What are the factors that cause intersymbol interference (ISI)?

Inter symbol interference is essentially caused by the dispersion of the channel. It can be avoided by leaving enough space in between the transmitted symbols. An obvious drawback of this primitive approach is that the throughput will decrease.

A smarter method is to use an equalizer, which acts like an “anti-channel”, limiting the dispersion. The first equalizers processed individual samples. More recent approaches take a block of samples and process these collectively. Modern communication systems, e.g. LTE have equalizers that process such blocks in the frequency domain, which is more efficient.

One of the goals of pulse shaping/windowing at the transmitter is to limit the out-of-band radiation. However, doing this shaping within the limits of the transmitted symbol (or block) itself will distort it (*). To avoid this, the symbol (or block) is extended in time and the shaping is perfomed on this extension. Obviously, introducing such an extension will increase the risk of overlap in time with other symbols (or blocks), causing ISI.

(*) for instance in a multicarrier block transmission like OFDM, a straightforward shaping of the transmitted blocks will lead to a loss of orthogonality between the carriers and thus intercarrier-interference (ICI).