Abstract: A method of constructing a spreading code associated with one user of a direct sequence code division multiple access digital transmission system includes at least one stage of concatenating a plurality of different sub-codes. Specifically, the spreading code is constructed by concatenating at least two different, in particular, cyclically different, basic sub-codes belonging to the same family of basic sub-codes, and at least one secondary sub-code obtained by circular permutation of one of the basic sub-codes. The constructed spreading code is then designated as being associated with the one user.

Abstract: A threshold detector for digital signal transmission systems using a transmission channel, in particular for transmission to mobiles, calculates a set of coefficients f.sub.n belonging to the group comprising a set of coefficients c.sub.n of the correlation between a first series of digital samples x.sub.n received by the receiver and a second series of reference digital samples p.sub.n known to the receiver, in order to acquire synchronization of the system, and a set of coefficients h.sub.n of an estimate of the impulse response of the transmission channel, when the system is already synchronized. It also calculates a decision variable A such that: ##EQU1## where L and N are predetermined integers. The device has many applications in mobile radio systems, for example for acquisition of synchronization in DS-CDMA systems or for optimizing the transmission channel estimate.