This paper proposes a new technique to identify the parameters of a digital predistorter based on iterative learning control (ILC). ILC is a well-established control theory technique that can obtain the inverse of a system. Instead of focusing on identifying the predistorter parameters, the technique proposed here first uses an iterative learning algorithm to identify the optimal power amplifier (PA) input signal that drives the PA to the desired linear output response. Once the optimal PA input signal is identified, the parameters of the predistorter are estimated using standard modeling approaches, e.g., least squares. To this end, in this paper, we present a complete derivation of an ILC scheme suitable for the linearization of PAs, which includes convergence conditions and the derivation of two learning algorithms. The proposed ILC scheme and parameter identification technique were demonstrated experimentally and compared with the indirect learning architecture (ILA) and direct learning architecture (DLA). The experimental results show that, even for the most difficult cases, the proposed ILC scheme can successfully linearize the PA. The experimental results also indicate that the proposed parameter identification technique is more robust to measurement noise than ILA and can provide better linearity performance when the PA nonlinearities are strong. In addition, the proposed parameter identification technique can achieve similar or better linearity performance than DLA but with a simpler identification process.

Iterative Learning Control for RF Power Amplifier Linearization

Heterogeneous patterns of brain atrophy in Alzheimer's disease.

In-phase/quadrature (I/Q) imbalance is one of the main sources of distortion in RF modulators. In this paper, a dual-input nonlinear model based on a real-valued Volterra series is proposed for compensation of the nonlinear frequency-dependent I/Q imbalance. First, different sources of distortion are identified from experimental measurements, then a dual-input nonlinear I/Q imbalance model is developed. Further, the inverse model is used for I/Q imbalance compensation. Finally, the performance of the I/Q imbalance compensator is evaluated with both simulations and experiments. In comparison with previously published results, the proposed I/Q imbalance compensator shows significantly improved performance. Thus, we prove that a complete nonlinear I/Q imbalance compensation can minimize the effects of the RF modulator in high-performance digital communication systems.

I/Q Imbalance Compensation Using a Nonlinear Modeling Approach

While the field of MIMO transmission has been explored over the past decade mainly theoretically, relatively few results exist on how these transmissions perform over realistic, imperfect channels The reason for this is that measurement equipment is expensive, difficult to obtain, and often inflexible when a multitude of transmission parameters are of interest This paper presents a flexible testbed developed to examine MIMO algorithms and channel models described in literature by transmitting data at 245 GHz through real, physical channels, supporting simultaneously four transmit and four receive antennas Operation is performed directly from Matlab allowing for a cornucopia of real-world experiments with minimum effort Examples measuring bit error rates on space-time block codes are provided in the paper

Vienna MIMO testbed

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Fernando Hugo Gregorio Name of the dissertation Analysis and compensation of nonlinear power amplifiers effects in multi-antenna OFDM systems Manuscript submitted 08.06.2007 Manuscript revised 27.09.2007 Date of the defence 19.11.2007 Monograph Article dissertation (summary + original articles) Department Electrical and Communications Engineering Laboratory Signal Processing Laboratory Field of research Digital Signal processing in wireless communication systems Opponent(s) Prof. Markus Rupp (Vienna University of Technology) and Docent Mikko Valkama (Tampere University of Technology) Supervisor Risto Wichman Instructor Stefan Werner Abstract The high peak to average power ratio (PAPR) levels associated with multicarrier systems require the use of linear power amplifiers (PA) with large dynamic range. Unfortunately, linearity and power efficiency are two conflicting specifications for practical PAs. However, systems with high power efficiency are required, for example, in case of battery operated mobile terminals.Thus, the PA needs to operate in a high power efficiency region which compromises the linearity constraints. The evaluation of the effect of nonlinear amplification on the system performance is an important issue that must be considered in a realistic system design. This thesis first addresses the performance analysis of multiuser OFDM systems (space division multiple access (SDMA)OFDM) employing nonlinear PAs. Bit error probability (BEP) and capacity expressions are derived for an OFDM system with diversity, assuming a memoryless PA. Moreover, BEP expressions are derived for an orthogonal space-time coded (OSTBC)-OFDM system that include the effect of a broadband nonlinear PA and imperfect memory compensation. Thereafter, techniques to reduce the effects of power amplifier distortion are proposed. We consider techniques that can be implemented either in the receiver, the transmitter, or as a combination of both, all with the same goal of relieving the linearity constaints and improving the power efficiency. A low PAPR OFDM implementation is addressed where subcarriers are allocated to different users in such a way that the generated intermodulation distortion is kept at minimum. Then, we propose the combination of multiuser detection and nonlinear distortion mitigation techniques. Initially, knowledge of the PA model (modelled as memoryless) is required for the implementation. The extension to broadband PAs and to PA model estimation in the receiver are also made. Efficient channel estimation strategies are developed that work in the presence of nonlinear distortion. Finally, a split predistorter structure to remove nonlinear distortion caused by a broadband PA is proposed. The idea is to reduce the transmitter complexity by distributing the predistorter tasks such that the transmitter compensates for the static nonlinearity, and the receiver equalizes the PA memory.

/pdf/analysis-and-compensation-of-nonlinear-power-amplifier-2f1fm9h0s5.pdf

Analysis and compensation of nonlinear power amplifier effects in multi-antenna OFDM systems

In this paper a comparison between a memoryless nonlinear power amplifier model and a special nonlinear model with memory (Wiener-model) based on measurements on a power amplifier is presented. With moderate signal bandwidths of only 2 MHz the nonlinear model with memory outperforms the memoryless model. The amplifier model is intended to be used for pre-distortion purposes. Furthermore, an adaptive algorithm for finding the model parameters is presented.

/pdf/modelling-and-identification-of-a-nonlinear-power-amplifier-384tsij2o8.pdf

Modelling and identification of a nonlinear power-amplifier with memory for nonlinear digital adaptive pre-distortion

This paper deals with iterative linearization of a nonlinear dynamical system. The methods proposed are suited for digital pre-distortion of power amplifiers working close to saturation, as it is common for efficiency reasons. Two methods (a well known successive approximation method based on a fixed-point formulation and a Newton method) are compared with respect to convergence speed, robustness against model uncertainties, and complexity. The presented methods are used to linearize a Volterra model of a measured power amplifier.

/pdf/iterative-linearization-methods-suited-for-digital-pre-32jw2d4ue4.pdf

Iterative linearization methods suited for digital pre-distortion of power amplifiers

The paper reports the development of a flexible and scalable testbed for the implementation and evaluation of signal processing algorithms for multiple antenna (MIMO) systems. At a maximum, four transmit and four receive antennas can be served. Two operational modes are targeted, non real-time block transmission and real-time operation.

Design of a flexible and scalable 4/spl times/4 MIMO testbed

Recently, many papers have been published on the designs of predistortion systems for power amplifiers (PA). Most of these studies are based on computer simulations. In this work, we examine our predistorter (PD) designs on a practical microwave PA in a laboratory setup. The objective is to validate two PDs of different design approaches. The first PD is modeled using a simplicial canonical piecewise linear (SCPWL) function while the second one is modeled using a polynomial function. The former exploits the properties of the SCPWL function to develop a reduced complexity PD identification method. The latter utilizes the secant method to iteratively search for the signal at the PD output, thus parameter identification is avoided. The implementation complexity and effectiveness of the PDs are compared. The limitations of the measurement system for predistorter design is also discussed briefly

/pdf/comparison-and-experimental-verification-of-two-low-uegbukme13.pdf

Comparison and Experimental Verification of Two Low-complexity Digital Predistortion Methods

Predistortion (PD) lineariser for microwave power amplifiers (PAs) is an important topic of research. With larger and larger bandwidth as it appears today in modern WiMax standards as well as in multichannel base stations for 3GPP standards, the relatively simple nonlinear effect of a PA becomes a complex memory-including function, severely distorting the output signal. In this contribution, two digital PD algorithms are investigated for the linearisation of microwave PAs in mobile communications. The first one is an efficient and low-complexity algorithm based on a memoryless model, called the simplicial canonical piecewise linear (SCPWL) function that describes the static nonlinear characteristic of the PA. The second algorithm is more general, approximating the pre-inverse filter of a nonlinear PA iteratively using a Volterra model. The first simpler algorithm is suitable for compensation of amplitude compression and amplitude-to-phase conversion, for example, in mobile units with relatively small bandwidths. The second algorithm can be used to linearise PAs operating with larger bandwidths, thus exhibiting memory effects, for example, in multichannel base stations. A measurement testbed which includes a transmitter-receiver chain with a microwave PA is built for testing and prototyping of the proposed PD algorithms. In the testing phase, the PD algorithms are implemented using MATLAB (floating-point representation) and tested in record-and-playback mode. The iterative PD algorithm is then implemented on a Field Programmable Gate Array (FPGA) using fixed-point representation. The FPGA implementation allows the pre-inverse filter to be tested in a real-time mode. Measurement results show excellent linearisation capabilities of both the proposed algorithms in terms of adjacent channel power suppression. It is also shown that the fixed-point FPGA implementation of the iterative algorithm performs as well as the floating-point implementation.

/pdf/prototype-implementation-of-two-efficient-low-complexity-9m9bv69ute.pdf

E. Aschbacher

Papers

Modelling and identification of a nonlinear power-amplifier with memory for nonlinear digital adaptive pre-distortion

Iterative linearization methods suited for digital pre-distortion of power amplifiers

Design of a flexible and scalable 4/spl times/4 MIMO testbed

Comparison and Experimental Verification of Two Low-complexity Digital Predistortion Methods

Prototype implementation of two efficient low-complexity digital predistortion algorithms