Frequency synchronization in global mobile satellite communications systems

Consider the frequency synchronization in global niobile satellite coiiiniunications systems. Let fc ( n , %: t ) be the t,raiismitt,ed ca.rrier frequency from the SAN t o the i-th user in the n-th spot bea.m, D c ( t ) be the fractional Doppler of t.he feeder link, fc6 be t,he translation frequency at. t.he sat.ellit,e, J f ( t ) be the fractional satel1it.e translation error, atid D,(n , i , t ) be t.he fractional Doppler of t,he 1110bile l ink for t,liF: IJT. The received carrier frequency by t,he UT is [l] A iiovel iiietliocl of frequency synchronization is proposed for global mobile satellite. coniiiiuxiicatioiis systems. It is sliowii that this iiietliod is optiiiial to liaiidlc-: dyiiiiiiiic frequency errors of large range doiiiiiiatetl by Doppler. Frequency syiicliroiiizatic?ii is also studied for diversity calls and systeiiis with large delays. I I nt r o d IIC t io 11 I I I glol)iil niol)ilc sat.ellit,e commuiiicat.ions syst,enis employing nietliuni cart.li-orbit, sa.t,ellit,es and low eartli-orbit, satellit.es. t h e satellites a.re moving fast. relat.ive to t,he Eart,h. ( 'ii using llop pl er fretlit enc y shift. . The Doppler frequency shift. is ra.ndonily clist,ribut,ecl in a large range and varying w i t h t,ime. l'lierefore. it. is challenging for frequency s\:nc:liroiiizat,ion t o hatidle t,he dynamic Doppler in large range along wit.11 ot,lier frequency errors such as t,he satel1it.e trailslation error and t,he oscillat,or drift.. A glolial iiiobile sat.ellite communicat.ions syst,eni consist,s of resourc-e management, centers, sat.el1it.e access nodes (SANS) radio frequency t,erminals ( RFTs) ~ a group of sat.elli tes and thousands of user t,erniinals (I!Ts). The resoiir(:e nianagmient. cent,er manage t.lie resources such as frequeiicy bancls. channels ancl t,ime slot,s. The SANS connect. the users t,erniinals i n t,he system t,lirougli t.lie R.FTs !vi t.h ot.lier net.works such as t,he public land mobile network. publicswit,ched t.elephone iiet,\vork. ancl public swit.checl c1at.a net.work. etc. The sat.ellit,es l ink t,he user t,ertiiinals to die SANS. T h e link carrying t,ransmissinii from the SAN to t,lie UT is called t,he forward link. a.nd t,liat, from t.he I T to the SAN is called t.he ret,urii link. The l i n k bet,ween a SXN and a sat.ellit,e is called the feecler link, while that. bet.ween a sa.tellit,e ancl a IiT is callrcl the niobile link. 'rhe inoliile link usually has hundreclh of spot Iiertms eii(:li serving many user termirials. f [ , T ( l i , i , t ) = f c ( n , i . t ) f c s + A f ~ r ( n , i , f ) (1) where Aft; ( r j ~ i , t ) = f c ( 71, i , t ) DC(t)-fcb M ( t ) + ( f c ( t i , i . t ) f cS)DJ(n~ i, t ) + d j c r is t,lie total frequency error in t,he forward link dominated by the Doppler and c l f r i = ( f c ( n . i . t ) D c ( t ) f cs : \J ( f ) )D, (n . i , t ) is usually negligible. The first. t,erin in t.he right. hand side of Af[,(n, i . t ) is called t,he forward feeder link Doppler. Usually it. is a. cont'inuous variable in t,he ra.nge of several dozens of kilohertz. It, will be zero when t,here is no relative motion bet,ween the sat.ellite ancl the sat.ellit,e access node. This happens when t,he sat,ellite is flying directly above t,he satellibe access node. The second term in the right hand side of ilf~;( n q i , f ) is called t,he sat,ellite translation error. LTsually the t,ranslat,ion frequency f c 6 is several gigahertz and the the fract.iona1 sat,ellite t,raiislat,ion error M ( t ) is a random number around So the satellite t,raiislation error is a random variable uniformly distributed in a range of several dozens of hert,z. The third t,erni is called t,he forward 1110bile link Doppler. It, is a random variable dist,ribut,ed in the range of several dozens of kilohert,z. When there is no relat.ive niot,ion between t.he satellit,e and the user t,errninal. the forward mobile link Doppler is zero, which happens when the satel1it.e is flying directly above the user t,erniinal. Therefore, t.he t,ot,al frequency error of A fc; ( n . i, t ) is a continuous rancloni variable in a range of several dozens of ki1ohert.z. If not, corrected properly, this large frequency error can easily cause being unable t,o be synchronized and failure of coni in unicat ion. 0-7803-5668-3/99/$10.00