OTDR Based Estimation of CO-OFDM CFO

Optical time-domain reflectometer (OTDR) is a primary test and measurement instrument for detecting, localizing and qualifying various fiber optic link events induced by breaks, splices and connectors. However, in spite of a number of innovative enhancements of the OTDR capabilities proposed throughout decades of its use in the communications test application area, few reports can be found about extending the OTDR capabilities beyond detecting and qualifying refractive and reflective events of the fiber under test, to also include prediction of performance and impairments specifically related to the coherent optical orthogonal frequency-division multiplexing (CO-OFDM) symbol transmission over fiber link. As large enough OTDR dynamic range provides alike optical signal-to-noise-ratio (OSNR) even at far end of the fiber, then the dominantly reflective OTDR trace pattern can be considered as the two-way power-delay profile (PDP) of the fiber. Furthermore, in this work, we also justifiably assume that the cyclic prefix (CP) is applied to guard the OFDM symbol against inter-symbol interference, as well as that the (formerly eventually large) peak-to-average power ratio (PAPR) is significantly reduced, e.g. by simple peak clipping at the transmitter. This finally retains the OFDM carrier frequency offset (CFO) as the major OFDM-inherent signal impairment to dominantly determine the bit error rate (BER) floor in this case. Accordingly, in our model, we abstracted the CFO by the additional delays added to the original OTDR trace pulses, which would produce equal BER increase as the CFO does with the original trace. Inversely, this enables indirect estimation of CFO by simple BER testing, rather than by using dedicated and complex test instrumentation such as vector signal analyzers (VSA), not always at hand in field conditions.