Interference

When working with waves, one plus one does not necessarily equal two. It can also result in zero.

Figure 2.9: Constructive and destructive interference.

This is easy to understand when you draw two sine waves and add up the amplitudes. When peak hits peak, you will have maximum results (1 + 1 = 2). This is called constructive interference. When peak hits valley, you will have complete annihilation ((1 + (-)1 = 0) - destructive interference.

You can actually try this with waves on water and two little sticks to create circular waves -you will see that where two waves cross, there will be areas of higher wave peaks and others that remain almost flat and calm.

In order for whole trains of waves to add up or cancel each other out perfectly, they would have to have the exact same wavelength and a fixed phase relation, this means fixed positions from the peaks of the one wave to the other's.

In wireless technology, the word Interference is typically used in a wider sense, for disturbance through other RF sources, e.g. neighbouring channels. So, when wireless networkers talk about interference they typically talk about all kinds of disturbance by other networks, and other sources of microwave. Interference is one of the main sources of difficulty in building wireless links, especially in urban environments or closed spaces (such as a conference space) where many networks may compete for use of the spectrum.

Whenever waves of equal amplitude and opposite phase cross paths, the wave is annihilated and no signal can be received. The much more common case is that waves will combine to form a completely garbled waveform that cannot be effectively used for communication. The modulation techniques and use of multiple channels help to deal with the problem of interference, but does not completely eliminate it.