The previous two chapters presented the fundamental concepts underlying the behavior of radio frequency propagation in a real-world environment. With these principles as a foundation, we return to the development of a solution to the general systems-engineering challenge posed in Chapter 1, that is, to design a wireless telecommunication system that will:
- Support the communication of information from various sources, including speech, text, data, images, music, and video, in urban, suburban, and rural environments with quality approximating that of wired communications
- Be capable of expanding in geographic coverage
- Allow for virtually limitless growth in the number of users
- Support endpoints that are not geographically fixed and that may in fact be moving at vehicular speeds
The challenge as we have stated it is broad and devoid of any quantitative specification of the system's attributes. Nevertheless, the problem statement is not unlike one that a company might pose in the initial stages of developing a product or service.
Product or service development requires the investment of money, physical resources, and people. Sound business practice requires a clear assessment of the benefits to be gained by making the investment. The financial “return on investment” is often used to determine the viability of an investment in a new product or service. Based on an assessment of the return on investment, an enterprise (or, more accurately, the investors in the enterprise) can make informed decisions about the efficacy of proceeding with a new venture. This assessment may include quantifying the market opportunity in terms of the likely number of subscribers and the projected rate of growth in that number, the subscribers' willingness to pay for the service, the projected rate of growth in that number, the subscribers' willingness to pay for the service, the time to market (how long it will take to develop the service or product), the estimated cost of development, the projected profit margin, the product life cycle, and the competitive ability of companies offering similar products or services.
As discussed in Chapter 1, systems engineers play an important role, especially in the early process stages. As members of the product definition team, they represent the technical community, providing the insights necessary to ensure that the product concept is realistic and well defined. Systems engineers work with the business team to provide a complete, detailed, and quantitative definition of the product or service to be developed. This usually requires a strong interaction among the members of the technical community to ensure that the required technical competencies are available, the budget and schedule are realistic, and the technical risks are noted and realistically appraised.
Once a product or service has been defined in sufficient detail, key product requirements are assessed and analyzed to determine the high-level design or “system architecture” that best supports the project goals, within the constraints of resources, budget, and schedule. Simulations may be performed to estimate performance and the trade-off of performance against cost for various architectural alternatives under project constraints.
A system architecture may include a high-level technical description of the system and major subsystems along with their key functions and parameters. Development of a system architecture is often led by systems engineers working in collaboration with appropriate members of the technical team and the business team. In the next section we consider the overall system approach, or architecture, that might be used to implement the key characteristics of the system identified in our problem statement.
Requirements Assessment and System Architecture
As we proceed to investigate solutions to our stated problem, we will introduce some of the specific parameters and characteristics encountered in modern systems. We will also introduce realistic constraints as they are needed and when sufficient background has been presented to make them meaningful.
As a first consideration, we note that the allowable frequency range over which a proposed system may operate is usually fixed by government regulation and, in some cases, by international treaties. This implies that the operating frequency bands are finite and predetermined. In the United States, civilian communications policy is administered by the FCC, an agency created by Congress in 1934. Communications regulations are published in Volume 47 of the Code of Federal Regulations (CFR). The regulations pertaining to unlicensed radio services appear in Part 15 (designated 47CFR15). Each licensed radio service has its own part. Wireless services are assigned specific frequency bands to limit interference and ensure the reliability of communication services. Emission of radio frequency energy outside of the assigned frequency band must be limited in accordance with the rules governing the license. Licenses may also impose restrictions that vary by geographic area.
In most cases a wireless service is assigned a continuous range of frequencies. Let us designate this range as ƒsl to ƒsu , where ƒsl and ƒsu are the lower and upper limits, respectively, of the operating frequency range. Let the system bandwidth be denoted Bsys , where Bsys ?½?' ƒsu ??'ƒsl . The fraction of this bandwidth to be used by an individual subscriber depends on the information source (speech, music, data, video, etc.) and on the quality of service to be supported. We suppose that the band ƒsl to ƒsu is divided into
Nchan subbands or “channels,” each of bandwidth Bchan . We understand
Bchan to be the minimum bandwidth required to convey the required information in both directions at the required quality of service (QoS) between two endpoints. Let the center frequency of each channel be given by:
To simplify our discussions we also assume that each channel can support only one two-way radio link between a pair of endpoints at any instant of time. Therefore only Ncha n simultaneous radio connections can be supported in the given spectrum Bsys . For systems of interest to
us Bchan is much smaller than Bsys , so there are many available channels.
Radio spectrum is a limited resource. Like land, “they aren't making any more of it.”
Although new technology allows the use of ever higher frequencies, the general pattern is that spectrum can be assigned to a new radio service only if it is taken away from an existing radio service. In 1983 the FCC allocated 40 MHz of spectrum for a new cellular telephone system. Six years later an additional 10 MHz was added. The allocated spectrum is in the bands 824″849 MHz and 869″894 MHz. This spectrum
had previously been allocated to UHF television but became available when the spread of cable television reduced some of the demand for large numbers of broadcasting channels.
Each channel in the Advanced Mobile Phone Service (AMPS) consists of a 30 kHz “forward” link from the base station to the mobile unit and a 30 kHz “reverse” link from the mobile unit to the base station. All of the reverse links are contained in the band 824″849 MHz, and all of the forward links are in the band 869″894 MHz. In each two-way channel, the forward and reverse subchannels are separated by 45 MHz.
Systems that divide an allocated spectrum into channels that are to be shared among many users (as described previously) are classified as frequency-division multiple access (FDMA) or
frequency-division multiplexing (FDM) systems. There are other ways to allocate channels among many users. For example, the entire spectrum allocation may be dedicated to a single user for a short period of time, with multiple users taking turns on a round-robin basis. In a later chapter we will discuss this approach and other alternatives for implementing multiple access. Our intention in this chapter is to focus on the cellular concept, a concept that is perhaps most easily understood in the context of an FDMA system.
Given a system bandwidth Bsys and a channel bandwidth Bchan , the number of distinct channels available is:
It should be self-evident that the number of subscribers that a system can support is related to the number of available channels. The actual relationship between number of users and Nchan , however, is not so obvious. In a telephone system no user occupies a channel continuously, so a channel can support more than one user. In general, the number of users that can be supported is much greater than the number of channels. We will find that user behavior plays an important role in determining the ultimate capacity of an
Nchan -channel system. In the early stages of development of the public telephone system, it was recognized that a direct connection between every pair of subscribers was impractical and unnecessary. The concept of “traffic engineering”–that is, planning the allocation of resources to allow these resources to be effectively shared–is central to the design of multiple-user systems. We will discuss the basics of traffic engineering later in this chapter, at which time we will answer the question of how many subscribers
Nchan channels can support.
The simplest architecture for a wireless system might be to locate a single base station near the center of a coverage area and to adjust the transmitted power level and antenna height so that signal levels are sufficient to ensure the required quality of service within the coverage area. This is the common configuration for radio systems serving police and fire departments and for towtruck and taxicab companies. Early mobile telephone systems were also designed along these lines. The Improved Mobile Telephone Service (IMTS), introduced in the 1950s and 1960s, is an example. Unfortunately by 1976 the IMTS system in New York City, operating with 12 channels, was able to serve only about 500 paying customers. There was a waiting list of over 3700 people, and service was poor owing to the high blocking (system busy) rate.
Although the number of subscribers (and the quality of service) could be increased by adding channels, systems of this design are limited in coverage area by the maximum practical transmitter power and the sensitivity of the receiver. This kind of system is referred to as being “noise limited.” The performance within a given coverage area can be increased by adding repeater base stations, but doing so increases the cost of the service without increasing the number of customers who will share that cost. Increasing the coverage area may increase the number of potential customers, but without increasing the system's ability to provide them service. Without additional spectrum, adding more customers will degrade the system blocking rate. Clearly, neither of the goals of being capable of expanding in geographic coverage
or allowing for virtually limitless growth in the number of users can be met with this approach.
Since spectrum is limited, the most promising avenue for increasing the number of users is to utilize the available spectrum more efficiently. Specifically, we wish to increase the number of subscribers who can simultaneously use a given channel within the specified coverage area. Normally, allowing multiple users to transmit on the same channel at the same time increases interference. The key, then, is to find ways to reduce, or eliminate, this so-called cochannel interference. One approach might be to geographically isolate the users of a given channel by physically separating them. This is the basic concept underlying development of the first cellular systems.
Suppose instead of a single high-powered, centrally located base station we envision many
low-powered base stations, each serving only a portion of the desired coverage area. The smaller
coverage area served by a single base station is called a cell . Since the area of a cell is small, each cell will contain only a limited number of subscribers. We can therefore distribute the available channels among the cells so that each cell has only a subset of the complete set of channels. Now if two cells are far enough apart, they can be assigned the same group of channels. This allows every channel to be reused, possibly many times, throughout the system's coverage area. If the same channel supports two different customers at the same time in different parts of the system, however, there is a potential for cochannel interference. We shall see that the system's ability to withstand cochannel interference and the size of the cells are the limiting factors in determining the number of subscribers that can be supported. A system with this configuration is said to be “interference limited.” An important consequence of dividing the service area into cells is the need to transfer a call from one base station to another as a mobile unit moves through the service area. This transfer is called a “handoff” or “handover” and will be discussed in a subsequent section of this chapter. In the next section we will describe the cellular concept in some detail and demonstrate its ability to provide substantial, if not limitless, growth in both coverage and user capacity.
Next: Cellular Concepts
About the Authors
Bruce A. Black completed his B.S. at Columbia University, his S.M. at Massachusetts Institute of Technology, and his Ph.D. at the University of California at Berkeley, all in electrical engineering. Since 1983 he has been on the faculty of the Department of Electrical and Computer Engineering at Rose-Hulman Institute of Technology in Terre Haute, Indiana, where he has been advisor to Tau Beta Pi and is advisor to the Amateur Radio club (W9NAA). In 2004 he was named Wireless Educator of the Year by the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Philip S. DiPiazza received a B.E.E from Manhattan College in 1964, an M.E. in electrical engineering from New York University in 1965, and a Ph.D. (electrical engineering) from the Polytechnic Institute of New York in 1976. Dr. DiPiazza was responsible for the system integration and test of the first North American deployment of AMPS.. He is currently an Adjunct Professor at the Rose-Hulman Institute of Technology and a Senior Consultant with Award Solutions, Inc. Dr. DiPiazza is an advisor and member of the Global Wireless Educational Consortium and a member of the IEEE.
Bruce A. Ferguson received the B.S., M.S., and the Ph.D. degree in electrical engineering from Purdue University, West Lafayette, Indiana in 1987, 1988, and 1992 respectively. He is currently a Communication System Engineer with Northrop Grumman Space Technology. He has worked with space and ground communication systems and photonics at TRW Space and Electronics (now NGST), and taught at Rose-Hulman Institute of Technology and The University of Portland in Oregon. Dr. Ferguson is a member Eta Kappa Nu and IEEE.
David R. Voltmer received degrees from Iowa State University (B.S.), University of Southern California (M.S.), and The Ohio State University (Ph.D.), all in electrical engineering. During nearly four decades of teaching, Dr. Voltmer has maintained a technical focus in electromagnetics, microwaves, and antennas. His more recent efforts are directed toward the design process and project courses. He has served in many offices of the ERM division of ASEE and in FIE. Dr. Voltmer is an ASEE Fellow and a Life Senior member of IEEE.
Frederick C. Berry received the B.S., M.S., and D.E. degrees from Louisiana Tech University in 1981, 1983, and 1988 respectively. He taught in the Electrical Engineering Department at Louisiana Tech University from 1982 to 1995. Currently Dr. Berry is Professor and Head of the Electrical and Computer Engineering Department at Rose-Hulman Institute of Technology. In 2007 he became Executive Director of the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Title: Introduction to Wireless Systems ISBN: 0132447894 Chapter 4: Radio Frequency Coverage: Sysetms Engineering and Design
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