Mitigation of RB noise in bidirectional fiber transmission systems based on different OFDM SSB techniques

The interferometric beat noise due to the reflections (RE) from passive devices and the Rayleigh backscattering (RB) in wavelength reuse bidirectional fiber optic transmission systems leads to degradation of the system if not employed with a suitable RB mitigation technique. These noises can be reduced by implementation of the modulation schemes which reduce the spectral overlap between the data signal and RB-RE signals. In this paper, a bidirectional architecture to reduce RB-RE noise in wavelength reused bidirectional Radio over Fiber (RoF) is proposed. Optical Orthogonal Frequency Division Multiplexing (OFDM) Single Sideband (SSB) techniques are used for upstream and downstream transmission. Two SSB millimeter wave signals viz. conventional SSB and modified SSB are generated by the MZM employing a hybrid coupler to generate different phase shifts (90° & 120°) of the RF signal applied to its electrodes. In our architecture, the spectral overlap between the data signal and RB-RE signals is reduced by transmitting the downstream and upstream signals on the higher and lower first order optical sidebands of the SSB signal respectively. A mathematical model is developed to analyze the transmission performance of the two SSB techniques based on RB-RE interferences. An m-QAM optical OFDM baseband modulation scheme is used to transmit 10 Gbps data over 75 km fiber using the two SSB modulation schemes. Our results show that the modified SSB eliminates 2nd order RB-RE harmonics reduces RB-RE crosstalk against the conventional SSB.