Introduction to Wireless Systems
On the night of April 14, 1912, the RMS Titanic, en route from Southampton, England, to New York, struck an iceberg and sank in the North Atlantic. Over fifteen hundred lives were lost when the ship went down, but fortunately for the more than seven hundred passengers and crew who were able to find accommodation in the ship's lifeboats, the Titanic was equipped with a wireless system. The Titanic's wireless included a 5 kW rotary spark transmitter built by the Marconi Wireless Company. Distress calls were heard by a number of ships at sea, including the Carpathia that arrived on the scene of the disaster several hours later, in time to rescue the survivors.
The wireless traffic between the Carpathia and shore stations in North America was widely monitored. News was passed to the press even before the fate of the Titanic's passengers was known. The widespread publicity given to this disaster galvanized public interest and propelled wireless communication into the forefront of attention. The age of wireless communication might be said to have begun with the sinking of the Titanic.
As social beings, humans have a fundamental need to communicate. As we grow and learn, so do our communication needs evolve. Dramatic advancements over the past century have made several facts about our evolving communication needs rather apparent: (1) The information that needs to be communicated varies widely; (2) the types and amount of information that needs to be communicated continuously change, typically toward higher complexity; and (3) current technology rarely meets communication demands, so technology evolves. These facts, along with a healthy worldwide economy, produced the wireless revolution in the late twentieth century. Wireless communication is here to stay, and the design principles used to create wireless technology differ enough from those used to create wired communication systems that a separate treatment is necessary.
In this text the process of designing a wireless communication system is presented from the perspective of a systems engineer. Two main goals of the text follow immediately: (1) to present the concepts and design processes involved in creating wireless communication systems, and (2) to introduce the process of systems engineering and the role of a systems engineer to provide an organizing framework under which to introduce the wireless system concepts. In the industrial world, the design process flows in an organized manner from problem definition, through conceptual and detailed design, to actual deployment. In this text, information from first principles to advanced topics is presented in a fashion compatible with systems-engineering design processes, which are required to manage the development of complex systems.
In Chapter 1 the problem of moving information wirelessly from any point A to any point B is introduced. In every engineering endeavor it is important to have a clear understanding of the problem to be solved before beginning, and so the system and its requirements are defined. The role of a systems engineer and the methods of systems engineering are introduced as the perspective for conducting our study and design.
Chapter 2 presents the most fundamental element of our wireless system, the radio link that connects points A and B. This chapter addresses two issues: how radio waves propagate in space, and how much power must be provided at point B to ensure a desirable quality of communication service. This chapter focuses on propagation in free space and the development of the range equation, a mathematical model familiar to both radio and radar engineers. We introduce the antenna as a system element and the antenna design engineer as a member of the design team. To answer the question of how much power is enough, we develop models and analysis tools for thermal noise and describe how thermal noise limits performance. Signal-to-noise ratio (SNR) is introduced as a measure of system performance. Finally, the concept of link budget, a fundamental tool for radio frequency (RF) systems engineering, is presented and supported with examples.
Chapter 3 focuses on signal propagation in the real world. Obstacles in the signal path and indeed the very presence of the Earth itself modify a signal as it travels between endpoints. Various terrestrial propagation models are presented and discussed. These models provide methods for predicting how a signal will propagate in various types of environments. The phenomena associated with shadow fading, Rayleigh fading, and multipath propagation are described as well as the effects of relative motion of a receiver and of objects in the environment. Statistical methods are developed that allow engineers to create robust designs in an unstable and changing environment. Receiver design and channel modeling are added to the list of design functions that a systems engineer must understand to competently interact with team members who specialize in these design disciplines.
Given a basic understanding of methods to ensure that an adequate signal can be conveyed between two endpoints, we discuss the concepts and complexities involved in allowing many users in a large area to share a common system. Geographic diversity and frequency reuse are discussed and used as the basis for developing the "cellular" concept in Chapter 4. The cellular concept is the fundamental basis for designing and deploying most wireless communication systems that must provide service for many users over a large geographic area. The chapter describes how engineers using cellular engineering techniques plan for growth in user capacity and coverage area. Traffic engineering and the use of the Erlang formula as tools for predicting and designing a system for user capacity are demonstrated. At this stage of the design, system-level concerns are well above the device or subsystem level.
In Chapter 5 we describe the methods used to convey information over the wireless link. The process for conveying information using a radio signal, called modulation, is described from a trade-off perspective. The characteristics of several digital modulation schemes are developed and their attributes are compared. The key design parameters of data throughput, error rate, bandwidth, and spectral efficiency are contrasted in the context of optimizing a system design. Also in this chapter we introduce spread-spectrum signaling. Spread spectrum is a modulation technique that broadens the bandwidth of the transmitted signal in a manner unrelated to the information to be transmitted. Spread-spectrum techniques are very effective in making signals resilient in the presence of interference and frequency-selective fading. Our study of spread-spectrum techniques continues in Chapter 6, as these techniques provide an important basis for multiple-access communications.
The first five chapters provide all of the fundamental elements of system design for providing radio coverage to many users over a large area and for designing the components that support the conveying of information at a given quality of service (QoS) across a wireless link between individual users. Chapter 6 introduces various methods that allow many users to access the system and to simultaneously use the resources it provides. In this chapter we introduce the classical methods of frequency-division and time-division multiple access, as well as spread-spectrum-based code-division multiple access which allows independent users to share the same bandwidth at the same time. In providing a multiple-access capability, a systems engineer unifies a variety of system-level design activities to make the system accessible to a varying number of users.
People wish to communicate different types of information, and the information they want to communicate comes from a variety of sources. Chapter 7 discusses several of the types and sources of information commonly communicated in contemporary wireless systems. The required QoS that is to be provided to a system's users must be accounted for in nearly every aspect of a system design. Users' perceptions of what constitutes good quality vary for different types and sources of information and always depend on how well a signal representing the information is preserved in the communication process. Chapter 7 discusses some of the fundamental relationships between the perceptual measures of QoS and specific system design parameters. Understanding these relationships allows a systems engineer to design for predictable QoS at minimum cost. As modern wireless systems are designed to carry information in digital form, a major part of this chapter is about efficient digitization of speech. We discuss two general categories of speech "coding": waveform coding and source coding. As an example of the waveform coding technique we examine traditional pulse code modulation (PCM). Our example of source coding is linear predictive coding (LPC). This latter technique has been extremely successful in providing high-quality, low-bit rate digitization of voice signals for cellular telephone applications. Following the discussion of speech coding, the chapter concludes with an example of coding for error control. Convolutional coding is used for this purpose in all of the digital cellular telephone systems. We introduce the coding method and the widely used Viterbi decoding algorithm.
Chapter 8 wraps up the presentation with a review of the lessons developed in the preceding chapters. This is followed by an overview of the generations of cellular telephone systems and a look into the future at the way wireless systems are evolving to provide an increasing array of services at ever-higher quality. As wireless systems evolve, they tend to become more complicated. Thus the role of the systems engineer in managing the design process and in understanding the myriad of design trade-offs and interactions becomes ever more important.