Quasi-orthogonal sequences for code-division multiple-access systems

In this paper, the notion of quasi-orthogonal sequence (QOS) as a means of increasing the number of channels in synchronous code-division multiple-access (CDMA) systems that employ Walsh sequences for spreading information signals and separating channels is introduced. It is shown that a QOS sequence may be regarded as a class of bent (almost bent) functions possessing, in addition, a certain window property. Such sequences while increasing system capacity, minimize interference to the existing set of Walsh sequences. The window property gives the system the ability to handle variable data rates. A general procedure of constructing QOS's from well-known families of binary sequences with good correlation, including the Kasami and Gold sequence families, as well as from the binary Kerdock code is provided. Examples of QOS's are presented for small lengths. Some examples of quaternary QOS's drawn from Family are also included.