0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 707

6 MHz

assigned channel band

Video (visual)

carrier

Luminance

signal 4.2 MHz

Audio

(FM)

Chrominance

signal

Chrominance carrier

3.579545 MHz

Audio carrier

4.5 MHz

1.25 MHz Color subcarrier

Figure 15.2.29Spectrum of a color television signal.

out signalsfY(t) and [fI(t)+fQ(t)], which is further processed by two synchronous detectors in
quadrature to recoverSI(t) andSQ(t). An appropriate matrix combinesfY(t),SI(t), andSQ(t) to yield
close approximations of the originally transmittedmR(t),mG(t), andmB(t). These three signals
control the three electron beams in the picture tube.
The output of the envelope detector is also applied to circuits that separate the sync signals
needed to lock in the horizontal and vertical sweep circuits of the receiver. The bursts of color
carriers are isolated such that a PLL can lock to the phase of the color carrier, and thereby provide
the reference signals for the chrominance synchronous detectors.

Mobil Radio Systems (Cellular Telephone Systems)

Today radio-based systems make it possible for mobile people to communicate via cellular
telephone systems while traveling on airplanes and motor vehicles. For radio telephone service,
the FCC in the United States has assigned parts of the UHF band in the range of 806–890 MHz.
For efficient use of the available frequency spectrum, especially in highly populated metropolitan
areas with the greatest demand for mobile telephone services, the cellular radio concept has been
adopted, in which a geographic area is subdivided intocells, each of which contains abase station,
as shown in Figure 15.2.31. Each base station is connected by telephone lines to amobile telephone
switching office(MTSO), which in turn is connected through telephone lines to a telephone central
office of the terrestrial telephone network.
When a mobile user (identified by the telephone number and telephone serial number assigned
by the manufacturer) communicates via radio with the base station within the cell, the base station
routes the call through the MTSO to another base station if the called party is located in another
cell, or to the central office if the called party is not mobile. Once the desired telephone number is
keyed and the “send” button is pressed, the MTSO checks the authentication of the mobile user and
assigns (via a supervisory control channel) an available frequency channel for radio transmission
of the voice signal from the mobile telephone to the base station. A second frequency is assigned
for radio transmission from the base station to the mobile user. Simultaneous transmission between
two parties is known asfull-duplex operation. In order to complete the connection to the called
party, the MTSO interfaces with the central office of the telephone network by means of wide-