## Direct sequence CDMA (cont.)

Multi-user operation is achieved in direct sequence CDMA by assigning each user a different spreading code or a different time alignment of a common spreading code. Only that portion of the wideband spectral energy that has been spread by the same code as used in the receiver will be detected. Users are thus able to coexist in the same bandwidth and time space on the channel. Like frequency hopping, spread spectrum CDMA overcomes the problem of frequency selective fading by ensuring that most of the spread signal energy falls outside the fading 'notches'. |

If there is some correlation between spreading codes, as is almost always the case, then there will be a small contribution to any individual de-spread user signal from all the other spread users on the channel. Ultimately this puts an upper limit on the number of users that can co-locate on the same piece of spectrum before the unwanted de-spread energy gives rise to unacceptable data errors. This interference factor also gives rise to one of the most stringent power control requirements of all access techniques, as it is clear that this mutual interference between users will be minimized for each user if they all operate to give an identical spread power level at the receiving site.

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