The filter
type choice is extremely important since this will shape the transmission bandwidth by slowing
down the phase trajectories and, in doing so, may add interference from neighboring modulation symbols.
This interference is due to additional memory inserted into the signal and is called ISI??”not to
mention the ISI caused by the coherence bandwidth of the wireless channel.
The receiver section will also contain filters to perform channel selectivity. The received modulated
signal will enter a decision or detection device that will be used in the extraction of the information
signal from the modulation. Hence, we must concentrate on overall filter impulse response??”by this,
we mean the transmitter plus receiver filters.
In Fig. 2.1, we show such an example, where there is a filter used at the transmitter and one used
in the receiver. The received signal quality is monitored and measured by viewing the receiver signal
constellation diagram or eye diagram. Let us discuss the latter of the two. Here we see, when considering
the overall impulse response, the received signal should have as little ISI as possible, in order
to use a low-complexity symbol-by-symbol-based detector. A rule of thumb typically used is that the
more filtering performed on the transmission signal to reduce emissions, the more ISI will be inserted
into the transmitted signal.
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