Experimental Comparison of Different BER Estimation Methods for Coherent Optical QPSK Transmission Systems

We experimentally compare several bit-error-rate (BER) estimation approaches in a 10-Gbaud coherent optical nonreturn-to-zero quadrature-phase-shift-keying (NRZ-QPSK) system. The probability density function (pdf) fitting via the generalized exponential function offers the most reliable estimation of the system performance, which agrees well with the error counting measurement. We also propose a novel definition of
Q
-factor applicable to QPSK constellations, which is more credible compared to the error vector magnitude (EVM).