mobile radio - an overview

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MobileRadio:An OvetviewTechnology is giving us impressive communications, information,and navigation systems. The social and economic impact will besubstantial.

Andy D. Kucar

h e f o c u s o f t h i s p r e s e n t a t i o n i s onterrestr ial and satell i te mobile radiocomm unications. This includes cel-lu la r rad io sys tems such as ex is t ingN o r t h A m e r i c a n A M P S , J a p a n e seMCS, Scandinavian NM Tand British

T A C S , a n d t h e p r o p o s e d G S M , D i g it a l A M P S ,and sp read spec t rum C DM A; cord less te lephonysys tems such as ex is t ing CT1 , CT 2 and t he p ro -p o s e d C T 2 P l u s , C T 3 , a n d D E C T ; m o b i l e r a d i oda ta sys tems such as ARDIS and R AM; p ro jec tsk n o w n a s P C N , P C S , a n d F P L M T S ; s a t e l l i t emobile radio systems such as existing INMA RSA Tand OmniTRACS and the p roposed INMARSATX, MSAT, I r idium, Globa lsta r , and O RBCO MM .

Followingabrie f prologue and historical overview,the p aper discusses such technical issues as the reper-toire of systems and services, managem ent of t he air-waves, the opera ting environm ent, service quality,networkissues and cell size, channel coding an d mod-ulation, speech coding, diversity, multiplex an d mul-t iple access (FD MA , TDMA , CDMA).

A l s o a d d r e s s e d a r e t h e p o t e n t ia l e c o n o m i cand sociological impacts of mobile radio c omm u-nications in the wa ke of the redistribution of airwavesa t the Wor ld Admin is t ra t ive Rad io Confe renceW A R C '92. Most existing mobile radio communi-ca t ion s sys tems co l lec t the in f o rm at ion on n e t -work behavior, users' positions, etc., with the pur poseof e n h a n c i n g t h e p e r f o r m a n c e of c o m m u n i c a -tions, improving handover proc edure s and increas-i n g t h e s y s te m c a p a c i t y . C o a r s e p o s i t i o n i n gusually is achieved inhere ntly, while mo re precisenavigation can be achieved by employing LOR AN -C a n d / o r G P S s i g na l s, o r s o m e o t h e r m e a n s , a tthe marg inal expense in cost an d complexity.

The traffic load peaks ofmany mobile radio com-munications systemscoincide n t ime and spacewithvehicular traffic congestio n and traffic accidents.I t migh t be expec ted tha t improved t ra f fic man-agement provided by vehicle information systemswould im prove the traffic safety, relieve t he thirstfor airwaves, and enhan ce the performance of m obiler a d i o c o m m u n i c a t i o n s s y s t e m s . R e c e n t d e v e l -o p m e n t s r e l a t e d t o t h e s e t o p i c s a r e b r i e fl y

~ n d yucaris President of

4U Communications

Research. described.

Prologue

Mobile radi o systems provide their users withoppo r tun i t i e s to t rave l f ree ly wi th in the se rv icearea an d simultaneously comm unicate with any tele-phone, fax, data modem , and electronic mail sub-s c r i b e r a n y w h e r e i n t h e w o r l d . T h e s e s y s t e m sallow users to determi ne heir ownpositions; o trackthe p rec ious ca rgo ; to improve the managementof fleets of vehicles and th e distribution of goods;to improve traffic safety; to provide vital com mu-n i c a t i o n l i n k s d u r i n g e m e r g e n c i e s , s e a r c h a n drescue o peration s, etc. These tieless (wireless, cord-less) comm unications, the exchange of information,determinatio n ofposit ion, course a nd distance trav-eled are mad e possible by the uniq ue property of theradio to employ an aerial (ante nna) for radiating andreceiving electrom agnetic waves.

W h e n t h e u s e r 's r a d i o a n t e n n a i s s ta t i o n a r yover a prolonged period of time, the term fixed radiois used. A radio transceiver capable of being car-r i e d o r m o v e d a r o u n d , b ut s t a t i o n a r y w h e n i n

operation, s called a portable radio. Aradio transceiv-er capa ble of being carried a nd used by a vehicleor by a person on the move is called mo bile radio.

I n d i vi d u a l r a d i o u s e r s m a y c o m m u n i c a t edirectly or via on e or mo re intermediaries, whichmay be passive radio rep eater(s) , base station(s)o r swi tch(es ) . When a l l in te rmed ia r ies a re loca t-e d o n t h e E a r t h , t h e t e r m s t e r re s t r ia l r a d i o s ys -t e m a n d r a d i o s y s te m h a v e b e e n u s e d . W h e n a tl e a st o n e i n t e r m e d i a r y i s s a t el l i t e - b o r n e , t h etermssatelliteradiosystemandsate llite system havebeen used . Accord ing t o the loca t ion o f a use r ,t h e t e r m s l a n d , m a r i t i m e , a e r o n a u t i c a l , s p a c e ,and deep-space radio systems have been used.

Th e second un ique p roper ty o f a l l t e r res t ria land sa te l l i te rad io sys tems is tha t they a l l sha rethe same natural resource: the airwaves (frequen-cy bands and the space).

Recen t developmen ts in microwave monolith-ic integrated circuit (MMIC), application specificin teg r a ted c i rcu i t (AS IC) , ana log id ig i ta l s igna lprocessing (N D SP ) and battery technology, sup-p o r t e d b y c o m p u t e r a i d e d d e s i gn ( C A D ) a n drobotic manufa cturing, allow a viable imple men-

7 2 0163-6804/91/$01 OO 1991@ IEEE IEEE Communications Magazine November 1991



1908 Public radio telephone between ships and the land in Japan was established.

1921

1945

1958

1964

1968

1971

1974

1976

1979

1982

1982

1982

1983

1984

1984

1985

1985

1987

1988

1988

1988

1990

.

Police car radio dispatch service was introduced n Detroit USA Police Department.

DuringWW I I , significant progresses n design and widespread use of mobile radio were made.

LORAN-C commercial operation started. The initial development was started during W w 11.Railway Telephone service on Japanese Tokaido bullet train was introduced.

Carterphone Decision. FCC allows non-Bell equipment to be connected o (Bell) network.

Fully automatic radiotelephone system, the B network, was introduced in West Germany. Later extended to thecorresponding networks in Austria, Luxemburg, and the Netherlands.

US Federal Communications Commission allocated40 MHz frequency band, paving the way for estabilishing what isnow known as advanced Mobile Phone Service (AMPS).

MARISAT consortium initiated commercial service for mobile maritime users, providing ull duplex voice, data, andteleprinter services, worldwide.

Mobile communications systems MCS-L1 introduced by NTT Japan.

The Conference of European Postal and Telecommunications Administrations (CEPT) established Groupe SpecialMobile (GSM) with the mandate to define future Pan-Europeancellular radio standard.

INMARSAT began providing similar services as MARISAT.

Cospas - 1 inclined orbit satellite was launched, with a search and rescue package compatible with Future GlobalMaritime Distress and Safety Sysem (FGMDSS) on board.

SARSAT search and rescue instrument package was placed on board of U.S. National Oceanic and AtmosphericAdministration satellite NOAA-8 and launched.

January 01, divestiture (breakup) of AT&T.

The first interagency tests of Global Positioning System (GPS) receivers conducted in California.

Total Access Communications System (TACS) was introduced n UK.

CD900 cellular mobile radio system was introduced n West Germany.

Japan launched its own experimental satellite ETS - Vsupported by extensive study and experimental work.

Geostar introduced its Link One radiodeterminationservices. The radiodetermination nformation is obtained from aLORAN-C receiver and sent over a L-band satellite payload oward the ground central station.

C?ualcomm/Omninet tarted its two way data communication and radiodetermination (using a LORAN-C receiver)OmniTRACS services.

The second high capacity land mobile communicationssystem (MCS-L2) was introduced in Japan.

Pegasus rocket has been launched from the wing of a 8-52; the rocket injected its 423 Ib payload into a 273 x 370nm 94O inclined orbit.

After almost two decades of studies and experiments, sponsored by Canadian and U.S. tax payers, North Americanmobile satellite systems MSAT is entering its realization stage,

The European community is sponsoring studies and experiments or Pan-European mobile satellite systemsPROSAT and PRODAT.

Future Australian AUSSAT satellites will include, among other services, L band payloads or mobile communicaitonsand receivers for radiodeterminationservices.

Experimental ield trials of CT2, CT3, DECT, GSM, CDMA, TDMA, FDMA mobile radio communicationssystems inprogress, worldwide.

T a b l e Z A s u m m a r y of e v e n t s r e la t e d t o t h e m o b i l e r a d i o c o m m u n i c a t i o n s

IEEE Communications Magazine November 1991 73



-Today,

cordless

(wireless,

fiberless)

radio

systems offer

telepoint

services

similar in

scope to

those

provided bythe public

telephone.

t a t ion o f min ia tu re rad io t ransce ivers . The con-tinuous flux of market f orces (excited by the pos-sibilities of a myriad of new services and g reat profi ts),in te rna t iona l and domes t ic s tandard fo rces (whomana ge comm on natural resource: the airwaves),and technologyforces (capable to createviablep rod-ucts) acted harmoniously and created a broad choiceof communications (voice and da ta), information,and navigation systems, which propelled a n explo-sive growth of mobile ra dio services for travelers.

Is space the limit?

A Glimpse of History

Many things have an epoch, in whichthey are found a t the same time in severalplaces, just as the violets appear on every

side in spring.Farkas Wolfgang Bolyai, in 1823.

Late in the nineteen th century Heinrich RudolfH e r t z , N i k o l a T e s l a , G u g l i e l m o M a r c o n i , a n do ther sc ien t i s t s exper imen ted wi th the t ransmis -sion and reception of electromagnetic waves. Theb i r t h o f m o b i l e r a d i o g e n e r a l l y is a c c e p t e d t ohave occurred in 1897, when Marc oni was credit-e d w i t h t h e p a t e n t f o r w i r el e s s t el e g r a p h . S i n c et h a t t i m e , m o b i l e r a d i o c o m m u n i c a t i o n s h a v e

provided safe navigation fo r ships and airplan es,s a v e d m a n y l i v e s , d i s p a t c h e d d i v e r s e f l e e t s o fvehicles, won battles, generated newbusinesses, etc .A sum mary of som e of the key historical develop-men ts re la ted to commerc ia l mobi le rad io com-munications is provided in Table 1.

Satell i te mobile radio systems launched in the1970s and ea r ly 1980s use U H F f r eq u e n c y b a n d sof approximately 400 MHz, with bands of approx-i m a te l y 1 . 5 G H z u s e d f o r c o m m u n i c a t i o n s a n dn a v i g a t i o n s e r v i c e s . I n t h e 1 9 5 0 s a n d 1 9 6 0 s ,numerous private mobile radio networks, cit izenband (C B) mobile radio, ham operator mobile radio,and por tab le hom e rad io te lephones used d iversetypes and b rands o f rad io equ ipm ent and chunkso f a i r w a v e s l o c a t e d a n y w h e r e i n t h e f r e q u e n c yband f rom near 30 MH z to 3 GHz.

In th e 1970s, E r icsson in t roduced the Nord icM o b i le T e l e p h o n e ( N M T ) s y s te m , a n d A T & TBel l Labora to r ies in t roduce d Advanced Mobi lePhone Serv ice (AMPS) . Th e impac t o f these twopublic land mobile telecommunication systems onthe standardization an d prospects of mobile radioc o m m u n i c a t i o ns m a y b e c o m p a r e d w i t h t h eimpact of Apple and IBM on he pe rsona l compute rindustry. In Eu rope systems resembling AM PS com-peted with N M T systems; in the rest of the world,AM PS, backed by Be l l Labora to r ies ’ repu ta t ionfo r techn ica l exce l lence and the c lou t o f AT&T ,b e c a m e d e f a c to a n d d e j u r e t h e t e ch n ic a l s t a n -dard . (The Br i ti sh TACS an d the Japanese MCS-L1 are based o n AMPS.)

I n 1 9 8 2 , t h e C o n f e r e n c e o f E u r o p e a n P o s t a land Telecommunications Administrations (CEP T)established Gro upe Special Mobile (GSM ) with the

manda te t o de f ine fu tu re Pan-European ce l lu la rradio standards . On January 1,1984, during the phaseof explosive growth of AM PS a nd similar cellularm o b i l e r a d i o c o m m u n i c a t i o n s s y s t e m s a n d s e r -vices, the divesti ture (breakup) of A T& T occurred.

Le roi est mort, vive le roi.

Panta Rhei

B a s e d o n t h e s o l i d f o u n d a t i o n e s t a b l i s h e d i nthe B D (before divestiture) era, the buildup of mobiler a d i o s y s t em s a n d s e r v i ce s i n t h e A D ( a f t e rdivesti ture) era is continuing at a 20 percent to 50p e r c e n t r a t e p e r y e a r w o r l d w id e . T e r r e s t r i a lmobile radio systems offer analog voice and low-to m e d i u m - r a t e d a t a s e r v ic e s c o m p a t i b l e w i t hexisting public switching eleph one network s n scope,but with poorer voice quality and lower data throug h-

put. Satell i te mobile ra dio systems currently offerana log voice , low- to m ed ium-ra te d a ta , rad iode-termina tion an d global distress safety services fort rave le rs . By the en d o f 1988 (1990) the re w ereapproximately 2 (4.5) mill ion cellular telephone sin Nor th A mer ica , and a n add i t iona l 2 (4 .5) mi l -lion in the rest of theworld. T her e are approximately20 million cordless phones and about nine mill ionp a g e r s in N o r t h A m e r i c a , a n d a b o u t t h e s a m enum ber in the rest of the world. Consider able progresshas been m ade in recent years.

Equipm ent miniaturization and price are impor-t a n t c o n s t r a i n t s o n t h e s y s t e m s p r o v i di n g t h e s ese rv ices . In th e ea r ly 1950s mobi le rad io equ i p -ment used a considerable a mou nt of a car’s trunkspace and challenged the capacity of a car’s alter-n a t o r / b a t t e r y s o u r c e w h i l e in t r a n s m i t m o d e ;today, the pocket-sized (7.7 ounces or 218 grams)h a n d h e l d c e l l u l a r r a d i o t e l e p h o n e p r o v i d e s 4 5minutes of talkcapacity. Th e average cost of the leastexpensive models of battery-powered cellular mobileradio telephon es has dropped proportionally, andhas broken the $500 barrier.

T h e r e i s a r a p id l y e x p a n d i n g m a r k e t f o rpor tab le communica t ions , p r imar i ly devo ted tothe indoor ( in building, around building) environ-ment. Today, these cordless (wireless, fiberless) adiosystems offer telepoint services similar in scope tothose provided by the public telephone booths. Theiro b j e c t iv e s a r e t o p r o v i d e a b r o a d r a n g e o f s e r -v i ce s s i m i l a r t o t h o s e c u r r e n t l y o f f e r e d b y t h ePub l ic Swi tched Te lephone N etwork (PSTN) andthe planned Integrated Services Digital Network( ISDN).

M o b i l e s a t e l l i t e s y s t e m s a r e e x p a n d i n g i n

many d i rec t ions : la rge an d powerfu l s ing le un i tgeostationary systems; medium-sized, low orbit mul-tisatellite systems; and small-sized, low orbit mul-tisatellite systems, launched from aplane. T he growthand profit potentials of the mobile radio com mu-n icat ions marke t a t t rac ted the majo r m anufac tu r -ers in the a reas of netw orks, systems, and switching.This caused profound changes in research and devel-opment , s tandard iza t ion , and the dec is ion-mak-ing processes in the m obile radio communicationsindustry. In the search for El D o r a d o t h e m o b i l eradio com munications industry is following two mainpaths: terrestrial and satellite. The terrestrial mob ileradio pioneers, now accom panied by large marketingteams, favor existing cellular radio systems concepts.Th e newcomerswith telephony, switching,and soft-ware backgrounds promo te cordless elephony ((ST),

personal communications networks (PCN), and pe r-s o n a l c o m m u n i c a t i o n s s y s t em s ( P C S ) . T h o s eindividualswith a background in administration pro-mote future public land mobile telecommunicationssystems (FPLM TS) concepts. The satell ite mobilerad io p ioneers bu i ld o n ex is ting and new geos ta -t i o n a r y s a t e l l i t e s y st e m s , w h i le t h e n e w c o m e r s

74 IEEE Comm unications Magazine November 1991



promote inc l ined o rb i t concep ts . The p romote rso f each concep t may fu r the r be subd iv ided in toanalo g and digital; frequency division multiple access(FDM A), t ime division multiple access (TDM A),a n d s p r e a d s p e c t r u m c o d e d i vi s io n m u l t i p l eaccess (CDMA ), etc .

Omnia mutantur, nos et mutamur in illis.

Repertoire of Systems andServices

T h e variety of services offered to ravelers essen-tially consists of informa tion in analog and/or dig-ital form. Although most of today's tra ffic consistsof analog voice transm itted by analog frequencycodulation F M (o r phase m odulation PM), digitalsignaling and a co mbination of analog an d digitaltraffic might provide superior freque ncyre-us e capac-ity, processing, and n etw ork interconnectivity. Byusing a powerful an d affordable microprocessor anddigital signal processing chips, a myriad of differ-ent services particularly well-suited to the n eedsof people on the move could be realized econom-ically. A brief description of a few elementary sys-temsiservices curre ntly available to travelers willfollow. Some of th ese elem entary services can becombinedwithin the m obile radio units for a marginalinc rease in cos t and complex i ty com pared wi th

the cost of a single service system. For example, amobile radio com municatio ns system can includea positioning receiver, digital map, etc.

Terrestrial Systems

I n a t e r r e s t r i a l m o b i l e r a d i o n e t w o r k , arepea te r usual ly was loca ted a t the n eares t sum -mit offering maximum service area coverage. As thenumber of users increased, the available freque n-cy spectrum became un able to handle the increasedt r a f fi c , a n d a n e e d f o r f r e q u e n c y r e - u s e a r o s e .T h e s e r vi c e a r e a w a s s p l i t i n t o m a n y s m a l l su b -areas called cells , and the term cellular radio wasb o r n . T h e f r e q u e n c y r e - u s e o f f e r s a n i n c r e a s e dsys tem capac i ty , wh i le the smal le r ce l l s ize canoffer an increas ed service quality, but at the ex penseof increased complexity of the user's termina l an d

network infrastructure. Th e rade-offs between reale s t a t e a v a il a b il i ty ( b a s e s t a t i o n s ) a n d c o s t , t h eprice of equipment (base and mobile), networkcom-plexity and impleme ntation dynamics dictate theshap e and size of cellular network.

Satell ite systems employ one o r mo re satell i testo serve as base station(s) and/or repea ter(s) in amobi le rad io ne twork . Th e pos i t ion o f sa te l li te sre la t ive to the se rv ice a re a i s o f c ruc ia l impor-tance for the coverage, service quality, price, andcomplexity of the overall network.

Wh en asatellite circles h e Earth in 24-hourperi-o d s , t h e t e r m g e o s y n c h r o n ou s o rb i t h a s b e e nused. An o rb i t tha t is inclined with respect t o thee q u a t o r i a l p l a n e i s c al l e d a n i n c l in e d o r b i t . A norbit with a 90" inclination is called a polar orbit.A c i rcu la r geosynchronous (24-hour ) o rb i t overthe equatorial plane (O'inclination) is known asge o-

s ta t ionary o rb i t ( s ince f rom any po in t a t the su r -f a c e o f t h e E a r t h t h e s a t e l l i t e a p p e a r s t o b estationary). This orbit is particularly suitable forthe land m obile services at low latitudes, and for mar -i t ime an d ae ronau t ica l se rv ices a t la t i tudes le ssthan 80".

S y s t e m s t h a t u s e g e o s t a t i o n a r y s a t e l l i t e si n c lu d e I N M A R S A T , M S A T , a n d A U S S A T . A nelliptical geosynchro nous orb it with the inclina-tion angle of 63.4"is known as Tu ndra orbit. An ellip-t ica l 12 -hour o rb i t wi th the inc l ina t ion ang le o f63.4" is known as Molniya orbit.

Both Tun dra an d Molniya orbits have been select-ed for the coverage of the northern lati tudes and th ea r e a a r o u n d t h e N o r t h P o l e . F o r u s e r s a t t h o s ela t i tudes the sa te l l ite s appear to wander a roundt h e z e n i t h f o r a p r o l o n g e d p e r i o d o f t i m e . T h e

coverage o f a pa r t icu la r reg ion ( reg iona l cover -a g e ) a n d t h e e n t i r e g l o b e ( g l o b al c o v e r a g e ) c a nbe p rov ided by d i f fe ren t cons te l la t ions o f sa te l -lites, including hose in inclined and polar orbits. Forexample , inc l ined c i rcu la r o rb i t cons te l la t ionsh a v e b e e n p r o p o s e d f o r G P S ( 1 8 - 2 4 s a t e ll i t es ,55"-63" inclination), Glo balst ar (48 satellites, 47"inclination), and Iridium (77 satell i tes , 90" incli-na t ion , po la r o rb i t s ) sys tems . Al l th ree sys temswi ll p rov ide the g loba l coverage . ORB CO M sys-tem employs Pegasus launchable low-orbit satellitesto provide uninterrupted coverage of the Ear th below+60"latitudes, an d an intermittent but frequent cov-erage over the polar regions.

Sa te l l i te an te nn a sys tems can have one ( s in -gle-beam global system) or more beams (multibeams p o t s y s t e m ) . T h e m u l t i b e a m s a t e l li t e s y s te m ,similar to th e terrestr ial cellular system, employs

a n t e n n a d i r e c ti v i ty t o a c h i e ve b e t t e r f r e q u en c yre-use, at th e expense of system complexity.

Rad io paging is a non-speech, one-way (from basestation tow ard travelers) personal selective call-ingsystemwith alert,without message orwith definedmessage such as numeric or alphanumeric. A p er-sonwishingtosend amessagecontactsasystemoper-a t o r b y p u b l i c s w i t c h ed t e l e p h o n e n e t w o r k( P S T N ) , a n d d e l i v e rs h i s m e s s a g e. A f t e r a nacceptable t ime (qu eueing delay), a system oper-ator forwards the message to the traveler by radior e p e a t e r ( F M b r o ad c a st i n g t r a n s m i tt e r , V H F or

U H F d e d i c a t e d t r a n s m i t t er , s a t e l li t e , c e ll u l a rr a d i o s y s t e m ) . A f t e r r e c ei v i n g t h e m e s s a g e , at rave le r ' s smal l ( rough ly the s ize of a c i g a r e t t epack) receiver (pager) s tores the message into i tsm e m o r y , a n d on d e m a n d e i t h e r e m i t s a l e r ti n g

tones or displays he message. Examples include theSwedish system, which uses a 57 kHz subcar r ie ron F M broadcasting transmitters; United States sys-t e m s t h a t e m p l o y 1 5 0 M H z , 4 5 0 M H z , a n d 8 0 0MH z mobi le rad io f requenc ies ; the R PCl sys temused in the United Kingdom, Un ited States, Aus-tralia, New Zealand, the People's R epublic of China,a n d F i n l a n d , w hi c h e m p l o y s 150 M H z m o b i leradio frequencies; and the J apanese system that o per-ates at approximately 250 MH z.

Global Distress Safety System (G DSS) geosta-tionary and inclined orbit satellites transfer emer-gency calls sent by vehicles to the c entral ea rth sta tion.E x a m p l e s a r e : C O S P A S , S e a r c h A n d R e s c u eS a t e l l it e A i d e d T r a c k i n g s y st e m ( S A R S A T ) ,Geos ta t ionary Opera t iona l Env i ronmenta l Sa te l -l i te s (GO ES) , and SEarch a nd REscue Sa te l l i te(SER ES) . The recommended f requency fo r th is

transmission is 406.025 MH z.

Global Positioning System (GPS)

United States Departm ent of Defense NavstarGP S 18-24 p lann ed sa te l l i te s in inc l incd o rb i t semi t Lband (L1= 1575.42 MH z, 12 = 1227.6 MHz )

-By using a

powerful and

affordable

micro-

processorand digital

signal

processing

chips, a van'-

ety of services

can be

offeredeconomically

topeople on

themove.

IEEE Communications Magazine November 1991 7 5



P a r a m e t e r I US 1 S w e d e n 1 J a p a n 1 Au st ra l ia

base

mobile

Duplexing method

I I I

TX freq. band,Mtiz I I I I

935-940 76.0-77.5 850-860 865.00-870.00851 -866 41 5.55-41 8.05896-901 81 .O-82.5 905-915 820.00-825.00

806-821 406.1 0-408.60

FDD/semi, FDD/semi 1 FDD/semi FDD/semi,

kH zRF channel rate,kbfsNumber of traffic

I full I I I fullRF channelbw. I 12.5 1 25.0 1 12.5 I 25.0

25.0 12.5

29.6 1.2 1.2 29.6

400 6 0 ? 799 200ch.Modulation type:voicedata

600

FM Fh4 Rvl FMFSK MSK-FM MSK-FM FSK

Table 2. Comparison of dispatch systems

' A M P S M C S - L 1 N M T C 4 5 0 T A C S G S M P C N IS-54MCS-LP

869-894 870-885 935-960 461-466 935-960 890-915 1710-1785 869-894824-849 925-940 890-915 451-456 890-915 935-960 1805-1880 824-849

spread sp ectrum s ignals f rom which an inte ll igentmicroprocessor-based receiver will be able to extracte xt r e m e ly pre c i s e t im e a nd f r c que n c y inform a -t ion. and accura te ly determ ine i tsown th ree-dimen -sional position. velocity, and acceleratio nwo rldwid e.Th e coarse accuracy of < 10 0 m available to com -mercial users has been dem ons trated by using a hand-

held receiver. An accuracy of meters or ccntimctersis possib le by using the precise (mili tary)codes and/ord i f f e re n t i a l G P S ( a d d i t i o n a l r e f e r e n c e ) p r i n c i -ples . Glonass is the USS R'scoun tcrpar t o f t h c Uni t -ed State 's GPS. Similar systems have been studiedbythe E uropean Space Agency(Navsat) and by WestGerman y (Granas , Popsat , and Navcom).

Lora n C is thc 100 kHz freque ncy navigat ion

FDMA

' F D D30.0

1

832

analog2:l

PM

;tl2

digitalFSK

Manch.&

10.0

BCH(40,28)

(48,36)

Ibase .

FDMA FDMA FDMA FDMA TDMA TDMA TDMAFDD Rx, FDD FDD FDD FDD FDC)

25.0 12.5 20.0 25.0 200.0 200.0 30.0

12.5 10.0

1 1 1 1 8 16 3600 1999 222 1000 12 5x 8 375x1 6 83 2x 3

analog analog analog analog REL P RELP VSELP

13.0 6.7 8.0

1200 444

2:l 2:l 2:l 2:l

PM PM PM PM GMSK GMSK d 4

i 5 &5 i 4 k9.5

- 270.8 270.8 48.6

digital digital digital digital digital digital digitalFSK FFSK FSK FSK GMSK GM SK d 4

Manch. NFV NRZ Manch. NRZ NFLZ NRZi4.5 i3.5 i2.5 k6.4

0.3 1.2 5.3 8.0 270.8 270.8 48.6

BCH B1 BCH BCH RS RS Conv.(43,31) burst (15,7) (40,28) (12,8) (12 8 ) 1 2

a.(43,31) burst (15,7) (48,36) (12,8) (12,8) 112

p.(l 1,07)

I Traffic channelsr RF channel

syllabic comp.speech rate, kblsmodulation

peak dev., kH zch. rate, kblsControl:modulation

bb waveformpeak dev., kHZch. rate , kbfsChanne l coding

base-+mobile

mobile-+base

system t h a t provides a positional accuracy bctween1 0 m and 150 m . A usc r ' s r c c c ive r m e a sure s thet ime difference between the master stat ion t rans-mit ter an d secondary s ta t ion s signals, and def ineshis hypcrbolic line of position. North Am erican Lo ranCcoverage includes the Grcat Lakes and the Atlanticand Pacific coasts, with decreasing signal strengthand accuracy as the user appro aches the Rocky Mou n-tains from the East.Similar radio-navigation systemsare thc 100 kHz D ecca and 10 kHz Om e ga .

RadioD cterminatio n Sate l l i te Service (RDSS)

uses L (1610.0 M H z to 1626.5 MH z) and S (2483.5M H z t o 2500.0 MHz) band spread spectrum codedivision multiple access (CD M A) signals translatedby d i s loc a te d s a te l li t e s to provide r a d iode te rm i -nat ion services on a primary basis and any associ-a ted no nvoice data services on an ancilla ry basis.Both the United Sta tcs GEOSTAR (which coversNorth America) and the future French CNES L O C -STARsystem (whichcovcrsEuropc,Afr ica ,and theMidd le-East) a re technica lly capable of providingRDSS. digital voicc. and dat a.

Th c Inmarsa t communicat ions system consis tsof three o pera t ional geosta l ionary payloads locat-e d a t 2 6 ' W ( A t l a n t i c O c c a n ) , 63" E ( I n d i a nOc e a n) , a nd 180" W (Pa c i f i c Oc e a n) . Th e S t a n -dardAL-bandsystem.byemployinga0.7Y m to 1.95

ni diameter pointingantenna and approximately200kg of a bove - /be lowd e c k e quipm e nt , c a n provide

analog voice tclep hony . telex, facsimile. up to 56 kb/sd a t a , g r o u p c a l l b r o a d c a s ti n g , a n d e m e r g e n c yc a l l s to m a r i t im e use r s . T h e S t a n d a r d B sys t cmwil l provide d ig i t a l vo ic e ( a b ou t Y.6 kbis ) . da taa nd t e lc x s e rv ic e s by c m p l o y i n g s m a l lc r e q u i p -mentth cn Standard A.The StandardCsystem.whichemploysasmall antcnna (about thcs izeo fa half l i te rc a n) a nd ii small transceiver (roughly the size of a

Table 3 . Comparison of cellular mobile radio systems



Table 4 Comparison of Digital Cordless Telephone Systems

Vote I : The capacity (in the num ber of voice channe ls) fo r a single isolated cell.

Vote 2: The capacity in parentheses may correspond to a 32 kbls vocoder. Re-use eficienc y and associate

capacities reflect our ow n estimates.

Source: 4C Communications Research, 1991.09.15.

t e lephone book d i rec to ry ) can o f fe r up to 600 b kd a t a . S t a n d a r d M s y st e m i s p l a n n e d f o r l a n dm o b i le a n d m a r i t i m e m o b i l e u s e r s . w h i l e a e r o -nautical systems will provide d ata a nd voice ser-\ ices to air travelers.

Vol na is a Soviet system of satellites which, inconjunctionuith the satelli te More, andwith L-handtranspo nders on the Raduga an d Gorizont satellites.will provide worldwide voice and data services to

a fleet of ships and aircraft.

Airphone isapublic,fully automatic air-to-groundtelephone system thatoperatesin the Y00MHzbandu s i n g h k H z S S B t r a n s m i s s io n . E a c h g r o u n dt r an \ ce ve r . b y e m i t t i n g a n e f f e c t i v e i s o t r o p i cr a d i a t e d p o w e r of 3 d B W . s e r v e s a c e l l w i t h ar a d i u s o f a b o u t 100 m . A n a i r c r a f t u s c s t w oblade antenn as, four transceivers (each radiating7 dBW). a te lcphonc se t and an a i rborne comput-cr th at d irects all call logistics.

Dispa tch tho-way rad io land mobi le o r sa te l -s t e m . \ ~ i t h o r w i t h o u t c o n n e c t i o no the PSTN.s t s of a n o p e r a t i n g c e n t e r c o n t i - o l li n g h e

o p e r a t i o n of ;I f lce t of veh ic les such as a i rc ra f t ,taxis . police cars, tracks. rail cars, etc . Th c sum -mary of some existing an d planned terrestr ial sys-tems . inc lud ing MO BITEX RA M and AR DIS , i sgiven in T a b l e 2 . OmniTRACS d ispa tch sys tem

employ s Ku-band geostationary satell i te locatedat 103" W t o provide tux-way digital messagingand posit ion reporting (derived from incorporat-e d sate I i t e ai d e d LO R A N C rcce vc r) . throu gh-o u t th e contiguous United States (CO NU S).

Cellular radio or public land m obile telephone

system offers a full range of services to t he travel-er that are equivalent to those provided by PSTN.Som e of the operating cellular radio systems are: theN o r t h A m e r i c a n A d v a nc e d M o b i l e P h o n e S e r -v ice (AMPS) , the Japanes e land mobi le com mu-n i c a t i o n s s y s t e m s M C S - L I a n d M C S - L ? , t h cNordic Mobile Tele phon e systems N M T - 4 5 0 a n dNMT-900, the Germ an C450, he Italian public landmobile radio comm unication system at 450 MH z,the F rench rad io te lephone mul t i se rv ice ne twork

a t 200 , 400 M H z R A D I O C O M 2 0 0 0 . a n d t h eUnite d Kingdom Total Access Communication Sys-t e m ( T A C S ) . T h e t e c h n i c a l c h a r a c te r i s t i c s o fs o m e e x i s ti n g a n d p l a n n e d s y s te m s a r e s u m m a -rized in Tab le 3.

Cordless TelephonyTh e first generation of the Un ited Kingdom's

cord less te lephones (coded C TI ) was deve lopedas the answer t o the large quantit ies of impor ted ,technically supe rior yet unlicensed mob ile radioequi pm ent. The simplicity and cost-effectivenessof CT1 a n a l o g r a d i o a n d b a s e s t a t i o n p r o d u c t susing eight K F channels and FD MA scheme s temfrom th eir applicationsli initcd to incoming calls froma limited num ber of mobile users to thc isolated tele-po in ts . As the num ber o f use rs g rew, s o d id the

CO -c h a n n c1 in te r e e n cc leve1s, w hi

e t h e qu a1 tyof the service dctcriorated . Anticipating this s i tu-ation. hc second generation digital cordless elecom-m u n i c a t i o n s r a d i o e q u i p m e n t a n d c o m n i o ii a i ri n t e r a c e s t a n d a r d s ( CT2iCA I ) , in c o m p 21 ti b ewith the CTI equipment. have been developcd.CT2

-Th e irst

generation of

the UK's

cordless

telephones(coded C T l )

was devel-

oped as the

answer to the

large quanti-

ties of

imported

mobile radio

equipment.



Whicle

in ormation

system is an

synonym Jor

the variety ofsystems an d

services

aimed

toward

tra fic safety

and location.

s c h e m e s e m p l o y d i g i t a l v o ic e , b u t t h e s a m eFDM A principles as CTIachcmes. Network an d frc-qucn cyre-us e issues necessary to accommo date antic-ipa ted re s identia l , business , and te lepo int t raff icgrowth have not been addressed adequate ly. Rec-ogniz ing the se l im i ta t ions a nd a nt i c ipa t ing them a rke t r e qui re m e n ts . d i f f e re nt f r e que nc y d iv i -s ion mult iple access (FD M A) , time divis ion mul-t i p l e a c c es s ( T D M A ) , c o d c d i v i si o n m u l t i p l ea c c e ss ( C D M A ) , a n d h y b r i d s c h e m e s a i m c d a tc e l l u l ar m o b i l e a n d D C T s e rv i c es h a v e b e e n

develop ed. The technica l character is t ics of someschemes are given in Ta ble 4. Future Public Land Mob ile Telecommunications

Systems (FPL MT S) is a huge international admin-i s t r a tive pro je c t . for whic h t a sks a n d obje c t ive sare p resented in CCIR Re por t 1153.It discusses dif-ferent te rres tr ia l and sa te l l i te mobile radio com-m u n i c a t i o n s a n d b r o a d c a s t in g s y s te m s , t h et r a nsm is s ion of da ta , vo ic e , a nd im a ge s , a t r a te sbetween8 kbisand 1,920 kbts ,and averyb roadrang co f s e r v ic e s a n d t e c h n i c a l a n d a d m i n i s t r a t i v eissues.

A m a t e u r s a t e ll i t e s e r v i c es s t a r t e d i n 1 9 6 5 ,when the OS CA R3 satellite was launched. SuccessiveOSCAII/AMSATsatcllitesused 144MHz ,432 MHz,I 2 7 0 M H z , a n d 2400 M H z c a r r i e r f r e q u e n c i e s.T h e USSR‘s I sk r a sa te l l it e s use 21/29 MHz a n dRS-3 sa te l li t e s use 145/29 MHz c a r r i e r f r e que n-

cics.Vc hic lc inform a t ion sys te m is a synonym for

thevariety ofsystems andservices aimed toward traf-fic safety and location . This includes: traffic man-a g e ine n . ve hic le id c n t i c a ion , d i g i t iz e d m a pinformation and navigation, radio-navigation, speeds e n s i n g , a n d a d a p t i v e c r u i s e c o n t r o l . c o l l i si o nwarning and prev ention, c tc . Some of the vehic leinformation systems can eas ily be inco rpora ted inmobile radio communications ransceivers to enhancet h e s e r v i c e q u a l i t y a n d c a p a c i t y o f r c s p e c t i v ecommunicat ions systems.

Embarrass du choix.

Airwaves ManagementTh e a irwaves ( the frequency spectru m and the

s p a c e s u r r o u n d i n g u s ) a r e a l i m i t e d n a t u r a lresource shared am ong severa l different radio users(military,governm ent, commercial, public, and ama-teur) . It s sharing (e.g., among different users. ser-vices described in the previous section, televisiona nd sound broa dc a s t ing , et c . ) , coordina t ion , a ndadministration is an ongoing process exercised onnational and international levels. National admin-i s t r a tions ( e . g . , the Uni te d S ta te s Fc de ra l C oni -m u n i c a t io n s C o m m i s s i o n ( F C C ) , t h e C a n a d i a nDepartm ent of Communicat ions (DO C) in Cana-da . e tc .) . in coopera t ion w ith users and industry,se t the rules and pro cedures for planning and ut i-l i z a t ion of s c a rc e f r e que nc y ba nds . Th e se p la nsa n d u t i l i z a ti o n s m u s t b e f u r t h e r c o o r d i n a t e dinternationally.

The Internat ional Telecommunicat ions Union( I T U ) s a s p e c i a l i z e d a g e n c y o f t h e U n i t e d

Nations . s ta tioned in Geneva, Switzer land. The I TUhas more than 150government m embers . respon-s ible for a l l polic ies re la ted to radio, te legraph,a nd t e le phone . Ac c ording to thc I T U , the worldis divided into three regions: Region I-Europe,including the Sovict Union, Ou ter M ongolia. Africa,

and the M iddle East wcst of I ran ; Region 2- thcAm e r ic a s a nd Gre e nla nd: a nd Re gion 3-As ia(excluding par ts west of I r an a n d O u t e r M o n g o -lia), Austra lia, and Oce ania. Historically, these threeregions have developed , more or less independently,the i r own f r e quc nc y p la ns , which be s t su i t loc a lpurposes .

With the advcnt of satcllite services and glob-alization trends, the coordination between differentr e gions be c om e s m ore urge nt . F re que nc y spe c -trum planning and coordination sperformed through

such ITU bodicsas: the Co mite Consultatif de Inter-national Radio (CCIR ), the International FrequencyRe gis t r a t ion Boa rd ( IFR B) , the Wor ld Adm ini s -tra tive Radio Conferencc (WA RC), and the R egion-a l Adminis trat ive Radio Conference (R ARC ).

T h r o u g h i ts s t u d y g r o u p s . C C I R d e a l s w it hte c hnic a l a n d ope ra t iona l a spe c t s o f r a d i o c o m -municatio ns. Results of these activities have beens u m m a r i z e d i n t h e f o r m o f r c p o r t s a n d r e c o m -mend ations published every four years .

Thc IF RB serves as a custodian of acom mon an dscarce natura l resource , namely, the a irwaves . I ni t s c apa c ity , the IF RB re c ord s r a d io f r e que nc ie s ,advises the mem bers on technical issues, and co n-tr ibutes o n othe r technica l matters . Based on th ework of CCIR , IFRB , a nd the na t iona l a dm ini s -t r a t io n s , I T U m e m b e r s c o n v e n e a t a p p r o p r i a t eR A R C a n d W A R C m e e ti n g s , w h e r e d o c u m e n t s

o n f requency planning and ut i l iza t ion ( the Rad ioRegulations) arc upd ated. Actions on a nationa l levelfollow.

The m a na ging of airwaves becomes even m oreintercs t ing. CCIR’s big bro the r , CCITT, be c a m einvolved with the mobile radio communicat ions .Restructuring of ITU ( including CCIR and IF RB)also has becn proposed.

Th e far-reaching impact of mobile radio com-municat ions on econom ies and the well-being of thethree main trading blocks. other developing and thirdworld countrics, potential manufacturers, and usersm a k e s t h e a i rw a y s ( f r e q u e n cy s p e c t r u m ) e v e nmore im portant. Whilc the battlc for the mobile radiomarket , thc a irwavcs and global competi t ivenessintensif ies . the World Ad minis tra tive R adio C on-ference , WARC ’92, s scheduled to be held in Gra na -d a , S p a i n . At tha t c onfe re nc e , the a i rwa ves m a pof the world is expectcd to be red rawn.

Si vis pacem, para bellum.

Operating EnvironmentWhile t ravel ing, a customer of cellular mobilc

radio systems may experience sudden changes in sig-na l qua l i ty c a use d by h i s m ove m e nts r e la t ive tothe correspo nding base s ta t ion and surroundings .m u l t i p a th p r o p a g a t i o n , a n d u n i n t en t i o n a l j a m -m i n g ( e . g . , m a n m a d e n o i s e , a d j a c e n t c h a n n e lin te r f e re nc e , a nd c o-c ha nne l in te r f e re nc e inh e r -ent to the ce l lular sys tems) . Such an enviro nmen tb e l o n g s t o t h e c l as s o f n o n s t a t i o n a r y r a n d o mfields, ofwhich exper imenta l data is difficult toob tain.Their behavior is hard to predic t and m odel sa t is -factorily. Wh en reflected signal compon ents bccom e

comp arable in level to the a t tenuated direc t com-ponent, an d their delays are comparable to th e inverseo f t h c c h a n n e l b a n d w i d t h , f r e q u e n c y s e l ec t i v efading occurs . The rcception is fur ther degradedb y m o v e m e n t s o f a u s e r , r e la t i v e t o r e f l e c t i o npoints and re lay s ta t ion, causing the Do ppler f re-

78 IEEE C o m m u n i ca t i on \ M a p z i n c * “ e m b e r I 9 9 1



quency shifts . The simplif ied mo del of this envi-ronme nt is known as the D oppler affected multipathRayleigh channel.

The existing and planned cellular mobile radiosystems employ sophisticated n arrowban d an d wide-band fi l tering, interleaving, coding, modulation,equalization, decoding, carrier an d timing recovery,and m ultiple access schemes. The cellular mobilerad io channe l invo lves a dynamic in te rac t ion o fs i g n a l a rr i v e d v ia d i f f e r e n t p a t h s , a d j a c e n t a n dc o - c h a n n el i n t e r f e r e n c e , a n d n o i s e . M o s t c h a n -

n e l s e x h i b it s o m e d e g r e e o f m e m o r y , w h ic hdescription requires higher ord er statis t ics of spa-tial and temporal m ultidimensional random vectors(amplitude, phase, multipath delay, Doppler fre-quency , e tc. ) to be em ployed . Th is may requ i r et h e e v a l u a t i o n o f u s e f u l n e s s of e x i s ti n g r a d i oc h a n n e l m o d e l s a n d e v e n t u a l d e v e l o p m e n t o fmore accura te ones .

Ce l l eng ineer ing , p red ic t ion o f se rv ice a rea ,and service quality, in an ever-changing mobile radiochannel env ironm ent, is a diff icult task. The aver-age path loss depends o n terrain microstructure with-in a cell, with considerablevariation between differentt y p e s of c e l l s ( i .e . , u r b a n , s u b u r b a n , a n d r u r a le n v i r o n m e n t s ) . A v a r i e t y of m o d e l s b a s e d on

experimental and theoretic work have been devel-oped to predict pat h radio propa gation losses in amobile channel. Unfortunately, none of them areu n i v e r sa l l y a p p l i c a b l e . I n a l m o s t a l l c a s e s , a nexcessive transmitting pow er is necessary t o providean adeq uate system performan ce. The curves of pathp r o p a g a t i o n l o s s L i n d e c i b e l s ( d B ) v e r s u s t h edistanced in kilometers (km),based on som e of thesemodels, are summ arized in Fig. 1.

T h e f i r st g e n e r a t i o n m o b i l e s a t e l li t e s y s t em semplo y geostationary satell i tes (or payloads pig-gybacked on a host satellite)with small 18dBi anten-nas cover ing the en t i re g lobe . When the sa te l l i tei s p o s i t i o n e d d i r e c t l y a b o v e t h e t r a v e l e r ( a tz e n i t h ) , a n e a r c o n s t a n t s i g n al e n v i r o n m e n t i sexperienced-the Gaussia n chan nel. Th e travel-er 's move ment relative to the satell i te is negligi-ble ( i .e ., D oppler frequency is practically equal toze ro ) . As t h e t r a v e l e r m o v e s ( n o r t h , s o u t h , e a s tor w e s t ) t h e s a t e l li t e a p p e a r s l o w e r o n t h e h o r i -

zon . In add i t ion to the direct path, many signifi-can t s t reng th - re f lec ted compone n ts a re p resen t ,r e s u l ti n g i n a d e g r a d e d p e r f o r m a n c e . F r e q u e n -cies of these components f luctuate due o movemento f t h e t r a v e l e r r e l a t i v e t o t h e r e f l e c t i o n p o i n t sand th e sa te l l i te . Th is env i ronmen t i s known asthe D oppler affected Ricean channel. An inclinedorbit satelli te located for aprolo nged period of t imeabove 45" lati tude north and 106" longitude westcould provide travelers all over the Un ited Statesa n d C a n a d a , i n c l u d i n g t h e f a r N o r t h , a s e r v i c equality unsurpasse d by either geostationary satelliteor terrestrial cellular radio. Similarly,a satellite locat-e d a t 4 5 " l a t i t u d e n o r t h a n d 1.5" l o n g i t u d e e a s tcould provide travelers in Euro pe with improved ser-vice quality.

A typ ica l re tu rn l ink budge t ( f rom a t rave le rto the central s tation via a satell i te) of the seco ndg e n e r a t i o n g e o s t a t i o n a r y s a t e l l i t e s y s t em f o rmedium ra te d a ta se rv ices (and d ig i tized voice )follows. The traveler 's m obile radio employs a IOW transmitter and a small 3 dBi gain antenna. On

i t s wa y t o w a r d a s a t e l l i t e ( u p l i n k ) , t h e 1 . 6 G H zsignal experiences a line-of-sight (LOS) loss of 188.9

L, dB

90

100

110

120

130

140

L

150

160

170

180

19 010 10 0

,,d B

- 9 0

100

110

12 0

130

140

150

16 0

17 0

180

19 0

d, km

Tigure 1 . The curves of path propagation loss L in decibels (db) vs. the distance

in kilometers (km)

dB . In a satellite, this signal is amplified, convert-ed to 12 GHz , then once again amplified, and retrans-mi t ted toward the ea r th con t ro l s ta tion su f fe r inga LOS loss of 205.8 d B in downlink, and a delayof 2 x 0.125 s.

H u g e LOS lo sses pu t an enormous bu rden onsatell i te energy resources, while long delays l imitvo ice t ransmiss ion qua l i ty , and the se lec t ion o fe f f ic ien t da ta p ro toco ls . Inc l ined o rb i t sa te l l i tesystems can offer a low startup cost, a near Gaus-sian channel environment a nd im proved service qual-ity. Low orbit satellites, positioned closer to t he servicea rea , can p rov ide h igh s igna l levels and sh or t (afew milliseconds long) delays, and offer co mpati-bil i ty with the cellular terres tr ial systems. Th eseadvantages need to be weighed against network com-plexity, intersatellite links, tracking facilities, etc.

Service QualityThe p r imary and the mos t impo r tan t measure

of service quality should be customer satisfaction.The customer's needs, both current and future, should

provide guidance to a service provider and an equip-ment m anufactu rer for both th e system concept andproduct design stages. Acknowledging the im por-tanceof eachstepofthecomplexserviceprocessand

a r c h i t e c t u r e , a t t e n t i o n i s l i m i t e d h e r e t o a f e wtechnical merits of quality.

Guaranteed quality level usually is related to ap e r c e n t a g e o f t h e s e r v i c e a r e a c o v e r a g e f o r a nadequate per centage of t ime.

Dat a service quality can be de scribed by the aver-age bit error ra te (e .g. , BER < packet BER( P B E R < 1 0 -2 ) , c o n t r o l P B E R ( C P E R <signal processing delay (1-10ms), multiple accesscollision pro bability (< 20 percent) , the probabil-ity of a false call (false alarm ), the probability of amissed ca l l (miss ) , the p roba b i l i ty o f a los t ca l l(synchroniz ation loss), etc.

Voice quality usually is expressed in term s ofthe me an opinion score (MOS) of subjectiv e eval-ua t ions by se rvice use rs . MO S marks a re : bad =0, poor = 1, fair = 2, good = 3, and excellent = 4.T h e M O S f o r P S T N v o i ce s e r v i c e , p o o l e d b yleading service providers, relates the poo r M OS markto a signal-to-noise ratio (SIN) in a voice chan nel

IEEE Comm unications Magazine November 1991 79



of S j N - 35 d B . w h i le a n e x c e l l e n t s c o r e c o r r e -sponds to SIN > 45 dB. Currently, users of mobileradio services are giving poor marks to th c voice qual-it y associated with a SIN = I 5 dB and an excellentinark for SIN > 25 dB. I t is evident tha t there is asignificant difference (20d B) between the PST N andm o b i l e s e r v i ce s . I f d i g i t a l s p e e c h i s e m p l o y e d ,both the speech and the speak er rccognition have tobe assessed. For m ore objective evaluation ofspeechq u a l i ty u n d e r r e a l c o n d i t i o n s ( w i t h n o i m p a i r -mcnts , in the presence of burs t e rrors durin g fad-

i n g, r a n d o m b i t e r r o r s a t B E R = 10-2, D o p p l e rf r q u e n c y o f f s e t s , t r u c k a c o u s t i c b ac kg r o u n dnoise. ignition no ise, etc.), additiona l tests such a:

ACSSB I Amplitude Companded Single Side BandAMAPKBLQAMBPSKCPFSKW MDEPSKDPMDPSKDSB-AMDS5-SCAMFFSKFM

FSKFSOOGMSKGTFMHMQAMIJFLPAMLRCLRECLSRCMMSKMPSKMQAMMQPRMQPRSMSKm hOQPSKPMPSKQAMQAPSKQPSKQORCSQAMSQPSKSQORCSSB

Amplitude Modulation -

Amplitude Phase Keying ModulationBlackman Quadrature Amplitude ModulationBinary Phase Shift KeyingContinuous Phase Frequency Shift KeyingContinuous Phase ModulationDifferentially Encoded PSK (with carrier recovery)Digital Phase ModulationDifferential Phase Shift Keying (no carrier recovery-Double SideBand Ampliiude ModulationDouble SideBand SuppressedCarrierAMFast Frequency Shift Keying (MSK)Freqency ModulationFreouencv Shift KevinaFrequency Shift Offset Quadrature modulationGaussian Minimum Shift KeyingGeneralized Tamed Frequency ModulationHamming Quadrature Amplitude ModulationIntersymhl Jitter Free ( S W R C )

L-ary Pulse Amplitude ModulationLT symbols long Raised Cosine pulse shapeLT symbols long Rectangularly Encoded pulse shapeLT symbols long Spectrally Raised Cosine schemeModified Minimum Shift KeyingM-ary Phase Shift KeyingM-ary Quadrature Amplitude ModulationM-aty Quadrature Partial ResponseM-ary Quadrature Partial ResDonse SvstemMinimum Shift Keyingmult i4 CPM

Offset (staggered) Quadrature Phase Shift KeyingPhase ModulationPhase Shift KevinaQuadrature AmbliGde ModulationQuadrature Amplitude Phase Shift KeyingQuadrature Phase Shift KeyingQuadrature Overlapped Raised CosineStaggered Quadrature Amplitude Modulation

Sinale Side BandS3MQAM

TS I QPSK

Staggered class 3 Quadrature Amplitude ModulationTamed Frequency Modulation

Crosscorrelated PSK

Class 3 Quadrature Amolitude Modulation

7d4 QPSK 7t/4 shift QPSK

3MQAM4MQAM12PM3

1 Class4 Quadrature Amplitude ModulationI 12state PM with 3 bit correlation

Table 5. Modulation schemes;glossary of terms

(Source: 4U CommunicationsResearch, 1991.09.15)

t h e d i a g n o s t i c a c c e p ta b i l i ty m e a s u r e ( D A M ) .d i a g n o s t i c r h y m e t e s t ( D R T ) , Y o u d c n s q u a r er a n k o r d e r i n g , a n d S i n o - G r a e c o - L a t in s q u a r etes ts , can be perform ed.

Network lssues and Cell SizeT o u n d e r s t a n d i d e a s a n d t e c h n ic a l s o l u t i o n s

offered in existing schemes. and in proposals sucha s c o r d l e s s t e l e p h o n y ( C T ) , d i g i t a l c o r d l e s ste le c om m unic a t ions (DCT) , pe r sona l c om m uni -

cations service (PCS), personal com munications net-work (P CN ) , etc . , on e a l so ne e ds to a na lyz e thereasons for the ir intro duction and success . Cellu-l a r m obi le s e rv ic e s a re f lour i sh ing a t a n a n nua lra te of 20 percent to 40 percent worldwide . Theses y s te m s ( e .g . , A M P S , N M T , T A C S , a n d M C S ) ,use frequency division multiple access ( F D M A ) an ddigi ta l mod ula t ion schem es for access, comman dand control purp oses and analog phaseifrequencym odula t ion s c he m e s for the t r a nsm is s ion of a nanalog voice. M ost of the network intelligence is con-c e nt ra te d a t fi x e le m e nts of the ne twork , inc lud-ing base s ta t ions , which seem to be well-suited t onetworks with a modest numb er ofmedium- to large-sized cclls.

T o s a t is f y t h e g r o w in g n u m b e r o f p o t e n t i a lcustome rs, more cells and base stations were createdby the cell splitting and frequency re-use process.

Technically, the sha pe and size of a particular celli s d i c t a t e d by t h e b a s e s t a t i o n a n t e n n a p a t t e r na nd the topo gra phy of the s erv ice a re a . Cur r c ntterrestrial cellular radi o systems employ cells witha r a d ius of 0.5 km to 50 k m . T h e m a x i m u m c e l lsize usually is dictated by the link budge t, particu-la rly the ga in of a m obi lc a n te nn a a nd a va i l a b leo u t p u t p o w e r . T h i s s i t u a t i o n a r i se s i n a r u r a le n v i r o n m e n t , w h e r e t h e d e m a n d on c a pa c i ty i svery low and cell splitting s not econom ical. Th e min-imum cell s ize usually is dic ta ted by the n eed fo ran in crcase in capacity, part icularly in downtowncores. Practical constraints (e.g.. real est ate avail-a b i li t y a n d p r i c e , a n d c o n s t r u c t i o n d y n a m i c s )limit the minimu m cell size to0.5km to 2 km . In suchtypes of networks, however, the complexity of thenetwork and the cost of service grow exponential-

ly with thc num be r of base stations, while the effi-c icnc y of pre se nt ha ndove r proc e d ure s be c om e sina de qua te .

Anten nas with an omnidirec t ional pa t tern in ahor iz onta l d i r e c t ion . but w i th a bout 10 dBi gainin vertical direction, provide the frequency re-usecfficiency ofNFDMA 1/12. Ba se s t a t ion a nte n-nas with s imilar direc t ivi ty in ver t ica l direc t ionan d 60" directivity in horizo ntal d irection (a ccll isd iv ide d in to s ix s e c tor s ) c a n provide the r e -u scefficiency NFp,A = 114. Th is res ults in a t h r c e -fold increase in the system capacity; if C D M A ise nip loyc d ins te a d of FD M A, a n inc re a se in r e -use efficiencyNF DM A 114+NCI,MA= 213may beexpected.

Recogniz ing some of the l imita t ions of cxis t-ing schemes and anticipat ing the mark et requirc-m c n t s , t h e r c s e a r c h i n t i m e d i v is i on m u l t i p l e

access (TD MA ) schemes a imed a t ce l lular mobileand D CT services , and in code divis ion mu lt ipleaccess (CD M A) schemes aimed toward mobile satel-lite systems. cellular and person al mo bile applica-t i o n s h a v c b e en in t a t ed . AI t hou g h e m p 1oy ngdifferent access schemes, TD MA (CD MA ) network

80 IEEE C o m m u n i c a t i o n \ Mapa7ine N o v c m b e r 1991



c o n c e p t s r e l y on a s m a r t m o b i l e i p o r t a b l e u n i ttha t scans t ime s lo ts (codes ) to ga in in fo rmat ionon network behavior, free slots (codes), etc., improv-i n g f r e q u e n c y r e - u s e a n d h a n d o v e r e f fi c i e n cywhile, hopefully, ke eping the complexity and costof the overall network a t reasonable levels.

Some of the propose d system concepts dependon low gain (0 dBi) base station antennas deployedin a l icense - f ree , unco ord in a ted fash ion ; smal lsize cells (10 m t o 1000 m in rad ius ) and an emi t -t e d i s o t r o p i c r a di a t e d p o w e r o f a b o u t 10 m W

(+ 10 d B m ) p e r 100 kHz have been an t ic ipa ted .A freque ncy re-us e efficiency of N = 119 to N =

1 / 3 6 h a s b e e n p r o j e c t e d f o r D C T s y s t e m s . N =

119corresponds t o the highest user capacitywith thelowest transmission quality , while N = 1 /36 hasthe lowes t use r capac i ty wi th the h ighes t t rans -mission quality. This significantly reduced frequenc yre-use capability of proposed system concepts willresult in significantly reduced system capacity,whichneeds to be comp ensated for by other means, includ-ing new spectra.

In practical networks, the nee d for acapacity (andfrequency spectrum) is distributed unevenly in spacea n d t i m e . In s u c h a n e n v i r o n m e n t , t h e c a p a c i tyand f requency r e -use e f f ic iency o f the ne two rkmay be im proved by dynamic channel allocation,where an inc rease in the capac i ty a t a pa r t icu la rhot spot may be traded for the decrease in the capac-ity in cells surro und ing the hot sp ot, the quality ofthe transmission, and network instability.

To cover the sam e area (space) with increasinglys m a l l e r ce l ls , o n e m u s t e m p l o y m o r e a n d m o r ebase stations. A linear increase in the num ber of bases t a t i o n s i n a n e t w o r k u s u a l l y r e q u i r e s a n e x p o -n e n t i a l i n c r e a s e i n t h e n u m b e r o f c o n n e c t i o n sbetween base stations, switches and netwo rk cen-ters. These connec tions can be realized by fixed radiosys tems (p rov id ing more f requency spec t ra wi l lbe available for this purpose), or, mo re likely, bya cord (w ire, cable, fiber, etc.). The refor e, the fol-lowing postulate can apply:

There will be a plenty of cord in cordlesstelephony. (PI)

T h e f i rs t g e n e r a t i o n g e o s t a t i o n a r y s a t e l l it e

system an tenna b eam covers the entire eart h ( i .e .,t h e c e l l r a d i u s e q u a l s = 6 5 0 0 k m ) . T h e s e c o n dgeneration geostationary satell i tes will use largermultibeam antennasproviding 10 o 20beam s (cells)with a radius of 800km to 1,600km .Low-orbit satel-lites (e.g., Iridium) w ill use up t o 37 beams (cells)w i t h a r a d i u s o f 6 7 0 k m . T h e t h i r d g e n e r a t i o ngeostationary satellite systemswill be able to useveryl a r g e r e f l e c t o r a n t e n n a s ( r o u g h l y t h e s i z e o f abaseball stadium), and provide 80 o 100beam s (cells)withacel l radiusof=200km. Ifsuch asatell i teisteth-e r e d t o a p o s i ti o n 40 0 k m a b o v e t h e e a r t h , t h ecell size will decreas e to =2 km in radius, which iscom parabl e in size with tnday's small s ize cell interrestrialsystems. Yet, such asatellitesystemm ighthave the potential t o offer an improved service qual-i ty due to i ts near optimal location with respect t ot h e s e r v i c e a r e a . S i m i l a r t o t h e t e r r e s t r i a l c o n -cepts, an increase in the num ber ofsatellites in anet -w o r k w i ll r e q u i r e a n i n c r e a s e i n t h e n u m b e r o fconnections between satellites and/or earth networkmanagement a nd satellite rackingcenters, etc. Addi-tional factors that ne ed to b e taken into consider-a t ion inc lude p r ice , ava i lab i li ty , re l iab i li ty , and

K P

@ re 2. The conceptual transmitter (m)nd

receiver (Ru) f a mobile system

t ime l iness of th e launch p rocedures , a few la rgeversus many sm all satellites, tracking stations, etc.

Coding and Modulation

The concep tua l t ransmi t te r (TX ) and receive r(RX ) of a mobile system are shown in Fig. 2. T h etransmitter s ignal processor (TX S P) accepts ana-log vo ice and /o r d a ta a nd t rans fo rms (by ana loga n d / o r d i g i t al m e a n s ) t h e s e s i g n a ls i n t o a f o r msuitable for a double-sided suppressed carrier ampli-t u d e m o d u l a t o r , a l s o c al l e d q u a d r a t u r e a m p l i -tude modula to r (QAM ). Bo th ana log and d ig i ta linput signals may be supported , and either analogor d ig i ta l modu la t ion may resu l t a t the t ransmi t -ter outpu t. Coding an d interleaving also can b e includ-ed. Often, the processes of coding and modulationar e perform ed jointly; we will call this joint pro-cess modulation.A list of typical mod ulation s cheme ss u i t a b le f o r t r a n s m i s s io n o f v o i c e a n d / o r d a t aover Doppler-affected Ricean channel, which canbe gen erated by this transmitter , is given in Table5 . These pa r t icu la r mod ula t ions , however , a l soc a n b e g e n e r a t e d by m e a n s d i f f e re n t t h a n t h a tsuggested in Fig. 2.

Existing cellular rad io systems such as AM PS,T A C S , M C S , a n d N M T e m p l o y h y b ri d ( a n a l o ga n d d i g i t a l ) s c h e m e s . F o r e x a m p l e , i n a c c e s smode AMPS uses a digital modulation schem e (BCHcoding and FSK m odula t ion) . Whi le in in fo rma-t i o n e x c h a n g e m o d e , t h e f r e q u e n c y m o d u l a t e dana log vo ice i s merge d wi th d isc re te SA T and /o rST signals and occasionally blanked to send a dig-i tal message . These hybr id m odula t ion schem esexhibit acon stan t envelope and as such allow the useof d.c . power-efficient nonlinear amplifiers . On

the receiver side, these schem es can be dem odulatedby an inexpensive but efficient limiter/discriminatordevice. They require m odest to high C/N = 10 d Bt o 20 dB, are very robust in adjacent ( a spectrumis concen t ra ted near the ca r r ie r ) an d co-channe linterference (up o C/I= 0 dB, due to capture effect)ce l lu la r rad io env i ronment , and reac t qu ickly tothe signal fade outages (no carrier , code or f ramesynchronization). Frequency-selective and Dop pler-affected mobile radio c hannels will cause modestto significant de gradations, known as the randomphase/frequency modulation.

T ight ly f i l te red modula t ion schemes , such as

IEEE Communications Magazine November 1991 81



-5 0 5 10 15 20 25 3 0

Figure 3. Error rate vs. Eb/No curvesfor digital 4QAM codulation schemes

xi4 QPSKadditionallyfiltered by a square root raisedc o s i n e f i l t e r , e x h i b i t a n o n c o n s t a n t e n v e l o p e ,w h i c h d e m a n d s ( q u a s i ) l i n e a r , l e ss d . c . p o w e r -e f f i c i e n t am p l i f i e r s t o b e e m p l o y e d . On t h ereceiver side, hese schemes require complex demod-

u la t ion rece ive rs , a l inea r pa th fo r s igna l de tec -tion, and a nonlinea r one for reference detection,d i f fe ren t ia l de tec t ion , o r ca r r ie r recovery . W hensuch a transceiver op erate s in a selective fading mul-t ipa th channe l env i ronment , add i t iona l coun te r -m e a s u r e s ( i n h e r e n t l y s l u g g is h e q u a l i z e rs , e t c . )are necessary to mprove the performance by reduc-ing the bit erro r rate foor. These modulation schemesr e q u i r e m o d e s t C I N = 8 d B t o 1 6 d B a n d p er -form modestly in adjacent and/or co-channel (upt o C i I = 8 dB) in te r fe rence env i ronment . Codu-lation schem es emplo yed in sprea d spectrum sys-tems use low -rate coding schemes and m ildly filteredmodulation schemes. When equipped with sophis-t i c a te d a m p l i t u d e g a i n c o n t r o l on t h e t r a n s m i tand receive side, and robu st rake receiver, theseschemes can p rov ide super io r C IN = 4 d B to 10dB and C /I < 0 dB performance.

The erro r rate performance of som e digitalcodu-lation schemes operating in three different mobilechanne l env i ronments i s summar ized in Fig. 3. Itmight be expected that acodulation scheme (in con-junction with an appropriate access scheme) tailoredto a pa r t icu la r loca l env i ronmen t ca n p rov ide as ign i f ican t improvem ent in pe r fo rmance and /o rcapacity over an ad ho c selected combination. There-fore, the following can apply:

For every mobile radio channel, there isan optimal codulation scheme. (P2)

For every codulation scheme, there is anoptimal mobile radio channel. (P3)

Speech Coding

H u m a n vo c a l t r a c t a n d v o i c e r e c e p t o r s , i nconjunction with language redundancy (coding), arewel l -su i ted fo r face - to - face conversa t ion . As thechannel changes (e .g. , from telephone channel tomobile rad io chann el) , different coding strategiesa r e n e c e s s a ry t o p r o t e c t t h e loss of in fo rmat ion .In (analog) companded PM iFM mobile radio sys-

tems, speech is limited to 4 kHz, comp ressed in ampli-tude (2 :1 ) , p re -emphas ized , and phase i f requencymodulated. At a receiver, nverse operations are per-formed . Degradation caused by these conversionsand chann el impairm ents results in lower voice qual-ity. Finally, the hum an ea r and brain have to performthe estimation an d decision processes on the receivedsignal.

In digital schemes sampling and digit izing of

an analog speech (source) are performed first . Then,by using knowledge of properties of the hu ma n vocal

tract an d th e lang uage itself , a spectrally efficientsource cod ing i s pe r fo rmed . A h igh ra te 64 kb is ,56 kbis and A DP CM 32 kbis digitizedvoice compliesw i t h C C I T T r e c o m m e n d a t i o n s f o r t o l l qu a l i t y ,but may be less practical for the mob ile environment.O n e is p r i m a r i ly i n t e r e s t e d i n 8 kb is to 16 kb /srate speech coders, which might offer satisfactoryq u a l i t y , s p e c t r a l e f f i c i e n cy , r o b u s t n e s s , a n da c c e p t a b l e p r o c e s s i n g d e l a y s i n a m o b i l e r a d i oe n v i r o n m e n t . A s u m m a r y o f t h e m a j o r s p e e c hcoding schemes is provided in Tab le 6 .

A t t h i s p o i n t , a p a rt i a l c o m p a r i s o n b e t w e e na n a l o g a n d d i g it a l v o ic e s h o u l d be m a d e . T h equality of 64 kbis digital voice, transmitted over atelephone line, is essentially the same a s the orig-i n a l a n a l o g v o i ce ( t h e y r e c e i v e n e a r l y e q u a lMO S) . What does th is nea r equa l M O S mean ina rad io env i ronment?

A mobi le ra d io conversa t ion cons is t s o f one( m o b i l e t o h o m e ) o r a m a x i m u m of two (mobi leto mobile) m obile radio paths, which dictate the qual-i ty o f t h e o v e r a l l c o n n e c t i o n . T h e r e s u l t s of acomparison between analog an d digital voice schemesin different artificial mobile rad io environments havebeen wide ly pub l i shed . Genera l ly , sys tems tha te m p l o y d i g i t a l v o i c e a n d d i g i t a l c o d u l a t i o ns c h e m e s a p p e a r t o p e r f o r m w e l l u n d e r m o d e s tconditions, while analog voice and analog codula-t i o n s y s t em s o u t p e r f o r m t h e i r d i g i t a l c o u n t e r -pa r t s in fa i r and d i f f icu l t (nea r th resho ld , in thepresence o f s t rong co-channe l in te r fe rence) con-ditions. Fortunately, present technology can offera viable impleme ntation of both anal og and digi-tal systems within the same m obilciportable radiotelep hon e unit. This would give every individual a

choice of either an anal og or digital schem e, bet-te r se rv ice qua l i ty and h igher cus tomer sa t i s fac -tion. Tradeoffs betwcc n the quality of digital speech ,the comp lexity of speech and chan nel coding, aswellas d.c . power consumption, must be assessed care-fully and com pared with analo g voice systems.

Macro and Micro Diversity

Macro Diversity

In a cellular system, the b ase station is usuallylocated in the barocente r of the service area (cen-ter of th e cell), as illustrated in Fig. 4a. Typically,the base an te nna i s omnid i rec t iona l in az imuth ,bu t wi th abou t 6 dB i to 10 dBi gain in elevation,an d se rves mos t o f th e ce ll a rea (e .g . , > 95 per -

cen t ) . Some par t s w i th in th e ce ll may exper iencea lower quality of service because th e direct path sig-n a l m a y b e a t t e n u a t e d d u e t o o b s t r u c ti o n losses

caused by buildings, hills, trees, etc .Th e closest neighboring base stations ( th e f irst

t ie r ) se rve co r respond ing ne ighbor ing a rea ce l l sby using different sets of frequ encies, eventually caus-

82IEEE Communications Magazine November 1991



i n g a d j a c e n t c h a n n e l i n t e r f e r e n c e . T h e s e c o n dc l o s e s t n e i g h b o r i n g b a s e s t a t i o n s ( t h e s e c o n dt ie r ) migh t use the sam e f requenc ies ( f requencyre-use) causing co-channel interfe rence. If the sam ereal estate (b ase stations) s used in conjunction with120" directional ( in azimuth) anten nas. the desig-na ted a rea may be se rved by th ree base s ta t ions ,as illustrated in Fig. 4b. In this configuration on ebase s tation serves three cells by using thr ee 120"d i r ec t io n a l a n t e n n a s . T h e r e f o r e , t h e s a m e n u m -berofexisting base stationsequipped with newdircc-

tional antennas and additional combining circuitryis requi red to se rve the same num ber of cells, yetin a different fashion.

The mode of oper ation in which twoor m ore basestationsserve the same area iscalled themacrodiver-sity . Statis t ically, thr ee base sta tion s a r e a b l e to

p r o v i d e a b e t t e r c o v e r ag e of a n a r e a s i m i l a r ins ize to the sys tem wi th a c e n t r a l l y l o c a t ed b a s estation. The directivityof a base station ante nna (120"o r e v e n 60") prov ides add i t iona l d isc r imina t ionagainst s ignals from neighboring cells , therefore,reducing adjacent andco-c hann el n terferen ce (i.e.,improving re-use efficiency an d capacity). Effec-tive improvement depe nds on th e terrain configu-ration and the combining strategy and efficiency.However, it requires mo re complex ante nna systemsand combining devices.

Micro DiversityM i c r o d i v e rs i ty r e f e r s t o t h e c o n d i t i o n i n

which two or more signals are received at onc site(base or m obile) .

Space diversity systems employ tw o or more anten-n a s s p a c e d a c e r t a i n d i s t a n ce a p a r t f r o m o n ea n o t h e r .A separation of only h/2 = 15 cm. whichis suitable for implementation on th e mobile side,c a n p r o v i d e a n o t a b l e i m p r o v e m e n t i n s o m emobile radio channel environ ments. Micro spacediversity is routinely use d on cellular base sites. Mac rodiversity is also a form of space diversity.

Field-comp onent diversity systems employ dif-ferent types of antenna s receiving either the elec-tricor thc magneticcomponent ofa n electromagneticsignal.

Frequency diversity systems employ two or mo re

different carrier frequencies to transmit the samein fo rma t ion . S ta t i s tica l ly , the sam e in fo rmat ions i g na l m a y o r m a y n o t f a d e a t t h e s a m e t i m e a tdifferent carrier frequencies. Frequency hoppin ga n d v e r y w i d e b a n d s i g n a l i n g c a n b e v i e w e d a sfrequency diversity te chniqu es.

Tim e diversity systems are used primarily fort h e t r a n sm i s s io n o f d a t a . T h e s a m e d a t a i s sen tthrough the channel as many tim es as necessary, untilthe require d quality of transmission is achieved (auto-

? 22)

Figure 4 The cellular system concepts: U ) Burocen-

tric base stations, center excited cellular system; b)

Corner base stations, corner excited cellular system.

ADMADPCMAClTAPCAPC-ABAPC-HQAPC-MQLAQATCBARCELPCVSDM

DAMDMDPCMDRTDSIDSPHCDMLDMLPCMPLPCMSQN CPVXCPWAQMFRELPRPESBCTASlTDHSVAPCVCELPVEPCVQVQLVSELPvxc

Adaptive Delta ModulationAdaptive DifferentialPulse Code ModulationAdaptive Code sub-band exclted Transform (GTE)AdaDtive Predictive CodingAPC with Adaptive Bit AllokationAPC with Hybrid QuantizationAPC with M-&mum Likelihood QuantizationAdaptive QuantizationAdaptive Transform CodingBackward Adaptive ReencodingCode Excited Linear PredictionContinuous Variable SlopeDelta Modulation

Diagnostic Acceptability MeasureDelta ModulationDifferentialPulse Code ModulationDiagnostic Rhyme TestDigital Speech InterpolationDigital Signal ProcessingHybrid Companding Delta ModulationLinear Delta ModulationLinear Predictive CodingMulti Pulse LPCMultipath Search CodingNearly Instantaneous CompandingPulse Vector excitation CodingPredicted Wordlength AssignmentQuadrature Mirror FilterResidual Excited Linear PredictionRegular Pulse ExcitationSub Band CodingTime Assigned Speech InterpolationTime Domain Harmonic ScallingVector Adaptive Predictive CodingVector Code Excited Linear PredictionVoice Excited Predictive CodingVector QuantizationVariable Quantum Level CodingVector-Sum Excited Linear PredictionVector excitation Coding

Table 6. Digitized voice; glossary of terms

mat ic rcpca t reques t A R Q ) . "Would y o u pleascrepeat your last sentencc" is a form of t imc diver-sity used in 3 speech transmission.

T h e i m p r o v e m e n t o f any diversity schcmc iss t rongly dependen t o n the combin ing techn iqucs

employe d, i .e . , the selective (switched ) combin-ing. the maximal ratio combining. the e q u a l gaincombining, the feedforward combining, he feedback(Gr anlu nd) combining, majority vote, etc.

Multiplex and Multiple Access

Comm unications networks for travclcrs havc twod i s t i n c t d i r ec t i o n s : t h e f o r w a r d l i nk ( f r o m t h cbase stationvia satellite to the travclcr) and the returnlink (from a travelerviasatcll i te to the base station).In the forward direction, a base station distr ibutesin fo rm at ion to t rave le rs accord ing to the p rcv i -ously established prot ocol, i.e.. n o multiplc accessis involved. In the reverse direction, many travel-e rs make a t tem pts to access one o f the base s ta -tions. This occurs in so-called control channels, ina particular t ime slot, a t particular frequency. orby using a particular c ode. If collisions occur. cus-tome rs mus t wa i t in a qu eue and t ry aga in un t i lsucccss is achieved. If successful (i.e., if no colli-s ion occur red) . a par t icu la r cus tom er wil l au to -matically exchange the necessary inform ation forcallsetup.Thcnetworkmanagementcentcr(NMC)




-The strengths

of F D M

schemes

seem to

be fullyexploited in

narrowband

channel

environ-

ments.

willveriCy the custo mer’s status, his cred it rating, e tc.Then the N MC may ass ign achannel f requency, imes lo t , o r c o de on whic h the c us tom e r w i ll be a b leto exchange information with his correspon dent .Th e optimizat ion of the forward an d reverse l inksmay require different coding, modulation schem es,and bandwidths in each direc t ion.

In forward link, there are thre e basic distribution(multiplex) schemes: one uses discrimination n fre-quency between different users and is ca l led fre-q u e n c y d i v i si o n m u l t i p l e x ( F D M ) ; a s e c o n d

discriminates in time and is called time division mul-tiplex (TDM ); and the third has different codes basedon spread spectrum signaling, which is known as co ded i v i s io n m u l t ip l e x ( C D M ) . I t s h o u l d b e n o t e dthat hybrid schem es using a combin ation of bas icschemes a lso can be developed.

I n r e v e r s e l i n k , t h e r e a r e t h r e e b a s i c a c c e s ssc he m e s : on e use s d i s c r im ina t ion in f r e que nc ybetween different users an d is called frequency divi-s ion mult iple access (FDM A); a second discr imi-nates in t im e and is ca l led t im e divis ion mult ipleaccess (TDMA); nd a third which has different codesbased o n spread spectrum signaling is known as codedivisionmultiple access (CD MA ). It should be notedtha t hybr id s c he m e s us ing c om bina t ion of ba s icschemes a lso can b e developed.

A p erformanc e comparison of mult iple accesss c h e m e s i s a d i f f ic u l t t a s k . T h e s t r e n g t h s o f

F D M A s c h e m e s s e e m t o b e f u ll y e x p l o it e d i nnarrowb and channel environments. To avoid the useof e qua l i z e r s , c ha nne l ba ndw idths a s na r row a sposs ib le should be e m ploy e d. Ye t , in suc h na r -rowband ch annels the quali ty of service is limitedby the m a xim a l e xpe c te d Dopple r f r e que nc y a ndpractical stability offrequency sources. Current prac-tical limits are approxim ately 5 kHz.

T h e s t r e n g th s of b o t h T D M A a nd C D M Aschemes seem to be fully exploited in wideband chan-nel environments . TDMAschemesneedmanyslots( a n d b a n d w i d t h ) t o c o l l ec t i n f o r m a ti o n o n n e t -work be ha vior . O nc e the e qua l i z a t ion i s ne c e s -s a r y ( a t b a n d w i d t h s > 2 0 k H z ) , t h e d a t a r a t eshould be made as high as poss ible to ncrease frameefficiency and freeze the frame o ease equalization.High data rates, however, require high RFpe akp ow -

e r s a n d a l a r g e a m o u n t o f s i g n a l p ro c e s s i n gpower, which may be difficult to achieve in handh elduni ts . Cu r re nt pra c t i c a l ba ndwidths a re a pproxi -mate ly 0.1 MHz to 1 O M H z .

C D M A s c he m e s n e e d l a r g e s p r e ad i n g ( p r o -c e ss ing) ga ins ( a nd ba ndwidth) to r e a l i z e spre a ds p e c t r u m p o t e n t i a l s , y et h i g h d a t a r a t e s a l s or e q u i r e a l a r g e a m o u n t o f s i g n a l p ro c e s s i n gpower,which may be difficult to achieve in handheldunits . Curren t prac t ica l bandw idths are approxi-mate ly 1. 2 M H z . N a r r o w f r e q u e n cy b a n d s s e e mto f a vor FD MA sc he m e s , s incc both TD M A a ndCD M A schemes require more spectra to ful ly devel-o p t h e i r p o t e n t i al s . O n c e t h e a d e q u a t e p o w e rspectrum is available, however, the la tter two schemesm a y be be t t e r su i t e d for a c om ple x (m ic ro)c el lu-lar network environm ent. Multiple access schemesalso are message sensi tive . Th e length an d type of

m e ssa ge , a n d the k ind of s e rv ic e w i ll in f lue nc ethe choice of multiple access, AR Q, fram e, and cod-ing. Th erefore , the fol lowing can apply:

For every mobile radio channel, there isan optimal access scheme. (P3)

For every access scheme, there is anoptimal mobile radio channel. (P4)

For every type of service, there is anoptimal access scheme. (P5)

For every access scheme, there is anoptimal type of service. (P6)

Hybrid SchemesFor var ious reasons anum ber ofexisting and pro-

posed m ultiple access schemes are hybrid. For exam-

ple , the GS M systememploysaTDMAschemewithe i g h t s l o ts p e r 2 0 0 k H z w i d e R F c h a n n e l ; c h a n -nels are further distributed in an FD M fashion. Thereis an optio nal frequency-hopping pattern (code divi-sion) available.The transmitter and receiver are sep-ara ted by 45 MH z, and frequency divis ion duplex(FD D) mod e of opera t ion is assumed. Therefo re ,th i s r a the r c om ple x hybr id s c h e m e i s de note d a sF D M I F D D I T D M N C D M .

A s l ight ly l e s s c om ple x sc h e m e i s the ne wlypropo sed No rth America n digi ta l ce l lular sys tem(de note d IS-54 in Ta ble 3). This system employsa T D M A s c h e m e w i th t h r e e slots p e r 30 k H zwide R F c ha nne l . Cha nne l s a re fur the r d i s tr ibut -ed in an FD M fashion, the transmitter an d receiv-er are separa ted by 45 MHz, and afrequ ency divisionduplex (FDD ) mode of opera t ion is assumed. There-f o r e , t h i s h y b r i d s c h e m e i s d e n o t e d as

F D M I F D D K D M A .Th e CT2P lus system propo ses a t ime division

d u p l e x ( T D D ) m o d e o f o p e r a t i o n i n 1 0 0 k H zwide R F c ha nne l . Cha nne l s a re d i s t r ibuted in a nFD M fashion, access channels use TDh4.4, and infor-mation channels use FD M mo de of operation. There-fore , this ra ther complex hybrid scheme isde note da s F DM i TD D iT D M N F D M .

Th e design and evalu ation process of such hybridschemes requires careful ba lancing between com-ple xity a nd t e c hnic a l a dva nta ge s a n d d i s a dva n-tages of each of the elementary schemes.

System Capacity

Th e re c e nt surge in the popu la r i ty of c e l lu la rradio, and mobile service in genera l , has resul ted

in an overall increase in traffic and a sh ortage of avail-able system capacity in la rge metropoli tan a reas .Cur rent cellular systems exhibit a wide range of traf-f ic densi t ies , f rom low in rura l a reas to o verload-ing in downto wn areas, with large daily variationsbetween pea k hou rs and quie t night hours . I t is agre a t sys te m e ngine e r ing c ha l l e nge to de s ign asystem that will mak e optimal use of the availablef r e que nc y sp e c t rum , of fe r ing a m a xim a l t r a f fi cthro ugh put ( e . g . , E r la ngs iMHz lse rv ic e a re a ) a tan accep table service quali ty, constra ined by theprice and s ize of the mobile equipmen t.

In a ce l lular en vironm ent, the overa l l sys temcapacity in a given service area (sp ace) is a prod -uct of many factors (with complex interrelation ships),inc luding the available f requency spectra , servicequali ty, t raff ic s ta t is t ics , type of t raff ic , type of

protoco l , shape and s ize of service area , se lec ted

anten nas, diversity, frequen cy re-use capability, spec-tral efficiency of coding and mod ulation schemes,efficiency of multiple access, etc.

In th e 1970s so-cal led analo g ce l lular sys temse m p l o y ed o m n i d i r e ct i o n a l a n t e n n a s a n d s i m p l eo r no diversi ty schemes offer ing mod est capacity,

a4 IEEE Comm unications Magazine November 1991



which satisfied a relatively low number of cust ome rs.Ana log ce l lu la r sys tems o f the 1990s employ upto 60" sec to r ia l an tennas a nd improved d ive rs i tyschemes. This latter combination resulted in a three-f o l d t o f i v e fo l d i n c r e a s e i n c a p a c i t y . A f u r t h e r( twofo ld ) inc r ease in capac i ty can be expec tedfrom narrow band analog systems (25 kH z + 12.5k H z ) , w h i c h w a s p r a c ti c a l ly d e m o n s t r a t e d i nJ a p a n (L I 4 L2 system). Howev er, sl ight degra-dation in service quality might be expected. Thesei m p r o v e m e n t s s p u r r e d t h e c u r r e n t g r o w t h i n

capacity, the overall success and prolonged life ofanalog cellular radio.

The re also are nume rous marketing results, wherea tenfold to wentyfold increase in capacity has beenc la imed (wa tch fo r sm al l p r in t ! ) . In th is k ind o fcampa ign new digital systems of the twenty-firtst cen-tury, operating under ideal conditions, usually arec o m p a r e d w i t h t h e o l d s y s te m s o f t h e 1970s,

operating under the worst conditions.

There are numerous ways of increasingthe capacity of cellular radio; acquiring

new frequency spectra is perhapsthe easiest one.

ConclusionIn this contribution, a broad reperto ire of ter-

restrial an d satellite systems and services for trav-elers is briefly describe d.The technical characteristicso f the d ispa tch , cel lu la r , and cord less te lephonysystems are tabulated for ease of comparison. Issuessuch as opera t in g env i ronment , se rv ice qua l i ty ,network complexity, cell size, chan nel coding an dmodulation (codulation), speech coding, macro andmicro d ive rs i ty , mul t ip lex an d mul t ip le access ,and the m obile radio communications system capac-ity , are discussed. Presen ted d ata reveals signifi-c a n t d i f f e r e n c e s b e t w e e n e x i s t in g a n d p l a n n e dterrestrial cellular mobile radio comm unications sys-t e m s , a n d b e t w e e n t e r r e s t r i a l a n d s a t e ll i t e s y s-t e m s . T h e s e s y s t e m s u s e d i f f e r e n t f r e q u e n c ybands , bandwid ths , codu la t ion schemes , p ro to -cols, etc., meaning they are not com patible.

What are the technical reasons for this incom-p a t i b il i t y? I n t h i s p a p e r , p e r f o r m a n c e d e p e n -

dence on multipath delay (related to the cell size andterrain configura tion), Dop pler frequency (relat-e d t o t h e c a r r i e r f r e q u e n c y , d a t a r a t e a n d t h espeed of vehicles) , and message length (may dic-tate the choice of multiple access) are briefly dis-cussed. A system optimized to serve the travelersi n t h e G r e a t P l a i n s m a y n o t p e r f o r m w e ll i nmounta inous Swi tze r land . A sys tem op t imizedfor downtown cores may not bewell-suited to a ruralenvironm ent. A system employing geostationary

( a b o v e e q u a t o r ) s a t e l l i t e s m a y n o t b e a b l e t oserve travelers at high latitudes adequate ly. A sys-t e m a p p r o p r i a t e f o r s lo w - m o v in g v e h i cl e s m a yfail to function properly in a high Dopp ler shift envi-ronm ent. Additionally, a system optimized for voicetransmission may not begood for data transmission.A s y s t e m d e s i g ne d t o p r o v id e a b r o a d r a n g e o fservices to everyone, everywhere, may not b e as goodas a sys tem des igned to p rov ide a pa r t icu la r se r -vice in aparticular ocal environment, ust as aworldchampiond ecathletemay not be as successful in com-

petitions with specialists in particular disciplines.However, the re a re many oppor tun i t ie s where

compatibility between systems, their integration , andfrequency sharing may offer improvements in ser-vice quality, efficiency, cost, and cap acity (and there -fore availabili ty) . Terrestr ia l systems offer a lowstartup cost and a mo dest cost-per-user in dense-lypopulated areas. Satellite systemsmay offer a highq u a l i ty o f s e r v i c e a n d m a y b e t h e m o s t v i a b lesolution o serve ravelers n scarcely populated areas,on oceans, and in the air. Terrestrial systems are con-fined to two dimensions, and rad io propagation occursi n t h e n e a r h o r i z o n t a l s e c t o r s . B a r o s t a t i o n a r ys a t e l l i t e sy s t e m s u s e t h e n a r r o w s e c t o r s i n t h euser'szenith nearly perpendicular to the Earth's sur-face having the potential for frequency re-use anda n i n c r e a s e i n t h e c a p a c i t y i n d o w n t ow n a r e a s

d u r i n g p e a k h o u r s . A c a l l s e t u p i n a f o r w a r ddirection (from the PST Nvia base station to the trav-eler) may be a cumbers ome process in a terrestr i-a l s y s t e m w h e n a t r a v e l e r t o w h o m a c a l l i si n t e n d e d i s r o a m i n g w i t h i n a n u n k n o w n c e l l .However, this is easilyrealized in aglobal beam satel-lite system.

England expects this day that every manwill do his duty.

Horatio Nelson, 21, IO,1805.

-Terrestrial

systems

offera low

startup cost

and a

modest

cost-per-user

in densely

populated

areas.

Biography

Andy D. Kucar (M '80, SM '90) received a Dipl . Tech degree (summ acum la ude) in e lect ron ics f ro m the Technica l Col lege R i jeka in LaGuardia'sFiume; in 1974a Dipl. Ing.degree(financialreward)inelectricalengineering, and in 1980 an M.S. in electrical engineer ing. bo th fromZagreb University; and in 1987 the Ph.D. degree in electrical engineer-

ing f romUn ive rs i tyo f0 t tawa .Canada .Between1971 and 1973. Dr. Kucarserved as a research and teaching ass istant (m icrowave rad io andradar systems) n the Department of RF and Microwaves at the facul tyof electrical engineering at Zagreb University. In 1974 he was an engi-n e e r f o r R a d i o i n d u s t r i j a Z a g r e b . B e t w e e n S e p t e m b e r 1 9 7 4 a n dSeptember 1982 he worke d for lskra Ljubljana From September 1982

to December 1988. he worked for Universi ty of Ottawa Digi tal Com-munic at ions Group. Dr. Kucar worked for Telesat Canada from 1985

to 1987. From 1987 t o 1990 he worked for Bell Nor thern Researchradio division. In November 1990 he established 4U Communicat ionsResearch, a comp any specializing in excellence in terrestrial and satel-l i t e rad io commu nicat ions. whe re he serves as a pres ident . He hasorganized and chai red numerous sessions on in ternat ional confer^ences including ICC, GLOBECOM, and VTC.


mobile radio - an overview

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