Ever since I found out that coding gain allows you to receive a signal that is well below the noise floor I have been fascinated with the idea of covert radio signals (low probability of detection). You could imagine a clandestine scenario with spies or political dissidents where not only the content of the radio message must be kept secret, but the mere existence of the communication must also be kept secret. Using spread-spectrum you can keep the power spectral density of your signal well bellow the noise floor, which would perhaps make the signal impossible to spot with a simple glance at a spectrum analyzer...
My questions are the following:
- How likely would someone be to detect such a signal by accident?
- Assuming your adversary did not know any details about the signal (its bandwidth, center frequency, modulation, etc.) but merely suspected the existence of covert signals with ultra-low PSD below the noise floor, how easy would it be to detect and locate?