Code Division Multiple Access spreads the sprectrum of the transmitted signal over many times the normal bandwidth so that the signal levels are extremely low over a very wide bandwidth,only just above the noise floor in some cases.the receiver despreads the signal using what are basically matched filters.The reason this is done is that the spreading and despreading function allows multiple individual channels to occupy the same bandwidth and appear only as width bandwidth white noise to any reciver which does not use the correct despreading code.they are mutualy orthogonal Pseudo-random numbers in effect.