Signals
Noise in time and frequency

A very important concept in communications systems, including WLANs, is noise. The word noise has a general meaning of undesirable sounds. However, in the context of telecommunications, noise can be better defined as undesirable voltages from both natural and technological sources. Since noise is just another signal that produces waves, the noise will be added to other signals, as previously discussed. If the affected signal is representing information in a communications system, the noise can change the information. Clearly, this is not acceptable.

Relevant to a WLAN, sources of noise in a WLAN include the electronics in the WLAN system, plus radio frequency interference (RFI), and electromagnetic interference (EMI) found in the WLAN environment. By studying noise, network designers can find ways to reduce the effects of noise on the WLAN system.

One form of noise is called Gaussian, or white noise. The spectrum analyzer graph of white noise is a straight line across all frequencies. Theoretically, Gaussian noise affects all different frequencies equally. In actuality, white noise does not follow such a simple pattern. However, this is still a very useful concept when studying communications systems. Because white noise would affect all of the frequencies in a radio signal equally, there are implications for both the transmitter and receiver circuitry.

Another form of noise is called narrowband interference. The term band refers to a grouping of frequencies. A narrowband has a relatively smaller range of frequencies. FM radio is an example of narrowband interference. While white noise would disturb all radio stations equally, narrowband interference would only interfere with some radio stations.

Both forms of noise are important in understanding WLANs. Since white noise would degrade the various channels equally, the various components of FHSS and DSSS would be equally affected. Narrowband interference might disrupt only certain channels or spread spectrum components. It might even be possible to use a different channel to avoid the interference entirely.