In the figure, the transmit filter frequency response HT( f ) can be seen as in cascade with the receive
filter frequency response HR( f ). The overall frequency response is given as H( f ) and defined as
(2.1)
In Fig. 2.2, we show an example of a received eye diagram assuming a cascade of transmit and
receive filters. This eye diagram shows the ISI present as well as timing jitter present. The ISI is present
due to the nonlinear phase filters used in the transmitter and receiver.
The received signal??™s eye diagram is oversampled at a rate of 8 times the symbol rate. The x-axis
shows two symbol times in duration [1].
H( f ) HT( f ) # HR( f )
51
2.1.1 ISI-Free System Requirements
In order to have an ISI-free system, we require the overall filter response to exhibit zero-ISI properties.
Before we present a typically used solution, we will first review the Nyquist theorem which we
will leverage in our discussion of ISI.
Nyquist??™s Theorem. Nyquist??™s theorem tells us the maximum possible transmission bit rate, given
a filter BW, while still achieving ISI-free conditions. Specifically, the maximum data rate in order to
achieve ISI-free transmission through an ideal brick wall filter with cutoff frequency (BW 1/2Ts)
is given as 1/Ts bps (see [2] and [3]).
Pages:
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112