Sturdy-multistage noise-shaping delta-sigma modulator with enhanced noise-coupling and NTF-optimized strategy for intermediate frequency digital mobile fronthaul

In this paper, digital mobile fronthaul (MFH) based on the enhanced noise-coupling structure for intermediate frequency (IF) scenarios and noise transfer function (NTF)-optimized strategy sturdy multi-stage noise-shaping delta-sigma (SMASH) modulator is proposed. Without the need for additional integrators, the SMASH modulator with enhanced noise-coupling (SMASH-NC) gains two extra zeros in the signal band, improving the second-order noise shaping capability and achieving a 13.6 dB signal-to-noise rate (SNR) improvement. The NTF-optimized strategy based on genetic algorithm (GA) is proposed to design SMASH-NC. Compared with the SMASH-NC obtained by traditional method, the stability of SMASH-NC of GA method is higher, which supports larger input signals, with a 4.1 dB SNR improvement. We experimentally demonstrate the transmission of orthogonal frequency division multiplexing (OFDM) signals in MFH with the fourth-order SMASH-NC modulator. The OFMD signals at the center frequency of 1.5 GHz are modulated to 5-Gbaud 4-level pulse amplitude modulation (PAM4) signals through the SMASH-NC modulator and transmitted over the intensity modulation direct detection (IM/DD) link and 20 km single-mode fiber (SMF). Experimental results show that OFDM signals can be transmitted up to 65,536 quadrature amplitude modulation (QAM) with the SNR of 52.41 dB. In addition, the fourth-order SMASH-NC scheme has 9.52 % less adders and 29.17 % less multipliers, respectively, in comparison to the traditional fourth-order single-loop delta-sigma modulator (SDSM) scheme.