The subcarrier transmission of quadrature-amplitude modulation radio frequency (RF) signals over a dispersive fiber link is analytically and numerically studied. It is well known that the double-sideband transmission of an RF subcarrier will suffer from frequency-dependent fading caused by fiber dispersion. The present analyses show that the fading of the RF amplitude depends on the modulation index in the optical signal, which causes distortion in the constellation map. We will show that this is attributed to higher order harmonics of modulation sidebands excited through the nonlinear response of the intensity modulators. Optical single-sideband transmission, which is believed to be free from dispersion-induced fading, is also analyzed and predicts a cyclic deviation of the symbols both in amplitude and phase.
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