56 Conventional FM generation method for 4-level FSK.
fc fc + fd fc + 3fd fc ??“ fd fc ??“ 3fd
|S( f )|
Hz
t
t
t
Data
I(t)
Q(t)
+3
??“3
+1
??“1
+1
??“1
+1
??“1
Tb
Tb
Tb
FIGURE 2.57 Simplified view of 4-level FSK modulation scheme.
FIGURE 2.58 Unfiltered complex envelope of 4-level FSK with 1 and 3.
for the two inner spectral components and 2 3 for the outer two spectral components. The corresponding
baseband waveforms are displayed in Fig. 2.58.
The three waveforms shown in Fig. 2.58 constitute the baseband frequency modulator input data
signal, and the analytic output signals. The waveforms are provided for a few values of the 4-level
random data.
We were able to present an equation to quantify the separation between the frequency deviations
provided by the unitless modulation index parameter . Next we want to understand if there are certain
values of the modulation index that exhibit properties that are in some way better than other
modulation indices [26]. The frequency deviations should be separated enough in order to be separable
in the receiver. At first thought, we would consider waveforms that are orthogonal to one another.
Let the two frequency deviations (assuming BFSK) be represented as
(2.
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