How does cell service work?

Mobile phones, equipped with instruments for calling and accessing the internet, became unavoidable tools for various tasks in everyday life. Despite that, not so many people actually know how they work.

Electromagnetic waves

Regardless of whether it's using the internet or calling someone, cell phone has to transmit signals wirelessly. This is done using electromagnetic radiation.
We know that waves have couple of properties, main ones being amplitude and wavelength. Let's look into the last one.
When wavelength is between 400 and 700 nanometers, that wave represents a color in white light, i.e. visible spectrum of light (more about visible spectrum in our article about rainbows at THIS LINK ).
When wavelength is below 400nm, that wave has much more energy, and can pass through some less dense materials. Those can be X-rays, used to see inside a human body.
Lastly, when wavelength exceeds 700nm, it is considered radio wave. These waves are able to travel bigger distances and bend around objects, so they have all usefull properties for their use in comunication. To achieve that, we need a way to manipulate their properties and carry information.

Parts of electromagnetic spectrum. Credits: https://www.differenttruths.com/science-technology/how-was-electromagnetic-radiation-radio-waves-discovered/

Manipulating amplitude and wavelength

One way to transmit information via radio waves if by changing their amplitude (or "strength") on some places, depending on the transmitted value. This is known as AM (amplitude modulation). This communication has advantages - it can work on long distances, and it is simplier - but also huge drawback, as it is extremelly susceptible to interferences.
Other way is by changing their wavelength (frequency), known as FM (frequency modulation). This method is far more reliable, hence used more often.

Amplitude and wavelength. Credits: https://pressbooks.bccampus.ca/kpupsyc1100/chapter/waves-and-wavelengths/

Transmission

Now we know how information is "packed" into waves, but which form does that information travel in? Since sending original audio signal is inefficient, it is converted into binary sequence. In that sequence, higher amplitude or wavelength can represent 1, and lower represent 0.
To make it even more efficient, reciever can distinguish between different phases of a wave connected to each other. One phase (full wave cycle) can start at the midpoint going upward, second at the midpoint going downward, third starting at the peak (crest) and fourth starting at the trough. Then, each of these can get binary value (00, 01, 10 or 11) asigned to it. This way we can pack twice as much data into the same wave. When each of these is represented in higher and lower amplitude, we quadripple amount of data. It doesn't have to stop there, so some of the most advanced computers can process up to 32 768 combinations, in 15-digit binary sequences.
When signal is ready, it is sent to the reciever via cell sites.

Cell sites

Cell sites are consisted of multiple towers and instruments for modulating (converting data to signal in radio wave) and demodulating (converting signal to sound or text). In order to enable access to these towers everywhere, land is splitted into hexagons, each having a cell site in its center. Additionally, signals from these sites reach far enough to overlap neighbouring sites' signals, thus enabling user in motion to change transmitter without lossing connection.

Code division multiple access

Originally, only 832 frequencies were allocated for cell service. From that number, 42 were used for communication between towers, leaving 790 for calls. As both inbound and outbound frequency were required, there were only 395 call channels. As no two neighbouring sites could use the same frequencies, each site had 56 call channels, meaning 52 users in a site range could use it simultaniously. While phone's popularity was increasing, so did the problems because of these limits, but were solved with "code division multiple access" (CDMA).
This system allowed large number of users to simultaniously use the same cell site or tower. It allocates unique code, so called "spreading code" to each user. Binary codes sent by all users together with their spreading codes are, using mathematical operations and algorythms, converted into a single binary sequence transmitted between towers. When signal is received, same operations are reversed, breaking single sequence into data sent by multiple users.

Today, we use our phones and internet thanks to these principles, and switch to faster and faster networks thanks to the ingenuity of people finding new ways to send more data using the same resources.