Noise, One of the Limitations of Wireless Transmissions

Wireless transmissions are ubiquitous in our everyday lives, encompassing phones, Wi-Fi, TV and GPS. Transmission speed is steadily increasing due to technological advances and the use of higher frequencies. Let’s take a look at one of the main inhibiting factors of this output: noise.

In layman’s terms, noise is one or more sounds that disrupt our hearing. It is the same for transmissions. To understand the origin of noise, we will look at its makeup, including temperature and electric current.

Temperature is related to the movement of particles. With no movement, temperature is minimal; at −273.15 °C (0 kelvin) it is known as absolute zero. Our usual temperatures therefore indicate considerable movement at the microscopic level.

As for an electric current, it is a flow of electrons circulating in a conductive material. This flow usually comes from a generator (battery, alternator, solar panel, etc.) and is “organized”, i.e. it is constant or follows a specific pattern (e.g. sinusoidal or square).

Now, let’s link the two notions. When a conductive material is at room temperature, its particles are in motion and electrons circulate inside, creating an electric current. The flow of electrons is not “organized”: the electrical signal is random and contains no information (see 1st figure). It is therefore a nuisance, which is why it is called “noise”, or thermal noise.

Therefore, all receivers create noise that is superimposed on the useful signals it receives. This noise is extremely low, in the order of a femtowatt (millionth of a billionth of a watt), but since signals received by wireless transmission are also very weak, the noise is certainly not negligible. As proof, the figure represents in reality a GPS signal masked by noise a hundred times more powerful. The direct consequence is the potential for errors during transmission; which makes it necessary to compromise between a desired data rate and a tolerable error level.

However, noise does not run amok either. Like any random phenomenon, such as the outcome of a dice roll or the size of an individual, it has certain rules related to probabilities and statistics. For example, when we observe the signal amplitude histogram (centre figure), i.e. the amplitude distribution, we see the well-known Gaussian pattern. Thanks to these rules, we know how to characterize noise and calculate its power in order to predict transmission performance and estimate achievable data rate.

Additional Information

This article was submitted to the science journalism competition Plume de science, organized by Science pour tous for the 24 hours of science 2018, and finished 3rd in the adult category. The winning articles can be found on the Science pour tous blog on the Agence Science Presse website.