14.1 SIGNALS AND SPECTRAL ANALYSIS 627(PM) is performed primarily at the transmitter. For example, for a radio station found at a
setting of AM820, the carrier wave transmitted by the radio station is at the frequency of
820 kHz.
The function of the receiver is to recover the message signal contained in the received
signal. If the message signal is transmitted by carrier modulation, the receiver performs carrier
demodulationto extract the message from the sinusoidal carrier.
The communication channel (transmission medium) is the physical medium that is utilized to
send the signal from the transmitter to the receiver. In wireless transmission, such as microwave
radio, the transmission medium is usually the atmosphere or free space. Telephone channels, on
the other hand, employ a variety of physical media such as wire lines and optical fiber cables.
Irrespective of the type of physical medium for signal transmission, the essential feature is that
the transmitted signal is corrupted in a random manner by a variety of possible mechanisms. For
simplicity, the effects of these phenomena (attenuation, distortion, interference, noise,etc.) are
shown at the center of Figure 14.1.1, since the transmission medium is often the most vulnerable
part of a communication system, particularly over long distances.
Attenuation, caused by losses within the system, reduces the size or strength of the signal,
whereas distortion is any alteration of the waveshape itself due to energy storage and/or non-
linearities. Contamination by extraneous signals causes interference, whereas noise emanates
from sources both internal and external to the system. To eliminate any one of these may pose a
challenge to the design engineer.
Successful information recovery, while handling the aforementioned problems, invariably
calls forsignal processingat the input and output. Common signal-processing operations include
the following:
- Amplificationto compensate for attenuation
- Filteringto reduce interference and noise, and/or to obtain selected facets of information
- Equalizationto correct some types of distortion
- Frequency translationorsamplingto get a signal that better suits the system characteristics
- Multiplexingto permit one transmission system to handle two or more information-bearing
signals simultaneously
In addition, to enhance the quality of information recovery, several specialized techniques,
such as linearizing, averaging, compressing, peak detecting, thresholding, counting, and timing,
are used.
Analog signals in ananalog communication systemcan be transmitted directly via carrier
modulation over the communication channel and demodulated accordingly at the receiver. Alter-
natively, an analog source output may be converted into a digital form and the message can be
transmitted via digital modulation and demodulated as a digital signal at the receiver. Potential
advantages in transmitting an analog signal by means of digital modulation are the following:
- Signal fidelity is better controlled through digital transmission than through analog trans-
mission; effects of noise can be reduced significantly. - Since the analog message signal may be highly redundant, with digital processing, redun-
dancy may be removed prior to modulation. - Digital communication systems are often more economical to implement.
Figure 14.1.2 illustrates the basic elements of adigital communication system. For each
function in the transmitting station, there is an inverse operation in the receiver. The analog input