K-band ROF System Non-biased Predistortion Circuit Research

In 5G radio over fiber(ROF)system,there are lasers,optical modulators and other non-linear components on the link from the base station to the central unit.These non-linear components will generate intermodulation distortion and high order harmonic distortion in the output of the ROF system and this distortion can reduce the reliability of the system.Among them,the third-order intermodulation distortion is the main distortion component becasuse it has highest amplitude and closest to the desired signal,which means it's toodifficult to filter out on the spectrum,and it is the most harmful distortion to the system.In order to guarantee the reliability of the system,we must to linearize system and reduce the harmcaused by the intermodulation component and harmonic component.especially third-order intermodulation component.ROF linearization technology research has become the key issue to ROF technology research.Linearization technology mainly has Digital predistortion technology and analog predistortion technology.Digital predistortion linearization technique has good linearzation effect for big dynamic range of input signal using the OFDM modulation,but it has a narrow bandwidth,besides it is difficult to meet the future needs of 5G communication system.Analog predistortion linearization technique is widely studied and applied to linearize ROF system linearization because of its wide band and quick response.This paper mainly study the K-band analog predistortion circuit in 5G ROF system,in the hope of realizing a pre-distortion circuit on the band 24GHz-27GHz.This paper designs a analog predistortion circuit test system to test the linearization effect of analog predistortion circuit for the amplifier.The analog predistortion module from HeJiaXing company in XiaMen,integrates the third generation of Scintera analog predistortion chip(SC 1894),the bandwidth is 1800 MHZ to 2700 MHZ.This paper carried out a nonlinear suppression test for the four different bandwidth input signal of different modes.the center frequency of input signal is 2.1 GHz.In the result,with the increase of signal bandwidth,the performance of the third order intermodulation(IMD3)compression is reduced slightly.In this paper,a non-biased analog predistortion circuit based on zero-bias diode is proposed to linearize 5G ROF system in K-band.The circuit design choose the reverse-parallel zero-bias schottky diode pair as the predistortion signal generator,bias source for schottky diodes and bias Tee are removed,and resistance connected with schottky diodes are set to adjustable resistance.In ADS simulation,electric absorption modulator are choosen to be the nonlinear components,the results show that the analog predistortion circuit can suppress third order intermodulation 10 dB-40 dB on 24 GHz to 27 GHz,and max IMD3 suppression 40.9 dB is achieved at 24.9 GHz.