A Geographical Location Based Satellite Selection Scheme for a Novel Constellation Composed of Quasi-Geostationary Satellites

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1 A Gographical Location Basd Satllit Slction Schm for a Novl Constllation Composd of Quasi-Gostationary Satllits Tarik Talb, Umith Dharmaratna, Ni Kato, and Yoshiaki Nmoto Graduat School of Information Scincs, Tohoku Univ., apan talb, kato@it.ci.tohoku.ac.jp, nmoto@nmoto.ci.tohoku.ac.jp Abstract In ordr to raliz th dram of global broadband covrag, th nd for satllit communication systms has grown rapidly during th last fw yars. Svral Low Earth Orbit (), Mdium Earth Orbit (MEO), and Gostationary(GEO) satllit constllations hav bn thus proposd in th rcnt litratur. Howvr, ths constllations ithr rquir a potntial numbr of satllits or ar unabl to provid data transmission with high lvation angls. This papr proposs a nw satllit constllation composd of Quasi Gostationary Satllits. Th main advantag of th constllation is in its ability to provid global covrag with a significantly small numbr of satllits whil, at th sam tim, maintaining high lvation angls. Sinc nd-trminals can b simultanously covrd by plural satllits in th proposd constllation, a schm is proposd to slct th most appropriat satllit for communication. Th slction is basd on th gographical location information of nd-trminals. Th fficincy of th proposd schm is vrifid through a st of simulations. Simulation rsults rval th good prformanc of th proposd mthod in rducing th dlay, th dlay variation, and ultimatly improving th ovrall quality of srvic. I. INTRODUCTION Nw multimdia srvics rquir mor cost-ffctiv, high-quality, and high-spd tlcommunication tchnologis. Larg-scal dploymnt of ths wid-band srvics in a mtropolitan ara with a potntially larg numbr of usrs is a challnging task for trrstrial tchnologis. Satllit ntwork systms ar sn as an attractiv solution to raliz th vision of a global broadband multimdia infrastructur []. This is bcaus of thir xtnsiv gographic rach, flxibl and rapid dploymnt faturs, and inhrnt multicast capabilitis. Furthrmor, givn th rcnt advancs and ongoing improvmnts in satllit tchnologis, broadband satllit basd multimdia srvics ar likly to opn a promising and strong markt for srvic providrs and oprators in th nar futur [] [3] [4]. In th rcnt litratur, a numbr of Low Earth Orbit (), Mdium Earth Orbit (MEO), and Gostationary (GEO) satllit constllations hav bn proposd to provid broadband srvics. For mor than two dcads, GEO satllit systms hav bn usd for providing commrcial srvics. Thy hav, howvr, faild in providing data transmission with high lvation angls ovr high latitud rgions. In Tokyo, for xampl, th lvation dos not xcd. In GEO systms, nd-trminals, mainly mobil usrs, xprinc consquntly frqunt cut-offs of propagation signals du to high buildings and mountains. Th proposd and MEO satllit constllations rquir, on th othr hand, a larg numbr of satllits for global covrag. Thy ar thus charactrizd by frqunt handovr occurrncs [5] [6]. Additionally, thir satllits can b usd for only a short lif span. As a rmdy to th abov issus, this papr proposs a novl constllation composd of long-lif span Quasi-Gostationary Orbit (Quasi-GSO) satllits. Th strngth of th constllation is in its ability to provid global covrag with a significantly small numbr of satllits whil, at th sam tim, maintaining high lvation angls. Th architctur of th proposd constllation is dynamic in its natur, yt xhibits significantly lss mobility than or MEO constllations. Nvrthlss, to dal with issus rlatd to th initial st-up of connctions or handovr phnomnon, nd-trminals should b acquird with th ability of slcting th most appropriat satllit. Applying th baslin satllit slction procdur [7], th most widly usd mthod for rsarch on and MEO satllit constllations, to th proposd constllation rsults in unncssarily longr dlays and highr lvls of dlay variation. To dal with such an issu, th covrag ara of th constllation is dividd into a numbr of rgions and a satllit slction schm is dvlopd basd on information rlatd to th gographical location of nd-trminals. Th rmaindr of this papr is structurd as follows. Sction II givs a brif dscription of th Quasi-GSO satllit systms and highlights som of thir main mrits. Th ky dsign philosophy and distinct faturs that wr incorporatd in th proposd constllation ar prsntd in Sction III. Following this, Sction IV portrays in dtail th satllit slction schm proposd for th considrd Quasi-Gostationary satllit constllation. In Sction V, th proposd satllit slction mthod is valuatd through a st of simulations. Simulation rsults ar discussd in th sam sction. Th papr concluds in Sction VI with a summary rcapping th main advantags and achivmnts of th proposd constllation. II. QUASI-GEOSTATIONARY ORBIT SATELLITE SYSTEM For mor than thr dcads, satllit systms hav bn succssful in providing som commrcial srvics. Currntly, thr ar two typs of broadband satllit systms: low-altitud arth orbit and gostationary satllit systms. Th formr rquirs a hug infrastructur invstmnt and has cratd som doubts on its conomical practicality mainly aftr th rcnt financial failur of th Iridium systm. Th lattr, on th

2 Systm ID Numbr Satllit Satllit Satllit B S Rgions 3 N A 4 Latitud Elvation Angl Tim (s) C 9 Fig.. Elvation angl variation of th thr satllits obsrvd from Tokyo othr hand, fails to provid a consistntly high-lvation angl and consquntly xprincs frqunt incidncs of signal propagation cut-off du to tall buildings or mountains. Nds for a systm whr satllits hav a clar lin of sight to th ground, in conjunction with covrag of high latitud rgions, hav sparkd th dvlopmnt of nw cost-ffctiv satllit communication systms calld Quasi-Gostationary Orbit satllit systms [8]. Quasi-GSO satllit systms provid constant covrag ovr a particular ara of th Earth through mploymnt of a sris of satllits. Th Quasi-GSO satllits complt on full orbit pr day in synchronization with th Earth s rotation, dscribing a north-south figur of ight locus cntrd around a point on th quator. Th Quasi-GSO satllit systm consists of at last thr satllits placd in circular orbits at an inclination angl of approximatly rlativ to th gostationary orbit. Th satllits ar placd in orbit such that on would b positiond almost dirctly abov th targt ara at any givn point in tim. Th Quasi-GSO satllits guarant a minimum angl and highr valus of lvation of lvation of at last angl can b achivd by using mor than thr satllits. Fig. shows th variation of th lvation angls of a QuasiGSO systm mad of thr satllits obsrvd by a trminal in Tokyo. Quasi-GSO satllit systms ar a promising altrnativ to convntional satllits in gostationary or low-altitud orbits. Thy can dlivr hug amounts of broadcasts at high spd with high quality, and without bing obstructd by tall buildings. Thy hav bn considrd fficint for vhicular satllit communications, frquncy sharing in fixd satllit communications, positioning systms, and north and south polar rgions obsrvation. In addition, thy ar particularly wll suitd to th provision of vido-on-dmand, a srvic whr signal propagation blockings ar not tolratd. It should b strssd that th inhrnt issus with latncy of QuasiGSO satllits should not pos challngs for dlivry of high quality multimdia. This papr aims to study how a constllation of ths satllits could b usd to provid global broadband covrag. III. OVERVIEW OF THE Q UASI -GSO S ATELLITE C ONSTELLATION Th abstract configuration of th constllation is concptually dpictd in Fig.. Th figur portrays th orbits of six E7 W3 W7 W E5 E Longitud No Covrag Covrag by on Satllit Covrag by two Satllits Covrag by thr Satllits Fig.. Illustration of th proposd Quasi-GSO constllation (This is an ditd imag, gnratd by th Satllit constllation Visualizr (SaVi), at Quasi-GSO systms. Each systm consists of thr satllits, giving ris to only 8 satllits in th whol constllation. Stting th minimum valu of th lvation angl to 4, th constllation can provid covrag to th whol glob. It should b mphasizd that th constllation can provid lvation angls largly highr than 4 ovr middl-latitud rgions. Th longr lif span of satllits in gosynchronous orbits, in conjunction with th small numbr of rquird satllits, maks th cost of th whol constllation mor rasonabl than most proposd or MEO systms. Morovr, du to th insignificant mobility charactristic of th constllation, th mobility managmnt rlatd cost of th systm bcoms chapr also. Concrning intr-satllit links, two typs ar considrd: Intra-Systm and Intr-Systm links. Intra-Systm links rfr to th thr links that connct th thr satllits of a givn systm, and ar dubbd Intra-Systm Intr-Satllit Links (Intra-SISLs) throughout this papr. Convrsly, Intr-Systm links rprsnt th thr links that joint btwn a satllit of a givn systm and its corrspondnt in th nighboring systm. Intr-Systm links ar rfrrd to as Intr-Systm Intr-Satllit Links (Intr-SISLs) throughout this papr. It should b notd that Intr-SISLs ar of fixd lngth, whras Intra-SISLs vary in lngth. For as of illustration of Fig., both Intr-Systm and Intra-Systm links ar not plottd. Systm Iridium NLS Tldsic Skybridg Clstri Globalstar Orbilink GEO(3) Quasi-GSO Orbital Latitud MEO GEO GEO Numbr Satllits Elvation Angl covrag (%) TABLE I C OMPARISON WITH OTHER SATELLITE CONSTELLATIONS

3 Tabl I compars th proposd constllation to othr satllit constllations. Th Quasi-GSO constllation is capabl of providing almost global covrag with lss numbr of satllits and high lvation angl. Th frquncy of handovr occurrncs in th constllation is significantly lowr compard to that of or MEO constllations (3 to 6 handovrs pr day). Th ntwork topology is hnc simpl and asy to manag. On th othr hand, th round trip propagation dlay is around. Th constllation is thus not suitabl for dlaysnsitiv applications. It can b, howvr, a good candidat for th provision of applications that ar not affctd by long latncy [9]. Notabl xampls ar Vido-on-Dmand (VoD), liv broadcasting, distanc larning, onlin radio, mssaging, and Global Positioning Systm srvics (GPS). Although th mobility of th proposd Quasi-GSO satllit constllation is insignificant in comparison with or MEO constllations, nd-trminals ar not always continuously connctd to th sam satllit in th constllation during th ntir communication tim. Handovr of Ground to Satllit Link (GSL) to a nw satllit may b thus rquird during th connction tim. Having th minimum lvation angl st to 4, divrsity covrag bcoms rif in this typ of constllation. Fig. 3 shows th variation of th divrsity covrag of on Quasi-GSO satllit systm ovr a tim intrval of ight hours. To cop with this divrsity covrag issu, th baslin satllit slction procdur simply slcts th satllit with th highst lvation angl during th initial stup of connctions or upon a handovr occurrnc [7]. Applying such a mchanism to th proposd constllation rsults in long dlays and significant dlay variations, mainly whn th connction is st btwn two gographically clos trminals. Furthrmor, sinc handovrs occur rarly in th proposd constllation, th slction of an inappropriat satllit may influnc th ovrall connction prformanc for a long priod of tim. In othr words, till th chosn satllit falls blow th lvation mask and th nxt handovr taks plac. In short, a mor accurat satllit slction schm is rquird for th considrd constllation. Th nxt sction dscribs in dtail th satllit slction schm proposd for th Quasi-GSO constllation. IV. SATELLITE SELECTION SCHEME FOR THE QUASI-GSO SATELLITES CONSTELLATION This sction givs an oprational ovrviw of th proposd satllit slction schm. Bfor dlving into dtails of th schm, th following obsrvations should b mad. Firstly, sinc th six Quasi-GSO systms, considrd in th constllation, ar similar, focus is on only on of thm. Th lngth of GSL links,, can b calculatd as follows: "! () # %$'&)(+*, whr and * dnot th satllit altitud and th lvation angl to a satllit from th trminal, rspctivly. Sinc th satllit altitud of th constllation is high, th lngth of GSL Th constllation rturns back to its initial position vry ight hours. varis drastically upon a small variation in th lvation angl. This ultimatly affcts th propagation dlay. Th lvation angl should b thus maximizd to rduc th propagation dlay variation. In th Quasi-GSO orbit, th vrtical spd of satllits rlativ to Earth is not stabl. For xampl, satllits mov fastr nar th cntr of th locus, and slow down at th orbit uppr and lowr tip. To dal with such an issu, th minimum lvation angl should b dynamic and should tak diffrnt valus for diffrnt latituds. Using on minimum lvation angl for th whol constllation, as in th baslin mthod, rsults in drastic dlay variation ovr th Quasi-GSO Satllit constllation. Th covrag ara of a singl Quasi-GSO systm is dividd into thr rgions such that ach rgion is covrd by only on satllit. Rcall that th covrag ara of on systm is in latitud and - in longitud from ach sid of th locus cntr. Basd on latitud, th thr rgions ar dtrmind as follows (Fig. ):. Rgion /3 : Trminals in th northrn part of th glob with latituds gratr than :<;78& (4 >= latitud). Rgion /? : Trminals with latituds within th 58A<9:<;7& and ( DC latitud C 4 ). Rgion /E GF Trminals in southrn part of th glob with latitud smallr than 58A<9:<;78& (latitud ) whr $ idntifis th numbr of th systm in th constllation. Northrn latitud and southrn latitud ar slctd as boundaris for th thr rgions as two satllits of th systm lin up on N or S vry four hours. Trminals in both rgions / 3 and /E prform handovr of GSL vry ight hours, whras usrs in rgion /I? prform handovr vry four hours. Upon handovr occurrncs, nd-trminals us function to slct th most appropriat satllit to communicat through. Th slction dpnds intuitivly on th rgion ndtrminals ar locatd in and th tim of th handovr. Function is formd from thr sub-functions. Each sub-function concrns nd-trminals in a spcific rgion. Trminals blonging to diffrnt rgions gt connctd always to diffrnt satllits. Th satllit slction function,, is dfind as: (LKNM OQPRM O9ST,)! U V WYX (L9 S,Z/ 3 X? (+9ST,[/? X E (+9ST,\/ E whr K M and P M dnot th longitud and latitud of trminal ], rspctivly. 9QS dnots th currnt tim of th day. Outputs of ar,, or. Ths valus corrspond to th ID of th satllit a trminal should gt connctd to at tim 9TS. Satllits ar idntifid as follows. In a systm with a locus cntr at longitud ^, th initial stat of a satllit (at tim _ ) and its corrspondnt idntifir ar givn in Tabl II. Dpnding on th initial positions of satllits and th tim rquird by ach ()

4 c o g (a) t=t (b) t=t+h (c) t=t+h (d) t=t+3h () t=t+4h (f) t=t+5h (g) t=t+6h (h) t=t+7h Fig. 3. Covrag variation of a Quasi-GSO Systm, T:Tim th constllation is in initial position, t:tim lapsd in hours (Ths ar ditd imags, gnratd by th Satllit constllation Visualizr (SaVi), at to cross a givn rgion (Fig. 3), functions X (L9 S,, X?`(+9 S,, and X Ea(+9 S, ar dfind as follows: X (L9 S,b!dcf 9 SaC _hg - i jlk gm i j 9Sa=n_hg (3) j whr! (+9 Spo _ o,qir sg t5ru<v -rw. X? (L9Sq,)!Dc 9 SaC _xg (+9 Spo _ o,qir sg t5ruyv -wz9 S =m_xg (4) X Ea(+9 S,)! 9S C _{g{ (+9S o _ o,qi t5ru<v -rw 9S}=m_{g{ (5) Fig. 4. Propagatoin Dlay (ms) Baslin Satllit Slction Gographical Location Basd Satllit Slction Longitud diffrnc btwn th dstination and sourc nods ( ο ) Dlay variation xprincd by UDP connctions in rgion Šˆ Œ Satllit ID Initial longitud Initial latitud of satllit of satllit ~ ~" ƒ N % ~ˆ S % TABLE II SATELLITES IDENTIFICATION AND THEIR INITIAL POSITIONS Information on th gographical location of trminals and th currnt tim ar rquird to slct th most appropriat satllit. In th rcnt litratur, svral approachs hav considrd th inclusion of gographical location information in trminal addrsss [] []. In addition, a satllit slction mthod basd on gographical location information is proposd in []. Currnt tim can b asily obtaind from th systm clock of th trminal. Th abov approach can b fairly implmntd in ndtrminals. Trminals will b accordingly abl to slct th most appropriat satllit to communicat via. In cas of fixd trminals, handovrs can b initiatd by satllits. Satllits should kp thus track of th nxt handovr tim. This opration can b programmd in advanc in th satllits. Handovrs occur whn two satllits of th sam systm lin up at latituds N or S. At this tim, th Intra-SISL dlay btwn th two satllits taks its shortst valu. This minimizs th communication ovrhad (in trms of tim and systm cost) causd by th handovr. In cas of mobil trminals, handovrs can b initiatd by trminals whn thy ntr th covrag ara of anothr satllit. V. EVALUATION OF THE PROPOSED SCHEME Having dscribd th proposd satllit slction schm, focus is now on valuating its prformanc. Evaluation rlis Fig. 5. Propagatoin Dlay (ms) Baslin Satllit Slction Gographical Location Basd Satllit Slction Longitud diffrnc btwn th dstination and sourc nods ( ο ) Dlay variation xprincd by UDP connctions in rgion Š)ŽlŒ on computr simulation, using Ntwork Simulator- (NS-) [3]. Sinc th Quasi-QSO satllit systm is symmtrical around th quator, simulations ar conductd considring th cas of only northrn latitud rgions / 3 and /?. A UDP packt is snt from a sourc to a dstination vry minut during a tim priod of 4 hours (44 packts). imum, minimum, and avrag path propagation dlays ar calculatd for th transmittd 44 packts. Similar simulations ar carrid out for various sourc and dstination pairs with diffrnt latituds. In rgion /?, sourc and dstination latituds ar varid from th Equator to northrn latitud with an intrval of (Fig. ). As for trminals within rgion /3, latituds ar varid from northrn latitud to northrn latitud with an intrval of. Th longitud diffrnc btwn a sourc and a dstination is varid from E5 to E55 with a intrval for ach sourc and dstination pair. Th longitud of th cntr of th Quasi-GSO systm locus is st at E4 (Fig. ). Th prformanc of th proposd schm is compard to that of th baslin satllit slction

5 approach. In all conductd simulations, th rsults show that th proposd schm achivs significant rduction in both dlay and dlay variation for trminals in both rgions / 3 and /?. Figs. 4 and 5 ar two xampls (among many) that illustrat th good prformanc of th proposd schm ovr th baslin approach in rducing th dlay and dlay variation. Th two figurs show th dlay and dlay variation of two UDP connctions stablishd ovr rgions / 3 and /?, rspctivly. In Fig. 4, th sourc is fixd at N3 and E5. Th latitud of th dstination is st at N45, whras its longitud is varid. Th X axis dnots th diffrnc btwn th longituds of th dstination and sourc nods. Th sourc in Fig. 5 is st at N5 and E5. Th latitud of th dstination is fixd at N5 whil its longitud is varid. Whn th baslin satllit slction schm is usd, trminals ar connctd via ithr th sam satllit or two diffrnt satllits. In cas of th baslin satllit slction schm, th maximum dlay valus in Figs. 4 and 5 dpict th dlay whn trminals ar connctd to diffrnt satllits, whil th minimum valus show th dlay whn trminals ar connctd to th sam satllit. Th two figurs show that th avrag dlay of th proposd mthod is always clos to th minimum dlay of th baslin mthod. This rsult indicats that th proposd schm succds in making trminals communicat via th sam satllit. In both cass, th proposd schm achivd a rduction of ovr 9% in th dlay variation. Rduction in th avrag dlay was of about %. Trminals blonging to two diffrnt rgions usually communicat via two diffrnt satllits. Consquntly, in cas of two trminals blonging to two diffrnt rgions, yt gographically clos to ach othr, an ISL will b rquird for communication. This will intuitivly lad to highr propagation dlays. A rmdy to this issu can b mad by including latitud diffrnc (btwn two trminals) as a paramtr in th satllit slction function. VI. CONCLUSION In this papr, a novl satllit constllation was proposd. Th constllation compriss six Quasi-GSO satllit systms. Each Quasi-GSO systm consists of thr satllits. Th positions of th systms ar dcidd in a mannr that most dns citis with high buildings (i. Nw York City) or mountainous rgions ar ntirly covrd by th systms. Th constllation would b abl thn to covr most populatd rgions of Earth with only satllits. Obsrving that th and r - q systms (Fig. ) covr th Pacific and Atlantic ocans rspctivly, and sinc signal blockings ar not an issu in such aras, th numbr of satllits in th constllation can b furthr rducd to by rmoving th two systms from th constllation. Th constllation maintains transmission with a minimum lvation angl of 4. High latitud rgions that hav bn dprivd from transmission with high lvation angls, should no mor xprinc signal blockings. Furthrmor, th constllation is fairly asy to manag bcaus of its small numbr of satllits and lss mobility charactristic. Th constllation can b sn as a good infrastructur for providing dlay-insnsitiv multimdia applications, such as VoD and distanc larning. A satllit slction schm for th Quasi-GSO constllation was also proposd. Th covrag ara of ach Quasi-GSO systm was dividd into thr rgions such that ach rgion is covrd by at last on satllit at any givn tim. Slction of th most appropriat satllit dpnds on th gographical location of trminals and th handovr occurrnc tim. Th prformanc of th proposd schm was valuatd and compard to that of th baslin satllit slction schm. Extnsiv simulation rsults lucidatd th bttr prformanc of th proposd schm in rducing both th ovrall dlay and dlay variation. Anothr crdit of th proposd schm consists in liminating th last hop ambiguity that xists in and MEO constllations. Indd, sinc trminals in a gographical location should b ncssarily connctd to a known satllit, packt forwarding among satllits can b prformd according to a gographical location basd routing protocol as proposd in []. This will significantly rduc th additional ovrhad that may b du to routing procdurs. Finally, it is our hop that th findings in this papr may contribut in th construction of a nw constllation and hlp to a bttr undrstanding of Quasi-GSO systms whil stimulating furthr work in th ara. REFERENCES [] P. Chitr and F. Ygnoglu, Nxt-Gnration Satllit Ntworks: Architcturs and Implmntations, IEEE Commun. Magazin, Vol. 37, No. 3, Mar. 999, pp [] F. Carducci and G. Losquadro, Th EuroSkyWay Worldwid Systm Providing Broadband Srvic to Fixd and Mobil End-Usrs, Int.. Satll. Commun., Vol. 7, No.-3, 999, pp [3] M. Holzbock, Y.F. Hu, A. ahn, and M. Wrnr, Evolution of Aronautical Communications for Prsonal and Multimdia Srvics, IEEE Commun. Magazin, Vol. 4, No. 7, ul. 3, pp [4] T. L-Ngoc, V. Lung, P. Takats, and P. Garland, Intractiv Multimdia Satllit Accss Communications, IEEE Commun. Magazin, Vol. 4, No. 7, ul. 3, pp [5] T. Talb, N. Kato, and Y. Nmoto, An Explicit and Fair Window Adjustmnt Mthod to Enhanc TCP Efficincy and Fairnss ovr Multi- Hops Satllit Ntworks., IEEE. Slct. Aras in Commun., Vol., No., Fb. 4, pp [6] T. Talb, N. Kato, and Y. Nmoto, A Rcursiv, Explicit and Fair Mthod to Efficintly and Fairly Adjust TCP Windows in Satllit Ntworks, in Proc. of IEEE Int. Conf. Commun., Paris, Franc, un. 4. [7] L. Wood, Intrntworking with Satllit Constllations, Ph. D thsis, Univ. of Surry, un.. [8] K. Kimura, Elvation Proprtis of Quasi-Znith Satllit Systms Using Circular Orbits, IEICE Trans. on Commun., Vol.E87-B, No.8, Aug. 4, pp4-5. [9] T. Talb, N. Kato, and Y. Nmoto, On-Dmand Mdia Straming to Hybrid Wird/Wirlss Ntworks ovr Quasi-Go Stationary Satllit Systms, Elsvir. on Computr Ntworks, Vol. 47, No., Fb. 5. pp [] T. Hain, An IPv6 Providr-Indpndnt Global Unicast Addrss Format, Intrnt Draft, draft-hain-ipv6-pi-addr-5.txt, Aug. 3. [] T. Hndrson, Ntworking ovr Nxt-Gnration Satllit Systms, Ph. D thsis, Univ. of California, 999. [] U. Dharmaratna, T. Tsunoda, N. Kato, and Y. Nmoto, A Satllit Slction Mthod for Walkr Dlta Satllit Ntworks, IEICE Trans. on Commun., Vol.E87-B, No.8, Aug. 4, pp [3] UCB/LBNL/VINT, Ntwork Simulator - ns (vrsion ),