In the well-recognized cloud radio access networks concept, radio-over-fiber (ROF) links can serve as an option to make fronthaul connections conveying multi-band radio signals. High-performance ROF links covering future higher frequency commonly rely on external modulation. However, externally-modulated ROF links suffer from inherent modulator nonlinearities. In order to address this issue, we investigated the multi-band nonlinear behavior based on multi-dimensional memory polynomial model. Based on this model, multi-dimensional digital predistortion is experimentally studied for multi-band externally-modulated ROF links. The experimental results show that the external modulator instead of photodetector contributes to the most of nonlinear distortions among which the cross-band distortions is dominant with respect to the in-band distortions. Moreover, the results also suggest us to implement a memoryless multi-dimensional digital predistortor with involving nonlinearity orders up to 5 for a practical multi-band externally-modulated RoF system. With the suggested parameters, the proposed multi-dimensional DPD technique is able to improve the RF input power tolerance by greater than 3 dB for each band in a two-band case.
['Chunjing Yin', 'Jianqiang Li', 'Hao Chen', 'Qiang Lv', 'Yuting Fan', 'Feifei Yin', 'Yitang Dai', 'Kun Xu']
Behavioral modeling and digital compensation of nonlinearity in multi-band externally-modulated radio-over-fiber links