Frequency Synchronization in Global Mobile Satellite Communications Systems

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Frequency Synchronization in Global Mobile Satellite Communications Systems

Qingchong

Liu Huglies

Network Systems

11'717

Esploration Lane Germantown,

M D 20876

Abstract 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 1110-

bile link for t,liF: IJT. T h e received carrier frequency by t,he UT is [l]

A iiovel iiietliocl of frequency synchronization is proposed for global mobile satellite. coniiiiuxiica- tioiis systems. It is sliowii that this iiietliod is op- tiiiial to liaiidlc-: dyiiiiiiiic frequency errors of large range doiiiiiiatetl by Doppler. Frequency syiicliro- iiizatic?ii is also studied for diversity calls and sys- teiiis 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. . T h e Doppler frequency shift. is ra.ndonily clist,ribut,ecl in a large range and vary- ing 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 satel- 1it.e trailslation error a n d t,he oscillat,or drift..

A glolial iiiobile sat.ellite communicat.ions syst,eni con- sist,s of resourc-e management, centers, sat.el1it.e access nodes (SANS) radio frequency t,erminals ( RFTs) ^~ a group of sat.elli tes a n d thousands of user t,erniinals (I!Ts). T h e 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. public- swit,ched t.elephone iiet,\vork. ancl public swit.checl c1at.a net.work. etc. The sat.ellit,es l i n k t,he user t,ertiiinals to die SANS. T h e link carrying t,ransmissinii from the SAN t o t,lie U T 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 m a n y 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 c S ) D J ( n ~ i, t )

+

^djcr ^ist,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 c s : \ J ( f ) ) D , ( n . i , t ) is usually negligible.

T h e first. t,erin in t.he right. hand side of A f [ , ( n , i . t ) is called t,he forward feeder link Doppler. Usually it. is a. con- t'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. T h e second term in the right hand side of i l f ~ ; ( 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. T h e third t,erni is called t,he forward 1110- bile 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,ernii- n a l . 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.

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Consider the Doppler frequency i n the ret,urn link. Let f s ( n , i , t ) be the trarismitt,ed carrier frequency by the i-th user t,erminal in the n-t,h spot beam. T h e received carrier frequency by the sat,ellite is

Let fsc be the satellite translation frequency from the mobile link t o the feeder link. Then the transmitted carrier frequency by the satellite is

where A f ~ ( n , i ~ t ) = fs(n,i,t)Ds(n,,i,t)

+

f s c M ( t )

+

( f s ( n . , i , t )

+

f s c ) D c ( f )

+ CUR

is the total frequency error for the retmurn link and

~ l f ~

= (fs(n,,i,t)Ds(n,i,t)

+

fsciLf(t))Dc(t)

is negligible. T h e first term in A f ~ ( n , i , t ) is the return mobile link Doppler, which is a random va.ri- able of a p p r o s i m a t ~ e l ~ t,he same range as the forward niobile link Doppler. The seconcl term is the satellite translation error. T h e t.hircl t er m is t.he feeder link Doppler.

T h e random Doppler frequency errors of very large range in the forivard link antl t,he return link must he compensated 60 achieve carrier frequency synchronizat,ion:

haye opt,irnum demodulat.ion performance, atid use system banclwidth efficiently.

Frequency synchronizat.ion in global mobile satel1it.e c.oinmunicatiotis systems have to be performed for control channels. rancloni access channels antl t,rakfic channels. In [ 11 the s!.ncilironizat,ion p-oblem for the control chaiiriels ancl random access channels has been considered. So we assume that, tlie user terininal can access t.he syst,em.

This paper st iiclies tlie freqiien(:y syriclironizat,ion in the presence of dytiainitr Dopplrr of large range in global 1110-

bile sat.el1it.e conitiiiinic.at,ioiis systems i n t,he process of set- t.ing u p calls ancl during calls.

I1 Traditioiial Method for Traffic Chaiiiiel

the access request, from the user terminal, t.he satellite access node allocates carrier frequencies 1.0 t,lie user terminal to transmit and receive, which are called the traffic chan- nels. These frequencies are carried t o the user t,erniinal through the control channel. Once the user terminal re- ceives these frequencies, it starts t o transmit. and receive.

Traditionally, the Doppler compensations for the traffic channels are performed only a t the satellite access node.

Let

fi

( n , i , t ) be the nominal carrier frequency for the traffic channel in the forward mobile link and f $ ( n , i, t ) be the corresponding frequency for the return mobile link. T h e user terminal s t a r t s t o transmit a t the carrier frequency f & ( n , i, t ) . T h e received carrier frequency a t t,he satellite is

f,Tdl(n,i,t) = f$(n,i,t)

+

AfT(n,i,t) (5) where A f ~ ( n , i , i ) = f$(n,i,t)D,(n;i,t) is the Doppler.

The received carrier frequency by the R F T is

fRFT(72,i,L) = f;f.(7l,i,L)+f~c+Af.R(71,il2) ( 6 ) where A f ~ ( n , i, t ) is tlie received frequency error at, the satellite access node, i.e., A f n ( n , i , f ) = A f ~ ( 7 1 , i , t )

+

ter Doppler correction for the return link should also lie perfornied by t,lie radio frequrticy terminal for all of the channels by deduct.ing A f l b ( t ) = -flt,Dc(2). where flb is the center carrier frequency i n the return feeder link. So t,he frequency error at, thc input. of the satellite tiasesta- t.ioti subsystem is A f , 9 B . 5 ( n 2 i , l ) = ~ l f ~ ( 7 1 . i , t ) + f s C A 4 ( t ) + (f$(n,i.t)

+

^{f S c}^-ftbD,(t))Dc(f)

+

o(f;D,D,), where

AfT(72. i, 1 ) dominates. This error will be corrected by tlie satellite basest,ation suhsyst,eni so t.liat, a t t,he input of t.he cleniotlulator t.here will tie 1 1 0 carrier frequency error.

T h e t,raditional met,liod works well for C’DMA syst,ems.

However. it is not good for FDhI.A/TDIClA syst,ems.

Consider any t.wo of adjacent traffic (:haiinels in the re- t,urn l i n k . Assume that t,he first, clianncl is t,he j - t h channel having carrier frequency f i and the second is t,he ( j + 1)-th channel having carrier frequency f j + l = f j

+

I4’, where lit;

is t.h? banclwitltlt of each channel. Usually

CV

is around 3OkHz. Assume t,he j-t,li chatinel is assigned to t,he n-t,h spot. Iieani antl the ( j

+

^1)-th^cliannelis assigned t o t,he 117- t 11 spot beam. It r a t i lie shown t.liat. t,lie tlist.ance bet,ween t h e two received ( w r i e r frequencies at. t,lic. radio frequency terniinal is

fscL44(t)

+

^(f;(n,^{i , i )}

+

f s c ) D c ( t )

+

o(f;:U,U,). T h e ^ten-

W h e n t,liere is t i 0 Doppler, t.Iit. (list atice between the t,wo carrier frrqueiiries for adjacent c:hannel.i is I,V, which is u s i ~ i i l l y cdioseii to n i a k e acl.jac.ent. c l i i ~ n w l s t,o be ort,hog- onal. 1 1 1 tlir worst msc of l>oppler, i t will happen that.

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U,(rn,k,t) = - D , ( n , z ’ , f ) = - i i i a s n , l D , ( n , i , t ) . T h e n we have

min c l f ~ ~ ~ = W

-

2 ^fJ trias _n_{. z} D, ( n , i , t ) (8) which can have the term ( 2 j j niax,,,, U , ( n , i , t ) ) almost as large as the system channel spacing W . T h e t,wo adjacent channels are almost totally overlapped, which will result in no signal can be received from eit.her channel. In many cases, the distance between t,wo carriers received for adjacent channels can be very small. hecause the absolute value of the second term in ( 7) can be fairly large even before reaching ma.simuni. Usually a sat,ellite ha.s hun- dreds of spot beams, which makes t,lie probabilit,y for tlie absolute value of t.he second t,erni in ( 7) t.0 be large no longer negligible. Therefore, failure of comiiiunicat,ion link can easily happen if t.lie Doppler is not compensated.

For t,he forward link traffic cha.nnel. t,radit,ionally t,he sat.ellit,e access.node transmits at, t,he following frequency

f T ( n , i . t ) = _f;(n,_{i , t )}

+

^fc,^-( J ” { , ( r i . i . f )

+

f c s ) D c ( t )

+ ; ( I ? , i , f ) U , ( I i . i . t )

+

f c , M ( t )

+

^y

where y = o ( f c , D c D , ) , ^sothat t,lie received carrier frequency by the user t,erminal will be t,hr nominad frequency f{(n? i , t ) . This met.hod works well for CDR1.A syst,eins, but. will cause problems for FDI\I.-I/TDR~I.\ syst,etns. Con- sider any two of atlja-cent t.raffic (-liaiinels j and ( j

+

1) for the forward link. Assume that. t.he j-t.11 thannel is assigned t o the ri-t,Ii spot. beam and t.he ( j + l)-.tIi channel is assigned t o the In-th spot b e m i . Then tlie dist.ance behveeli the t.wo carrier frequencies t.ransmit.t~etl by t.he satellite access node

1s

nal as the reference point, for t,raffic channels so t h a t both of the transmitted and the received carrier frequencies at the user terminal are nominal. T h i s choice is based on the assumption t h a t user terminals d o not know their Doppler frequencies. ‘Thus, the satellite access node, which knows all the elements to calculate Doppler, has t o correct the Doppler for all of the user terminals. However, as seen in the previous subsection, this approach causes big waste of system bandwidth in F D M A / T D M A systems, because tlie Doppler is a random variable distributed in a large range.

T h e best reference point of Doppler correct,ioii is the satellite for the traffic channels and access channels. In other words, Doppler should be correct.ed in such ways t,hat t8he received carrier frequencies at t,he &tellit,e are al- ways the nominal frequencies. By doing this, there is no need tBo employ guard ba.nds and the demodulat,ion perfor- niances will not be degraded both on board t,he satellite and at the user t,erminal and t,he sate1lit.e access node.

This will be very desirable by ilew generation of satellite communications systems, which are aiming at bett,er uti- lization of syst,eni bandwidth and having deniodulation on board sat,ellites.

Using the sat,ellite as the reference point.. t.he feeder link residual Doppler a.nd the sa.tellite taranslat ion error should be correct,ed by t.he sat,ellite basest,at,ion s u b

feeder link ceni,er Doppler shoulcl be correc-t.ed by tlie radio frequency t.erniinal; and t.he mobile link Doppler should he conipensat.ec1 by t,he user t.erniinal guided by tlie sat,ellite access node. For t,Iie, t#raffic channels, tlie satellit,e access node needs t.0 send two pairs of frecjuenci~s. The first pair is ( f $ ( n 3 i.f).df+(n. i . t ) ) ! where f $ ( t i , i . f ) i s the nominaI carrier frequeiicy for tlie user t,erminal t o transniit atid

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111 t,he worst. case, t,lie t.eriii ( j J ( i i . i . t ) ( U , ( t n * j . 2 ) - D, ( 7 1 , i , t ) ) ) can be as large a s t h e syst.em channel spacing

LV.

When t,liis happens. t.lir, user terminals involved can not. receive any signal.

In order t o overcome t,he ranclotn 1)oppler of large range.

some systems resort t,o the t’ratlit.io:ial way of employing guardbands. However, this approach brings a big wast,e of syst,ern banclwidt.h, because t.lic, 1)oppler is very large in mobile satellite comiiiunicat.ioiis syst,erns using fast moving satellit,es.

I11 Optimum Method for Traffic Channel

T h e traditional method to ⁽oiiiliat Doppler for mohtle satellite c ~ m n i u n i c a t i ~ ~ i zyst e t i i ~ i lioowz the u w r terini-

is the corresponding correction for Doppler T h e seconcl pair is ( j J ( t i . i . t ) . d f J ( ) j . i , t ) ) for t.he usrr tcrrniiiial to receive. where

d f $ ( u , i , t ) = - f i ( t l . i q t ) D 5 ( t ) . ^{i . t j} (11) is tlie Doppler correct ion and D, ( n . i . f ) is t lie fractional Doppler of the user t.erniinal measured froni i t a v w s s request signal,

111.1 Return Link

For tlie traffic c~hannel in t.he r e t u r n link t h e w c ~ r teriiiinal starts to transniit at f T ( r i . i . f ) = ( f F ( t 1 . i I ) + d f ; . ( r t . i . I ) .

I . ? . .

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So t,he received carrier frequency at, the satellite is the nominal frequency f&(n, i, t ) . This guarantees t h a t de- modulation performance on board satellite will not be degraded by the Doppler frequency error. T h e received carrier frequency at the radio frequency. terminal is

which can be written as

where the third term in the right hand side is the center Doppler of the feeder link, the fourth term is the residual Doppler, the fifth term is the satellite trarislation error and the sixth term is negligible. This received carrier frequency a t the radio frequency terminal does not contain tlie Doppler int.roduced in the mobile link. T h e distance between two received carrier frequencies for any two adjacent channels is (14'

+

W D , ( t ) ) : where bC' is the channel spacing for the case of no Doppler arid W D J t ) is usually less t,han 0.5Hz. Therefore, our method guarantees there is no overlap between any t,wo signals i n a,djac:ent channels.

For each channel, the radio frequency terminal performs c-eiit,er Doppler correction and downconversion; the satel- 1it.e basestation subsystem performs the correctmion for both of lie residiial Doppler in the feeder link and the satellite translation error [ 11.

111.2 Forward Link

For t,he t,raffic channel i n t h e forward link! t h e sat.ellite ii('(:ess node dioulcl t.ransniit at t h e carrier frequency

Sitice the tlist.ance bet.ween t h r rarrier frequencies for atiy two adjacent channels is (1.1; ^-o( L l ~ D ~ ~ ( / ) ) ) , where o( II.D,(t))) is usually less than 0.3Hz. i n this method t.he Doppler does not cause an!. int.rrfercnce l,ct.\veen t,wo iicl- j acen t (111 a ii ti e Is.

Aft er t.hr translation from t h e feeder l i n k bo the mobile

l i i i k . tlie carrier frequrncy m i t . by t h r 3 sntellitr to the u s e r twtiiinal is tlir nominal frequency j ; ( n . i . t i . ~ ~receivecl i e c,arric\r frecluetic-y a t t h e user teriiiiital is

j / . r ( n . i . t ) = j ; . ( / , . i . t ) ( 1

+

/ l v ( i i , i . t ) ) (15) .I.; I lit, IISPI' teriiiinal h a s rc:cteiwtl I lif, I ) o p ~ ) l ~ r trorrecct.ioii

lllrollgll t l l f t.on1rol c:llallncl. n ' f { . ( / , . i . I ) = - f $ D q . t .l l f1

user t,erniinal can perform a perfect Doppler correc-t>ion by adding S f i ( n , i, t ) to the received carrier frequency

fL+,

i , t ) .

IV Doppler Compensation for Di- versity Call

In global mobile satellite communications systems, cliver- sity is usually used t o maintain call continuity, hest. path and difficult links. When diversity exists, two pat.hs via two different satellites are reserved for the same user k r - rninal to communicat,e with the same sate1lit.e i ~ c e s s node.

T h e first path is set, up i n the ways discussed i n tlhe previous sections, i.e., the user terminal sends access request signal t o the satellite access node, and the satellite access node assigns traffic channels t o the user terminal via t,lie control channel, then tlie user terminal starts t o t,rxiisrnit, and receive. To set u p the diversity call, the noriiinal carrier frequencies of the traffic channels for t,he secoird pa.t.li are sent. t,o the user terminal via the traffic cliatinel o f t>lie first p a t h . Unlike tlie first path, no Doppler corrwtioii for the second path will b e sent, to the user termiiial. So t h e user terimiiial has t,o fincl them out by it,self and (.orred them propcrly.

Assuiiie the second pa.t,h is i n the 17i-th spot brain o f the seconcl satrllit.e. Let f,,, ( m , i ) he the noinitial c a r r i e r frc- yiiency of the rontrol channel in this spot heatn. & ( m , t ) be the fract.ional Doppler on its median Doppler l i t w a.ncl

D s ( r n . i : t ) be tlie fract,ioiial Doppler of the sanic user terminal i n tlie n-t,li spot, beam of t,he seconcl sat,ellit.r.. I t (.an be shown t.hat t h e receivecl carrier frequency of t hr: cont,rol channel for the m-th spot, )learn of the second sat.ellite is

which is at. most. few kilohert#z away from t hr nonii- tial frequency fc,,, ( / n . t ) . T h e user t.erniinal can easily tune to this freqiiency and cleniodrilate the inforinatmion of L ) / , ( n i . 1 ) . wliidi is carried by t#he c-ont8rol c-hatitirl. By generat.irig a c.arrier a t t.he nominal freciuenc-y j r , , , ( 7 7 1 , t ) a n d ( w i n p a r i n g it, wit,h the receivecl frequency fr?(.-.( r r j , i , I ) . t.lie user t erinitial (ran find t.he followittg pa.ranietkr

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t o receive from the traffic channel i n the second p a t h . T h e user terminal t,iines to

(19) f;(?n,i,f)

- - f;;

( m , ;, t ) f o r , (nz, t )

l + D s ( I n , i , t ) 1 + D b ( m , t ) f R C ( m , i , t ) to t,ransniit for t,he second traffic channel. Therefore, ideal carrier frequency synchronization can be achieved for the diversitmy path without passing the Doppler for the second path to the user terminal.

V Real T h e Doppler Compensa- t ioii

During a call t.he cwrier frequency from the user t,erminal niay vary because of it.s motion and oscillat,or drift,. T h i s frequency error can be detectmecl by t,he sat.ellit,e 1iasest.ation subsyst.em and preclictmecl by a Iialman filt,er implementecl itisicle t,he sat.ellit,e basestat,ion subsystem. The sat,ellite basest at.ion subsystem sends t,he error t,o t.he user terminal.

Then 6lie user t,erminal c0rrect.s t,he error atrc~orclingly.

For syst,enis where t,lie t,ot.a.l t,ransmissioti delay from t.he satel1it.e basest,at,ion subsystem t,o t.he user t,erniinal is not negligible, t.he implemetit,at~ion of t.he proposed met,liods should be nioclifiecl in t.lie follo\ving ivays. For t h e access chaiiiicl. t.he coiit.rol channel should cont.aiii t h e iio~ninal carrier frequency f ~ A ( n . ) , t.he instant.aneous mobile link fratrt,ional Doppler Db( 1 1 , t ) on t,he meclian Doppler line antl it.s rat.e ofcliange relat.ive t.o t,ime . r ( t ) = wit.li t,ime s t a m p . Assume at, t,ime instant, t a user t,ermiiial \vant,s to send an access request t.0 t.he satel1it.e access iiotle ancl the lat~est fract.ional Doppler receivecl by the user terminal was st,ampetl at. the time inst.ant, t o . The user terminal calculat,es the Doppler frequency shift at t.he t.inie instant.

t by applying the following formula

Then the user t.ermina1 sends access request at. the frequency ( f R , 4 ( ? 7 )

+

b f ~ ~ ( n . , f ) ) .

For traffic clia.nnel, the sat,ellit.e access tiotle sciiclh t o t lie user terminal the assigned carrier freqiteticries. t.lie titea-

surecl fract,ional Doppler and its rat.($ of c:hatigc I v i t I1 I hc time s t a m p . i.e.,

(I rl t

( f i f . ( i L ! i , t ) , f . ; : ( / i , % , t ) , ~ , ( n . i. 1 ) . - ~ ~ s ( / i . i . / ) . I )

For simplicit.y, the rate of change of the fractional Doppler

$D. ( n , i , t ) of the m e r terminal can be well approsimat,ed by t h e rate of change 011 the median Doppler line, i.e.,

latter can be calculated from t h e orbit, of the satellite.

Assume t h a t a t the time instant (t

+

^{A t ) ,}t,lie user terminal wants t o send the first burst of signal for the traffic channel. T h e user terrnin.al c0nstruct.s its iiistantaneous fract,ional Doppler by applying

x D s ( n , i , t ) d x -$Db(n,t). T h e error is negligible. T h e

d

D , ( n , i , t + A t ) = D , ( n , i , t ) + A t - D , ( n , i , t ) . dt (21) T h e n the user t,erminal calculates its inst,antaneous Doppler frequency shift b f + ( n , i, t ) for the return traffic channel by applying the equat,ion ( 10). T h e user terminal t.ransriiits at, t,he carrier frequency (f$(n,

%,

t ) + b f $ ( n , i . t ) ) and tunes t.0 (jif.(n., i , t )

+

bf$(n,i,t)) to receive, where b f $ ( n , is calculated by employing the equation ( 11).

VI Coiiclusioiis

Optimal carrier frequeiicy synchronizat,ion can be acliieved for global mobile sat.ellite communications syst,ems t.hrougli measuring t h e clynamic frequency errors by t.he sat.ellit,e access nodes (SANS) ancl correcting t8hem accordingly t)y t,lie SANS and the user t,erminals. In performing frequency syncl~rotiizat.ion, t.he satel1it.e should he the referenc-e point t,o which all of the received ancl t~ransmit ted frequencies shoulcl be nominal. T h e clynamic Doppler in t,he mcbile link shoulcl he compensated by the users terminals g i t i d d by t,he SAN..;. This makes sure the system liandwidth can lie usecl optimally ancl s!.ticlironizat,ion can lie mai~itainc-cl.

Frequency Synchronization in Global Mobile Satellite Communications Systems