Intersymbol Interference (ISI)
InterSymbol Interference (ISI) is a kind of distortion that occurs when one or more symbols (pulses in digital baseband transmission) interfere with subsequent signals. This can cause noise in the signal which can cause the output to be less than ideal.
ISI occurs when there is multipath propagation and/or nonlinear frequency in the channels. These causes can be reduced which can help eliminate ISI from the system to achieve an ideal output.
The following equation mathematically represents the receiver output y(t) sampled at time t_i=i*T_b (with i representing integer values):
The first term in the above equation, µ*a_i, is produced by the ith transmitted bit. The second term represents the residual effect or ISI of all other transmitted bits on the decoding of the ith bit.