Communication Systems 667invention of the vacuum tube was particularly instrumental in the development of radio commu-
nication systems. Amplitude modulation (AM) broadcast was initiated in 1920, while frequency
modulation (FM) broadcast was developed commercially by the end of World War II. Commercial
television broadcasting began in 1936 by the BBC (British Broadcasting Corporation), and the
FCC (Federal Communications Commission) authorized television broadcasting five years later
in the United States.
The growth in communications over the past 60 years has been phenomenal. The invention of
the transistor in 1947 and the integrated circuit and laser in 1958 have paved the way tosatellite
communication systems. Most of the wire-line communication systems are being replaced by
fiber-optic cables (providing extremely high bandwidth), which makes the transmission of a wide
variety of information sources (voice, data, and video) possible. High-speed communication
networks linking computers and the greater need for personal communication services are just
the beginning of the modern telecommunications era.
Todaydigital communication systemsare in common use, carrying the bulk of our daily
information transmission through a variety of communication media, such as wire-line telephone
channels, microwave radio, fiber-optic channels, and satellite channels. Even the current analog
AM and FM radio and television broadcasts will be replaced in the near future by digital transmis-
sion systems. High-speed integrated circuits (ICs), programmable digital signal processing chips,
microelectronic IC fabrication, and sophisticated digital modulation techniques have certainly
helped digital communications as a means of transmitting information.
In spite of the general trend toward digital transmission of analog signals, a significant amount
of analog signal transmission still takes place, especially in audio and video broadcasting. His-
torically,analog communication systemswere placed first, and then came digital communication
systems.
In any communication system, the communication channel provides the connection between
the transmitter and the receiver. The physical channel (medium) may be any of the following:
- A pair of wires, which carry the electric signal
- Optical fiber, which carries the information on a modulated light beam
- An underwater ocean channel, in which the information is transmitted acoustically
- Free space, over which the information-bearing signal is radiated by using an antenna
- Data storage media, such as magnetic tape, magnetic disks, and optical disks.
The available channel bandwidth, as well as the noise and interference, limit the amount of
data that can be transmitted reliably over any communication channel.
Figure 15.0.1 illustrates the various frequency bands of the electromagnetic spectrum (radio
and optical portions) along with types of transmission media and typical applications.
Wire-line channelsare used extensively by the telephone network for voice, data, and video
transmission. Twisted-pair wire lines (with a bandwidth of several hundred kHz) and coaxial
cable (with a usable bandwidth of several MHz) are basically guided electromagnetic channels.
Fiber-optic channelsoffer a channel bandwidth that is several orders of magnitude larger than
coaxial cable channels. The transmitter or modulator in a fiber-optic communication system is a
light source, such as a light-emitting diode (LED) or a laser, whose intensity is varied (modulated)
with the message signal. The light propagates through the fiber as a light wave and is amplified
periodically along the transmission path to compensate for signal attenuation. At the receiver
end, the light intensity is detected by a photodiode, whose output is an electric signal that varies