Advanced architectures and the required technologies for next-generation communications satellite...
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Acta Astronautica Vol. 20, pp. 185-195, 1989 0094-5765/89 $3.00 + 0.00 Printed in Great Britain Pergamon Press pie ADVANCED ARCHITECTURES AND THE REQUIRED TECHNOLOGIES FOR NEXT-GENERATION CONNUNICATIONS SATELLITE SYSTENS Pay Arnold Director, Communications and Information Systems Division NASA Headquarters Washington D.C., USA F. Michael Naderi Manager, ACTS Program NASA Headquarters Washington D.C., USA ABSTRACT To be competitive with other modes of information-transport systems, the next-generatlon communications satellites will use new technologies including multiple-beam antennas, and onboard processing and switching. This paper disc,,~ses these technologies and illustrates their use by providing a description of three satellite systems currently under development. The paper is organized in three sections: Section l describes advanced architectures for future satellite systems and the required technologies to enable these systems; Section 2 gives examples of such systems by briefly describing the American ACTS, the Japanese ETS-VI, and the Italian ITALSAT; and finally Section 3 provides some simplified design equations that relate some of the key top-level parameters involved in the design of such systems. INTRODUCTION With the e~ergence of fiber optic networks, the satellite industry has the need for technological innovations to create more capable satellites and marketing innovations to conceive and provide services best suited to satellite system, and thus beyond the reach of the competition. One such market involves thin route point- to-multipoint, as well as muitipoint- to-multipoint voice, video, and data communi- cations. To serve this market, satellite networks using Very Small Aperture Terminals (VSATs) have received much consideration in recent years. To address a wide market, a VSAT satellite network should be capable of providin s voice, data, and video at a variety of information rates from tens of kilobits to one or two am|shits per second. And it should d~ so with acceptable propagation delay and with VSATs of affordable cost. This, however, is not possible with current VSAT networks, which are arranged in a star configur- ation with a terrestrial-based switching hub, and which use power-limited satellites of today. These networks, while efficient for low-speed data communications, fall short in providing voice services. This is because in the present star networks all traffic from remote nodes is routed to the hub. Therefore, node-to~node connectivity can only be accomplished through two satellite hops, which most people find objectionable for voice communications. Additionally, network throughput efficiency and satellite power limitation also argue against using voice or higher-rate data in today's VSAT networks. In the following, various techniques, tech- nologies, and system architectures that would overcome many shortcomings of the present genera- tion VSAT networks are described. This includes a discussion of high-EIRP multiple-beam antennas, onboard processing and switching, and various access techniques. TECHNOLOGIES FOR THE NEXT-GENERATION SYSTL~S Multlple-geamAntennas To accoemodate the higher bit rate transmission needed for single- or multlple-channei voice and video, and still get by with small, low-oust VSATs, high satellite EIRP is needed. Most present-generatlon domestic satellites have antennas that produce a single shaped beam covering the region of interest. For U.S. domestic satellites, thls area is continental U.S. (CONS). For these U.S. satellites, the peak antenna gain is nominally 33 dH, which drops to about 27 dg at the edges of the country. This results in an edge-of-coverage EIRP of some 35 dEW for C-band satellites and &0--43 dBW for Ku-band satellites that use higher power transmitters. Replacing the single CONUS beam with multiple spot bean~J (Figure 1) could provide 10-20 dE more gain. As an example, approximately 100 spot beams of 0.3-delree beamwidths could provide complete coverage of CONUS with an edge-of-coverage gain of 50 dB. In addition to providin$ considerably more gain, au~ltibeam satellites have a further advantage in that by uJing spatial separation they can reuse the smne frequency band in 8eo|raphically separated beams, thus increasing the bandwidth availability by several fold-over, single-beam satellites. However. due to the cost and complexity involved, this evolution from single to multiple beams will be gradual, starting with a few and growing possibly to as nmny as 100 beams. The beams shown in Figure I can be fixed or geographically hopping. In a hopping-beam system, each of N independent beams continuously hop to N different locations, in effect producing an N x N virtual-beam system. For example, the I00 spots shown in Figure I can result from I0 beams, each hopping to I0 locations. The hopping rate is fast, typically in microseconds. The advantage of a hopping-beam system is that the dwell time in each location can be dynamically adjusted to match the instantaneous demand for traffic at that location, thus making optimum use of the system capacity. Additionally, with hopping beams the nu;nber of required satellite 185
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Transcript of Advanced architectures and the required technologies for next-generation communications satellite...
Acta Astronautica Vol. 20, pp. 185-195, 1989 0094-5765/89 $3.00 + 0.00 Printed in Great Britain Pergamon Press pie
ADVANCED ARCHITECTURES AND THE REQUIRED TECHNOLOGIES FOR NEXT-GENERATION CONNUNICATIONS SATELLITE SYSTENS
Pay A r n o l d
D i r e c t o r , Communica t i ons a n d I n f o r m a t i o n Sys t ems D i v i s i o n NASA H e a d q u a r t e r s W a s h i n g t o n D . C . , USA
F. Michae l N a d e r i
M a n a g e r , ACTS P rog ram NASA H e a d q u a r t e r s W a s h i n g t o n D . C . , USA
ABSTRACT
To be c o m p e t i t i v e w i t h o t h e r modes of i n f o r m a t i o n - t r a n s p o r t systems, t he n e x t - g e n e r a t l o n c o m m u n i c a t i o n s s a t e l l i t e s w i l l u s e new t e c h n o l o g i e s including m u l t i p l e - b e a m a n t e n n a s , and o n b o a r d p r o c e s s i n g and s w i t c h i n g . T h i s p a p e r d i s c , , ~ s e s t h e s e t e c h n o l o g i e s and i l l u s t r a t e s t h e i r u se by p r o v i d i n g a d e s c r i p t i o n o f t h r e e s a t e l l i t e s y s t e m s c u r r e n t l y u n d e r d e v e l o p m e n t . The p a p e r i s o r g a n i z e d in t h r e e s e c t i o n s : S e c t i o n l d e s c r i b e s a d v a n c e d a r c h i t e c t u r e s f o r f u t u r e s a t e l l i t e s y s t e m s and the r e q u i r e d t e c h n o l o g i e s to e n a b l e t h e s e systems; S e c t i o n 2 gives examples o f s u c h systems by b r i e f l y d e s c r i b i n g the Amer i can ACTS, t h e J a p a n e s e ETS-VI, and the I t a l i a n ITALSAT; and f i n a l l y S e c t i o n 3 p r o v i d e s some s i m p l i f i e d d e s i g n e q u a t i o n s t h a t r e l a t e some o f t he key t o p - l e v e l p a r a m e t e r s i n v o l v e d in t he d e s i g n o f s u c h s y s t e m s .
INTRODUCTION
With the e ~ e r g e n c e of f i b e r o p t i c n e t w o r k s , t he s a t e l l i t e i n d u s t r y has t he need f o r t e c h n o l o g i c a l i n n o v a t i o n s to c r e a t e more c a p a b l e s a t e l l i t e s and m a r k e t i n g i n n o v a t i o n s to c o n c e i v e and p r o v i d e s e r v i c e s b e s t s u i t e d to s a t e l l i t e s y s t e m , and t h u s beyond the r e a c h of t he c o m p e t i t i o n .
One such m a r k e t i n v o l v e s t h i n r o u t e p o i n t - t o - m u l t i p o i n t , a s w e l l a s m u i t i p o i n t - t o - m u l t i p o i n t v o i c e , v i d e o , and d a t a communi- c a t i o n s . To s e r v e t h i s m a r k e t , s a t e l l i t e n e t w o r k s u s i n g Very Smal l A p e r t u r e T e r m i n a l s (VSATs) have r e c e i v e d much c o n s i d e r a t i o n i n r e c e n t y e a r s . To a d d r e s s a wide m a r k e t , a VSAT s a t e l l i t e n e t w o r k s h o u l d be c a p a b l e of p r o v i d i n s v o i c e , d a t a , a n d v i d e o a t a v a r i e t y o f i n f o r m a t i o n r a t e s f rom t e n s o f k i l o b i t s to one o r two a m | s h i t s p e r s e c o n d . And i t s h o u l d d~ so w i t h a c c e p t a b l e p r o p a g a t i o n d e l a y and w i t h VSATs of a f f o r d a b l e c o s t . T h i s , h o w e v e r , i s no t p o s s i b l e w i t h c u r r e n t VSAT n e t w o r k s , which a r e a r r a n g e d i n a s t a r c o n f i g u r - a t i o n w i t h a t e r r e s t r i a l - b a s e d s w i t c h i n g hub , and wh ich use p o w e r - l i m i t e d s a t e l l i t e s o f t o d a y .
These n e t w o r k s , w h i l e e f f i c i e n t f o r l o w - s p e e d d a t a c o m m u n i c a t i o n s , f a l l s h o r t in p r o v i d i n g v o i c e s e r v i c e s . Th i s i s b e c a u s e in t he p r e s e n t s t a r n e t w o r k s a l l t r a f f i c f rom r emote n o d e s i s r o u t e d to t h e hub . T h e r e f o r e , n o d e - t o ~ n o d e c o n n e c t i v i t y can o n l y be a c c o m p l i s h e d t h r o u g h two s a t e l l i t e h o p s , wh ich most p e o p l e f i n d o b j e c t i o n a b l e f o r v o i c e c o m m u n i c a t i o n s . A d d i t i o n a l l y , n e t w o r k t h r o u g h p u t e f f i c i e n c y and s a t e l l i t e power l i m i t a t i o n a l s o a r g u e a g a i n s t u s i n g v o i c e o r h i g h e r - r a t e d a t a in t o d a y ' s VSAT n e t w o r k s .
In t h e f o l l o w i n g , v a r i o u s t e c h n i q u e s , t e c h - n o l o g i e s , and s y s t e m a r c h i t e c t u r e s t h a t would overcome many s h o r t c o m i n g s o f t h e p r e s e n t g e n e r a - t i o n VSAT n e t w o r k s a r e d e s c r i b e d . Th i s i n c l u d e s a d i s c u s s i o n of h igh-EIRP m u l t i p l e - b e a m a n t e n n a s , o n b o a r d p r o c e s s i n g and s w i t c h i n g , and v a r i o u s a c c e s s t e c h n i q u e s .
TECHNOLOGIES FOR THE NEXT-GENERATION SYSTL~S
M u l t l p l e - g e a m A n t e n n a s
To a c c o e m o d a t e t h e h i g h e r b i t r a t e t r a n s m i s s i o n needed f o r s i n g l e - o r m u l t l p l e - c h a n n e i v o i c e and v i d e o , and s t i l l g e t by w i t h s m a l l , l o w - o u s t VSATs, h i g h s a t e l l i t e EIRP i s needed. Most p r e s e n t - g e n e r a t l o n d o m e s t i c s a t e l l i t e s have a n t e n n a s t h a t p r o d u c e a s i n g l e s h a p e d beam c o v e r i n g the r e g i o n o f i n t e r e s t . For U.S. d o m e s t i c s a t e l l i t e s , t h l s a r e a i s c o n t i n e n t a l U.S . ( C O N S ) . For t h e s e U .S . s a t e l l i t e s , t h e peak a n t e n n a g a i n i s n o m i n a l l y 33 dH, wh ich d r o p s to a b o u t 27 dg a t t h e e d g e s o f t h e c o u n t r y . Th i s r e s u l t s i n an e d g e - o f - c o v e r a g e EIRP of some 35 dEW f o r C-band s a t e l l i t e s and &0--43 dBW f o r Ku-band s a t e l l i t e s t h a t u s e h i g h e r power t r a n s m i t t e r s .
R e p l a c i n g t h e s i n g l e CONUS beam w i t h m u l t i p l e s p o t bean~J ( F i g u r e 1) c o u l d p r o v i d e 10 -20 dE more g a i n . As an e x a m p l e , a p p r o x i m a t e l y 100 s p o t beams of 0 . 3 - d e l r e e beamwid ths c o u l d p r o v i d e c o m p l e t e c o v e r a g e o f CONUS w i t h an e d g e - o f - c o v e r a g e g a i n o f 50 dB. In a d d i t i o n t o p r o v i d i n $ c o n s i d e r a b l y more g a i n , au~l t ibeam s a t e l l i t e s have a f u r t h e r a d v a n t a g e in t h a t by u J i n g s p a t i a l s e p a r a t i o n t h e y can r e u s e the smne f r e q u e n c y band i n 8 e o | r a p h i c a l l y s e p a r a t e d beams , t h u s i n c r e a s i n g t h e b a n d w i d t h a v a i l a b i l i t y by s e v e r a l f o l d - o v e r , s i n g l e - b e a m s a t e l l i t e s . However . due to t h e c o s t and c o m p l e x i t y i n v o l v e d , t h i s e v o l u t i o n f rom s i n g l e to m u l t i p l e beams w i l l be g r a d u a l , s t a r t i n g w i t h a few and g r o w i n g p o s s i b l y to a s nmny a s 100 beams.
The beams shown i n F i g u r e I c a n be f i x e d o r g e o g r a p h i c a l l y h o p p i n g . In a h o p p i n g - b e a m s y s t e m , e a c h o f N i n d e p e n d e n t beams c o n t i n u o u s l y hop to N d i f f e r e n t l o c a t i o n s , i n e f f e c t p r o d u c i n g an N x N v i r t u a l - b e a m s y s t e m . For e x a m p l e , t he I00 s p o t s shown i n F i g u r e I can r e s u l t f rom I0 beams , e ach h o p p i n g to I0 l o c a t i o n s . The h o p p i n g r a t e i s f a s t , t y p i c a l l y i n m i c r o s e c o n d s .
The a d v a n t a g e of a h o p p i n g - b e a m s y s t e m i s t h a t t he d w e l l t ime in e a c h l o c a t i o n c a n be d y n a m i c a l l y a d j u s t e d t o ma tch the i n s t a n t a n e o u s demand f o r t r a f f i c a t t h a t l o c a t i o n , t h u s making optimum use o f t he s y s t e m c a p a c i t y . A d d i t i o n a l l y , w i t h hopp ing beams the nu;nber of r e q u i r e d s a t e l l i t e
185
MULlllqJ.MAM u*TILUTE ¢UIMINT OQNEsnc sATnul~
186 R A y ARNOLD a n d F. MICHAEL NaoFatI
Figure 1. Coverage by current single-beam satellite vs. future multiple-beam satell£te
r e c e i v e r s , t r a n s m i t t e r s , and o t h e r s a t e l l i t e h a r d w a r e i s r e d u c e d - - in t h e above example f rom 100 to 10. However , u s i n g h o p p i n g beams a l s o n e c e s s i t a t e s u s i n g t ime d i v i s i o n m u l t i p l e a c c e s s (TDMA), w h i c h f o r a h i g h - b u r s t - r a t e s y s t e m c o u l d l e ad to c o s t l y g r o u n d t e r m i n a l s .
F r e q u e n c y A d d r e s s a b l e Seams. H i g h e r £IRP and f r e q u e n c y r e u s e c a n be p r o v i d e d by means o t h e r t h a n d i s c r e t e s p o t beams a s d e p i c t e d i n F i g u r e 1. An a l t e r n a t i v e c o n c e p t , c a l l e d f r e q u e n c y a d d r e s s a b l e f a n beams, has been r e c e n t l y p r o p o s e d by Hughes A i r c r a f t Co. In t h i s c o n c e p t , t h e s a t e l l i t e a n t e n n a s y s t e m a c t s a s a mic rowave p r i s m . J u s t a s in t he c a s e o f an o p t i c a l p r i s m , where a n i n c i d e n t w h i t e l i g h t i s d e f r a c t e d a t d i f f e r e n t a n g l e s d e p e n d i n g on i t s c o n s t i t u e n t s ' w a v e l e n g t h s , in a f a n beam a n t e n n a s y s t e m t h e i n p u t f r e q u e n c y band i s d i r e c t e d i n a s w e e p i n g f a s h i o n a c r o s s t h e c o v e r a g e a r e a s u c h t h a t t h e low end of t h e band i s d i r e c t e d a t one e d g e o f t he c o v e r a g e and the h i g h end o f t h e beam i s d i r e c t e d a t t he o t h e r edge o f t h e c o v e r a g e . T h i s c a n be a c c o m p l i s h e d t h r o u g h u s e o f a n a r r a y a n t e n n a where the e l e m e n t s a r e d r i v e n by f r e q u e n c y - p h a s e - s h i f t e d s i g n a l s c a u s i n g the a n t e n n a t o d i r e c t i t s be am t o d i f f e r e n t l o c a t i o n s a t d i f f e r e n t f r e q u e n c i e s . The s i g n a l s d r i v i n g t h e a n t e n n a e l e m e n t s a r e o b t a i n e d t h r o u g h a b e a m - f o r m i n g n e t w o r k t h a t c o n s i s t s o f c o u p l e r s and t i m e - d e l a y e l e m e n t s ( R e f e r e n c e 1 ] .
To b e t t e r u n d e r s t a n d t h e a b o v e c o n c e p t , c o n s i d e r t h e p a r t i c u l a r i m p l e m e n t a t i o n p r o p o s e d i n R e f e r e n c ~ 2. A 500--Ms band i s d i v i d e d i n t o 16 s u b b a n d s , e a c h 30 ~0[a w i d e . When the e n t i r e band i s p r e s e n t e d t o t h e a n t e n n a , e a c h o f t h e 16 zubbands i s d i r e c t e d Co a d i f f e r e n t l o c a t i o n i n the c o u n t r y , a s shown i n F i g u r e 2 . The f r e q u e n c y sweep c a n be done once a c r o s s t h e c o v e r a g e a r e a o r r e p e a t e d s e v e r a l t i m e s f o r f r e q u e n c y r e u s e . As shown in F i g u r e 2 , i n t h e p r o p o s e d Hughes s y s t e m a & - f o l d f r e q u e n c y r e u s e i s a c c o m p l i s h e d by d i v i d i n g the c o u n t r y i n t o & z o n e s . The e n t i r e band i s s w e p t a c r o s s e a c h zone s u c h t h a t t h e low end of t h e band i s d i r e c t e d t o t h e wes t edge o f t he zone and t h e h i g h end i s d i r e c t e d to t h e e a s t edge o f t h e z o n e . In t h i s way a t t h e b o u n d a r y where zones m e e t , t h e h i g h f r e q u e n c y subband i n one zone o v e r l a p s the low f r e q u e n c y subband i n t h e a d j a c e n t zone w i t h o u t i n t e r f e r e n c e .
F i g u r e 3 shows the a p p r o x i m a t e - 2 dB beam c o n t o u r c o r r e s p o n d i n g t o some o f t h e f a n beams a s t h e y sweep a c r o s s t h e c o u n t r y . The e d g e - o f - c o v e r a g e g a i n v a r i e s a p p r o x i m a t e l y be tween 39 to &l dR, wh ich i s more t h a n 10 dB h i g h e r t h a n t o d a y ' s s i n g l e - b e a m s a t e l l i t e s .
Ouboard S w i t c h i n S
A l t h o u g h m u l t i p l e s p o t beams have many a d v a n t a g e s , p r o d u c i n g s u c h be -m~. s t a t i o n a r y o r h o p p i n g , r e q u i r e s r e l a t i v e l y l a r g e s a t e l l i t e a n t e n n a s w i t h complex b e a m - f o r m i n g n e t w o r k s . Hore i m p o r t a n t l y , t o p r o v i d e i n t e r b e Am c o n u u n i c a t i o n s , • r o u t i n g mechanism i s n e e d e d o n b o a r d t h e s p a c e c r a f t . In t he b e n t - p i p e , s i n g l e - b e a m s a t e l l i t e s o f t o d a y , t he u p l i n k f rom a n y g r o u n d t e r m i n a l i s down c o n v e r t e d and b r o a d c a s t t h r o u g h o u t t h e c o v e r a g e a r e a , wh ich means t h e s i s ~ a l can be r e c e i v e d by a n y o t h e r t e r m i n a l . In a m u l t i p l e - b e a m s a t e l l i t e h o w e v e r , i n t e r b e a m c o n n e c t i v i t y i s a c h i e v e d a t a c o s t o f f u r t h e r s a t e l l i t e c o m p l e x i t y i n t h e fo rm o f o n b o a r d r o u t i n g , which c a n be e i t h e r d y ~ m i c o r s t a t i c (Tab le I ) . Dyuamic r o u t i n g c a n be a c c o m p l i s h e d u s i n g f a s t s w i t c h e s , o r u s i n g t ime o r f r e q u e n c y m u l t i p l e x e r s , t o s y s t e m a t i c a l l y c o n n e c t t h e u p l i n k f rom one beam co t h e d o v n l i n k on a n o t h e r . S t a t i c , o r p r e a s s i g n e d , r o u t i n g c a n be done u s i n g 8 m a t r i x o f f i l t e r s and c r o s s s t r a p p i n g o f t r a n s p o n d e r s . In g e n e r a l t h e r o u t i n g c a n be done a t p a c k e t , c i r c u i t , o r m u l t i - c i r c u i t l e v e l s . A d e s c r i p t i o n of s e v e r a l a p p r o a c h e s f o l l o w s .
Baseband v s . IF S w i t c h i n | . In F i g u r e ~ , c o n s i d e r a n m beam s y s t e m i n w h i c h g r o u n d t e r m i n a l s i n beam A need t o communica te w i t h t e r l m l n a l s i n beem B and a l s o w i t h t e r m i n a l s i n beam C. In t h e f o l l o w i n g d i s c u s s i o n a Tl~v~ s y s t e m i s c o n s i d e r e d f i r s t .
An m x m Nic rowave S w i t c h ~ a t r l x (NSM) o n b o a r d t h e s p a c e c r a f t c a n p r o v i d e c o n n e c t i v i t y b e t w e e n t h e beams . The TONA f rame i s s u b d i v i d e d i n t o d i f f e r e n t t ime s l o t s , e a c h c o r r e s p o n d i n g to a s p e c i f i c s a t e l l l t e s w i t c h c o n f i g u r a t i o n . At a p a r t i c u l a r t ime s l o t , when t h e MSM i s c o n f i g u r e d t o p r o v i d e a c r o s s c o n n e c t i o n be tween beams A and B, a l l t e r m i n a l s i n beam A n e e d i n g to coaznun ica t e w i t h t e r m i n a l s i n beam g t r a n s m i t u p l i n k b u r s t s to t h e s a t e l l i t e . In t he s a t e l l i t e , t h e s e b u r s t s a r e dow~ c o n v e r t e d t o IF and a r e r o u t e d t h r o u g h t h e HSN t o t h e d o b ' n l i n k p o r t f o r beam S.
The t e r m i n a l s i n b e a A must w a i t f o r a s econd t ime s l o t , when t h e MSM i s c o n f i g u r e d f o r beam A to beam C c o n n e c t i o n , t o t r a n s m i t t h e u p l i n k s i g n a l s d e s t i n e d f o r t h e t e r m i n a l s i n beam C. T h i s s y s t e m , c a l l e d s a t e l l i t e - s w i t c h e d (sS/TDNA), iS e q u i v a l e n t to d i s t r i b u t e d t i m e - s p a c e - t i m e s w i t c h i n g , i n wh ich t h e s p a c e S w i t c h i n g i s done o n b o a r d t h e s a t e l l i t e and t h e t ime s w i t c h i n g i s done by t h e g r o u n d t e r m i n a l s a t e a c h end o f t h e l i n k . The o r i | i n a t i n | g r o u n d t e r m i n a l s e r v e s a s a t ime s w i t c h by o r g a n i z i n g t r a f f i c b u r s t s so t h a t e a c h b u r s t a r r i v e s a t t h e s a t e l l i t e d u r i n g t h e i n t e r v a l i n wh ich t h e NSN i s c o n f i g u r e d t o r o u t e t h e r e c e i v e d b u r s t t o i t s d e s t i n a t i o n downbeem; the MSN s e r V e s a s 8 s p a c e s w i t c h by d y u a m i c a l l y c o n n e c t i n g upbeams w i t h downbeams; and f i n a l l y , t he d e s t i n a t i o n g r o u n d t e r m i n a l s e r v e s a s t he f i n a l t ime s w i t c h when i t s
Next.generation communications satellite systems 187
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r e c e i v e r opens a n a p e r t u r e t o c a p t u r e t h e d o v n l i n k b u r s t .
As d e s c r i b e d . SS/TIMA r e q u i r e s t h a t i n a s i n g l e f r a m e , a g r o u n d s t a t i o n p r o v i d e s e v e r a l , p l i n k
b u r s t s . , up t o a maximum of N where N i s t h e number
of the beams in the system. The overhead, such as preambles and guard times associated w i th each of the many r e s u l t a n t bu rs ts , makes th is technique r e l a t i v e l y i n e f f i c i e n t p a r t i c u l a r l y ~or a system having . ~ n y VS&T t e r : i n a l s c a r r y i n g low t r a f f i c l o a d s t ha t a r e s p l i t i n t o many beams . B u t , when i a t e r b e a m t r a f f i c i s heavy and among a r e l a t i v e l y s m a l l number o f t e r m i n a l s (< 100) t h e o v e r h e a d a s s o c i a t e d w i t h m u l t i p l e b u r s t s i s n o t a s o b j e c t i o n a b l e .
I n c r e a s i n g t h e f r a m e d u r a t i o n i m p r o v e s t h e SS/TIh~A £ ra~e e f f i c i e n c y , b e c a u s e t h e r a t i o o f i n ( o r m a t i o n t r a f f i c t o t h e o v e r h e a d i n c r e a s e s . ~owever~ d o i n g so a l s o a d d s d e l a y . Because o f t h e f o r e g o i n g r ~ t i o n a l e , SS/TIh~ n o r m a l l y i s u sed i n n e t w o r k s c a r r y i n g medium t o h e a v y t r u ~ k s i n v o l v i n g less t h a n 100 terminals.
An a l t e r r ~ t i v e t o t he above i s t o p u t t he e n t i r e s w i t c h i n g f u n c t i o n , i . e . , b o t h t ime and space switching, o n b o a r d the satellite. This c a n be done w i t h b o t h h o p p i n g o r s t a t i o n a r y beams , and r e q u i r e s o n h o a r d s t o r a g e and b a e e b e n d s w i t c h i n g c a p a b i l i t y , a s shown in F i g u r e 5 . The s a t e l l i t e d e m o d u l a t e s t h e u p l i n k s i g n a l f rom e a c h beam t o r e c o v e r e a c h c h a n n e l and s t o r e s the c o n t e n t o f e a c h c h a n n e l i n a n i n p u t memory b i n . An m x m b a s e b a n d s w i t c h r o u t e s t h e c o n t e n t s o f t h e i n p u t memory b i n s t o o u t p u t memory b i n s so t h a t a t l c h a n n e l s i n t e n d e d f o r a g i v e n beam can be m u l t i p l e x e d , r e m o d u l a t e d , u p c o n v e r t e d , and t r a n s m i t t e d in a s i n g l e T ~ b u r s t .
For t h i s system, which i n t h i s p a p e r i s r e f e r r e d to as Onboard S t o r e d Baseband S w i t c h e d
188 RAY ARNOLD a n d F. MICHAEL NADERI
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C o n v e n t i o n a l S i n g l e F u t u r e M u l t i p l e P a r a m e t e r Beam S a t e l l i t e s Beam S a t e l l i t e s
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High S p a t i a l d i v e r s i t y enables f r e q u e n c y r e u s e among s p o t beams
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Complex o n b o a r d p r o c e s s i n g and s w i t c h i n g t o a l l o w s i g n a l f l o w among beams
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TI~A, o r OSBS/TI~V~, t h e ~ f r a m e i s d i v i d e d i n t o a number o f s u b f r a m e s , e a c h a l l o c a t e d to a g i v e n beam. A~ s e e n i n F i g u r e 6 , i n t h e a p p r o p r i a t e s u b f r a m e a s s i g n e d t o beam A, e a c h t e r m i n a l w i t h i n t h e beam p r o v i d e s one u p l i n k b u r s t c o n t a i n i n $ a l l o f the t e r m i n a l ' s t r a f f i c t o a l l o t h e r t e r m i n a l s i n a l l beams. The b u r s t i s composed o f a s i n B l e p r e a m b l e and many t i m e - l u l t i p l e x e d c h a n n e l s , e a c h a d d r e s s e d t o a d i f f e r e n t t e r m i n a l . The t e r m i n a l w i l l depend on t h e s a t e l l i t e t o s t o r e t h i s u p l i n k b u r s t and s o r t o u t and r o u t e a p p r o p r i a t e c h a n n e l s t o a p p r o p r i a t e beams .
I n t e r e s t i n g l y , a b a s e b a n d s w i t c h i n g s a t e l l i t e n e t w o r k i s a l s o c o n f i g u r e d in a s t a r t o p o l o g y , a s a r e t o d a y ' s VSAT n e t w o r k s ; h o w e v e r , t h e f u n c t i o n o f t h e c e n t r a l hub i s l i f t e d and p l a c e d o n b o a r d the s a t e l l i t e . Hence t h e s e s a t e l l i t e s a r e somet imes r e f e r r e d t o as s w i t c h b o a r d s in t h e s k y .
A b a s e b a n d s w i t c h i n g s a t e l l i t e i • i n h e r e n t l y a r e g e n e r a t i v e s a t e l l i t e . The d e m o d u l a t i o n r e m o d u l a t i o n r e q u i r e d in t h e s w i t c h i n g p r o c e s s
w i l l a l s o p r o v i d e s e v e r a l dB o f l i n k i m p r o v e m e n t , wh ich makes c o m ~ n i c a t l o n s w i t h l o w - c o s t t e r m i n a l s e a s i e r . There a r e a l s o d i s a d v a n t a g e s to b a s e b a n d s w i t c h i n g . Baseband s w i t c h e s a r e h e a v i e r , more complex , and r e q u i r e more power t h a n IF swi tches. A l s o , a f t e r t h e s a t e l l i t e i s l a u n c h e d g r o u n d t e r m i n a l s a r e r e s t r i c t e d in t h e i r u s e o f m o d u l a t i o n t ype and more i m p o r t a n t l y in t h e r a t e a t wh ich t h e y c a n t r a n s m i t . The c a r t o o n s in F i g u r e 7 d e p i c t t he • pace and t h e g r o u n d s e l m e n t f o r SS and OSHS •atellites.
FDM/TDNA Systems
In a ~ system, t h e n e t w o r k t h r o u l h p u t i n c r e a s e s a s t h e b u r s t r a t e i s i n c r e a s e d . However , s i n c e more p o w e r f u l HPAs and l a r E e r a n t e n n a s a r e n e e d e d , h i g h - b u r s t - r a t e TI)ffA s y s t e m s r e s u l t i n c o s t l y | r o u n d s t a t i o n s . Fo r r e a s o n s o f economy, and to a l l o w s o l i d s t a t e i m p l e m e n t a t i o n , r e s t r i c t i n s t h e t e r m i n a l BPA t o less t h a n I0 w a t t s and p r e f e r a b l y c l o s e r t o 2-5 w a t t s i s d e s i r a b l e . S i m i l a r l y , t h e t e r m i n a l a n t e n n a i s r e s t r i c t e d t o 1.2 -1 .8 m e t e r s .
These r e s t r i c t i o n s l i m i t t h e t e r m i n a l b u r s t r a t e and t h u s t h e s y s t e m t h r o u g h p u t . T h e r e f o r e , a s t he n e t w o r k t r a f f i c g r o w s , t h e s y s t e m must i n t r o d u c e a d d i t i o n a l T D ~ c h a n n e l s t o accun lnoda te t he i n c r e a s e d t r a f f i c . Ne tworks u s i n $ s e v e r a l f r e q u e n c y - m u l t i p l e x e d ~ c h a n n e l s a r e known a s FIM/TI~qA o r s i m p l y a s c h a n n e l i z e d TIN~A s y s t e m s .
To i l l u s t r a t e t h e above p o i n t s , c o n s i d e r a f u t u r e m u l t i p l e - b e a m s y s t e m a s d e p i c t e d in F i g u r e 1. Assume t h e s a t e l l i t e t o have 10 h o p p i n g b e ~ , e a c h h o p p i n g t o 10 d i f f e r e n t l o c a t i o n s f o r a t o t a l o f I00 v i r t u a l b e a s t . Assuming 500-MHz b a n d w i d t h a v a i l a b i l i t y and &-subband f r e q u e n c y r e u s e t o p o l o g y , e a c h be - s i s a l l o c a t e d 125 ~ l z . Us ing b a n d w i d t h e f f i c i e n t m o d u l a t i o n w i t h 1 .25 § z / b i t e f f i c i e n c y , a p e r beam t r a n s m i s s i o n r a t e of 100 Mbps c a n be a c h i e v e d . However . a 100 Mbps ~ l r o u n d t e r m i n a l , even i n q u a n t i t i e s and even when c o m n m n i c a t i n g w i t h a h i g h - p o w e r e d s a t e l l i t e , can COlt many times the $10,000 cost t a rge t of the cu r ren t VSAT t e rm ina l s .
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An a l t e r n a t i v e approach would be to replace the s i n s l e lO0-Sbps TD~ channel in every bess v i t h 20 f requency-mul t ip lexed. 5-Mbps ~ channels. In doing so, the s y s t e • throughput i s ~ . In t a lned as before vh i le the cost of the ground terminals i s reduced considerably . However, the d i sadvan ta | e i s a more complex • a t e l l i t e . This point can be made more c lea r by r e f e r r i n g to Figure 5. In a lO-bess system, implementation of a s i ng l e lO0-4qbps
channel per bess r equ i re • 10 demodulators.
The lover burst system, however, requires 20 demodulators per bess, one per T~I4A channel, fo r a t o t a l of 200 demodulators onboard the s a t e l l i t e . This increases woilht and power requirements fo r the s a t e l l i t e payload.
Bulk Demodulators
Reducing the number of the demodulators onboard the spacecra f t can be achieved by us ins bulk d a m d u l a t o r s . These are devices capable of de~odulating many channels s imul taneous ly . Bulk demodulators can be Implemented in analos form usin8 su r face acous t i c wave (SAW) devices or in diS•as1 form - - i n s FFT. In e i t h e r case , a bulk demodulator accepts many F~4 channels and demodulates the• s imul taneously , o u t p u t t i n S the baseband channels in a TIM s t r e s s , Reference 3 provides a lood overview of bulk demodulator technolosy and a l so d i scusses the use of nml t lp lexcrs fo r onboard s v i t c h i n | end r ou t i n l .
The ,,to of bulk demodulators can be pushed to the extreme with implementation of an i l l system where ground terminals operate in a s i n l l e - c h e n n e l - p e r - c a r r l e r (SCPC) mode. In app l ica t ions vhere o n l y o n e or rye voice channels per | round t e ru ina l ore needed. SC?C t r a n s • i s s i o n leads to lowest cost implementation. X• such • system, the channel t ransmiss ion r a t e can be 32-64 Kbpe, or two orders of mean/rude l e s s than the TlX4A s y s t e • exmnple l iven above.
Xn an a l l SCPC s y s t e • using baseband s w i t c h l n | , many thousands of individual channels need to be
190 RAY ,~NOLD and F. M~CHA~ NXD~RI
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demodulated and routed onboard the s a t e l l i t e . Reference 3 d e s c r i b e s a fu tu re e y s t e J where so re than 10,000 channels a re demodulated wi th 100 bulk demodulators .
EXANPLES OF PLANNED SYSTlh~q
In t h i s s e c t i o n th ree planned s a t e l l i t e e y s t e e ~ , which inco rpo ra t e many of the t e c h n o l o g i e s d i scussed in the p rev ious sec t i on , a r e d e s c r i b e d . These s a t e l l i t e s y s t e ~ a r e the American ACTS, the Japanese ETS-VI, and the I t a l i a n ITALSAT.
The American A4vanced Coamunications Technolosy S a t e l l i t e (ACTS)
ACTS, an exper imental advanced technology s a t e l l i t e sponsored by NASA and scheduled fo r launch in May 1992, i n c o r p o r a t e s many of the t e c h n o l o s i e s d i s cus sed in t h i s paper . The ACTS prosram o b j e c t i v e i s to develop and f l i s h t q u a l i f y h i g h - r i s k t e chno log i e s and t echn iques for the nex t gene ra t ion of conJnercial c o I m m i c a t i o n s s a t e l l i t e s . (For a d e t a i l e d d e s c r i p t i o n of ACTS see ae fe rence ~ . )
Spot Beau, CoveraEe. Figure 8 d e s c r i b e s beam coverage by the ACTS mul t ip le -beam antenna , which
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Figure 8. Coverase area for NASA's ACTS satelli~e
produces th ree s t a t i o n a r y and two fas t -hopp ing spot beams. One of the two hopping beams can hop to s i x d i s c r e t e l o c a t i o n s and anywhere w i t h i n a cont iguous s e c t o r . The second hopping beam can hop to seven d i s c r e t e l o c a t i o n s and anywhere w i th in a second contiSuoua s e c t o r .
On ACTS, the ha l f -power h e . w i d t h for both the s t a t i o n a r y and the hopping beams i s approxiemtely 0,3 degrees° To extend the coverage beyond t h a t
Next-genera t ion communica t ions satellite systems 191
shown in Fllpare 8 , a m e c h a n i c a l l y s t e e r a b l e an t enna w i th • l . O - d e g r e e b e ~ t d t h has a l s o been l n c o r p e r a t e d i n t o ACTS. This beam, ~ot shown in F lgurn 8 , can be p o i n t e d anywhere w i t h i n the d i s k of the E a r t h as seen i rma the l O 0 - d e | r e e l o n g i t u d e l o c a t i o n of ACTS. Cross c o n n e c t i o n s onhoard ACTS al low t r a f f i c from the mechanically steered a n t e n n a , u p l i n k o r downl ink , to be r o u t e d to any of the b e l 8hotm in F i g u r e 8 .
Onboard S w i t c h i n | . Two types of e w i t c b l n s , IF and baseband , have been i n c o r p o r a t e d in ACTS. The IF s w i t c h i n t e r c o n n e c t s the t h r e e s t a t i o n a r y beams in a SS/TI~A mode of o p e r a t i o n . The baseband swi t ch p r o v i d e s the i n t e r c o n n e c t i o n f o r the two hoppin S beams u s i n g OSBS/TDMA.
F i | u r e 9 i s a s i m p l i f i e d b lock d i ag ram of the s a t e l l i t e pay load t h a t s u p p o r t s t h e s e two modes of o p e r a t i o n s . A f t e r down c o n v e r s i o n , the si&~al i s rou ted th rough ACTS by e i t h e r the baseband p r o c e s s o r o r the IF s w i t c h m a t r i x .
In the baseband s w i t c h e d mode, a f t e r down c o n v e r s i o n to IF, the u p l i n k from each of the two hoppin S beams i s demodula ted by the baseband p r o c e s s o r end s t o r e d in an i n p u t memory (F igure 10). A baseband s w i t c h w i t h i n the p r o c e s s o r p r o v i d e s the i n t e r c o n n e c t i o n s between the two u p l i n k and the two downlink beams by r o u t i n S d a t a from the i n p u t memory to the p r o p e r ou tpu t memory, where a l l d a t a a d d r e s s e d to the same spo t l o c a t i o n i s t i m e - d i v i s i o n m u l t i p l e x e d , modula ted , and t r a n s m i t t e d on the a p p r o p r i a t e downlink beam. The mod u l a t i o n used in t h i s mode of o p e r a t i o n i s S e r i a l Minimum S h i f t Keying (SMSK). The u p l i n k d a t e r a t e can be 110 o r 27.5 ,Tops, whi le the downl ink t r a n s m i s s i o n r a t e i s 110 Mops.
In the OSBS/TD~ mode, ACTS s w i t c h e s i n d i v i d u a l b~-k'bps c i r c u i t s onboa rd . T h i s , a l o n S w i th the use of h i g h e r EIRP, e n a b l e s ACTS to p r o v i d e m u l t i p l e v o i c e channe l s to VSATe end to do so in a s i n g l e s a t e l l i t e hop - - n e i t h e r of which i s p o s s i b l e wi th c u r r e n t VSAT n e t w o r k s . ACTS can suppo r t a VSAT th roughpu t of 1.5 ~bpe - compared
w i th t o d a y ' s VSAT t h r o u g h p u t o f l e s s t h i n 9 .6 Kbp8 - with terminals having a 1.i--meter antenna and a lO-wat t RPA.
In add i t ion to baseband meitchtnR, ACTS also uae l I 3 x 3 IF s w i t c h i n g m a t r i x i n a SS/T/N~ a r r a n g e m e n t . This mode i s n o m i n a l l y used w i th t h r e e s t a t i o n a r y beams, a l t h o u g h two h o p p i q beams can be kept s t a t i o n a r y and used i n c o n j u n c t i o n with the Cleveland s ta t ionary beam. (The master control s ta t ion fo r ACTS is located in Cleveland.) The system i s d~eia~ed to o p e r a t e a t a nominal b u r e t r a t e of 220 Nbpe, bu t lower r i t e s a r e a18o possible. In th is mode, |round termtna l i are not res t r i c ted in the i r use of modulation technique.
Ks-band. ACTS uses l~a-band - - 30-GRs up l ink and 20-GSz downlink transmission. To combat fading at the I~-band, adaptive coding and power cont ro l techniques are used. In OSBS/T~A mode, termiruLls e x p e r i e n c i n g fade d y n a m i c a l l y reduce b u r e t r a t e s by a f a c t o r of & and invoke a r a t e I / 2 c o n v o l u t l o n a l code to add a t o t a l of 10 dB of e x t r a marg in to the S-dR c l e a r weather m e r l i n in the ACTS l i n k . In the SS/TI~A mode the s p a c e c r a f t t r a n s m i t t e r power i s a d a p t i v e l y i n c r e a s e d from a nominal i0 wails per beam to ~0 w a t t s , t h e r e b y p roduc ln S 6 dS of a d d i t i o n a l marg in .
The Italian ITALSAT
The Italian National Research Council is i pon lo r in S in advanced technology l l t e l l i t e , cal led ITALSAT, which w i l l employ I number of advanced technologies also planned for use in NASA'e ACTS. These include multiple-beam antennae, onboard regenerat ion, onhoard swi tching, end use of Ks-bend t r a n s m i s s i o n [Refe rence 5 ] .
ITALSAT, which i s e s t i m a t e d to c o s t approx i~ , te l y $I.00 M, inc luding launch end a11 the ground t e r m i n a l s , i s s chedu led f o r l aunch on an Ar i an r o c k e t in 2une 1990. The s a t e l l i t e , c o n s i d e r e d to be e x p e r l m e n t a l / p r e o p e r e t l o n a l , has a c a p a c i t y of 20,000 t e l e p h o n e c i r c u i t s and • l i f e e x p e c t a n c y of 5 y e a r s . A f t e r l aunch i t w i l l be
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Figure 9. ACTS beam i n~e rconnec t i ons
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o p e r a t e d by T e l e s p a z i o and w i l l be i n t e g r a t e d i n t o e x i s t i n s I t a l i a n t e l e p h o n y n e t w o r k . ITALSAT's p r ime c o n t r a c t o r i s S e l e n i a S p a z i o .
Coverage and M u l t i p l e gelun A n t e n n a . The c o v e r a g e f o r ITALSAT, shown i n F i g u r e 11, i n c l u d e s s i x 0 . 5 - d e g r e e beams and one a l l - I t a l y beam. The s p o t beams a r e p r o d u c e d by two a n t e n n a s , e a c h h a v i n g a 2-m d i r e c t - o f f s e t - f e d r e f l e c t o r . Each a n t e n n a p r o d u c e s t h r e e o f t h e s p o t beams . The a n t e n n a s p r o d u c e f a i r l y h i g h s i d e l o b e l e v e l s ( -19dB) and t h e r e a r e no p r o v i s i o n s f o r r e u s e o f the s p e c t r u m among the s i x beams . Each beam p r o d u c e s 57 dBw of EIRP u s i n g a 2 0 - w a t t t r a n s m i t t e r .
The a l l - I t a l y beam i s elliptical and p r o d u c e s h6 dBw EIRP. A d d i t i o n a l l y t h e r e a r e , 0 - a n d 50-CBz p r o p a g a t i o n b e a c o n s b r o a d c a s t i n 3 - -degree b e ~ .
Onboard S w i t c b l n s . ITALSAT i s a r e g e n e r a t i v e s a t e l l i t e e m p l o y i n g d i f f e r e n t i a l QFSK d e m o d u l a t i o n and m o d u l a t i o n o n b o a r d . A f t e r r e c o v e r i n s t h e b a s e b a n d s i g n a l , a 6x6 b a s e b a n d m a t r i x s w i t c h
~ ' ~
• ~ w e t
Figure 11 . Italy's ITALSAT beam layout
o n b o a r d the s a t e l l i t e i n t e r c o n n e c t s t h e s i x s p o t b e a m .
Ks -band T r a n s m i s s i o n . The s a t e l l i t e employs a l o w - n o i s e , 30-GBz r e c e i v e r w i t h a ~ . 8 - d S n o i s e f i g u r e and 9~0 ~s of b a n d w i d t h . On the t r a n s m i s s i o n s i d e , t h e 20-CHz T~rA a r e 2 0 - w a t t c l a s s and have 700 ~ z o f b a n d w i d t h .
The a p p r o x i m a t e l y 100 g r o u n d s t a t i o n s i n t h e s y s t e m have 3.5- to 5--m a n t e n n a s and 5 0 - w a t t HPA. The t r a n s m i s s i o n b u r s t r a t e i s l&7 Mbps and the m u l t i p l e access i s SS/TD~A w i t h 32.-ms f r a m e d u r a t i o n .
Compar i son w i t h ACTS. ITALSAT p r o d u c e s o n l y s t a t i o n a r y s p o t beams , w h i l e ACTS employs e l e c t r o n i c a l l y h o p p i n g as w e l l a s s t a t i o n a r y beams . The a d v a n t a g e o f e l e c t r o n i c a l l y h o p p i n s beams i s t h a t by a d a p t i v e l y c o n t r o l l i n g t h e d w e l l t ime o f t he h o p p i n g beam i n a n y one g e o g r a p h i c a l l o c a t i o n , t h e c a p a c i t y o f t h e s a t e l l i t e c a n be e f f i c i e n t l y ma tched t o t h e i n s t a n t a n e o u s demand f o r t r a f f i c .
A d d i t i o n a l l y t h e b a s e b a n d p r o c e s s o r on ACTS i s c a p a b l e o f b o t h t ime and s p a c e s w i t c h i n g , where as ITALSAT c a n do o n l y s p a c e s w i t c h i n g . Th i s a d d i t i o n a l c a p a b i l i t y a l l o w s ACTS to s w i t c h i n d i v i d u a l v o i c e c i r c u i t s o n b o a r d the s a t e l l i t e . T h i s i s a d v a n t a g e o u s when | r o u n d s t a t i o n s w i t h low t o medium t r a f f i c a r e e m p l o y e d . Th i s i s t y p i c a l l y t h e c a s e in n e t w o r k s e m p l o y i n g c u s t o m e r p r e m i s e s t e r m i n a l s o r VSATs.
Another difference between the two satellites i s t h a t ACTS r e u s e s t h e f r e q u e n c y band among i t s s p o t beam a n t e n n a s , where a s ITALSAT l a c k s t h i s c a p a b i l i t y .
ITALSAT, which represents a s i g n i f i c a n t advancement in European satellite technology, will be l a u n c h e d a p p r o x i m a t e l y two y e a r s b e f o r e ACTS.
The J a p a n e s e E n K i n e e r i n g T e s t S a t e l l i t e Vl (ETS-VI)
J a p a n Ls d e v e l o p i n g ETS-VI a s a f o r e r u n n e r to f u t u r e l a r g e o p e r a t i o n a l c o m m u n i c a t i o n s s a t e l l i t e s ,
Next-generation communications satellite systems 193
which may be i n t h e 2 - t o n , &-kw c l a s s . Whi l e ETS-VI i t s e l f w i l l n o t be q u i t e t h a t l a r g o , i t w i l l r e ~ l i s e many o f t h e r e q u i s i t e t e c h n o l o g i e s . The s a t e l l i t e I s boLuS d e v e l o p e d b y T o s h i b a , b u t v i i i u t i l i z e many p a y l o n d c o m p o n e n t s b u i l t by NTT a n d bus h a r d w a r e f rom l l L C O , amon S o t h e r s . L a u n c h i s s c h e d u l e d f o r 8 u m m r o f 1992 on on B-IX r o c k e t [ R e f e r e n c e 6] .
The [TS-VI c o m m m i n a t i o n a p a y l o a d i n c l u d e s f i x e d and m o b i l e s a t e l l i t e c o m m u n l c a t l o n s , S - b a n d a n d Ks -band m u l t i p l e - b e a m a n t e n n a s , o n b o e r d s w i t c h i n g (SSITI~A), and O-band and op t i ca l commnications.
The m u l t i p l e - b e a m a n t e n n a d e s t S u i s d r i v e n b y the f a c t Chat mos t c o m m u n i c a t i o n s t r a f f i c i n J a p a n i s c o n c e n t r a t e d i n two a r e a s , n a m e l y Tokyo and Osaka. These c i t i e s are separated by only 0.7 d e g r e e s , a s s e e n f rom a g e o s t a t i o r m r y o r b i t , so i n o r d e r t o r e u s e the f r e q u e n c y band i n t h e s e two a r e a s , t h e s p o t beams s h o u l d be no l a r g e r t h a n h a l f t h i s a m o u n t , o r a p p r o x i m a t e l y 0 . 3 d e g r e e s . T h i s would a l l o w t h e Tokyo and Osaka beams t o be s e p a r a t e d by • t l e a s t one b e a m w i d t h . A c c o r d i n g l y , ETS-VI u s e s 2 . 5 - m and 3.5--m a n t e n n a s a t 30 a n d 20 GHz, respect ive ly . Each p r o d u c e s 0.3-desree be&me.
F i g u r e 12 shows t h e c o v e r a g e a r e a o f • p o t e n t i a l o p e r a t i o n a l conmunications s a t e l l i t e based on STS-VI technology. ETS-VI w i l l not p r o d u c e as many beams a s shown in t h i s f igure . Only f o u r K a - b a n d beams w i l l be p r o d u c e d - - the Tokyo and Osaka beams , wh ich w i l l r e u s e a f r e q u e n c y b a n d , and two other bea~s to be determined.
S e l e c t e d ETS-VI t e c h n o l o g y f e a t u r e s a r e d i s c u s s e d b e l o w and compared w i t h s i m i l a r ACTS technologies.
M u l t i p l e - B e a m A n t e n n a . ETS-VI w i l l p r o d u c e f o u r s t a t i o n a r y K s - b a n d beams f rom • 3 .S -m 20-GSz
t r a n s m i t a n t e n n a a n d 8 2 . 5 - 1 , 30-GSs a n t e n n a . (ACTS v i i i a l s o p r o d u c e t h e s m s i z e h e m , b u t i t v i i i b a y s t h r e e s t a t i o n a r y bemm a n d two f a s t b o p p i n g b o o m s . )
P o i n t i n g a c c u r a c y on r ~ s - v I i s 0 . 0 1 5 d e s r e m s , o r 1 / 2 0 t h o f t h e beam d i a m e t e r . (On ACTS i t i s o n l y 0 . 0 2 5 d e g r e e s . ) The ETS-VI m i n r e f l e c t o r c a n be a d j u s t e d m e c h a n i c a l l y on o rb i t t o a c h i e v e p a i n t i n | p r a c i i l o u . ( T h i s c a n a l s o be done on ACTS.)
C l u s t e r f e e d t e c h n o l o g y i s u s e d t o a c h i e v e very' low s i d e l o b e s o f -35 dS. ( T h i s i s b e t t e r t h a n what ACTS a c h i e v e s . ACTS a l s o u s e s c luster f e e d s . )
ETS-VI a n t e n n a s w i l l o p e r a t e at m u l t i p l e f r e q u e n c i e s . N o t a b l y , i t s 20-GHa a n t e n n a w i l l a l s o be u s e d t o p r o d u c e S - b a n d beams f o r m o b i l e c o m m u n i c a t i o n s . ( T h e r e a r e no s u c h p l a n s f o r ACTS.)
Onboard S w l t c h i n S. ETS-VI w i l l u s e an o n b o a r d IF s w i t c h i n g m a t r i x t o i n t e r c o n n e c t i t s m u l t i p l e beams . The IF s w i t c h i s 16 x 12 a n d i s b a s e d on m o n o l i t h i c GaAs s w i t c h t e c h n o l o g y w i t h s w i t c h i n g s p e e d o f 50 n a n o s e c o n d s . (ACTS u s e s a h x ~ s w i t c h w i t h a speed o f lO0 n a n o s e c o n d s . )
ETS-VI has no o n b o a r d r e g e n e r a t i o n o r c i r c u i t s w i t c h i n g . (ACTS u s e s a b a s e b a n d p r o c e s s o r t o r e g e n e r a t e t h e b a s e b a n d s i g n a l and a c h i e v e o n b o a r d c i r c u i t s w i t c h i n g . )
Ks -band T e c h n o l o x y . ETS-VI u s e s BENT t e c h n o l o g y f o r t h e LNA and NNIC f o r t h e r e c e i v e r . (ACTS u s e s GaAs LNA; ~5IC i s n o t used f o r t h e r e c e i v e r . )
O t h e r A s p e c t s . An o p t i c a l p a y l o a d i s p l a n n e d f o r ETS-VI. (The ACTS o p t i c a l p a y l o a d was d e l e t e d due to f u n d i n g c o n s t r a i n t s . ) ETS-VI w i l l u s e ion engines f o r n o r t h - s o u t h sta t ion k e e p i n g . (ACTS w i l l u s e h y d r a z l n e . ) The ETS-VI p a y l o a d w i l l we igh 660 l b . and consume 1 .25 kw. (ACTS numbers a r e 1100 l b . and 950 kw. ) ETS-VI i s S c h e d u l e d f o r l a u n c h i n t h e e m m e t o f 1992 , t h e some a s ACTS.
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Figure 12. Japan's ETS-VI beam layout
DESIGN TRADE-OFFS
In t h i s s • c t l o n we i d e n t i f y some t o p - l e v e l d e s l g n p a r a m e t e r s and p r o v i d e s i m p l e s y s t e m d e s i g n e q , , a t l o n s t h a t i n t e r r e l a t e t h e s e p a r a m e t e r s . The f o l l o w i n g d i s c u s s i o n f o c u s e s on ~ s y s t e m s ; h o w e v e r , when a p p r o p r i a t e , p a r a m e t e r s r e l a t i n g to SCPC s y s t e m s a r e a l s o d i s c u s s e d .
P a r a m e t e r s
Sys tem P a r a m e t e r s . Assume t h a t i n o r d e r to p r o v i d e c o v e r a g e t o a r e g i o n o f i n t e r e s t we need N h o p p i n g beams , e a c h h o p p i n g t o n d i f f e r e n t l o c a t i o n s ( c a l l e d d w e l l areas) r • s u l t i n | i n • t o t a l o f N x n v i r t u a l beams . (Fo r a s y s t e m h a v i n g o n l y s t a t i o n a r y beams n = 1 . ) F o r ~ s y s t e m s , assume the f r ame d u r a t i o n t o be T s e c o n d s and t h e m u l t l p l e - a c c e s s scheme be s u c h t h a t I t results in a f r ame e f f i c i e n c y o f e .
Assume a t o t a l b a n d w i d t h a v a i l a b i l i t y o f g Ha. F u r t h e r • s smue t h a t t h e s i d e l o b e s o f t h e s a t e l l i t e m u l t i b e a m a n t e n n a a r e s u c h t h a t t h e y w i l l a l l o w
194 RAY ARNOLD and F. M]CHASL NADERI
s p a t i a l r e u s e o f t he f r e q u e a c y , and l e t i be t he number o f r e u s a b l e f r e q u e n c y s ubba nds . T h i s neons t ha t t h e r e w i l l he I l l i s a v a i l a b l e to e a c h beam. F i n a l l y assume t h a t the s y s t e m I s d e s l a n e d such t ha t i t con acco~oda te k FINq channels per beam. This on/ms t ha t e a c h b e e s w i l l have k FI~4/TI)P~ channels ( o r k SCPC channels i f ~ i s used) .
~ i t h S t a t i o n P a r a m e t e r s . Let t h e r e be m e a r t h s t a t i o n s p e r d w e l l a r e a and M = N x n x m t o t a l e a r t h s t a t i o n s i n t he s y s t e m . Let r ' d e n o t e the s l l l e s t a s s i ~ a b l e c a p a c i t y t h a t any e a r t h s t a t i o n u s e r can r e q u e s t . T h i s , f o r example , can be a I 2 8 - b i t p a c k e t , a 64-kbps SCPC c h a n n e l , o r l 32-kbps ~ c h a n n e l . Assume the number of a v a i l a b l e c h a n n e l s pe r e a r t h s t a t i o n to be C, i . e , assume t h e r e i s s u f f i c i e n t ha rdware to have C u n i t s of $CPC c h a n n e l s pe r e a r t h s t a t i o n , o r in t he c a s e of TIMA, l e t C be t he nomina l nmnber of c h a n n e l s (o r s l o t s ) t h a t an e a r t h s t a t i o n may r e q u e s t . This t hen s a y s t h a t each e a r t h s t a t i o n can p roduce an i n f o r m a t i o n r a t e of r = C x r ' . Let the t r a n s m i s s i o n b u r s t r a t e be deno ted by R and assume the e a r t h s t a t i o n u s e s 8 m o d u l a t i o n scheme w i t h e f f i c i e n c y of E b i t s / H z .
S a t e l l i t e P#rame. te r s . For a r e l e n e r a t i v e s a t e l l i t e , l e t t he number of d e m o d u l a t o r s onboard be d = k x N. A d d i t i o n a l l y l e t S in b i t s d e n o t e the onboard s t o r a s e c a p a b i l i t y o f the s a t e l l i t e .
I n t e r r s l a t i n a t h e P a r s s m t e r s
The l e f t hand colusm in Tab l e 3 s a t i a t e the d e f i n e d p a r s / m e e t s , l ~ s a d on t h e s e d e f i n i t i o n s , i t i s c l e a r t ha t e a c h s a r a h s t a t i o n I t n a r e t e s r x T b i t s of i n f o r l s t i o n d u r t n ~ e a c h f r ame d u r a t i o n and t ha t i t ha l (e x T ) l ( n x m) seconds to t r a n s m i t t ha t i n f o r m a t i o n . This t hen l U l l t l t l t ha t the number of FI~ c h a n n e l s needed p e r beam i s l i v e n by:
r x T
k - ~e x T ) / ( n x m ) . A
C x r ' x m x n 8 s x f l
r x m x n e x 2
( l )
For a s y s t e m w i t h s t a t i o n a r y beams and SCPC o p e r a t i o n , n = l , r ' = R, and • = I r e s u l t i n j i n k = C x m, i s e x p e c t e d .
For a v i d e r a n | e of f rame d u r a t i o n s and a c c e s s schemes t h e f r a / e e f f i c i e n c y • i n Eq. 1 w i l l be c o n f i n e d be tween 0 .6 end 0 . 9 . T h e r e f o r e , i t i s assumed t h a t k w i l l v a r y by no more than a f a c t o r of 2 due to any v a r i a t i o n in t h e f rame d u r a t i o n o r access scheme.
Table 2. Some key system parameters in the desi&n of multiple-beam satellite systems with onboard s~orase and baseband switchln E
P a r a m e t e r s Example
System P a r a m e t e r s
N = Number of h o p p i n | beams 6 n - Number of dwe l l l o c a t i o n s p e r beam 10 T • Frame d u r a t i o n (sec) 10 °3 • = F r m e f f i c i e n c y 0 . 8 m = Number of e a r t h s t a t i o n s p e r dwe l l 50 M = T o t a l number of e a r t h s t a t i o n s in t h e sys t em 3000 B = T o t a l a v a i l a b l e bandwid th (Hz) 109 i • Number of r e u s a b l e f r e q u e n c y subbands r ' = S m a l l e s t a s s i s n a b l e c a p a c i t y ( b i t s / s a c ) 32 x I03 k = Number of F ~ c h a n n e l s pe r beam TDD
E a r t h S t a t i o n P a r a m e t e r s
C = Number of c h a n n e l s ( i . e . , no. of r ' ) p e r e a r t h s t a t i o n 10
r = I n f o r m a t i o n r a t e pe r e a r t h s t a t i o n (C • r ' ) ( b i t s l s e c ) 32 x 10 ~
R = T r a n s m i s s i o n b u r s t r a t e ( b i t s / s a c ) 2 x SO 9
E = H o d u l a t i o n e f f i c i e n c y ( b i t s l H z ) 1 .5
S a t e l l i t e P a r a m e t e r s
d = Number of d e m o d u l a t o r s needed on boa rd TBD S = On-board storase ( b i t s ) T~D
Next-generation communications satellite systems 195
The p a r m s e t e r s i n [ q . 1 c a n be s e l e c t e d o n l y a f t e r s a t i s f y i n s c e r t a i n c o n s t r a i n t s . One s u c h c o n s t r a i n t is the a v a i l a b i l i t y o f f r e q u e n c y s p o e t r m n . T h i s c o n s t r a i n t i s shown b e l o w :
k • I S (2 ) z ( " T
A d d i t i o n a l l y , due t o c o m p l e x i t y , v e i s h t , and p o w e r , one m i s h t c h o o s e t o c o n s t r a i n t h e r e q u i r e d o n b o a r d s t o r a s e c a p a c i t y . This c o n s t r a i n t is shown b e l o w :
4 ( a x T x k x N) ~ S (3 )
The f a c t o r 4 i n Eq. 3 a l l o w s f o r s t o r a g e b o t h b e f o r e end a f t e r o n b o a r d s w i t c h i n s ( s e e F i s u r e 1 0 ) , and a l l o w s f o r t he f l e x i b i l i t y o f w r i C i n S i n t o one l o c a t i o n w h i l e r e a d i n S f rom a n o t h e r .
Example
One c a n s y n t h e s i z e t h e f o l l o w i n S o p e r a t i o n a l s y s t e m for t h e U.S . Assume 6 h o p p i n S beams , e a c h h o p p i n s t o 10 l o c a t i o n s . T h i s r e s u l t s i n c o v e r a J e of t he t o p 60 m e t r o p o l i t a n a r e a s . Assume • TDHA s y s t e m w i t h a f r a m e d u r a t i o n o f 1 me a n d a n a c c e s s scheme r e s u l t i n s i n a f r a m e e f f i c i e n c y o f 0 . 8 . The c a p a c i t y i s a s sumed to be r e q u e s t e d , a n d to be a s s i g n e d , i n u n i t s o f 32 kbps c i r c u i t s . F u r t h e r , we assume t h a t e a c h e a r t h s t a t i o n , on t h e a v e r a s e , r e q u i r e s t e n 3 2 - k b p s c h a n n e l s .
A t o t a l o f 50 e a r t h s t a t i o n s p e r d w e l l a r e a i s a s s u m e d , w h i c h r e s u l t s in 3000 t o t a l e a r t h s t a t i o n s and 3 0 , 0 0 0 t o t a l c h a n n e l s i n t h e s y s t e m . To keep t h e c o s t o f e a r t h s t a t i o n s r e a s o n a b l e , t h e t r a n s m i s s i o n b u r s t r a t e is k e p t a t 2 . 0 Hbps . To c o n s e r v e f r e q u e n c y , a m o d u l a t i o n scheme w i t h e f f i c i e n c y o f 1 .5 b i t s / H z is c o n s i d e r e d .
Assumin S t r a n s m i s s i o n in t h e K s - b a n d , t h e t o t a l a v a i l a b l e b a n d w i d t h i s c o n s t r a i n e d t o 1 . 0 GHz. However , i t i s assumed t h a t t h e s a t e l l i t e a n t e n n a d e s i s n p e r m i t s r e u s e o f t h e f r e q u e n c y w i t h a &-subband f r e q u e n c y r e u s e t o p o l o s y .
F u r t h e r , i t i s a s sumed t h a t t h e o n b o a r d s t o r a g e c a p a b i l i t y o f t h e s a t e l l i t e i s t o be c o n s t r a i n e d t o no mere t h a n 50 ,~o.
The a b o v e a s s u m p t i o n s a r e s m m a r i z e d i n t h e r i g h t hand c o l u ~ i n T a b l e 2 . With t h e s e a s s u m p t i o n s , we use Eq. 1 to c a l c u l a t e the requ i red number o f FI~/TDNA channels per beam.
k - 410) x (32 x 10 +3) x (50) x (10) = 100 (4)
( 0 . 8 ) x (2 x 10 ÷6)
The number o f t h e d e m e d u l a t o r s n e e d e d o n b o a r d i s d = k x N • 600 . T h i s s u s s e s t s t h a t b u l k t y p e d e m e d u l a t o r s need to be u s e d .
As f o r s a t i s f y i n s t h e c o n s t r a i n t s s i r e n b y Eq. 2 we have:
k x_~RR . (100) x (2 x 10 *6 ) = 133 x 10+6 E 1.5
H 1 0 ÷ 9 c T " - ' ~ " 250 x 10 ÷6
(5)
The o n b o a r d s t o r a s e c o n s t r a i n t I s s a t i s f i e d a s shown b e l o w :
4 (k x T x k x N) - 4 (2 x 10 +6)
(10 - 3 ) (100) ( 6 ) (6 )
- 48 x 10 +6 < $0 x 10+6
CONCLUSION
To e x p a n d t h e m a r k e t b a s e f o r VSAT s a t e l l i t e n e t w o r k s , mere s o p h i s t i c a t e d s a t e l l i t e s a r e n e e d e d . Such s a t e l l i t e s c o u l d employ h i s h - E I R P s p o t beam antennas to overcome the low power c o n s t r a i n t of t h e p r e s e n t s a t e l l i t e s , and a n o n b o a r d s w i t c h b o a r d t o r o u t e i n d i v i d u a l t e l e p h o n e c i r c u i t s on a s e n i l e s a t e l l i t e bop . O~board s i g n a l r e s e n e r a t i o n c o u l d be a c c o m p l i s h e d u s i n S b u l k d e m o d u l a t o r s .
To e x p e r i m e n t w i t h t h e above t e c h n o l o g i e s t h r e e s a t e l l i t e systems are now under cons t ruc t i on in three p a r t s o f the w o r l d . These independant p r o s r ~ are ACTS ( U . S . ) , ITAJ.SAT ( I t a l y ) , and ETS-VI ( J a p a n ) .
Look ing f u r t h e r i n t o t h e f u t u r e , an o p e r a t i o n a l s y s t e m c o u l d be e n v i s i o n e d t h a t would have 3 0 , 0 0 0 c i r c u i t s and would u s e b u l k d e m o d u l a t o r s t o e f f e c t i v i l y d e m o d u l a t e 600 ~ c h a n n e l s o n b o a r d t h e s a t e l l i t e . Such a s a t e l l i t e would a l s o need • 4 8 - m e s a b i t s t o r a s e c a p a c i t y .
REFERENCES
1- Hosen, Haro ld , "Mobi le S a t e l l i t e Conmunications System,, '' Presented a t Symposium on NASA Communications, Washinston D.C. , May 26, 1987.
2- Hushes Co~mmicst ions Ga l laxy I n c . , "FCC A p p l i c a t i o n s F i l e No. 1820-DSS-P/LA-83," October 7, 1986.
3- Hu t s , l en t M ichae l , "Advanced Space C o ~ m a n i c a t i o n s A r c h i t e c t u r e S t u d y , " TRW C o n t r a c t NAS3-24743. Prepared f o r NASA L e w i s , 1986.
4- Nader i , Michael and Campanella, Joseph, "NASA's ACTS; An Overview of the S a t e l l i t e , the Network and the Under l y in S Techno los ies , " Proceedinss of AIAA 12th I n t e r n a t i o n a l Coun~micetion S a t e l l i t e Systems Conference, pp. 204-224, Washinston D.C., March 1988 .
5 ° M o r e l l i , G. and M e r i t , i . T . , "The ITALSAT S a t e l l i t e P r o g r a m , " P r o c e e d i n s s o f AIAA 12th I n t e r n a t i o n a l C o e l u n i c a t i o n S a t e l l i t e Sys tems C o n f e r e n c e , pp . 1 1 2 - 1 2 1 , W a s h i n g t o n D . C . , March 1988 .
6 - N a k a s a w a , Kazuo, Kawai , Makoto and T a n a k a , Masayoshi, " F i x e d and Mobi l e S a t e l l i t e C o m m u . i c e t i o n s f o r £ T S - V I , " Proceedinss o f AI~A 12th I n t e r n a t i o n a l Communication S a t e l l i t e Systems Conference. pp. 612-616, Washinston D.C. , March 1988 .
