15.2 ANALOG COMMUNICATION SYSTEMS 685
Nao=
kTsysBNGa(f 0 )
Lr
(15.1.30)
The signal-to-noise power ratio at the system output is an excellent measure of performance
for many communication systems. The available signal power at pointAin Figure 15.1.11(a) is
given by
SA=
PtGtGrλ^2
( 4 π)^2 R^2 LtLtaLchLra
(15.1.31)
based on Equation (15.1.24). The available noise power at pointAis given by
NaA=kTsysBN (15.1.32)
Thus, the system performance with noise is measured by the signal-to-noise power ratio,
(
S
N
)
A
=
PtGtGrλ^2
( 4 π)^2 R^2 LtLtaLchLrakTsysBN
(15.1.33)
15.2 Analog Communication Systems
Ananalog messageis a continuum of possible amplitudes at any given time, and analog signals
are continuous in time and in amplitude, such as audio and video signals. When the message to
be sent over a communication system is analog, we refer to the system as analog. The transmitted
waveform must be some function of the message so that the receiver could decipher the message.
Usually the transmitted waveform is the result of varying either the amplitude, phase, or frequency
of a basic signal called acarrier. Combinations of amplitude, phase, and frequency variations are
also possible.
The carrier usually is sinusoidal of the formAccos(ωct+φc), whereAc,φc, andfc=ωc/ 2 π
are the carrier’s amplitude, phase, and frequency, respectively. WhenAis varied as a linear function
of the message,amplitude modulation(AM) occurs. Inphase modulation(PM) a phase term that
is a linear function of the message is added to the carrier. When the added phase is a linear function
of the integral of the message, the result is known asfrequency modulation(FM). Note that the
carrier’s frequency in FM is a linear function of the message, because instantaneous angular
frequency is the time derivative of instantaneous phase. Thus, FM and PM are closely related.
Every communication system has amodulatorat the transmitting station to structure the
transmitted waveform, and ademodulator(detector) at the receiving end to recover the message
from the received signal. Radio (AM and FM) and television broadcasting are the most familiar
forms of communication through analog signal transmission. The FCC in the United States
regulates the carrier-frequency assignments in order to minimize the interference between nearby
stations. Commercial AM radio broadcasting utilizes the frequency band of 535 to 1605 kHz for
the transmission of voice and music. The carrier-frequency allocations range from 540 kHz to
1600 kHz, with 10-kHz spacing. Each station can occupy a channel bandwidth of only 10 kHz
centered on its carrier. Even though the baseband message signal is limited to a bandwidth of about
5 kHz, the AM broadcasting system adequately meets the need for low-cost mass communication
and general audio entertainment, in spite of lacking high-fidelity behavior.
Commercial FM radio broadcasting utilizes the frequency band of 88 to 108 MHz for the
transmission of music and voice signals. The carrier frequencies are separated by 200 kHz, and the
peak frequency deviation is fixed at 75 kHz. Each station (out of the possible 100) broadcasts in
a channel bandwidth of 200 kHz centered on the carrier. The FM system is capable of presenting
higher quality audio to the user than AM because of the larger audio band allowed from 50 Hz to
