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KEY TOPICS: Covers every fundamental aspect of wireless communications, from cellular system design to networking, plus the world-wide standards, including ETACS, GSM, and PDC.
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KEY TOPICS: Covers every fundamental aspect of wireless communications, from cellular system design to networking, plus the world-wide standards, including ETACS, GSM, and PDC.
MARKET: For cellular radio engineers and technicians.
Technical introduction to the fastest growing engineering field, cellular radio and wireless personal communication. Combines many disciplines to present principles of design fundamentals, mobile radio propagation, modulation techniques, channel and speech coding, multiple access techniques and networking. Describes and compares existing and proposed world-wide cellular cordless and personal communication systems. Compiles the major wireless standards. Some familiarity with probability, communication theory and electromagnetics is assumed.
As the demand for service increases (i.e., as more channels are needed within a particular market), the number of base stations may be increased (along with a corresponding decrease in transmitter power to avoid added interference), thereby providing additional radio capacity with no additional increase in radio spectrum. This fundamental principle is the foundation for all modern wireless communication systems, since it enables a fixed number of channels to serve an arbitrarily large number of subscribers by reusing the channels throughout the coverage region. Furthermore, the cellular concept allows every piece of subscriber equipment within a country or continent to be manufactured with the same set of channels so that any mobile may be used anywhere within the region.
Figure 3.1 illustrates the concept of cellular frequency reuse, where cells labeled with the same letter use the same group of channels. The frequency reuse plan is overlaid upon a map to indicate where different frequency channels are used. The hexagonal cell shape shown in Figure 3.1 is conceptual and is a simplistic model of the radio coverage for each base station, but it has been universally adopted since the hexagon permits easy and manageable analysis of a cellular system. The actual radio coverage of a cell is known as the footprint and is determined from field measurements or propagation prediction models. Although the real footprint is amorphous in nature, a regular cell shape is needed for systematic system design and adaptation for future growth. While it might seem natural to choose a circle to represent the coverage area of a base station, adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping regions. Thus, when considering geometric shapes which cover an entire region without overlap and with equal area, there are three sensible choices—a square, an equilateral triangle, and a hexagon. A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell. For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three. Thus, by using the hexagon geometry, the fewest number of cells can cover a geographic region, and the hexagon closely approximates a circular radiation pattern which would occur for an omnidirectional base station antenna and free space propagation. Of course, the actual cellular footprint is determined by the contour in which a given transmitter serves the mobiles successfully.
When using hexagons to model coverage areas, base station transmitters are depicted as either being in the center of the cell (center-excited cells) or on three of the six cell vertices (edge-excited cells). Normally, omnidirectional antennas are used in center-excited cells and sectored directional antennas are used in corner-excited cells. Practical considerations usually do not allow base stations to be placed exactly as they appear in the hexagonal layout. Most system designs permit a base station to be positioned up to one-fourth the cell radius away from the ideal location.
To understand the frequency reuse concept, consider a cellular system which has a total of S duplex channels available for use. If each cell is allocated a group of k channels (k < S), and if the S channels are divided among N cells into unique and disjoint channel groups which each have the same number of channels, the total number of available radio channels can be expressed as
S = kN
The N cells which collectively use the complete set of available frequencies is called a cluster. If a cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of capacity and is given by
C = M kN = MS
As seen from Equation (3.2), the capacity of a cellular system is directly proportional to the number of times a cluster is replicated in a fixed service area. The factor N is called the cluster size and is typically equal to 4, 7, or 12. If the cluster size N is reduced while the cell size is kept constant, more clusters are required to cover a given area, and hence more capacity (a larger value of C) is achieved. A large cluster size indicates that the ratio between the cell radius and the distance between co-channel cells is small. Conversely, a small cluster size indicates that co-channel cells are located much closer together. The value for N is a function of how much interference a mobile or base station can tolerate while maintaining a sufficient quality of communications. From a design viewpoint, the smallest possible value of N is desirable in order to maximize capacity over a given coverage area (i.e., to maximize C in Equation (3.2)). The frequency reuse factor of a cellular system is given by 1/N, since each cell within a cluster is only assigned 1/N of the total available channels in the system.
Due to the fact that the hexagonal geometry of Figure 3.1 has exactly six equidistant neighbors and that the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees, there are only certain cluster sizes and cell layouts which are possible [Mac79]. In order to tessellate—to connect without gaps between adjacent cells—the geometry of hexagons is such that the number of cells per cluster, N, can only have values which satisfy Equation (3.3).
N = i 2 + ij + j 2
where i and j are non-negative integers. To find the nearest co-channel neighbors of a particular cell, one must do the following: (1) move i cells along any chain of hexagons and then (2) turn 60 degrees counter-clockwise and move j cells. This is illustrated in Figure 3.2 for i = 3 and j =2 (example, N = 19).
Total bandwidth = 33 * MHz
Channel bandwidth = 25 kHz 2 simplex channels = 50 kHz/duplex channel
Total available channels = 33,000/50 = 660 channels
(a) For N = 4,
total number of channels available per cell = 660/4 = 165 channels.
(b) For N = 7,
total number of channels available per cell = 660/7 = 95 channels.
(c) For N = 12,
total number of channels available per cell = 660/12 = 55 channels.
A 1 MHz spectrum for control channels implies that there are 1000/50 = 20 control channels out of the 660 channels available. To evenly distribute the control and voice channels, simply allocate the same number of voice channels in each cell wherever possible. Here, the 660 channels must be evenly distributed to each cell within the cluster. In practice, only the 640 voice channels would be allocated, since the control channels are allocated separately as 1 per cell.
(a) For N = 4, we can have five control channels and 160 voice channels per cell. In practice, however, each cell only needs a single control channel (the control channels have a greater reuse distance than the voice channels). Thus, one control channel and 160 voice channels would be assigned to each cell.
(b) For N = 7, four cells with three control channels and 92 voice channels, two cells with three control channels and 90 voice channels, and one cell with two control channels and 92 voice channels could be allocated. In practice, however, each cell would have one control channel, four cells would have 91 voice channels, and three cells would have 92 voice channels.
(c) For N = 12, we can have eight cells with two control channels and 53 voice channels, and four cells with one control channel and 54 voice channels each. In an actual system, each cell would have one control channel, eight cells would have 53 voice channels, and four cells would have 54 voice channels.
In a fixed channel assignment strategy, each cell is allocated a predetermined set of voice channels. Any call attempt within the cell can only be served by the unused channels in that particular cell. If all the channels in that cell are occupied, the call is blocked and the subscriber does not receive service. Several variations of the fixed assignment strategy exist. In one approach, called the borrowing strategy, a cell is allowed to borrow channels from a neighboring cell if all of its own channels are already occupied. The mobile switching center (MSC) supervises such borrowing procedures and ensures that the borrowing of a channel does not disrupt or interfere with any of the calls in progress in the donor cell.
In a dynamic channel assignment strategy, voice channels are not allocated to different cells permanently. Instead, each time a call request is made, the serving base station requests a channel from the MSC. The switch then allocates a channel to the requested cell following an algorithm that takes into account the likelihood of future blocking within the cell, the frequency of use of the candidate channel, the reuse distance of the channel, and other cost functions.
Accordingly, the MSC only allocates a given frequency if that frequency is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference. Dynamic channel assignment reduce the likelihood of blocking, which increases the trunking capacity of the system, since all the available channels in a market are accessible to all of the cells. Dynamic channel assignment strategies require the MSC to collect real-time data on channel occupancy, traffic distribution, and radio signal strength indications (RSSI) of all channels on a continuous basis. This increases the storage and computational load on the system but provides the advantage of increased channel utilization and decreased probability of a blocked call....
|1||Introduction to Wireless Communication Systems||1|
|2||The Cellular Concept - System Design Fundamentals||25|
|3||Mobile Radio Propagation: Large-Scale Path Loss||69|
|4||Mobile Radio Propagation: Small-Scale Fading and Multipath||139|
|5||Modulation Techniques for Mobile Radio||197|
|6||Equalization, Diversity, and Channel Coding||299|
|8||Multiple Access Techniques for Wireless Communications||395|
|10||Wireless Systems and Standards||483|
|App. A Trunking Theory||555|
|App. B Noise Figure Calculations for Link Budgets||565|
|App. C Gaussian Approximations for Spread Spectrum CDMA||569|
|App. D Q, erf & erfc Functions||593|
|App. E Mathematical Tables||599|
|App. F Abbreviations and Acronyms||607|
|App. G References||617|
The ability to communicate with people on the move has evolved remarkably since Guglielmo Marconi first demonstrated radio's ability to provide continuous contact with ships sailing the English channel. That was in 1897, and since then new wireless communications methods and services have been enthusiastically adopted by people throughout the world. Particularly during the past ten years, the mobile radio communications industry has grown by orders of magnitude, fueled by digital and RIP circuit fabrication 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 deployment 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 to deal with the techno-political aspects of cellular radio and personal communications, techno-politics are a fundamental driver in the evolution of new technology and services, since radio spectrum usage is controlled by governments, not by service providers, equipment manufacturers, entrepreneurs, or researchers. Progressive involvement in technology development is vital for a government if it hopes to keep its own country competitive in the rapidly changing field of wireless personal communications.
Wireless communications is enjoying its fastest growth period in history, due to enabling technologies which permit wide spread deployment. Historically, growth in the mobile communications field has come slowly, and has been coupled closely to technological improvements. The ability to provide wireless communications to an entire population was not even conceived until Bell Laboratories developed the cellular concept in the 1960s and 1970s INob62J, [Mac79J, [You79]. With the development of highly reliable, miniature, solid-state radio frequency hardware in the 1970s, the wireless communications era was born. The recent exponential growth in cellular radio and personal communication systems throughout the world is directly attributable to new technologies of the 1970s, which are mature today. The future growth of consumer-based mobile and portable communication systems will be tied more closely to radio spectrum allocations and regulatory decisions which affect or support new or extended services, as well as to consumer needs and technology advances in the signal processing, access, and network areas.
The following market penetration data show how wireless communications in the consumer sector has grown in popularity. Figure 1.1 illustrates how mobile telephony has penetrated our daily lives compared with other popular inventions of the 20th century. Figure 1.1 is a bit misleading since the curve labeled "mobile telephone" does not include nontelephone mobile radio applications, such as paging, amateur radio, dispatch, citizens band (CB), public service, cordless phones, or terrestrial microwave radio systems. In fact, in late 1990, licensed noncellular radio systems in the U.S. had over 12 million users, more than twice the U.S. cellular user population at that time LFCC91]. Figure 1.1 shows that the first 35 years of mobile telephone saw little market penetration due to high cost and the technological challenges involved, but how, in the past decade, cellular telephone has been accepted by consumers at rates comparable to the television, and the video cassette recorder.
In 1934, 194 municipal police radio systems and 58 state police stations had adopted amplitude modulation (AM) mobile communication systems for public safety in the US. It was estimated that 5000 radios were installed in mobiles in the mid 1930s, and vehicle ignition noise was a major problem for these early mobile users [Nob62]. In 1935, Edwin Armstrong demonstrated frequency modulation (FM) for the first time, and since the late 1930s, FM has been the primary modulation technique used for mobile communication systems throughout the world. World War II accelerated the improvements of the world's manufacturing and miniaturization capabilities, and these capabilities were put to use in large one-way and two-way consumer radio and television systems following the war. The number of U.S. mobile users climbed from several thousand in 1940 to 86,000 by 1948, 695,000 by 1958, and about 1.4 million users in 1962 [Nob621. The vast majority of mobile users in the 1960s were not connected to the public switched telephone network (PSTN), and thus were not able to directly dial telephone 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. population. Research in 1991 estimated between 25 and 40 million cordless telephones were in use in the U.S., and by the turn of the century this is certain to double [Rap9lcl. 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 conventional wireline customers throughout the world!
1.2 Mobile Radiotelephone in the U.S.
In 1946, the first public mobile telephone service was introduced in twentyfive 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 mode (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 of the difficulty 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 factor of 4 increase in spectrum efficiency due to technology advances from 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 I Ca1881. 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 twelve channels and could serve only 543 paying customers. There was a waiting list of over 3,700 people [Ca188], 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 telecommunications companies throughout the world developed the theory and techniques of cellular radiotelephony the concept of breaking a coverage zone (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 different channels in a region of the country, and then reallocates those same channels 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 allocated 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) [You791. According to FCC rules, each city (called a market) was only allowed to have two cellular radio system providers, thus providing a duopoly within each market which would assure some level of competition. As described in Chapters 2 and 10, the radio channels were split equally between the two carriers. AMPS was the first U.S. cellular telephone system, and was deployed in late 1983 by Ameritech in Chicago, IL [Bou91]. In 1989, the FCC granted an additional 166 channels (10 MHz) to U.S. cellular service providers to accommodate the rapid growth and demand. Figure 1.2 illustrates the spectrum currently allocated for U.S. cellular telephone use. Cellular radio systems operate in an interference-limited environment and rely on judicious frequency reuse plans (which are a function of the market-specific propagation characteristics) and frequency division multiple access (FDMA) to maximize capacity. These concepts will be covered in detail in subsequent chapters of this text.
In late 1991, the first U.S. Digital Cellular (USDC) system hardware was installed in major U.S. cities. The USDC standard (Electronic Industry Association Interim Standard IS-54) allows cellular operators to replace gracefully some single-user analog channels with digital channels which support three users in the same 30 kHz bandwidth [EIA90]. In this way, U.S. carriers can gradually phase out AMPS as more users accept digital phones. As discussed in Chapters 8 and 10, the capacity improvement offered by USDC is three times that of AMPS, because digital modulation (7L/4 differential quadrature phase shift keying), speech coding, and time division multiple access (TDMA) are used in place of analog FM and FDMA. Given the rate of digital signal processing advancements, 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 (IS-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 to operate with a much smaller signal-to-noise ratio than conventional narrowband FM techniques allows CDMA systems to use the same set of frequencies m every cell, which provides a large improvement in capacity [Gi191]. Unlike other digital cellular 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 developed 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-SMR) network in the 800 MHz band that could provide capacity and services similar to cellular. Using Motorola's integrated radio system (MIRS), SMR integrates voice dispatch, cellular phone service, messaging, and data transmission capabilities on the same network [Fi195].
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 on 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 differences 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) signaling at 512 bps, 1200 bps, and 2400 bps. New paging systems, such as FLEX and ERMES, provide up to 6400 bps transmissions by using 4-level modulation 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 antennas mounted on street lights or on sides of buildings. The CT2 system uses battery efficient frequency shift keying along with a 32 kbps adaptive differential pulse code modulation (ADPCM) speech coder for high quality voice transmission. 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 data and voice transmissions for office and business users. In the U.S., the PACS standard, developed by Bellcore and Motorola, is likely to be used inside office buildings as a wireless voice and data telephone system or radio local loop. The Personal Handyphone System (PHS) standard supports indoor and local loop applications in Japan. Local loop concepts are explained in Chapter 9.
The world's first cellular system was implemented by the Nippon Telephone and Telegraph company (NTT) in Japan. The system, deployed in 1979, uses 600 FM duplex channels (25 kHz for each one-way link) in the 800 MHz band. In Europe, the Nordic Mobile Telephone system (NMT 450) was developed in 1981 for the 450 MHz band and uses 25 kHz channels. The European Total Access Cellular System (ETACS) was deployed in 1985 and is virtually identical to the U.S. AMPS system, except that the smaller bandwidth channels result in a slight degradation of signal-to-noise ratio (SNR) and coverage range. In Germany, a cellular standard called C-450 was introduced in 1985. The first generation 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 Mobile) which was first deployed in 1990 in a new 900 MHz band which all of Europe dedicated for cellular telephone service [Ma1891. 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 world. 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 systems used in everyday life. Garage door openers, remote controllers for home entertainment equipment, cordless telephones, hand-held walkie-talkies, pagers (also called paging receivers or "beepers"), and cellular telephones are all examples of mobile radio communication systems. However, the cost, complexity, performance, 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 (e.g. a cellular telephone in a fast moving vehicle) whereas the term portable describes a radio terminal that can 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 a mobile or portable user because in most mobile communication 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 terminals. The mobiles communicate 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 elements of wireless communication systems.
Mobile radio transmission systems may be classified as simplex, halfduplex 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 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 simultaneous radio transmission and reception between a subscriber and a base station, by providing two simultaneous but separate channels (frequency division duplex, or FDD) or adjacent time slots on a single radio channel (time division duplex, or TDD) for communication to and from the user.
Frequency division duplexing (FDD) provides simultaneous radio transmission channels for the subscriber and the base station, so that they both may constantly transmit while simultaneously receiving signals from one another. At the base station, separate transmit and receive antennas are used to accommodate the two separate channels. At the subscriber unit, however, a single antenna is used for both transmission to and reception from the base station, and a device called a duplexer is used inside the subscriber unit to enable the same antenna to be used for simultaneous transmission and reception. To facilitate FDD, it is necessary to separate the transmit and receive frequencies by about 5% of the nominal RF frequency, so that the duplexer can provide sufficient isolation while being inexpensively manufactured.
In FDD, a pair of simplex channels with a fixed and known frequency separation is used to define a specific radio channel in the system. The channel used to convey traffic to the mobile user from a base station is called the forward channel, while the channel used to carry traffic from the mobile user to a base station is called the reverse channel. In the U.S. AMPS standard, the reverse channel has a frequency which is exactly 45 MHz lower than that of the forward channel. Full duplex mobile radio systems provide many of the capabilities of the standard telephone, with the added convenience of mobility. Full duplex and half-duplex systems use transceivers for radio communication. FDD is used exclusively in analog mobile radio systems and is described in more detail in Chapter $.
nine division duplexing (TDD) uses the fact that it is possible to share a single radio channel in time, so that a portion of the time is used to transmit from the base station to the mobile, and the remaining time is used to transmit from the mobile to the base station. If the data transmission rate in the channel is much greater than the end-user's data rate, it is possible to store information bursts and provide the appearance of full duplex operation to a user, even though there are hot two simultaneous radio transmissions at any instant of time. TDD is only possible with digital transmission formats and digital modulation, and is very sensitive to timing. It is for this reason that TDD has only recently been used, and only for indoor or small area wireless applications where the physical coverage distances (and thus the radio propagation time delay) are much smaller than the many kilometers used in conventional cellular telephone systems.
1.4.1 Paging Systems
Paging systems are communication systems that send brief messages to a subscriber. Depending on the type of service, the message may be either a numeric message, an alphanumeric message, or a voice message. Paging systems are typically used to notify a subscriber of the need to call a particular telephone number or travel to a known location to receive further instructions. In modern paging systems, news headlines, stock quotations, and faxes may be sent. A message is sent to 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). Simulcast 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 necessitates 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 station. Figure 1.3 shows a diagram of a wide area paging system.
1.4.2 Cordless 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 telephone 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. Modern cordless telephones are sometimes 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 travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters. Figure 1.4 illustrates a cordless telephone system.
1.4.3 Cellular Telephone Systems
A cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system. Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often comparable to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another.
Figure 1.5 shows a basic cellular system which consists of mobile stations, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO), since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile communicates via radio with one of the base stations and may be handedoff to any number of base stations throughout the duration of a call. The mobile station contains a transceiver, an antenna, and control circuitry, and may be mounted in a vehicle or used as a portable hand-held unit. The base stations consist of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC. The MSC coordinates the activities of all of the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions, as well. In large cities, several MSCs are used by a single carrier.
Communication between the base station and the mobiles is defined by a standard common air interface (CAD that specifies four different channels. The channels used for voice transmission from the base station to mobiles are called forward voice channels (FVC) and the channels used for voice transmission from mobiles to the base station are called reverse voice channels (RVC). The two channels responsible for initiating mobile calls are the forward control channels (FCC) and reverse control channels (RCC). Control channels are often called setup channels because they are only involved in setting up a call and moving it to an unused voice channel. Control channels transmit and receive data messages that carry call initiation and service requests, and are monitored by mobiles when they do not have a call in progress. Forward control channels also serve as beacons which continually broadcast all of the traffic requests for all mobiles in the system. As described in Chapter 10, supervisory and data mes18.104.22.168 How a Cellular Telephone Call is Made
When a cellular phone is turned on, but is not yet engaged in a call, it first scans the group of forward control channels to determine the one with the strongest 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 number 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 cellular system. The mobile identification number (MIN), which is the subscriber's telephone number, is then broadcast as a paging message over all of the forward control channels throughout the cellular system. 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 acknowledgment 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 cell's 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 occur 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 telephone 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 validates 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.
All cellular systems provide a service called roaming. This allows subscribers to operate in service areas other than the one from which service is subscribed. 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 accomplished over the FCC, since each roamer is camped on to a FCC at all times. Every several minutes, the MSC issues a global command over each FCC in the system, asking for all mobiles which are previously unregistered to report their MIN and ESN over the RCC. New unregistered mobiles in the system periodically report back their subscriber information upon receiving the registration request, and the MSC then uses the MIN/ESN data to request billing status from the home location register (HLR) for each roaming mobile. If a particular roamer has roaming authorization for billing purposes, the MSC registers the subscriber as a valid roamer. Once registered, roaming mobiles are allowed to receive and place calls from that area, and billing is routed automatically to the subscriber's home service provider. The networking concepts used to implement roaming are covered in Chapter 9.
1.4.4 Comparison of Common Mobile Radio Systems
Table 1.5 and Table 1.6 illustrate the types of service, level of infrastructure, cost, and complexity required for the subscriber segment and base station segment of each of the five mobile or portable radio systems discussed earlier in this chapter. For comparison purposes, common household wireless remote devices are shown in the table. It is important to note that each of the five mobile radio systems given in Table 1.5 and Table 1.6 use a fixed base station, and for good reason. Virtually all mobile radio communication systems strive to connect a moving terminal to a fixed distribution system of some sort and attempt to look invisible to the distribution system. For example, the receiver in the garage door opener converts the received signal into a simple binary signal which is sent to the switching center of the garage motor. Cordless telephones use fixed base stations so they may be plugged into the telephone line supplied by the phone company - the radio link between the cordless phone base station and the portable handset is designed to behave identically to the coiled cord connecting a traditional wired telephone handset to the telephone carriage.
Notice that the expectations vary widely among the services, and the infrastructure costs are dependent upon the required coverage area. For the case of low power, hand-held cellular phones, a large number of base stations are required to insure that any phone is in close range to a base station within a city. If base stations were not within close range, a great deal of transmitter power would be required of the phone, thus limiting the battery life and rendering the service useless for hand-held users.
Because of the extensive telecommunications infrastructure of copper wires, microwave line-of-sight links, and fiber optic cables - all of which are fixed - it is highly likely that future land-based mobile communication systems will continue to rely on fixed base stations which are connected to some type of fixed distribution system. However, emerging mobile satellite networks will require orbiting base stations.
1.5 Trends in Cellular Radio and Personal Communications
Since 1989, there has been enormous activity throughout the world to develop personal wireless systems that combine the network intelligence of today's PSTN with modern digital signal processing and FR technology. The concept, called Personal Communication Services (PCS), originated in the United Kingdom when three companies were given spectrum in the 1800 MHz to develop Personal Communication Networks (PCN) throughout Great Britain [Rap91c]. PCN was seen by the U.K. as a means of improving its international competitiveness in the wireless field while developing new wireless systems and services for citizens. Presently, field trials are being conducted throughout the world to determine the suitability of various modulation, multiple-access, and networking techniques for future PCN and PCS systems.
The terms PCN and PCS are often used interchangeably. PCN refers to a wireless networking concept where any user can make or receive calls, no matter where they are, using a light-weight, personalized communicator. PCS refers to new wireless systems that incorporate more network features and are more personalized than existing cellular radio systems, but which do not embody all of the concepts of an ideal PCN.
Indoor wireless networking products are steadily emerging and promise to become a major part of the telecommunications infrastructure within the next decade. An international standards body, IEEE 802.11, is developing standards for wireless access between computers inside buildings. The European Telecommunications Standard Institute (ETSI) is also developing the 20 Mbps HIPERLAN standard for indoor wireless networks. Recent products such as Motorola's 18 GHz Altair WIN (wireless information network) modem and AT&T's (formerly NCR) waveLAN computer modem have been available as wireless ethernet connections since 1990 and are beginning to penetrate the business world [Tuc93J. Before the end of the 20th century, products will allow users to link their phone with their computer within an office environment, as well as in a public setting, such as an airport or train station.
A worldwide standard, the Future Public Land Mobile Telephone System (FPLMTS) renamed International Mobile Telecommunication 2000 (IMT-2000) in mid-1995 - is being formulated by the International Telecommunications Union (ITU) which is the standards body for the United Nations, with headquarters in Geneva, Switzerland. The technical group TG 8/1 standards task group is within the ITU's Radiocommunications Sector (ITU-R). ITU-R was formerly known as the Consultative Committee for International Radiocommunications (CCIR). TG 8/1 is considering how future PCNs should evolve and how worldwide frequency coordination might be implemented to allow subscriber units to work anywhere in the world. FPLMTS (now IMT-2000) is a third generation universal, mufti-function, globally compatible digital mobile radio system that would integrate paging, cordless, and cellular systems, as well as low earth orbit (LEO) satellites, into one universal mobile system. A total of 230 MHz in frequency bands 1885 MHz to 2025 MHz and 2110 MHz to 2200 MHz has been targeted by the ITU's 1992 World Administrative Radio Conference (WARC). The types of modulation, speech coding, and multiple access schemes to be used in IMT-2000 are yet to be decided.
Worldwide standards are also required for emerging LEO satellite communication systems that are in the design and prototyping stage. Due to the very large areas on earth which are illuminated by satellite transmitters, satellitebased cellular systems will never approach the capacities provided by land-based microcellular systems. However, satellite mobile systems offer tremendous promise for paging, data collection, and emergency communications, as well as for global roaming before IMT-2000 is deployed. In early 1990, the aerospace industry demonstrated the first successful launch of a small satellite on a rocket from a jet aircraft. This launch technique is more than an order of magnitude less expensive than conventional ground-based launches and can be deployed quickly, suggesting that a network of LEOs could be rapidly deployed for wireless communications around the globe. Already, several companies have proposed systems and service concepts for worldwide paging, cellular telephone, and emergency navigation and notification [IEE91 1.
In emerging nations, where existing telephone service is almost nonexistent, fixed cellular telephone systems are being installed at a rapid rate. This is due to the fact that developing nations are finding it is quicker and more affordable to install cellular telephone systems for fixed home use, rather than install wires in neighborhoods which have not yet received telephone connections to the PSTN.
The world is now in the early stages of a major telecommunications revolution that will provide ubiquitous communication access to citizens, wherever they are [Kuc91], [Goo91], [ITU94]. This new field requires engineers who can design and develop new wireless systems, make meaningful comparisons of competing systems, and understand the engineering trade-offs that must be made in any system. Such understanding can only be achieved by mastering the fundamental technical concepts of wireless personal communications. These concepts are the subject of the remaining chapters of this text.
The purpose of this text is to initiate the newcomer to cellular radio and wireless personal communications, one of the fastest growing fields in the engineering world. Technical concepts which are at the core of design, implementation, research, and invention of wireless communication systems are presented in an order that is conducive to understanding general concepts, as well as those specific to particular cellular and personal communication systems and standards. This text is based upon my experiences as an educator, researcher, and consultant, and is modeled from an academic course developed for electrical engineering students as well as a self-study course for practicing engineers and technicians, developed at the request of the Institute of Electrical and Electronics Engineers (IEEE). References to journal articles are used liberally throughout this text to enable the interested reader to delve into additional reading that is always required to master any field. However, for handbook or classroom use, or for those who find it difficult to pursue outside reading, this text has been written as a complete, self-contained teaching and reference book. Numerous examples and problems have been provided to help the reader solidify the material.This book has been designed for the student or practicing engineer who is already familiar with technical concepts such as probability, communication theory, and basic electromagnetics. However, like the wireless communications industry itself, this book combines material from many different technical disciplines, so it is unlikely that any one person will have had introductory courses for all of thetopicscovered. To accommodate a wide range of backgrounds, important concepts throughout the text are developed from first principles, so that readers learn the foundations of wireless communications. This approach makes it possible to use this book as a handbook within industry, or as a teaching tool in a classroom setting.
The material and chapter sequence in this text have been adapted from an entry-level graduate course which I first taught in 1991 at the Virginia Polytechnic Institute and State University. Chapter 1 demonstrates the rapid growth of cellular radio throughout the world and provides a glimpse into the future. Chapter 2 covers cellular radio concepts such as frequency reuse and handoff, which are at the core of providing wireless communication service to subscribers on the move using limited radio spectrum. Chapter 2 also demonstrates how interference between mobiles and base stations affects the capacity of cellular systems. Chapter 3 presents radio propagation path loss and log-normal shadowing and describes different ways to model and predict the large-scale effects of radio propagation in many operating environments. Chapter 4 covers small-scale propagation effects such as fading, time delay spread, and Doppler spread, and describes how to measure and model the impact that signal bandwidth and motion have on the instantaneous received signal through the multipath channel. Radio wave propagation has historically been the most difficult problem to analyze and design for, since unlike a wired communication system which has a constant, stationary transmission channel (i.e., a wired path), radio channels are random and undergo shadowing and multipath fading, particularly when one of the terminals is in motion.
Chapter 5 provides extensive coverage of the most common analog and digital modulation techniques used in mobile communications and demonstrates trade-offs that must be made in selecting a modulation method. Issues such as receiver complexity, modulation and demodulation implementation, bit error rate analysis for fading channels, and spectral occupancy are presented. Channel coding, adaptive equalization, and antenna diversity concepts are presented in Chapter 6. In portable radio systems where people communicate while walking or driving, these methods may be used individually or in tandem to improve the quality (that is, reduce the bit error rate) of digital mobile radio communications in the presence of fading and noise.
Chapter 7 provides an introduction to speech coding. In the past decade there has been remarkable progress in decreasing the needed data rate of high quality digitized speech, which enables wireless system designers to match end-user services to network architectures. Principles which have driven the development of adaptive pulse code modulation and linear predictive coding techniques are presented, and how these techniques are used to evaluate speech quality in existing and proposed cellular, cordless, and personal communication systems are discussed. Chapter 8 introduces time, frequency, and code division multiple access, as well as more recent multiple access techniques such as packet reservation and space division multiple access. Chapter 8 also describes how each access method can accommodate a large number of mobile users and demonstrates how multiple access impacts capacity and the network infrastructure of a cellular system. Chapter 9 describes networking considerations for wide area wireless communication systems, and presents practical networking approaches that are in use or have been proposed for future wireless systems. Chapter 10 unites all of the material from the first nine chapters by describing and comparing the major existing and proposed cellular, cordless, and personal communication systems throughout the world. The trade-offs made in the design and implementation of wireless personal communications systems are illuminated in this final chapter. The compilation of the major wireless standards makes Chapter 10 particularly useful as a single source of information for a wide range of systems.Appendices which cover trunking theory, noise calculations, and the Gaussian approximation for spread spectrum code division systems provide details for those interested in solving practical wireless communications problems.
For industry use, Chapters 1-4 and 8 will benefit working engineers in the cellular system design and radio frequency (RF) testing/maintenance/measurement areas. Chapters 5-7 are tailored for modem designers and digital signal processing (DSP) engineers new to wireless. Chapters 9 and 10 should have broad appeal to network operators and managers, as well as working engineers.To use this text at the undergraduate level, the instructor may wish to concentrate on Chapters 1-5, or Chapters 1-4, and 8, leaving the other chapters for treatment in a second semester undergraduate course or a graduate level course. Alternatively, traditional undergraduate courses on communications or network theory may find in Chapters 1, 2, 3, 5, 7, 8, and 9 useful material that can be inserted easily into the standard curriculum. In using this text at the graduate level, I have been successful in covering most of the material in Chapters 1-8 during a standard half-year semester. In Chapters 9 and 10, I have attempted to cover important but rarely compiled information on practical network implementations and worldwide standards.
Without the help and ingenuity of several former Virginia Tech graduate students, this text could not have been written. I am pleased to acknowledge the help and encouragement of Rias Muhamed, Varun Kapoor, Kevin Saldanha, and Anil Doradla - students I met in class while teaching the course Cellular Radio and Personal Communications. Kevin Saldanha also provided camera ready copy for this text (which turned out to be no small task!). The assistance of these students in compiling and editing materials for several chapters of this text was invaluable, and they were a source of constant encouragement throughout the project. Others who offered helpful suggestions, and whose research efforts are reflected in portions of this text, include Scott Seidel, Joe Liberti, Dwayne Hawbaker, Marty Feuerstein, Yingie Li, Ken Blackard, Victor Fung, Weifang Huang, Prabhakar Koushik, Orlando Landron, Francis Dominique, and Greg Bump. Zhigang Rong, Jeff Laster, Michael Buehrer, Keith Brafford, and Sandip Sandhu also provided useful suggestions and helpful reviews of early drafts.This text benefits greatly from practical input provided by several industry reviewers. Roman Zaputowycz of Bell Atlantic Mobile Systems, Mike Bamburak of McCaw Communications, David McKay of Allen Telecom Group, Jihad Hermes of Cellular One, Robert Rowe of AirTouch Communications, William Gardner of Qualcomm, and John Snapp of Blue Ridge Cellular provided extremely valuable input as to what materials were most important, and how they could best be presented for students and practicing engineers. Marty Feuerstein of U.S. West NewVector and Mike Lord of Cellular One provided comprehensive reviews which have greatly improved the manuscript. The technical staff at Grayson Electronics also provided feedback and practical suggestions during the development of this text.From the academic perspective, a number of faculty in the wireless communications field provided useful suggestions which I readily incorporated. These reviewers include Prof. J. Keith Townsend of North Carolina State University and Prof. William H. Tranter of the University of Missouri-Rolla. Professors Jeffrey Reed and Brian Woerner of Virginia Tech also provided excellent recommendations from a teaching perspective. I am grateful for the invaluable contributions from all of these individuals.
I am pleased to acknowledge the support of the National Science Foundation, the Advanced Research Project Agency, and the many sponsors and friends of the Mobile & Portable Radio Research Group, who have supported our research and educational activities in wireless communications since 1990. It is from the excellent faculty at Purdue University, particularly my advisor, Clare D. McGillem, that I formally learned about communications and how to build a research program. I consider myself fortunate to have been one of the many graduate students who was stimulated to pursue a dual career in engineering and education upon graduation from Purdue.Finally, it is a pleasure to acknowledge my family and students, who put up with my preoccupation on this project, Barbara Coburn and Jill Cals of the IEEE, who championed the IEEE self-study course on the same subject, and Karen Gettman and Camille Trentacoste of Prentice Hall, who commissioned this work and helped me bring this text to you.
—Theodore S. Rappaport