An analysis of bi-directional use of frequencies for satellite communications

N a t i o n a l Aeronaut ics and Space A d m i n i s t r a t i o n Lewis Research Center Cleveland, Ohio 44135 Thomas S u l l i v a n and John E. M i l l e r ORI, I n c . Landover, Maryland 20785 The b i d i r e c t i o n a l use o f f r equenc ies a l l o ca ted f o r space communications has t h e p o t e n t i a l t o doub le t h e o r b i t / s p e c t r u m c a p a c i t y a v a i l a b l e . T h i s s tudy examines t h e t e c h n i c a l f e a s i b i l i t y o f r eve rse band use (KBU) a t C-band ( 4 GHz u p l i n k s and 6 GHz down l i nks ) . The a n a l y s i s i d e n t i f i e s t h e c o n s t r a i n t s under which b o t h f o rwa rd and r e v e r s e band use s a t e l l i t e systems can share t h e same f r e quencies k i t h t e r r e s t r i a l , l i n e o f s i g h t t ransmiss i o n systems. The r e s u l t s o f t h e a n a l y s i s show t h a t RBU s a t e l l i t e systems can be s i m i l a r l y s i z e d t o f o rwa rd band use (FBU) s a t e l l i t e systems. I n a d d i t i o n , t h e o r b i t a l sepa ra t i on requ i rements between RBU and FBU s a t e l l i t e systems a r e examined. The a n a l y s i s shows t h a t t h e r e a r e no unso lveab le t e c h n i c a l problems. The a n a l y s i s shows t h a t a c a r r i e r t o i n t e r f e rence r a t i o o f 45 dB can be ma in ta ined between RBU and FBU s a t e l l i t e s separa ted by l e s s than 0.5', and t h a t a c a r r i e r t o i n t e r f e r e n c e r a t i o o f 42 dB can be ma in ta ined i n t h e a n t i p o d a l case. Ra in s c a t t e r p ropaga t i on a n a l y s i s shows t h a t RBU and FBU e a r t h s t a t i o n s r e q u i r e sepa ra t i on d i s tances o f l e s s t han 10 km a t a r a i n r a t e o f 13.5 mmlhr escal a t i n g t o l e s s than 100 km a t a r a i n r a t e o f 178 mmlhr f o r e a r t h s t a t i o n antennas i n t h e 3 t o 10 m range. The a n a l y s i s a l s o shows t h a t t h e p resen t i n t e r f e r e n c e l i m i t s f o r s t a t i o n s i n t h e f i x e d s e r v i c e sha r i ng a l l o c a t i o n s w i t h t h e FSS a re adequate t o p r o t e c t bo th se rv i ces under c e r t a i n t e c h n i c a l s i t i n g c o n s t r a i n t s . 1. I n t r o d u c t i o n The f requenc ies t h a t a re most w i d e l y used f o r communications s a t e l l i t e s a re p r e s e n t l y used u n i d i r e c t i o n a l l y , 6 GHz earth-to-space and 4 GHz space-to-earth, f o r example. B i d i r e c t i o n a l use o f t h e f r equenc ies has been proposed as a means o f i n c r e a s i n g t h e o r b i t / s p e c t r u m resource a v a i l a b l e f o r space co~nmunicat ions. T h i s i n n o v a t i v e use o f t h e f requency spectrum has been d iscussed i n t h e C C I R s i n c e 1978,132 I n February 1985, a paper by Pagones and ~ r a b h u , ~ showed t h a t b i d i r e c t i o n a l f requency use by s a t e l l i t e s would i nc rease t h e i n t e r f e r e n c e exper ienced by 4 GHz t e r r e s t r i a l systems. The paper presented here c o n t r i b u t e s f u r t h e r t o t h e a n a l y s i s o f t h e p o s s i b i l i t y o f b i d i r e c t i o n a l use o f f r equenc ies f o r s a t e l l i t e communications. The b i d i r e c t i o n a l use o f f r equenc ies a l l o c a t e d f o r space communications has t h e p o t e n t i a l t o double t h e o r b i t l s p e c t r u m c a p a c i t y a v a i l a b l e . T h i s paper examines t h e t e c h n i c a l f e a s i b i l i t y o f r eve rse band use (RBU) a t C-band ( 4 GHz u p l i n k s and 6 GHz down l i nks ) . F i g u r e 1 shows s a t e l l i t e s o p e r a t i n g w i t h bo th f o rwa rd band use (FBU) and RBU. I n o r d e r f o r b i d i r e c t i o n a l use t o achieve a d o u b l i n g o f t h e o r b i t l s p e c t r u m capac i t y , RBU s a t e l l i t e s w i l l need t o be i n t e r l e a v e d among FBU s a t e l l i t e s w i t h o u t caus ing s i g n i f i c a n t i n t e r f e r e n c e t o e i t h e r system. W i i h t h e FCC c o n s i d e r i n g t h e implementa t ion o f a 2 spac ing p l a n f o r domest ic C-band s a t e l l i t e s , RBU s a t e l l i t e s would need t o be i n t e r l e a v e d a t l o separa t ion , f r om FBU sate1 1 i t e s . The a n a l y s i s i d e n t i f i e s t h e o r b i t a l sepa ra t i on requ i rements between RBU and FBU s a t e l l i t e systems. I n a d d i t i o n , t h e c o n s t r a i n t s under which b o t h f o r ward and reve rse band use s a t e l l i t e systems can share f r equenc ies w i t h t e r r e s t r i a l , l i n e o f s i g h t t r ansm iss ion systems a re examined. Another i n t e r f e r e n c e p a t h which would r e s u l t f rom RBU implementa t ion i s i n t e r f e r e n c e between FBU and RBU e a r t h s t a t i o n s . The a n a l y s i s uses two approaches t o determine c o o r d i n a t i o n d i s tances between e a r t h s t a t i o n s based on ( 1 ) r a i n s c a t t e r t r ansm iss ion l o s s and ( 2 ) g r e a t c i r c l e p a t h propaga t i on . I n t e r f e r e n c e f rom s c a t t e r paths i s s h o r t t e rm w i t h s t a t i s t i c s t h a t a re p r e d i c a t e d on those of t h e s c a t t e r i n g media (e.g., hydrometeors and a i r c r a f t ) . The g r e a t c i r c l e p a t h a n a l y s i s de te r mines r e q u i r e d l osses f o r v a r i o u s pa th con f i gu ra t i o n s (e.g., pa ths over water, pa ths i n suburban areas, paths i n urban areas) i n d i f f e r e n t c l i m a t e r e g i o n s (e.g., c o n t i n e n t a l temperature, m a r i t i m e sub t rop i c , e q u a t o r i a l , d e s e r t ) . The f i n a l i n t e r f e r e n c e paths i n v o l v e t e r r e s t r i a l r a d i o r e l a y s t a t i o n s w i t h RBU systems. These paths i n c l u d e i n t e r f e r e n c e t o t e r r e s t r i a l system r e c e i v e r s f r om bo th RBU e a r t h s t a t i o n and s a t e l l i t e t r a n s m i t t e r s . There a l s o e x i s t i n t e r f e r e n c e paths f r o m t e r r e s t r i a l t r a n s m i t t e r s i n t o RBU e a r t h s ta t i o n s and s a t e l l i t e r e c e i v e r s .