Intersymbol Interference(ISI)

Refer to Wikipedia ISI article.

In telecommunication, intersymbol interference(ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effects as noise, thus making the communication less reliable.

ISI is usually caused by multipath propagation and the inherent non-linear frequency response of a channel. ISI arises because of imperfections in the overall frequency response of the system.
The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore in the design of the transmitting and receiving filters, the objective is to minimize the effect of ISI.

Ways to fight against intersymbol interference include adaptive equalization and error correcting codes.

Multiplath Propagation
Multipath propagation means a wireless signal from a transmitter reaches the receiver via many different paths. The cuase of this include reflection, refraction and atmospheric effects. Since all of these paths are different lengths, this results in the different versions of the signal arriving at different times. This delay means that part or all of a given symbol will be spread into the subsequent symbols, thereby interfering with the correct detecting of those symbols. Additionally, the various paths often distort the amplitude and/or phase of the signal thereby causing further interference with the received signal.

Bandlimited Channels
A bandlimited channel means the frequency response is zero above or below a certain frequency(the cutoff frequency). Passing a signal through such a channel results in the removal of frequency component above and below this cutoff frequency.
This filtering of the transmitted signal affects the shape of the pulse that arrives at the receiver. Besides it is spread out over the subsequent symbol periods. When a message is transmitted through a channel, the spread pulse of individual symbol will interfere with following symbols.
As opposed to multipath propagation, bandlimited channels are present in both wired and wireless communications. The limitation is often imposed by the desire to operate multiple independent signals through the same area/cable; due to this, each system is typically allocated a piece of the total bandwidth available.
The bandlimiting can also be due to the physical properties of the medium.
Communication systems that transmit data over bandlimited channels usually implement pulse shaping to avoid interference caused by the bandwidth limitation. Often the channel response is not known beforehand, and an adaptive equalizer is used to compensate the frequency response.