In this case certain system design trade-offs can be made in order to benefit the entire communication
link.
4.2 COHERENT DETECTION
In a typical consumer electronic product, the received signal is spectrally down converted to base
band. Due to inaccuracies in the implementation and signal processing algorithms, residual frequency
and phase offsets will exist. A general block diagram of a coherent receiver is given below assuming
a super heterodyne architecture (see Fig. 4.1). First, note the following block diagram is simplified
174 CHAPTER FOUR
BPF
LPF
LPF
Carrier
Recovery
Slicer
Block
90?° ~
I(t)
Q(t)
LNA
fc
with the intention of conveying the present message. The received signal is first amplified by a low
noise amplifier (LNA). It is then spectrally shifted to an intermediate frequency (IF) where the band
pass filter (BPF) will extract the desired information signal. Then the signal is brought down to DC
or Zero IF (ZIF) so the signal processing algorithms can operate on the baseband signal.
The general principle is that the received signal is down converted to baseband producing the complex
envelope signal, I(t) j Q(t). This complex envelope will have a residual frequency and phase
offset.
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