[madengr]

Well that is sort of a misnomer, as it depends on your receiver bandwidth. They always say such and such modulation (say JT65) is "under the noise floor". Of course it is when your bandwidth definition is 2.5 KHz (an HF SSB channel). But the symbol rate for JT65 is maybe 10 Hz, so if you filter to 10 Hz, it isn't under the noise floor.

Same with GPS, sure it's way under the noise floor of your receiver BW is at the 2 MHz, but once it is de-spread to the information bandwidth, it is not under the noise floor.

You can pretty close to Shannon's limit, which I suppose is under the noise floor at it's -1.6 dB limit, but in practicality you need extra margin, then you can usually see the signal with the proper filtering.