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

22 SIGNALS AND SIGNAL SPACE
the trouble of dealing with many decimal places when signal power is large or small. As a
convention, a signal with average power of P watts can be said to have power of
[10 - log 10 P] dBw or [30 + 10 · log 10 P] dBm
For example, -30 dBm represents signal power of 1 o- 6 W in normal decimal scale.
Example 2. 1 Determine the suitable measures of the signals in Fig. 2.2.
The signal in Fig. 2.2a approaches O as l t l --+ oo. Therefore, the suitable measure for this
signal is its energy E g
, given by
00
f oe fo 1
-oo - 1 0
E g = g 2 (t) dt = (2) 2 dt + 4e -1 dt = 4 + 4 = 8
The signal in Fig. 2.2b does not approach O as l tl --+ oo. However, it is periodic, and
therefore its power exists. We can use Eq. (2.3) to determine its power. For periodic signals,
we can simplify the procedure by observing that a periodic signal repeats regularly each
period (2 seconds in this case). Therefore, averaging g 2 (t) over an infinitely large interval
is equivalent to averaging it over one period (2 seconds in this case). Thus
1
1
1
1
1
P g
= - / g 2 (t) dt = - / t 2 dt = -
2 -1 2 -1 3
Fi g ure 2.2
Signal for
Example 2.1.
(a)
- 1 0 2 4 1 -
Recall that the signal power is the square of its rms value. Therefore, the rms value of this
signal is I/ ,./3.
2.2 CLASSIFICATION OF SIGNALS
There are various classes of signals. Here we shall consider only the following pairs of classes,
which are suitable for the scope of this book.
1. Continuous time and discrete time signals
2. Analog and digital signals