> But digital communication, especially when encrypted, would completely blend with the background noise.. right? Not exactly. The important part is that how t

> But digital communication, especially when encrypted, would completely blend with the background noise.. right? Not exactly. The important part is that how the signal is modulated, or more precisely, whether the spread spectrum of the signal has been modulated or not. Every signal modulation has a unique power spectral density graph, disambiguating it from random noise. However spread spectrum modulation gives a twist on top of that. Frequency hopping (FHSS) slices the transmission in smaller fragments, transmitting them in order at pseudorandom frequencies. Spectral diagram would show a bunch of spikes, which still could be identified as the output of some civilization or burst transmissions of a star. For example, Bluetooth uses this technique. https://www.youtube.com/watch?v=6qNWQxRKoss https://www.youtube.com/watch?v=6qNWQxRKoss Direct-sequence spread spectrum (DSSS) distributes the energy of a channel on a wider bandwidth, reducing energy to background noise level, making it hard to disambiguate from random noise, unless you know where to look at and what to look for. Used by older (802.11b) Wifi, GSM, ... Newer (g, n, ac, ...) Wifi uses OFDM signal modulation, which again has its characteristic spectral diagram.