Even for a small frequency
offset of 200 Hz, a fraction of a dB degradation exists.
336 CHAPTER SIX
Q-Channel Q-Channel
I-Channel I-Channel
( )8
FIGURE 6.47 Phase state transitions due to raising /4-DQPSK to the power of 8.
2NEC Performance with Frequency Offsets
1.E??“01
1.E??“05
1.E??“04
1.E??“03
1.E??“02
4 6 8 10 11 12
Eb/No (dB) BER
2NEC(0 Hz)
2NEC(200 Hz)
2NEC(400 Hz)
2NEC(600 Hz)
2NEC(800 Hz)
9 7 5
FIGURE 6.48 The BER performance of the 2NEC receiver for various frequency offsets.
This system used the /4-DQPSK modulation in the AWGN channel. The symbol rate used in the
simulation was set to 24.3 ksps, which for a frequency offset of 400 Hz produced a normalized frequency
offset of . foff # Ts 0.016461
Next we wanted to show the performance degradation of the earlier described detectors as the frequency
offset is increased. Here we see for the zero offset case, widening the observation window
when making a decision improves overall performance. However, when there is frequency offset present
the behavior is different. First we compare the performance of 1NEC or MLDD to the differential
detector, we notice that there is gain in widening the observation window as long as the residual frequency
offset is less than approximately 600 Hz, for this particular system simulation.
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