B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

172 AMPLITUDE MODULATIONS AND DEMODULATIONS
The velocity Ve depends on the actual vehicles (e.g. spacecrafts, airplanes, cars). For example,
if the mobile velocity Ve is 108 km/ph, then for a carrier frequency at 100 MHz, the maximum
Doppler frequency shift would be 10 Hz. Such a shift of every frequency component by a fixed
amount 1::iw destroys the harmonic relationship between frequency components. For !::if = 10
Hz, the components of frequencies 1000 and 2000 Hz will be shifted to frequencies 1010
and 2010 Hz, respectively. This upsets their harmonic relationship and the quality of nonaudio
signals.
It is interesting to note that audio signals are highly redundant, and unless !::if is very large,
such a change does not destroy intelligibility of the output. For audio signals /::if < 30 Hz
does not significantly affect the signal quality. !::if > 30 Hz results in a sound quality similar
to that of Donald Duck. But the intelligibility is not completely lost.
Generally, there are two ways to recover the incoming carrier at the receiver. One way is
for the transmitter to transmit a pilot (sinusoid) signal that can be either the exact carrier or
directly related to the carrier (e.g., a pilot at half the carrier frequency). The pilot is separated
at the receiver by a very narrowband filter tuned to the pilot frequency. It is amplified and used
to synchronize the local oscillator. Another method, in which no pilot is transmitted, is for the
receiver to use a nonlinear device to process the received signal, to generate a separate carrier
component that can be extracted using narrow bandpass filters. Clearly, effective and narrow
bandpass filters are very important to both methods. Moreover, the bandpass filter should also
have the ability to adaptively adjust its center frequency to combat significant frequency drift
or Doppler shift. Aside from some typical bandpass filter designs, the phase-locked loop (PLL),
which plays an important role in carrier acquisition of various modulations, can be viewed as
such a narrow and adaptive bandpass filter. The principles of PLL will be discussed later in
this chapter.
4.7 FREQUENCY DIVISION MULTIPLEXING (FDM)
Signal multiplexing allows the transmission of several signals on the same channel. In
Chapter 6, we shall discuss time division multiplexing (TDM), where several signals timeshare
the same channel. In FDM, several signals share the band of a channel. Each signal is
modulated by a different carrier frequency. These carriers, referred to as subcarriers, are adequately
separated to avoid overlap ( or interference) between the spectra of various modulated
signals. Each signal may use a different kind of modulation (e.g., DSB-SC, AM, SSE-SC,
VSB-SC, or even frequency modulation or phase modulation). The modulated signal spectra
may be separated by a small guard band to avoid interference and facilitate signal separation
at the receiver.
When all the modulated spectra are added, we have a composite signal that may be
considered to be a baseband signal to further modulate a radio-frequency (RF) carrier for the
purpose of transmission.
At the receiver, the incoming signal is first demodulated by the RF carrier to retrieve
the composite baseband, which is then bandpass-filtered to separate all the modulated signals.
Then each modulated signal is demodulated individually by an appropriate subcarrier to obtain
all the basic baseband signals.
One simple example of FDM is the analog telephone long-haul system. There are two
types of long-haul telephone carrier system: the legacy analog L-carrier hierarchy systems and
the digital T-carrier hierarchy systems in North America (or the E-carrier in Europe). 3 Both
were standardized by the predecessor of the International Telecommunications Union known
(before 1992) as the CCITT (Comite Consultatiflnternational Telephonique et Telegraphique).