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Telecomunications

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Elektrotehnička škola „Nikola Tesla“ Beograd,Kraljice Natalije 31

Maturski rad iz Sistema prenosa Tema: Satelitski mobilni sistemi

Mentor:Dejana Lučić,prof Učenik:Miloš Milenković

Beograd,Jun 2013.

Sadržaj

Uvod.................................................................................................2
Osnovne komponente satelitskih sistema.........................................4
Globalni javni mobilni telekomunikacioni servisi(GMPCS)...........6
Sistem GLOBALSTAR...................................................................8
Kosmički Segment...........................................................................10
Frekvencijski opseg.........................................................................11
Napajanje.........................................................................................14
Zemaljska Stanica............................................................................15
Zemaljski Segment..........................................................................17
HUB centralna stanica.....................................................................17
VSAT terminali...............................................................................18
Zaključak.........................................................................................19
Literatura.........................................................................................20

♦Uvod:

Satelitske komunikacije su veoma napredne i predstavljaju budućnost. Guglielmo Marconi jednom je rekao :“Nemoguće je postaviti granice bežičnog prenosa“. Satelitske komunikacije funkcionišu kao globalne bežične mreže veze sa satelitima kao repetitorskim stanicama.
Obezbedjuju mikrotalasnu vezu izmedju dve geografski udaljene lokacije.
Sateliti se nalaze na velikim visinama iz jednog razloga,a to je da bi se pokrilo što veće područje na Zemlji.
-Karakteristike Satelitskih komunikacija:
Široko je područje pokrivanja Zemljine površine;
Difuzan prenos;
Velika širina propusnog opsega kanala;
Nezavisnost cene veze od rastojanja te veze i satelita;
Veće kašnjenje pri prenosu.
Mobilni satelitski sistemi (LAND MOBILE SATELLITE), u sve većoj meri postaju značajniji deo treće i četvrte generacije bežičnih sistema.Oni su značajni za analizu ,zbog toga što imaju mogućnost da po niskoj ceni opslužuju veliki broj korisnika u velikim oblastima.
Takođe omogućuju korišćenje nekih naprednih servisa koji nisu moguću kod zemaljskih mobilnih sistema,kao što je npr.Land Mobile Terrestrial-LMT.
Kako kvalitet LMS ,pre svega,zavisi od prenosnog kanala između satelita i mobilnog korisnika.od interesa je postaviti tačan i pouzdan statistički model datog kanala,koji bi se koristio za procenu preformansi celokupnog sistema.
U literaturi modeli LMS kanala se mogu klasifikovati na pojedinačne i kombinovane.

Kod pojedinačnih modela,kanal je određen jednom odgovarajućom statističkom raspodelom,dok se kod kombinovanih modela kanal opisuje sa 2 ili više statističke raspodele,uz odgovarajuće težinsko sumiranje. Pojedinačni modeli su pogodni za stacionarnih uslova u kanalu kod kojih statistika kanala ostaje konstantna tokom vremenskog intervala od interesa,dok kombinovani modeli odgovaraju nestacionarnim kanalima čija se statistika u velikoj meri menja u vremenu.

♦Osnovne komponente satelitskih sistema:

Svaki komunikacioni satelit u najjednostavnijoj formi,bez obzira u kojoj putanji orbitira podrazumeva transmisiju signala od izvorne zemaljske stanice ka satelitu i retransmisiju signala od satelite nazad ka Zemlji. Pri tome se na zemlji može odabrati neka konkretna zemaljska stanica ili se može raditi o difuznoj radio-televizijskoj mreži. Otuda je jasno da sateliti moraju imati prijemnik(prijemnu antenu),predajnik(predajnu antenu) i neke uređaje za povezivanje ovih uređaja,te izvor napajanja koji pokreće ceo sistem i svu tehnologiju. Stvarna priroda ovih komponenata će se razlikovati ,zavisno od orbite i sistema arhitekture,ili ipak svaki satelit mora imati te osnovne uređaje.

Prijemnik i predajnik na satelitu se nazivaju zajedničkim imenom transponder. On može biti ponavljač koji pojačava signal i frekvencijski ga pomera,a može biti i kompleksniji pa tada izvodi i neke dodatne operacije,kao što su detekcija,demultipleksiranje ,demodulacija,itd. Predajnik u okviru transpondera,je zasnovan na pojačavaču snage koji radi na granici zasićenja,da bi obezbedio maksimalnu iskorišćenost DC napajanja. Tipični nivoi snage su 10W,a koeficijent korisnog dejstva samo 30 %.
Snaga koju predajnik na satelitu treba imati zavisi u prvom redu od visine orbite. S obzirom da su geostacionarne orbite oko 100 puta udaljenije od LEO orbita,biće potrebno mnogostruko veća snaga. Na sreću,mogu se promeniti neke druge stvari što će doprineti smanjanju te ogromne razlike u potrebi za ogromnom snagom,pri čemu se misli na antenu. Međutim,pojam velike snage na satelitu podrazumeva i snagu od 200W ,za razliku od pojma velike snage predajnika u zemaljskom radio sistemu gde je to reda više kW.

♦Globalni javni mobilni telekomunikacioni servisi-GMPCS

* Oni obezbeđuju vezu sa mobilnim terminalima. * Obezbeđuju kompletan opseg uskopojasnih i širokopojasnih usluga u bilo koje vreme i bilo gde na svetu. * Činiće važnu komponentu Globalne Informacione Infrastrukture- GII (Global Information Infrastructure).

* Komercijalna primena od 1999.godine. * 200 zemalja potpisale su licence za korišćenje GMPSC-a. * Većina su LEO i MEO globalni sistemi * IRIDUM * GLOBALSTAR * ICO * ODYSSEY * THURAYA(GEO) * Osnovcni princip : ostvariti vezu putem satelita sa personalnim mobilnim terminalima * Mane u odnosu na ćelijski sistem: * GMPSC sateliti imaju ograničen život * Slabije performanse( veće kašnjenje,slabiji signal,itd) * Skupa infrastruktura * Rešenje je kooperacija sa ćelijskim sistemom * DUAL MODE * Raspoloživost * Profitabilnost ( Ćelijski sistem uzima deo profita od satelitskih poziva)

| LEO | MEO | GEO | Cena Satelita | Srednja do visoke | Niska | Niska | Cena opreme na zemlji | Srednja do visoke | Srednja | Niska | Vek satelita (godina) | 5-7.5 | 10-15 | 10-15 | Složenost funkcionisanja | Visoka | Visoka | Niska | Globalna Komunikacija | Zahtev međuveze preko zemaljskih stanica i satelita | Zahtev međuveze preko zemaljskih stanica | Nije moguća | Tipično kašnjenje govora | Neprimetno | Neprimetno | Primetno | Gubitak poziva | Čest | Redak | Nikad |
Tabela:Poređenje LEO,MEO i GEO satelitskih sistema.

♦Sistem GLOBALSTAR

* Satelitski LEO digitalni komunikacioni sistem * Sistem je namenjen za : * Područja koja nisu pokrivena ćelijskim sistemom * Poslovna putovanja * Države u razvoju * Mogućnost opsluživanja 9 miliona pretplatnika * Usluge GLOBALSTAR-a: * Digitalizovani govor * Prenos podataka do 9.6 kbit/s * SMS * Faks grupe 3 * Određivanje tačne pozicije * Konstacija Satelita: * 48 LEO satelita + 4 rezervna * 8 orbitalnih ravni -52˚ u odnosu na Ekvator * Visina 1414 km * Postavljeno je 38 zemaljskih stanica širom sveta * Sistem pokriva preko 100 zemalja * DUAL-MOD-ni ručni telefoni-Globalstar i GSM ili čak trostruko modni za AMPS * Telefoniranje: telefon automatski traži pogodnu ćelijsku mrežu; ako ćelijska mreža nije dostupna ,telefon se automatski prebacuje na GLOBALSTAR-ov satelitski mod i poziv se rutira na najbliži satelit. * Satelit prosleđuje poziv najbližoj zemaljskoj stanici. * Zemaljska stanica prebacuje poziv u lokalnu PTSN za to područje i šalje ga na krajnju destinaciju * Svaka zemaljska stanica može da obezbedi konekcije za više od 1000 korisnika istovremeno as PSTN. * Podržano pružanje usluga u područjima koja nemaju fiksnu komunikacionu strukturu. * Korisnik uspostavlja poziv preko fiksnog satelitskog telefona koji je sličan telefonskoj govornici. * Ovi fiksni satelitski telefoni se mogu koristiti od strane jednog korisnika ili povezani preko glavnog interfejsa mogu opsluživati jednu lokalnu mrežu.

Princip rada sistema GLOBALSTAR

* PoDasd

♦Kosmički segment:

* -Satelit-RF primopredajnik. * -Satelit mora biti uvek u vidnom polju zemaljskih stanica. * -Prima signal na jednoj frekvenciji,pojačava ga i šalje na drugoj frekvenciji. * -Satelitski transponder ima kapacitet oko 100 Mbit/s. * -Pomeranje položaja satelita-centralna stanica ima sistem za automatsko praćenje satelita.

GLOBALSTAR satelit.

♦Frekvencijski opseg

-Na svom putu od Zemlje do satelita i obrnuto radio-talasi moraju da prođu kroz atmosferu. Iz teorije radio-komunikacija poznato je da atmosfera unosi dodatno slabljenje signala,koje ima selektivni karakter. Naime, istraživanja su pokazala da u celom opsegu frekvencija radio-talasa postoje dva „prozora“. Na talase čije su frekvencije unutar tih prozora ,atmosfera ne unosi slabljenje ili je ono neznatno.
-Jedan od prozora predstavlja opseg frekvencija koji se nalazi u okolini vidljivih zraka i infracrvenih zraka. Taj opseg nije interesantan za telekomunikacije,za razliku od drugog. Na granične frekvencije ovog prozora utiču jonosfera i troposfera. Ispitivanje uslova prostiranja kroz jonosferu pokazala su jedan interesantan fenomen. Postoji kritična frekvencija jonosfere. Ona deli ceo opseg radio-talasa na dva dela: talasi nižih frekvencija od kritične reflektuju se od jonosfere ka zemlji,a talasi viših frekvencija prodiru kroz nju. Donja granična frekvencija drugogo prozora je kritična frekvencija jonosfera. Kritična frekvencija jonosfere,koja zavisi od mnogo faktora kao što su: doba dana i noći,solarna aktivnost,trajektorija prostiranja,geografska pozicija zemaljske stanice,itd. I obično kreće između 2 i 70 MHz.
-Na gornju graničnu frekvenciju drugog prozora utiče troposfera u kojoj kiša i gasovi apsorbuju jedan deo energije talasa. Ta apsorpcija postaje naročito značajna za frekvencije veće od nekoliko desetina GHz. Konstatovano je da gornja granična frekvencija u izuzetno nepovoljnim okolnostima može da iznosi oko 10 GHz,u normalnim uslovima se nalazi oko 20 GHz,a u ređim slučajevima može da se popne i do 50 GHz. Prema tome,opseg frekvencija između nekoliko desetina MHz i 20 GHz predstavlja opseg koji definiše ovaj prozor u kosmos.
-Satelitske veze mogu biti uspostavljene u različitim frekvencijskim opsezima,a koriste drugačije noseće frekvencije za prijem (zemaljska stanica-satelit) i predaju (satelit-zemaljska stanica). U Tabeli je dat prikaz najčešće korišćenih frekvencijskih područja.

-Frekvencijsko područje C (4GHz) je naviše korišćeno kod satelitskih komunikacija prve generacije,međutim to frekventno područje se danas koristi za zemaljske mikrotalasne veze. Današnji trend je korišćenje sve viših i viših frekvencija i to iz područja Ku i Ka (11 i 20 GHz),ali pri tim frekvencijama velike probleme prave atmosferske nepogode kao što su kiša,magla i sneg.
-Izbor najpogodnijeg RF opsega za satelitske telekomunikacije je složen problem,jer je potrebno naći kompromis između raznih,često oprečnih ekonomsko-tehničkih zahteva. Za komercijalnu upotrebu telekomunikacionog satelita prvi i osnovni uslov jeste ekonomska rentabilnost. Visoka cena transpondera i lansiranje satelita u orbitu mogu se opravdati samo u onim slučajevima kad se obezbedi veliki informacijski kapacitet satelita. Informacioni kapacitet zavisi od dva faktora: širine frekvencijskog opsega i odnosa signal/šum,pa se povećanje informacionog kapaciteta može postići povećavanjem frekvencijskog opsega. Međutim, cena telekomunikacionih sklopova (filtara,pojačavača..) proporcionalna je relativnoj širini njihovog opsega. Smatra se da je neki sistem uskopojasan (A to znači da mu je cena razumna),ako je zadovoljen uslov B/fc<<1, gde je fc-centralna frekvencija propusnog opsega,B-širina propusnog opsega. To znači da potrebnu širinu propusnog opsega možemo utoliko ekonomičnije realizovati,ukoliko je viša centralna frekvencija sistema. Ovaj zaključak se koristi u strategiji planiranja svih telekomunikacionih sistema,a za satelitske telekomunikacije je to uslov sine qua non(bez kojeg se ne može). Frekvencije koje se koriste u satelitskoj komunikaciji su podeljenje u četiri opsega: Frekventno područje | Prijem(GHz) | Predaja(GHz) | Nedostaci | C | 4(3,7-4,2) | 6(5,925-6,425) | Interferencija sa zemaljskim vezama | Ku | 11(17,7-12,2) | 14(14,0-14,5) | Gušenja prilikom kiša | Ka | 20(17,7-21,7) | 30(27,5-30,5) | Visoka cena opreme | L/S | 1,6(1,610-1,625) | 2,4(2,483-2,500) | Interferencije sa ISM područjem |

-S obzirom na slabljenje RF signala na satelitskoj deonici,pokazalo se da je najpogodnija frekvencija od 10 GHz. Međutim,na tim frekvencijama rade zemaljski radio-relejni (RR) sistemi,što predstavlja problem za satelitske telekomunikacije,ali i prednost,s obzirom da se mogu koristiti tehnologije koje su razrađene za RR sisteme. Zbog toga se zaključuje da je za satelitske komunikacije koristiti frekvencije iz opsega 4-10 GHz. Da bi se izbeglo uzajamno ometanje,donesene su međunarodne preporuke o zauzimanju pojedinih kanala o limitiranju efektivne snage predajnika,kako na zemlji tako i na satelitu.
-Međutim,i pored toga primećuje se tendencija „preseljavanja“ u više frekventno područje,tako da već INTELSAT V nosi nekoliko transpondera koji rade na 14 GHz na ulaznoj deonici i na 11Ghz na silaznoj deonici,a radi se i na tome da se osvoje frekvencije između vrhova apsorpcionog opsega slabljenja izmedju 21-58 i 60-118 Ghz. Izborom veće frekvencije smanjuju se dimenzije svih sklopova ( antena,talasovoda,skretnica,itd) što je od posebne koristi. Većim dobitkom antene se može kompenzovati povećano slabljenje na satelitskoj deonici.
-Relacija B/fc<<1 se u praksi svodi na B/fc0,1 i ako se ona primeni na satelite koji radi sa nosiocem 4/6 GHz ,dobili bi širinu opsega B (400-600)MHz,a za RF 14/11 GHz bi to bilo 1100-1400 MHz. Međutim,stvarna širina propusnog opsega jednog satelitskog kanala ograničena je pojavom šuma,kanalne i međukanalne interferencije i frekvencijski selektivnim fedingom.

♦Napajanje:

-Ako se pitate ,zašto se problem slabog signala sa satelita ne reši naprosto većom snagom transpondera ,odgovor je: transponder nije priključen na postrojanja zemaljske elektrodistribucije,već na bateriju koju pune solarne ćelije! A to znači da nema mnogo energije na raspolaganju na satelitu ,pa ono što ima mora biti racionalno iskorišćeno. Naime,postoje praktična ograničenja koliko velik može biti solarni panel koji nosi solarne ćelije i postoji ograničenje koliko one mogu konvertovati sunčeve energije u električnu. Tu je i dodatni problem u samom predajniku koji ima relativno nizak koeficijent iskorišćenja ,pa od 1000W snage napajanja,on u koristan signal pretvori jedna 150 do 200 W ( 10 do 15 %). Pored toga,solarne ćelije na satelitu mogu praktično dati samo nekoliko hiljada W električne snage. Osim toga,satelit mora biti sposoban da radi i u vreme kad mu sunce nije vidljivo,kad zemlja prolazi između sunca i satelita. To zahteva postojanje baterija koje se pune za vreme aktivnosti solarnih ćelija ,kad se jedan deo energije skladišti za upotrebu kad satelit bude u senci.

♦Zemaljska stanica

-Zemaljske stanice se najčešće sastoje od sledećih pet podsistema: antena,kombinovani niskošumni prijemnik/down-konvertor,kombinovani up-konvertor i pojačavač snage,srednjefrekventni up-konvertor i lokalni oscilator.
-Inače veličina zemaljske stanice zavisi najviše od toga na kojoj visini se nalazi satelit sa kime ona održava vezu. Investicije za njihovu izgradnju mogu biti veoma velike. Najdelikatniji je antenski sistem. Da bi se kompenzovala relativno mala usmerenost antenskog sistema, grade se ogromne antene koje imaju paraboloidni reflektor čiji prečnik otvora iznosi i do 30m. Dobitak takvih antena na frekvencijama iz opsega 6GHz je oko 60 dB. Snaga na izlazu predajnika je oko 1kW do 5kW,pa EIRP iznosi oko 5000 MW. Najveći problem je u tome što ovakav antenski sistem teži do 400 tona,a morao bi biti pokretan! Zaista se ne bi isplatilo da se za svaki novolansirani satelit ,koji može da ima drugi pologaš,gradi nov antenski sistem. Osim toga postoji i problem „klizanja“ satelita zbog uticaja gravitacija drugih nebeskih tela. Osim tipova zemaljskih stanija postoje i drugi,zavisno od visine orbite ciljanog satelita.
-Primer jedne male satelitske stanice bi se mogao ilustrovati na osnovu sistema USAT koji radi u komunikacionom sistemu u Ka frekvencijskom opsegu. Napredne tehnologije omogućavaju korisnicima zemaljskih stanica da održavaju zadovoljavajući protok informacija i uz upotrebu malih antena i predajnika malih snaga,što omogućava laganu konfiguraciju,integraciju i održavanje. USAT stanice imaju antenu prečnika 35 do 60 cm,pojačavače snage od 0.25 i 4 W. Šum u prijemnicima je 4dB,a korisnički interfejs je na frekvenciji od 70 MHz. Down-konvertor prima signale i konvertuje ih sa 20 GHz na 70 MHz sa poajačanjem od 26dB. Up-konvertor za veliku snagu je konstruisan tako da kombinuje drugi stepen konverzije:sa UHF na Ka opseg od 30 GHz sa pojačanjem do 4 W. Dimenzije antena zemaljskih stanica USAT-a su 35 cm do 60 com,a ređe 1.2 metra.

Jedna od najvećih razlika između LEO i GEO satelitskih sistema je u antenama,jer GEO satelit zahteva 10 000puta jaču snagu predajnika,ako su svi drugi parametri i komponente isti. To se rešava povećanjem dijametra antene,tako,ako se on udvostruči,time će se smanjiti površina zraka na ¼,što se iskazuje gainom ili dobitkom antene.Gain nam kaže koliko više snage će pasti na 1 cm² sa tom antenom,nego da je snaga predajnika rasejana uniformno u svim smerovima.Ova veća antena će imati gain četiri puta veći od manje antene. S druge strane,GEO satelitski sistem ima prednost jer ne zahteva značajno prilagođavanje antene zemaljske stanice položaju satelita,budući da je položaj satelita gotovo fiksan,za razliku od položaja LEO satelita ,za kojim sa zemaljske stanice se mora tragati.

♦Zemaljski segment

*HUB- centralna stanica : * Lokacija u blizini direkcije korisnika i HOST računara. * Obezbeđuje vezu svih VSAT terminala –veza sa HOST računarom i bankom podataka. * RF terminal : * Antena prečnika 4.5-11m * Konvertori (naviše,naniže) * Niskošumni prijemnici * Pojačavači snage (50-300W) * Procesor za upravljanje mrežom,potpuno kontroliše rad mreže.

*VSAT Terminali * Spoljašnja jedinica * Mala parabolična antena (3,6-1,2 ni manje) * Prijemnik –niskošumni konvertor koji pojačava signal i menja frekvenciju iz opsega 4,11,12 GHz 1 GHz * Unutrašnja jedinica * Predajnik (tranzistorski pojačavač 1-2W) * Interfejs za vezu sa korisničkim delom terminala * Modemi * Prijemni i predajni moduli. * Potrošnja(energija iz energetske mreže) 15-300 W.

♦VSAT Terminal

*Softver VSAT: * Dominantna uloga * Snaga sistema se određuje softverom za upravljanje * Organizuje ,upravlja i optimiziju ukupni saobraćaj * Predstavlja 80% razvojnih troškova sistema i glavni je parametar mogućnosti sistema.
*Dobre strane VSAT-a: * Raspoložvost preko 96% (dobro projektovanje 99%) * Kvalitet –mala verovatnoća greške * Mogućnost uvođenja novih i renoviranje postojećih telekomunikacionih servisa * Prenos podataka do 2Mbit/s ( 34Mbit/s) * Kratko vreme instaliranja i uspostavljanja servisa.
*Loše strane VSAT-a:
-Zavisi od svakog terminala posebno.
-Opšti slučaj: * Tajnost informacija se mora štititi dodatnim merama * Slabljenje usled kiše (11/12, 14 GHz) * Ometanje satelitskih veza od strane zemaljskih radio-relejnih stanica (2,4GHz;8GHz; 23GHz) rešava se propisima.

Zaključak:
-Uvideli smo da je budućnost komunikacije u satelitskom prenosu. Sigurniji i lakši prenos je ono ka čemu se teži.Prenos će kroz budućnost biti sve više bolji i sigurniji. Napredak satelitske komunikacije čovečanstvu omogućava sve bolji prenos,lakše komunikacije i više opcija.Ova vrsta komunikacije je u napretku,i samim tim kvalitet,pouzdanost,sigurnost su više dostupni čoveku radi poboljšanja komunikacija i korišćenja.

Literatura: http://www.link-elearning.com/lekcija-Satelitske-komunikacije_5706 http://muricmilorad.files.wordpress.com/2011/11/satelitskekomunikacije.pdf http://www.mtt-serbia.org.rs/microwave_review/pdf/No5-03-TMangir.pdf http://ktios.net/moodle/pluginfile.php/699/mod_resource/content/1/SatelitskeKomunikacije.pdf

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