Inter Symbol Interference (ISI):-
In Tele-communications, Inter symbol 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 effect as noise. Thus making the communication less reliable.
i.e, the spreading of the pulse beyond its allotted time interval causes it to interfere with neighboring pulses.
ISI is usually caused by the two factors
- Multi path propagation.
- The inherent Non-linear frequency response of the channel.
These two factors causes successive symbols to “blur” together.i.e, the channel is always band-limited. A band limited channel disperses (or) spreads a pulse wave form passing through it.
when the channel Band width is much larger than the Pulse width the spreading of the pulse will be slight.
when the channel band width is close to the signal band width, the spreading will exceed a symbol duration and causes signal pulses to over lap. This over lapping is called as ISI.
ISI causes system degradation, it is particularly insidious form of interference because raising the signal power to overcome the interference will not always improve the error performance. when considering the filtering aspects of a typical digital communication system there are various filter throughout the system in the transmitter, Receiver and in the channel.
At the transmitter :- the information symbols x1,x2,x3 are characterized as impulses (or) voltage levels, are filtered to comply with some bandwidth constraint.
if the system is a Base band system—–> channel has distributed reactances(acts as a filter) that distort the pulses .
if the system is Pass band system—–> wireless systems are characterized by fading channels, that behave like undesirable filters causes distortion in the pulses.
At the Receiver:- when the receiving filter is configured to compensate for the distortion caused by both the transmitter and channel it is often referred to as equalizing filter.
The equivalent model for the system, lumping all the filtering effects into one overall equivalent system transfer function
is given in the figure.
now, due to filtering effects, the received pulses can overlap one another as shown the tail of a pulse can “smear” into adjacent symbol intervals, there by interfering with the detection process and degrading the error performance, such interference is termed (ISI). Even in the absence of noise , the effects of filtering and channel- induced distortion leads to ISI.
Nyquist investigated the problem of specifying a received pulse shape so that no ISI occurs at the detector. The minimum band width required to detect Rs symbols/Sec without ISI is BWmin = Rs /2 Hz. This occurs when H(f) is rectangular.