|
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
|
ISBN: (0132447894)
By: Bruce A. Black,
Philip S. DiPiazza,
Bruce A. Ferguson,
David R. Voltmer,
Frederick C. Berry.
|
Reproduced from the book
Introduction to Wireless Systems. Copyrightã 2008, Pearson Education,
Inc., 800 East 96th Street, Indianapolis, IN 46240.
|
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 unpredictable 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 realworld 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.
|
