Pardis,
As far as the question of Aliasing it is slightly different from ISI.
ISI is when two consequetive symbols overlap and the receiver might decode the signal wrongly.
An d Aliasing is when the signal is sampled usually below the Nyquest criterion and this results in incorrect decoding of the signal. Like the blades of Halicopter seem to rotate slowly in the direction opposite to their actuall. this is because our eye’s sample at a lower rate that required to perceive the movement of the halicopter blades