Analog Predistortion Research Articles

Simple pre-distortion schemes for improving the power efficiency of SOA-based IR-UWB over fiber systems

In this paper, we investigate the usage of SOA for reach extension of an impulse radio over fiber system. Operating in the saturated regime translates into strong nonlinearities and spectral distortions, which drops the power efficiency of the propagated pulses. After studying the SOA response versus operating conditions, we have enhanced the system performance by applying simple analog pre-distortion schemes for various derivatives of the Gaussian pulse and their combination. A novel pulse shape has also been designed by linearly combining three basic Gaussian pulses, offering a very good spectral efficiency (>55%) for a high power (0dBm) at the amplifier input. Furthermore, the potential of our technique has been examined considering a 1.5Gbps-OOK and 0.75Gbps-PPM modulation schemes. Pre-distortion proved an advantage for a large extension of optical link (150km), with an inline amplification via SOA at 40km.

Clipping Distortion Suppression of Directly Modulated Multi-IF-over-Fiber Mobile Fronthaul Links Using Shunt Diode Predistorter

Herein, we demonstrate clipping distortion suppression of directly modulated multi-IF-over-fiber links using a simple shunt diode predistorter. The dynamic range of a directly modulated analog fiber optic link is limited by nonlinear distortions caused by laser-diode clipping. We investigate the link performance in the context of carrier-to-noise and distortion ratio (CNDR) and error vector magnitude (EVM) requirements when supporting LTE-A services. We also design an analog predistorter with a shunt-diode structure, and demonstrate experimentally that the predistorter has the ability to suppress clipping-induced third-order intermodulation distortions of the link by at most 14 dB. It also improves the CNDR and EVM of the 4-IF-multiplexed LTE-A carriers by 7 dB and 2.9%, respectively.

New Broadband Predistortion Linearizer Using Balanced Circuits Structure for Satellite TWTAs

The power amplifies is a key component influencing the power efficiency of communication system. The travelling wave tube is widely used in satellite system because of its high efficiency, apposite weight and broadband characteristic, but its strong nonlinear characteristic generates intermodulation distortion, which degrades the data quality of transmitted signal and causes interference in adjacent channels. The analogue predistortion structure using Schottky diodes could improve the linearity well, but parasite element in the Linearizer limits predistortion broadband, reluctant to satisfy the growing high volume data transmission requirement. This paper presents a new balanced structure of Linearizer for the travelling wave tube amplifier. We have analyzed the model of linearizer and the TWTA and used to calculate the intermodulation distortion and optimize the Linearizer nonlinear expansion characteristic. A frequency bandwidth of 1.2 GHz Linearizer has been demonstrated with a K-band TWTA between 19.8 and 21.0 GHz. The measurement result shows that the linearizer delivers about 2~6dB of gain expansion and 20~60 degrees phase shift expansion, and the cascaded LTWTA shows good linearity improvement of 17.8dB at 6dB input power back off point.

A dual-branch vector synthesis millimeter-wave predistortion linearizer

In view of the strong correlation between amplitude and phase characteristics of Traveling Wave Tube Amplifier (TWTA) analog predistortion technology, a dual-branch vector synthesis predistortion linearizer is proposed, which consists of branch line hybrid couplers, an adjustable attenuator and a reflective diode-based predistortion circuit. This linearizer is fabricated and measured according to the theoretical analysis and simulation results. The measurement results show that the linearizer can compensate the nonlinearity of the TWTAs. Once the two branches bias voltages are rationalized, the phase expansion can change 30 degrees, while the change of the amplitude expansion is less than 1 dB, which effectively reduces the correlation between the amplitude and the phase characteristics of the analog predistortion linearizer.

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.

Analog predistortion linearizer on reducing corrected amplitude overshoot

This paper presents a dual-nonlinear branch linearizer for reducing the corrected amplitude overshoot of conventional single nonlinear branch linearizer. Theoretical analysis is carried out, the analysis is verified by simulation, and a prototype of Ka band 25.28∼26.08 GHz dual nonlinear branch linearizer is achieved. It indicates that the corrected amplitude overshoot is less than 0.5 dB, the C/I3 improvement is more than 10 dB related to a single carrier IBO 9 dB, when it is linked and tested for 50 W spacebrone Travelling Wave Tube Amplifier (TWTA).

Linearization Technologies for Broadband Radio-Over-Fiber Transmission Systems

Linearization technologies that can be used for linearizing RoF transmission are reviewed. Three main linearization methods, i.e. electrical analog linearization, optical linearization, and electrical digital linearization are presented and compared. Analog linearization can be achieved using analog predistortion circuits, and can be used for suppression of odd order nonlinear distortion components, such as third and fifth order. Optical linearization includes mixed-polarization, dual-wavelength, optical channelization and the others, implemented in optical domain, to suppress both even and odd order nonlinear distortion components, such as second and third order. Digital predistortion has been a widely used linearization method for RF power amplifiers. However, digital linearization that requires analog to digital converter is severely limited to hundreds of MHz bandwidth. Instead, analog and optical linearization provide broadband linearization with up to tens of GHz. Therefore, for broadband radio over fiber transmission that can be used for future broadband cloud radio access networks, analog and optical linearization are more appropriate than digital linearization. Generally speaking, both analog and optical linearization are able to improve spur-free dynamic range greater than 10 dB over tens of GHz. In order for current digital linearization to be used for broadband radio over fiber transmission, the reduced linearization complexity and increased linearization bandwidth are required. Moreover, some digital linearization methods in which the complexity can be reduced, such as Hammerstein type, may be more promising and require further investigation.

Wideband LTE power amplifier with integrated novel analog pre-distorter linearizer for mobile wireless communications.

For the first time, a new circuit to extend the linear operation bandwidth of a LTE (Long Term Evolution) power amplifier, while delivering a high efficiency is implemented in less than 1 mm2 chip area. The 950 µm × 900 µm monolithic microwave integrated circuit (MMIC) power amplifier (PA) is fabricated in a 2 µm InGaP/GaAs process. An on-chip analog pre-distorter (APD) is designed to improve the linearity of the PA, up to 20 MHz channel bandwidth. Intended for 1.95 GHz Band 1 LTE application, the PA satisfies adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM) specifications for a wide LTE channel bandwidth of 20 MHz at a linear output power of 28 dBm with corresponding power added efficiency (PAE) of 52.3%. With a respective input and output return loss of 30 dB and 14 dB, the PA’s power gain is measured to be 32.5 dB while exhibiting an unconditional stability characteristic from DC up to 5 GHz. The proposed APD technique serves to be a good solution to improve linearity of a PA without sacrificing other critical performance metrics.

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.

Process, Voltage and Temperature Compensation Technique for Cascode Modulated PAs

This paper presents a process, voltage and temperature (PVT) compensation method for a cascode modulated polar power amplifier (PA). It is shown that it is possible to create a baseband replica circuit of the PA that has the same AM-AM nonlinearity as the PA itself. The replica circuit, that represents a transistor level model (empirical model) of the cascode modulated PA, is utilized in a PA analog predistorter. The analog predistorter linearizes and compensates for PVT variation of the cascode modulated PA. The empirical model is placed in the negative feedback of an operational transconductance amplifier. The predistorted varying envelope signal is applied to the cascode gate of the PA. It is shown that the proposed PVT compensation technique significantly reduces the PVT spread of the PA linearity indicators and improves the PA linearity. Simulations were performed in a 0.13 μm CMOS process.

An Adjustable Linearization for RF Power Amplifier

This paper presented an adjustable analog predistortion linearization method for RF power amplifier. The linearization includes an attenuator, a phase shifter and an anti-parallel Schottky diode pair with DC bias, through adjusting these components, we can adjust predistortion characteristic of the linearization to be complementary to distortion characteristic of RF power amplifier. By cascading this linearization with power amplifier, we can improve linearity of amplifier. Firstly, we simulated the proposed predistortion linearization with software ADS, then the linearization was manufactured and tested together with RF power amplifier PTF080551. Experimental results show that we can adjust predistortion characteristic of the linearization to be closely complementary to distrotion characteristic of power amplifier PTF080551 as desired. After cascading the linearization with power amplifier PTF080551, more than 10dB improvement in IMD3 of the amplifier can be achieved. Furthermore, the linerarizaion is flexible and can be adjusted to linearize different amplifiers.

Ultrabroadband Linear Power Amplifier Using a Frequency-Selective Analog Predistorter

This brief presents an ultrabroadband (2- to 600-MHz band) linear power amplifier using a compact frequency-selective analog predistorter, which includes a capacitor having an optimized capacitance and a biased Schottky diode connected in series. It is shunted at the input of the main amplifier and has a frequency-selective third-order intermodulation generation capability using its optimized capacitance. A two-stage push-pull power amplifier, which has an ultrabroadband operation range from 2 to 600 MHz, was implemented and linearized using the proposed analog predistorter. It exhibited high 1-dB compression point (P1 dB) and power-added efficiency characteristics of over 43 dBm and 32% at each P1 dB for the entire operating band, respectively. At an average output power level of 36 dBm for the two-tone signal input, which has a tone spacing of 1 MHz, its third-order intermodulation distortion over the entire band is no higher than -39.1 dBc after linearization, as compared with -34.2 dBc before linearization.

A highly linear fully integrated CMOS power amplifier with an analog predistortion technique

A transformer-based CMOS power amplifier (PA) is linearized using an analog predistortion technique for a 2.5-GHz m-WiMAX transmitter. The third harmonic of the power stage and driver stage can be cancelled out in a specific power region. The two-stage PA fabricated in a standard 0.18-μm CMOS process delivers 27.5 dBm with 27% PAE at the 1-dB compression point (P1dB) and offers 21 dB gain. The PA achieves 5.5 % EVM and meets the spectrum mask at 20.5 dBm average power. Another conventional PA with a zero-cross-point of gm3 bias is also fabricated and compared to prove its good linearity and efficiency.

Linearity enhanced Doherty power amplifier using analog predistortion

We propose a linearity-enhanced Doherty power amplifier using an analog predistortion (APD) technique, which incorporates an intermodulation (IM) generator.Because the IM generator is configured with antiparalleled diode structure, the resulting Doherty power amplifier behaves differently from a conventional one. Using antiphased IM generated by the hybrid coupler before the auxiliary amplifier, linearity for the whole power amplifier is improved because the IM from the main amplifier can be cancelled out at the combining section. The output power of the proposed Doherty power amplifier is observed 28 dBm at the maximum in measurement, whereas the adjacent channel leakage ratio is 18 dB improved at 5 MHz offset compared with that of the Doherty power amplifier without APD. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:403–404, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25701

Analog predistortion linearization of Doherty power amplifiers using bandwidth reduction of error signal

AbstractThis article represents analog predistortion linearization of Doherty power amplifier (DPA) using bandwidth reduction of error signal. To verify our methods, two DPAs are implemented using a push‐pull GaN HEMT and a push‐pull Si LDMOS at 2.14 GHz and tested using the memory‐compensated analog predistorter with transistor‐based error generators and three‐branch nonlinear paths. From the measured four‐carrier WCDMA results, the proposed analog predistortion DPAs show the significant linearity improvement. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1313–1316, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/mop.25190

A High-Linearity Wideband Power Amplifier With Cascaded Third-Order Analog Predistorters

A high-linearity wideband analog predistortion power amplifier (PA) is presented. The memory effects of the PA are reduced by a high drain bias voltage as well as a wide bias line and several decoupling capacitors. The simple cascaded third-order analog predistorters (CTAPDs) are used to cancel the third- and fifth-order intermodulations (IM3s and IM5s) of the low-memory PA. For verification, the 45 W class-AB PA is implemented and tested using a two-tone signal and a four-carrier wideband code division multiple access (WCDMA) signals at 2.14 GHz. From the measured results, the low-memory PA with the CTAPDs delivers significant IM3 and IM5 cancellation and superior ACLR improvement.

Analog Pre-Distortion Linearizer Using Self Base Bias Controlled Amplifier

An analog pre-distortion linearizer employing a radio frequency (RF) transistor with a self base bias control circuit is proposed. The self base bias control circuit extracts the envelope from the modulated input RF signal of the RF transistor and automatically controls its base current according to the extracted envelope. As a result, the proposed linearizer realizes positive gain deviation at high input power level. By adding a resistor between the RF transistor and the self base bias control circuit, the negative gain deviation can be derived. The design of the proposed lineaizer is described with taking the envelope frequency response of the self base bias control circuit into consideration. The fabricated linearizer achieves the adjacent channel power leakage ratio (ACLR) improvement of 8.1dB for a 2GHz-band, 10W-class GaAs FET high-power amplifier (HPA) with negative gain deviation for W-CDMA base stations. It also achieves the ACLR improvement of 8.3dB for a LDMOS HPA with positive gain deviation for the same application.

Highly linear analog predistortion power amplifier with low memory effects at high drain bias voltage

AbstractIn this article, we propose a highly linear analog predistortion power amplifier (PA). To successfully use the analog predistorter, memory effects of the PA are significantly reduced using drain bias circuit with high drain bias voltage as well as widened λ/4 bias line and several decoupling capacitors. For a two‐tone signal and a two‐carrier wideband code division multiple access signal, the analog predistortion PA shows the significant IM3 cancellation and ACLR improvement by delivering low memory effects. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 295–299, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24909

A Transistor-Based Analog Predistorter With Unequal Delays for Memory Compensation

A transistor-based analog predistorter (APD) with memory effect compensation is represented. The nonlinearity of a gain amplifier generated by a low supply voltage is used as an error generator. The predistorted signal by the unequal-delayed nonlinear paths improves the linearity by compensating for memory effects of the power amplifier (PA). For verification, the proposed APD with three-branch nonlinear paths is implemented with a 30-W class-AB PA and tested using two-tone and various wideband code division multiple access (WCDMA) signals at 2.14 GHz. From the measured results, the proposed APD significantly improves the linearity of the PA with memory effects.

Realization of an analog predistortion circuit for RF optical fiber links

This paper presents an analog predistortion circuit for RF optical fiber links. The circuit consists of two source-coupled differential transconductance amplifiers which could provide linear and nonlinear transfer characteristics by adjusting the bias voltage and the transistor sizes. The circuit was designed and realized in a standard 0.18-μm CMOS technology of SMIC. The chip occupies 0.48 × 0.24 mm2. The DC supply is 3.3 V. Using this circuit, the third-order intermodulation distortion (IMD) suppression of a directly modulated RF optical fiber link can be improved by 9–16 dBc at relatively low cost.