[zamalek]

I think a combination of LoRa, bluetooth, and WiFi might be the alternative. I've seen videos of LoRA functioning below the noise floor (perfect for evading RF triangulation), and at 200km (perfect for reaching past physical borders). The major weakness is line of sight (and availability), but bluetooth and WiFi can help there.

[klysm]

Theoretically it can't be below the noise floor right? I don’t know much about radio stuff fwiw that just seems impossible by definition

[picture]

Communication protocols that incorporate spread spectrum (code/direct sequence for GPS and chirp spread spectrum for LoRa) get a "processing gain" at the stage of the receiver where the signal is despreaded. The resulting signal will have an SNR roughly equivalent to a narrow band (non spread spectrum) signal with otherwise the same parameters. You will have a generally equivalent bit error rate for the same SNR.

It's also possible to receive non spread-spectrum signals below the noise floor, if you can observe it over longer time and get additional "processing gain" that way

Additionally, it is a bad idea to use spread spectrum as a means of concealment because if the adversary is physically near enough, your signal will show up above the noise floor. Due to the inverse square law etc, you have a narrow zone of enough power to be received by your remote recipient, but not enough power for closer adversaries to detect you. You are also reliant on the unlikely situation of an adversary without more advanced RF hardware with lower noise receivers.

[staticfloat]

Other posters have pointed out that this is incorrect, but I wanted to give a bit of intuition as to how signals can be received when they are below the noise floor.

First, as a definition, below the noise floor means that the power of my signal at any given time is smaller than the power of the ambient noise in my channel, and usually this implies that you're only interested in a particular segment of frequency spectrum (e.g. within the 10MHz band centered at 1.8GHz). If we were doing a simple frequency-shift keying or amplitude-modulated signal, once the noise power exceeds the signal power, there is basically no hope of recovering anything useful, as those are both demodulation schemes that rely upon obtaining instantaneous estimates of the frequency or amplitude of the signal of interest.

However, spread-spectrum methods make a time/frequency tradeoff, where the signal of interest is "spread" across multiple points in time and frequency. A very simple example of this is to say "if I want to transmit a 1, instead of transmitting one cycle of a sinusoid at 18.GHz, I will transmit 10 cycles". Then, at the decoder stage, you average across 10 cycles of your carrier in order to detect whether a signal was sent or not. By doing this averaging across time, you get a 10x gain versus the noise which is expected to cancel itself out as often as not.

True spread-spectrum techniques are more advanced than this, they actually use wave shapes that are more complicated than just a sinusoid to make it easier to detect when they start and stop (whereas with a sinusoid there's a fair amount of ambiguity if you shift one period to the left or right) but the fundamental idea of averaging across time is the same.

Through this mechanism we are able to rescue out signals from far below the noise floor, although it reduces your maximum transmission rate. When dealing with digital radio systems we can even rescue out signals from below our quantization floor, although not too much lower, as eventually you lose the ability to average out a signal that is fluctuating by significantly less than a single bit.

Whenever I talk about making tradeoffs in transmission speed to aid in reception, I am reminded of the ELF systems in submarines [0]. While they did not use spread-spectrum techniques, (they just jumped between two frequencies, 76Hz and 80Hz) they still correlated across time to boost up their effective SNR. [0] https://en.wikipedia.org/wiki/Communication_with_submarines#...

[codersfocus]

Or an even more concise example: fountain codes

[zamalek]

https://www.thethingsnetwork.org/docs/lorawan/rssi-and-snr/

> If the RSSI is below the noise floor, it is impossible to demodulate the signal. However, LoRa can demodulate signals that are below the noise floor.

[masfuerte]

GPS signals are below the noise floor when they arrive at your phone.

[raluk]

Simplest explation is: If you take a closer look to Shannon–Hartley theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore... you can see, that in theory you can have arbitrarly low signal/nose ratio for message to get trough.

[datadrivenangel]

If you know where to look for the signal, you can often find it