orthogonal codes
orthognal codes are codes which satisfy :
cross correlation between any two=0
auto correlation is finite.
they are used in communication systems as is CDMA , where spread spectrum is used "for multiple access".
when spreading a certain signal with one of these codes it cannot be de-spread unless by the same "nearly" code, so users "in CDMA" are using the same spectrum "Band" but each with an identifying code, and hence the multiple access.
types: walsh - PN "short-long"
methode: they are formed "i think" by using shift registers , where they must follow the above mentioned conditioned.