This report presents a study of digital predistortion of analog amplifiers and the simulation of a digital predistorter. Predistortion can be of significant benefit in wireless transmitters. As power consumption is becoming a greater concern, digital predistortion helps maximize power efficiency of power amplifiers by allowing them to operate in their most efficient regions while not corrupting data. Many wireless standards use channels with frequency carriers in close proximity to other channels. This leads to the necessity to ensure that spectral regrowth, which adds interference to neighboring frequencies, does not occur. Digital predistortion is a promising method to achieve this. Some major aspects of a predistortion system are discussed. These include the analog or digital predistorter implementation, simulating designs with memory or memoryless power amplifiers, baseband or high frequency predistortion, and training a predistorter. / text

This thesis presents a transmitter with digital baseband predistorter to improve linearity of power amplifier. The architecture adopts the look-up table predistorter base on a polar scheme, and realizes the digital processor using FPGA. The predistortion for AM/AM and AM/PM nonlinear relations in a 0.5£gm GaAs pHEMT class-AB power amplifier has been performed. One-tone, two-tone and multi-tone continuous waves generated by baseband signal are used for linearity test. The results demonstrate that the digital predistortion technique can effectively improve intermodulation distortion. With this technique, the power amplifier can operate in the higher power range to achieve a similar linearity and a higher efficiency.

This thesis utilizes the digital predistortion technique to improve the linearity of high power amplifiers for WiMAX basestation applications. The power amplifier with the proposed technique can achieve higher output power and efficiency than with the conventional output power backoff technique. Owing to a very high PAPR value of the OFDM modulation signals applied to WiMAX basestation, the linearity requirement for the basestation power amplifiers is very strict. This thesis adopts a look-up table predistorter based on a polar scheme and realizes the digital predistorter circuitry using FPGA. As a result, the implemented digital predistorter successfully enhances the linearity of a 15W Lateral MOSFET Class-AB power amplifier operating in 2.6 GHz WiMAX band. Under the conditions that satisfy the spectrum mask and EVM requirement, the power amplifier with the digital predistorter can operate at higher output power and conversion efficiency than without the digital predistorter.

Radio frequency power amplifiers (PAs) are the most challenging part of the design of radio systems since they dictate the overall system's performance in terms of power efficiency and distortion generation. The performance is further challenged by modern modulation schemes which are characterized by highly varying signal envelopes. In order to meet the spectrum mask requirements, PAs are usually operated at high power back-off to ensure linearity, at the cost of efficiency. To tackle this issue, many efficiency enhancement techniques have been presented in the literature. In fact, these techniques do increase the PA power efficiency at back-off, however, efficiency enhancement techniques do not ensure the linearity of the PA. Furthermore, these techniques may lead to additional distortion. On the other hand, several linearization techniques have been developed to mitigate the PA nonlinearity problem and allow the PA to operate at less back-off. Digital Pre-Distortion (DPD) technique is gaining more attention, as compared to other linearization techniques, thanks to its simple concept and advancements in digital signal processors (DSP) and signal converters. DPD technique consists of introducing a nonlinear function before the PA so that the overall cascaded system behaves linearly. It was clear from the literature that this technique showed good performance. Yet, it has primarily been validated using commercial test equipment, which has good capabilities, and far from the real world environment in which this technique would be implemented. Indeed, DPDs would need to be implemented in signal processors characterised by limited resources and computational accuracy. This thesis presents an implementation of several DPD models, namely look-up table (LUT), memoryless polynomial and memory polynomial (MP), on a field programmable gate array (FPGA). A novel model reformulation made this implementation possible in fixed-point arithmetic. Measurements were collected to validate the DPD models' implementation and an improvement of the signal quality was recorded in terms of error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR). As many wireless access technologies must continue to coexist, multi-standard radio systems are required to reduce the cost while maintaining the interoperability. This thesis presents a development platform for multimode radio which comprises mixed-signal modules. The platform provides the capacity for hardware and software development. In fact, the FPGA under investigation allowed for the implementation of a baseband transceiver and DPD schemes. In addition, a software tool was developed as a dashboard to control and monitor the system. The radio system in the platform was optimized through the equalization of the feedback receiver frequency response performed through a simultaneous measurement of the amplitude ripple of the transmitter and receiver. Furthermore, a phase-coherent frequency synthesizer was designed to bring more flexibility by allowing the transmitter's carrier frequency to be different from the receiver's frequency.

Radio frequency power amplifiers (PAs) are the most challenging part of the design of radio systems since they dictate the overall system's performance in terms of power efficiency and distortion generation. The performance is further challenged by modern modulation schemes which are characterized by highly varying signal envelopes. In order to meet the spectrum mask requirements, PAs are usually operated at high power back-off to ensure linearity, at the cost of efficiency. To tackle this issue, many efficiency enhancement techniques have been presented in the literature. In fact, these techniques do increase the PA power efficiency at back-off, however, efficiency enhancement techniques do not ensure the linearity of the PA. Furthermore, these techniques may lead to additional distortion. On the other hand, several linearization techniques have been developed to mitigate the PA nonlinearity problem and allow the PA to operate at less back-off. Digital Pre-Distortion (DPD) technique is gaining more attention, as compared to other linearization techniques, thanks to its simple concept and advancements in digital signal processors (DSP) and signal converters. DPD technique consists of introducing a nonlinear function before the PA so that the overall cascaded system behaves linearly. It was clear from the literature that this technique showed good performance. Yet, it has primarily been validated using commercial test equipment, which has good capabilities, and far from the real world environment in which this technique would be implemented. Indeed, DPDs would need to be implemented in signal processors characterised by limited resources and computational accuracy. This thesis presents an implementation of several DPD models, namely look-up table (LUT), memoryless polynomial and memory polynomial (MP), on a field programmable gate array (FPGA). A novel model reformulation made this implementation possible in fixed-point arithmetic. Measurements were collected to validate the DPD models' implementation and an improvement of the signal quality was recorded in terms of error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR). As many wireless access technologies must continue to coexist, multi-standard radio systems are required to reduce the cost while maintaining the interoperability. This thesis presents a development platform for multimode radio which comprises mixed-signal modules. The platform provides the capacity for hardware and software development. In fact, the FPGA under investigation allowed for the implementation of a baseband transceiver and DPD schemes. In addition, a software tool was developed as a dashboard to control and monitor the system. The radio system in the platform was optimized through the equalization of the feedback receiver frequency response performed through a simultaneous measurement of the amplitude ripple of the transmitter and receiver. Furthermore, a phase-coherent frequency synthesizer was designed to bring more flexibility by allowing the transmitter's carrier frequency to be different from the receiver's frequency.

This thesis aims to use a hybrid quadrature polar modulator (HQPM) for enhancing average efficiency of 3G mobile transmitter with power control. The HQPM consists of a quadrature modulator instead of a phase modulator in the polar modulator for processing the RF modulated carrier and a Class-S modulator for processing the envelope signal. In addition, the instantaneous magnitude of the quadrature modulated signal is propotional to the instantaneous envelope magnitude. As a result, the output feed-through and gain-compression phenomenon in the polar modulator can be improved. The digital baseband processor realized by FPGA can generate CDMA2000 1x baseband signal with excellent modulation accuracy. For enhancing the average transmit efficiency, the output PA is realized as Class-E design. But the Vdd/AM and Vdd/PM nonlinear effects of the Class-E PA distort the output signal. To solve this problem, a digital predistorter is presented to compensate the nonlinear distortions. The proposed HQPM-based transmitter can simultaneously achieve high efficiency and high linearity over a wide modulated output power range.

This dissertation presents a hybrid quadrature polar modulator (HQPM) to drive the power amplifier (PA) highly efficiently in a wireless RF transmitter with good potential for multi-mode operation. For enhancing the efficiency, a Class-E PA is used in the transmitter. The HQPM consists of a quadrature modulator for processing the RF modulated carrier and a Class-S modulator for processing the supply-voltage signal. The quadrature modulator and the Class-S modulator deliver the output signals with envelope variation before being inserted into the RF-input terminal and the supply-voltage terminal of Class-E PA, respectively, causing the double envelope modulation to distort the modulated RF signal at the PA output. Therefore, a digital predistorter is proposed to be embedded in the HQPM for compensation. The use of such predistorted HQPM techniques can help reducing the average DC and RF input powers and the output feed-through levels so as to enhance power added efficiency and adjacent channel power rejection quite remarkably.

This master’s thesis deals with integrating simulations using Agilents Electronic Design Automation tool ADS with customized Matlab scripts, for solving complex analog and digital radio architectures. In particular, it addresses predistortion, realized in the digital domain, of power amplifiers, modeled in the analog domain. The former is implemented in Matlab while the latter is implemented in ADS. Two versions of integrating the two systems have been tested: 1) Iterative approach on sample basis and 2) Scheduled batch solution by matrix inversion. The concept has been tested on two different PA configurations: 1) a standard class-AB PA and 2) a Doherty PA architecture. Evaluation has also been done on ADS ability to correctly simulate memory effects in PAs and on the actual DPD-algorithms ability to compensate for these memory effects.  An integrated simulation environment for ADS and Matlab was successfully established within the work of this thesis. Matlab scripts, used in predistortion algorithms in the digital domain, could interact directly with component-based PA models, in an enclosed simulation system.  The simulation results show that sample basis method is the most accurate, fast and reliable method to linearize a PA. The PA1 proved to be easier than the DPA to linearize, except for when being close to saturation where better IMD-suppression was achieved with the DPA.  ADS is clearly able to simulate memory effects in the analog domain. At low gain-levels the applied compensating memory-algorithms showed a great improvement to the linearization of the output signal of the PA. At higher gain-levels though, the compensation for memory effects lost their efficiency because the non-linearity of the PA became too significant.

The purpose of this thesis is to linearize a power amplifier using digital predistortion. A power amplifier is a nonlinear system, meaning that when fed with a pure input signal the output will be distorted. The idea behind digital predistortion is to distort the signal before feeding it to the power amplifier. The combined distortions from the predistorter and the power amplifier will then ideally cancel each other. In this thesis, two different approaches are investigated and implemented on an FPGA. The first approach uses a nonlinear model that tries to cancel out the nonlinearities of the power amplifier. The second approach is model-free and instead makes use of a look-up table that maps the input to a distorted output. Both approaches are made adaptive so that the parameters are continuously updated using adaptive algorithms. First the two approaches are simulated and tested thoroughly with different parameters and with a power amplifier model extracted from the real amplifier. The results are shown satisfactory in the simulations, giving good linearization for both the model and the model-free technique. The two techniques are then implemented on an FPGA and tested on the power amplifier. Even though the results are not as well as in the simulations, the system gets more linear for both the approaches. The results vary widely due to different circumstances such as input frequency and power. Typically, the distortions can be attenuated with around 10 dB. When comparing the two techniques with each other, the model-free method shows slightly better results.

Digital predistortion is a signal processing technique used to remove undesired distortions caused by nonlinear system effects. This method is predominately used to linearize power amplifiers in communication systems in order to achieve efficient transmitter circuits. However, the technique can readily be applied to cancel undesired nonlinear behavior in other types of systems. This thesis investigates the effectiveness of digital predistortion in the context of a wideband arbitrary waveform generator. A theoretical foundation discussing nonlinear system models, predistortion architectures and system identification methods is complemented with a simulation study and followed by verification on a real system. The best predistorter is able to fully suppress the undesired distortions for any fixed two-tone sinusoidal signal. Furthermore, the results indicate the existence of a wideband predistorter which yield acceptable suppression over a frequency range of several hundred MHz. / Digital predistorsion är en signalbehandlingsteknik som används för att undertrycka oönskade distorsioner orsakade av icke-linjära effekter i elektriska system. Denna metod används i huvudsak för att linjärisera effektförstärkare i kommunikationssystem för att erhålla effektiva sändarkedjor men tekniken kan utan större problem även tillämpas på andra typer av icke-linjära system. Denna uppsats undersöker i vilken utsräckning digital predistorsion kan användas för att undertrycka oönskade signaldistorsioner i en bredbandig signalgenerator. Uppsatsen presenterar en bakgrund som utgår ifrån teorin om icke-linjära systemmodeller, arkiteturer för predistorsion och systemidentifieringsmetoder. En kvantitativ studie i en simuleringsmiljö åtföjs av en utvärdering på ett verkligt system. Det bästa predistorsionssystemet åstadkommer en fullständig linjärisering i testfallet med en fix tvåtonssignal. Resultaten indikerar även att det existerar ett system som linjäriserar signaler i ett frekvensområde som uppgår till hundratals MHz.