Suppression Of3rd-order Intermodulation Distortion In RoF Based On Feedfoward Method

Radio over Fiber(RoF) technique is a new technology of transporting radio-frequency (RF) signal over optical fiber. RoF not only has optical fiber communication advantages of wide bandwidth and low loss, but also has RF wireless communication merits of flexibility. RoF is a promising technology which can be used for mobile communication, wireless local area network, intelligent transportation system, cable TV, military applications and so on. However, the electrical-to-optical (E/O) conversion nonlinearity leads to third-order intermodulation distortion (IMD3), which restricts the RF power, the spurious-free dynamic range (SFDR) and the applications of RoF to some extent. The communication system with higher SFDR has stronger anti-inference ability and better distributing ability, so it is important to reduce IMD3and to increase SFDR of the communication system. The aim of this thesis is to reduce IMD3using feedfoward(FF) method, and to optimize some parameters in the feedfoward RoF system for larger SFDR.A feedfoward architecture is designed, which can be used for RF signal fiber optic transportation with larger SFDR. The architecture incorporates two loops, the signal cancellation loop and the distortion cancellation loop. The signal cancellation loop cancels two-tone signal to isolate the distortion, and the distortion cancellation loop cancels the distortion. Moreover, the two-tone signal modulation in RoF system is analyzed to demonstrate the existence of IMD3. At last, the equations of SFDR,3rd-order Intercept Point(IP3) and the noise in RoF system are derived to show the theoretical basis of calculating SFDR.Three parameters, including optical power, modulator position and optical splitting ratio are optimized by using OptiSystem7.0software to increase SFDR in FF RoF system.Some experiments are performed using the optimized simulation parameters. It is found that a FF RoF system with90:10optical splitting ratio has larger SFDR than that with50:50optical splitting ratio, which is consistent with the simulation results. Under manual control, IMD3suppression of20dB and SFDR of114dB/Hz213are demonstrated with two tone signal centered at350MHz and two tone separation of100KHz. IMD3suppression of lOdB and SFDR of110dB/Hz2/3are also demonstrated with two tone signal centered at350MHz and two tone separation of500KHz.