-Until now, we have seen that the propagation channel weakens the signal emitted by the antenna, that is to say the signal transporting information. We will call it the useful signal. We also saw that the propagation channel limited the available frequency space for the transmission. This is the bandwidth notion and the fact that the emitted signal must be adapted to this bandwidth for the transmission. We will see in this sequence that the propagation channel also perturbs the useful signal by adding unwanted radio waves called noise. There can be multiple noise sources. Some are of natural origin, others of artificial origin. For example, in the atmosphere, in the case of storms, lightning and thunder can generate radio waves. The Earth but also the different stars in the Milky Way generate radio waves, especially the Sun, which is a powerful radio wave generator. If all these radio waves are received by the reception antenna in addition to the useful signal, they perturb it. Until now I only talked about natural radio wave sources. But these waves can also be created by human activity. A device supplying power or a device that needs some will also generate radio waves that can be received by the reception antenna and thus perturb the useful signal. Electronic devices within the transmitter, the receiver, the satellite, such as amplifiers, antennas, also generate parasite radio waves. This phenomenon is slightly different. It is the electronic excitement, the disordered excitement of electrons under the influence of heat that generates these parasite radio waves. If we want to measure the damage created by this noise, we can define the signal-to-noise ratio which represents the ratio between the power of the signal and the power of noise. It is usually expressed in dB. We know for example that a 0 dB signal-to-noise ratio corresponds to equal signal and noise powers. A positive signal-to-noise ratio corresponds to the signal being more powerful than noise. A negative signal-to-noise ratio corresponds to noise being more powerful than the signal. This noise we just talked about, all these parasite radio waves received by the antenna or generated by the receiver, the transmitter, the satellite, are usually grouped as a single source called noise. This is usually additive noise. It is added to the useful signal. It is usually white. Whiteness is a spectral property. It is an analogy with white light. White light is a light in which all frequencies can be found. A white noise is a noise with a constant energy distribution whatever the frequency. So it will perturb all the frequencies in the useful signal in the same way. Noise is also usually Gaussian. The fact that it is Gaussian is a time property. Its amplitudes are distributed following a certain Gaussian or normal law. Here are a few signal examples. The first, that you now know quite well, is a NRZ-type signal. It is the useful signal emitted by the antenna which includes the binary information to transport. The second signal, in green here, is a received signal attenuated by the propagation channel. You see that the shape of the signal has not changed. But its dynamics has changed, it has been reduced. The signal has been weakened. The next two signals, yellow and blue, are noisy signals. The emitted signal has been perturbed by a certain amount of radio waves in the propagation channel that were added and perturbed it. We see that both signals are not perturbed in the same way. This is normal since the signal-to-noise ratio is not the same. The signal-to-noise ratio equals 10 dB for the yellow signal, thus a signal ten times more powerful than noise. The signal-to-noise ratio equals 0 dB for the blue signal, that is to say equivalent noise and signal powers. The perturbation is more important when noise is more powerful. Remember that the quality of a digital transmission depends on the bit error rate. Of course, the bit error rate is a function of the signal-to-noise ratio. But we can have an acceptable bit error rate with noise. You will see this when talking about channel coding next week.
