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This thesis is concerned with the performance limits arising in optical code-division multiple-access (OCDMA) networks due to the mixing of light from the independent sources of each user. The scheme of spectral-amplitude OCDMA is shown here to be significantly limited by the phase-induced intensity noise arising from such mixing, and the corresponding signal-to- noise ratio and network capacity limits are of the same order as those arising in coherence-multiplexing systems, also due to such noise. Mixing can only occur between spatially coherent light, and this typically takes place through the combination of the signals into a single-mode fibre. The use of multimode fibre instead of single-mode can thus significantly reduce the levels and effects of phase-induced intensity noise, and this is experimentally demonstrated. It is shown that in general, assuming independent sources, there are only four possible ways in which to alleviate or eliminate noise limits of the form found for spectral-amplitude OCDMA. These are to separate the signals from each user in either the temporal, spectral, or spatial domains, or else to coherently despread the received signal. Neither spectral separation nor coherent de spreading are practical for spectral-amplitude OCDMA, but spatial separation via multimode fibre may be applied. The use of pulse-position modulation (PPM) with spectral-amplitude OCDMA is shown to be able to improve the performance beyond the limits found earlier, and this is because of the temporal separation it introduces. However, unlike the direct reduction of source duty cycle, PPM signalling can be applied without increasing, relative to the bit rate, the modulation or detection bandwidths, nor the dispersion sensitivity. Such PPM signalling and the associated decoding can also be applied to other similarly limited systems, including those based upon coherence multiplexing. If an OCDMA system is incoherent, has independent sources of the same spectrum, and has only a single-mode fibre to and from each user, then it can only avoid the significant noise limits found for spectral-amplitude OCDMA by the temporal separation of the signals from each user. This is the case for incoherent unipolar OCDMA systems, since the sparse codes of these systems rely on such temporal separation. Bipolar codes are not sparse, and in bipolar systems there is ordinarily no significant temporal separation between the signals from each user. Consequently, assuming sole single-mode-fibre paths and independent sources with identical spectra, every incoherent bipolar OCDMA system must encounter performance limits at least as bad as those found for spectral-amplitude OCDMA. These worst-case limits are identified for each of the incoherent bipolar OCDMA proposals to date.
Scottish registered charity number: SC000278
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In this thesis few new code sets and a multi-user interference (MUI) cancellation scheme have been proposed for Optical Code Division Multiple Access (OCDMA) systems, which can be employed in the next generation of global communication networks to enhance their existing systems’ performance dramatically. The initial evaluation of the proposed code sets shows that their implementation improves the performance, decreases the BER and increases security considerably. Also the proposed MUI cancellation scheme totally removes all the cross-talk and interference between the active users within the network. These novel schemes and codes can be easily implemented in the optical packet switched networks. Optical switching has the ability of bandwidth manipulation at the wavelength level (e.g. with optical circuit/packet/burst switching); the capability to accommodate a wide range of traffic distributions, and also to make dynamic resource reservations possible.
This thesis first gives a brief overview of co-channel interference reduction in OCDMA networks, then proposes two novel code sets, Uniform Cross-Correlation Modified Prime Code (UC-MPC) and Transposed UC-MPC (T-UCMPC), along with their evaluation and analysis in various systems, including IP routing over an OCDMA network. Thereafter, the new MUI cancellation scheme is proposed and then the proposed code sets and the MUI cancellation scheme are implemented and analysed in a laboratory-based experimental test bed. Finally the conclusion of this research is discussed.
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In this thesis, firstly we have reviewed both previous and current state of optical CDMA (OCDMA) technologies. Search for appropriate spreading codes is one of the main challenges of OCDMA applications and hence is an important topic which is heavily addressed in the literature. Existing codes have restrictions on code-lengths, weights and correlation properties where the number of generated codes is severely limited. Secondly, we have paid a particular attention to proposing a novel spreading code, hereby referred to as Double Padded Modified Prime Code (DPMPC) which suppresses the multiple-access interference and also enhances the network capacity. Then, we have applied the DPMPC to both coherent and incoherent time-spreading OCDMA transceivers and analysed their overall performances. We have also proposed novel transceivers which are power-efficient, simple and able to accommodate great number of simultaneous users. Accordingly, an advanced two-dimensional frequency-polarization modulation for OCDMA has been introduced, for the first time, to elevate the system security as well as the performance. Finally, the application of OCDMA in the passive optical network leading to the OCDMA-PON architecture has been established including the optical line terminal and network units. Since Internet protocol is currently the dominant network protocol, IP-over-OCDMA network node configuration ha
Thesis | Electronic Filter | Resonance
Optical code division multiple access (OCDMA) allows signal to be transmitted in an asynchronous manner, and promises high flexibility and simplicity with considerable security. OCDMA harnesses an enormous bandwidth that provides high spectral utilization and efficiency to support multiple, simultaneous active users. However, this multiple access technique also has impairments such as multiple user interference (MUI) and signal dispersion over distance. Thus, traditional metropolitan area network (MAN) must be developed by considering system performance optimization. Optimization is typically achieved by increasing code length and system complexity, or by applying a high level of medium access control (MAC) supervision. Numerous code set developments have been proposed to overcome the aforementioned impairments. However, the proposed code sets have not been demonstrated in an actual optical system that supports multiple users, particularly in a MAN, which has various noise sources and MUI effects. Thus, the feasibility of these code sets cannot be verified with absolute certainty. Conventional OCDMA architecture and encoding stage also exhibit various deficiencies, including rigid synchronization, fiber delay line (FDL) accuracy issues, stability resulting from environmental fluctuations, high costs, difficulty in fabrication, lack of compactness, and spectral resolution issues. In this work, an optimized model is presented as an enhanced solution to the limitations of the existing approach to be applied in a MAN. iii This work proposes an integrated formulation that can be realized by three key aspects: differential modulation, optimal optical code design, and the MUI reduction. This work also addresses the development of an optimized OCDMA architecture that includes a transmitter and a receiver, designed for a MAN. This thesis presents new grating configurations for fiber Bragg grating (FBG) and investigates the possibility of achieving system performance optimization in a MAN while considering the effects of various noise sources, non-linearity, and dispersion. The encoding and decoding schemes are designed based on the bipolar phase shift by using FBG. This work presents optimal grating configurations that embodies the orthogonality aspect and demonstrates consistent performance in different scenarios: intensity modulation-direct detection, external modulation, non-return-to-zero (NRZ)-differential phase shift keying and NRZ-differential quadrature phase shift keying, thus obtaining satisfactory agreement with the theoretical analysis. This work requires minimum MAC supervision and eliminates the need for FDL for queuing, which is in line with the requirement of real-time delivery. System optimization is achieved by using an integrated formulation that embodies the differential modulation approach, optimal grating configurations, and the MUI reduction. This optimization approach is verified by the design parameters, and enhancement is demonstrated by the performance parameters, which are over 2, 4, 8, and 12 users at bit rates of 1.25 Gbps, 2.5 Gbps, 10 Gbps, and 40 Gbps, respectively, spanning from 5 km to 100 km. This integrated optimization formulation extends the maximum allowable singlemode fiber span, which offers error-free transmission for BER ≤ 10-9 and accommodates higher aggregated traffic capacity than the conventional approach.