Effects of Rayleigh backscattering in single-source bidirectional fiber-optic systems using different modulation formats for downstream and upstream transmission

Statistical properties of Rayleigh backscattering in bidirectional fiber-to-the-home passive optical networks based on on–off keying (OOK), frequency-shift keying (FSK), and differential phase-shift keying (DPSK) modulation formats have been studied. The error probability (BER) for bidirectional transmission systems using FSK or DPSK format for downstream and OOK format for upstream has been examined in the optical line terminal (OLT) and optical network unit (ONU). Also, the probability density function of the received current for the bidirectional system using OOK format for both downstream and upstream transmission is derived, and the BER has been evaluated. Our results show that the required receiver power for bidirectional system using FSK/OOK and DPSK/OOK topology in OLT is lower than that for the system using OOK/OOK topology by about 9 and 10 dB m to reach the error probability of 10−9 for nonreturn-to-zero (NRZ) signal, respectively. In addition, we have found that using return-to-zero (RZ) modulation format can lead to significant performance improvement for OOK/OOK, FSK/OOK, and DPSK/OOK topologies at the cost of increased bandwidth.