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Suppose I created a private network of peer-to-peer cellphones and started handing phones out to people globally. Now, these phones don't have to use the common cell phone bands of the EM spectrum... I know some bands are better than others for building penetration, etc... the users of this network are willing to put up with many annoyances, so maybe they use radio or something more exotic. Each peer needs a transmission distance of maybe 50 meters, tops... not powerful broadcasts.

At first, since few people will have these devices, there would just be bursts of transmission, but over the course of time, the transmissions would rise globally to essentially continuous chatter. Because of encryption, this network will essentially be broadcasting white noise globally on a given channel of the EM spectrum on as narrow a band as possible with current technology.

What transmission band could I use to have the longest period without being detected by government and corporate interests? It has to work to support cell phone type communication net, so maybe I'm restricted to the channels already in use for cell phones, but I'm hoping not. I have heard that between science researchers and government spies, there's essentially global monitoring of the whole spectrum, so someone is going to notice eventually. I'm just trying to figure out a space where that notice can plausibly slip by as long as possible.

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    $\begingroup$ Signals intelligence is the least of your worries. How are you hiding the act of distributing phones globally? Especially how are distributing them so thoroughly each peer is only ~50m away from the next while still staying secret? $\endgroup$
    – Ryan_L
    Sep 1 at 5:24
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    $\begingroup$ How's peer-to-peer communicators (they aren't cell phones because cell phones use an immense network of cell towers connected to backhaul) going to work globally? Peer-to-peer only works if you have an unbroken chain of peers between each city or some other connection to backhaul to give you connection between cities. $\endgroup$
    – GrumpyYoungMan
    Sep 1 at 5:47
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    $\begingroup$ "Because of encryption, this network will essentially be broadcasting white noise globally": maybe they will indeed emit something which resembles white noise, if that's how the physical layer of the transmission works; but encryption has nothing to do with it. Yes, there are signal encoding schemes which are designed to like white noise to naive receivers, with the purpose of squeezing more signals onto one channel, but they can carry any payload, encrypted or not. $\endgroup$
    – AlexP
    Sep 1 at 6:29
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    $\begingroup$ There really isn't any channel that's both useful and could go a while without being detected. SDRs exist, and make it trivial to watch wide swaths of the spectrum simultaneously, and militaries/governments have high-powered SIGINT software packages which can identify basically any type of radio transmission and flag unusual events instantly. Noise (or an encrypted signal) powerful enough to be useful would be noticed within days, and triangulated and hunted soon afterwards (assuming you are in a first world country). $\endgroup$
    – Dragongeek
    Sep 1 at 8:19
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    $\begingroup$ Pity the lone chain of rural persons between two cities whose phone batteries only last 20 minutes because they are relaying thousands of simultaneous calls between the cities. $\endgroup$
    – user535733
    Sep 1 at 13:08
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Don't pick a single band: use all of them.

Spread spectrum technology can be used to send/retrieve signals below the noise floor. This makes it's hard to detect. The wider the available frequency spectrum you use, the less power devoted to any one frequency and so the harder it is to identify that anyone is actually transmitting.

Let's imagine you transmit at 2.4 GHz normally at 1 watt with a channel width of 10 kHz:

  • Spread that 1 watt over 100 kHz and now a 10 kHz band only gets 100 mW.
  • Spread it over 1 MHz and that 10 kHz band gets 10 mW.
  • Spread it over 10 MHz and you're down to 1 mW detectable signal.
  • 100 MHz → 100 µW, 1 Ghz → 10 µW.

This is the RF power at the antenna. By the time you're a couple of meters away, detecting a 10 µW signal is very hard. But your receiver with its 1 GHz spread spectrum system still listens like you're transmitting at 1 W.

Current spread spectrum is relatively narrow band (Wi-Fi is spread spectrum), but I can imagine that if your system spanned from, say 100 MHz to 1 GHz it would be nearly impossible to detect. The reason it's so narrow (apart from regulatory considerations) is because engineering a broad spectrum transmitter and receiver is hard (probably borderline infeasible a couple of years back). There are also odd practical considerations: the different frequencies will bounce/absorb differently. And if you're doing long range comms, the different signals will arrive at different times (IIRC a lightning strike at the north pole arrives at the south pole as an ascending chirp rather a single pulse!). Given sufficient motivation, I'm sure there is a solution out there.

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    $\begingroup$ This is a really good idea. I can coordinate a rotating schedule within regions. I think this is the best idea. Thank you. $\endgroup$
    – SRM
    Sep 2 at 1:39
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    $\begingroup$ For reference, spread spectrum doesn't need a 'rotating schedule'. Each transmitter is broadcasting (effectively) on many frequencies at all times! Also, so long as you have different modulation seeds you can even have multiple transmissions at the same time! $\endgroup$
    – sdfgeoff
    Sep 2 at 6:25
  • $\begingroup$ The only downside of these below-noise-power transmissions is an abysmal Baud rate (data transmission speed roughly), but depending on the use case this is ok $\endgroup$
    – Hobbamok
    Sep 2 at 15:27
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    $\begingroup$ Re: "100 MHz to 1 GHz", this is basically en.wikipedia.org/wiki/Ultra-wideband . The "continuous chatter" still raises the noise floor and that frequency range is so heavily used that it will be noticed almost immediately, particularly by cell network operators. Spread spectrum helps with making the transmitter hard to find (it was originally tech for military comms) but by the same token, to counter it, there has been lots of research into tech for detecting and locating spread spectrum transmitters; see, for example, eprints.utas.edu.au/22401/1/whole-Vlok-thesis-2014.pdf $\endgroup$
    – GrumpyYoungMan
    Sep 2 at 16:27
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    $\begingroup$ right, it's not a rotating schedule, like "let's all use 100MHz between 1am and 2am and then use 1334MHz between 2am and 3am" because the government would find it during that one hour. It's more like "let's use 100MHz for one microsecond and then 101.5MHz for one microsecond and then 97.2MHz for one microsecond and (in your hypothetical extreme scenario) 64221MHz for one microsecond" $\endgroup$
    – user253751
    Sep 3 at 14:52
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If the transmission distance only needs to be 50 meters, tops, just use wifi. Each phone is a hotspot. Don't make the network name discoverable. Or do; it's perfectly legal to set up wifi hotspots, so it's no big deal if someone discovers the broadcast. You could automatically assign names to each hotspot designed to be similar to names people normally assign to wifi networks, to make it less conspicuous.

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    $\begingroup$ This could work. Pick a ssid name that nobody will want to connect to "FBI_pedofinder_van_17" seems to work quite well for me. I've never have anyone even try to connect to that, for some obscure reason. $\endgroup$
    – PcMan
    Sep 1 at 11:21
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    $\begingroup$ @PcMan how would you know if they did try? $\endgroup$
    – user253751
    Sep 1 at 14:27
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    $\begingroup$ @user253751: Read the logs? Some of us do it, you know. $\endgroup$
    – AlexP
    Sep 1 at 14:40
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    $\begingroup$ "Each phone is a hotspot." I am so vexed that wireless ad hoc networks effectively elude common knowledge, although that technology is more than twenty years old, is used in vast non-commercial networks and every standard Wi-Fi device should be able to be switched into ad hoc mode (although you would still need additional software for multi-hop routing). $\endgroup$
    – Dubu
    Sep 1 at 17:35
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    $\begingroup$ There is Briar, which implements exactly this. $\endgroup$
    – Simon Richter
    Sep 1 at 19:27
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Other answers and comments have addressed the peripheral issues of

  • Distributing hardware
  • Peer-to-peer communication
  • Encryption doesn't necessarily mean it looks like white noise

...so I won't. Here's your answer: 8-meter, i.e. 40MHz. There's several reasons why.

  1. 8-meter enjoys excellent sporadic E, i.e. the radio waves can bounce off ionized clouds in the upper atmosphere which can allow signals to go well over the horizon, perhaps farther. A popular party trick with North American radio amateurs is to use 8m in the summer to make contacts in Hawaii, northern parts of South America, and western Africa. It's worth noting that sun spots and other solar activity can greatly improve this. IRL, we're actually moving into an active cycle and expect it to get a lot better for a couple years.

  2. 8-meter is virtually dead, like most low-frequency bands. In the USA and most of the world, the 8m band is reserved for amateurs because it isn't terribly useful for serious applications. The frequency is too low for modern high-bandwidth digital communications. Even modern RADAR systems work on much higher frequencies. If 8m ever sees widespread commercial/industrial/military application it will be a low-bandwidth automated ping, not any sort of serious communications.

  3. The people who care about 8-meter are generally ignored. I used to watch the office for a lab manager when I was a university instructor. His office included the interface for the university's 8m repeater, so I'd spend hours listening to the chatter. It's mostly conspiracy theorists who, once their conversation moved to how Obama was going to microchip all the Christians, agreed to switch to morse code to make it harder for the NSA to monitor their conversation. These are the people who'd actually notice your "white noise" and they'd likely attribute it to a government conspiracy. The less crazy people would perhaps report it to the FCC (or comparable local regulatory body) but frankly nobody is likely to care that much. As long as the noise is relatively narrow and doesn't interfere with old folks discussing Bill Gates and the lizard people Deep State, no regulatory body is going to expend the resources to investigate it.

EDIT: One more thing! Global white noise signals on specific frequency bands are actually not uncommon, especially among the lower bands. If it does indeed look like white noise and is indeed global, it might get written off as some sort of stellar phenomena.

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  • $\begingroup$ Huh. I'm an Amateur Extra and didn't even realize we had 8 meters. The ARRL band chart doesn't even include it. $\endgroup$
    – chrylis -cautiouslyoptimistic-
    Sep 3 at 5:34
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    $\begingroup$ @chrylis-cautiouslyoptimistic- we don't. There's a petition to assign it as secondary (don't hold your breath) and just recently an STA issued to one station for experimentation. The band is federal (NTIA). Europe is also in the "thinking about it" stage; there are a very few countries with assignments, but mostly 40MHz is just a spot in the middle of VHF Low Band. $\endgroup$
    – hobbs
    Sep 3 at 14:53
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As of 2021, I'd guess probably somewhere >=100 GHz. Anything below that is allocated to and being heavily used by somebody; take a look at the frequency allocation charts at https://electronics.stackexchange.com/questions/304919/what-unlicensed-frequencies-can-or-should-be-used-for-proprietary-wireless-com . The problem is, of course, that components that work at those frequency ranges are uncommon and expensive. You might try to use the highest EHF frequency band allocated to amateur radio your radios can reach to reduce the chance of attention as long as possible.

Don't count on even those frequency bands remaining unused much longer though, either; the need for bandwidth is infinite and engineers and researchers are already looking at how to use those frequencies for commercial and industrial purposes already, including 6G cellular networks. See, for example, https://www.youtube.com/watch?v=yz9aaVEXztE

As an aside, broadcasting encrypted signals that look like white noise hides nothing because "continuous chatter" raises the noise floor. Anybody with a spectrum analyzer that covers the right frequency range can see that unexpected bump and will start wondering why and government communications agencies will be highly motivated to find out once they notice it occurring.

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  • $\begingroup$ You seem to be thinking that the purpose of the signal being white noise is to delay detection. But I think that the reason for the OP positing white noise is that it would be a side effect of encryption. $\endgroup$
    – Acccumulation
    Sep 1 at 6:13
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    $\begingroup$ @Acccumulation: Whether the trasmitted signals look like white noise or not has nothing to do with encryption. Payload encryption and physical transmission happen at in different layers of the communications stack. Yes, there are signal encoding schemes which are designed to like white noise to naive receivers, with the purpose of squeezing more signals onto one channel, but they can carry any payload, encrypted or not. $\endgroup$
    – AlexP
    Sep 1 at 6:32
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    $\begingroup$ @Acccumulation: No, it cannot. Even if the data payload is absolutely random, the transmitted signal won't look like white noise at all, unless the transmission scheme was specifically designed to make it so; and if the transmission scheme is specifically designed to make the transmitted signal look like white noise it will look like white noise (to naive receivers) even if the payload is absolutely non-random, such as a string of zeros. $\endgroup$
    – AlexP
    Sep 1 at 6:57
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    $\begingroup$ @Acccumulation - transmitting in a narrow band is never going to look like white noise. $\endgroup$
    – Jon Custer
    Sep 1 at 13:29
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    $\begingroup$ Seriously -- do you think that any TLA pays attention to the noise floor in the 2.4GHz band? I don't think they'd notice the OP's scheme over their own office WLAN! $\endgroup$
    – Shalvenay
    Sep 2 at 2:51
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  1. Ultra wideband. It's a modulation that broadcasts signal in short pulses but with very wide spectrum. It won't interfere with usual narrowband radio. It is in fact already supported in some phones but only for personal area network, no idea if it is feasible for longer distances.

  2. Hide in some existing signal. For example repeat wifi broadcasts and hide data there to mimic legitimate traffic. The signal would superficially look like ordinary reflection. The receiver would extract data by comparing original and copy.

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Use a band that already lets everyone and their mom do this

The 2.4GHz ISM band is usable worldwide for low-powered unlicensed signals, is certainly suitable for communications at the range that you describe, and another wideband signal in that noise floor simply won't get noticed among all the WLAN/BT/ISM signals already present.

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  • $\begingroup$ Especially if you don't use WiFi/bluetooth so it doesn't get picked up by the WiFi/Bluetooth mapping projects. $\endgroup$
    – user1937198
    Sep 4 at 10:45
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No EM. Use sound.

50 meters is only half of a football field! I can read your Tshirt at 50 meters (it says "Cured by Ivermectin"). You can speak in moderate tones and be heard. Your devices can use infrasound or ultrasound.

https://www.youtube.com/watch?v=qNf9nzvnd1k&t=47s

On this video I did not hear anything until 210 HZ and then could not hear anything again over 11000 HZ. Your devices are microphone / speakers that take the pitch of the users voice either over or under what humans can hear, amplify it and broadcast it. Acting as a receiver it lowers or raises the frequency of what it hears to be within human hearing range. Alternatively it could just be a parabolic dish with a microphone and speaker if you don't care who hears you.

Dogs may do a lot of howling. Whales might show up wondering what is going on. Spies will be none the wiser. Unless they are whale or dog spies.

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    $\begingroup$ 210 Hz is stupidly high. Either your hearing is terrible, or whatever speakers you were using are terrible. (Probably the speakers; cheap and/or small speakers are well known to be unable to handle low frequencies. I have a small woofer and can easily hear ~35-40 Hz. Heck, I can sing notes with a root frequency of around 100 Hz, which is still on the bass clef. That said, you need a pretty big woofer to produce tones that low, too, so as far as the question, infrasound is probably not the answer.) $\endgroup$
    – Matthew
    Sep 1 at 19:39
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    $\begingroup$ @Matthew / Either your hearing is terrible, or whatever speakers you were using are terrible. / Can it be both? $\endgroup$
    – Willk
    Sep 1 at 21:07
  • $\begingroup$ Maybe... if you've got a woofer or high end headphones and you really get nothing under 210, might want to go see someone... As a different data point, however, I tried the same test on my tablet (Galaxy Tab S7+) and phone (Galaxy S20 Ultra), and got zilch until 110 Hz and 150 Hz, respectively. My ATH M50x, on the other hand, comfortably growl all the way down to 20 Hz. (Dang, I thought my JBL speakers were better than that!) If you're listening on cheap laptop speakers, I would definitely check with another audio device first. $\endgroup$
    – Matthew
    Sep 1 at 22:04

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