Systems Engineering and the Role of the Systems Engineer
Wireless communications, and communications in general, are specializations in the discipline of systems engineering. Our approach to the study of wireless communications from the perspective of a systems engineer is therefore a study in a specialized field of systems engineering. It is fitting, then, that we begin our study by discussing systems engineering at a general level.
Some type of system supports nearly every aspect of our daily life. Systems help us to travel anywhere on the Earth (and beyond), create memos, solve complex problems, store and retrieve vast arrays of information, cook our food, heat and light our homes, entertain ourselves and our friends, and of course communicate with each other. There is no universally accepted standard definition of a "system," but the purposes of this discussion are served by the working definition: A system is any collection of elements or subsystems that operate interdependently to perform some specified function or functions.
An automobile, an airplane, a personal computer, a home, a television or radio, an ordinary telephone, and a microwave oven are all common examples of systems. But at a lower level, an automobile engine or transmission, an airplane's hydraulic system, a computer chip or microprocessor, a home air-conditioning or heating unit, or the video circuitry of a television or audio circuitry of a radio are also systems by definition. Depending on the context of discussion, a system may often be referred to as a subsystem, since it may perform only a few of the intended or auxiliary functions of the overall system of which it is a part. For example, an automobile is a system that conveys passengers and/or objects to arbitrary geographic locations. Its subsystems are the engine, transmission, braking system, steering system, chassis, dashboard, and so on, all of which are necessary for an automobile to perform the functions we have come to expect. Likewise, an engine is a collection of other subsystems such as the ignition system, fuel system, and emission control system. At some level the terms system and subsystem become less relevant. For example, a passive circuit may be considered a system, but considering resistors, capacitors, and inductors as subsystems has little relevance. In such instances we may choose to use the term component or element.
In the previous sections we introduced a simplified block diagram for a generic wireless system. Each block may be considered a subsystem of the overall system. Furthermore, each block performs some direct or auxiliary function needed to meet the general requirements of the system application. Regardless of the intended wireless application, the designs of the various blocks and their functions are founded on principles derived from distinct specialty areas. To identify a few of these specialty areas:
- Antennas and electromagnetic wave propagation
- Microwave circuit theory and techniques
- Signals, systems, and signal processing
- Noise and random processes
- Statistical nature of the environment and its effects on a propagating signal
- Communication theory
- Traffic theory
- Switching and networking theory
Depending on complexity and scale, the design and development of a system usually require knowledge and professional expertise in a number of distinctly different disciplines. For example, the development of large-scale wireless systems, such as personal communication systems or advanced radar systems, requires the participation of specialists in such diverse disciplines as
- Antenna design
- RF propagation and radio environment modeling
- Microwave circuit design
- Transmitter design
- Low-noise amplifier (LNA) design
- Modulator/demodulator (modem) design
- Digital circuit/system design
- Signal processing
- Real-time, non-real-time, and embedded software development
- Power systems and power management
- Switching, networking, and transmission
- Mechanical structures and packaging
- Human factors engineering
- Manufacturing engineering
- Reliability and quality engineering
- And, last but not least, systems engineering
Successful development companies usually have processes (sequences of well-defined steps and procedures) and information systems that allow development teams to work and communicate effectively; track progress; manage schedule, budget, and other resources; control changes; and maintain and improve product quality. Highly successful companies also review their processes, constantly seeking ways to reduce development costs and schedule time while improving product quality, customer satisfaction, and cost competitiveness. In fact, process engineering and improvement is an important area of specialization. A strong and continuously improving development process is often vital to a company's ability to compete in a given market.
Development processes may vary among companies, but they all possess common phases of particular emphasis, for example,
- Product/system definition
- Integration and system test
- Product life-cycle management
The specific activities in each phase may vary significantly, and many of the phases may, and often do, run concurrently.
One of the most important factors contributing to the successful development of a system is a complete, well-directed, and stable product definition. The product definition, sometimes called "functional product requirements" (FPR), is usually developed by a marketing or market research organization in concert with members of the technical development community, especially systems engineering. In addition to specifying the required attributes of the system from a customer perspective, an FPR also defines all the information necessary to ensure a viable financial return for the investors, including cost to manufacture the product, time to market, development budget, projected manufacturing ramp-up and life-cycle volumes, key competitive attributes, and so forth.
The design and development phase of any system usually begins with a system design. It is one of the most important products of a systems-engineering effort. A system design consists of all the requirements, specifications, algorithms, and parameters that a development team uses to design and develop the hardware and software necessary to implement and manufacture a product in accordance with an agreed-upon product definition. System-level documentation may include
- System-level requirements—a high-level technical document that translates the needs expressed from a customer perspective into technical constraints on system functions, performance, testing, and manufacture
- System architecture—a specification that defines all of the parameters and subsystem functions necessary to ensure interoperability among subsystems and meet system requirements, including distribution of system-level functions among the subsystems, definition of subsystem interfaces, and specification of system-level controls
- Supporting analyses—appropriate documentation of all analyses, simulations, experimentation, trade-off studies, and so on, that support the choice of key technical parameters and predict and/or verify system-level performance
As it relates to a system design, the responsibilities of a systems engineer are to
- Translate customer-level functional requirements into technical specifications at a system level
- Develop a system architecture and determine specific parameters to ensure that the system will meet the desired level of functionality and performance within specified constraints
- Perform trade-off analyses among the system elements to ensure that the implementation requirements can be met within the specified constraints and technology limitations
- Develop and negotiate specific requirements for each of the subsystems based on analysis, modeling, experimentation, and simulation
These functions are the focus of this text and are the basis for many other functions that systems engineers perform. These other functions might include
- Interacting with potential customers
- Developing human-interface specifications
- Developing plans, methods, and criteria for system integration and verification
- Interfacing with government and legal entities
- Specifying deployment, maintenance, and operations procedures
- Competitive analysis
- Supporting regulatory and standards development
Depending on the complexity of the system being developed, a team of systems engineers, each of whom has a particular area of expertise, may be required to fully perform all of the systems-engineering functions of a development.
This text develops the major systems aspects of personal communication systems while demonstrating the application of relevant theory and principles and introducing students to some of the real-world aspects of the wireless systems-engineering profession. It is fitting, therefore, that the subject matter be presented in the context of a solution to a general systems-engineering problem. To be specific, we seek to design a wireless telecommunication system that will
- Support the communication of information of various types, including speech, text, data, images, and video, in urban, suburban, and rural environments and 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, in fact, may be moving at vehicular speeds
Many of the attributes of this system, as stated previously, were in fact the major objectives underlying the development of the very first cellular mobile telephone systems. Our discussions of principles and concepts are presented as motivation for solving this systems-engineering problem in particular. In view of the continued advances in digital technologies and the directions of modern communication systems, our emphasis will be on digital wireless communications, although many of the principles apply to both analog and digital systems.
Since the advent of the first mobile phone systems, the meanings of some commonly used terms have become blurred by marketing and advertising efforts to provide some level of distinction between early first and later generations of systems. Specifically, the terms cellular and PCS are often used to identify the cellular frequency (850 MHz) or personal communication systems (or services) (1.9 GHz) frequency bands. The term cellular, however, originally referred to the technology underlying nearly all of the systems that may be classified as personal communication systems. We will endeavor to ensure that the meaning is always clear in context; however, it is important to recognize that most modern systems are capable of operating in either band.