Deliver all those messages
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.
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.
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.
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.
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.
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.
Slices of time and spectrum
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).
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).
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.
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.
End of line for 3G
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.
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.
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.