High-fidelity digital-analog hybrid RoF fronthaul link enabled by nonlinear radio signal shaping.

We propose a simple and effective method to enhance the fidelity of radio signals in a digital-analog radio-over-fiber (DA-RoF) system by introducing nonlinearity into the transmitted signal. This nonlinearity is introduced via an arctangent function for radio signal shaping (RSS), while the corresponding tangent function is used for signal recovery. Nonlinear shaping spreads near-zero amplitude signals and compresses far-from-zero ones. After nonlinear recovery, noise in near-zero signals is suppressed, while noise in far-from-zero signals is amplified. Since the radio signal follows a Gaussian distribution, with most amplitudes near zero, the overall SNR is improved. In our experiments, we demonstrate the proposed radio signal shaping technology using a DA-RoF system based on intensity modulation with a direct detection communication system, achieving a 1.8-dB SNR gain. We achieve an SNR of 34.17 dB for QAM-1024 signals at a DA-RoF symbol rate of 104 Gbaud over 1 km of fiber. This corresponds to a CPRI-equivalent rate of 1560 Gbps and an equivalent fronthaul channel capacity of 294.85 Gbps. Our proposed scheme offers a promising solution for high-fidelity, high-capacity RoF links in Beyond 5G (B5G) and 6G networks.