On February 22, AT&T is expected to turn off its 3G cellular network. T-Mobile is scheduled to shut down on July 1, and Verizon is expected to follow suit on December 31.

The vast majority of cell phones in service operate on 4G/LTE networks, and the world has begun the transition to 5G, but up to 10 million phones in the United States still rely on 3G service. Additionally, the cellular network functions of some older devices like Kindles, iPads, and Chromebooks are tied to 3G networks. Similarly, some older Internet-connected systems, such as home security, car navigation and entertainment systems, and solar-panel modems, are 3G-specific. Consumers will need to upgrade or replace these systems.

So why are telcos turning off their 3G networks? As an electrical engineer who studies wireless communications, I can explain. The answer starts with the difference between 3G and later technologies such as 4G/LTE and 5G.

Imagine a family trip. Your spouse is on the phone arranging activities to do at the destination, your teenage daughter is streaming music and chatting with her friends on her phone, and her younger brother is playing an online game with his friends. All of these separate conversations and data streams are communicated over the cellular network, seemingly simultaneously. You probably take this for granted, but have you ever wondered how the cellular system can handle all these activities at the same time, from the same car?

Communicate all these messages

The answer is a technological trick called multiple access. Imagine using a sheet of paper to write messages to 100 different friends, one private message for each person. The multiple access technology used in 3G networks is like writing each message to each of your friends using the whole sheet of paper, so all the messages are written on top of each other. But you have a special set of different colored pens that allow you to write each message in a unique color, and each of your friends has a special pair of glasses that only reveal the color intended for that person.

However, the number of colored pens is fixed, so if you want to message more people than the number of colored pens you have, you will need to start mixing colors. Now when a friend applies their special lenses, they will see a few messages to other friends. They won’t see enough to read the other messages, but the overlap might be enough to blur the message intended for them, making it harder to read.

The multiple access technology used by 3G networks is called Code Division Multiple Access, or CDMA. It was invented by Qualcomm founder Irwin M. Jacobs along with several other leading electrical engineers. The technique is based on the concept of spread spectrum, an idea that dates back to the early 20th century. Jacobs’ 1991 paper showed that CDMA can increase cellular capacity many times over on systems of the time.

CDMA allows all cellular users to send and receive their signals at any time and on any frequency. So if 100 users want to initiate a call or use cellular service at around the same time, their 100 signals will overlap across the entire cellular spectrum for the duration of their call.

Overlapping signals create interference. CDMA solves the interference problem by leaving each user with a unique signature: a code sequence that can be used to retrieve each user’s signal. The code 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.

Time slices and spectrum

Instead of allowing users to share the entire cellular spectrum at all times, other multiple access techniques divide access based on time or frequency. Time slicing creates time slots. Each connection can last several time slots spread over time, but each time slot is so short – a matter of milliseconds – that the mobile phone user does not perceive the interruptions of the alternating time slots. The connection appears to be continuous. This time slicing technique is time division multiple access (TDMA).

The division can also be done in frequency. Each connection receives its own frequency band in the cellular spectrum, and the connection is continuous throughout its duration. This frequency slicing technique is frequency division multiple access (FDMA).

In our paper analogy, FDMA and TDMA are like dividing paper into 100 strips in two dimensions and writing each private message on a strip. FDMA would be, for example, horizontal stripes, and TDMA would be vertical stripes. With individual tapes, all messages are separated.

4G/LTE and 5G networks use Orthogonal Frequency Division Multiple Access (OFDMA), a very efficient combination of FDMA and TDMA. In the paper analogy, OFDMA is like drawing strips in both dimensions, dividing the whole paper into multiple squares and assigning each user a different set of squares based on their data needs.

End of line for 3G

You now have a basic understanding of the difference between 3G and the latest 4G/LTE and 5G. You could still reasonably ask why 3G needs to be stopped. It turns out that due to these differences in access technology, the two networks are built using completely different equipment and algorithms.

3G handsets 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 slices of the spectrum. These two systems require completely different sets of hardware, from the cell tower antenna to the components in your phone.

So if your phone is a 3G phone, it cannot connect to a 4G/LTE or 5G tower. For a long time, cellular service providers maintained their 3G networks while building a completely separate network with new tower equipment and servicing new handsets using 4G/LTE and 5G. Imagine you are incurring the cost of operating two separate networks at the same time for the same purpose. Finally, you have to go. And now, as carriers begin to roll out 5G systems in earnest, that time has come for 3G.

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

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