digital c nh ngha nh l mt hm theo thi gian v ch c mt tp hp cc tr gi ri rc. Nu dng sng digital l dng sng nh phn, th ch c hai tr gi. Dng sng analog l mt hm theo thi gian c khong cc tr gi lin tc. Mt h thng thng tin digital in t thng c cc in th v dng in vi dng sng digital. Tuy nhin, n vn c th c cc dng sng analog. Th d, tin tc t mt ngun nh phn c th pht n sink bng cch dng mt sng sin 1000Hz din t bit 1 v mt sng sin 500Hz din t bit 0. y ngun tin tc digital c pht n sink bng cch dng cc sng analog, nhng vn c gi l h thng vin thng digital. Xa hn na, sng analog ny c gi l tn hiu digital v n m t 1 ngun tin digital. Tng t, mt tn hiu analog m t mt ngun tin analog . T quan im ta thy mt k s Vin thng digital cn hiu lm sao phn tch cc mch analog cng nh cc mch digital. Vin thng digital c nhng li im: - Cc mch digital tng i r c th c dng. - Khong tc ng ln hn. ( Khong gia cc tr ln nht v nh nht ). - D liu t ting ni, hnh v cc ngun d liu khc c th c trn ln v truyn i trn cng mt h truyn digital. - Trong cc h truyn vi khong cch xa, nhiu khng chng cht t repeater n repeater. ( Trm pht li ). - Sai s trong d liu c phn tch th nh, d khi c mt lng nhiu ln trn tn hiu thu c. - Nhiu c th c sa cha ( corrected ) bng cch dng s m ha. Nhng n cng c nhng bt li: - Thng thng, n cn mt h rng dy tn ( Band width ) ln hn h analog. - Cn n s ng b ha. Vi nhiu u im, cc h digital tr nn ngy cng ph bin. Sng xc nh v sng ngu nhin. Trong cc h Vin thng, ta phn cc dng sng lm hai loi ln: Xc nh v Ngu nhin. - nh ngha: Mt dng sng xc nh c th c m hnh ha nh mt hm hon ton ring bit ca thi gian. Th d: Nu w(t) = A cos ( 0t + o ) Din t mt dng sng , vi A, 0 , o l cc hng bit. Th dng sng w(t) c ni l c xc nh. - nh ngha: Mt dng sng ngu nhin khng th c chuyn bit ha hon ton nh l nt hm theo thi gian v phi m hnh ha 1 cch xc xut. Cc dng sng biu din mt ngun khng th xc nh c. Th d, trong h vin thng digital, ta c th gi tin tc ng vi bt k mt mu t no - Mi mu t c biu din bng mt dng sng xc nh. Nhng khi ta xt dng sng c pht t ngun ta thy rng l dng sng ngu nhin, v ta khng bit chnh xc nhng k t s c pht. 5. C s vin thng Phm Vn Tn Trang I.4 Do , ta thc s cn thit k h vin thng dng dng sng ngu nhin v tt nhin bt k nhiu no c a vo s cng c m t bng mt dng sng ngu nhin. K thut ny cn n nhng khi nim v xc sut v thng k. ( S lm vic phn tch v thit k phc tp hn ). Nhnng may thay , nu ta trnh by tn hiu bng dng sng tiu biu xc nh, th ta vn c th c hu ht, nhng khng tt c cc kt qu. S KHI MT H THNG VIN THNG. Hnh 1.1 S khi ca mt h thng vin thng. Ch ch mt h Vin thng l truyn mt tin tc t ngun, k hiu l s(t), n Sink. Tin tc ly ra t Sink k hiu l (t); tin tc c th l digital hay analog, ty vo h c dng. N c th l tin tc v Video, audio hay vi loi khc. ~s Trong cc h multiplex ( a hp ), c th s c nhiu ngun vo v nhiu Sink. Ph ca s(t) v (t) tp trung quanh f = 0. Chng c gi l nhng tn hiu bng gc ( base band ). ~s Khi x l tn hiu: my pht ty iu kin ngun sao cho s truyn c hiu qu. Th d: Trong 1 h digital, n l mt vi x l. Trong h analog, n khng g hn l 1 lc h thng. Trong h lai, n l mch ly mu tin tc vo ( analog ) v digital - ha c mt bin iu m xung ( Pulse code modulation ) PCM. Tn hiu ra ca khi XLTH my pht cng l tn hiu bng gc v cc tn s tp trung gn f = 0. Khi sng mang: my pht i tn hiu bng gc x l thnh mt bng tn truyn a vo knh truyn. Th d: Nu knh gm mt cp dy xon ( twisted - pair ) telephone, ph ca sm(t) s nm trong dy m tn ( audio ), t 300 -> 3.700Hz. Nhng nu knh gm cp quang, ph ca sm(t) s l tn s nh sng. - Nu knh truyn i nhng tn hiu bng gc, khng cn dng khi sng mang v sm(t) c th l tn hiu ra ca khi XLTH. - Khi sng mang th cn khi knh c th ch truyn cc tn s thuc 1 bng xung quanh fc , vi fc >> 0. Trong trng hp ny sm(t) c gi l tn hiu dy thng ( Band pass Signal ). V n c thit k c nhng tn s thuc 1 bng quanh fc. Th d, mt i pht bin iu AM vi mt tn s kt hp 850 KHz c sng mang fc = 850 KHz. S p tn hiu bng gc dng sng s(t) thnh tn hiu dy thng sm(t) c gi l s bin iu ( modulation ). ( s(t) l tn hiu audio trong i pht AM ). Tn hiu dy thng bt k c dng: sm(t0 = s (t) cos [ c(t) + (t) ] Vi c = 2fc, fc l tn s sng mang. Nu s(t) = 1 v (t) = 0 th sm(t) s l mt tn hiu hnh sin thun ty vi f = fc v bng tn bng 0. 6. C s vin thng Phm Vn Tn Trang I.5 Trong s bin iu bi mch sng mang, sng vo s(t) lm cho R (t) v/hoc (t) thay i nh l mt hm ca s(t). S thay i trong R (t) v (t) lm cho sm(t) c mt kh bng ph thuc vo nhng tnh cht ca s(t0 v vo hm p c dng pht ra R (t) v (t). Cc knh truyn: C th phn chia lm 2 loi: dy mm ( softwire ) v dy cng (hardwire). Vi loi knh dy mm tiu biu nh: Khng kh, chn khng v nc bin. Vi loi knh truyn dy cng: Cp dy xon telephone, cp ng trc, ng dn sng v cp quang. Mt cch tng qut, knh truyn lm gim tn hiu, nhiu ca knh truyn v / hoc nhiu do my thu khin cho ~s (t) b xu i so vi ngun. Nhiu ca knh c s gia tng t ngun in, dy cao th, s nh la hoc nhiu do s ng ngt ca mt computer. Knh c th cha b phn khuch i tc ng, th d: H thng repeater trong telephone hoc nh v tinh tip chuyn trong h thng vin thng trong khng gian. D nhin, cc b phn ny cn thit gi cho tn hiu ln hn nhiu. Knh cng c th c nhiu ng ( multiple paths ) gia input v output v chng c thi gian tr ( time delay ), tnh cht gim bin ( attenuation ) khc nhau. Nhng tnh cht ny c th thay i theo thi gian. S thay i ny lm thay i bt thng ( fading ) tn hiu ng ra ca knh. ( Ta c th quan st s fading khi nghe khi nghe 1 i sng ngn xa ). My thu nhn tn hiu ng ra ca knh v i n thnh tn hiu bng gc. S phn chia cc vng tN s (Frequency Allocations). Trong cc h thng tin dng khng kh lm knh truyn, cc iu kin v giao thoa v truyn sng th ph thuc cht ch vo tn s truyn. V mt l thuyt, bt k mt kiu bin iu no (Am, Fm, mt bng cnh - single sideband, phase shift keying, frequency shift keying...) u c th c dng cho bt k tn s truyn no. Tuy nhin, theo nhng qui c quc t, kiu bin iu rng bng, loi tin c truyn cn c xp t cho tng bng tn. Bng sau y cho danh sch cc bng tn, k hiu, iu kin truyn v cng dng tiu biu ca chng. Bng tn K hiu t tnh truyn Nhng ng dng tiu biu 3 - 30KHz VLF very low frequency Sng t. Suy gim t ngy v m. Nhiu khng kh cao Thng tin di nc 30- 300KHz LF low frequency Tng t VLF. t tin cy. B hp thu vo ban ngy Hng dn radio cho hi hnh 300- 3000KHz MF Medium frequency Sng t v sng tri ban m. Suy gim t vo ban v nhiu vo