To solve the nonlinear impairment during transmitting high-order quadrature amplitude modulation (QAM) signals, we propose a high-order QAM transmission system based on Delta-Sigma modulation (DSM). With the commercialization of 5G networks, the number of mobile internet traffic terminals has surged to increase the demand for fronthaul network transmission rates. At present, the mobile fronthaul network interface common public radio interface (CPRI) usually adopts 15-bits sampling width, which means that at least 15 times of oversampling is required, which results in low spectral efficiency (SE). To this end, researchers proposed a fronthaul network based on DSM technology. Under ten times of oversampling, one-bit quantized DSM can provide a signal-to-noise ratio (SNR) of about 33 dB and support 1024QAM mobile fronthaul. Therefore, facing ever-increasing traffic demands, we study the transmission performance of broadband DSM signals to provide certain references for the future design of high-speed fronthaul networks based on DSM technology.

To study the transmission performance of broadband DSM signals, we carry out the transmission experiment of 100 Gbaud DSM signal. For the power fading caused by chromatic dispersion that often occurs in intensity modulation/ direct detection (IM/DD) systems, we have chosen the O-band transmission. For the insufficient bandwidth for the transmission system, we employ pre-equalization technology to address the high-frequency fading caused by narrow-band channels. We adopt the on-off-keying (OOK) signal as the training sequence and carry out 100 Gbaud optical back-to-back (BTB) transmission. Then, the frequency domain corresponding to the tap coefficient in the steady state of the constant modulus algorithm (CMA) in the digital signal processing (DSP) algorithm is taken as the inverse response of the channel. Afterward, the finite impulse response (FIR) is generated according to the tap coefficients of CMA. Finally, the generated FIR filter is utilized to filter the transmission signal after two-up-sampling to complete the pre-equalization. Through O-band transmission and pre-equalization technology, the transmission of 100 Gbaud DSM signals is successfully realized.

For the transmission of 50 Gbaud DSM signals, the corresponding experimental results are shown in Fig. 4. In the case of 25 km transmission, when the received optical power (ROP) is -2 dBm, the DSM-OOK signal can realize the error-free transmission. In the case of 50 km fiber transmission, error-free transmission is realized when ROP is 0 dBm. Compared with BTB transmission, the transmissions of 25 km and 50 km introduce 2 dB and 4 dB of power penalty respectively. For the transmission of 100 Gbaud DSM signals, the corresponding experimental results are shown in Fig. 6. When the ROP is 2 dBm, the BER of the DSM-OOK signal after BTB transmission and 10 km transmission is 10^-5 and 3×10^-5 respectively. However, in the case of 15 km transmission, the conventional DSP algorithm can only achieve BER of 10^-4. Therefore, by adding post-filtering (PF) and maximum likelihood sequence estimation (MLSE) algorithm after the traditional DSP, the BER is successfully reduced to 10^-5. The final experimental results are summarized in Table 1.

This study realizes the 50/15 km transmission of 50/100 Gbaud DSM signals on the O-band IM/DD link by utilizing one-bit quantized DSM and pre-equalization technology. Thanks to one-bit quantization DSM with SNR of 33 dB, the system supports up to 4096QAM signal transmission that meets the soft decision threshold of 4.0×10^-2, and the EVM of 1024QAM signal meets the EVM standard of 2.5% for mobile fronthaul. In the transmission of 50 Gbaud DSM signals, the 1024QAM mobile fronthaul with a rate of 5×10×127/256=24.8 Gbit/s on 50 km fiber is achieved. In the transmission of 100 Gbaud DSM signals, by utilizing PF and MLSE, the 1024QAM signal mobile fronthaul with a rate of 10×10×127/256=49.6 Gbit/s on 15 km fiber is realized. In addition, the system supports a maximum net bit communication rate of 12×10×127/256×0.75=44.6 Gbit/s. The broadband DSM signal transmission system demonstrated in the experiment provides a solution for high-order QAM signal transmission in the IM/DD link and also provides references for future design of high-speed mobile fronthaul approaches based on DSM technology.