With the continuous enrichment of mobile services and the explosive growth of internet traffic, the architecture of cloud radio access networks (C-RAN) has garnered significant attention. Mobile fronthaul is a crucial part of the C-RAN architecture. As we step into the sixth generation of mobile communications technology (6G), the fronthaul system is required to support data rates equivalent to the common public radio interface (CPRI) exceeding terabits, along with high-order modulation formats such as 1024 quadrature amplitude modulation (QAM) and above, to meet the demands for large-capacity and high-speed transmission in the 6G era. However, traditional digital radio-over-fiber and analog radio-over-fiber schemes both have shortcomings, and it has been found that they struggle to meet these requirements. In recent years, the widely studied digital-analog radio-over-fiber (DA-RoF) technology has demonstrated a signal to noise ratio (SNR) gain of over 12 dB at the cost of sacrificing half of the spectral efficiency. Meanwhile, due to the cost sensitivity of operators in mobile fronthaul scenarios, intensity-modulation and direct-detection (IM-DD) technology shows great potential because of its cost advantage. Against this background, relying on the DA-RoF system based on IM-DD to balance cost, explore transmission capacity, and optimize fidelity has become a key issue in the mobile fronthaul field. We propose a high-capacity DA-RoF scheme based on space-division multiplexing (SDM). Focusing on the IM-DD system, we adopt an improved modulation factor optimization strategy to achieve an SNR gain exceeding 15 dB, which facilitates the stable transmission of 1024-QAM signals and promotes the development of 6G fronthaul technology.

The terabit DA-RoF communication system proposed in this work is primarily based on DA-RoF modulation technology and an improved modulation factor optimization strategy, combined with the space-division multiplexing technology of multi-core fiber. At the transmitting end, the 1024-QAM modulated signal generated by the digital signal processing (DSP) module is converted into an orthogonal frequency division multiplexing (OFDM) signal through the inverse fast Fourier transform to simulate the wireless waveform. Then, DA-RoF modulation is applied to divide the wireless waveform into a digital part with probabilistic constellation shaping and an analog part with quantization residual error. Joint optimization is performed on the amplitudes of the digital part, the analog part, and the original wireless waveform to achieve the maximum recovered SNR. At the receiving end, a third-order Volterra equalizer is used to compensate for the link impairments, and finally, the corresponding DA-RoF demodulation and OFDM demodulation are carried out to recover the original signal. The experimental setup uses 10 km of single-mode fiber, 10 km of 7-core fiber, and 1 km of 7-core fiber to explore the effectiveness of the proposed method in different scenarios. Through the experimental implementation, terabit DA-RoF transmission of the 1024-QAM signal has been successfully realized.

The optimization outcome of the modulation factor under a back-to-back configuration is presented in Fig. 5. When considering the 1024-QAM signal with a 2.5% error vector magnitude (EVM) threshold, the received optical power (ROP) sensitivity reaches -11 dBm. Upon transmission through a 10-km single-mode fiber, the recovered SNR of the 1024-QAM signal exceeds 32 dB, which features a symbol rate of 40 GBaud and a CPRI equivalent data rate of 313 Gbit/s, thus fulfilling the 1024-QAM transmission criteria, as illustrated in Fig. 6(b). Fig. 6(c) depicts the variation in SNR corresponding to different symbol rates. Evidently, as the symbol rate rises, the in-band noise increases, causing the recovered SNR to correspondingly diminish. After the 30 GBaud 1024-QAM signal is transmitted over a 10-km 7-core fiber, the average recovered SNR of the signals across all cores is measured at 33.54 dB. This indicates that the proposed scheme attains an SNR gain exceeding 15 dB and accomplishes a CPRI equivalent rate-distance product of 16.4 Tbit/(s·km^-1) (Fig. 8). Moreover, the 60 GBaud 1024-QAM signal has been successfully transmitted over a 1 km 7-core fiber, achieving a CPRI equivalent data rate of 3.28 Tbit/s (Fig. 10).

We propose and experimentally demonstrate a large-capacity, high-fidelity mobile fronthaul transmission scheme utilizing a C-band IM-DD configuration. By employing DA-RoF modulation technology and jointly optimizing the modulation factors, an SNR gain of over 15 dB is achieved, which enables the successful transmission of 1024-QAM signals. The system capacity is significantly enhanced through the use of SDM technology with 7-core fibers. Specifically, the experiment shows the transmission of 1024-QAM signals with a symbol rate of 30 GBaud over 10 km of 7-core fiber, achieving a record 16.4 Tbit/(s·km^-1) CPRI equivalent rate-distance product. Additionally, the transmission of 1024-QAM signals with a symbol rate of 60 GBaud over 1 km of 7-core fiber is also demonstrated, achieving a record 3.28 Tbit/s CPRI equivalent data rate. These experimental results indicate that the proposed IM-DD DA-RoF scheme offers a promising large-capacity, high-fidelity, and cost-effective solution for future beyond 5G/6G mobile fronthaul scenarios.