Direct-Sequence Spread Spectrum Transceiver:
When you transmit a modulated signal via an electromagnetic communication systems, the signal has to be above the 'noise floor' in the receiver or you can't detect and read your signal. In any communications system, the receiving system always includes noise from thermal energy, other signal transmissions, and the microwave background radiation left over from the Big Bang.
The fact that your signal must be higher than the surrounding noise means that your signal can be detected, which is a problem in military communications systems. But when you transmit digital data on a radio signal, the amount of frequency spectrum that it uses, the 'bandwidth', is determined by the bitrate...transmitting high bitrate signals requires more bandwidth.
In a direct-sequence spread spectrum system, each data bit '1' or '0' is encoded using two separate, very long, psuedorandom code sequences - one coded pattern representing a '1' and a dfferent coded pattern representing a '0'.
This greatly increases the bitrate, and therefore bandwidth, that the transmitted signal uses, so much so that the power at any one frequency is so low that the signal is effectively buried in the noise.
If your receiver hardware knows the specific pseudorandom patterns for the '1' and the '0', you can still detect and 'lock-on' to the signal buried in the noise and extract the original data. However, anyone who does not know the specific random patterns for the '1' and '0' will instead just see the signal itself as more noise - they not only can't intercept and read the signal, but they can't even detect that the signal is present.
This was a super interesting project to work on. My specific role was to work with 2 other engineers on the Radio-Frequency (RF) transmitter and receiver parts of the system. Our entire team of new engineers was able to build a fully functioning transceiver in only 3 months which successfully transmitted signals with a -11 dB Signal to Noise Ratio (which means the signal strength was less than 1/10 of the noise level).