Time-Domain Spread Modulation in Radio-over-Fiber Systems for Robust Low-SNR Hardware Compensation

Abstract

In radio-over-fiber (RoF) systems, digital signal processing (DSP) is concentrated at the central station, leaving the remote antenna unit with only a power amplifier and an antenna. Cost-constrained commercial-grade components along the signal chain introduce nonlinear impairments that degrade the effective signal-to-noise ratio (SNR). Conventional compensation either requires digital hardware at the remote antenna unit that is too costly, or loses accuracy at low SNR. A time-domain spread modulation and demodulation model is proposed that disperses multi-bit information across continuous in-phase and quadrature sequences. A wavelet-based transmitter shapes the waveform and a Transformer-based receiver recovers bits from the continuous channel context. Under a dynamic hardware-impaired RoF channel, an end-to-end comparison shows that the proposed system reduces the normalized area under the pre-forward-error-correction (pre-FEC) bit error rate curve (nAUC) by 79.2%, 72.8%, and 54.9% at 4, 6, and 8 bits/symbol compared with a conventional quadrature amplitude modulation (QAM) system using digital pre-distortion and pilot-based DSP reception. Even under conventional DSP reception, the proposed waveform reduces nAUC by 70.1%, 63.2%, and 46.2% at the three spectral efficiencies. This robustness makes the approach promising for low-cost deployment in low-SNR, hardware-impaired RoF links.