Incoherent optical code division multiple access systems

Classical code division multiple access (CDMA) principles can be translated to the optical domain, resulting in all-optical derivatives of electronic systems. There are a number of ways of realising an all-optical CDMA system, classified as incoherent, coherent and approaches based on encoding in the frequency spectrum. This paper reviews the progress made to date in the incoherent regime, the most straightforward implementation based on the power summation of optical pulses. The review covers the basic principles of incoherent CDMA, the nature of the codes used in this approach and the resultant limitations on system performance with respect to the number of stations (code cardinality), the number of simultaneous users (correlation characteristics of the families of codes) and hardware requirements. The paper presents the latest developments with respect to the integration of conventional time spread codes, used in the realisation of these networks to date, with wavelength division concepts, commonplace in optical networking. These new families of codes exhibit an unprecedented set of characteristics when compared to existing unipolar codes in terms of their efficiency and correlation properties, allowing the goal of a large, asynchronous high speed network to become more feasible.