In a point-to-point mode, that is, one transmitter and one receiver, I understand how a spread spectrum receiver can read the transmitted signal from a spread spectrum transmitter by knowing the code. But I cannot understand how different codes can be sufficient to separate signals from multiple transmitters on the same bandwidth.
When there are multiple transmitters transmitting at the same frequency and at the same time, even if they have different codes, don't their signals still interfere with each other? How come codes are not affected by interference?
Best Answer
There are (at least) two types of spread-spectrum.
Frequency hopping and direct sequence spread spectrum (DSSS)
In DSSS all transmitters share the same frequency but with a different spreading code.
Multiple transmitters do interfere but after doing a cross-correlation using the specific code the unwanted signals get reduced by the cross-correlation factor. Ideally there will be zero cross-correlation but in practice, the codes used are not perfect and other effects such as doppler and range attenuation will make things worse.