Partial-response signaling can also be used when the input to the channel takes more than two amplitudes.
a. Describe a duobinary system with a four-level input alphabet. Give a decision-feedback demodulator including the detection rule.
b. Can a precoder be used in place of the decision-feedback demodulator? If so, describe it.
c. Can one choose a duobinary system for an eight-ary PSK signal constellation? d. If so, can a precoder be used?