CDMA technique

Finally ,codes with certain characterstics can be applied to the transmission to enable the use of code division multiplexing (CDM).Code division multiplexing access(cdma) systems use exactly these codes to seperate different users in code space and to enable acess to a shared medium without interferances .The main problem is how to find good codes and how to seperate the signal from noise generated by other signals and the envoirnment.
Earlier ,how the codes for spreading a signal could be used.The code directly controls the chipping sequence .But is a god code for CDMA ?.A code for a certain user should have a good autocorreleation and should be orthogonal to other codes.Orthogonal in code space has same meaning as in standard space(in three- dimensional space).Think of systems of co-ordinates and vectors starting at origin ,i.e in (0, 0 ,0).Two vectors (2,5,0)and (0,0,17) = 0+0+0.But also vectors like (3,-2,5) and (-2, 3,3) are orthogonal :(3,-2,5)*(-2,3,3) = -6 -6 +12 =0, thus they are orthogonal.
They must also be or hav an auto- correleation between them.
The barker code for example has(+1,-1,+1,-1,+1,-1,-1,-1), has a good auto co-releation i.e the inner product with intself is large, the result is 11.This code is used for ISDN and IEEE 802.11 .But as soon as this Barker code is shifted 1 chip further, the co-releation drops to an absolute value of 1. It says at this low value until the code matches itself again perfectly.This helps ,for example , to synchronize a receiver with incoming data stream.The peak in matching process helps the receiver to reconstruct the original data precisely,even if noise distorts the original signal up to certain level.