Experimental study on multi-dimensional digital predistortion for multi-band externally-modulated radio-over-fiber systems

Multi-dimensional digital predistortion based on memory polynomial model is experimentally studied for multi-band externally-modulated radio-over-fiber (RoF) systems, in terms of nonlinearity order, memory length, maximum tolerable RF input power, and distorted signal feedback approach for model coefficients adaptation. 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. Based on the investigations, it is feasible to employ an auxiliary photodetector at the transmitter side to feedback the distorted signal for adapting the model coefficients. 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.