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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#... |