Development of novel ocdma codes for FTTH network
Thana, Hussein Abd
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Optical Code Division Multiple Access (OCDMA) technique enables many subscribers to share a network simultaneously and asynchronously by allocating a specific code to each subscriber. The Multiple Access Interference (MAI) is considered as the dominant degradation factor in the OCDMA system. Intelligent code sequence design is important to reduce the contribution of MAI. Over the last decade, many codes were proposed for the OCDMA. However, these codes have several restrictions on choosing the code parameters (code length, weight, cross-correlation properties). In addition, the number of accommodating users is severally limited. In this thesis two novel codes, namely Multi Diagonal (MD) and Dynamic Cyclic Shift (DCS) code have been proposed for the OCDMA system, to suppress the MAI consequently mitigate the phase induced intensity noise (PIIN), accommodate large number of users and enhanced the optical network capacity. The DCS code have been developed based on cyclic shift and Galois field method to obtain low cross-correlation property and minimise the code length with low weight value. On the other hand, MD code was developed based on multi diagonal matrixes to achieve zero cross-correlation property which significantly contributes to the elimination of the MAI and thus improved system performance of OCDMA network. The study of both codes, firstly derived mathematically, and subsequently carried out using simulation experiment utilizing optical simulator OptisystemTM version 9.0. The study focused on the effect of distance, bit rate, data modulation format, input power, chip spacing and amplifier ratio on the systems performance. The DCS code has been compared with codes that have cross-correlation value between their code words such as Modified Quadratic Congruence, (MQC), Modified Frequency Hopping (MFH), Enhanced Double Weight (EDW) and Modified Double Weight (MDW) codes. The DCS code showed the ability to accommodate a higher number of simultaneously users. This code could support 160 users simultaneously, carrying out 622Mb/s with a permissible bit error rate of 10-11. Power required was lower at the photo detector which means the detection system required less power by applying DCS code. Accordingly, the MD code system showed better performance than the former codes with zero cross-correlation property such as Zero Cross-correlation (ZCC) and Random Diagonal (RD) codes. It could accommodate 240 simultaneous users with 2.5 Gb/s at low transmitting power comparing to the 160 active user of the ZCC code at the same transmitting date rate. The development of the DCS and MD code has contributed to the OCDMA system improvement by reducing the cross-correlation value between code words and thus mitigating the PIIN. By adapting these codes in the OCDMA network, larger number of users can be accommodated with lower system complexity.