The Revolution Rapid multinational progress will soon make global wireless communication a ubiquitous reality.

Theodore 5. Rappaport

ver the past three years, the inter- formance, more flexibility, user options, etc.) est in wireless communications has than a present-day cellular telephone. been nothing less than spectacular. Cellular radio systems around the world have been enjoying 33 percent Current Demand to 50percentgrowth rates. Manypag- The premise that wireless personal communi- ing services have been gaining customers at a rate cations is emerging as a key, wide-sweeping tech- of 30 percent to 70 percent or more per year, and nology that will dramatically impact our society is within the last two years there has been intense supported in numerous sources, including world- corporate research and development aimed at com- wide trade journals and government agency mercializing new wireless communication services reports. As an example, in the called PCS (personal communications services). there were more than 6.3 million cellular tele- Meanwhile, new digital cellular technologies have phone users as of September 1991 [18]. This com- been installed in Europe, and developing nations are pares with 25,000 users in 1984, and 2.5 million beginning to install cellular infrastructure. U.S. users in late 1989 [l]. It is clear to most The first wide-scale adoption of a wireless per- industry experts that the Cellular Telephone sonal communications system was in citizens Industry Association's (CTIA) 1989 projections band (CB) radio during thelate 1960s and early 1970s. of 10 million United States cellular users by 1995 will Although it was a victim of its own success due to be exceeded in late 1992, and cellular radio carri- a rapid and uncontrolled saturation of the radio spec- ers are enjoying exponential increases in service sub- trum, and suffered severely from lack of traffic man- scriptions. This demand for mobileiportable agement and services, the staggering acceptance telecommunications is a worldwide trend and is par- of CB radio was a clear indication that consumers ticularly acute in Europe. For example, cellular tele- wanted an inexpensive, portable means of com- phone in Sweden already enjoys a 6.6 percent municating. A more modern wireless device that has adult market penetration [18], and this figure has enjoyed widespread popularity is the cordless been increasing by more than 0.1 percent per month. telephone. By the time you read this article, it is like- Finland, Norway, and France have been experiencing ly that more than 65 million cordless telephones similar growth rates. In Hong Kong, more than will have been sold in the United States alone 50percent of the adult population own or have oper- (although it is estimated that almost halfof the cord- ated cellular telephones. Although no spectrum less phones sold have been discarded or are not used). has yet been allocated to emerging PCS in the The convenience of portable telecommunications United States, industry analysts already are pro- offered by a device as simple as the cordless jecting annual U.S. revenues to be between $33 phone is clearly in demand. Although the design solu- billion and $55 billion by the year 2000 [17]. tions for a ubiquitous wireless communications In the United Kingdom, viewed by many as network will be tremendously complex, all mar- the leading country for PCS initiatives, three ket indications show that consumers wish to have major companies are investing hundreds of millions a small device, similar in size to a cordless phone, of dollars to install an infrastructure that may that would allow them to make and receive phone eventually allow citizens to use small 10 mW portable calls wherever they are. The idea behind wireless terminals to place and receive calls in populated areas personal communications networks (PCNs) is to throughout the country. At worst, PCS will pro- make communications truly personal, so that any- vide relief for users who operate in congested 900 one can place a call to anyone else. Much like the MHz United Kingdom cellular markets. At best PCS stereo Walkman'" or thelaptopcomputer, PCNwill will offer customers the ability to use a single is U permit truly ubiquitous access regardless of the loca- wireless communications unit for home, office, or faculty member of the tion of the user at the time of access. Today, the terms automobile, thereby obviating the need for a tra- Virginia Polytechnic Institute, PCN and PCS often are used interchangeably. ditional wired phone to the home. Two of the the State Uniwrsit) in PCN refers to a concept by which a person can three companies involved, Microtel and Unitel, Blacksbq, virgmia. and at use a single communicator anywhere in a large are actually consortia consisting of many leading tlie Mobile and Portable geographic area. PCS refers to a service that may telecommunications companies. The third United Radio Research Group not embody all of the PCN concepts,but is more per- Kingdom PCN service provider, Mercury, is the lead- (.MPRG). sonalized (i.e., lightweight terminal, better per- ing non-wireline service provider of England's

52 0163-68O4/91/$Ol .OO 1991' IEEE IEEE Communications Magazine November 1991 present analog cellular system. These PCN com- Technology (VT)which features digital cellular tech- - panies are expected to become some of the biggest nologies. As indicated in the VT special issue, it is advertisers in the United Kingdom throughout conceivable that 50 million to 75 million sub- In lase this decade, as they strive to pioneer and market scribers could be using wireless systems for vari- the revolutionary PCN service [19]. ous types of personal communications by the United States The pervasiveness of PCN will be made available mid-1990s. This is corroborated by a recent Mor- by an immense infrastructureof low-power, suitcase- gan Stanley report that predicts cellular and PCN sized base stations which will provide portable systems will achieve at least 12 percent market markets, subscribers with wireless access to the local telephone penetration in many developed countries by the end loop in populated areas. PCN hopes to be able to of this century. the 832 offer wireline communications quality using radio as a transmission medium. Base stationswill be placed Recent Events in Wireless cellular on lamp posts, roofs, and ceilings of buildings and InlargeUnitedStatesmarketssuch asLosAnge- concourses, and in other locationswhere people con- les and New York, where hundreds of thousands gregate. Backbone links that connect PCN base of users can access the cellular radio spectrum, ana log-FA4 stations to other PCN base stations and to the the 832 cellular analog-FMvoice channels are unable public switched telephone network (PSTN) will to accommodate the number of users, and meth- channels are be supplied by one of two methods: either via the ods to improve capacity and cellular system existing telephone wire or fiber plant, or via line- design are desperately needed. For those not of-sight microwave links. Of course, the wide- familiarwith cellular radio, in each market (i.e., city) unable to scale deployment of such an extensive high-grade there are two cellular service providers: the radio wireless telephone system will require engineer- common carrier (non-wireline provider), called accommo- ing tools and techniques and antenna designs that the A channel provider, and the wireline, or B allow rapid and accurate propagation prediction and channel, provider. Each of the two service system design. The radio coverage of each base providers are allocated 416 duplexvoice channels in date the station will intentionally be limited by low transmitter a 25 MHz spectrum allocation. Each voice chan- power, so that the same frequencies can be re- nel is comprised of a 30 kHz base-to-mobile link and number of used many times within a few city blocks. Depend- a 30 kHz mobile-to-base link. ingon regulatory decisions by the British Post Office, Over the past four years, numerous standards users. PCN could compete directly with wired residen- have been proposed for digital cellular radio tial telephone service in the United Kingdom. Thus, communication interfaces throughout the world. it is conceivable that the customers could bypass Digital modulation offers improved spectral effi- the telephone company and use a single PCN ter- ciency and simultaneouslyoffers better speech intel- minal for communications at home or in the office. ligibility for a given carrier-to-interference ratio (CII). In the United States, more than 65 experimen- More importantly, digital modulation accommo- tal licenses have been issued within the last year dates powerful digital speech coding techniques that to regional bell operating companies (RBOCs), non- further reduce the spectrum occupancy of voice users. wireline cellular service providers, manufacturers, Using digital transmission formats, service cable companies, and new start-up companies providers will be able to offer customers addi- hoping to pioneer PCN service. Experimental tional features, such as dynamically allocated FCC licenses that allow limited use of radio data services, encryption, etc. transmissions for portable FAX service, wireless In early 1990, the CTIA and the Telecommu- spread spectrum PABX systems, and microcellu- nications Industry Association (TIA) approved Inter- lar and personal communication services have im Standard 54, which specifies a dual-mode cellular been granted and trials are in various phases. radio transceiver that uses both the analog FM To increase the competition and development (the present-day United States Advanced Mobile time of new wireless technologies and services, Phone System, or AMPS, standard) and a linearized the FCC created a special incentive, called the n/4 Differential Quadrature Phase Shift Keying mod- pioneer’spreference, that offers the exclusive use of ulation format with a Code Excited Linear Pre- reallocated radio spectrum to companies that dictive (CELP) speech coder (called the US. Digital first demonstrate new technologies or concepts Cellular, or USDC standard) [20]. for PCN or other new wireless services. Many of The USDC standard offers roughly three the UnitedStatesPCNexperimenters havefiledpeti- times the capacity improvement over AMPS bypro- tions for rulemaking to the FCC requesting new ded- viding three voice channels that are time-division icated or shared spectrum for PCN services, and hope multiplexed (TDMA) on a single AMPS 30 kHz FM to secure an advantageous position through pioneer’s voice link. With further speech coding improvements, preference. Industry and government experts six times capacity is likely to be achieved by 1994. view the demand for wireless personal communi- The dual mode equipment will allow a graceful tran- cations to be sogreat that the FCC has indicated that sition from analog FM to digital cellular, since underutilized portions of existing radio spectrum cellular operators will be able to change out ana- could be subject to reallocation in order to accom- log channels to digital channels depending on modate consumer demand. their geographic capacity demands. In this man- Several good tutorial articles that describe the ner, customers with analog phones will be assured impetus and technological challenges behind cel- service in any market until some announced time lular radio and PCN can be found in the August 1990 in the future. and September 1990 issues of the IEEE Commu- A rural cellular carrier that does not suffer nications Magazine. The different second generation great capacity demand would likely stay with standards that have emerged around the world AMPS for as long as possible, probably several years are described in several invited papers in the May (until the mid-l990s), while a metropolitan oper- 1991 Issue of the IEEE Transactions on Vehicular ator would likely change out AMPS for USDC more

IEEE Communications Magazine November 1991 61 - rapidly (within two years). It is interesting (and COMM proposal received immediate attention. also a bit troubling to those who invested agreat deal Today, many United States companies (e.g., In contrast of time and money into developing the IS-54 American Personal Communications, PCN Amer- standard and who plan to abide by it) that since ica, and Omnipoint Data) are lookingat theviability with USDC, the introduction of the USDCstandard IS-54, numer- of overlaying TDMA, FDMA, and spread spectrum ous vendors have introduced their own compet- mobile radio systems on existing point-to-point ing standards that are incompatible with IS-54[20]. microwave links, used by utility companies, banks, the Euro- Now virtually all major cellular radio serviceproviders and public safety offices,in the 1850 MHz - 1990 MHz are conducting field trials to evaluate new, competing band. There are many others who are aggressive- pean di@tal standards, so it isunclear if US. digital cellular radio ly conducting research and experimentation to deter- systems will be completely compatible through- mine overlay possibilities, and their experimental sys- out the country, even though this was the major results may be obtained at the FCC office in cellular goal of USDC. Washington, D.C. Recent FCC experiment reports In contrast to USDC, the European digital filed by American Personal Communications [36] tem (Group cellular system (called Group Special Mobile, or and Telesis Technology Laboratories [37] indi- GSM) was developed for a brand new spectrum allo- cate that even in the largest urban United States mar- Special cation in the 900 MHz band. That is, GSM was devel- kets, it may be possible to break the existing 1850 oped to ensure that a single access and equipment MHz to 1990 MHz band into 5 MHz segments which standard would be used throughout the Euro- could be used simultaneously by line-of-sight Mobile, or pean continent. microwave users and PCS providers. Unlike USDC, which hoped to make a seam- The concept behind overlay is that low-power GSM) was less transition of America’s analog system to digi- PCN service could be offered directly on top of tal, GSM was developed from scratch and made the existing terrestrial microwave band. The rea- pan-European compatibility its primary objective. soning is that since existingmicrowave links use direc- developed Today, in most European countries, the cellular sys- tional antennas, and thus do not radiate over a tems and standards are unique to the individualcoun- large geographical area, it could be possible to inter- for a new tries. In fact, at the time of this writing, a cellular twine PCS frequencies over a geographical area such phone that works in France cannot be used in that existing microwave links are not interferedwith spectrum England. GSMoperates in new spectrum at 900 MHz by overlaid PCS. Recent experiments and analysis dedicated for use throughout Europe. Details of the seem to suggest that existing microwave links GSM specification are given in [21,25] and equip- have not been engineered throughout large markets allocation ment will be available to the pan-European com- to take full advantage of the spectrum. munity by the time this article is published. References [23] and [24] discuss the concept in the GSM is the world’s first TDMA cellular sys- behind overlay, and some techniques that could tem standard, and uses a constant envelope mod- be used to minimize interference between exist- 900 ulation format to gain power efficiency (constant ing and new users. Recent propagation measure- MHz envelope modulation allows more efficient class- ments to test the levels of interference caused by C power amplifiers to be used) over spectral effi- PCN subscriber units to existing fixed microwave band. ciency (constant envelope modulation has a users were presented in [24] and more recently larger bits per hertz of RF occupancy than does have been detailed in FCC reports [37, 381 as part linear modulation). GSM uses an equalizer and slow of experimental PCN licenses. frequency hopping to overcome multipath effects Presently, major telecommunication compa- that cause intersymbol interference and, thus, nies are researching the effectiveness of overlay irreducible bit error rates. As the world’s first dig- systems from a capacity standpoint. If a sufficient ital cellular radio standard that has been adopted grade of service could be offered through spec- by a large market, GSM is viewed as a front-run- trum sharing between new wireless service ner for early implementation of PCN throughout the providers and existing line-of-sight microwave world. Depending on the success of United King- licensees, then the FCC would be able to instant- dom 1800 MHz PCN initiatives, GSM equipment ly accommodate market demand for wireless cel- could be implemented on a worldwide scale if spec- Iularb’CNwithout a major new spectrum allocation. trum allocationsare made availablein other countries. The fear, of course, is that there could be an In February 1990,just after CTIA adopted IS-54, unsatisfactory degradation of service to both the QUALCOMM, Inc. proposed to CTIA the use of existing microwave users and the overlayed PCN spread spectrum and sophisticated base station users. The overlay concept is being tested bynumer- signal processing to offer capacity improvements ten ous companies in the United States under the times greater than AMPS (see the paper by FCC experimental license program. At present Gilhousen, et. al. in the May 1991 IEEE Trans. the FCC has not made a decision on their position VT special issue on Digital Cellular Technolo- on the matter. gies). Major cellular radio serviceproviders and man- The cable industry also has been watching the ufacturers have steadily supported QUALCOMM sudden growth in cellularPCN throughout the world. as they develop prototype spread spectrum cellu- Cable companies have amassive RFnetworkinstalled lar phones that will be ready by mid-1992 [22]. throughout populated regions, and it is obvious Work conducted two decades ago [26], and more that telecommunication services could be offered recently [29], confirms that spread-spectrum over the existing cable plant. In fact, PCN base holds great promise for accommodating huge capac- stations could be installed easily in residential itywith simple frequency management, although the areas by splicing existing cable runs and tapping capacity is highlydependent on radio path losswith- on base stations mounted on lamp posts or inside in the service area. Of course, higher capacity means buildings. Indeed, several United States cable higher revenues and less churn (loss of cus- companies are conducting experiments to determine tomers) for cellular service providers, so the QUAL- the feasibility of PCS using the cable plant. When

62 IEEE Communications Magazine November 1991 one realizes that United States cable operators istration (NASA), through its research programs - already have spectrum allocated for microwave feeds in space communications and mobile satellite in the 2 GHz and 13 GHz bands, the possibility of communications experiments (MSAT-X), and more NASA has utilizing the cable spectrum for PCN services instead recently through its Advanced Communications of point-to-point feeders presents a lucrative new Technology Satellite (ACTS) program, has provided provided the business opportunity for the cable industry. the bulk of United States federal research fund- For local loop, or premises applications, which ing in the mobile communications area. Although merge voice and data, Bellcore [15] and the mobile satellite systems do not promise nearly bulk of European Technological Standards Institute (ETSI) the same capacity (users per MHz) as do land- have proposed digital TDMA standards that offer based microcellular and PCS systems,there is remark- United States between 400 kilobits per second (kbis) and 1100kbis able commonality between the technologies used data rates in office and residential environments. in satellite and cellular mobile systems. Factors such federal Although the widespread deployment of such as modulation, coding, multiple access, antenna standards likelywill rely on new dedicated spectrum design, network control, and propagation are fun- for the services, significant engineering manpow- damental to both types of wireless communica- research er has been devoted to develop the standards, tion systems, as are the techniques used to design and a great deal can be learned from the research. and analyze such systems. finding in Bellcore’s system exploits the slow time-vary- Through the NASA MSAT program alone, at ing nature of indoor channels and uses antenna polar- least 11 United States universities and numerous ization diversity to improve link performance between companies have been able to conduct multiple- the mobile a base station (port) and mobile terminal (portable). year research projects that have produced both howl- Bellcore’s proposal limits the data rate to 450 edgeable graduates and innovative technologies. communica- kbis based on an extensive measurement program Today, many graduates of NASA-funded com- that determined worst case multipath channels in munications research programs are working in tions area a large number of buildings, houses, and cities. the satellite or cellular industries. Examples of Bellcore’s approach minimizes the complexity two different satellite systems for personal com- and battery drain of the portable, since power- munications are described in [38] and [39]. hungry adaptive equalizers can be avoided. Also, Until recently the National Science Founda- Bellcore’s system uses a novel oversampling demod- tion has shown little interest in funding experimental ulation technique that allows areceiver tolockcoher- or theoretical work applied to wireless communi- entlyonto the incomingmodulationwith only a couple cations. It is ironic that an NSF-sponsored proj- of bits of overhead. Reference [15] provides addi- ect at in the mid-1970s provided tional details about the Bellcore system, and [21, the first study of aspread spectrum cellular radio sys- 25,34, and 351 provide additional information about tem [26], a study that created intense interest in spread ETSI and the DECT standard. spectrum access approaches for cellular radio communicationswhich now (17years later) are being Funding in Wireless extensively commercialized by numerous compa- nies. This summer, however, NSF awarded Rut- The burgeoning wireless communications indus- gers Universitywitha cooperative UniversityDndustry try has created an interesting problem for wire- Center. The Rutgers Wireless Information Network less manufacturers and service providers throughout (WINLAB), founded by Professor David Goodman the world. Because the field of cellular radio andper- in 1988-1989,isworking on network and access solu- sonal communications is changing so rapidly, and tions for third generation wireless personal com- since the field involves system concepts seldom taught munication systems, and was the first United at universities, wireless companies are having dif- States academic laboratory formed for wireless per- ficulty finding entry-level graduates with suffi- sonal communications education and research. cient education to make an immediate contribution Steadily, more universities are recruiting faculty in research or design. In particular, a large num- interested in the wireless communications field. ber of universities presently do not offer under- The Defense Advanced Research Projects Agen- graduate or graduate courses on the topics of mobile cy (DARPA) has funded projects to develop radio propagation or wireless communication sys- small, low-powered, wireless devices, and is sup- tem design. Consequently, recruiters are forced porting research that will lead to technology to raid competing companies for more senior development for rapidly deployable local area personnel, and resign themselves to spending six communication systems. New design and fabrica- months to two years to train new graduates in the tion technologies range from advanced silicon art and science of mobile and portable radio. integrated circuits, to enhanced speech coding algo- In an informal survey of some of the largest rithms and circuit fabrication, to software tools cellular radio companies in the United States, the for propagation prediction and installation. The per- author has learned that engineers with knowl- sonnel development, knowledge base, resulting tech- edge of mobile radio communications change nologies, and system design tools from these projects jobs often, and are in extremely high demand. will not only improve the United States militarycapa- Particularly in the past two years, since cellular radio bility, but also has relevance to the U.S. con- has enjoyed a50percent annual growth rate and new sumer wireless personal communications industry. digital systems have been proposed and tested, engi- ’s Mobile and Portable Radio neers with experience in cell-site design or computer Research Group (MPRG) is a new group within simulation expertise in mobile radio propagation, the university’s Bradley Department of Electrical traffic modeling, antenna design, and digital sig- Engineering. Founded in the spring of 1990,it iscon- nal processing have been highly sought after by ducting basic and applied research in the areas of the wireless industry. radio propagation measurement and prediction, The National Aeronautics and Space Admin- communication system design using measure-

IEEE Communications Magazine November1991 63 - ment-based propagation models, and simulation This year, the Ontario province and Canadian of various digital modulation, diversity, and radio industry will provide more than $6 million for In Canada, equalizer techniques. The group’s mission is to university communications research in Ontario. help the United Stateswireless industry through the A significant portion of those funds is being used numerous development of analysis tools and computing for mobile and satellite systems research for wire- techniques for emerging wireless personal com- less personal communications. The mobile and satel- munication systems,while providing quality research litesystemsprogram supportsmore than 50graduate initiatives are opportunities for graduate and undergraduate students at five universities, and involves approxi- students. mately 20 Canadian communications companies. under way to MPRG also is providing opportunities for Other Canadian provinces also are providing research technical interchange. An EE graduate student support for regional universities active in wireless encourage lecture series in the fall of 1989 featured key personal communications. researchers from the cellular radio industry, and the On a federal level, The Canadian Institute for First VirginiaTech Symposium on Wireless Personal Telecommunications Research (CITR) was estab- academic Communicationswas held June 1991 in Blacksburg, lished in 1989, and is providing research grants to Virginia. The symposium featured 20 invited speak- universities throughout Canada. CITR’s budget is participation ers and panel discussions by industry experts over more than $4 million annually (overhead is waived a three-day period. It was attended by 175 people on CITR funding) and is focusing on two major areas: from 22 states and nine countries. The sympo- broadband and wireless communications. Examples in research sium, now scheduled as an annual June event on of CITR projects in the antennas and propaga- the Virginia Tech campus, provided an opportu- tion area include propagation and diversity tech- for wireless nity for students and engineers to learn both fun- niques for indoor wireless communications at damentals and trends in wireless communications. millimeterwaves, fading issues for 20 GHz to 30 GHz persona 1 The necessity for academic research groups like personal satellite communications systems, and new MPRG becomes clear when one realizes the cellular system design techniques. Universities enormous, yet sudden, activity in the wireless involved with CITR wireless research include communica- field, and the growing demand for young gradu- Carleton University. Concordia, Ecole Polytechnic, ates who can make immediate contributions to a Lava1 University, McGill, Queens, Simon Frazier tions. dynamic field. University, the University of British Columbia, It is worth noting that in contrast with the the University of Calgary, the University of United States government agencies, the Euro- Ottawa, the University of Toronto, the University pean, Canadian, and Japanese governments have of Victoria, the University of Waterloo, and the Uni- made substantial funding commitments to research versity of Western Ontario. laboratories and university programs focusing on Japanese research programs in wireless personal wireless personal communications and related tech- communications abound, and there are numerous nologies. For example, in Europe, the RACE universities active in the area that are making program (Research and Development in Advanced fundamental contributions. As an example, Kyoto Communications in Europe) has committed over University has been a major contributor in the $100 million per year to the European communi- wireless communications field, and has an active ty during the period 1987-1992 [27]. graduate program in propagation and communi- A significantportion of those funds has been spent cation system design. Federal funding for aca- on collaborative industryluniversity research in wire- demic research in Japan often involves interaction less communications, with the goal of concurrent- with industrial laboratories [27]. It seems clear ly expanding the knowledge base and pool of technical that most federal governments view the tremendous experts. RACE appears to be yielding large divi- impact that wireless communications will make dends. The European community is widely recog- on the world’s economy as agolden opportunity, and nized as the world leader in creating and accepting are hoping their investments will result in new new digital cellular radio system techniques technologies and abody of experts who can engineer (many European countries presently enjoy more and develop new wireless personal communica- than a 5 percent cellular market penetration, tion systems. comparedwith approximately3 percent in the Unit- Early efforts to obtain government funding from ed States), and it is where the concept of personal the National Science Foundation failed, but communications was first put into wide scale industry response has been excellent and after practice. one year, 15 major wireless companies (including Casual conversationswith researchers across the many regional Bell Operating Companies, major world indicate that more than 50 European Ph.D. radio manufacturers, and computer manufacturers), students are tackling dissertations dealingwith anten- DARPA, and the FBI have provided a funding nas and propagation for emerging wireless per- base of over $1 million. Collaborative research sonal communication systems. This does not include projects with Purdue University, Northeastern Uni- other areas of PCN, such as modulation, coding, versity, and the University of California at San Diego diversity, and system design. have provided cross-fertilization of ideas and knowl- In Canada, numerous initiatives are under way edge, and are helping to make the United States to encourage academic participation in research for approach to wireless communications research more wireless personal communications. The Telecom- synergistic. munications Research Institute of Ontario (TRIO) At MPRG, students receive an educational expe- program is enhancing the technological competi- rience that provides them with a solid under- tiveness of Canadian industry through standing of the theory and practice of mobile university/industry partnerships in telecommuni- radio communications and emerging personal com- cations research. TRIO has grown steadily since munication systems.At the same time, they are tasked it was founded in 1987. with developing andvalidating analyses, models, and

64 IEEE Communications Magazine November 1991 research tools for the wireless industry. These Propagation Measurement - tools help transfer knowledge from the MPRG laboratory into industry and academia. A large part of MPRG research has dealt with A large part In 1990, the first year of MPRG, five M.S.E.E. measuring, and then statistically modeling, the studentsgraduatedwith expertise in RFfilter design, path loss and time dispersion of multipath radio chan- of MPRG indoor radio propagation measurement and pre- nels. Measurements and models have been made diction, adaptive noise-cancellation techniques, and in many different environments: traditional urban urban radio propagation prediction. Presently, there cellular radio channels with base station antenna research has are 18 graduate and six undergraduate MPRG heights exceeding many tens of meters [l,21; students pursuingdegreeswith an emphasisonwire- urban cellular radio channels with lower antenna dealt with less communications. As a result of student the- heights, on the order of 15 to 20 meters [2]; and ses, several analysis and simulation software tools in-building channels within sports arenas, facto- measuring, have been developed for internal use, and are ries, and office buildings [3, 41, and open plan also being used for research and development by office buildings [5]. Also, impulsive noise mea- a number of companies and universities. An elec- surements have been made inside many types of and then trical engineering graduate course dedicated to buildings at three bands between 900 MHz and the topics of cellular radio and personal commu- 4.0 GHz [28], includingtwoof the license-free Indus- statistically nications was offered in the spring of 1991, and trial, Scientific, and Medical (ISM) bands. enjoyed an enrollment of 34 students, making it For typical urban mobile radio channels, it has the most popular graduate course in the EE cur- been reported that the coherence bandwidth (the modeling, riculum at Virginia Tech that term. Senior elec- bandwidth over which the received signal strength tive courses on radio wave propagation and willlikelybewithin90percent of anyother frequency the path satellite communications also are popular EE from the same source) is between 10 kHz to 500 courses at Virginia Tech. kHz [6].Consequently, to sufficiently resolve loss and Although the research and educational mis- multipath components (in the time domain) that sion of MPRG concerns itself with more than just cause frequency selective fading, a channel propagation, the group received its first research sounder for urban channels should possess an RF time disper- contracts in the area of propagation measure- bandwidth several times larger than the maxi- ment and prediction, and continues to maintain mum coherence bandwidth. This thinking led us sion of an active program in this area. There is extreme inter- to use a500nsprobingpulse (4MHzRFbandwidth) est in and demand for propagation measurements in [I, 21. For indoor measurements, we have used and models for the proper design of emerging probes that have durations on the order of 5 ns, there- multipa th wireless services such as PCN, and more powerful by providing measurements that span over 400 MHz site-specific channel modeling techniques and and which resolve individual multipath echoes to radio tools are needed. about 1.6 m separation distance. (The baseband pulse The United States wireless industry, however, is DSB modulated so there is a bandwidth expansion channels. often finds conducting their own measurements and factor of two at RF. Thus, the baseband complex propagation research to be a time-consuming and envelope response has a 200 MHz resolution expensive task, and many industrial players view bandwidth). Time domain techniques rather than it as an expensive luxury that involves high-priced swept frequency techniques are used since it is consultants. MPRG has emerged as a sensible easy to identify the location and intensity of and cost-effective alternative, since it has an reflecting objects in the channel. established equipment arsenal and provides research This information is vital for successful devel- expertise in the area of propagation measure- opment of site-specific propagation models that can ment and prediction. By pooling resources in an recreate the channel impulse response. Our mea- industrial affiliates program, MPRG is generating surement systems are easy to assemble, test, and useful tools and basic propagation models that deploy, and provide instantaneous channel mea- can be shared by all affiliate members, and the result- surementswith excellent time delay resolution. Post- ing value of the research is much greater than the detection integration is employed to improve the cost to an individual member. All results are pub- signal to noise ratio of the measured power delay lished so the entire research community benefits profiles. Our approach, though, requires more peak from the knowledge base. power than the direct sequence systems used in Here, we briefly present some of the basic param- [7,8, and 91 for a specifiedcoverage distance. Broad- eters used to describe multipath channel charac- band discone, yagi, and helical antennas have teristics, and show results from propagation been used to ensure no pulse spreading is attribut- measurements in urban, microcellular, and indoor ed to impedance mismatch. channels. These measurements involve wideband Important parameters that indicate multipath characterizations, where avery short RFburst is sent dispersion in mobile radio channels are illustrat- and the echoes are received, as well as narrow- ed in Fig. 1. Therms delay spread (oT)measures how band (CW) characterizations that measured sig- spread of the channel power delay profile about nal fading over large temporal and spatial spans. the centroid, and the excess delay spread (XdB) indi- As is subsequently shown, antenna experiments have cates the maximum excess delay at which multi- revealed that polarization can dramatically reduce path energy falls to X dB below the peak received the multipath time delay spread and the fluctua- level. These parameters are useful measuresfor com- tion of delay spread in mobile channels. Resultspre- paring different multipath channels and have sented in this article are not meant to be interpreted been used to determine approximate bit error as definitive work in mobile radio propagation. rates for digital modulation schemes without On the contrary, it is hoped that this articlewill spark equalization. interest and ongoing discussionsin propagation mod- Historically, time dispersion parameters (e.g., for eling and prediction for personal communications. ionospheric channels) were computed by using a

IEEE Communications Magazine November 1991 65 - Particularly i in indoor I channels, I individua 1 ,- 1 E 1- Mean Excsss Deley = 45.05 n6 = 7 multipath ~P, -30 I,,, , , , , , , , -50 0 50 103 150 zoo 250 300 350 400 450 I components Lppp Excess Delay (ns) Figure 1. Example of a power delay profile illustrat- f I fade vey lit- ing important channel parameters 1.0 10.0

~ temporal average of the channel impulse response T-R~ Separat!on. Ikilometersi ~ -2 tle between during a time period overwhich the channel appeared Figure 2. Scatter plot of wideband path loss wide-sense stationary. In mobile systems, howev- measured in cellular and microcellular channels in er, the time variation of the impulse response is Europe[2] two fied due primarily to motion, so the parameters may imcnts using computer simulation instead of pro- be computed over a spatial average duringwhich the totype hardware [MI. terminals or channel appears wide-sensc stationary [9, 301. Historically, path loss has been found to be closc- Particularly in indoor channels, individual ly linked to the separation distance between terminals multipath components fade very little between transmitter and receiver, so a simple model for two fixed terminals or terminalsmoved along asmall the path loss at some distance r>ro from a trans- area [31. 321. Statistical processing on an exten- mitter can be expressed as moved along sive indoor propagation data base showed that P(r) = P(r,) 60 irIn (1) individual multipath components fade in a log- The exponent 11 in Equation (1) represents the a small area. normal sense over small temporal and spatial best-fit (in a mean square sense) average power intervals, with a standard deviation of only a cou- law at which signal power decays with respect to a ple of dB. Simultaneous CW measurements free space measurement at ro, the close-in reference showed that the narrowband fading between two distance. Measurements have shown that field mea- fixed terminals is Ricean, but the CW fading can surements are generally log-normally distributed be either Ricean, log-normal, or Rayleigh when about the average distance-depcndent power law the receiver is moved over a small area. Deep given in (I). independent of r. fades of individual multipath components arc pri- Different time dispersion and path loss results marily due to shadowing as a terminal is moved. for urban microcellular measurements reported or results from the phasor sum of unrcsolvable in [2] are shown in Fig. 2 and Table 1. The mea- multipath components within a resolution cell. surements were made throughout several existing Knowledge of the channel time dispcrsion to ccllular markets in Germany using a 500 ns prob- temporal resolutions much greater (smaller dura- ing pulse and existing cellular base station anten- tion) than the bit durations of a communication na heights that ranged from 20 m to 93 m in signal, and how thc time dispersiveness changes over height [2].Data in Fig. 2 and Table 1 assume ro space, is important because these factors deter- is 100 m. Using the assumption that path loss is mine the instantaneous bit error floor that occurs log-normally distributed about the mean power law, because of data smearing. By performing the the standard deviation 0 in dB completcly speci- time convolution of transmitted data bits with fies the distribution of received power as a func- accurate spatially (time) varying impulse response tion of distance. For a best-fit path loss exponent, models. it becomes possible to predict burst owill be minimized over the entire scatter plot. errors and conduct real-timc system design expcr- Reference [29] shows that the selection of the

66 IEEE Cornmunications Magazine Novemher 1991 - I I, MPRG research has focused on measuring indoor and microcellular

a) Excess Delay lnsl channels and develop-

1''' ' ' ' ':'n=4 ing models

:'n=J for such n=2.1 channels.

...... ,

I ... *.-I . 3 .,L.U"" t '1 1.5 2 3 4 5 6 100 lo T-R Separalion (m) 'igure 3 (These waveforms are examples produced bq ., CIM. SIRCIMprovides the user with datafiles that contain amplitudes, phases, and time delays of the channel impulse response, and computesfading statistics, large-scale path loss, best fit exponent and standard dev. User can specify building type, topography (LOS or OBS), and T-R separation distance. SIRCIM is based on extensive measurements made in more than 10 different buildings. close-in reference distance is important for sys- and microcellular channels and developing mod- tem design considerations and capacity analysis. The elsfor such channels, since it is our belief that indoor close-in reference distance is a leverage point environments and street-level systems will serve from which a best-fit mean path loss exponent extends. the largest number of wireless users in the com- Historically, a reference distance of 1 km was ing decades. Extensive indoor propagation mea- used as early land mobile systems strived to pro- surements have been and continue to be made vide coverage over tens of kilometers. Emerging with the goal of deriving site-specific modeling microcellular systems, however, will cover delib- approaches and installation tools based on physi- erately smaller regions so that spectrum re-use cal descriptions of building interiors [S, 131. can be employed more extensively in a specific Along the way, we have used statistical model- market. By diminishing the size of the radio cells, ingprocedures [lo] to reproduce, on apersonal com- more users can be accommodated within a given puter, extensive propagation measurements given spectrum allocation; consequently capacity can in [ 131 so that research can focus on indoor radio be increased at the expense of more base stations communication system design using realistic com- and infrastructure. Thus, path loss models must puter-generated impulse responses. Also, more use a close-in reference distance that is several times recent measurements [3,4] and measurements smaller than the distance of the most distant user reported in the literature [ 141 have been used togen- within a coverage zone. erate (on a computer) impulse response and path Using field measurements, it has been shown that loss measurements in traditional-partitioned and the value of the path loss exponent can change soft-partitioned (Herman-Miller office parti- depending on the free space reference distance cho- tions) office buildings. The statistical channel sen, which indicates that simple dn path loss models are useful for determining, through simu- models are subject to interpretation of the close- lation, irreducible bit error rates, modulation per- in leverage point selected. For microcellular and formance, diversity implementations, and robust PCN systems, reference distances on the order of equalization methods. 1 m to 100 m are appropriate. The propagation simulator, called SIRCIM (Sim- MPRG research has focused on measuring indoor ulation of Indoor Radio Channel Impulse response

IEEE Communications Magazine November 1991 67 -~~ All Soft Partitioned Environments 130 7-p 160 b Office Building 1 4th Floor 1 50 0 Office Building 1 51h Floor 0 Office Building 2 2nd Floor 1 I 90 ~ -~ 80 I m Soft Porlltlon - 1 39 dE Elo0 Concrete = 2 38 dE --/ ~ (0 o=4ldB 4 90 1 50 80

U a~ 70 ~ 3 ;60 a

40 I /’ il 30 1 30 40 50 60 70 BO 90 100 110 120 130 I I Measured Path Loss (dB) 80 Figure 4. Best fit line for a simple two-parameter Figure 5. Measured and predicted signal strength model used to predict signal loss due to obstructions contours based on site-specific model [S] Measurements). is a valuable tool for MPRG experiences dictate. communications research. and also is being used A useful result from [3]is that propagation by 40 companies and universities. The models characteristics are very similar at both 1.3 GHz used in SIRCIM are detailed in [IO]. Although and 4.0 GHz, which means the SIRCIM models similar in nature to the SURP simulation pro- (based on measurements at 1.3 GHz) will hold up gram for urban radio propagation [11], SIRCIM to at least 4.0 GHz, and probably at somewhat is based on measurements made over small scale dis- higher frcquencies. Typical examples of the data pro- tances. and thus allowssynthesisof the phase of indi- duced bySIRClMareshownin Fig.3. Datafiles that vidual multipath components based on the contain the amplitudes,phases, time delays, and path Doppler shift and a random scattering model. By loss for individual multipath components are pro- computing the spatially varying phasor sum of duced by SIRCIM and written to disk for later multipathcomponents, SIRCIMrecreatcsCWfad- use in bit error simulation. These files may then ing envelopes identical in nature to those mea- be used to test bit error rates and system designswith- sured in the field. MPRG plans to update the software out prototype hardware. as more measurements become available. and as user Narrowband measurements have shown how the path loss exponent, and the dcviation about the best- fit average path loss model. can be affected by build- ing type, or location within a building. Table 2, extracted from [5],indicates that in different buildings, the floors can offer different values of attenuation. Measured attenuation factors for various obstacles in indoor environments are presented in Tables 3 and 4 [4, 51. In [SI, a simple two- paramcter statistical model was developed to modcl the loss due to each partition or each wall encoun- tered between a transmittcr and receiver locatcd within a building. A scatter plot of path loss measurements made on three different floors of two different office buildings, and the predicted path loss based on the simplc model in [5]is shown in Fig. 4. The agreement between nieasured and predicted path loss is very good for the most part, and has an overall standard deviation of 4 dB. A standard deviation much greater than 10 dB usually results when only distance (and no site-specific informa- tion) is used to predict signal strength from a data base of several different types of buildings. For measurements in Fig. 4. the transmitter anten- nawasmounted 1.8 m abovc ground, and the receiv- er was located at desk height. slightly shadowed by movablc office cubicles (soft partitions) and obstructed by concrete walls. Using a simple model that assumes 1.4 dB loss for each cubicle wall and 2.4 dB for each concrete wall (the walls did not span the entire floor), and free space Table 2. Attenuation values for different buildings andjloors propagation cverywherc else, it was possible toclose- ITEM LOSS (dB) ly predict signal strength contours. - Figures 5, 6, and 7 illustrate how the simple site-specific attenuation model can accurately In Office Buildings Narrowband predict coverage throughout the work space. A schematic of a typical 59 m x 59 m open plan Concrete Block Wall 13 measure- office building with movable cloth partitions and concrete walls is shown in Fig. 6. Dark lines in Loss From One Floor 20-30 Fig. 6 denote concretewalls that extended from floor ments have to ceiling. Lighter lines represent 2.0 m tall soft Loss From One Floor and one 40-50 partitions. Fig. 5 and 7 can be overlayed on Fig- Wall shown how ure 6, and show measured and predicted signal strength contours based on the site-specific Fade observed when 10-15 the path loss model in [5]. A simple distant-dependent path transmitter turned a right angle loss model would provide circular contours of corner in a corridor constant radius about the transmitting antenna. exponent Although this modeling work is preliminary, Light Textile inventory 3-5 it demonstrates that simple descriptions about can be the building topography could be used to predict Chain link fenced in area 20 ft. 5-12 coverage areas and interference zones with much high which contains tools, affected by better accuracy than models used today. MPRG inventory, and people researchers are continuing measurements that will help to develop accurate site specific models. building These models will then be incorporated into an auto- Metal Blanker-I2 square feet 4-7 mated system design tool that will optimally locate base stations for minimum interference Metallic Hoppers which hold 3-6 type, or and consequently maximum capacity. MPRG iswork- scrpa metal for recycling-I 0 ing to exploit knowledge of the propagation envi- square feet location ronment to improve and automate system installation without measurements. Small Metal Pole-6 in. diameter 3 within a Work in [4] has shown that antenna polariza- tion can play a big part in reducing the delay Metal Pulley System used to 6 spread (i.e., improving the bit error perfor- hoist metal inventory4 sq. ft. building. mance). In [15], Cox describes the Bellcore UDPC system as using polarization diversity to open Light Machinery c 10 sq. ft. 1-4 the eye in digital modulation techniques. Our work [4] shows that, indeed, polarization diversity General Machinery 5-10 can be used to select the best channel at a partic- - ular location. Our work also shows that circularly 10-20 square feet polarized (C-P) directional antennas, when used in line-of-sight channels, can provide a much Heavy Machinery > 20 sq. ft. 10-12 lower delay spread than linear polarized anten- nas with similar directionality [4]. Metal catwalkktairs 5 Figure 8 shows how the instantaneous rms delay spread changes as a mobile receiver is Light Textile 3-5 moved over a 2.5h track. The identical trackwas tra- versed with a receiver using omnidirectional and Heavy Textile inventory 8-1 1 directional linear polarized antennas, and direc- tional circular polarized antennas. Note that the Arerea where workers inspect 3-12 metal finished products for defects.

Metallic inventory 4-7

Large I-beam - 16-20 in 8-10

Metallic inventory racks - 4-9 B square feet

Empty Cardboard inventory 3-6 30x8s

Soncrete Block Wall 13-20

Ceiling Duct 1-8 "able3 Measured signal loss due to common

I obstructions in buildings* Figure 6. Blueprint of open plan building measured * These data were collected by comparing signal in Fig. 5 strengths on either side of the obstruction

IEEE Communications Magazine November1991 69 - I 60 60 70 80 90 1 MPRG is conducting additional measure- ments up to 30 GHz around campus to provide Site-specific data for building penetration loss, floor-to-floor loss for different-shaped buildings, the correlation of sig- propagation nal strengths over small distances, and the impor- tance of antenna pattern and polarization on system design. models Referring back to Fig. 2, there is a large amount of scatter about the best fit, indicating that predict, that surrounding buildings have a large impact on the measured path loss between a transmitter with good and receiver. Site-specific propagation models that predict, with good accuracy, the shadowing loss- es and the diffraction effects in urban canyons accuraq, the are needed for system design, and good progress is being made by numerous researchers through- shadowing Figure 7. Predicted contour plot of signal strength out the world. A recent paper shows the viability using best fit model of Fig. 3. Agreement with mea- of ray tracing and shadowing for accurate propa- losses and sured data is better than simply using T-R separation gation prediction for microcellular systems [ 161. alone [S] Additionally, unpublished work at MPRG shows that in fact only a few rays and simple diffraction the difiuc- C-P helical antenna offers much less delay spread, methods can be used most of the time to get sur- and smaller delay spread variability, than do the prisinglygoodprediction (within3 dB) of measured L-P omni or yagi antennas. Also, directional signals in microcellular environments. tion effects antennas reduce the delay spread when com- pared with omni antennas. We also have seen Conclusion in urban this on cross-campus links that illuminate several buildings at a time. In outdoor links especially, it The wireless personal communications age is appears that when aligned off-axis, directional C- coming soon. This paper has attempted to put inper- canyons are P antennas offer much more multipath resistance spective some recent trends andevents that areshap- than linear polarized antennas. The multipathreduc- ing the wireless personalcommunications revolution. needed for tion is likely due to cancellation of odd-bounce mul- The paper also gave insight into the research and tipath, and offers a significant performance gain educational activities at Virginia Tech, and point- system since it reduces the time dispersion of the chan- ed out some tools and techniques used to charac- nel. Further, this finding indicates that an accu- terize radio propagation. New models for rate propagation prediction tool must consider propagation prediction will lead to appropriate spec- design. polarization effects. trum allocations and robust system designs. One area of research that is of great impor- Obstacle Description I Attenuation 1 tance but was not mentioned is environmental safety.As important as research aimed at developing wireless personal communications, good, objec- 2.5 m storage rack tive in-depth experiments must be performed with small metal parts jointly and immediately by engineers and medical scientists to determine the health risks associated 4 m metal box 10-12 with continuous and pulsed microwave radiation. Although worldwide standards exist, it is unclear 5 m storage rack with to many if those standards actually represent safe paper products levels for humans. If a cause and effect relation- ship exists between cancers and microwave radia- tion, this must be made known immediately and 5 m storage rack with the physical mechanisms must be learned to combat paper products radiation effects in the future. A universal, responsible approach to this potential problem, and 5 m storage rack with the public's perception of the problem, should be large metal parts undertaken immediately by government, indus- try, and academia. Within a decade, low-level RF radiation at microwave frequencies will be near Semi-automated our bodies and all around us. In 20 to 30 years, Assemblv Line wireless communications probably will be as per- 12-14 vasive as utilitylines and housewiring are today. We must be certain that the wireless personal com- munications age will be an environmentallysafe age, Stainless Steel as well. Piping for Cook-Cool Process References 8-15 [ll T. S. Rappaport, S. Y. Setdel. and R. Singh. "300 MHz Multipath Propagation Measurements for US. Digital Cellular Radiotele- Concrete floor I 10 -1 phone," /E€€ Trans. Veh. Technol., pp. 132-39. May 1930. 121 S. Y. Seidel. eta/.. "Path Loss, Scattering, and Multipath Delay Table 4 Shadowing eflects of commonfactory equip- Statistics in four European Cities for Digital Cellular and Micro- ment cellular Radiotelephones," lEEE Trans. Veh. Technol., vol. 40, no. 4. Nov. 1931.

70 IEEE Communications Magazine November 1991 - 100 ...... ,...... [3] D. A. Hawbaker and T. S. Rappaport, "Indoor Wideband Radiowave o Omni-Vertical : Propagation Measurements at 1.3 GHz and 4.0 GHz " IEEE Omni-Horizontal : ElectronicsLetters, vol.26,no.21,pp. 1800-02,OctoberlO. 1990. 0 Directive -Vert. j [4] D. A. Hawbaker, "Indoor Radio Propagation Measurements, Mod- 80 ' . m Directive - Hor els, and Communication System Design Issues." Masters Thesis in Electrical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, May 1991. Also see paper in Globecom '91 60 [51 S. Y. Seidel and T. S. Rappaport, "900 MHz Path Loss Measure- ments and Prediction Techniques for In-Building Communica- tion System Design." 1991 IEEE Vehicular Technology Conference, St. Louis, MO, May 21, 1991. To be published in IEEE Trans. Antennas Propag. early 1992 40 [61 R. W. Lorenz "Impact of Frequency-Selective Fading on Digital Land Mobile RadioCommunicationsatTransmission Ratesof Sev- eral Hundred kbitJs," IEEE Trans. Veh. Technol.. vol. VI-35, no. 3, pp. 122-28. Aug. 1987. 20 [7] R. J. C. Bultitude, "Propagation Characteristics on Microcellular Urban Mobile Radio Channels at 910 MHz," IEEEJ. in Se/. AreasCommun.. vol. 7, no. 1. pp. 31-39. Jan. 1989 OBS Topography - 32 6 m mean 1-R separation [E] D. M. 1. Devasirvatham, "Multipath Time Delay Spread in the Digi- tal Portable Radio Environment," IEEE Commun. Magazine. 025 050 075 1 00 1 25 1 50 1.75 2.00 2.25 2.50 vol. 25. no. 6, pp. 13-21, June 1987. [91 D. C. Cox. "Time and Frequency-domain Characterizations of Mul- Distance (wovelengths) tipath Propagation at 910 MHz in a Suburban Mobile-Radio Fimre 8 Variation of RMS delay spread along a 2.5 ;1 track for omni and direc- Environment,"RadioSci., MI. 7, no. 12, pp. 1069-81, Dec. 1972. - -- [lo] T. S. Rappaport, S. Y Seidel. and K. Takamizawa, "Statistical tional antennas. Gain of omni antenna was 1.5 dei, gains of linear directional Channel Impulse Response Models for Factory and Open Plan and CP directional antennas were approximately 10 dBi. Frequency = 1.3 GHz. Building Radio Communication System Design " IEEE Trans on Commun. vol. 39, no. 5, pp. 794-807. May 1991. L281 K. L. Blackard. T. S. Rappaport, and C. W. Bostian, "Radio Fre- [l11 H. Hashemi. "Simulation of the Urban Radio Propagation Chan- quency NoiseMeasurementsand Modelsfor Indoor Wireless Com- nel," /E€€ Trans. Veh. Techno/., vol. VI-28. pp. 213-25, Aug. 1979. munications at 918 MHz. 2.44 GHz. and 4.0 GHz," Proc. of 1121 J. I. Smith, "A Computer Generated Multipath Fading Simulation 1991 IEEE Int'l Commun. Conf., Denver, CO, pp. 19.3-.5, June for Mobile Radio." IEEE Trans. Veh. Technol.. vol. 24, no. 3, 23, 1991. pp. 39-40. Aug. 1975. L291 1. S. Rappaport and L. B. Milstein, "Effects of Path Loss and [131 T. S. Rappaport, "Characterization of UHF Multipath Radio Chan- Fringe User Distribution in CDMACellular FrequencyReuse," Proc. nels in Factory Buildings."lEEE Trans. AntennasandPropag.,vol. of 1990 IEEE Global Commun. Conf., San Diego, CA, pp. 500- 37, no. 8. pp. 1058-69, Aug. 1989. 06, Dec. 2, 1990. [141 D. Molkdar, "Review on Radio Propagation Into and Within Build- [301 T. S. Rappaport, "Indoor Radio Communications for Factories of ings,"lEEEProc.-H. vol. 138. no. 1. pp. 61-73, Feb. 1991. the Future," IEEE Commun. Magazine, vol. 27, no. 5, pp. 15- [151D.C.Cox."ARadioSystemProposalforWidespread Low-PowerTeth- 24, May 1989. erless Communications," IEEE Trans. Comm., vol. 39, no. 2, pp. [311 T. S. Rappaport and C. D. McGillem. "UHF Fading in Factories," 324-35. Feb. 1991. IEEEJ. Se/. Areas Commun., vol. 7, no. 1, pp. 40-48, Jan. 1989. I161 F. Ikegami, T. Takeuchi, and S. Yoshida, "Theoretical Prediction of [321 R. Ganesh and K. 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IEEE Communications Magazine November 1991 71