High capacity of optical code division multiple access in visible light communication system using catenated OFDM modulation technique

Visible light communication (VLC) system is a new technology as an alternative for crowded wireless communication using radio frequency spectrum. This technology offers unregulated, unlicensed and huge bandwidth to meet the growing demand in future indoor communication system. To date, the major challenge in VLC systems has been in improving transmission speed of VLC system with optimize bandwidth capacity. One of the method to improve spectral usage is using advanced modulation formats such as orthogonal frequency division multiplexing (OFDM). In this work, a new approach for modulation technique based on modification of conventional OFDM has been developed in VLC system which capable to improve the spectral efficiency and optimize the data rate. This work also contributes to the development of mathematical model of optical code division multiple access (OCDMA) based on zero cross correlation code (ZCC) using a new modulation technique. This combination demonstrates an efficient access control in backbone design network to enhance the channel capacity, to upgrade the system data rate and at same time to improve sensitivity of photodetector. In particular, this thesis begins with the construction of a new modulation technique, namely catenated-OFDM that is developed based on multiband OFDM. In order to fulfill the next generation demand for higher data rate system, conventional OFDM-VLC is modified to get the optimum performance. A new mathematical framework of this modulation technique to analyze SNR and BER performance in VLC system has been derived. Simulation are carried out to evaluate some parameters that are not included in the numerical analysis. Different selection of parameter values such as bit rates, number of bands, effective power and propagation distance will have different effect on the system performance. Validation of theoretical results and simulation results are also conducted. Numerical analysis shown that the catenated-OFDM outperforms conventional OFDM by five times spectral efficiency improvement and eight times spectral efficiency improvement at their five and eight number of bands respectively. It is also shown that the proposed catenated-OFDM design is working well even at very high bit rate of 15 Gbps. In the combination of OCDMA-VLC using catenated-OFDM, the theoretical analysis and simulation model are also developed to evaluate and analyze its performance. This system theoretically demonstrates cardinality enhancement with double-fold, triple-fold and quadruple increase in number of user for (Band=2), (Band=5) and (Band=8) respectively compared to previous OCDMA ZCC-VLC system design. This result reveals improvement in term of receiver sensitivity which the power penalty of 7.3 dB compared to previous work. In addition, this integration provides higher receiver sensitivity; an approximately –17.7 dBm with 8 number of weight and fixed code length of 20. It is also found that, to accommodate more user, the system requires higher effective power at the receiver. This results exhibit the feasibility of the scheme to be one powerful modulation technique as a candidate for future access network. Thus, with a larger capacity, high data rate and enhanced security the OCDMA-VLC based on catenated-OFDM is predicted to be new considerable for upcoming OCDMA-VLC mobile communication system design.