Intersymbol Interference (ISI)
Definition: In communication system, when the data being transmitted in form of pulses i.e bits, the output produced at the receiver due to other bits or symbols interferes with the output produced by the desired bit. This is known as intersymbol interference (ISI). The intersymbol interference will introduce errors in the detected signal.
Factors responsible for intersymbol interference (ISI)
- The ISI arises due to the imperfection in the overall frequency response of the system.
- When a short pulse is transmitted through a bandlimited system, then the frequency components contained in the input pulse are differentially attenuated and more importantly differentially delayed by the system.
- Due to this the output pulse get dispersed.
- Due to this dispersion, the symbol will interfere with eachother when transmitted over a communication channel.
- This will result in the intersymbol interference (ISI).
Effect of Intersymbol Interference
- In the absence of ISI and noise, the transmitted bit can be decoded correctly at the receiver.
- The presence of ISI will introduced errors in the decision at the receiver output.
- Hence, the receiver can make an error in deciding whether it has received a logic 1 or a logic 0.
Remedy to reduce ISI
- It has been proved that the function which produce a zero intersymbol interference is a sinc function. Hence, instead of a rectangular pulse if we transmit a sinc pulse then the ISI can be reduced to zero.
- This is known as nyquist pulse shaping. the sinc pulse transmitted to have a zero ISI.
- Further, we know that Fourier transform of a sinc pulse is a rectangular function. Hence, to preserve all the frequency components, the frequency response of the filter must be exactly flat in the pass band and zero in the attenuation band.
Eye Pattern: Practical way to study ISI
- Eye pattern is a pattern displayed on the screen of cathode ray oscilloscope (CRO).
- The shape of this pattern resembles the patten of eye. Hence, it is known as eye pattern.
- Eye pattern is the practical way to study the intersymbol interference and its effects on PCM or data communication system.
- The eye pattern is obtained on CRO screen by applying received signal to vertical plate of the CRO and sawtooth wave to the horizontal plate of CRO.
- This is how we obtain eye pattern on CRO screen.