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

Abstract

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 similar to 26 dBc in the IMD3 and similar to 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.