High Efficient Delta-Sigma Modulator Utilizing Multi-Stage Noise Shaping Structure

Recently, several Delta-Sigma modulators (DSMs) with ultra-high quadrature-amplitude-modulation (QAM) order larger than one million, e.g., 1048576 and 4194304 QAM are reported. As different DSM works were implemented with different oversampling rates (OSRs), and pursuing a higher DSM QAM order seems to deviate from its original idea to achieve high spectral efficiency. Up to the authors' knowledges, there is still no comprehensive comparison of the intrinsic efficiencies of different DSM works. In this work, we first summarize the recent DSM works and compare their efficiencies. We also propose and demonstrate a 1-bit DSM and a 2-bit multi-stage noise shaping (MASH) DSM for the baseband-over-fiber system. The modified 1-bit DSM can achieve record efficiencies of 0.589 and 0.665 under the hard-decision (HD) and the 25% soft-decision (SD) forward error correction (FEC) requirements, respectively. The 2-bit MASH DSM can also achieve record efficiencies of 1.025 (HD) and 1.150 (SD) respectively.