SIGNALS CONVEY INTELLIGENCE
A simplified model of a fiber-optic link.
A simplified model of a fiber-optic link is illustrated in Figure 2.13. In this figure, the
driver conditions the electrical baseband signal prior to modulation of the light signal;
the optical source is the transmitter where the light signal is generated and modulated;
the fiber-optic transmission medium consists of a fiber strand, connectors, and splices; the
optical detector is the receiver where the light signal is detected and demodulated; and
the output circuit conditions the resulting electrical baseband signal for transmission to
the electrical line (Ref. 3).
A more detailed discussion of fiber-optic systems will be found in Chapter 9.
Up to now we have discussed guided transmission. The signal is guided or conducted
down some sort of a "pipe." The "pipes" we have covered included wire pair, coaxial
cable, and fiber-optic cable. Radio transmission, on the other hand, is based on radi-
The essential elements of any radio system are (1) a transmitter for generating and
modulating a "high-frequency"
carrier wave with an information baseband, (2) a trans-
mitting antenna that will radiate the maximum amount of signal energy of the modulated
carrier in the desired direction, (3) a receiving antenna that will intercept the maximum
amount of the radiated energy after its transmission through space, and (4) a receiver
to select the desired carrier wave, amplify the signal, detect it, or separate the signal
from the carrier. Although the basic principles are the same in all cases, there are many
different designs of radio systems. These differences depend upon the types of signals
to be transmitted, type of modulation (AM, FM, or PM or a hybrid), where in the fre-
quency spectrum (see Figure 2.6) in which transmission is to be affected, and licensing
restrictions. Figure 2.14 is a generalized model of a radio link.
The information transport capacity of a radio link depends on many factors. The first
factor is the application. The following is a brief list of applications with some relevant
Line-of-sight microwave, depending on the frequency band: 2, 5, 10, 20, 30, 40,
SCADA (system control and data acquisition): up to 12 kHz in the 900-MHz band.
In the context of this book, "high-frequency" takes on the connotation of any signal from 400 MHz to 100 GHz.