Simulink Tutorial on Digital Modulation Methods - Cengage Learning

602 CHAPTER 13. SIMULINK TUTORIAL ON DIGITAL MODULATION
Figure 13.40: Signal x(t) at matched filter input
Figure 13.41: Demodulated baseband signal s_r(t) at matched filter output
SOLUTION
Figure 13.42: Sink signal v(t)
1. The zero ISI condition is met only at the transmitter because the cascade of
two raised-cosine pulses (matched filter!) results in ISI if α �= 0. Also, the
horizontal eye opening is not maximum. We would observe maximum horizontal
eye opening at the transmitter for α = 1 (see <strong>Tutorial</strong> Problem 13.4).
2. The detectability would be optimum after the transmit filter. However, after
the receive filter we encounter ISI, resulting in a reduced vertical eye opening.
Therefore, the raised-cosine pulses and a matched receive filter are not optimum.
The raised-cosine pulse shaping has to be split evenly between the transmit and
receive filters in order to simultaneously meet the matched filter condition, which
maximizes the SNR at the receive filter output and the zero ISI condition. Therefore,
we need to apply a square-root raised-cosine pulse. Then, the zero ISI
condition is met at the receiver rather than at the transmitter.
3. This is due to the raised-cosine pulse shape. The range of the signal-space trajectory
is the same as in the vertical axis of the eye pattern.
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