Introduction to Wireless Systems--A Tutorial--Part I

• 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 ƒ_sl to ƒ_su, where ƒ_sl and ƒ_su are the lower and upper limits, respectively, of the operating frequency range. Let the system bandwidth be denoted B_sys, where B_sys ý½ý' ƒ_su ýý'ƒ_sl. 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 ƒ_sl to ƒ_su is divided into N_chan subbands or "channels," each of bandwidth B_chan. We understand B_chan 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 N_chan simultaneous radio connections can be supported in the given spectrum B_sys. For systems of interest to us B_chan is much smaller than B_sys, so there are many available channels.

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