B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

2
SIGNALS AND SIGNAL
SPACE
I
n
this chapter we discuss certain basic signal concepts. Signals are processed by systems.
We shall start with explaining the terms signals and systems.
Signals
A signal, as the term implies, is a set of information or data. Examples include a telephone or
a television signal, the monthly sales figures of a corporation, and closing stock prices (e.g., in
the United States, the Dow Jones averages). In all these examples, the signals are functions of
the independent variable time. This is not always the case, however. When an electrical charge
is distributed over a surface, for instance, the signal is the charge density, a function of space
rather than time. In this book we deal almost exclusively with signals that are functions of
time. The discussion, however, applies equally well to other independent variables.
Systems
Signals may be processed further by systems, which may modify them or extract additional
information from them. For example, an antiaircraft missile launcher may want to know the
future location of a hostile moving target, which is being tracked by radar. Since the radar
signal gives the past location and velocity of the target, by properly processing the radar signal
(the input), one can approximately estimate the future location of the target. Thus, a system
is an entity that processes a set of signals (inputs) to yield another set of signals (outputs).
A system may be made up of physical components, as in electrical, mechanical, or hydraulic
systems (hardware realization), or it may be an algorithm that computes an output from an
input signal (software realization).
2. 1 SIZE OF A SIGNAL
Signal Energy
The size of any entity is a quantity that indicates its strength. Generally speaking, a signal
varies with time. To set a standard quantity that measures signal strength, we normally view
a signal g (t) as a voltage across a one-ohm resistor. We define signal energy E 8
of the signal
g (t) as the energy that the voltage g(t) dissipates on the resistor. More formally, we define E 8
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