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You know how phones, computers and other electronic devices that use wireless communication use radio waves to communicate?...well since almost everyone has gotten a smart phone wouldn't radio waves from different smart phones interfere with each other...well...they don't (other wise we wouldn't be able to call people)... But I want to know why radio waves from different electronic devices or from two different sources don't interfere with each other... if there are two explanations then pls tell me both of them (specially if one of them is a quantum mechanical theory)..help will be appreciated...thanx in advance

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Why don't radio waves sent by electronic devices intefere with each other?

Who says they don't? If you set up a broad-band antenna, the voltage across the antenna's terminals will be a weighted, instantaneous sum of all of the signals being sent by all of the devices in your neighborhood and your region.

Electronic communication devices use three different methods, often in combination with each other, to share the "airwaves."

  • https://en.wikipedia.org/wiki/Frequency-division_multiplexing, in which the receiver contains a resonator that is tuned to the same frequency as a "carrier wave" that is sent by the transmitter. The resonator does not respond to the carrier waves sent by other transmitters operating on different frequencies. This is the oldest of the three techniques.
  • https://en.wikipedia.org/wiki/Time-division_multiplexing, typically is used in conjunction with frequency-division: It essentially means, transmitters operating on the same frequency use the channel in short bursts and either (a) they coordinate with each other and take turns, or (b) they transmit at random times, and even though some messages "collide", statistically, enough get through to enable reliable communication.
  • https://en.wikipedia.org/wiki/Code-division_multiple_access, This one, I can't explain because I don't have a strong enough mathematical background. You can do a google search if you want to learn more.
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Indeed, there are two aspects to this question. One is more physical, one more technical:

  1. Why don't electromagnetic waves interfere? They do, they always do, they just don't destroy any information in the process. This is the superposition principle in classical electrodynamics, but if you are keen on a more quantum mechanical explanation, the selfsame superposition principle is also at work there, and depending on the textbook it is even etched into the fundaments of quantum mechanics as one of the axioms. If you want to go as deep as quantum field theory, the fact the electromagnetic waves can travel undisturbed from one another comes from the fact the photons - as opposed to electrons or any of the quarks - carry no charge (neither electrical nor color etc.), so they do not interact with one another. So in summary, there is interference, and the electromagnetic waves lie on top of one another just like waves in a pool.

  2. That brings us to the more technical point of your question. If we all send out electromagnetic signals all of the time, how can any device take this chaotic intereference and read out any valuable information out of it? That is pretty much like floating in a swimming pool, closing your eyes, and measuring the clothing size of the next person to jump in from the waves they create. The way around that is summarized nicely in the following link: https://www.explainthatstuff.com/cellphones.html The key idea is that when multiple senders and receivers are present in an area, like with conventional radio devices, they use different frequencies to communicate. If the frequencies are far enough apart, it is easily possible to decompose a mixed signal (containing signals on the whole frequency spectrum) into separated signals at different frequencies (key word is the Fourier transform, 3Blue1Brown made a great video about that, if you care for it). However, the more people send and receive messages, the separation between neighboring transmission frequencies must grow smaller and smaller, until it becomes increasingly hard to tell neighboring signals apart. And there you would get your intereference. The way around that is sort of a divide and conquer approach. Instead of finding frequencies for all the phones (in the world or a country or a large city), where the available frequency ranges are not sufficient, one partitions the network into small units called cells (hence the name: cell phone). In each cell, the available frequencies are sufficient for all devices in the area. Each devices sends a weak signal that does not leave the area to a mast. The mast repeats a weak signal to another device at the correct frequency. With that, the two devices can communicate. That is communication within one cell. If you call someone from another cell, the signal received by the mast is transmitted to their cell (usually via land line) and repeated by a mast near them.

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You are basically asking, "How does all of telecommunication multiplexing work", which is a question too large to answer comprehensively, so a vignette or two will have to do.

There is frequency multiplexing on radio and television, in which various signals are transmitted a different frequencies, commonly referred to as "stations" in the former and "channels" in the latter.

Your stereo (if those still are a thing) uses "space division" multiplexing, in which the left and right signals are carried on physically separate cables from source to speaker.

Civilian GPS uses code division multiplexing. All the GPS satellites transmit at the same frequency (and bandwidth), but each satellite has a pseudo random binary code which XORs the phase of their signal. The received signal can then be correlated with pseudo random code to recover each of the satellites data simultaneously in parallel, albeit at a lower bandwidth.

Another common method is Time Division Multiplexing, whose most familiar applications are CPU sharing on "the mainframe", or worse, time-share properties, in which multiple people (signals) share a vacation house (channel).

Many other more complex and/or subtle forms of multiplexing exists, but those are the basics.

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  • $\begingroup$ thanx for the answer but can you tell me why radio waves of different frequencies don't intefere with each other? $\endgroup$ Dec 30, 2020 at 15:19
  • $\begingroup$ filters......... $\endgroup$
    – JEB
    Dec 31, 2020 at 0:30
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For a significant and stable interference pattern, two such waves would have to be of the same frequency and of comparable amplitude. There is an area in North Miami where I get poor reception on my car radio. I think the several radio and TV transmitters in the area are over-driving the circuits in my radio.

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  • $\begingroup$ Another explanation is that there are reflecting bodies (buildings, etc) that are place in just the places that diffraction from them causes a node. This explanation would be more likely if you experience the signal fading in and out as you drive slowly. $\endgroup$
    – garyp
    Dec 30, 2020 at 15:55

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