Alien crosstalk (AXT) is electromagnetic noise that can occur in a cable that runs alongside one or more other signal-carrying cables. The term "alien" arises from the fact that this form of crosstalk occurs between different cables in a group or bundle, rather than between individual wires or circuits within a single cable.
Alien crosstalk can be particularly troublesome because, unlike the simple crosstalk caused by a single interfering signal, it cannot be eliminated by phase cancellation. Alien crosstalk arises from multiple signals, and includes mixing products in which phantom signals at innumerable sum and difference frequencies blend with the originating signals. The result is a "hash" of electromagnetic noise that is too complex to be dealt with by phase-cancellation measures. Because it resembles noise rather than signals, alien crosstalk degrades the performance of a communications system by reducing the signal-to-noise ratio (S/N).
Alien crosstalk can be minimized or eliminated by avoiding configurations in which cables are bundled together or run parallel to one another in close proximity. If cables must be run parallel to each other, each cable can be surrounded by a grounded metal braid (electromagnetic shield) to prevent electromagnetic fields from entering or leaving the cable. This in effect isolates the cables from one another. However, it is an expensive solution and it can also increase cable loss per unit length.
Alien crosstalk should not be confused with simple crosstalk or near-end crosstalk (NeXT). Simple crosstalk is caused by the electric or magnetic fields of one telecommunication signal affecting a signal in an adjacent circuit. For example, in a telephone circuit, simple crosstalk can result in your hearing part of a voice conversation from another circuit. NEXT is a form of simple crosstalk that sometimes occurs when connectors are improperly attached to twisted pair cabling, resulting in crossed or crushed wire pairs. The term NEXT is also used in reference to simple crosstalk that occurs between two twisted pairs within a single cable.