An improved feed-forward linearisation of optical transmitter for radio over fiber system

Abstract

Radio over fiber (RoF) technology provides an efficient and cost effective solution to increase the capacity of mobile communication systems and the coverage of wireless broadband communication systems. This is due to the growing data traffic and bandwidth demands. However, the performance of an RoF system is limited by the linearity of the optical transmitter that contributes to major nonlinear distortions in the links. Meanwhile, in multichannel applications, high linearity is required to avoid a channel interference due to nonlinear distortions. The feedforward linearisation technique offers a number of advantages compared to other techniques such as a broadband distortion reduction at high frequencies and simultaneous reduction in all orders of distortion. Therefore, it is suitable for linearisation of the optical transmitter. But, the existing design uses many electronic components causing difficulty for the amplitude and phase matching to achieve the optimum distortion reduction and also generate the parasitic parameter at high frequency. This thesis aims to improve the performance of the optical feedforward transmitter by reducing the electrical component without decreasing the distortion reduction performance; thus reducing the difficulty for the amplitude and phase matching and the parasitic parameter at high frequency. System characterisation is carried out by the mathematical analysis using Volterra series approach and simulation using a commercial optical design software which is validated by the practical measurement. It shows that the IMD3 reduction is good for high frequency. More than 15 dB IMD3 reduction is achieved over the carrier frequency 1.2 to 2.4 GHz using the proposed system practically. In addition, the proposed system is simpler and less sensitive in the amplitude and phase matching to obtain the optimum distortion reduction since the distortion suppression is influenced by less electrical parameters compared to other works.