Intersymbolinterferenzen

Intersymbolinterferenzen, often abbreviated as ISI, is a phenomenon that occurs in telecommunications and digital signal processing. It describes the distortion of a signal where one symbol interferes with subsequent symbols. This interference arises because the response of a channel to a transmitted pulse or symbol does not decay rapidly enough. As a result, the tail of a previous symbol's signal overlaps with the current symbol's signal, corrupting its intended shape and potentially leading to errors in detection.

ISI is a major limiting factor in achieving high data rates over many communication channels, especially those with limited bandwidth or that exhibit frequency-selective fading. Factors contributing to ISI include channel imperfections, such as multipath propagation where signals arrive at the receiver via multiple paths of different lengths, and the presence of filters at the transmitter or receiver that have non-ideal responses. The effect of ISI is particularly pronounced when symbols are transmitted at a high rate, as the time available for the channel's response to decay between symbols becomes shorter.

To mitigate ISI, various techniques are employed. Equalization is a common method, where a filter is used at the receiver to counteract the distortion introduced by the channel. Adaptive equalizers can adjust their characteristics over time to compensate for changing channel conditions. Other approaches include the use of specific pulse shaping techniques at the transmitter, such as Nyquist pulse shaping, which aims to create pulses that do not interfere with each other when sampled at the symbol rate. Error correction codes can also help to recover from the data errors caused by ISI.