Editing Intersymbol interference (section)

== Causes ==

=== Multipath propagation ===
{{Main|Multipath propagation}}
One of the causes of intersymbol interference is [[multipath propagation]] in which a wireless signal from a transmitter reaches the receiver via multiple paths. The causes of this include [[reflection (physics)|reflection]] (for instance, the signal may bounce off buildings), [[refraction]] (such as through the [[foliage]] of a tree) and atmospheric effects such as [[atmospheric ducting]] and [[ionospheric reflection]]. Since the various paths can be of different lengths, this results in the different versions of the signal arriving at the receiver at different times. These delays mean that part or all of a given symbol will be spread into the subsequent symbols, thereby interfering with the correct detection of those symbols. Additionally, the various paths often distort the [[amplitude]] and/or [[phase (waves)|phase]] of the signal, thereby causing further interference with the received signal.

=== Bandlimited channels ===

Another cause of intersymbol interference is the transmission of a signal through a [[bandlimited]] channel, i.e., one where the [[frequency response]] is zero above a certain frequency (the cutoff frequency). Passing a signal through such a channel results in the removal of frequency components above this cutoff frequency. In addition, components of the frequency below the cutoff frequency may also be attenuated by the channel.

This [[Filter (signal processing)|filtering]] of the transmitted signal affects the shape of the pulse that arrives at the receiver. The effects of filtering a rectangular pulse not only change the shape of the pulse within the first symbol period, but it is also spread out over the subsequent symbol periods. When a message is transmitted through such a channel, the spread pulse of each individual symbol will interfere with following symbols.

Bandlimited channels are present in both wired and wireless communications. The limitation is often imposed by the desire to operate multiple independent signals through the same area/cable; due to this, each system is typically allocated a piece of the total [[Bandwidth (signal processing)|bandwidth]] available. For wireless systems, they may be allocated a slice of the [[electromagnetic spectrum]] to transmit in (for example, [[FM radio]] is often broadcast in the 87.5–108&nbsp;[[MHz]] range). This allocation is usually administered by a [[government agency]]; in the case of the United States this is the [[Federal Communications Commission]] (FCC). In a wired system, such as an [[optical fiber cable]], the allocation will be decided by the owner of the cable.

The bandlimiting can also be due to the physical properties of the medium - for instance, the cable being used in a wired system may have a cutoff frequency above which practically none of the transmitted signal will propagate.

Communication systems that transmit data over bandlimited channels usually implement [[pulse shaping]] to avoid interference caused by the bandwidth limitation. If the channel frequency response is flat and the shaping filter has a finite bandwidth, it is possible to communicate with no ISI at all. Often the channel response is not known beforehand, and an [[adaptive equalizer]] is used to compensate the frequency response.