自从人类了解了电磁波，通信技术的发展也是一波接着一波。
AT&T is scheduled to decommission its 3G cellular network, On February 22, 2022. T-Mobile is scheduled to be shut down on July 1, 2022, and Verizon is set to follow suit on December 31, 2022.
AT&T计划于2022年2月22日停用其3G蜂窝网络。T-Mobile计划于2022年7月1日关闭，Verizon也将于2022年12月31日关闭。
The vast majority of cell phones in the service run on 4G/LTE networks, and the world has begun the transition to 5G, but as many as 10 million phones in the US still rely on 3G service. In addition, the cellular network functionality of some older devices such as Kindles, iPads, and Chromebooks are connected to 3G networks. Similarly, some legacy internet-connected systems such as home security, in-car navigation and entertainment systems, and solar panel modems are for 3G. Consumers will need to upgrade or replace these systems.
该服务中的绝大多数手机都运行在4G/LTE网络上，世界已经开始向5G过渡，但美国仍有多达1000万部手机依赖3G服务。此外，Kindle、iPad和Chromebook等旧设备的蜂窝网络功能也连接到3G网络。类似地，一些传统的互联网连接系统，如家庭安全、车载导航和娱乐系统，以及太阳能电池板调制解调器也适用于3G。消费者需要升级或更换这些系统。
So why are carriers turning off their 3G networks? As an electrical engineer studying wireless communications, I can explain. The answer starts with the difference between 3G and newer technologies like 4G/LTE and 5G.
那么，运营商为什么要关闭3G网络呢？作为一名研究无线通信的电气工程师，我可以解释。答案从3G与4G/LTE和5G等较新技术之间的差异开始。
Imagine a family trip. Your wife is on the phone arranging activities to do at the destination, your teenage daughter is streaming music and talking with her friends on her phone, and her younger brother is playing an online game with his friends. All these separate conversations and data streams are connected over the cellular network, seemingly simultaneously. You might take this for granted, but have you ever wondered how a cellular system can handle all of these activities at the same time, from the same vehicle?
想象一次家庭旅行。你的妻子在电话上安排在目的地的活动，你十几岁的女儿在流媒体音乐，在手机上和朋友聊天，她的弟弟在和朋友玩网络游戏。所有这些独立的对话和数据流似乎同时通过蜂窝网络连接。你可能会认为这是理所当然的，但你有没有想过，一个蜂窝系统如何在同一辆车上同时处理所有这些活动？
when everyone in your car is using voice and cellular data at the same time
1 Deliver all those messages 传递所有这些信息 （码分多址 CDMA）
The answer is a technological trick called multiple access. Imagine using a sheet of paper to write letters to 100 different friends, one private message per person. The multiple access technology used in 3G networks is like writing each message to each of your friends using the whole paper, so that all messages are written on top of each other. But you have a special set of pens in different colors that allow you to write each message in a unique color, and each of your friends has a special pair of glasses that reveal only the color assigned to that person.
答案是一种称为多址接入的技术技巧。想象一下，用一张纸给100个不同的朋友写信，每人发一条私人信息。3G网络中使用的多址接入技术就像是用整张纸把每条信息都写给你的每个朋友，这样所有的信息都被写在彼此的上面。但是你有一套不同颜色的特殊钢笔，可以让你用一种独特的颜色写每条信息，你的每个朋友都有一副特殊的眼镜，只显示分配给那个人的颜色。
However, the number of colored pens is fixed, so if you want to send messages to more people than the number of colored pens you have, you will need to start mixing the colors.
然而，彩色笔的数量是固定的，所以如果你想发送信息给更多的人，而不是你拥有的彩色笔的数量，你需要开始混合颜色。
Now when a friend applies his own lenses, he will see few messages to other friends. They won’t see enough to read other messages, but the interference may be enough to blur the message meant for them, making it hard to read.
现在，当一个朋友使用自己的镜片时，他几乎看不到给其他朋友的信息。他们看不到足够多的信息来阅读其他信息，但干扰可能足以模糊为他们准备的信息，使其难以阅读。
The multiple access technology used by 3G networks is called Code Division Multiple Access, or CDMA. It was invented by Qualcomm founder Irwin Jacobs along with many other notable electrical engineers. The technology is based on the concept of spread spectrum, an idea that can be traced back to the early 20th century. Jacobs’ 1991 paper showed that CDMA could increase cellular capacity several times over systems at the time.
3G网络使用的多址技术被称为码分多址（CDMA）。它由高通公司创始人欧文·雅各布斯（Irwin Jacobs）和许多其他著名的电气工程师共同发明。这项技术基于扩频的概念，这个概念可以追溯到20世纪初。雅各布斯1991年的论文表明，CDMA可以将当时的系统的蜂窝容量提高数倍。
CDMA allows all cell users to transmit and receive their signals at all times and over all frequencies. So if 100 users want to initiate a call or use a cellular service at about the same time, their 100 signals will overlap each other across the entire cellular spectrum for the entire time they are communicating.
CDMA允许所有蜂窝用户在所有时间和频率上发送和接收信号。因此，如果100个用户想要在大约相同的时间发起呼叫或使用蜂窝服务，那么他们的100个信号将在整个蜂窝频谱中在他们通信的整个时间内相互重叠。
Overlapping signals create interference. CDMA solves the problem of interference by allowing each user to obtain a unique signature: a code sequence that can be used to retrieve each user’s signal. The symbol corresponds to the color in our paper analogy. If there are too many users on the system at the same time, the codes may overlap. This leads to interference, which gets worse as the number of users increases.
重叠信号会产生干扰。CDMA通过允许每个用户获得唯一的特征码来解决干扰问题：一个可用于检索每个用户信号的码序列。这个符号对应于我们纸上的颜色。如果系统上同时有太多用户，代码可能会重叠。这会导致干扰，随着用户数量的增加，干扰会变得更严重。
2 Slices of time and spectrum 时间和光谱切片 （时分多址 TDMA、频分多址FDMA）
Rather than allowing users to share the entire cellular spectrum at all times, other multiple-access technologies segment access by time or frequency. Segmentation over time creates time slots. Each connection can proceed over multiple time slots distributed in time, but each time slot is so short – a matter of milliseconds – that the mobile user is unaware of the interruptions caused by the alternating time slots. The connection seems to be going on. The slicing technology this time is Time Division Multiple Access (TDMA).
其他多址技术不允许用户随时共享整个蜂窝频谱，而是按时间或频率分段接入。随着时间的推移，细分会产生时间段。每个连接都可以在时间上分布的多个时隙上进行，但每个时隙都很短（仅为毫秒），以至于移动用户不知道交替时隙造成的中断。这种联系似乎正在继续。这次的切片技术是时分多址（TDMA）。
Frequency division can also be performed. Each communication is given its own frequency band within the cellular spectrum, and the communication is continuous for its duration. This frequency slicing technology is Frequency Division Multiple Access (FDMA).
也可以进行分频。在蜂窝频谱中，每个通信都有自己的频带，并且通信在其持续时间内是连续的。这种频率切片技术就是频分多址（FDMA）。
In our paper analogy, FDMA and TDMA are like dividing the paper into 100 bars in either dimension and writing each private message on one strip. For example, FDMA will be horizontal bars, and TDMA will be vertical bars. Using individual strips, all messages are separated.
在我们的纸张类比中，FDMA和TDMA就像是将纸张在任意维度上分成100条，并将每条私人消息写在一条纸带上。例如，FDMA将是水平条，TDMA将是垂直条。使用单独的条带，所有消息都被分隔开。
4G/LTE and 5G networks use orthogonal frequency division multiple access (OFDMA), a highly efficient combination of FDMA and TDMA. In the paper analogy, OFDMA is like drawing strips along both dimensions, dividing the entire paper into many squares, and assigning a different set of squares to each user according to their data need.
4G/LTE和5G网络使用正交频分多址（OFDMA），这是FDMA和TDMA的高效组合。在纸张类比中，OFDMA就像是沿着两个维度绘制条带，将整个纸张分割成许多正方形，并根据每个用户的数据需求为他们分配一组不同的正方形。
3 End Of 3G? 3G时代的终结？
Now you have a basic understanding of the difference between 3G, 4G/LTE and later 5G. You may still reasonably ask why 3G should be turned off. It turns out that due to these differences in access technology, the two networks are built using completely different equipment and algorithms.
现在，您已经基本了解了3G、4G/LTE和更高版本的5G之间的区别。你可能仍然会合理地问为什么应该关闭3G。事实证明，由于接入技术的这些差异，这两个网络是使用完全不同的设备和算法构建的。
3G phones and base stations operate on a broadband system, which means they use the entire cellular spectrum. 4G/LTE and 5G operate on narrowband or multi-carrier systems, which use segments of the spectrum. These two systems require completely different sets of hardware, from the antenna on your cell tower to the components in your phone.
3G手机和基站在宽带系统上运行，这意味着它们使用整个蜂窝频谱。4G/LTE和5G在窄带或多载波系统上运行，这些系统使用频谱段。这两个系统需要完全不同的硬件，从手机发射塔上的天线到手机中的组件。
So if your phone is a 3G phone, it can’t connect to a 4G/LTE or 5G tower. For a long time, cellular providers kept their 3G networks going while building a completely separate network with new tower equipment and servicing new phones with 4G/LTE and 5G. Imagine incurring the cost of running two separate networks at the same time for the same purpose. Ultimately, one has to go. And now, with carriers starting to roll out 5G systems in earnest, it’s time for 3G.
因此，如果你的手机是3G手机，它就无法连接到4G/LTE或5G基站。很长一段时间以来，手机运营商一直在维持其3G网络的运行，同时使用新的塔式设备构建一个完全独立的网络，并使用4G/LTE和5G为新手机提供服务。想象一下，为了同样的目的同时运行两个独立的网络会产生成本。最终，一个人必须离开。现在，随着运营商开始认真推出5G系统，是时候结束3G了。
ref:
https://www.lengzhishi.net/profile/hunziyue
https://webghost.bid/end-of-3g-electrical-engineer-explains/
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