Fiber-optic ladder networks for inverse decoding coherent CDMA

Coherent fiber-optic networks are used to improve upon incoherent CDMA schemes. If a single-channel network is used between transmitters and receivers, standard coherent correlation takes place. If several parallel spatial channels (fibers and/or polarizations) are used, ideal inverse decoding can be implemented, with perfect δ-function response of the matched decoder. The two-channel case is studied in detail. The rules for finding inverses of a given coding network and the relationship between matched networks for one-channel decoding and two-channel inverse decoding are given. Phase and frequency differences among stations imply that unwanted signals add up incoherently. The ideal two-channel network with inverse decoding has no loss due to encoding/decoding, only inevitable loss due to the broadcast network. In experiments using a single fiber between a transmitter and a receiver by orthogonal polarizations, δ-function inverse response was observed