Digital Mobile Fronthaul Based on Delta-Sigma Modulation and Chirp-Managed Signal Transmission

A novel digital mobile fronthaul (MFH) architecture combining delta-sigma modulation (DSM) and the chirp-managed technique is proposed to enhance spectral efficiency and dispersion tolerance. In the proposed MFH, one-bit DSM based on an optimized 4th-order cascaded resonator feedback (CRFB) circuit is utilized as the digitization interface to quantize the high-order 1024/4096 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signals. The digitized on-off keying (OOK) signals are transmitted over an optical intensity modulation-direct detection (IM-DD) link, which is chirp-managed by a directly modulated laser (DML) and an optical spectral re-shaper. Simulations of chirp-managed lasers (CML) are conducted employing VPI software to confirm that the optical spectral re-shaper exerts similar effects when placed before and after 50 km standard single-mode fiber (SSMF) when the transmitted (Tx) power is below ∼10 dBm. For self-phase modulation (SPM)-dominated high Tx power (>10 dBm), the post filter outperforms the front one and improves the transmitted power dynamic range. Furthermore, the proof-of-concept experimental demonstration of the proposed MFH based on the DSM and chirp-managed technology is performed over 50-km 20-Gbaud OOK DML-based IM/DD transmission with an information data rate of 10.55 Gbps, which reinforces that the cost-effective and high-spectral-efficiency DSM-CML approach is anticipated to be a promising candidate for digital MFH in B5G/6G, especially for cost-sensitive and high-fidelity applications.