# 3.2 Intersymbol interference (ISI)

## The problem of intersymbol interference

#### With any practical channel, the inevitable filtering effect will cause a spreading (smearing) of individual data symbols passing through the channel. For consecutive symbols, this spreading causes part of the symbol energy to overlap with neighbouring symbols, causing intersymbol interference (ISI). Additionally, filtering in the transmitter or receiver units themselves may also introduce ISI degradation. |

| #### Unless very careful design steps are taken, intersymbol interference can significantly degrade the ability of the data detector to differentiate a current symbol from diffused energy of adjacent symbols. Even with no noise present in the channel this can lead to detection errors, termed an irreducible error rate, and at the very least will degrade the bit and symbol error rate performance in the presence of noise.

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