I. Introduction
Optical wireless communications [1]–[3] have been attracting attention as a promising short-distance communication method. In optical wireless communications, there is a problem that the error rate characteristic significantly deteriorates by background-light noise and ambient-light noise from lighting apparatuses and other sources. As a solution to this problem, optical-wireless code shift keying (OCSK) system [4]–[6], which is one of the multi-level transmission methods using plural 0,1-pseudo-noise (PN) codes, has been proposed.