Direct Sequence Code Division Multiple Access.
Spreading the Signal
Let’s pretend that the end-user wants to transmit a file over the mobile network. We see the narrowband signal to be transmitted (event 1). Between the 1st and the 2nd event the signal has been spread with a code. The spreading means that each bit in the original message is multiplied with the spreading code. This means that the spread signal consists of more bits than before the spreading.
The signal is then modulated (MOD) with QPSK (Quaternary Phase Shift Keying) and sent over the air. The “WBIs” are other wideband users (interferes), and the “NBI” is a narrowband signal interfering as well.
De-Spreading
Interference is strong in the system, since all users are transmitting data at the same time. In event 3, we see what has reached the receiver. As you can see our signal is far below the noise level. In order to get it out and find out he original message, the receiver has to do de-spreading.
De-spreading is done like spreading, by multiplication with the same code that was used between event 1 and 2. In this way the original message is turned back into a narrowband signal and the interfering signals stay spread. The interfering narrow band signal is spread out to wideband, thus lowering its impact on the noise.
Finally the signal is passed though a filter that takes away the wideband interference (event 5).
