3.
As you would expect, this PA output constellation exhibits significant distortion, which is visible
in the time domain. The frequency domain must contain spectral spreading and regrowth to dual the
temporal variations.
Let us briefly turn our attention to the receiver where we will introduce the demodulation operations.
A simplified block diagram of the receiver is shown in Fig. 2.29, where we place emphasis on
the channel derotation or compensation aspect.
QPSK NLA Output Signal Constellation
??“1.5
??“1.5
??“1
??“0.5
??“0.5
0
0.5
0.5
1
1.5
1.5 ??“1 0 1
I-Channel
Q-Channel
FIGURE 2.28 PA output signal constellation diagram for SRC-filtered QPSK with roll-off
factor 0.3.
Let us walk through this receiver; first, the received signal enters a BPF to reduce the noise and
adjacent channel interference. This filtered signal is then spectrally down converted to produce the
received complex envelope signal. Note that we have previously drawn the quadrature demodulator
with LPFs internal to the block. The LPFs shown in Fig. 2.29 are in addition to the quadrature demodulation
filtering. The received signal constellation will have a time-varying phase offset (the amplitude
will vary as well; but for sake of discussion, let??™s keep this simple) which will be estimated by the
channel estimation (CE) block.
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