Noise Temperature - Application To Communication Systems

Application To Communication Systems

A communications system is typically made up of a transmitter, a communications channel, and a receiver. The communications channel may consist of a combination of different physical media, resulting in an electrical signal presented to the receiver. Whatever physical media a channel consists of, the transmitted signal will be attenuated and corrupted with additive noise.

The additive noise in a receiving system can be of thermal origin (thermal noise) or can be from other noise-generating processes. Most noise processes will have a white spectrum, at least over the bandwidth of interest, identical to that of thermal noise. Since they are indistinguishable, the contributions of all noise sources can be lumped together and regarded as a level of thermal noise. The noise power spectral density generated by all these sources can be described by assigning to the noise a temperature as defined above:

In a wireless communications receiver, the equivalent input noise temperature would equal the sum of two noise temperatures:

The antenna noise temperature gives the noise power seen at the output of the antenna. The noise temperature of the receiver circuitry represents noise generated by noisy components inside the receiver.

Note that refers not to the noise at the output of the receiver after amplification, but the equivalent input noise power. In other words, the output of the receiver reflects that of a noiseless amplifier whose input had a noise level not of but of . Thus the figure of merit of a communications system is not the noise level at the speaker of a radio, for instance, since that depends on the setting of the receiver's gain. Rather we ask how much noise the receiver added to the original noise level, which is thus given by the factor . If a signal is present, then this factor represents the decrease in signal to noise ratio incurred using the receiver system with a noise temperature of .