In code division multiplexing (CDM), the signals are encoded with individual code sequences, transmitted together via a transmission channel or transmission medium and then filtered out of the signal mixture using suitable filtering techniques.
For filtering out the code sequences, a specific code pattern is assigned to each subscriber. With this pattern, the differently coded signals can be kept apart and separated from each other. The codes must be independent of each other so that they do not interfere with each other. The use of code sequences increases the amount of data to be transmitted because the signal is spread out.
The CDM technique allows simultaneous communication of different connections or sessions over one physical line or over one radio link. This technique can be used to distinguish signals from transmission channels that are broadcast simultaneously by several transmitters in the same frequency band. Code division multiplexing can therefore be used as an access method (CDMA) for the use of radio channels in point-to-multipoint connection (P2MP) and mobile radio systems.
The code patterns are bit patterns. However, not any arbitrary bit pattern may be assigned, since otherwise mutual interference of the coded signals could occur. The codes must be independent of each other, which is referred to as orthogonal.