Why don’t cell phone calls get mixed up?

An EarthSky Facebook friend asked us, “Why don’t cell phone conversations on my phone get mixed with my neighbor’s?” To find out, we asked Andrea Goldsmith, author of the book Wireless Communications. Goldsmith said each call gets a unique assignment in the frequency spectrum – that’s the range of frequencies that are used in wireless communications – kind of like the visible light spectrum, only with radio waves. She explained:

Andrea Goldsmith: You can break up the frequency spectrum in some way whether it’s time or frequency or codes so you assign it to users so they don’t interfere with each other.

Goldsmith said most second-generation, or 2G, cell phones use a combination of time division and frequency division. In time division, Goldsmith said that you’re sharing a frequency channel with other phones, but the reason your conversation stays exclusive is that your call gets assigned a specific timeslot on that channel at the start of your call. The nearest cellular base station transmits your call in chopped up fractions of seconds, as if it’s flipping through a rolodex and your card keeps coming up. While this division of the frequency spectrum works, said Goldsmith, it’s not infallible.

Andrea Goldsmith: There’s not enough spectrum to assign every phone in the world its own dedicated channel so we have to reuse the channels and that causes interference between users. Sometimes that’s why calls get dropped or why you get poor quality.

The base station assigns your call a unique combination of time, frequency or code that determines how it accesses radio frequencies.

Andrea Goldsmith: Some amount of time it transmits to the first phone, some amount of time it transmits to the second, and some amount of time to the third phone, and then it comes back to the first phone, and because this is going very quickly you don’t notice the delay. It’s the same radio signal but at different times.

Meaning your call is assigned a unique time, frequency or code that determines how you access radio frequencies and that keeps your call separated from others. Second generation phones, which incorporate all of these techniques to break up the frequency spectrum, are the most widespread mobile devices. Most of these use the GSM network– short for Global System for Mobile Communications.

Andrea Goldsmith: GSM phones use a combination of time and frequency. You have the base station of the cellular system that’s transmitting to mobile phones. Some amount of time it transmits to the first phone, some amount of time it transmits to the second, and some amount of time to the third phone, and then it comes back to the first phone, and because this is going very quickly you don’t notice the delay. It’s the same radio signal but at different times.