Noise

Author: Edmund A. Laport

Signal intelligibility is always compromised by the presence of noise. Interference can also be regarded as noise having different statistical properties from receiver noise, or to the broad group of radiations classed generally as static, whether natural or man-made.

Every type of communication system has a certain signal-to-noise ratio below which the intelligibility of the signal is insufficient for communication. If the incoming signal-to-external-noise ratio is below this minimum value, obviously the only thing that can be done to obtain serviceable operation at the optimum working frequency is to increase effective radiated power over the path of propagation. Under some circumstances, advantage may be taken of the directivity of the receiving antenna, making it responsive to the signal and blind to the sources of noise. This works only when signal and noise directions are actually different and when suitable directivity can be obtained. We assume that, beyond the antenna, the receiver bandwidth is no more than necessary to accept the spectrum of the incoming signal, so that no unnecessary extraband noise is admitted. The received signal-to-external-noise ratio is very often improved by directive antennas, but there are times when this is not at all true. In high-noise regions when signal-to-external-noise ratios are typically low, it is difficult to do anything in the way of antenna engineering that will improve the ratio. Almost any antenna appears to give the same performance. Any reduction of response to the noise makes the system equally unresponsive to signal on the same frequency, with no change in signal-to-noise ratio. Any measures that increase the received power merely reduce the gain required in the receiver, without any net improvement in circuit performance.

In very low noise regions there is a different set of prevailing conditions. Not limited by external noise, very weak signals may be utilized by employing adequate receiver gain. Receiver noise may then be the controlling factor. In this case, signal-to-noise ratio can always be improved by any means that will increase the signal power delivered to the receiver. Directivity will always be beneficial (assuming no angular deviations), and there is reason to be careful with antenna-feeder impedance matching at the working frequency so that system loss will be minimized. These practices are relatively useless in regions of high ambient noise.

In speaking about these extremes it is necessary to remember that the majority of practical cases occur somewhere in between; also, that at any one location it is possible for both extremes to be encountered occasionally. The practices adopted are then based on the percentage of time they occur. Tropical or arctic techniques may prevail according to which is most typical of the location.