What Is 3G Cellular Service and Why Is It Being Shut Down? An Electrical Engineer Explains

The sun is setting on 3G networks. Credit: Ted/Flickr, CC BY-NC

On February 22, 2022, AT&T began shutting down its 3G cellular network. T-Mobile is scheduled to shut it down on July 1, 2022, and Verizon is scheduled to follow suit on December 31, 2022.

The vast majority of mobile phones in service operate on 4G/LTE networks and the world has started the transition to 5G, but as many as 10 million phones in the US still rely on 3G service. Additionally, some older devices such as Kindles, iPads, and Chromebooks have cellular network capabilities tied to 3G networks. Similarly, some legacy internet-connected systems such as home security, car navigation, entertainment systems, and solar panel modems are 3G specific. Consumers will need to upgrade or replace these systems.

So why are telcos shutting down their 3G networks? As an electrical engineer studying wireless communications, I can explain. The answer starts with the difference between 3G and later technologies like 4G/LTE and 5G.

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

How does it work when everyone in your car is using mobile voice and data service at the same time, and many of the people in the cars around you are too?

Communicating all of these messages

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

However, the number of colored pencils is fixed. So if you want to send messages to more people than you have colored pencils, you need to start mixing colors. Now when a friend uses their special lenses, they see a small portion of the messages sent to other friends. They won’t see enough to read the other messages, but the overlap might be enough to blur the message meant for them, making it difficult 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 prominent electrical engineers. The technique is based on the concept of spread spectrum, an idea that can be traced back to the early 20th century. Jacobs’ 1991 publication showed that CDMA can increase cellular capacity many times over systems of the time.

CDMA enables all mobile phone users to transmit and receive their signals at any time and over any frequency. So if 100 users want to make a call or use cellular service at roughly the same time, their 100 signals will overlap throughout the communication time across the cellular spectrum.

The overlapping signals create interference. CDMA solves the interference problem by giving each user a unique signature: a code sequence that can be used to recover 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 can overlap. This leads to interference that 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 by time or frequency. The division over time creates time windows. Each connection can last for several time-slots spread out in time, but each time-slot is so short – a matter of milliseconds – that the mobile phone user is not aware of the breaks of alternating time-slots. The connection seems to be continuous. This time slicing technique is time division multiple access (TDMA).

The division can also be made according to frequency. Each connection is assigned its own frequency band within the cellular spectrum and the connection is continuous for its duration. This frequency slicing technique is frequency division multiple access (FDMA).

In our paper analogy, FDMA and TDMA are like dividing the paper into 100 strips in each dimension and writing each private message on one strip. For example, FDMA would be horizontal stripes and TDMA would be vertical stripes. In the case of individual strips, all messages are separated.

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 all the paper into many squares, and allocating a different set of squares for each user according to their data needs.

Various techniques for sharing access to wireless network resources. Entropy 2019, 21(3), 273, CC BY-SA

Terminus for 3G

Now you have a basic understanding of the difference between 3G and the later 4G/LTE and 5G. You might still be wondering why 3G needs to be shut down. It turns out that because of these differences in access technology, the two networks are built with completely different devices and algorithms.

3G cell 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 using parts of the spectrum. These two systems require completely different hardware, from the antenna on the cell tower to the components in your phone.

So if your phone is a 3G phone, it cannot connect to a 4G/LTE or 5G tower. Cell phone providers have been keeping their 3G networks running for a long time while building a completely separate network with new tower equipment and maintaining new cell phones with 4G/LTE and 5G. Imagine bearing the cost of running two separate networks simultaneously for the same purpose. At some point someone has to go. And now that carriers are seriously starting to deploy 5G systems, the time has come for 3G.

Written by Mai Vu, Associate Professor of Electrical and Computer Engineering, Tufts University.

This article was first published in The Conversation.