What Is a Wireless System?
In the most general sense, a wireless system is any collection of elements (or subsystems) that operate interdependently and use unguided electromagnetic-wave propagation to perform some specified function(s). Some examples of systems that fit this definition are
- Systems that convey information between two or more locations, such as personal communication systems (PCS), police and fire department radio systems, commercial broadcast systems, satellite broadcast systems, telemetry and remote monitoring systems
- Systems that sense the environment and/or objects in the environment, including radar systems that may be used for detecting the presence of objects in some region or volume of the environment and measuring their relative motion and/or position, systems for sensing or measuring atmospheric conditions, and systems for mapping the surface of the Earth or planets
- Systems that aid in navigation or determine the location of an object on the Earth or in space
Each of these systems contains at least one transmitting antenna and at least one receiving antenna. In the abstract, an antenna may be thought of as any device that converts a guided signal, such as a signal in an electrical circuit or transmission line, into an unguided signal propagating in space, or vice versa. We note in passing that some systems do not need to transmit and receive simultaneously. For example, the WiFi local area network computer interface uses a single antenna that is switched between transmitter and receiver. Specifically, a pulse of energy is transmitted, after which the antenna is switched to a receiver to detect the response from the network access point.
As the examples show, some systems may be used to convey information, whereas others may be used to extract information about the environment based on how the transmitted signal is modified as it traverses the path between transmitting and receiving antennas. In either case, the physical and electromagnetic environment in the neighborhood of the path may significantly modify the signal. We define a channel as the physical and electromagnetic environment surrounding and connecting the endpoints of the transmission path, that is, surrounding and connecting the system's transmitter and receiver. A channel may consist of wires, waveguide and coaxial cable, fiber, the Earth's atmosphere and surface, free space, and so on. When a wireless system is used to convey information between endpoints, the environment often corrupts the signal in an unpredictable1 way and impairs the system's ability to extract the transmitted information accurately at a receiving end. Therein lies a major difference between wired and wireless systems. To provide a little further insight, we compare some of these differences.
The signal environment or channel characteristics of a single-link wired system are rather benign.
- At any instant of time, the path between endpoints is well known and many of its degrading effects upon a signal can be measured and compensated for.
- Signal dropout (signal loss), momentary or otherwise, is very rare.
- Random effects such as "thermal noise" and "interference" are fairly predictable and controllable and therefore less likely to corrupt the signal to the extent of unintelligibility.
- The signal environment does not change or changes very slowly with time.
- The endpoints do not move.
In contrast, the signal environment of a wireless system is hostile.
- The direction of the signal cannot be completely controlled, and the path between endpoints is not unique.
- The path between endpoints is time-varying.
- Signal dropouts are frequent.
- Noise and interference levels are often difficult to predict and time-varying.
- Objects in the path between and surrounding the endpoints affect the signal level and its content.
- Variations in the signal environment change with geographic location, seasons, and weather.
- For mobile systems, as in cellular and PCS systems, at least one of the endpoints may be moving at an unknown and sometimes significant speed.
As an everyday example, the differences between wired and wireless systems may be compared to the difference between carrying on a conversation with someone in the environment of your living room versus conversing in the environment of a busy airport runway. The same principles of communication theory apply to the design of both wired and wireless communication systems. In addition to those specific functions associated with the unguided propagation of signals, however, the most profound differences between the implementations of wired and wireless communication systems relate to overcoming the signal impairments introduced by a changing wireless channel and, for mobile systems, compensating for the possible motion of the endpoints.
In addition to providing the fundamental basis for the design of wireless communication systems, the principles of communication theory, RF engineering, and propagation in real-world environments also apply to a host of other applications. As examples, these principles apply to a multitude of radar applications, including object or target detection, location and ranging, speed/velocity measurement, terrain mapping, weather monitoring, and navigation. In fact, many of the techniques used to develop modern personal communication systems were originally developed and proved for radar applications. In contrast to wireless communication systems that convey information between endpoints, radar systems analyze the way transmitted signals are reflected and modified by the presence of objects or variations along the signal path to extract information about the objects or the environment that the signal traverses. As a simple example, consider that a narrow pulsed-RF signal is transmitted in a given direction. Objects within the transmission path reflect some fraction of the signal incident upon them. If a receiver colocated with the transmitter detects an approximate replica of the transmitted signal sometime after the transmitted signal is sent, it is reasonable to assume that an object is located in the direction of transmission and the distance to the object is proportional to the time delay between transmitted and received signals. If no signal is detected within a specified period of time, it is assumed that there are no reflecting objects in the path of the signal, over a given range.
Clearly our general definition of a wireless system fits a vast range of seemingly unrelated applications. It is profoundly important, however, to recognize that all of these applications are founded on a common set of enabling principles and technologies encompassing communication theory, RF engineering, and RF propagation. Although the focus of this text is personal communication systems, the principles and techniques to be presented provide a strong foundation for study of other wireless system applications.
General Architecture, Basic Concepts, and Terminology
At a high level, every communication system is described by a common block diagram. In this section we present a basic functional description of each of the blocks to introduce some of the terminology of wireless systems and to help motivate later discussion of each of the functions.
We begin by considering the general block diagram of a wireless system for a generic application as shown in Figure 1.1. Many of the blocks and their functions apply to both wired and wireless communication systems. Note, however, that the blocks contained within the dashed outline are fundamental and necessary to wireless systems. With the exception of the antennas, all of the remaining blocks may also be found in wired system applications.
Figure 1.1 A Wireless System
The box labeled "Information Source" includes all functions necessary to produce an electrical signal that adequately represents the actual information to be communicated between end users. The term end user refers to a person or device that is the source or recipient (sink) of the information to be communicated. The term endpoint refers to the location of the transmitters and receivers in the communication path. End users may or may not be colocated with the endpoints. The functions of the Information Source box might include
- Creation of an analog waveform representing speech derived from a microphone, or creation of a digital bit stream resulting from sampling of an analog waveform
- Formatting digital information such as data, text, sampled audio, images, video, and so forth
The signals from information sources are typically bandlimited; that is, they contain frequencies from DC (or near DC) to some nominal cutoff frequency. They are termed baseband signals.
The box labeled "Signal Processing" encompasses all operations necessary to convert information signals into waveforms designed to maximize system performance. Signals may be processed to increase capacity, throughput, intelligibility, and accuracy and to provide other auxiliary functions. In modern wireless systems, many of the signal-processing functions are aimed at improving signal reception by mitigating the corrupting effects of the transmission medium or environment. Signal-processing functions on the transmitting end may include
- Converting analog signals to digital signals of a specific type
- Shaping signals to minimize the corrupting effects of the environment or transmission medium
- Compressing and coding signals to remove redundancies and improve throughput
- Coding signals to aid in the detection and correction of errors caused by the environment
- Encryption of signals for privacy
- Multiplexing information from several sources to fully utilize the channel bandwidth
- Adding information that simplifies or enhances access and control for the endpoints or end users
Signal processing may also include digital modulation, a technique used to spread the signal spectrum by coding one or more bits into a substantially longer bit stream. We will say more about digital spread spectrum and its benefits in a later chapter.
Signal processing, especially digital signal processing (DSP), has dramatically enabled rapid advances in the state of the art of communications in general and wireless personal communications in particular. The majority of topics to be covered in this text, as in any text on modern communications, will focus on some aspect of signal processing.
The efficient radiation of an electrical signal as an electromagnetic wave requires that the physical size of the antenna be comparable in size to the wavelength of the signal. This is also true for the reception of such an electromagnetic wave. This physical limitation renders the radiation of baseband signals impractical. As an example, consider the size requirement for radiating a 10 kHz signal. Recall from basic physics that the wavelength of a signal is related to its frequency by
where c is the speed of light in free space, 3 x 108 m/s. The wavelength of a 10 kHz signal is about 98,000 feet. If a typical quarter-wavelength (l/4) antenna were used, it would be 24,600 feet or 4.7 miles in length. In contrast, l/4 antennas in the cellular (900 MHz) or PCS (2 GHz) bands are 3.3 inches and 1.5 inches long, respectively. For this reason, practical wireless systems employ high-frequency or radio frequency sinusoidal signals called carriers to transport (or carry) information between endpoints.
The laws and regulations of the countries in which the systems are to be deployed govern and constrain the radiation of electromagnetic waves. Various frequency bands are allocated by law for specific applications; for example, there are AM, FM, and TV broadcast bands; public safety bands; airport communication, radar, traffic control, and maritime applications bands; and others. Furthermore, the laws may regulate transmitted power, transmitted spectrum and spectrum characteristics, modulation method, geographic location, tower height, and so on. Figure 1.2 shows some of the spectrum allocations in the ultra-high-frequency (UHF) band from 300 MHz to 3 GHz. A detailed chart of spectrum allocations in the United States is available from the National Telecommunications and Information Administration (NTIA).2 In the United States, the Federal Communications Commission (FCC) is the agency entrusted with the responsibility for administering the use of the radio spectrum, granting licenses, and working with government and private industry to develop fair and equitable regulatory rules and standards.
Figure 1.2 Some Spectrum Allocations in the UHF Band (January 2008)
Information signals are imposed upon a carrier signal by modulating (varying) its amplitude, frequency, and/or phase in direct relation to the variations of the information signal. At the receiving end, an information signal is extracted from the carrier by a process of demodulation. The boxes labeled "Modulation" and "Demodulation" refer to any of a wide range of techniques that may be used to impose/extract an information signal upon/from a carrier. As we will discuss later, the choice of modulation scheme is strongly influenced by a number of factors, including available frequency spectrum, spectrum rules and regulations, required throughput, channel characteristics, and QoS requirements. In the context of a wireless system (or a "broadband" wired system that employs coaxial cable, waveguide, or fiber as a transmission medium), a modulator translates the spectrum of a baseband signal to a bandpass spectrum centered about some high "radio" frequency appropriate for the intended application and consistent with spectrum regulations.
Many wired systems (for example, "plain old telephone service" [POTS]) use transmission media that allow the system to operate effectively at baseband. For such systems, a modulator translates an information signal into waveforms (sometimes called line codes) that are optimized for the given transmission medium and application. For example, a line code may convert a binary bit stream (1s and 0s) into a bipolar or multilevel voltage waveform, or it may convert a bit stream to positive and negative voltage transitions.
For wireless systems, a transmitter is essentially an RF power amplifier and appropriate bandpass filter. A transmitter drives a transmitting antenna (often through a coaxial cable or waveguide) and ensures that the modulated RF signal is radiated at a power level, and within a bandwidth, specific to the application and applicable regulations. Wired systems, on the other hand, often use transmitters termed line drivers that ensure that transmitted signals have sufficient energy to overcome the line losses in the path to the receiving end.
The power intercepted and absorbed by a receiving antenna is usually much lower than the transmitted power. For example, when a cellular base station transmits with a power of one watt, the received signal two miles away may be only a few tenths of a nanowatt. In fact, a receiver may be located so far from the transmitter that the signal level is comparable to system noise. System noise is a random signal that arises from a number of sources such as galactic radiation, engine ignitions, and the very devices used to amplify a received signal. In particular, we will discuss thermal noise, which is a random signal arising from the thermal agitation of electrons in the receiving antenna and its downstream interconnections and circuitry. The difference between transmitted and received power is inversely related to the distance (raised to some power) between the transmitting and receiving antennas and is termed path loss.
A receiver is essentially an amplifier designed to optimally reproduce the transmitted signal and remove the carrier. As such, a receiver is matched to the characteristics of the transmitted signal. Receivers usually employ high-gain, low-loss front-end amplifiers that are designed to minimize the level of thermal noise that they will pass to downstream functional blocks.
Signal processing on the receiving end seeks to restore the signal originating at the source. It converts the signal from the receiver into the form required for the endpoint recipient, that is, the Information Sink. In modern digital communication systems, signal processing at the receiving end is aimed at the reliable detection of bits. This may include error detection and correction, depending on the coding used to transmit the original signal, and also may include digital demodulation of a spread-spectrum signal.