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The independent optical dual-single-sideband (dual-SSB) signal generation and detection can be achieved by an optical in-phase/quadrature (I/Q) modulator and one single photodiode (PD). The dual-SSB signal is able to carry two different information. After PD detection, the optical dual-SSB signal can be converted into an electrical millimeter-wave (mm-wave) signal. Therefore, the optical dual-SSB signal generation and detection technique can be employed in the radio-over-fiber (RoF) system to achieve higher system spectral efficiency and reduce system architecture complexity. However, the I/Q modulator's nonideal property results in the amplitude imbalance of the optical dual-SSB signal, and then the crosstalk can occur. Moreover, after PD detection, the generated mm-wave signal based on the optical dual-SSB modulation has a relatively low signal-to-noise ratio (SNR), which restricts the system performance. In this paper, we propose an optical asymmetrical dual-SSB signal generation and detection scheme based on the probabilistic shaping (PS) technology, to decrease the influence of the optical dual-SSB signal's amplitude imbalance and to enhance the system performance in the scenario of the limited SNR. The dual-SSB in our scheme is composed of the left sideband (LSB) in probabilistic-shaping geometric-shaping 4-ary quadrature amplitude modulation (PS-GS4QAM) format and the right sideband (RSB) in quadrature phase-shift keying (QPSK) format. The transmitter digital signal processing (DSP) generates a dual-SSB signal to drive the optical I/Q modulator. The I/Q modulator implements an electrical-to-optical conversion and generates an optical dual-SSB signal. After PD detection, the optical dual-SSB signal is converted into a PS-16QAM mm-wave signal. In our simulation, compared with the normal 16QAM scenario, the PS-16QAM scenario exhibits a ∼1.2 dB receiver sensitivity improvement at the hard-decision forward error correction (HD-FEC) threshold of 3.8×10
Yan et al. (Tue,) studied this question.