Taijun Liu

A Novel Analog Broadband RF Predistortion Circuit to Linearize Electro-Absorption Modulators in Multiband OFDM Radio-Over-Fiber Systems

We propose and demonstrate a novel and simple predistortion circuit using reflective antiparallel diodes for linearization of electro-absorption modulators (EAMs) in multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio-over-fiber systems for UWB signals from 3.1 to 4.8 GHz. In this novel predistortion circuit, neither amplifiers, nor phase shifters are used, while only two antiparallel connected diodes are used, and the predistortion signal is adjusted by tuning the diodes dc-bias current. The simple architecture makes the circuit cost effective and suitable for UWB system applications. By adding the predistortion circuit before the EAM, more than 7-dB suppression in third-order intermodulation distortion and 11-dB improvement in spurious-free dynamic range are achieved over the frequency range of 3.1-4.8 GHz, i.e., bandgroup 1 of MB-OFDM UWB. Correspondingly, 1-dB improvement in error vector magnitude is obtained experimentally when the designed predistortion circuit is used.

Optical Single-Sideband Modulation With Tunable Optical Carrier to Sideband Ratio in Radio Over Fiber Systems

We show that an optical modulator that consists of an integrated dual parallel Mach-Zehnder modulator (dMZM) can be used for obtaining not only optical single-side band modulation but also tunability of optical carrier to sideband ratio (OCSR) simultaneously. Such a modulator will be vital for optimizing the performance of radio over fiber links by improving modulation efficiency and receiver sensitivity and by removing fiber chromatic dispersion induced RF power fading. It is shown that a wide range of OCSR tunability can be obtained by altering bias voltage of dMZM, and optimum OCSR, to maximize the output RF power, depends on RF modulation index and extinction ratio of the integrated dMZM. Good agreement between theory, simulation, and experiment is obtained. For typical extinction ratio and low modulation index, it is found that an OCSR of 0 dB is optimum to maximize RF carrier power. However, for multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio, the best error vector magnitude (EVM) of -21.8 dB is obtained experimentally at an OCSR of ~5.4 dB due to avoidance of clipping induced nonlinear distortion.

Experimental Demonstration of Mixed-Polarization to Linearize Electro-Absorption Modulators in Radio-Over-Fiber Links

We experimentally investigate optical mixed polarization for the linearization of an electro-absorption modulator (EAM) in radio-over-fiber links. A linear polarizer is used before and after a polarization-dependent EAM. Thus, both transverse-electric and transverse-magnetic polarized lights are simultaneously modulated by different amounts. By carefully optimizing two polarization angles, the third-order intermodulation distortion (IMD3) is suppressed. The linearization leads to suppression of the IMD3 by more than 16 dB and improvement of spurious-free dynamic range by 8.1 and 9.5 dB for back-to-back and after 20 km of fiber transmission, respectively, compared to the nonlinearization case.

Impact of Electro-Absorption Modulator Integrated Laser on MB-OFDM Ultra-Wideband Signals Over Fiber Systems

This paper presents investigation of the impact of electro-absorption modulator integrated laser (EML) on performance of multiband-orthogonal frequency-division-multiplexing (MB-OFDM) ultra wideband (UWB) signal over single-mode fiber transmission by theoretical and experimental analysis, considering various parameters, such as fiber length, bias voltage, modulation power, bias current, and temperature of EML. The effect of the EML modulation nonlinearities on the OFDM subcarriers in MB-OFDM UWB over fiber system is investigated considering intermodulation distortion (IMD) and adjacent channel power ratio. It is shown that MB-OFDM UWB over fiber is limited by SNR for small modulation power and EML modulation nonlinearities induced IMD for high modulation power. Fiber transmission length is mainly degraded by EML phase noise converted relative intensity noise and power fading. The developed theory is verified by experiments.

Mach-Zehnder: A Review of Bias Control Techniques for Mach-Zehnder Modulators in Photonic Analog Links

The Mach-Zehnder modulator (MZM) has been widely used for broadband photonic analog links and high-speed digital optical fiber communication systems because it possesses large modulation bandwidth, low driving voltage, and low chirp. The MZM is a very important optical modulator for photonic applications. In an MZM, the input light is split into two paths, each of which is modulated by an electrical signal. Then the two arms are combined to generate an intensity-modulated light or a phase-modulated light at the output of the MZM. An MZM can be made of lithium niobate (LiNbO3), gallium arsenide (GaAs), or indium phosphide (InP), materials that exhibit some anisotropy in their dielectric properties. Theoretically, the relation of output optical field and driving voltage is a cosine function, i.e., nonlinear transfer function. For RF photonics, the MZM has typically two applications: optical harmonic generation for optical frequency multiplication and optical subcarrier modulation for optical signal modulation. For optical frequency multiplication, high transfer-function nonlinearity is preferred. In contrast, high transfer-function linearity is preferred for optical subcarrier modulation. It is well known that a cosine transfer function can present high or low nonlinearity dependent on operation voltage. Specifically, bias voltages determine the degree of nonlinearity or linearity of the MZM transfer function. For optical frequency multiplication such as millimeter-wave generation, the MZM should be biased at some specific bias points, such as minimum transmission, maximum transmission, and quadrature bias points, to enhance nonlinearity [1]?[2]. For optical subcarrier modulation, biasing an MZM in its linear region such as quadrature bias points allows transmitting broadband RF signals with multioctave bandwidth and improves spurious free dynamic range (SFDR). Therefore, care must be taken to maintain and control the MZM bias point for a specific application.

Impact of Laser Relative Intensity Noise on a Multiband OFDM Ultrawideband Wireless Signal Over Fiber System

We experimentally investigate and theoretically analyze the performance of a multiband orthogonal frequency division multiplexing (MB-OFDM) ultrawideband (UWB) wireless signal over fiber system considering the impact of laser relative intensity noise (RIN). Two types of RIN are considered: laser intrinsic RIN and laser phase-noise-converted RIN due to fiber dispersion. To reduce the impact of intrinsic RIN, laser output power of 2 dBm and beyond should be used. With the increase of fiber length and/or laser linewidth, UWB wireless signal over fiber is more degraded due to the converted RIN. For a given 0.6 dB degradation of error vector magnitude (EVM), the allowed laser linewidth is 30 MHz for 20 km of standard single-mode fiber for the first three bands. However, the converted RIN has much greater impact on higher-frequency UWB channels, depending on fiber length. A laser with a linewidth of less than 1 MHz will ensure that the EVM penalty due to the converted RIN is less than 1 dB for all 14 bands of MB-OFDM UWB.

Gap Waveguide-Based PMC Packaging for via Holes-Caused Nonsmooth PEC Surface

We investigate the application of perfect magnetic conductor (PMC) packaging for a nonsmooth perfect electric conductor surface, caused by metalized via holes. The PMC shielding is artificially built by a metal lid of pins, based on the gap waveguide (GW) technology. As a foundation, the effects of via hole in a unit cell are studied first considering its diameter, substrate thickness, pin size, pin height, as well as relative position between the via hole and the pin. The via adjacent space and pin rows are also considered for a general grounded coplanar waveguide along with ground vias. The effectiveness of package for such a transmission line containing metalized via holes is then verified experimentally. It is shown that the measured performance agrees well with the simulation. This paper presents a guideline for the GW-based PMC packaging technology.

Generalized Two-Box Cascaded Nonlinear Behavioral Model for Radio Frequency Power Amplifiers With Strong Memory Effects

Accurately modeling the memory effects is critical in modeling and nonlinearity pre-compensation for wideband radio frequency (RF) power amplifiers (PAs) with strong memory effects. In this paper, the memory effects are divided into two categories: lagging memory effect and leading memory effect, which are defined based on the input samples located after and before the current output samples, separately. Considering the above two types of memory effects, a generalized two-box cascaded (GTBC) nonlinear behavioral model for RF PAs with strong memory effects is proposed: one box for a static nonlinearity block that is used to include the strong static nonlinearities and the other box for a dynamic nonlinearity block which is used to predict the mild dynamic nonlinearities. The modeling accuracy of the proposed model, in terms of the normalized mean square error (NMSE) and the memory effect intensity (MEI), is compared to the generalized memory polynomial (GMP), the augmented Hammerstein (AH) and the enhanced Hammerstein (EH) models for a 450-470 MHz Doherty PA with a three-carrier WCMDA input signal and a 895-935 MHz inverse Class-F Doherty PA with a single-carrier FDD-LTE OFDM input signal. The validation demonstrates that the proposed model leads to high-accuracy with fewer modeling coefficients and floating point operations (FLOPs).

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.