SAC-OCDMA in free space optical networks using multi-wavelength laser source

Free space optical (FSO) networks offer high speed and secure communication system. To utilize the bandwidth of FSO networks efficiently, spectral amplitude coding optical code division multiple access (SAC-OCDMA) is proposed as a solution. The reason is that OCDMA in general and especially SAC have robustness against atmospheric effects. So far, few investigations have been done to evaluate the performance of the SAC-OCDMA in FSO. However, Most of them used broadband sources as transmitter which is not suitable for outdoor FSO networks due to their wide beam divergence (about 8 mrad) and low modulation bandwidth (about 200 MHz). On the other hand, multi-wavelength laser source are becoming more available with regards to cost and variety. The work in this thesis focuses on design, development and enhancement of the SAC system in FSO networks deploying multi-wavelength laser source. Three main contributions are presented which have been proved mathematically and using simulation software. The feasibility of the proposed system is also implemented using proofof- concept hardware experiment. As the first contribution, a mathematical model that represents the characteristics of SAC system in FSO networks is designed and developed. In addition to the impact of turbulence, influences of several system noises, such as, relative intensity noise (RIN), optical beat interference (OBI), shot noise and thermal noise have been studied. From the numerical results, it is found that the proposed system could achieve up to 2.7 km of transmission distance with 10 users. As the second contribution, the performance of the aforementioned system is improved by proposing a new code namely double weight zero-cross correlation (DW-ZCC). It is shown that in comparison with modified double weight (MDW) and ZCC codes, DW-ZCC can improve the performance and transmission distance by at least 350 m and 200 m respectively. As the third contribution, the study of the system is extended to quality of service (QoS) area. For this purpose, based on the promising results of DW-ZCC in theoretical analysis, a variable weight code is proposed namely multi-service ZCC (MS-ZCC) which can provide service differentiation capability. Moreover, in comparison with previous code namely variable weight ZCC, the results shows that MS-ZCC can improve the system performance in term of number of users with 10 and 22 users for weights W = 4 and W = 8 respectively. For the next stage, the proposed system is implemented using Optisystem version 10 simulation software and hardware experimentation. Simulation results approve the mathematical approach and it is shown that DW-ZCC provides better performance compared to MDW and ZCC. In the experimental section, DW-ZCC is compared with MDW in back-to-back setting with broadband source deployment. With regards to required input power to attain threshold of 10-3 BER, MDW needs -2dBm, while this value is -13 dBm for DW-ZCC which shows 11 dBm improvement for DW-ZCC