Photonic CDMA Networking with Spectrally Pseudo-Orthogonal Coded Fiber Bragg Gratings

Jen-Fa HUANG  Dar-Zu HSU  Yih-Fuh WANG  

IEICE TRANSACTIONS on Communications   Vol.E83-B   No.10   pp.2331-2340
Publication Date: 2000/10/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Advanced Internetworking based on Photonic Network Technologies)
fiber-optic code-division multiple-access (FO-CDMA),  fiber Bragg grating (FBG),  multiple-access interference (MAI),  maximal-length sequence codes (m-sequence codes),  pseudo-orthogonal codes,  

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An optical spectral coding scheme is devised for fiber-optic code-division multiple-access (FO-CDMA) networks. The spectral coding is based on the pseudo-orthogonality of FO-CDMA codes properly written in the fiber Bragg grating (FBG) devices. For an incoming broadband optical signal having spectral components equal to the designed Bragg wavelengths of the FBG, the spectral components will be reflected and spectrally coded with the written FO-CDMA address codes. Each spectral chip has different central wavelength and is distributed over the spectrum of the incoming light source. Maximal-length sequence codes (m-sequence codes) are chosen as the signature or address codes to exemplify the coding and correlation processes in the FO-CDMA system. By assigning the N cycle shifts of a single m-sequence code to N users, we get an FO-CDMA network that can theoretically support N simultaneous users. To overcome the limiting factor of multiple-access interference (MAI) on the performance of the FO-CDMA network, an FBG decoder is configured on the basis of orthogonal correlation functions of the adopted pseudo-orthogonal codes. An intended receiver user that operates on the defined orthogonal correlation functions will reject any interfering user and obtain quasi-orthogonality between the FO-CDMA users in the network. Practical limiting issues on networking performance, such as non-flattened source spectra, optical path delay, and asynchronous data accesses, are evaluated in terms of the bit-error-rate versus the number of active users. As expected, the bit-error-rate will increase with the number of active users. Increasing the flatness parameter of optical signal will lead to a lower average error probability, since we are working in a part of the more flattened optical spectrum. In contrast, reducing the encoded bandwidth will reduce the total received power, and this will necessitate higher resolution of fiber Bragg gratings.