Wideband Delta-Sigma Radio-Over-Fiber Embedding Complex-Valued Pulse-Distortion Model for 5G and Beyond

We propose a wideband model-embedding delta-sigma radio-over-fiber (DS-RoF) design for keeping the adjacent channel leakage low. This RoF design is derived on the basis of the DS-RoF embedding a pulse-distortion model (PDM). To enable wideband modulation with low adjacent channel leakage, we also propose an optimization method using an objective function simulating the adjacent channel leakage. By refining the digital down-converter from previous work, we additionally propose an extended PDM that generalizes its parameters from real numbers to complex numbers, which allow for fine-tuned delay adjustments in the PDM. A RoF demonstration using commercially available small form-factor pluggable (SFP) modules and a 10-km single mode fiber (SMF) with a 400-MHz-bandwidth orthogonal frequency division multiplexing (OFDM) signal, including a calibration path for modeling pulse distortion, revealed that the proposed design with our extended complex-valued PDM improved the signal-to-noise ratio (SNR) by 0.8 dB, normalized mean-squared error (NMSE) by 0.3 dB and adjacent channel leakage ratio (ACLR) by 0.4 dB compared with a RoF design with the original real-valued PDM. These results indicate that the complex-valued PDM, which enables fine-tuned delay adjustments, increases modeling accuracy, leading to enhanced compensation performance. Moreover, the proposed DS-RoF design with our complex-valued PDM also achieved an ACLR below -31 dBc. This result indicates that, to the best of our knowledge, we are the first to realize a 400-MHz-bandwidth modulation required for 5G millimeter wave as a DS-RoF system.