[♪ INTRO] By now, it seems like we’d have ironed out all the kinks in cell phone communication.

But sometimes, we still find ourselves on a call or browsing our phones somewhere with plenty of reception when suddenly, the call drops or the internet slows way down. While that might seem like just another puzzling failure of technology, the reason might not be a faulty cell tower, your surroundings, or even your phone.

Instead, the culprit might be interference. And it’s a byproduct of how cell phones work in the first place, at least when lots of us want to use them. It turns out whether we get decent phone reception comes from what the rest of us are doing and how we all share the airwaves.

What’s more, as our collective appetite for data keeps growing and 5G comes into play, cell phone interference could actually become an even bigger problem. Luckily, engineers are really good at getting around the limitations inherent to cell phone technology. So today, we’re going to talk about how your phone network sort of works against itself, and how we make them work anyway.

It all comes down to the basic way a cell phone works. Cell phones are basically just complicated radios. They send radio waves carrying a signal back and forth between radio towers, which are connected in a network with other towers, sending radio signals to phones.

Those signals carry data, like a phone call, a text message, or my face on your screen. The complicated part is actually encoding data on a radio wave. Radio waves, like the rest of the electromagnetic spectrum, have two key properties: amplitude, which is how large the wave is, and frequency, the number of times a wave cycles in a given time.

By changing both of those properties across a wave, we can encode information in them, although most of the focus is on the frequency part. Broadly speaking, cell phones use a slice of the radio frequency spectrum in the 800 to 4000 Megahertz range. By using different frequencies we can encode the kinds of data we want onto the wave.

For instance, we could use a series of high and low frequencies to represent a stream of 1s and 0s, encoding digital data. The crucial thing is that it takes a range of different frequencies to do this, which we call a band. The width of that band, or in other words, the amount of frequencies we can use for a single phone or even a whole network, is called bandwidth.

And this all works just fine in the case of one phone on one tower. The tricky part is when other phones come into play. If lots of cell phones all use the same band of frequencies in the same way to communicate with a cell tower, their signals overlap with each other and become garbled.

That’s because radio waves can add and subtract from one another to create new waves, which we call interference. If the peaks of two waves line up, they add up to a higher peak. If a peak and a trough line up, you get a flat bit.

That kind of thing. Interference isn’t always a bad thing, but when our signals get mixed up by it, a cell tower has no hope of disentangling all the incoming waves. And the same thing can happen for our phone receiving signals that use the same bands.

So how do we connect lots of phones to a single cell tower, and vice versa? The simplest solution is to give everyone a different band of frequencies to use. That way, a cell tower can associate each phone with its own unique band of frequencies, and they don’t interfere with each other when being received by the tower.

The signals from each phone get distinguished by their bands, which a cell tower can separate out to send and relay across the network. While that sounds great, the kicker is we only have so many frequencies we can use. Remember, we’re only looking at some parts of the spectrum between 800 to 4000 megahertz, and you can only split them up so far.

This is partly because of engineering constraints, but also because radio waves have lots of different uses, like air and sea navigation, television broadcasts, and even hobbyists. So not all the frequencies are up for grabs. In the earliest days of cell phones, some cities only had two megahertz of bandwidth to use, which was only enough for about thirty people to use at the same time.

A lot of the effort to improve phone networks over the years has come from tackling this problem. So here’s where we get to “gees”. The letter G in 4G or 5G refers to the generation of technology used for sending radio waves around.

Naturally, it all started with “1G”. The users close to a given cell tower would each be given a certain, distinct band of frequencies to use. Engineers call this Frequency Domain Multiple Access, or FDMA.

But a key breakthrough involved allocating how the frequencies were used in physical space too. The idea is to divide up areas into a kind of honeycomb pattern where each hexagon is called a cell. And each of those cells uses a different cell tower or base station to transmit to.

In case you’re wondering, yes, that’s why we call them cell phones! The trick is that nearby cells are allocated different frequencies to use, but distant ones can use the same frequencies without overlapping. By distributing frequencies in cells like this, you reduce the odds of interference because people using the same bands of frequencies are likely to be communicating with totally different cell towers.

In short, this means that we can reuse certain frequencies, letting more people access the network since there’s more bandwidth we can use over and over. Then 2G came along and made things even more efficient. And part of that success was because of Time Division Multiple Access, or TDMA.

In TDMA, phones were still being allocated certain frequency bands. But by shuffling bits of their signals in time, different phones could use the same bands. To see how this works, imagine being in a room with two people speaking to you with similar voices.

If they speak over each other, you can’t make out what they’re both saying. But imagine, weirdly, that they take turns saying the individual words of their messages. So you might hear “cell” “I” “phones” “love” “are” “Scishow” “cool”.

It sounds like, well, nonsense. But if you write it down and take every other word you get “Cell phones are cool” and if you take the words in between you get “I love SciShow”, giving you each person’s message. That’s basically what TDMA does, but with more than just 2 people.

While it’s confusing for a person, machines can easily keep track of the different messages in time, so that everyone can use the same channel by chunking signals into bits that take turns and then get reassembled on the receiving end. It happens so fast you don’t notice the interruption, ideally. And since many people can now use the same frequency bands, we get even more efficient use of bandwidth without interference.

And with more bandwidth comes more data. Because of that, 2G let us do more than voice calls, letting us text and even send… picture messages. Then came 3G, which took things even further.

With 3G, we came to rely heavily on what’s called Code Division Multiple Access, or CDMA. Engineers really like these kinds of acronyms. This is a little more complex, and involves letting phones use some of the same frequencies and even letting them happen at the same time… but encoding the signals in such a way that they can still be made out distinctly on the other end.

In engineering terms the codes are orthogonal, meaning that they can’t get mixed up with one another. To get an idea for this, let’s revisit our room analogy. This time, imagine four people are in a room with two people speaking at the same time, but one of them is speaking in Chinese and the other in Spanish.

At the other end of the room, a person fluent in Spanish and another fluent in Chinese are both listening to the speakers. Because Chinese and Spanish sound very different, each person tunes out the language they don’t understand and just focuses on the one they do, and hears their respective message clearly. In the world of cell phones, base stations can be programmed to listen to lots of signals and decode them in different ways at the same time, simultaneously playing the roles of different “translators”.

So CDMA allows signals from different phones, sent at the same frequencies at the same time, to be picked apart at the other end. Once again, that lets us make even better use of frequency bands and send more data with more people. With 3G, we gained the ability to send emails, browse the web and even watch videos.

I know! In reality, sometimes the codes in CDMA are similar enough to each other that they cause issues. In the room analogy, the “languages” might be more like Spanish and Portuguese.

They’re different languages, but they share bits of grammar and vocabulary, which can be confusing to listen to at once. Engineers came up with a few other tricks to deal with this issue, like changing the power of the signal used to communicate with the different cell towers, to help distinguish different signals. Then, along came 4G, which, you guessed it, squeezed even more bandwidth from the available frequencies to accommodate rapidly growing numbers of cell phone users, all hungry for data.

Engineers implemented what’s called Orthogonal Frequency Division Multiplexing, or OFDM. Even with everything we’ve mentioned so far, bits of a signal sent over a continuous band can actually interfere with themselves. That’s because frequencies that are similar to each other might distort a little as they travel through the air and bounce around, getting muddled up.

The bigger the band, the more chance of this happening. To get around this issue, OFDM chops up a single band into small divisions called carriers whose frequencies are less likely to interfere with each other, because they’re orthogonal, much like the codes of CDMA. In 4G, the upshot of all this is being able to use wide bands of frequencies enabled by OFDM, for sending more data to phones.

With all these techniques, we could reuse pretty much the whole spectrum of frequencies for cell phone communication, in every cell serviced by a tower, providing even more bandwidth for everybody. But it also brought a few problems. At the edge of two cells using the same frequencies, we end up with interfering signals once again.

It’s not an issue when you’re close to just one tower, but when you’re between two different cell towers, the one communicating with your phone might be using similar frequencies to its neighbor. And at the cell boundary, the strength of each of their signals might be roughly the same, so that workaround doesn’t fly. When they’re both using the same tricks to use every bit of available frequency, unfortunately, there’s a chance that their signals interfere and neither one provides you with reception!

We call this inter-cell interference, or ICI. For now, it’s an unavoidable problem with modern cell phone networks. There are a few tricks that help that though.

Since ICI crops up on cell boundaries, where there aren’t as many users around, we can temporarily let a tower drop out and create a combined, larger cell with more area and fewer boundaries overall. Fewer boundaries means less chance of interference! We can also help the towers communicate with each other to coordinate how they send and receive signals to and from you, to avoid your phone getting confused about where it’s getting signals from, and even make use of both towers at once.

This is one of the features of 4G that also helps reduce ICI. But even then, there’s a new stumbling block on the horizon that could make ICI an even greater problem: 5G. One of the features of 5G, and all its promised data glory, is a network of what are called “ultra-dense networks” consisting of lots of smaller cells, providing reception to many users.

The problem is that with lots of tiny cells, we end up with more boundaries, increasing the amount of ICI! That doesn’t spell total doom for your phone reception just yet. Engineers are aware of the problem and are already trying to develop techniques to expand bandwidth without creating loads of interference, just as they did for one to four G.

But in the near term, it’s possible your cell phone reception might drop every so often while you’re out and about. While that’s annoying, it might also be a good opportunity to switch off for a little while and put down the phone… if only until your reception comes back. Thanks for watching this episode of SciShow.

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