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Wireless. . Communications PRINCIPLES AND PRACTICE Second Edition THEODORE 5, RAPPAPORT Prentice Hall Communications Engineering and Emerging Technologies Series Thendore S. Rappaport, Series Editor CHAPTER 1 Introduction to Wireless Communication Systems T. ability to communicate with people on the move has evolved remarkably since Guglielmo Marconi first demonstrated radio's ability to provide continuous centact with ships sailing the English chan- nel. That was in 1897, and since then new wireless communications methods and services have been enthusiasticaily adopted by people throughout the world. Particularly during the past ten years, the mobile radio communications indus- try has grown by orders of magnitude, fueled by digital and RF circuit fabrica- tion improvements, new large-scale circuit integration, and other miniaturization technologies which make portable radio equipment smaller, cheaper, and more reliable. Digital switching techniques have facilitated the large scale depioyment of affordable, easy-to-use radio communication networks. These trends will continue at an even greater pace during the next decade. 1.1 Evolution of Mobile Radio Communications A brief history of the evolution of mobile communications throughout the world is useful in order to appreciate the enormous impact that cellular radio and personal communication services (PCS) will have on all of us over the next several decades. It is also useful for a newcomer to the cellular radio field to understand the tremendous impact that government regulatory agencies and service competitors wield in the evolution of new wireless systems, services, and technologies. While it is not the intent of this text ta deal with the techno-politi- cal aspects of cellular radio and personal communications, techno-polítics are a fundamental driver in the evolution of new technology and services, since radio spectrum usage is controlled by governments, not by service providers, equip- ment manufacturers, entrepreneurs, or researchers. Progressive involvement in www.vsofts.net oldroad Evolution of Mobile Radio Commurications 3 =—3 + == T AD É — Video cos keconder 1 / Year o€ da luction = 1979 Cable Television | Year of Iniroducti a: / Televisiof me Il cio! — Yearot Prucia1 946 . E Los Rd . | E / rd / E! / / / | & Atomobfe f | 2 / ear ofÂmtroduction = 1900 ! ; E É t A f | co / E 1 A Telephone E / Year of Introduction = j 877 fa J / E - Mobile Telephone / Year of Introduction = 1946 rá | 04 : y j A 20 30 ao so 60 70 Nurber of years after the first commercial deployment Figure 1.1 Figure illustrating the growth of mobile telephony as compared to other popular inventions of this cen tury. 86,000 by 1948, 695,000 by 1958, and about 1.4 million users in 1962 [Nob62]. The vast majority of mobile users in the 1960s were not connected to the public siwitched telephone network (PSTN), and thus were not able to directly dial tele- phone numbers from their vehicles. With the boom in CB radio and cordless appliances such as garage door openers and telephones, the number of users of mobile and portable radio in 1995 was about 100 million, or 37% of the U.S. pop- ulation. Research in 1991 estimated between 25 and 40 million cordless tele- phones were in use in the U.S., and by the tum of the century this is certain to double [Rap91c]. The number of cellular telephone users grew from 25.000 in 1984 to about 16 million in 1994, and since then, wireless services have been experiencing customer growth rates well in excess of 50% per year. By the end of 1997, there will be nearly 50 million U.S, cellular users. In the first couple of decades of the 21st century, there will be an equal number of wireless and con- ventional wireline customers throughout the world! www.vsofts.net oldroad 4 Ch. 1 Intraduction to Wireless Communication Systems 1.2 Mobile Radiotelephone in the U.S. In 1946, the first public mabile telephone service was introduced in twenty- five major American cities. Each system used a single, high-powered transmitter and large tower in order to cover distances of over 50 km in a particular market. The early FM push-to-talk telephone systems of the late 1940s used 120 kHz of RF bandwidth in a half-duplex made (only one person on the telephone call could talk at a time), even though the actual telephone-grade speech occupies only 3 kHz of baseband spectrum. The large RF bandwidth was used because cf the dif- ficulty in mass-producing tight RF filters and low-noise, front-end receiver amplifiers. In 1950, the FCC doubled the number of mobile telephone channels per market, but with no new spectrum allocation. Improved technology enabled the channel bandwidth to be cut in half to 60 kHz. By the mid 1960s. the FM bandwidth of voice transmissions was cut to 30 kHz. Thus, there was only a fac- tor of 4 increase in spectrum efficiency due to technology advances frem WW II to the mid 1960s. Also in the 1950s and 1960s, automatic channel trunking was introduced and implemented under the label IMTS Improved Mobile Telephone Service). With IMTS, telephone companies began offering full duplex, auto-dial, auto-trunking phone systems [Cal88]. However, IMTS quickly became saturated in major markets. By 1976, the Bell Mobile Phone service for the New York City market (a market of about 10,000,000 people) had only tweive channels and could serve only 543 paying customers. There was a waiting list of over 3,700 people [Cal88), and service was poor due to call blocking and usage over the few channels. IMTS is still in use in the U.S,, but is very spectrally inefficient when compared to today's U.S. cellular system. During the 1950s and 1960s, AT&T Bell Laboratories and other telecom- munications companies throughout the world developed the theory and tech- nigues of cellular radiotelephony — the concept of breaking a coverage zong (market) into small cells, each of which reuse portions of the spectrum to increase spectrum usage at the expense of greater system infrastructure [Mac79]. The basic idea of cellular radio spectrum allocation is similar to that used by the FCC when it allocates television stations or radio stations with dif- ferent channels in a region of the country, and then reallocates those same chan- neis to different stations in a completely different part of the country. Channels are only reused when there is sufficient distance between the transmitters to prevent interference. However, cellular relies on reusing the same channels within the same market or service area. AT&T proposed the concept of a cellular mobile system to the FCC in 1968, although technology was not available to implement cellular telephony until the late 1970s. In 1983, the FCC finally allo- cated 666 duplex channels (40 MHz of spectrum in the 800 MHz band, each channel having a one-way bandwidth of 30 kHz for a total spectrum occupancy of 60 kHz for each duplex channel) for the U.S. Advanced Mobile Phone System (AMPS) [You79]. According to FCC rules, each city (called a market) was only www.vsofts.net oldroad 6 Cn. 1: Introduction to Wireless Communication Systems speech coding technology will increase the capacity to six users per channel in the same 30 kHz bandwidth within a few years. A cellular system based on code division multiple access (CDMA) has been developed by Qualcomm, Inc. and standardized by the Telecommunications Industry Association (TIA) as an Interim Standard (18-95). This system supports a variable number of users in 1.25 MHz wide channels using direct sequence spread spectrum. While the analog AMPS system requires that the signal be at least 18 dB above the co-channel interference to provide acceptable call quality, CDMA systems can operate at much larger interference levels because of their inherent interference resistance properties. The ability of CDMA ta operate with a much smaller signal-to-noise ratio than conventional narrowband FM tech- niques allows CDMA systems to use the same set of frequencies in every cell, which provides a large improvement in capacity [G1]91]. Unlike other digital cel- tular systems, the Qualcomm system uses a variable rate vocoder with voice activity detection which considerably reduces the required data rate and also the battery drain by the mobile transmitter. In the early 1990s, a new specialized mobile radio service (SMR) was devel- oped to compete with U.S. cellular radio carriers. By purchasing small groups of radio system licenses from a large number of independent private radio service providers throughout the country, Nextel and Motorola have formed an extended SMR (E-SMB) network in the 800 MHz band that could provide capacity and ser- vices similar to cellular. Using Motorola's integrated radio system (MIRS), SMR integrates voice dispatch, cellular phone service, messaging, and data transmis- sion capabilities on the same network [Fil95]. New Personal Communication Service (PCS) licenses in the 1800/1900 MHz band were auctioned by the U.S. Government to wireless providers in early 1995, and these promise to spawn new wireless services that will complement, as well as compete with, cellular and SMR. One of the stipulations of the PCS license is that a majority of the coverage area be operational before the year 2000. Thus, there is pressure en PCS licensees to “build-out” each market. As many as five PCS licenses are allocated for each major U.S. city (see Chapter 10). 1.3 Mobile Radio Systems Around the World Many mobile radio standards have been developed for wireless systems throughout the world, and more standards are likely to emerge. Table 1.1 through Table 1.3 lists the most common paging, cordless, cellular, and personal communications standards used in North America, Europe, and Japan. The dif- ferences between the basic types of wireless systems are described in Section 1,5, and are covered in detail in Chapter 10. The world's most common paging standard is the Post Office Code Standard Advisory Group (POCSAG) [CCI86], [San82]. POCSAG was developed by British Post Office in the late 1970s and supports binary frequency shift keying (FSK) www.vsofts.net oldroad Mobile Radia Systems Around the World Table 1.1 Major Mobile Radio Siandards in North America Standard Type Year of Multiple Frequency Modula- Channel Introduction | Access Band tion Bandwidth AMPS Cellular 1988 FDMA 824-894 MHz | FM 30 kHz NAMPS Cellular 1992 FDMA 8274-894 MHz | FM 10 kHz USDC Cellular 1991 TDMA 824-894 MHz | n/4- 30 kHz DAPSK CDPD Celiular 1993 FH/ 824-894 MHz | GMSK | 30kHz Packet 18.95 Cellular” | 1993 CDMA | 824-894 MHz | QPSK/ | 1.25 MHz PCS 1.8-2.0 GHz BPSK GSC Paging 1970 Simplex | Several FSK 12.5 kHz POCSAG | Paging 1970s Simplex | Several FSK 12.5 kHz FLEX Paging 1993 Simplex | Several 4-FSK | 15kHz DCS- PCS 1994 TDMA 1.85-1.99 GMSK | 200 kHz 1900 GHz (GSM) PACS Cordless” | 1994 TDMA! |1.85-1.99 w/d- 300 kHz PCs FDMA | GHz DQPSK MIRS SMR/PCS | 1994 TDMA Several 16- 25 kHz QAM signaling at 512 bps, 1200 bps, and 2400 bps. New peging systems, such as FLEX and ERMES, provide up to 6400 bps transmissions by using 4-level modu- Jation and are currently being deployed throughout the world. The CT2 and Digital! European Cordless Telephone (DECT) standards developed in Europe are the two most popular cordless telephone standards throughout Europe and Asia. The CT2 system makes use of microcells which cover small distances, usually less than 100 m, using base stations with anten- nas mounted on street lights or on sides of buildings. The CT2 system uses bat- tery efficient frequency shift keying along with a 32 kbps adaptive differential pulse code modulation (ADPCM) speech coder for high quality voice trangmis- sion. Handoffs between base stations are not supported in CT2, as it is intended to provide short range access to the PSTN. The DECT system accommodates www.vsofts.net oldroad Examples of Mobile Radio Systems 9 to the US. AMPS system, except that the smaller bandwidth channels result in a slight degradation of signal-to-noise ratio (SNR) and coverage range. In Ger- many, a cellular standard called 0-450 was introduced in 1985. The first genera- tion European cellular systems are generally incompatible with one another because of the different frequencies and communication protocols used. These systems are now being replaced by the Pan European digital cellular standard GSM (Global System for Mobilei which was first deployed in 1990 in a new 900 MHz band which all of Europe dedicated for cellular telephone service [MalS9]. As discussed in Chapter 10, the GSM standard is gaining worldwide acceptance as the first universal digital cellular system with modern network features extended to each mobile user, and is a strong contender for PCS services above 1800 MHz throughout the warld. In Japan, the Pacific Digital Cellular (PDC) standard provides digital cellular coverage using a system similar to North America's USDC. 1.4 Examples of Mobile Radio Systems Most people are familiar with a number of mobile radio communication sys- tems used in everyday hife. Garage door vpeners, remote controllers for home entertainment equipment, cordless telephones, hand-held walkie-talkies, pagers (also called paging receivers or “beepers”), and cellular telephones are all exam- ples of mobile radio communication systems. However, the cost, complexity, per- formance, and types of services offered by each of these mobile systems are vastly different. The term mobile has historically been used to classify any radio terminal that could be moved during operation. More recently, the term mobile is used to describe a radio terminal that is attached to a high speed mobile platform (eg.a cellular telephone in a fast moving vehicle) whereas the term portable describes a radio terminal that «an be hand-held and used by someone at walking speed (e.g. a walkie-talkie or cordless telephone inside a home). The term subscriber is often used to describe u mobile.or portable user because in most mobile commu- nication systems, each user pays a subscription fee to use the system, and each user's communication device is called a subscriber unit. In general, the collective group of users in a wireless system are called users or mobiles, even though many of the users may actually use portable terminais. The mobiles communi- cate to fixed base stations which are connected to a commercial power source and a fixed backbone network. Table 1,4 lists definitions of terms used to describe ele- ments of wireless communication systems. Mobile radio transmission systems may be classified as simplex, half- duplex or full-duplex. In simplex systems, communication is possible in only one direction. Paging systems, in which messages are received but not acknowledged, are simplex systems. Half-duplex radio systems allow two-way communication, but use the same radio channel for both transmission and reception. This www.vsofts.net oldroad 10 Ch. 1 « Introduction to Wireless Communication Systems Table 1.4 Wireless Communications System Definitions Base Station A fixed station in a mobile radio system used for radio communica- tion with mahile stations. Base stations are located at the center or on the edge of a coverage region and consist of radio channels and transmitter and receiver antennas mounted on a tower. Control Channel Radio channels used for transmission of call setup, call request, call initiation, and other beacan or control purposes. Forward Channel Radio channel used for transmission of information from the base station to the mobile. Full Duplex Communication systems which allow simultaneous two-way commu- Systems nication. Transmission and reception is typically on two different channels (FDD) although new cordless/PCS systems are using TDD. Half Duplexz Communication systems which allow two-way communication by Systems using the same radio channel for both transmission and reception. At any given time, the user can only either transmit or receive infor- mation. Handoff The process of transferring a mobile station from one channel or base station to another. Mobile Station A station in the cellular radio service intended for use while in motion at unspecified locations. Mobile stations may be hand-held personal units (portables) or installed in vehicles (mobiles). Mobile Switching Switching center which coordinates the routing of'calls in a large Center service area. In a cellular radio system, the MSC connects the cellu- lar base stations and the mobiles to the PSTN. An MSC is also called a mobile telephone switching office (MTSO). Page A brief message which is broadcast over the entire service area, usu- ally in a simulcast fashion by many base stations at the same time. Reverse Channel Radio channel used for transmission of information from the mobile to base station. Roamer A mobile station which operates in a service area (market) other than that from which service has been subscribed. Simplex Systems Communication systems which provide only one-way communica- tion, Subsecriber A user who pays subscription charges for using a mobile commuáica- tions system. Transceiver 4 device capable of simultaneously transmitting and receiving radio signals. means that at any given time, a user can only transmit or receive information. Constraints like “push-to-talk” and “release-to-listen” are fundamental features of half-duplex systems. Full duplex systems, on the other hand, allow simulta- neous radio transmission and reception between a subscriber and a base station, by providing two simultaneous but separate channels (frequency division duplex, wwwvsofts.net oldroad 12 €h. 1 + Introduction so Wireless Cammunication Systems number or travel to à known location to receive further instructions. In modern paging systems, news headlines, stock quotations, and faxes may be sent. À mes- sage is sent ta a paging subscriber via the paging system access number (usually a toll-free telephone number) with a telephone keypad or modem. The issued message is called a page. The paging system then transmits the page throughout the service area using base stations which broadcast the page on a radio carrier. Paging systems vary widely in their complexity and coverage area. While simple paging systems may cover a limited range of 2 km to 5 km, or may even be confined to within individual buildings, wide area paging systems can provide worldwide coverage. Though paging receivers are simple and inexpensive, the transmission system required is quite sophisticated. Wide area paging systems consist of a network of telephone lines, many base station transmitters, and large radio towers that simultaneously broadcast a page from each base station (this is called simulcasting). Simuicast transmitters may be located within the same service area or in different cities or countries. Paging systems are designed to provide reliable communication to subscribers wherever they are; whether inside a building, driving on a highway, or flying in an airplane. This necessi- tates large transmitter powers (on the order of kilowatts) and low data rates (a couple of thousand bits per second) for maximum coverage from each base sta- tion. Figure 1.3 shows a diagram of a wide area paging system. City [ei Landline link Paging Terminal PSTN City 2 Paging Landline link i Control Paging Terminal Center Da : di nas cs N Paging Terminal Satelkite link Figure 1.3 Diagram ví a wide area paging system, The paging control center dispatches pages received from the PSTN throughout several cities at lhe same time. www.vsofts.net oldroad Examptes of Mobile Radio Systems 13 Example 1.1 Paging systems are designed to provide ultra-reliable coverage, even inside buildings. Buildings can attenuate radio signals by 20 or 30 dB, making the choice of base station locations difficult for the paging companies. For this rea- son, paging transmitters are usually located on tall buildings in the center of a city, and simulcasting is used in conjunction with additional base stations located on the perimeter of the city to flood the entire area. Small RF band- widths are used to maximize the signal-to-noise ratio at each paging receiver, so low data rates (6400 bps or less) are used. 1.4.2 Cordiess Telephone Systems Cordless telephone systems are full duplex communication systems that use radio to connect a portable handset to a dedicated base station, which is then connected to a dedicated telephone line with a specific telephone number on the public switched telephone network (PSTN). In first generation cordless tele- phone systems (manufactured in the 1980s), the portable unit communicates only to the dedicated base unit and only over distances of a few tens of meters. Early cordless telephones operate solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for in-home use. Second generation cordless telephones have recently been introduced which allow subscribers to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modem cordless telephones are some- times combined with paging receivers so that a subscriber may first be paged and then respond to the page using the cordless telephone. Cordless telephone systems provide the user with limited range and mobility, as it is usually not possible to maintain a call if the user traveis outside the range of the base sta- tion. Typical second generation base stations provide coverage ranges up to a few hundred meters. Figure 1.4 illustrates a cordless telephone system. Publie . wireless Fixed i Switched Poa bnk Telephone Network (Base (PSTN) Station) = E] — 1 ' . Ni Figure 1.4 Cordless Handset Diagram of a cordless telephone system. www.vsofts.net oldroad Examptes of Mobile Radio Systems 15 Figure 1.5 An illustration of a cellular system. The towers represent base stations which provide radio acress between mobile users and the Mobile Switehing Center (MSC). sages are sent in a number of ways to facilitate automatic channel changes and handoff instructions for the mobiles before and during a call. Example 1.2 Cellular systems rely on the frequency reuse concept, which requires that the forward control channels (FCCs) in neighboring cells be different. By defining a relatively small number of FCCs as part of the common air interface, cellular phones can be manufactured by many companies which can rapidly scan all of the possible FCCs to determine the strongest channel at any time. Once find- ing the strongest signal the cellular phone receiver stays “camped” to the pur- ticular FCC. By bruadcasting the same setup data on all FCCs at the same time, the MSC is able to signal all subscribers within the cellular system and can be certain that any mobile will be signaled when it receives a call via the PSTN. 1.4.3.1 How a Cellular Telephone Cailis Made When a cellular phone is turned on, but is not yet engaged in a call, it first scans the group of forward contro! channeis to determine the one with the stron- gest signal, and then monitors that control channel until the signal drops below a usable level. At this point it again scans the control channels in search of the strongest base station signal. For each cellular system described in Table 1.1 through Table 1.3, the control channels are defined and standardized over the entire geographic area covered and typically make up about 5% of the total num- www.vsofts.net oldroad 16 Ch. 1 Introduction to Wireless Communication Systems ber of channels available in the system (the other 95% are dedicated to voice and data traffic for the end-users). Since the control channels are standardized and are identical throughout different markets within the country or continent, every phone scans the same channels while idle. When a telephone call is placed to a mobile user, the MSC dispatches the request to all base stations in the cellu- lar system. The mobile identification number (MIN), which is the subscriher's telephone number, is then broadcast as a paging message over all of the forward control channels throughout the cellular syatem. The mobile receives the paging message sent by the base station which it monitors, and responds by identifying itself over the reverse control channel. The base station relays the acknowledg- ment sent by the mobile and informs the MSC of the handshake. Then, the MSC instructs the base station to move the call to an unused voice channel within the cell (typically, between ten to sixty voice channels and just one control channel are used in each cells base station). At this point the base station signals the mobile to change frequencies to an unused forward and reverse voice channel pair, at which point another data message (called an alert) is transmitted over the forward voice channel to instruct the mobile telephone to ring, thereby instructing the mobile user to answer the phone. Figure 1.6 shows the sequence of events involved with connecting a call to a mobile user in a cellular telephone system. All of these events oceur within a few seconds and are not noticeable by the user. Once a call is in progress, the MSC adjusts the transmitted power of the mobile and changes the channel of the mobile unit and base stations in order to maintain call quality as the subscriber moves in and out of range of each base station. This is called a handoff. Special control signaling is applied to the voice channels so that the mobile unit may be controlled by the base station and the MSC while a call is in progress. When a mobile originates a call, a call initiation request is sent on the reverse control channel. With this request the mobile unit transmits its tele- phone number (MIN), electronic serial number (ESN), and the telephone number of the called party. The mobile also transmits a station class mark (SCM) which indicates what the maximum transmitter power level is for the particular user. The cell base station receives this data and sends it to the MSC. The MSC vali- dates the request, makes connection to the called party through the PSTN, and instructs the base station and mobile user to move to an unused forward and reverse voice channel pair to allow the conversation to begin. Figure 1.7 shows the sequence of events involved with connecting a call which is initiated by a mobile user in a cellular system. AU cellular systems provide a service called roaming. This allows subscrib- ers to operate in service areas other than the one from which service is sub- seribed. When a mobile enters a city or geographic area that is different from its home service area, it is registered as a roamer in the new service area. This is www.vsofts.net oldroad PSUBNQUIOS 6! 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