FIBEROPTIC LADDER NETWORKS FOR INVERSE DECODING COHERENT CDMA

Coherent fiber-optic networks can be 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 from transmitters to receivers, ideal inverse decoding can be implemented, with perfect delta-function response of the matched decoder. The two-channel case is studied in detail, with passive coding/decoding being provided by ladder networks. The rules for finding inverses of a given coding network are given. The relationship between matched networks for 1-channel decoding and 2-channel inverse decoding is 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 one version, each transmitter needs only one encoder; receivers are addressed by changing phases in the ladder, which provides a practical means of changing address in fiber optic CDMA. Experiments were performed with a single fiber between a transmitter and a receiver by using orthogonal polarizations; delta-function inverse response has been observed.