Ran Zhu

Broadband Predistortion Circuit Using Zero Bias Diodes for Radio Over Fiber Systems

A low-cost broadband analog predistortion circuit (PDC) is designed and experimentally verified in a radio over fiber (RoF) system. A miniature hybrid microwave integrated circuit technique is used to reduce the size and parasitics of PDC. Only two zero bias GaAs beam lead detector diodes and capacitors are used in the PDC. The diodes can be biased at due to zero bias characteristic so that power consumption is reduced. No broadband matching network is required because of high series resistance of the zero bias diodes. The linearization performance is evaluated using an RoF system without and with the PDC. It is shown that the PDC improves the input 1-dB compression power by 0.4-2.2 dB from 7 to 18 GHz; and spurious free dynamic ranges are improved by ~ 10 dB from 7 to 14 GHz and ~ 6 dB from 15 to 18 GHz, limited by fifth order nonlinear distortion.

Linearization of radio-over-fiber systems by using two lasers with different wavelengths

A linearization technique using two lasers with different wavelengths is further studied to improve output signal and suppress second and third order nonlinearities in radio-over-fiber (RoF) systems simultaneously, if an optical device or a subsystem generates nonlinear distortion and also has wavelength dependent transmission characteristic. In this paper, for an RoF system using an EAM that has wavelength dependent transmission, it is found that the optimum suppression of second order nonlinearity can be achieved by adjusting the power ratio of the two lasers. Compared to using single laser at 1552.6 nm, 1.6 dB improvement of signal power, 23 dB suppression of second order nonlinearity, and 2.1 dB suppression of third order nonlinearity are obtained experimentally. More than 11.5 dB improvement of spurious free dynamic range, 3 dB improvement of input power at 1 dB compression (P1dB), and 4 dB improvement of output P1dB are achieved.

Broadband Predistortion Circuit Design for Electro-Absorption Modulator in Radio over Fiber System

A low-cost broadband predistortion circuit using zero bias diodes is designed to remove 3rd order intermodulation distortion of electro-absorption modulator in radio-over-fiber system. Experiments show that input power at 1-dB compression point is improved by 0.8-3.8 dB from 8 to 17 GHz; and spurious free dynamic range is improved by ~9 dB from 7 to 14 GHz and ~4 dB from 15 to 18 GHz, limited by 5th order intermodulation distortion. The power consumption is reduced and no matching network is required because of characteristics of zero bias diode.

Novel broadband analog predistortion circuit for radio-over-fiber systems

A novel broadband analog predistortion circuit (PDC) is designed to linearize radio-over-fiber (RoF) systems. Dual Schottky diode is used as predistorter. The PDC with high bandwidth and small dimension is achieved by removing quarter-wavelength transformers and power dividers. Measured S-parameters show that the PDC can work from 10 MHz to up to 40 GHz. The PDC is verified using two-tone and WiFi signals in an RoF system. The spur-free dynamic range with respect to third order intermodulation distortion is improved by more than 10 dB from 1 to 5 GHz and more than 5 dB from 1 to 30 GHz, and it is found that the RoF system is limited by fifth order nonlinearity. Error vector magnitude is improved by 1 dB for WiFi signals at 2.4 and 5 GHz by using the PDC.

Ultra Broadband Predistortion Circuit for Radio-over-Fiber Transmission Systems

A dual Schottky diode-based ultrabroadband predistortion circuit (PDC) is designed and experimentally verified to linearize both directly and externally modulated radio-over-fiber (RoF) transmission systems. A monolithic dual Schottky diode is used in the PDC to reduce the footprint and improve the bandwidth. The measured S-parameters show that the circuit is able to work for up to 40 GHz. The theory of third order intermodulation distortion (IMD3) generation by the PDC is established, and calculated IMD3 agrees well with the measured. Two measures are used to verify the PDC: spurious-free dynamic range (SFDR) and error vector magnitude (EVM) for WiFi signal at 2.4 GHz. For a directly modulated RoF transmission linearized by the PDC, the SFDR is improved by ~12 dB. For an externally Mach-Zehnder modulator (MZM) modulated RoF transmission linearized by the PDC, the SFDR is improved by higher than 5 dB from 2 to 30 GHz, and in particular higher than 12 dB from 2 to 5 GHz. For 2.4-GHz WiFi signal over the externally modulated RoF transmission linearized by the PDC, the EVM for WiFi signal is improved by up to 5.1 dB for back-to-back and 3.5 dB for 20-km SMF transmission.

Analog Pre-Distortion Circuit for Radio Over Fiber Transmission

This letter presents a novel analog predistortion circuit based on two Schottky diodes that can suppress both the third- and fifth-order nonlinear distortions simultaneously. This circuit is simple, of low cost, and of low power consumption. The predistortion circuit is verified in two radio over fiber (RoF) transmission systems. It is shown that spurious free dynamic range is improved significantly from 1 to 6 GHz. Using the circuit for Wi-Fi wireless signal over RoF, error vector magnitude is improved by up to 3.9 dB.

Analysis of Dual Wavelength Linearization Technique for Radio-Over-Fiber Systems With Electro-Absorption Modulator

The dual wavelength linearization (DWL) technique using two lasers with different wavelengths is for the first time comprehensively studied theoretically and experimentally to improve output signal power and suppress second- and third-order nonlinearities in radio-over-fiber (RoF) systems using an electro-absorption modulator (EAM). Spurious-free dynamic ranges (SFDRs) are calculated and measured. Both the calculation and experiments show that not only the RF signal power is significantly increased, but also both the second- and third-, as well as fifth-order nonlinearities are suppressed. For our considered EAM, when the second-order nonlinearity is suppressed maximally, the SFDRs with respect to second-order harmonic distortion and second-order intermodulation distortion are improved by 11.5 and 8.5 dB, respectively. In the mean time, the SFDRs with respect to third-order harmonic distortion (HD3) and third-order intermodulation distortion (IMD3) are improved by 1.8 and 1.3 dB, respectively. When the third order is suppressed maximally, the SFDRs with respect to HD3 and IMD3 are improved by 8.1 and 20.4 dB, respectively. The fifth-order intermodulation distortion is also suppressed at the same time. The RoF transmissions for WiFi signals are also verified. As a result, 3.5 dB at 2.4-GHz improvement and 2.8 dB at 5-GHz improvement of error vector magnitude are obtained for an RoF system by using the DWL technique.

Linearization of radio-over-fiber systems using directly modulated and electro-absorption modulator integrated lasers

A low-cost linearization technique is proposed to improve RF signal power and suppress third order intermodulation distortion (IMD3) in radio-over-fiber (RoF) transmission systems. A directly modulated laser (DML) and an electro-absorption modulator integrated laser (EML) in C-band are both used for optical subcarrier modulation. The IMD3s induced by the two optical subcarrier modulations are suppressed by each other through adjusting the bias voltage of the electro-absorption modulator in the EML. In our initial experiments, the spurious free dynamic range (SFDR) is improved by more than 3.5 dB and output power at 1 dB compression point is improved by 4.3 dB.