Probably the simplest explanation I could give would be: when you're communicating, you can always repeat your message to have a better chance of it being received. The example I like to use to remember this concept is one of people speaking in noisy places. When someone is having trouble understanding
you, some options are to talk louder, talk slower, or repeat what you said. However, in the example given, the power is fixed, so talking louder isn't an option.
A more complicated explanation: the fundamental reason why this is possible is due to Shannon's channel capacity theorem [0]. This theorem tells us that the parameter that tells us whether we can communicate reliably is not the signal-to-noise ratio (SNR) but is instead the Energy-per-bit to Power Spectral Density ratio (Eb/N0). The difference is that Eb/N0 accounts for the total energy dedicated to sending a bit, whereas SNR only accounts for the rate at which you send that energy. The channel capacity theorem further tells us that the minimum Eb/N0 required to communicate reliably is about -1.6dB [1]. In the context of Olivia MFSK, the article claims that this communication scheme can
communicate at -10dB SNR, which is possible as long as the waveform does something to increase its Eb/N0. The article says that Olivia MFSK uses error correction codes, which is one way to increase Eb/N0. Essentially, error correction codes add redundancy to the transmitted bit stream to correct errors. The simplest example of error correction is the repetition code in which, for every bit that you want to send, you send an agreed-upon number of copies. The more copies you send, the less likely it is that over half of them will be wrong. As you might imagine, there are also much more complicated error correction codes. Another way to increase your Eb/N0 is through Direct Sequence Spread Spectrum (DSSS), which is the technique that Craig mentioned.
If you're interested, [2] is a good reference book on digital communications, and [3] is a detailed, but still very readable, text on information theory.
A more complicated explanation: the fundamental reason why this is possible is due to Shannon's channel capacity theorem [0]. This theorem tells us that the parameter that tells us whether we can communicate reliably is not the signal-to-noise ratio (SNR) but is instead the Energy-per-bit to Power Spectral Density ratio (Eb/N0). The difference is that Eb/N0 accounts for the total energy dedicated to sending a bit, whereas SNR only accounts for the rate at which you send that energy. The channel capacity theorem further tells us that the minimum Eb/N0 required to communicate reliably is about -1.6dB [1]. In the context of Olivia MFSK, the article claims that this communication scheme can communicate at -10dB SNR, which is possible as long as the waveform does something to increase its Eb/N0. The article says that Olivia MFSK uses error correction codes, which is one way to increase Eb/N0. Essentially, error correction codes add redundancy to the transmitted bit stream to correct errors. The simplest example of error correction is the repetition code in which, for every bit that you want to send, you send an agreed-upon number of copies. The more copies you send, the less likely it is that over half of them will be wrong. As you might imagine, there are also much more complicated error correction codes. Another way to increase your Eb/N0 is through Direct Sequence Spread Spectrum (DSSS), which is the technique that Craig mentioned.
If you're interested, [2] is a good reference book on digital communications, and [3] is a detailed, but still very readable, text on information theory.
[0] https://en.wikipedia.org/wiki/Channel_capacity [1] https://en.wikipedia.org/wiki/Eb/N0 [2] Proakis, John, Masoud Salehi. "Digital Communications." (2008). [3] Cover, Thomas M., Joy A. Thomas. "Elements of Information Theory." (2006).