Fractional-Order Digital-Analog Radio-Over-Fiber System for Continuous Scaling Between SNR and Spectral Efficiency

Digital-analog radio-over-fiber (DA-RoF) is regarded as a promising solution for fronthaul due to its efficient scaling between the recovered signal-to-noise ratio (SNR) and spectral efficiency (SE). However, its drawback lies in discrete bandwidth expansion only at an integer multiple, which lacks flexibility for various channel conditions. In this paper, we propose and experimentally demonstrate a fractional-order DA-RoF (FDA-RoF) scheme, which enables the continuous adjustment of bandwidth ratios while maintaining the efficient scaling law of approximately 10 dB SNR gain per additional bandwidth. A theoretical analysis of FDA-RoF is derived, which establishes the mathematical relationship between the recovered SNR and fractional bandwidth ratio. The numerical results with different incremental steps of the bandwidth ratio validate that FDA-RoF can realize continuous bandwidth expansion and retain efficient SNR scaling across various channel SNRs. In the experiment, an SNR gain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>2.5 dB can be achieved as the bandwidth ratio increases linearly with a step of 0.25, consistent with the scaling law observed in the integer-order DA-RoF (IDA-RoF). FDA-RoF can bridge the gap in discrete bandwidth expansion of IDA-RoF through finer-grained adjustment, offering a viable pathway for future flexible and spectrally-efficient fronthaul networks.