On the Use of Delay-Decorrelated I/Q Test Sequences for QPSK and QAM Signals

We report on important limitations of the commonly used method to generate complex optical signal constellations from delay-decorrelated copies of a common bit sequence to modulate in-phase (I) and quadrature (Q) components of a signal constellation. Using closed form analytical solutions, numerical simulations, and an experimental verification, we show that I/Q delays shorter than the combined channel and adaptive equalizer length can result in artificial improvements of the measured bit error ratio.