Investigation of Transmission Impairments in Radio Over Fiber (ROF) Systems
Where, the attenuation of RF signals in traditional transmission media increases rapidly when the frequencies of the signal increase, Radio Over Fiber (RoF) system has been proved as a promising transmission medium in which RF modulated optical carriers can be transmitted and distributed with very low loss. This thesis introduces optical and radio communication systems, and provides a review of the RoF systems for wireless access networks. The research then explores the effects of different fiber non-linearities and finds out Stimulated Brillouin Scattering (SBS) imposes primary limit to the system where high transmission power is required. The investigation starts by generating 10 GHz MQAM RF signal with bit rate 1 Gbps. The RF signal was further modulated using an external optical modulator (Mach-Zehnder modulators). The SBS threshold was initially measured as 11.5 dBm for the setup. Then the modulation index was optimized. Afterwards, the optimum bias voltage measured as 0.4 V, which increased the SBS threshold upto 12.7 dBm, while maintaining BER of 10-8.
System performance was enhanced by reducing the dispersion effect applying an optical filter to convert the generated double sideband signal (DSB) to single sideband signal (SSB). The filter position was compared between before and after the fiber reel and optimized position was before the fiber with an eye opening factor of 3.92. Finally the most influential noise source was identified as Relative intensity noise (RIN) and its allowable range was calculated as -120 dB/Hz.