## Characteristics of noise

#### Noise is usually classified as white or coloured depending on the spectral density of the noise power with frequency. White Noise is defined as having a flat power spectral density over all frequencies of interest, with a value usually denoted as N0 Watts/Hz. |
| #### Coloured noise has a non-uniform spectral distribution; however, over a finite bandwidth corresponding perhaps to a single communications channel, the power spectral density might appear flat, hence the term bandlimited white Gaussian noise. Not only is it necessary for the spectrum to be flat, but the statistics of the noise must be such that the envelope distribution of the bandlimited noise must be Gaussian in nature to fully satisfy the Shannon condition. Fortunately this holds approximately true for the majority of practical narrowband communications systems.