Low-complexity Adaptive Frequency-domain Nonlinear Equalization for Analog RoF Mobile Fronthaul Using FFT/IFFT-assisted Channel Aggregation

Addressing the nonlinearity induced signal degradation is of critical importance for harvesting the transparent benefits of analog radio over fiber (A-RoF architecture, to support mobile fronthaul (MFH with high spectral efficiency and reduced latency. Here, for linearizing the low-latency A-RoF MFH using digital-domain fast Fourier transform /inverse fast Fourier transform (FFT/IFFT assisted channel aggregation/de-aggregation (CA/DA, we propose and experimentally demonstrate a compatible adaptive frequency-domain (FD nonlinear equalization scheme with much lower computational complexity, while delivering comparable performance to the popular time-domain Volterra nonlinear equalizer. The proposed low-complexity equalization scheme is implemented using the FD two-box cascaded Hammerstein structure, which is capable of simplifying coefficients estimation, achieving computational efficient equalization, reusing the FFT/IFFT calculation in channel DA and sharing legacy simple one-tap FD linear equalization strategy in OFDM system. Experimental validations of our proposal are carried out in typical electroabsorption modulator (EAM and Mach-Zehnder modulator (MZM based intensity modulation and direct detection (IMDD A-RoF links. Cost-effective fiber-optic (with EAM and optical up-conversion enabled V-band millimeter-wave (mm-wave fiber-wireless (with MZM transmissions are demonstrated. Experimental results show that the proposed scheme can effectively improve the performances of both EAM and MZM based 10-km fiber-optic A-RoF MFH links, in terms of adjacent channel leakage ratio reduction (>8 dB, error-vector magnitude amelioration (>10 dB and EVM-compliant input IF power range enhancement (>5 dB, with an input signal aggregating 8 125-MHz 64-QAM OFDM channels. Furthermore, our scheme enables the transmission of a 5-Gbit/s aggregated intermediate-frequency 64-QAM signal over 10-km fiber link and 1.2-m 60-GHz wireless channel, wherein a >12 dB improved input power margin is observed.