Simple Electrical Predistortion Method Using Schottky Diode for Radio-Over-Fiber Systems

A simple low-current consumption analog predistortion circuit is designed to enhance linearity. The circuit is experimentally verified in a radio-over-fiber (RoF) systems. A Schottky diode is used to compensate for the nonlinearity of RoF systems. An L-I curve for a laser diode is compared with the I-V curve of the Schottky diode and the nonlinear characteristics are analyzed. The predistortion circuit is composed of two paths: 1) the nonlinear circuit with a Schottky diode to compensate for the laser diode nonlinearity and 2) the electrical delay line for a time delay. These two paths are recombined to suppress the IMD3 (third order intermodulation distortion) of the laser diode. Experimental results show that enhancements of ~26 dBc in the IMD3 and ~7.5 dB in the spurious-free dynamic range are achieved at 2.4 GHz. The proposed predistortion system has a significant reduction in current consumption compared with previous linearization techniques. In addition, error vector magnitude is satisfied under 4% with 64-quadratic-amplitude modulation.