Analog Predistorter for RoF Fronthaul Link Excited by a 400MHz Bandwidth 5G NR Signal

The radio-over-fiber (RoF) system has been widely used for the fronthaul transmission of remote radio units (RRUs) in recent decades. With the development of the mobile communication, the instantaneous bandwidth is increasing. In order to enhance the linearity of the RoF fronthaul link when transmitting a 5G new radio (5G NR) signal, a new analog predistorter (APD) consisting of two Schottky diodes cascaded with a field-effect transistor (FET) is proposed in this work. The FET is employed to enhance the linearization performance. And in order to achieve linearization for instantaneous broadband modulation signals, a 3dB coupler is employed in the proposed APD to match the characteristics of the two Schottky diodes. Experimental measurements demonstrate that the proposed APD significantly improves the adjacent channel leakage ratio (ACLR) of the 3.5 GHz RoF fronthaul link with a 16-quadrature amplitude modulation (16-QAM) of 400 MHz bandwidth 5G NR signal by more than 16 dB. Additionally, the error vector magnitude (EVM) of the fronthaul link is improved by 7. 15%.