How is it that so many wireless signals

The simplest means of having different signals available in the same area is by using different frequencies of radio waves. Think of a radio being tunable to different stations.

With cell phones, because there are so many of them, this becomes impractical. One way to get around it is ti is called spread spectrum. In this case, the signal is sent with lower power on a random set of frequencies, and the receiver picks it out by looking only at the correlation of these different channels. This allows many more signals to be sent on the same set of frequencies.

Much of the complexity of a receiver is in picking out the signal from the interference or noise.

Bob

 

Interference? Yes absolutely, so they are designed that way.
We spread them out. Spread out over frequency, time and space. And then when we have to, or for those times when they do interfere, we design them to be interference tollerant.
As BobK says, the main way to separate the radio channels is frequency. Frequency is the oscilation rate, just as there are colours in the rainbow. We give each one a block of frequencies to work in. All those AM radio stations have low frequencies about 1MHz, we give each one its own range one million oscillations per second. Then we give each one its own range, like giving one a band of red to use and another a band of yellow. Actual light, by the way is much higher frequency, so just using it as an analogue. FM stations are higher than AM stations, so each would get its oew different part of the spectrum.

https://gcn.com/Articles/2012/10/09/Explainer-WiFi-11ac-spectrum-allocation.aspx?m=1
Has a chart showing some of the bands allocated to various services. Gives you an idea of how much work goes into giving each use a reasonable part to use. Then the rules say any equipment you make can't mess with those bands next to yours.

There's also separation by space. If two guys are not close by, as in other ends of the state or country, we can give them the same frequency band, as, if they don't use really high power their signals won't reach each other and interfere.

A lot of the internet is wired. Here we constrain most of the signal to travel in or between wires, so it doesn't radiate out much and also gives it some resistance to other signals radiating in and interfering.
Interference is often usually controled by turning the power down to the minimum required to get a message through. Cellular systems use this. They started with a few high power base stations covering large areas of the city, sharing in frequency and time, and as Bobk said, sometimes in spreading method/code. Then when there were more cellular users they put in more cellular base stations and turned the power down. So each one only uses some frequency bands and only reaches some areas of the city. Then as more came along, they shrank again, cells become microcells, became pico cells. That's why if you look, you see so many cellular base stations, not because they can't reach further, but because if you make them not reach further, they you can reuse the same frequency band again, just a few blocks away. So when you see them putting in yet another cellular base station, maybe just a small thoing up a pole or one a rooftop, they are reacting to, or anticipating just as you said, that area, at least at some times of day, at least has "overload"ed, reached capacity. Users don't like not being able to connect, get heir music streamed, make that emergency call. Providers don't get revenue. Often worth putting in more capacity.

Another use of radio is those microwave links you see that do long-haul across the city or country. They go from one place to another: not designed to be picked up by anyone else. Here they use those large dish antennas to make it as directive as possible: point all the signal energy one direction where it is wanted. That way it doesn't tend to mess with others and doesn't get interference from others.

Then there's those times when we just don't have enough frequency rooom, or space, so we design to share. BobK mentioned one way of sharing, spreading things out. We can also let them share in time. Here there are a lot of schemes, but many phone systems send information in brief packets, then go quiet for most of the time. They do this so quickly that you feel as if it is always there. This is also what happens when you stream things on the internet. With everyone using short packets, you ocan share in time. Some system give each guy a time slot, so they take turns. Some systems have everyone shout out when they need to send something and just send it again if they collide and don't get through.
Most systems have a measure of each of these methods. Your WIFI shares in frequency, allocating bands or channels: you can look at the bands you are using at the moment in a WIFI analyser. Also in those bands it shares in time. You've probably seen times in shared WIFI when it's just slow. When lots of people are trying to use it, so the total demand is greater than it's capacity, so they interfere, some get through, others don't get through and have to be resent, so it sems slow to get things through.

One of the biggest interferers in your home is likely your microwave oven. In frequency, It's right there near the band you want to use for WIFI and Bluetooth. What's more, it's designed to produce microwaves of really high power. Just what you don't want in or near a communications system. So WIFI and Bluetooth are designed to choose frequencies that are not being used at the time and then choose or just try the times to send, till the message gets through. Another important part of this is that their service is non-essential. We can afford to miss a few milliseconds of music, or delay a packet of that track we are listening to by a few milliseconds, and resend before it got to the front of the queue to actually be played.

Another thing is that radio systems reflect off things and bounce round. This means that some small transmitter such as your cellphone when you're in some back alley and can't even see that base station, bounces off the buildings, the fences, the bus and gets there. Cell phones systems will try turning the power up if they find the signals are low. They also swap frequency bands/channels. This bouncing happens on many scales: it is also used on a big scale with higher power systems to bounce of layers of the atmosphere to get you the ability to communicate round the curvature of the earth.
This bouncing round also means that you get small areas where you own radio signal interferes with itself, sometimes to make areas where the signal is very low and doesn't work. You sometimes see this, it just doesn't work well, you move a small distance and it work much better. This is less common now, because modern phones detect when they are not getting through and change frequencies till they find something that works.
This bouncing round can also make small areas of higher signal.

Fly by wire planes have requirements that are not really non-essential. Their services tend to be essential, at least in the opinion of those currently on the plane. The risk of interference must be very low.

With older, higher power phones, on planes, the phones tend to be a long way from the cellular base station, so the phones try to turn up the power. The plane is metal, reflective, so much bouncing round, much self-interference: so there just could be some small areas of stronger signal. There are essential services on planes. If all those things happened at the wrong time, wrong combinations, so that the total of all phone signals and places and times and frequencies interferered with an onboard system, then the results could be bad. In early times, with much older design phones, there were reports of interference with navigation systems from onboard use of phones. So the ban is just a precaution, even with much-improved technology. You'll note that most planes are now rated for WIFI and Bluetooth ok in flight, but still not cellular, which is higher power, as an added precaution.
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This is also where you sometimes see a dangerous part of human behaviour. You get people who will say "But I've used it all the time on the plane and nothing happens". It's like saying, I've crossed the road many times and never been hit, so it can't ever happen. IT's the rare combonation of things that turns out just wrong that they precaustions are for.

So, good question. You have BobK's exlcellent brief answer and my rambling one. Hope something works for you.

 

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