INTERSYMBOL INTERFERENCE (ISI)
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Intersymbol interference (ISI) occurs when a pulse spreads out in such a way that it interferes with adjacent pulses at the sample instant.
Example: assume polar NRZ line code. The channel outputs are shown as spreaded (width Tb becomes 2Tb) pulses shown (Spreading due to bandlimited channel characteristics).
If the rectangular multilevel pulses are filtered improperly as they pass through a communications system, they will spread in time, and the pulse for each symbol may be smeared into adjacent time slots and cause Intersymbol Interference.
Three strategies for eliminating ISI:
Use a line code that is absolutely bandlimited.
Would require Sinc pulse shape.
Can’t actually do this (but can approximate).
Use a line code that is zero during adjacent sample instants.
It’s okay for pulses to overlap somewhat, as long as there is no overlap at the sample instants.
Can come up with pulse shapes that don’t overlap during adjacent sample instants.
Raised-Cosine Rolloff pulse shaping
Use a filter at the receiver to “undo” the distortion introduced by the channel.
Nyquist’s First Method for Zero ISI
Since pulses are not possible to create due to:
Infinite time duration.
Sharp transition band in the frequency domain.
The Sinc pulse shape can cause significant ISI in the presence of timing errors.
If the received signal is not sampled at exactly the bit instant (Synchronization Errors), then ISI will occur.
We seek a pulse shape that:
Has a more gradual transition in the frequency domain.
Is more robust to timing errors.
Yet still satisfies Nyquist’s first method for zero ISI.