In this section, we will discuss some of the special properties this modulation scheme has. Given
a BFSK modulation with index 0.5, we can quickly draw the constellation diagram in Fig. 2.60,
knowing the allowable phase change in a bit time is equal to .
The corresponding one-dimensional complex envelope waveforms can be drawn based on the
above modulation scheme parameters (see Fig. 2.61). These baseband MSK waveforms were generated
using the baseband FM modulator presented earlier.
86 CHAPTER TWO
I-Channel
Q-Channel
+1 Phase
Change
??“1 Phase
Change
FIGURE 2.60 MSK signal constellation diagram.
t
t
t
Data
I(t)
Q(t)
+1
??“1
+1
??“1
+1
??“1
Tb
Tb
Tb
FIGURE 2.61 Unfiltered complex envelope of MSK.
Next we will briefly present another method to generate MSK, called offset bandpass filtering of
BPSK [27]. This method consists of first generating a BPSK signal and then sending this signal
through a BPF whose center frequency is offset by a specific amount. The transmission block diagram
is shown in Fig. 2.62.
MODULATION THEORY 87
m(t) X
~ cos(w1t)
G( f ) S(t)
BPSK
Signal
MSK
Signal
FIGURE 2.62 Offset bandpass filtering method of generating an MSK
waveform.
+
X
X
cos(wct)
??“sin(wct)
I(t)
Q(t)
a(t)
Quadrature Modulator
p/2
??t
b(t)
S/P
Tb
b(t??“Tb)
m(t) PA
S(t)
X
2Tb
cos
X
??›
???
??›
???
FIGURE 2.
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