Performance Evaluation of a SCM Optical Transmission System

This paper focuses on the impact of Optical Beat Interference (OBI) on the performance of subcarrier multiplexed optical transmission system for the application on radio link via optical fiber. Performance results are evaluated for QPSK modulation of Microwave subcarriers with digital NRZ coded random data patterns. A suitable bandwidth of 890 – 950 MHz is selected for subcarriers and channel bandwidth of 200 KHz and carrier separation of 250 KHz are considered. By measuring the Optical Bit Interference (OBI) performance limitations of the subcarrier multiplexing optical transmission system is investigated. The OBI for 10 channels for input power 1 dB is -40 dB whereas for 110 channels the OBI is -20 dB. The SIR is also investigated to evaluate the performance. The achievable SIR for NRZ–QPSK subcarrier multiplexed transmission system is about 5 dB corresponding to 110 channels whereas the corresponding SIR is only 28 dB for 10 channels for input power 1 dB. The SIR also degrades as the input power is increased. Investigation of BER shows that for 10 channels the BER is 10 whereas for 110 channels BER is around 10. This performance also degrades as the input power is increased to 20 dB.