手机信号为何不会互相干扰?

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:

节目组收到Facebook网友提问：“为什么我的手机通话不和邻居的串音呢？”为了找到这个问题的答案，记者采访了《无线通信》作者安德里亚．戈德史密斯。戈德史密斯说，这是因为每个用户都得到了频谱当中的特定频段。所谓频谱，是指无线通信当中应用的频率范围，类似于可见光谱，只是带无线电波。她解释道：

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.

安德里亚．戈德史密斯：人们可以采取某种方式来划分频谱，不管是通过时间、频率或者编码加以划分，然后将频段分配给用户，使他们之间的通话不受干扰。

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.

戈德史密斯说，大多数第二代手机或2G手机采用时间划分和频率划分相结合的方式。就时间划分而言，手机用户共享一个频段，但各自通话不串音的原因在于用户通话起始会得到使用该频的特定时间段。最近的蜂窝基站将用户通话内容按秒数分割，如同高速旋转一台罗勒德斯名片架，能随时出现各种各样的名片。戈德史密斯表示，这种频谱划分方式尽管行之有效，却并非尽善尽美。

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.

安德里亚．戈德史密斯：由于频谱资源有限，我们很难让每个用户享有特定的频段，只能不断地将它们回收利用，导致用户之间出现通信干扰。这也就是为什么通话当中有时会出现掉话或音质差的原因。