Digital Communications

4Receiver System Noise

In Chapters 2 and 3, we studied the prediction of the signal power received under free‐space propagation conditions and in the presence of the Earth, that is, obstacles, mountains, hills, buildings, and so on. Consequently, we predicted the received signal power due to reflection, refraction, diffraction and scattering of electromagnetic waves by the obstacles in the propagation path. However, the SNR, the principal figure‐of‐merit of a communication system, is determined not only by the received signal level but also the received noise and interference levels. Noise is defined as a random time‐varying electromagnetic phenomenon which may be superimposed on, or combined with, a wanted signal. Noise varies randomly with time and does not carry information. [1] There are many sources of noise in the RF band that are internal and external to the receiving system. Internal noise is mostly of thermal origin and is generated by the receiver itself. Antenna losses, feeder lines connecting the antenna to the receiver and the receiver itself are the major contributors of the internal noise. The external noise is collected by the antenna and may be due to: [1][2][3]

1. Emissions from atmospheric gases and hydrometeors (water vapour, oxygen, nitrogen, rain, snow);
2. Radiation from extra‐terrestrial sources such as galaxies, sun, moon and stars;
3. The Earth’s surface;
4. Atmospheric noise due to lightning discharges (atmospheric noise due to lightning);
5. Man‐made noise due ...