how do radio signals (4G, WiFi etc.) manage to retain their information after passing through trees, buildings and other obstacles? And how are they not mixed up, intertwined?
In: Technology
First the not intertwined bit:
Different types of radio signals are allocated to certain bands. Some bands are a free for all (like 2.4 GHz for cordless phones, bluetooth and early wifi routers), others are strictly regimented. In the states this is by the FCC. You (as a device manufacturer or a broadcaster) send waves out in a frequency band you aren’t supposed to you get slapped big. Fines or even revoke your device licence. Each band of frequency is allocated for specific purposes (this band is UHF old school TV, this band is FM radio channels 97-108), another band is radar, another air traffic control, another is CDMA old school cell, another is 4G EDGE GSM whatever cell traffic. Other countries have same or similar, and have their own FCC and “spectrum” allocation which may or may not be similar.
So within each band, it may be further broken into channels. Most cell, radio, ATC communication is like this. AM/FM and broadcast TV too. Groups like the FCC make sure that one station on a channel doesn’t clobber or cross talk with another by ensuring geographical separation. Two cities may both have a 107.9 FM station, but they won’t be geographically near one another… or it is ensured that on most days the broadcast transmitter of one doesn’t reach the broadcast area of another.
For communication channels, more traffic gets stacked into a single channel using digital multiplexing. There’s various ways to do this but its akin to letting phone 1 talk during timeslot 1, phone 2 during timeslot 2 and so on (thats time division multiplexing).
Next passing through stuff:
Different frequency signals travel through different things to different degrees. Low frequency waves generally travel very far thru stuff, high frequency waves travel shorter distances or are “attenuated” more by various mediums. Whales use low frequencies and can hear each other over hundreds of miles. The loudest chipmunk in the world will only be heard a few hundred yards. AM signals are lower than FM signals, thus can be heard hundreds of miles away, only a few dozen miles for FM; given the same height of tower and strength of transmitter (and atmospheric conditions etc. etc.).
Now add in error correction etc. – analogue signals travelling through walls, trees etc., well not much you can do to correct that – you get static. Digital signals however, you can use error correction. For broadcast signals, again, you can’t ask for the last second to be rebroadcast, but perhaps there is some extra redundant data being sent and you can reconstruct part of the msg that is missing. Or, in the case if digital wifi or cell signals, if you didn’t get a msg correctly you can ask for it again.
Also don’t discount that a lot of these signals can travel through windows and bounce down hallways and around corners. [Here’s a map someone did that shows how wifi signals get weaker as you move further away from the router in an apartment.](https://i.dailymail.co.uk/i/pix/2014/08/27/1409153044123_wps_3_How_To_Perfectly_Locate_Y.jpg) Note that some signal is getting around corners but further and the more corners, less straight line you are the weaker the signal.
In wider area networks, this is where devices called repeaters come in. All they do is take the wifi or wireless signal and “repeat” it. Like all those relay fires in LoTR, signalling Gondor for aid. Emergency radio networks and public internet and cell networks all use repeaters to deep inside things like tunnels, the subway, really large buildings etc.
A bit more on traffic congestion in certain frequency bands: there are some frequency bands where traffic is largely unregulated. The 2.4GHz band is one. Anyone who wants to broadcast there, go right ahead. No FCC van will pull up and give you a fine (within limits). So thats why in this band we have: cordless phones, baby monitors, early wifi routers, bluetooth runs here (I think)… and really cheap microwaves. My wifi would go down everytime I microwaved something. There are “channels” that you can change your device to, like your wifi things… if your wifi traffic sucks, try changing your router away from “auto” to use a particular wifi channel (0-11), see if it helps.
Radio wavelengths are quite large (0,1m-meters) and get less attenuated compared to say, optical wavelengths. But there is reduction of the radio waves, as it will be absorbed along the way. But as long as the signal is intact (in phase), it can communicate. And the needed error correction algorithms of course.
It depends on the wavelength.
What you have to understand is that the signals you’re talking about are made of electromagnetic waves. Imagine a series of regular waves, with peaks and troughs. These waves have certain properties, including amplitude (the difference in height between a peak and a trough), and wavelength (the distance between two peaks). Waves with a short wavelength carry lots of energy but can’t go very far. Waves with a long wavelength carry less energy but can go much further. And by varying the amplitude of an electromagnetic wave (but keeping the same wavelength), you can transmit information.
Now, when an electromagnetic wave runs into an obstacle, such as a wall, it may or may not be absorbed by the atoms and molecules in that wall. See, atoms and molecules love to get more energy, but they can only take in specific set amounts, like lego bricks. If the incoming wave has just the right amount of energy, just one lego brick, then it will be absorbed by the wall and won’t be able to pass through. But if it carries less energy than what the atoms & molecules need to absorb, like half a lego brick, then it won’t be absorbed by the wall (the atoms and molecules can’t use half a lego brick, so they leave it alone).
Radio signals, like the radio in your car, have quite long wavelengths. This means that they can travel over long distances (easily hundreds of meters) without being much affected by obstacles in the way, like trees, walls and such (not enough energy to absorb). However, because the distance between peaks is larger, they can also convey less information at a time (the amplitude can’t vary very fast). Voices are fine, but forget full hd video.
Signals with shorter wavelengths, such as Wifi (microwave range), can convey much more information, but are more easily stopped by obstacles such as walls, which is why you need a wifi router in your home and not 200m away.
Now, as to why the signals don’t mix.
Between two radio signals: When you set your car radio to a certain station, you’re actually telling it to decode information coming along on one specific wavelength, and ignoring what’s happening on all the other wavelengths. Broadcasters have agreements between them, to say, I’ll broadcast on this wavelength, you can use a slightly shorter one, and a third broadcaster will use a slightly longer wavelength. That way you can listen to one broadcaster on each wavelength. Sometimes, signals will mix, because broadcasters haven’t agreed with each other and are broadcasting on the same wavelength!
Between a radio signal and wifi: these are along completely different wavelengths, so far apart that there is no chance of them mixing.
(For the lurking physicists – I know I’m oversimplifying like hell – sorry – but I really don’t want to go into quantum physics & scattering on ELI5)
Most information is encoded in the frequency of the wave, and as a wave transfers through an object the frequency is not altered. The edited and wavelength will change if the material is altered but the frequency is the same.
This is because the frequency measures how the atoms/field are being disturbed. For example a water wave pushed up against a boundary material like loose sand sets a wave of on the same as well. If the frequencies altered what will happen is that the water wave will be pushed up, while the same is being pushed down. Since the motion of the wave coming in is the cause for the wave in the same this can’t happen.
This means information encoded as slight changes in frequencies don’t have to worry about it getting scrambled.
Amplitude can get altered by passing through materials as that measures how far the medium is displaced, and thicker mediums don’t shift as far.
This is why am radio is often more staticy than fm.
Throw in the fact that the waves we choose to communicate (radio and microwave light waves) find most materials we build out of to be transparent, and bately even interact with them
They are, the key is there are error correction and detection algorithms that work to counter the problem. In bad environments they don’t always work.
Also, many newer devices actually utilize the attenuation and reflections to improve signal quality. Take a look into [MIMO](https://en.wikipedia.org/wiki/MIMO) technology. They do what is called [beamforming](https://en.wikipedia.org/wiki/Beamforming) where they vary the signal being output on multiple antennae to create hot spots in the signal at the receiving antenna(e) to improve reception. When multiple devices are connected it will actually change the beam forming per device moment to moment depending on the destination. It’s really cool stuff.
Edits: Links, typos.
Edit 2: Figured I could expand as well. When you’re dealing with reflections or other signals that could potentially interfere, they are usually a bit weaker than the main signal. This means that a receiver can filter them out by simply ignoring signals that are weaker than a certain level. This is called the noise floor of the radio. Think of it like being in a room full of people talking and ignoring all the background conversations because they’re harder to hear and focusing on your current one.
Attenuation of the main signal works similarly. As long as the signal isn’t attenuated below the noise floor, it will work. If the operating environment for the radios is bad enough, it may be impossible to get a strong enough signal to overcome the noise floor, and then nothing works.
So it basically breaks down to “waves are freaking persistent” ^^,
I read all of your finely crafted replies and am a little bit wiser for it. Thanks everyone.
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