This last /4-DQPSK signal constellation diagram in Fig. 2.47 corresponds to the output of a nonlinear
PA operating in the nonlinear region. Notice the constellation amplitude has almost been
mapped to a circle of constant radius. Also note that the eight phase states are not readily visible now;
hence there is some potential for in-band BER degradation.
MODULATION THEORY 79
??/4-DQPSK NLA Output Signal
Constellation
??“2
??“1
0
1
2
??“2 ??“1.5
??“1.5
??“1 ??“0.5
??“0.5
0 0.5
0.5
1 1.5
1.5
2
I-Channel
Q-Channel
FIGURE 2.47 Nonlinear PA output PSD of /4-DQPSK.
2.2.6 Frequency Shift Keying (FSK)
Next we would like to temporarily leave the topic of PSK and discuss the topic of FSK. We do this
for a number of reasons. First, FSK is a commonly used modulation scheme and deserves the attention.
Second, we can discuss a particular modulation scheme that can essentially be a member of both
the PSK and FSK family. Lastly, this will be used as a foundation when we discuss controlling the
constellation phase trajectories. Let us start with BFSK (see Fig. 2.48); here we vary the carrier
frequency in proportion to the information signal m(t). The transmitted signal is represented as
(2.32) s(t) A # cos c2pfct 2p 3
t
`
fd # m(t) dt d
which can be generated using the typical FSK transmission method of applying a time-varying signal
to a Voltage Controlled Oscillator (VCO).
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