Fiber-optic code division multiple access : multi-class optical orthogonal codes, optical power control, and polarization encoding

Ever since the mid- 1980s when the single-mode fiber-optic media were believed to become the main highways of future telecommunications networks for transporting high-volume high-quality multipurpose information, the need for all-optical multi-access networking became important. An all-optical multi-access network is a collection of multiple nodes where the interconnection among various nodes is via single- or multi-mode fiber optics and for which they perform all their essential signal processing requirements such as switching, add-drop, multiplexing/demultiplexing and amplification in the optical domain. Optical CDMA networking is one possible technique that allows multiple users in local area networks to access the same fiber channel asynchronously with no delay or scheduling. Optical CDMA networks are not without their own problems. Search for codes suitable to the optical domain is one of the important topics addressed in the literature on optical CDMA. Existing codes developed in the late 80's are limited to single class traffic or can support multiclass traffic but with restrictions on code lengths and weights. Also the number of generated codes is severely limited due to orthogonality issues. In this thesis, we pay particular attention to propose new codes that can support multiclass traffic with arbitrary code weights and lengths. Therefore, data sources with varying traffic demands can be accommodated by optical CDMA networks using the proposed codes. We also present a simple generation technique for the proposed multiclass codes and analyze their performance. The number of users supported by the proposed multiclass codes will be limited since it is an extension of existing code designs with such limitation. We then propose the use of polarization dimension in order to double the number of supported users. On the other hand, incoherent optical CDMA systems are considered as positive systems meaning that only unipolar codes can be considered for such systems. Therefore, multiple access interference will be quite high in optical CDMA due to the nature of incoherent power detection. Reducing the effect of the interference on the performance of optical CDMA is an important topic. We propose the use of power control to decrease the effects of interference in optical star networks in which users' fiber lengths and data rates are not equal. We consider the case of optically amplified network with amplifier noise as the main source. We then elaborate by considering the nonlinearity in the photodetection process and propose the use of an iterative algorithm to find the solution of the non-linear optical power control problem. Finally, we propose an optical CDMA system based on polarization encoding. Since the encoding is performed in the spatial domain, therefore, positive and negative levels can be realized. This approach leads to increasing the number of supported users of optical CDMA by the use of known codes, such as Gold and Hadamard codes, with enhanced performance.reviewe

Multi-class optical orthogonal codes for multi-service optical CDMA networks

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Earth fault distance estimation using travelling waves provided with triacs-based reclosing in distribution networks

This study presents an earth fault distance determination algorithm for distribution networks using active travelling waves. Three triacs are used in parallel with a three-phase breaker poles to overcome the mechanical inequality of the poles' reclosing times, so that the three phases are simultaneously reclosed. As the proposed fault location technique is an active type with controllable reclosing instant, the arrival time of the reflected surge from the fault point should be stamped precisely. For this purpose, three different travelling-wave detection algorithms are evaluated including the discrete wavelet transform, Hilbert transform, and signal derivative. The fault location performance is evaluated under different fault conditions such as fault distances, fault resistances, and busbar faults. Due to utilising the reclosing transients, the proposed fault location function successfully estimates the fault distance for different earthing concepts such as unearthed, compensated, and earthed networks. This study is accomplished via simulating a typical 20 kV distribution network by the ATP/EMTP program. The results ensure the superior performance of the proposed fault distance estimation algorithm for earth faults in distribution networks.Peer reviewe