Mobile Radio - An Overview

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Mobile Radio: An Ovetview
Technology is giving us impressive communications, information, and navigation systems. The social and economic impact will be substantial.
Andy D. Kucar

~ n d Kucaris President of y 4U Communications Research.

h e focus of this presentation is on terrestrial and satellite mobile radio communications. This includes cellular radio systems such as existing North American AMPS, Japanese MCS, Scandinavian NMTand British TACS, and the proposed GSM, Digital AMPS, and spread spectrum CDMA; cordless telephony systems such as existing CT1, CT2 and t h e proposed CT2Plus, CT3, and DECT; mobile radio data systems such as ARDIS and RAM; projects known a s P C N , PCS, a n d F P L M T S ; s a t e l l i t e mobile radio systems such as existing INMARSAT and OmniTRACS and the proposed INMARSAT X, MSAT, Iridium, Globalstar, and ORBCOMM. Followingabrief prologue and historical overview, the paper discusses such technical issues as the repertoire of systems and services, management of the airwaves, the operating environment, service quality, networkissues and cell size, channel coding and modulation, speech coding, diversity, multiplex and multiple access (FDMA, TDMA, CDMA). Also addressed a r e t h e potential economic and sociological impacts of mobile radio communications in thewake of the redistribution of airwaves at the World Administrative Radio Conference WARC '92. Most existing mobile radio communications systems collect the information on network behavior, users' positions, etc., with the purpose of enhancing the performance of communications, improving handover procedures and increasing t h e system capacity. C o a r s e p o s i t i o n i n g usually is achieved inherently, while more precise navigation can be achieved by employing LORANC and/or GPS signals, o r some other means, at the marginal expense in cost and complexity. The traffic load peaks of many mobile radio communications systemscoincide in time and spacewith vehicular traffic congestion and traffic accidents. It might be expected that improved traffic management provided by vehicle information systems would improve the traffic safety, relieve the thirst for airwaves, and enhance the performance of mobile radio communications systems. Recent develo 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 briefly described.

Prologue
Mobile radio systems provide their users with opportunities t o travel freely within the service area and simultaneously communicate with any telephone, fax, data modem, and electronic mail subscriber anywhere in t h e world. These systems allow users to determine their ownpositions; to track the precious cargo; to improve the management of fleets of vehicles and the distribution of goods; to improve traffic safety; to provide vital communication links during emergencies, search a n d rescue operations, etc. These tieless (wireless, cordless) communications, the exchange of information, determination ofposition, course and distance traveled are made possible by the unique property of the radio to employ an aerial (antenna) for radiating and receiving electromagnetic waves. When t h e user's radio antenna is stationary over a prolonged period of time, the term fixed radio is used. A radio transceiver capable of being carried o r moved a r o u n d , but stationary when in operation, is called a portable radio. Aradio transceive r capable of being carried and used by a vehicle or by a person on the move is called mobile radio. Individual r a d i o u s e r s may c o m m u n i c a t e directly or via one or more intermediaries, which may be passive radio repeater(s), base station(s) or switch(es). When all intermediaries are located on the Earth, the terms terrestrial radio system and radio system have been used. When at least o n e intermediary is satellite-borne, t h e termssatelliteradiosystem andsatellite system have been used. According to the location of a user, the terms land, maritime, aeronautical, space, and deep-space radio systems have been used. T h e second unique property of all terrestrial and satellite radio systems is that they all share the same natural resource: the airwaves (frequency bands and the space). Recent developments in microwave monolithic integrated circuit (MMIC), application specific integrated circuit (ASIC), analogidigital signal processing ( N D S P ) and battery technology, supp o r t e d by c o m p u t e r aided design ( C A D ) a n d robotic manufacturing, allow a viable implemen-

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IEEE Communications Magazine November 1991

1908 Public radio telephone between ships and the land in Japan was established. 1921 Police car radio dispatch service was introduced in Detroit USA Police Department. 1945 During WW II, significant progresses in design and widespread use of mobile radio were made. 1958 LORAN-Ccommercial operation started. The initial development was started during W w 11. 1964 Railway Telephone service on Japanese Tokaido bullet train was introduced. 1968 Carterphone Decision. FCC allows non-Bell equipment to be connected to (Bell) network. 1971 Fully automatic radiotelephone system, the B network, was introduced in West Germany. Later extended to the corresponding networks in Austria, Luxemburg, and the Netherlands. 1974 US Federal Communications Commission allocated 40 MHz frequency band, paving the way for estabilishingwhat is now known as advanced Mobile Phone Service (AMPS). 1976 MARISAT consortium initiated commercial service for mobile maritime users, providing full duplex voice, data, and teleprinter services, worldwide. 1979 Mobile communications systems MCS-L1 introduced by NTT Japan. 1982 The Conference of European Postal and Telecommunications Administrations (CEPT) established Groupe Special Mobile (GSM) with the mandate to define future Pan-Europeancellular radio standard. 1982 INMARSAT began providing similar services as MARISAT. 1982 Cospas - 1 inclined orbit satellite was launched, with a search and rescue package compatible with Future Global Maritime Distress and Safety Sysem (FGMDSS) on board. 1983 SARSAT search and rescue instrument package was placed on board of U.S. National Oceanic and Atmospheric Administration satellite NOAA-8 and launched. 1984 January 01, divestiture (breakup) of AT&T. 1984 The first interagency tests of Global Positioning System (GPS) receivers conducted in California. 1985 Total Access Communications System (TACS) was introduced in UK. 1985 CD900 cellular mobile radio system was introduced in West Germany. 1987 Japan launched its own experimental satellite ETS - Vsupported by extensive study and experimental work. 1988 Geostar introduced its Link One radiodeterminationservices. The radiodeterminationinformation i obtained from a s LORAN-C receiver and sent over a L-band satellite payload toward the ground central station. 1988 C?ualcomm/Omninetstarted its two way data communication and radiodetermination (using a LORAN-C receiver) OmniTRACS services. 1988 The second high capacity land mobile communicationssystem (MCS-L2) was introduced in Japan. 1990 Pegasus rocket has been launched from the wing of a 8-52; the rocket injected its 423 Ib payload into a 273 x 370 nm 9 O inclined orbit. 4 After almost two decades of studies and experiments, sponsored by Canadian and U.S. tax payers, North American mobile satellite systems MSAT is entering its realization stage, The European community is sponsoring studies and experiments for Pan-European mobile satellite systems PROSAT and PRODAT. Future Australian AUSSAT satellites will include, among other services, L band payloads for mobile communicaitons and receivers for radiodeterminationservices. Experimentalfield trials of CT2, CT3, DECT, GSM, CDMA, TDMA, FDMA mobile radio communicationssystems in progress, worldwide.
Table Z A s u m m a r y of events related t o the mobile radio communications

.

IEEE Communications Magazine November 1991

73

Today, cordless (wireless, fiberless) radio systems offer telepoint services similar in scope to those provided by the public telephone.

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tation of miniature radio transceivers. The continuous flux of market forces (excited by the possibilities of a myriad of new services and great profits), international and domestic standard forces (who manage common natural resource: the airwaves), and technologyforces (capable to createviableproducts) acted harmoniously and created a broad choice of communications (voice and data), information, and navigation systems, which propelled an explosive growth of mobile radio services for travelers.
Is space the limit?

Panta Rhei
Based o n the solid foundation established in the BD (before divestiture) era, the buildup of mobile r a d i o systems a n d services in t h e A D ( a f t e r divestiture) era is continuing at a 20 percent to 50 p e r c e n t r a t e p e r y e a r worldwide. T e r r e s t r i a l mobile radio systems offer analog voice and lowto medium-rate d a t a services compatible with existingpublic switching telephone networks in scope, but with poorer voice quality and lower data throughput. Satellite mobile radio systems currently offer analog voice, low- to medium-rate data, radiodetermination and global distress safety services for travelers. By the end of 1988 (1990) there were approximately 2 (4.5) million cellular telephones in North America, and an additional 2 (4.5) million in the rest of theworld. There are approximately 20 million cordless phones and about nine million pagers in N o r t h America, and about t h e same number in the rest of the world. Considerable progress has been made in recent years. Equipment miniaturization and price are important constraints on the systems providing these services. In the early 1950s mobile radio equipment used a considerable amount of a car’s trunk space and challenged the capacity of a car’s alternator/battery source while in transmit m o d e ; today, the pocket-sized (7.7 ounces or 218 grams) handheld cellular radio telephone provides 45 minutes of talkcapacity. The average cost of the least expensive models of battery-powered cellular mobile radio telephones has dropped proportionally, and has broken the $500 barrier. T h e r e is a rapidly e x p a n d i n g m a r k e t f o r portable communications, primarily devoted to the indoor (in building, around building) environment. Today, these cordless (wireless, fiberless)radio systems offer telepoint services similar in scope to those provided by the public telephone booths. Their objectives a r e to provide a broad range of services similar t o those currently offered by t h e Public Switched Telephone Network (PSTN) and the planned Integrated Services Digital Network (ISDN). Mobile satellite systems a r e expanding in many directions: large and powerful single unit geostationary systems; medium-sized, low orbit multisatellite systems; and small-sized, low orbit multisatellite systems, launched from aplane. The growth and profit potentials of the mobile radio communications market attracted the major manufacturers in the areas of networks, systems, and switching. This caused profound changes in research and development, standardization, and the decision-making processes in the mobile radio communications industry. In the search for El Dorado the mobile radio communications industry is following two main paths: terrestrial and satellite. The terrestrial mobile radio pioneers, now accompanied by large marketing teams, favor existing cellular radio systemsconcepts. The newcomerswith telephony, switching,and software backgrounds promote cordless telephony ((ST), personal communications networks (PCN), and personal communications systems (PCS). T h o s e individualswith a background in administrationpromote future public land mobile telecommunications systems (FPLMTS) concepts. The satellite mobile radio pioneers build on existing and new geostationary satellite systems, while the newcomers

A Glimpse of History
Many things have an epoch, in which they are found at the same time in several places, just as the violets appear on every side in spring. Farkas Wolfgang Bolyai, in 1823.

Late in the nineteenth century Heinrich Rudolf Hertz, Nikola Tesla, Guglielmo Marconi, and other scientists experimented with the transmission and reception of electromagnetic waves. The birth of mobile radio generally is accepted to have occurred in 1897, when Marconi was credite d with the patent for wireless telegraph. Since that time, mobile radio communications have provided safe navigation for ships and airplanes, saved many lives, dispatched diverse fleets of vehicles, won battles, generated newbusinesses, etc. A summary of some of the key historical developments related to commercial mobile radio communications is provided in Table 1. Satellite mobile radio systems launched in the 1970s and early 1980s use U H F frequency bands of approximately 400 MHz, with bands of approximately 1.5 G H z used for communications and navigation services. I n t h e 1950s a n d 1960s, numerous private mobile radio networks, citizen band (CB) mobile radio, ham operator mobile radio, and portable home radio telephones used diverse types and brands of radio equipment and chunks of airwaves located anywhere in the frequency band from near 30 MHz to 3 GHz. In the 1970s, Ericsson introduced the Nordic Mobile Telephone ( N M T ) system, and A T & T Bell Laboratories introduced Advanced Mobile Phone Service (AMPS). The impact of these two public land mobile telecommunication systems on the standardization and prospects of mobile radio c o m m u n i c a t i o n s may b e c o m p a r e d with t h e impact of Apple and IBM on the personal computer industry. In Europe systems resembling AMPS competed with NMT systems; in the rest of the world, AMPS, backed by Bell Laboratories’ reputation for technical excellence and the clout of AT&T, became d e facto and d e jure the technical standard. (The British TACS and the Japanese MCSL1 are based on AMPS.) In 1982, the Conference of European Postal and Telecommunications Administrations (CEPT) established Groupe Special Mobile (GSM) with the mandate to define future Pan-European cellular radio standards. On January 1,1984, during the phase of explosive growth of AMPS and similar cellular mobile radio communications systems and services, the divestiture (breakup) of AT&T occurred.
Le roi est mort, vive le roi.

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November 1991

promote inclined orbit concepts. The promoters of each concept may further be subdivided into analog and digital; frequency division multiple access (FDMA), time division multiple access (TDMA), a n d s p r e a d s p e c t r u m c o d e division m u l t i p l e access (CDMA), etc. Omnia mutantur, nos et mutamur in illis.

Repertoire of Systems and Services
The variety of services offered to travelers essentially consists of information in analog and/or digital form. Although most of today's traffic consists of analog voice transmitted by analog frequency codulation FM (or phase modulation PM), digital signaling and a combination of analog and digital traffic might provide superior frequencyre-use capacity, processing, and network interconnectivity. By using a powerful and affordable microprocessor and digital signal processing chips, a myriad of different services particularly well-suited to the needs of people on the move could be realized economically. A brief description of a few elementary systemsiservices currently available to travelers will follow. Some of these elementary services can be combinedwithin the mobile radio units for a marginal increase in cost and complexity compared with the cost of a single service system. For example, a mobile radio communications system can include a positioning receiver, digital map, etc.
Terrestrial Systems I n a t e r r e s t r i a l mobile r a d i o network, a repeater usually was located at the nearest summit offering maximum service area coverage. As the number of users increased, the available frequency spectrum became unable to handle the increased traffic, and a need for frequency re-use arose. T h e service area was split into many small subareas called cells, and the term cellular radio was born. T h e frequency re-use offers an increased system capacity, while the smaller cell size can offer an increased service quality, but at the expense of increased complexity of the user's terminal and network infrastructure. The trade-offs between real estate availability (base stations) and cost, the price of equipment (base and mobile), networkcomplexity and implementation dynamics dictate the shape and size of cellular network. Satellite systems employ one o r more satellites to serve as base station(s) and/or repeater(s) in a mobile radio network. The position of satellites relative to the service area is of crucial importance for the coverage, service quality, price, and complexity of the overall network. When asatellite circlesthe Earth in 24-hourperiods, t h e t e r m geosynchronous orbit has b e e n used. An orbit that is inclined with respect to the equatorial plane is called an inclined orbit. An orbit with a 90" inclination is called a polar orbit. A circular geosynchronous (24-hour) orbit over the equatorial plane (O'inclination) is known asgeostationary orbit (since from any point at the surface of 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 e stationary). This orbit is particularly suitable for the land mobile services at low latitudes, and for maritime and aeronautical services at latitudes less than 80".

Systems t h a t use g e o s t a t i o n a r y satellites include INMARSAT, MSAT, and AUSSAT. An elliptical geosynchronous orbit with the inclination angle of 63.4"is known as Tundra orbit. An elliptical 12-hour orbit with the inclination angle of 63.4" is known as Molniya orbit. Both Tundra and Molniya orbits have been selected for the coverage of the northern latitudes and the area around t h e North Pole. For users at those latitudes the satellites appear to wander around t h e zenith for a prolonged period of time. T h e coverage of a particular region (regional coverage) and the entire globe (global coverage) can b e provided by different constellations of satellites, including those in inclined and polar orbits. For example, inclined circular orbit constellations have been proposed for G P S (18-24 satellites, 55"-63" inclination), Globalstar (48 satellites, 47" inclination), and Iridium (77 satellites, 90" inclination, polar orbits) systems. All three systems will provide the global coverage. ORBCOM system employs Pegasus launchable low-orbit satellites to provide uninterrupted coverage of the Earth below +60"latitudes, and an intermittent but frequent coverage over the polar regions. Satellite antenna systems can have one (single-beam global system) or more beams (multibeam spot system). T h e multibeam satellite system, similar to the terrestrial cellular system, employs antenna directivity to achieve better frequency re-use, at the expense of system complexity. Radio paging is a non-speech, one-way (from base station toward travelers) personal selective callingsystemwith alert,without message orwith defined message such as numeric or alphanumeric. A personwishingtosend amessagecontactsasystemopera t o r by public switched t e l e p h o n e network ( P S T N ) , a n d delivers his message. A f t e r a n acceptable time (queueing delay), a system operator forwards the message to the traveler by radio repeater (FM broadcasting transmitter, V H F or U H F dedicated transmitter, satellite, cellular radio system). After receiving t h e message, a traveler's small (roughly the size of a cigarette pack) receiver (pager) stores the message into its memory, and on demand either emits alerting tones or displays the message. Examples include the Swedish system, which uses a 57 kHz subcarrier on FM broadcasting transmitters; United States systems that employ 150 MHz, 450 MHz, and 800 MHz mobile radio frequencies; the RPCl system used in the United Kingdom, United States, Australia, New Zealand, the People's Republic of China, and Finland, which employs 150 M H z mobile radio frequencies; and the Japanese system that operates at approximately 250 MHz. Global Distress Safety System (GDSS) geostationary and inclined orbit satellites transfer emergency calls sent by vehicles to the central earth station. Examples are: COSPAS, Search A n d Rescue Satellite A i d e d Tracking system ( S A R S A T ) , Geostationary Operational Environmental Satellites (GOES), and SEarch and REscue Satellite (SERES). The recommended frequency for this transmission is 406.025 MHz.
Global Positioning System (GPS) United States Department of Defense Navstar G P S 18-24 planned satellites in inclincd orbits emit Lband (L1= 1575.42MHz, 1,2 = 1227.6 MHz)

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By using a powerful and affordable microprocessor and digital signal processing chips, a van'ety of services can be offered economically to people on themove.

IEEE Communications Magazine November 1991

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Parameter

I
I
I

U S Sweden

1
I

1
I
I

Japan

1
I
I

Australia

TX freq. band,

Mtiz
base mobile Duplexing method RF channel bw. kHz RF channel rate, kbfs Number of traffic ch. Modulation type: voice data

I

935-940 76.0-77.5 850-860 865.00-870.00 851-866 41 5.55-418.05 896-901 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, full I I I full 12.5 1 25.0 1 12.5 I 25.0 25.0 12.5 29.6 1.2 1.2 29.6 400 600 60? 799 200

I

FM FSK

F4 h
MSK-FM

Rvl
MSK-FM

FM FSK

Table 2. Comparison of dispatch systems

spread spectrum signals from which an intelligent microprocessor-based receiver will be able to extract extremely precise time and frcquency information. and accurately determine itsown three-dimensional position. velocity, and accelerationworldwide. The coarse accuracy of < 100 m available to commercial users has been demonstrated by using a handheld receiver. An accuracy of meters or ccntimcters is possible by using the precise (military)codes and/or differential GPS (additional reference) principles. Glonass is the USSR'scountcrpart ofthc United State's GPS. Similar systems have been studied bythe EuropeanSpace Agency(Navsat) and by West Germany (Granas, Popsat, and Navcom). Loran C is thc 100 kHz frequency navigation

system that provides a positional accuracy bctween 1 0 m and 150 m . A uscr's rccciver measures the time difference between the master station transmitter and secondary stations signals, and defines his hypcrbolic line of position. North American Loran Ccoverage includes the Grcat Lakes and the Atlantic and Pacific coasts, with decreasing signal strength and accuracy as the user approaches the Rocky Mountains from the East. Similar radio-navigation systems are thc 100 kHz Decca and 10 kHz Omega. RadioDctermination Satellite Service (RDSS) uses L (1610.0 MHz to 1626.5 MHz) and S (2483.5 MHz to 2500.0 MHz) band spread spectrum code division multiple access (CDMA) signals translated by dislocated satellites to provide radiodetermination services on a primary basis and any associated nonvoice data services on an ancillary basis. Both the United Statcs GEOSTAR (which covers North America) and the future French CNES LOCSTARsystem (whichcovcrsEuropc,Africa,and the Middle-East) are technically capable of providing RDSS. digital voicc. and data. Thc Inmarsat communications system consists of three operational geostalionary payloads located a t 26'W ( A t l a n t i c O c c a n ) , 63" E ( I n d i a n Ocean), and 180" W (Pacific Ocean). The StandardAL-bandsystem.byemployinga0.7Y m to 1.95 ni diameter pointingantenna and approximately200 kg of above-/belowdeck equipment, can provide analogvoice tclephony. telex, facsimile. up to 56 kb/s d a t a , g r o u p call broadcasting, and emergency calls to maritime users. The Standard B systcm will provide digital voice (about Y.6 kbis). data and telcx services by cmploying smallcr equipmentthcn Standard A.The StandardCsystem.which employsasmall antcnna (about thcsizeofa halfliter can) and ii small transceiver (roughly the size of a

' AMPS

MCS-L1
MCS-LP

NMT

C450

TACS

GSM
890-915 935-960 TDMA

PCN

IS-54
869-894 824-849 TDMA
FDC)

I base

.

869-894 824-849 FDMA ' F D D 30.0 1 832
analog 2:l PM

I Traffic channels r RF channel
syllabic comp. speech rate, kbls modulation peak dev., kHz ch. rate, kbls Control: modulation bb waveform peak dev., kHZ ch. rate, kbfs Channel coding base-+mobile mobile-+base

870-885 925-940 FDMA FDD 25.0 12.5 1 600 1200 analog 2:l
PM i5 digital FSK Manch. i4.5 0.3

935-960 890-915 FDMA
Rx,

12.5 1 1999
analog 2:l PM &5 digital FFSK

461-466 451-456 FDMA FDD 20.0 10.0

935-960 890-915 FDMA FDD 25.0 1 1000
analog 2:l PM k9.5

1710-1785 1805-1880 TDMA

FDD
200.0

FDD
200.0 16 375x16
RELP

30.0

1 222 444 analog 2:l
PM i4 digital FSK

8 125x8
RELP

3
832x3
VSELP

13.0 GMSK 270.8 digital GMSK

6.7 GMSK 270.8 digital GMSK NL FZ 270.8

8.0 d4 48.6 digital d4

;tl2
digital FSK Manch.

-

NFV
i3.5 1.2 B 1 burst burst

NRZ
i2.5 5.3 BCH (15,7) (15,7)

&

10.0 BCH BCH (40,28) (43,31) (48,36) a.(43,31) p.(l 1,07)

digital FSK Manch. k6.4 8.0 BCH (40,28) (48,36)

NRZ
270.8

NRZ
48.6 Conv. 112 112

RS
(12,8) (12,8)

RS
(12 8 ) (12,8)

Table 3. Comparison o cellular mobile radio systems f

Table 4 Comparison of Digital Cordless Telephone Systems Vote I : The capacity (in the number of voice channels) for a single isolated cell. Vote 2: The capacity in parentheses may correspond to a 32 kbls vocoder. Re-use eficiency and associate capacities reflect our own estimates. Source: 4C Communications Research, 1991.09.15.

Thefirst generation of the UK's cordless telephones (coded C T l ) was developed as the answer to the large quantities of imported mobile radio equipment.

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telephone book directory) can offer up to 600 b k d a t a . S t a n d a r d M system is p l a n n e d f o r land mobile and maritime mobile users. while aeronautical systems will provide data and voice ser\ices to air travelers. Volna is a Soviet system of satellites which, in conjunctionuith the satellite More, andwith L-hand transponders on the Raduga and Gorizont satellites. will provide worldwide voice and data services to a fleet of ships and aircraft. Airphone isapublic,fully automatic air-to-ground telephone system thatoperatesin the Y00MHzband using h k H z SSB transmission. E a c h g r o u n d t r a n \ c e i ve r . by emitting an effective isotropic radiated power of 3 dBW. serves a cell with a radius o f a b o u t 100 k m . A n a i r c r a f t uscs two blade antennas, four transceivers (each radiating 7 dBW). a telcphonc set and an airborne computcr that directs all call logistics. Dispatch tho-way radio land mobile o r satelstem.\~ithorwithoutconnection the PSTN. to sts of an operating center conti-olling the operation of ;I flcet of vehicles such a s aircraft, taxis. police cars, tracks. rail cars, etc. Thc summary of some existing and planned terrestrial systems. including MOBITEX RAM and ARDIS, is given in Table 2. OmniTRACS dispatch system employs Ku-band geostationary satellite located at 103" W t o provide tux-way digital messaging and position reporting (derived from incorporate d sate I I i t e - ai d e d L ORAN C rcce ivc r). throughout the contiguous United States (CONUS). Cellular radio o r public land mobile telephone

system offers a full range of services t o the traveler that are equivalent to those provided by PSTN. Some of the operating cellular radio systems are: the North American Advanced Mobile Phone Service (AMPS), the Japanese land mobile communications systems MCS-LI a n d MCS-L?, t h c Nordic Mobile Telephone systems NMT-450 and NMT-900, the German C450, the Italian public land mobile radio communication system at 450 MHz, the French radiotelephone multiservice network a t 200, 400 M H z R A D I O C O M 2000. a n d t h e United Kingdom Total Access Communication System (TACS). T h e technical characteristics of some existing and planned systems are summarized in Table 3.

Cordless Telephony
The first generation of the United Kingdom's cordless telephones (coded C T I ) was developed as the answer to the large quantities of imported, technically superior yet unlicensed mobile radio equipment. The simplicity and cost-effectiveness of CT1 analog radio and base station products using eight K F channels and FDMA scheme stem from their applicationsliinitcd to incoming calls from a limited number of mobile users to thc isolated telepoints. As the number of users grew, so did the CO -c h an n c 1 inter fer e n cc leve 1s, w h i I e t he q u a1i t y of the service dctcriorated. Anticipating this situation. thc second generation digital cordless telecommunications radio equipment and comnioii air inter face stand a rds ( CT2iC A I ) , i nco m p 21 ti b I e with the CTI equipment. have been developcd. CT2

Whicle information system is a synonym Jor the variety of systems and services aimed toward trafic safety and location.
n

s c h e m e s employ digital voice, b u t t h e s a m e FDMA principles as CTI achcmes. Network and frcqucncyre-use issues necessary to accommodate anticipated residential, business, and telepoint traffic growth have not been addressed adequately. Recognizing these limitations and anticipating the market requirements. different frequency division multiple access (FDMA), time division multiple access ( T D M A ) , c o d c division multiple access ( C D M A ) , and hybrid schemes aimcd at cellular mobile and D C T services have b e e n developed. The technical characteristics of some schemes are given in Table 4. Future Public Land Mobile Telecommunications Systems (FPLMTS) is a huge international administrative project. for which tasks and objectives are presented in CCIR Report 1153. It discusses different terrestrial and satellite mobile radio comm u n i c a t i o n s and broadcasting systems, t h e transmission of data, voice, and images, at rates between8 kbisand 1,920 kbts,andaverybroadrangc of services a n d technical a n d administrative issues. A m a t e u r satellite services s t a r t e d in 1965, when the OSCAR3 satellite was launched. Successive OSCAII/AMSATsatcllitesused 144MHz,432 MHz, I270 MHz, and 2400 MHz carrier frequencies. The USSR‘s Iskra satellites use 21/29 MHz and RS-3 satellites use 145/29 MHz carrier frequencics. Vchiclc information system is a synonym for thevariety ofsystems andservices aimed toward traffic safety and location. This includes: traffic manage in en t . vehicle id c n t i f i c a t ion , digitized m a p information and navigation, radio-navigation, speed sensing, and adaptive cruise control. collision warning and prevention, ctc. Some of the vehicle information systems can easily be incorporated in mobile radio communicationstransceivers to enhance t h e service quality and capacity of rcspective communications systems. Embarrass du choix.

Airwaves Management
The airwaves (the frequency spectrum and the space s u r r o u n d i n g u s ) a r e a limited n a t u r a l resource shared among several different radio users (military,government, commercial, public, and amateur). Its sharing (e.g., among different users. services described in the previous section, television and sound broadcasting, etc.), coordination, and administration is an ongoing process exercised on national and international levels. National administrations (e.g., the United States Fcderal Conimunications Commission (FCC), the Canadian Department of Communications (DOC) in Canada. etc.). in cooperation with users and industry, set the rules and procedures for planning and utilization of scarce frequency bands. These plans and utilizations must be f u r t h e r coordinated internationally. The International Telecommunications Union ( I T U ) is a specialized agency of t h e U n i t e d Nations. stationed in Geneva, Switzerland. The ITU has more than 150 government members. responsible for all policies related to radio, telegraph, and telephone. According to thc ITU, the world is divided into three regions: Region I-Europe, including the Sovict Union, Outer Mongolia. Africa,

and the Middle East wcst of Iran; Region 2-thc Americas and Greenland: and Region 3-Asia (excluding parts west of Iran and Outer Mongolia), Australia, and Oceania. Historically, these three regions have developed, more or less independently, their own frequcncy plans, which best suit local purposes. With the advcnt of satcllite services and globalization trends, the coordination between different regions becomes more urgent. Frequency spectrum planning and coordination i s performed through such ITU bodicsas: the Comite Consultatif de International Radio (CCIR), the International Frequency Registration Board (IFRB), the World Administrative Radio Conferencc (WARC), and the Regional Administrative Radio Conference (RARC). Through its study groups. C C I R deals with technical and operational aspects o f radio communications. Results of these activities have been summarized in the form of rcports and recommendations published every four years. Thc IFRB serves as a custodian of acommon and scarce natural resource, namely, the airwaves. In its capacity, the IFRB records radio frequencies, advises the members on technical issues, and contributes o n other technical matters. Based on the work of CCIR, IFRB, and the national administrations, ITU members convene at appropriate R A R C and WARC meetings, where documents o n frequency planning and utilization (the Radio Regulations) arc updated. Actions on a national level follow. The managing of airwaves becomes even more intercsting. CCIR’s big brother, CCITT, became involved with the mobile radio communications. Restructuring of ITU (including CCIR and IFRB) also has becn proposed. The far-reaching impact of mobile radio communications on economies and the well-being of the three main trading blocks. other developing and third world countrics, potential manufacturers, and users makes the airways (frequency spectrum) even more important. Whilc the battlc for the mobile radio market, thc airwavcs and global competitiveness intensifies. the World Administrative Radio Conference, WARC’92, i s scheduled to be held in Granada, Spain. At that conference, the airwaves map of the world is expectcd to be redrawn. Si vis pacem, para bellum.

Operating Environment
While traveling, a customer of cellular mobilc radio systemsmay experience sudden changes in signal quality caused by his movements relative to the corresponding base station and surroundings. multipath propagation, and unintentional jamming (e.g.,manmade noise, adjacent channel interference, and co-channel interference inherent to the cellular systems). Such an environment belongs t o the class of nonstationary random fields, ofwhich experimental data is difficult toobtain. Their behavior is hard to predict and model satisfactorily. When reflected signal components bccome comparable in level to the attenuated direct component, and their delays are comparable to the inverse o f thc channel bandwidth, frequency selective fading occurs. The rcception is further degraded by movements of a user, relative t o reflection points and relay station, causing the Doppler fre-

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quency shifts. The simplified model of this environment is known as the Doppler affected multipath Rayleigh channel. The existing and planned cellular mobile radio systems employ sophisticated narrowband and wideband filtering, interleaving, coding, modulation, equalization, decoding, carrier and timing recovery, and multiple access schemes. The cellular mobile radio channel involves a dynamic interaction of signal arrived via different paths, adjacent and co-channel interference, and noise. Most channels exhibit s o m e d e g r e e of m e m o r y , which description requires higher order statistics of spatial and temporal multidimensional random vectors (amplitude, phase, multipath delay, Doppler frequency, etc.) to be employed. This may require t h e evaluation of usefulness of existing r a d i o channel models a n d eventual development of more accurate ones. Cell engineering, prediction of service area, and service quality, in an ever-changing mobile radio channel environment, is a difficult task. The average path loss depends on terrain microstructurewithin a cell, with considerablevariation between different types of cells (i.e., urban, suburban, and rural environments). A variety of models based on experimental and theoretic work have been developed to predict path radio propagation losses in a mobile channel. Unfortunately, none of them are universally applicable. In almost all cases, a n excessive transmitting power is necessary to provide an adequate system performance. The curves of path propagation loss L in decibels (dB) versus t h e distanced in kilometers (km), based on some of these models, are summarized in Fig. 1. T h e first generation mobile satellite systems employ geostationary satellites (or payloads piggybacked on a host satellite) with small 18dBi antennas covering the entire globe. When the satellite is p o s i t i o n e d directly above t h e t r a v e l e r ( a t zenith), a near constant signal environment is experienced-the Gaussian channel. The traveler's movement relative to the satellite is negligible (i.e., Doppler frequency is practically equal to zero). As the traveler moves (north, south, east or west) the satellite appears lower on the horizon. In addition to the direct path, many significant strength-reflected components are present, resulting in a degraded performance. Frequencies of these components fluctuate due to movement of the traveler relative t o t h e reflection points and the satellite. This environment is known as the Doppler affected Ricean channel. An inclined orbit satellite located for aprolonged period of time above 45" latitude north and 106" longitude west could provide travelers all over the United States and Canada, including t h e far North, a service quality unsurpassed by either geostationary satellite or terrestrial cellular radio. Similarly,a satellite located at 45" latitude north and 1.5" longitude east could provide travelers in Europe with improved service quality. A typical return link budget (from a traveler to the central station via a satellite) of the second generation geostationary satellite system for medium rate data services (and digitized voice) follows. The traveler's mobile radio employs a IO W transmitter and a small 3 dBi gain antenna. On its way toward a satellite (uplink), t h e 1.6 G H z signal experiences a line-of-sight (LOS) loss of 188.9

L, dB
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100 110 120

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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, converted to 12 GHz, then once again amplified, and retransmitted toward the earth control station suffering a LOS loss of 205.8 dB in downlink, and a delay of 2 x 0.125 s. Huge LOS losses put an enormous burden on satellite energy resources, while long delays limit voice transmission quality, and the selection of efficient data protocols. Inclined orbit satellite systems can offer a low startup cost, a near Gaussian channel environment and improved service quality. Low orbit satellites,positioned closer to the service area, can provide high signal levels and short (a few milliseconds long) delays, and offer compatibility with the cellular terrestrial systems. These advantages need to be weighed against networkcomplexity, intersatellite links, tracking facilities, etc.

Service Quality
The primary and the most important 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 equipment manufacturer for both the system concept and product design stages. Acknowledging the importanceof eachstepofthecomplexserviceprocessand architecture, attention is limited here t o a few technical merits of quality. Guaranteed quality level usually is related to a percentage of t h e service area coverage for a n adequate percentage of time. Data service quality can be described by the averpacket BER age bit error rate (e.g., BER < (PBER < 10-2), control PBER ( C P E R < signal processing delay (1-10 ms), multiple access collision probability (< 20 percent), the probability of a false call (false alarm), the probability of a missed call (miss), t h e probability of a lost call (synchronization loss), etc. Voice quality usually is expressed in terms of the mean opinion score (MOS) of subjective evaluations by service users. MOS marks are: bad = 0, poor = 1, fair = 2, good = 3, and excellent = 4. T h e M O S f o r PSTN voice service, p o o l e d by leading service providers, relates the poor MOS mark to a signal-to-noise ratio (SIN) in a voice channel

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of SjN- 35 dB. while an excellent score corresponds to SIN > 45 dB. Currently, users of mobile radio services are giving poor marks to thc voice quality associated with a SIN =I5 dB and an excellent inark for SIN > 25 dB. It is evident that there is a significant difference (20dB) between the PSTN and mobile services. I f digital speech is employed, both the speech and the speaker rccognition have to be assessed. For more objective evaluation ofspeech quality under real conditions (with no impairmcnts, in the presence of burst errors during fading, random bit errors at BER = 10-2, Doppler f r e q u e n cy offsets, truck acoustic b ac kg r o u n d noise. ignition noise, etc.), additional tests such a:

t h e diagnostic acceptability measure ( D A M ) . diagnostic rhyme test ( D R T ) , Youdcn square rank ordering, and Sino-Graeco-Latin square tests, can be performed.

Network lssues and Cell Size
T o understand ideas and technical solutions offered in existing schemes. and in proposals such as c o r d l e s s t e l e p h o n y ( C T ) , digital cordless telecommunications (DCT), personal communications service (PCS), personal communications network (PCN), etc., one also needs to analyze the reasons for their introduction and success. Cellular mobile services are flourishing at an annual rate of 20 percent to 40 percent worldwide. These systems (e.g., AMPS, NMT, TACS, and MCS), use frequency division multiple access (FDMA) and digital modulation schemes for access, command and control purposes and analog phaseifrequency modulation schemes for the transmission of an analog voice. Most of the network intelligence is concentrated at fix elements of the network, including base stations, which seem to be well-suited to networks with a modest number of medium- to largesized cclls. T o satisfy the growing number of potential customers, more cells and base stations were created by the cell splitting and frequency re-use process. Technically, the shape and size of a particular cell is dictated by the base station antenna pattern and the topography of the service area. Currcnt terrestrial cellular radio systems employ cells with a radius of 0.5 km to 50 km. T h e maximum cell size usually is dictated by the link budget, particularly the gain of a mobilc antenna and available o u t p u t power. This situation arises in a rural environment, where the demand on capacity is very low and cell splitting is not economical. The minimum cell size usually is dictated by the need for an incrcase in capacity, particularly in downtown cores. Practical constraints (e.g.. real estate availability a n d price, and construction dynamics) limit the minimum cell size to0.5 km to 2 km. In such types of networks, however, the complexity of the network and the cost of service grow exponentially with thc number of base stations, while the efficicncy of present handover procedures becomes inadequate. Antennas with an omnidirectional pattern in a horizontal direction. but with about 10 dBi gain in vertical direction, provide the frequency re-use = cfficiency ofNFDMA 1/12. Base station antennas with similar directivity in vertical direction and 60" directivity in horizontal direction (a ccll is divided into six sectors) can provide the re-usc efficiency NFp,,A = 114. This results in a thrcefold increase in the system capacity; if CDMA is eniploycd instead of FDMA, an increase in re= use efficiencyNFDMA 114 + NCI,MA= 213 may be expected. Recognizing some of the limitations of cxisting schemes and anticipating the market requircmcnts, t h e rcsearch in time division multiple access (TDMA) schemes aimed at cellular mobile and D C T services, and in code division multiple access (CDMA) schemes aimed toward mobile satellite systems. cellular and personal mobile applicat ions h avc b e en in i t i at e d. AI t h o u g h em p 1oy i ng different access schemes, TDMA (CDMA) network

ACSSB AM APK BLQAM BPSK CPFSK WM DEPSK DPM DPSK DSB-AM DS5-SCAM FFSK FM FSK FSOO GMSK GTFM HMQAM IJF LPAM LRC LREC LSRC MMSK MPSK MQAM MQPR MQPRS MSK m-h OQPSK PM PSK QAM QAPSK QPSK QORC SQAM SQPSK SQORC

I Amplitude Companded Single Side Band

Amplitude Modulation Amplitude Phase Keying Modulation Blackman Quadrature Amplitude Modulation Binary Phase Shift Keying Continuous Phase Frequency Shift Keying Continuous Phase Modulation Differentially Encoded PSK (with carrier recovery) Digital Phase Modulation Differential Phase Shift Keying (no carrier recoveryDouble SideBand Ampliiude Modulation Double SideBand SuppressedCarrier AM Fast Frequency Shift Keying (MSK) Freqency Modulation Freouencv Shift Kevina Frequency Shift Offset Quadrature modulation Gaussian Minimum Shift Keying Generalized Tamed Frequency Modulation Hamming QuadratureAmplitude Modulation Intersymhl Jitter Free (SWRC) L-ary Pulse Amplitude Modulation LT symbols long Raised Cosine pulse shape LT symbols long RectangularlyEncoded pulse shape LT symbols long Spectrally Raised Cosine scheme Modified Minimum Shift Keying M-ary Phase Shift Keying M-ary Quadrature Amplitude Modulation M-aty Quadrature Partial Response M-ary Quadrature Partial ResDonse Svstem Minimum Shift Keying multi4 CPM Offset (staggered) Quadrature Phase Shift Keying Phase Modulation Phase Shift Kevina Quadrature AmbliGde Modulation Quadrature Amplitude Phase Shift Keying Quadrature Phase Shift Keying Quadrature Overlapped Raised Cosine Staggered Quadrature Amplitude Modulation

SSB
S3MQAM

Sinale Side Band Staggered class 3 Quadrature Amplitude Modulation Tamed Frequency Modulation

TSI QPSK
Crosscorrelated PSK 7t/4 shift QPSK 3MQAM Class 3 Quadrature Amolitude Modulation 4MQAM 1 Class 4 Quadrature Amplitude Modulation 12PM3 I 12 state PM with 3 bit correlation Table 5. Modulation schemes; glossary o terms f (Source: 4U Communications Research, 1991.09.15)

7d4 QPSK

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concepts rely on a smart mobileiportable unit that scans time slots (codes) to gain information on network behavior, free slots (codes), etc., improving frequency re-use a n d handover efficiency while, hopefully, keeping the complexity and cost of the overall network at reasonable levels. Some of the proposed system concepts depend on low gain (0 dBi) base station antennas deployed in a license-free, uncoordinated fashion; small size cells (10 m to 1000 m in radius) and an emitted isotropic radiated power of a b o u t 10 m W (+ 10 dBm) per 100 kHz have been anticipated. A frequency re-use efficiency of N = 119 to N = 1/36 has been projected for D C T systems. N = 119corresponds to the highest user capacitywith the lowest transmission quality, while N = 1/36 has the lowest user capacity with the highest transmission quality. This significantly reduced frequency re-use capability of proposed system concepts will result in significantly reduced system capacity,which needs to be compensated for by other means, including new spectra. In practical networks, the need for acapacity (and frequency spectrum) is distributed unevenly in space and time. In such an environment, t h e capacity and frequency re-use efficiency of t h e network may be improved by dynamic channel allocation, where an increase in the capacity at a particular hot spot may be traded for the decrease in the capacity in cells surrounding the hot spot, the quality of the transmission, and network instability. T o cover the same area (space) with increasingly smaller cells, o n e must employ more and more base stations. A linear increase in the number of base stations in a network usually requires a n exponential increase in t h e number of connections between base stations, switches and network centers. These connections can be realized by fixed radio systems (providing more frequency spectra will be available for this purpose), or, more likely, by a cord (wire, cable, fiber, etc.). Therefore, the following postulate can apply:
There will be a plenty of cord in cordless telephony. (PI)

K

P

I

@re 2. The conceptual transmitter receiver (Ru)f a mobile system o

(m) and

timeliness of the launch procedures, a few large versus many small satellites, tracking stations, etc.

Coding and Modulation
The conceptual transmitter (TX) and receiver (RX) of a mobile system are shown in Fig. 2. The transmitter signal processor (TX SP) accepts analog voice and/or data and transforms (by analog and/or digital means) these signals into a form suitable for a double-sided suppressed carrier amplitude modulator, also called quadrature amplitude modulator (QAM). Both analog and digital input signals may be supported, and either analog or digital modulation may result at the transmitter output. Coding and interleaving also can be included. Often, the processes of coding and modulation are performed jointly; we will call this joint process modulation. A list of typical modulation schemes suitable for transmission of voice and/or data over Doppler-affected Ricean channel, which can be generated by this transmitter, is given in Table 5 . These particular modulations, however, also can b e generated by means different than that suggested in Fig. 2. Existing cellular radio systems such as AMPS, TACS, MCS, and N M T employ hybrid (analog a n d digital) schemes. F o r example, in access mode AMPS uses a digital modulation scheme (BCH coding and FSK modulation). While in information exchange mode, the frequency modulated analog voice is merged with discrete SAT and/or S T signals and occasionally blanked to send a digital message. These hybrid modulation schemes exhibit aconstant envelope and as such allow the use of d.c. power-efficient nonlinear amplifiers. On the receiver side, these schemes can be demodulated by an inexpensive but efficient limiter/discriminator device. They require modest to high C/N = 10 d B to 20 dB, are very robust in adjacent (a spectrum is concentrated near the carrier) and co-channel interference (up to C/I = 0 dB, due to capture effect) cellular radio environment, and react quickly to the signal fade outages (no carrier, code or frame synchronization). Frequency-selective and Doppleraffected mobile radio channels will cause modest to significant degradations, known as the random phase/frequency modulation. Tightly filtered modulation schemes, such as

T h e first generation geostationary satellite system antenna beam covers the entire earth (i.e., t h e cell radius equals = 6500 km). T h e second generation geostationary satellites will use larger multibeam antennas providing 10to 20beams (cells) with a radius of 800 km to 1,600km. Low-orbit satellites (e.g., Iridium) will use up to 37 beams (cells) with a radius of 670 km. T h e third generation geostationary satellite systemswill be able to usevery large reflector antennas (roughly t h e size of a baseball stadium), and provide 80 to 100beams (cells) withacellradiusof=200km. Ifsuch asatelliteistethered t o a position 400 km above t h e earth, the cell size will decrease to =2 km in radius, which is comparable in size with tnday's small size cell in terrestrialsystems. Yet, such asatellitesystemmight have the potential to offer an improved service quality due to its near optimal location with respect to the service area. Similar to the terrestrial concepts, an increase in the number of satellites in anetwork will require a n increase in t h e number of connections between satellites and/or earth network management and satellite trackingcenters, etc. Additional factors that need to be taken into consideration include price, availability, reliability, and

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-5

0

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Figure 3. Error rate vs. Eb/No curvesfor digital 4QAM codulation schemes xi4 QPSKadditionallyfiltered by a square root raised cosine filter, exhibit a nonconstant envelope, which demands (quasi) linear, less d.c. powerefficient amplifiers t o b e employed. On t h e receiver side, these schemes require complex demodulation receivers, a linear path for signal detection, and a nonlinear one for reference detection, differential detection, or carrier recovery. When such a transceiver operates in a selective fading multipath channel environment, additional countermeasures (inherently sluggish equalizers, etc.) are necessary to improve the performance by reducing the bit error rate foor. These modulation schemes require modest CIN = 8 d B to 16 d B a n d perform modestly in adjacent and/or co-channel (up to CiI = 8 dB) interference environment. Codulation schemes employed in spread spectrum systems use low-rate coding schemes and mildly filtered modulation schemes. When equipped with sophisticated amplitude gain control on the transmit and receive side, and robust rake receiver, these schemes can provide superior CIN = 4 d B to 10 dB and C/I < 0 d B performance. The error rate performance of some digitalcodulation schemes operating in three different mobile channel environments is summarized in Fig. 3. It might be expected that acodulation scheme (in conjunction with an appropriate access scheme) tailored to a particular local environment can provide a significant improvement in performance and/or capacity over an ad hoc selected combination. Therefore, the following can apply:
For every mobile radio channel, there is an optimal codulation scheme. (P2) For every codulation scheme, there is an optimal mobile radio channel. (P3)

tems, speech is limited to 4 kHz, compressed in amplitude (2:1), pre-emphasized, and phaseifrequency modulated. At a receiver, inverse operations are performed. Degradation caused by these conversions and channel impairments results in lower voice quality. Finally, the human ear and brain have to perform the estimation and decision processes on the received signal. In digital schemes sampling and digitizing of an analog speech (source) are performed first. Then, by using knowledge of properties of the human vocal tract and the language itself, a spectrally efficient source coding is performed. A high rate 64 kbis, 56 kbis and ADPCM 32 kbis digitizedvoice complies with CCITT recommendations for toll quality, but may be less practical for the mobile environment. O n e is primarily interested in 8 kbis to 16 kb/s rate speech coders, which might offer satisfactory quality, s p e c t r a l efficiency, robustness, a n d acceptable processing delays in a mobile radio environment. A summary of t h e major speech coding schemes is provided in Table 6. A t this point, a partial comparison between analog and digital voice should be made. T h e quality of 64 kbis digital voice, transmitted over a telephone line, is essentially the same as the original a n a l o g voice ( t h e y receive nearly e q u a l MOS). What does this near equal MOS mean in a radio environment? A mobile radio conversation consists of one (mobile to home) o r a maximum of two (mobile to mobile) mobile radio paths, which dictate the quality of t h e overall connection. T h e results of a comparison between analog and digital voice schemes in different artificial mobile radio environments have been widely published. Generally, systems that employ digital voice a n d digital c o d u l a t i o n schemes appear to perform well under modest conditions, while analog voice and analog codulation systems outperform their digital counterparts in fair and difficult (near threshold, in the presence of strong co-channel interference) conditions. Fortunately, present technology can offer a viable implementation of both analog and digital systems within the same mobilciportable radio telephone unit. This would give every individual a choice of either an analog or digital scheme, better service quality and higher customer satisfaction. Tradeoffs betwccn the quality of digital speech, the complexity of speech and channel coding, aswell as d.c. power consumption, must be assessed carefully and compared with analog voice systems.

Macro and Micro Diversity
Macro Diversity In a cellular system, the base station is usually located in the barocenter of the service area (center of the cell), as illustrated in Fig. 4a. Typically, the base antenna is omnidirectional in azimuth, but with about 6 dBi to 10 dBi gain in elevation, and serves most of the cell area (e.g., > 95 percent). Some parts within the cell may experience a lower quality of service because the direct path signal may be attenuated due to obstruction losses caused by buildings, hills, trees, etc. The closest neighboring base stations (the first tier) serve corresponding neighboring area cells by using different sets of frequencies, eventually caus-

Speech Coding
H u m a n vocal tract a n d voice receptors, in conjunction with language redundancy (coding), are well-suited for face-to-face conversation. As the channel changes (e.g., from telephone channel to mobile radio channel), different coding strategies are necessary to protect the loss of information. In (analog) companded PMiFM mobile radio sys-

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ing adjacent channel interference. T h e second closest neighboring base stations ( t h e second tier) might use the same frequencies (frequency re-use) causing co-channel interference. If the same real estate (base stations) is used in conjunction with 120" directional (in azimuth) antennas. the designated area may be served by three base stations, as illustrated in Fig. 4b. In this configuration one base station serves three cells by using three 120" directional antennas. Therefore, the same numberofexisting base stationsequipped with newdircctional antennas and additional combining circuitry is required to serve the same number of cells, yet in a different fashion. The mode of operation in which two or more base stationsserve the same area iscalled themacrodiversity. Statistically, three base stations are able to provide a better coverage of an area similar in size t o the system with a centrally located base station. The directivityof a base station antenna (120" or even 60") provides additional discrimination against signals from neighboring cells, therefore, reducing adjacent andco-channel interference (i.e., improving re-use efficiency and capacity). Effective improvement depends on the terrain configuration and the combining strategy and efficiency. However, it requires more complex antenna systems and combining devices.

ADM ADPCM AClT APC APC-AB APC-HQ APC-MQL AQ ATC BAR CELP CVSDM DAM DM DPCM DRT

DSI
DSP HCDM LDM LPC MPLPC MSQ NIC PVXC PWA QMF RELP RPE SBC TASl TDHS VAPC VCELP VEPC VQ VQL VSELP

Micro Diversity
M i c r o diversity r e f e r s t o t h e condition i n which two or more signals are received at onc site (base or mobile). Space diversity systems employ two or more antenn a s spaced a certain distance a p a r t from o n e another. A separation of only h/2 = 15 cm. which is suitable for implementation on the mobile side, can provide a n o t a b l e i m p r o v e m e n t in s o m e mobile radio channel environments. Micro space diversity is routinely used on cellular base sites. Macro diversity is also a form of space diversity. Field-component diversity systems employ different types of antennas receiving either the electricor thc magneticcomponent ofan electromagnetic signal. Frequency diversity systems employ two or more different carrier frequencies to transmit the same information. Statistically, the same information signal may or may not fade at the same time at different carrier frequencies. Frequency hopping and very wide band signaling can be viewed as frequency diversity techniques. Time diversity systems are used primarily for the transmission of data. T h e same data is sent through the channel as many times as necessary, until the required quality of transmission is achieved (auto-

vxc

Adaptive Delta Modulation Adaptive DifferentialPulse Code Modulation Adaptive Code sub-band exclted Transform (GTE) AdaDtive Predictive Coding APC with Adaptive Bit Allokation APC with Hybrid Quantization APC with M-&mum LikelihoodQuantization Adaptive Quantization Adaptive Transform Coding Backward Adaptive Reencoding Code Excited Linear Prediction Continuous Variable Slope Delta Modulation Diagnostic Acceptability Measure Delta Modulation DifferentialPulse Code Modulation Diagnostic Rhyme Test Digital Speech Interpolation Digital Signal Processing Hybrid Companding Delta Modulation Linear Delta Modulation Linear Predictive Coding Multi Pulse LPC Multipath Search Coding Nearly Instantaneous Companding Pulse Vector excitation Coding Predicted Wordlength Assignment Quadrature Mirror Filter Residual Excited Linear Prediction Regular Pulse Excitation Sub Band Coding Time Assigned Speech Interpolation Time Domain Harmonic Scalling Vector Adaptive Predictive Coding Vector Code Excited Linear Prediction Voice Excited Predictive Coding Vector Quantization Variable Quantum Level Coding Vector-Sum Excited Linear Prediction Vector excitation Coding

Table 6. Digitized voice; glossary of terms

matic rcpcat request ARQ). "Would y o u pleasc repeat your last sentencc" is a form of timc diversity used in 3 speech transmission. T h e improvement o f any diversity schcmc is strongly dependent o n the combining techniqucs employed, i.e., the selective (switched) combining. the maximal ratio combining. the equal gain combining, the feedforward combining,the feedback (Granlund) combining, majority vote, etc.

Multiplex and Multiple Access
Communications networks for travclcrs havc two distinct directions: the forward link (from thc base stationvia satellite to the travclcr) and the return link (from a travelerviasatcllite to the base station). In the forward direction, a base station distributes information to travelers according to the prcviously established protocol, i.e.. n o multiplc access is involved. In the reverse direction, many travelers make attempts to access one of the base stations. This occurs in so-called control channels, in a particular time slot, at particular frequency. or by using a particular code. If collisions occur. customers must wait in a queue and try again until succcss is achieved. If successful (i.e., if no collision occurred). a particular customer will automatically exchange the necessary information for callsetup.Thcnetworkmanagementcentcr(NMC)

2 Figure 4 The cellular system concepts: U ) Burocentric base stations, center excited cellular system; b) Corner base stations, corner excited cellular system.

2)

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The strengths of F D M schemes seem to be fully exploited in narrowband channel environments.

-

willveriCy the customer’s status, his credit rating, etc. Then the NMC may assign achannel frequency, time slot, o r code on which the customer will be able to exchange information with his correspondent. The optimization of the forward and reverse links may require different coding, modulation schemes, and bandwidths in each direction. In forward link, there are three basic distribution (multiplex) schemes: one uses discrimination in frequency between different users and is called frequency division multiplex ( F D M ) ; a s e c o n d discriminates in time and is called time division multiplex (TDM); and the third has different codes based on spread spectrum signaling, which is known as code division multiplex (CDM). It should be noted that hybrid schemes using a combination of basic schemes also can be developed. In reverse link, there a r e three basic access schemes: one uses discrimination in frequency between different users and is called frequency division multiple access (FDMA); a second discriminates in time and is called time division multiple access (TDMA);and a third which has different codes based on spread spectrum signaling is known as code divisionmultiple access (CDMA). It shouldbe noted that hybrid schemes using combination of basic schemes also can be developed. A performance comparison of multiple access s c h e m e s is a difficult task. T h e s t r e n g t h s of F D M A schemes s e e m t o b e fully exploited in narrowband channel environments. T o avoid the use of equalizers, channel bandwidths as narrow as possible should be employed. Yet, in such narrowband channels the quality of service is limited by the maximal expected Doppler frequency and practical stability offrequency sources. Current practical limits are approximately 5 kHz. T h e s t r e n g t h s of b o t h T D M A a n d C D M A schemes seem to be fully exploited in wideband channel environments. TDMAschemesneedmanyslots (and bandwidth) to collect information o n network behavior. Once the equalization is necessary ( a t bandwidths > 20 k H z ) , t h e d a t a r a t e should be made as high as possible to increase frame efficiency and freeze the frame to ease equalization. High data rates, however, require high RFpeakpowe r s a n d a l a r g e a m o u n t of signal processing power, which may be difficult to achieve in handheld units. Current practical bandwidths are approximately 0.1 MHz to 1.O MHz. C D M A schemes need large spreading (processing) gains (and bandwidth) to realize spread spectrum potentials, yet high d a t a r a t e s also r e q u i r e a large a m o u n t of signal processing power,which may be difficult to achieve in handheld units. Current practical bandwidths are approximately 1.2 MHz. Narrow frequency bands seem to favor FDMA schemes, sincc both T D M A and CDMA schemes require more spectra to fully develo p their potentials. O n c e t h e adequate power spectrum is available, however, the latter two schemes may be better suited for a complex (micro)cellular network environment. Multiple access schemes also are message sensitive. The length and type of message, and the kind of service will influence the choice of multiple access, ARQ, frame, and coding. Therefore, the following can apply:
For every mobile radio channel, there is an optimal access scheme. (P3)

For every access scheme, there is an optimal mobile radio channel. (P4) For every type of service, there is an optimal access scheme. (P5) For every access scheme, there is an optimal type of service. (P6)

Hybrid Schemes
For various reasons anumber of existing and proposed multiple access schemes are hybrid. For example, the GSM systememploysaTDMAschemewith eight slots per 200 kHz wide R F channel; channels are further distributed in an FDM fashion. There is an optional frequency-hopping pattern (code division) available.The transmitter and receiver are separated by 45 MHz, and frequency division duplex (FDD) mode of operation is assumed. Therefore, this rather complex hybrid scheme is denoted as FDMIFDDITDMNCDM. A slightly less complex scheme is the newly proposed North American digital cellular system (denoted IS-54 in Table 3). This system employs a T D M A s c h e m e with t h r e e slots p e r 30 k H z wide R F channel. Channels are further distributed in an FDM fashion, the transmitter and receiver are separated by 45 MHz, and afrequency division duplex (FDD) mode of operation is assumed. Theref o r e , this hybrid s c h e m e is d e n o t e d as FDMIFDDKDMA. The CT2Plus system proposes a time division duplex ( T D D ) m o d e of operation in 100 k H z wide R F channel. Channels are distributed in an FDM fashion, access channels use TDh4.4, and information channels use FDM mode of operation. Therefore, this rather complex hybrid scheme is denoted as F D MiTDDiTD M N F D M . The design and evaluation process of such hybrid schemes requires careful balancing between complexity and technical advantages and disadvantages of each of the elementary schemes.

System Capacity
The recent surge in the popularity of cellular radio, and mobile service in general, has resulted in an overall increase in traffic and a shortage of available system capacity in large metropolitan areas. Current cellular systems exhibit a wide range of traffic densities, from low in rural areas to overloading in downtown areas, with large daily variations between peak hours and quiet night hours. It is a great system engineering challenge to design a system that will make optimal use of the available frequency spectrum, offering a maximal traffic throughput (e.g., ErlangsiMHzlservice area) at an acceptable service quality, constrained by the price and size of the mobile equipment. In a cellular environment, the overall system capacity in a given service area (space) is a product of many factors (with complex interrelationships), including the available frequency spectra, service quality, traffic statistics, type of traffic, type of protocol, shape and size of service area, selected antennas, diversity, frequency re-use capability, spectral efficiency of coding and modulation schemes, efficiency of multiple access, etc. In the 1970s so-called analog cellular systems employed omnidirectional antennas and simple or no diversity schemes offering modest capacity,

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IEEE Communications Magazine

November 1991

which satisfied a relatively low number of customers. Analog cellular systems of the 1990s employ up to 60" sectorial antennas and improved diversity schemes. This latter combination resulted in a threefold to fivefold increase in capacity. A further (twofold) increase in capacity can b e expected from narrowband analog systems (25 kHz + 12.5 kHz), which was practically d e m o n s t r a t e d in Japan (LI 4 L2 system). However, slight degradation in service quality might be expected. These improvements s p u r r e d t h e c u r r e n t growth i n capacity, the overall success and prolonged life of analog cellular radio. There also are numerous marketing results, where a tenfold to twentyfold increase in capacity has been claimed (watch for small print!). In this kind of campaign new digital systems of the twenty-firtst century, operating under ideal conditions, usually are c o m p a r e d with t h e old systems of t h e 1970s, operating under the worst conditions.
There are numerous ways of increasing the capacity of cellular radio; acquiring new frequency spectra is perhaps the easiest one.

Conclusion
In this contribution, a broad repertoire of terrestrial and satellite systems and services for travelers is briefly described.The technical characteristics of the dispatch, cellular, and cordless telephony systems are tabulated for ease of comparison. Issues such as operating environment, service quality, network complexity, cell size, channel coding and modulation (codulation), speech coding, macro and micro diversity, multiplex and multiple access, and the mobile radio communications system capacity, are discussed. Presented data reveals significant differences between existing and planned terrestrial cellular mobile radio communications systems, and between terrestrial and satellite systems. T h e s e systems use different frequency bands, bandwidths, codulation schemes, protocols, etc., meaning they are not compatible. What are the technical reasons for this incompatibility? In this paper, performance d e p e n dence on multipath delay (related to the cell size and terrain configuration), Doppler frequency (relate d t o t h e carrier frequency, d a t a rate a n d t h e speed of vehicles), and message length (may dictate the choice of multiple access) are briefly discussed. A system optimized to serve the travelers in t h e G r e a t Plains may not p e r f o r m well in mountainous Switzerland. A system optimized for downtown cores may not bewell-suited to a rural environment. A system employing geostationary

(above e q u a t o r ) satellites may not b e able t o serve travelers at high latitudes adequately. A system appropriate for slow-moving vehicles may fail to function properly in a high Doppler shift environment. Additionally, a system optimized for voice transmission may not begood for data transmission. A system designed t o provide a broad range of services to everyone, everywhere, may not be as good as a system designed to provide a particular service in aparticular local environment, just as aworld championdecathletemay not be as successful in competitions with specialists in particular disciplines. However, there are many opportunities where compatibility between systems, their integration, and frequency sharing may offer improvements in service quality, efficiency, cost, and capacity (and therefore availability). Terrestrial systems offer a low startup cost and a modest cost-per-user in denselypopulated areas. Satellite systemsmay offer a high quality of service and may b e t h e most viable solution to serve travelers in scarcelypopulated areas, on oceans, and in the air. Terrestrial systems are confined to two dimensions, and radio propagation occurs in t h e near horizontal sectors. Barostationary satellite systems use t h e narrow sectors in t h e user'szenith nearly perpendicular to the Earth'ssurface having the potential for frequency re-use and an increase in t h e capacity in downtown areas during p e a k hours. A call s e t u p in a forward direction (from the PSTNvia base station to the traveler) may be a cumbersome process in a terrestrial system when a t r a v e l e r t o whom a call is i n t e n d e d is roaming within a n unknown cell. However, this is easilyrealized in aglobal beam satellite system.
England expects this day that every man will do his duty. Horatio Nelson, 21, IO,1805.

Terrestrial systems offera low startup cost and a modest cost-per-user i densely n populated areas.

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Biography
Andy D. Kucar (M '80, SM '90) received a Dipl. Tech degree (summa cum laude) in electronics f r o m the Technical College Rijeka in La Guardia'sFiume; in 1974a Dipl. Ing.degree(financialreward)inelectrical engineering, and in 1980 an M.S. in electrical engineering. both from Zagreb University; and in 1987 the Ph.D. degree in electrical engineeringfromUniversityof0ttawa.Canada. Between 1971 and 1973. Dr. Kucar served as a research and teaching assistant (microwave radio and radar systems) in the Department of RF and Microwaves at the faculty of electrical engineering at Zagreb University. In 1974 he was an engineer f o r Radioindustrija Zagreb. Between September 1 9 7 4 and September 1982 he worked for lskra Ljubljana From September 1982 to December 1988. he worked for University of Ottawa Digital Communications Group. Dr. Kucar worked for Telesat Canada from 1985 to 1987. From 1987 t o 1990 he worked for Bell Northern Research radio division. In November 1990 he established 4U Communications Research, a company specializing in excellence in terrestrial and satellite radio communications. where he serves as a president. He has organized and chaired numerous sessions on international confer^ ences including ICC, GLOBECOM, and VTC.

IEEE Communications Magazine

November 1991

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