How do we separate source signals in spread spectrum?

Question

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?

Answer 1

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.

Answer 2

CDMA (code division multiple access)

There are a multitude of frequency slots and any one transmitter occupies several of that multitude. The precise slots occupied are based on the "code word" and so another transmitter coming along is very unlikely to occupy any significant portion of the slots already "owned". The relevant receiver understands the code word and can demodulate the signal based on the knowledge of what slots are intended to be used by its partner radio.