43 AFC-One block diagram.
334 CHAPTER SIX
Q-Channel
I-Channel
I2 I1
Q2
Q1
q1
q2
Error
Error
FIGURE 6.44 The effect of frequency offsets on detected constellation states.
Differential
Detector
X
cos(x)
RI(t)
Quantizer
VCO
RQ(t) P/S
sin(??“x)
+
T
I2 + jQ2 I1 + jQ1
Output
Bits
LPF
Loop Filter
Error Signal
??“
Phase
Discriminator
X X
FIGURE 6.45 AFC-Two block diagram.
In fact, there are quite a few options available to the system designer, the reader is encouraged to
refer to [38] for further discussions and techniques.
Next we will discuss another form of the AFC without using the differential detector in the phase
discriminator operation. In some publications this has been called Digital Tan lock Loop (DTL) [39].
The basic premise of the loop is the same, with the difference coming from the method used in the
phase discriminator. For /4-DQPSK modulation, the error signal is expressed as
(6.120)
(6.121)
Since (t) k . /4 where k 0, 1, . . . , 7 then 8 (t) k . 2, which places all 8 phase states onto
a single state on the real axis. By raising the modulated symbol stream to the power of 8, we have
essentially removed the modulation. Similarly, QPSK can be raised to the power of 4 to remove the
modulation present on the received signal (see Fig.
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