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In this letter, we analyze a 16-ary quadrature amplitude modulation (16-QAM) optical signal generated by applying four-level electrical signals to a single in-phase/quadrature (IQ)-modulator. The amplitude of optical output of the Mach-Zehnder modulator (MZM) in the IQ-modulator needs to be carefully adjusted, and the limited extinction ratio of MZM results in unwanted phase offset. Required optical signal-to-noise ratio is estimated in the 112-Gb/s polarization-division-multiplexed (PDM) 16-QAM signal by numerical simulations with various amplitudes of the electrical modulation signals. Optimal operating conditions are found to guarantee the best performance of the received PDM 16-QAM signal.