手机通话为何不串音？(双语有声)

An EarthSky Facebook friend asked us, “Why don’t cell phone conversations on my phone get mixed up with my neighbor’s?” To find out, we asked Andrea Goldsmith, author of the book Wireless Communications. Goldsmith said that 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 that you assign it to users so they don’t interfere with each other.

Goldsmith said that 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 time slot 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. So sometimes that’s why calls get dropped or why you get poor quality.