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المستخلص
This thesis presents an overview on optical code division multiple access (OCDMA) communication systems of a star configuration. Three basic implementations of the encoding – decoding techniques used in OCDMA systems were studied: time domain encoding using optical delay lines, spectral intensity encoding, and fast frequency hoping CDMA (FFH-CDMA) encoding.
Several OCDMA receiver structures were considered. A comparison between them was presented in terms of integration time, electronic bandwidth, and receiver complexity. We then studied the effect of receiver noise on the system performance for the three basic techniques of OCDMA encoding. Each technique gives some advantages and drawbacks in system capacity, and system performance in terms of bit error rate (BER). In the time domain OCDMA, the system is very simple but the performance is limited by splitting the power in the encoder and decoder, and the maximum number of simultaneous active users is limited due to the effect of multiple access interference. In frequency domain OCDMA, the system performance is limited by the ratio between encoded bandwidth and the 3dB bandwidth required to achieve a maximum number of active users, and the effect of multiple access interference is very low. In FFH-OCDMA, system performance is limited by the spacing between fiber Bragg grating (FBG), but this system is simpler than frequency domain system and achieves better performance than time domain OCDMA to a certain threshold value of active number of users after this threshold the time domain OCDMA encoding technique has better performance than FFH-OCDMA.
Different types of optical fiber delay lines are then studied, including its basic features and characteristics. Focus oriented to FBGs for use as delay line elements to provide different delays with the same attenuation, we found that outside the FBG stop band the transmitivity can be adjusted to unity at different values of the grating length.
A statistical model based on the photon counting technique was developed and used to analyze the fiber-OCDMA system using real time passive encoders-decoders with FBG as delay elements. In this case, the code sequence can be obtained with the same chip amplitudes at the output of the encoder. This system has been compared to the system using conventional optical fibers as delay elements in terms of BER. The high detuning for FBG gives the same transmittivity for all required grating lengths, in contrast to the case of optical fiber which gives different values of attenuation in the code sequence at the output of the encoder. As the code length F increases the transmittivity of optical fiber delay lines decreases which degrade the BER performance of the network. For FBG delay lines, a near ideal transmittivity is obtained, achieving better performance. From the results obtained herein, it can be concluded that at low transmitted power and when the number of active users is less than a certain threshold which in our simulations was around 25 the system using FBG outperforms that using optical fibers as delay elements.