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Sec. 5–9 Binary Modulated Bandpass Signaling 353
is such that adjacent channels are assigned only to distant locations and there is little overlap
of their coverage areas. Consequently, there is little IBOC interference with the assigned
adjacent channel stations. For more information on this technique and lists of stations that are
transmitting FM IBOC, search the internet for IBOC FM.
5–9 BINARY MODULATED BANDPASS SIGNALING
Digitally modulated bandpass signals are generated by using the complex envelopes for
AM, PM, FM, or QM (quadrature modulation) signaling that were first shown in Table 4–1
and then studied in previous sections. For digital modulated signals, the modulating signal
m(t) is a digital signal given by the binary or multilevel line codes that were developed in
Chapter 3. In this section, details of binary modulated signals are given. In Sections 5–10
and 5–11, multilevel and minimum-shift–keyed (MSK) digitally modulated signals are
described.
The most common binary bandpass signaling techniques, illustrated in Fig. 5–19, are as
follows:
• On–off keying (OOK), also called amplitude shift keying (ASK), which consists of
keying (switching) a carrier sinusoid on and off with a unipolar binary signal. OOK
is identical to unipolar binary modulation on a DSB-SC signal [Eq. (5–13)]. Morse
code radio transmission is an example of this technique. OOK was one of the first
modulation techniques to be used and precedes analog communication systems.
• Binary phase-shift keying (BPSK), which consists of shifting the phase of a sinusoidal
carrier 0 or 180 with a unipolar binary signal. BPSK is equivalent to PM signaling
with a digital waveform and is also equivalent to modulating a DSB-SC signal with a
polar digital waveform.
• Frequency-shift keying (FSK), which consists of shifting the frequency of a sinusoidal
carrier from a mark frequency (corresponding, for example, to sending a
binary 1) to a space frequency (corresponding to sending a binary 0), according to
the baseband digital signal. FSK is identical to modulating an FM carrier with a
binary digital signal.
As indicated in Sec. 3–6, the bandwidth of the digital signal needs to be minimized to
achieve spectral conservation. This may be accomplished by using a premodulation raised
cosine-rolloff filter to minimize the bandwidth of the digital signal and yet not introduce
ISI. The shaping of the baseband digital signal produces an analog baseband waveform that
modulates the transmitter. Figure 5–19f illustrates the resulting DSB-SC signal when a premodulation
filter is used. Thus, the BPSK signal of Fig. 5–19d becomes a DSB-SC signal
(Fig. 5–19f) when premodulation filtering is used.
On–Off Keying
The OOK signal is represented by
s(t) = A c m(t) cos v c t
(5–70)