NTF-Improved Delta-Sigma Modulation Supported 65536 QAM Signal for Mobile Fronthaul

In this article, a support 65536 QAM signal mobile fronthaul (MFH) architecture based on the noise transfer function (NTF)-improved delta-sigma modulation (DSM) was proposed and demonstrated by experiment. In the NTF-improved DSM, the extended Lee's rule was used to optimize the NTF generated by the Closed-Loop Analysis of Noise-Shaping Coders (CLANS) method to achieve greater gain of signal-to-noise rate (SNR). The performance of traditional DSM and NTF-improved DSM under different oversampling ratios (OSRs) and different quantization levels were simulated and compared. The results show that the NTF-improved DSM can achieve about 7 dB SNR improvement at 8 times OSR compared with the traditional DSM. Based on NTF-improved DSM, the orthogonal frequency-division multiplexing (OFDM) signal with high-order quadrature amplitude modulation (QAM) is quantized into 4-level pulse amplitude modulation (PAM4) to evaluate the scheme performance. The quantized PAM4 signal was transmitted over 25-km single-mode fiber (SMF) in C-band 25G baud intensity modulation direct detection (IM/DD) system. At the receiver, the SNR of the recovered OFDM signal after filtering reaches 51.7 dB. In the case of transmitting 65536 QAM OFDM signals, the error vector magnitude (EVM) of the recovered signal is 0.25%, which is below the 0.34% EVM threshold of 65536 QAM MFH. The experimental results show that the NFT-improved DSM can achieve 65536 QAM MFH when the OSR is 8. Compared with the traditional DSM that requires an OSR of 10, the spectral efficiency (SE) has been greatly improved.