Jesus de Mingo

Efficient feedforward linearization technique using genetic algorithms for OFDM systems

Feedforward is a linearization method that simultaneously offers wide bandwidth and good intermodulation distortion suppression; so it is a good choice for Orthogonal Frequency Division Multiplexing (OFDM) systems. Feedforward structure consists of two loops, being necessary an accurate adjustment between them along the time, and when temperature, environmental, or operating changes are produced. Amplitude and phase imbalances of the circuit elements in both loops produce mismatched effects that lead to degrade its performance. A method is proposed to compensate these mismatches, introducing two complex coefficients calculated by means of a genetic algorithm. A full study is carried out to choose the optimal parameters of the genetic algorithm applied to wideband systems based on OFDM technologies, which are very sensitive to nonlinear distortions. The method functionality has been verified by means of simulation.

Design and Applications of a 300–800 MHz Tunable Matching Network

In this paper, the optimized design, characterization and applications of a broadband 300-800 MHz (~ 91% fractional bandwidth) digitally-controlled tunable matching network is presented. The design employs PIN diodes as switching components and a repetitive structure of basic cells using lumped reactive elements. After an intensive and complex optimization process, a Smith chart coverage (return losses better than 10 dB and losses lower than 2 dB) above 60% is obtained in all the bandwidth reaching 75% in the middle of the band (400-700 MHz). The potential of the manufactured tunable matching network for antenna mismatch compensation, antenna bandwidth extension and power amplifier efficiency improvement in back-off is showed.

Digital predistortion based on Zernike polynomial functions for RF nonlinear power amplifiers

Predistortion linearizer for high nonlinear power amplifiers (PAs) is an important topic of research in wireless applications. Behavioral models based on polynomial functions are widely used for both PAs and predistorters. However, they often suffer from numerical instability problems when we include high nonlinear-order in the mathematical model. In this work, we propose a new predistorter model considering the use of an orthogonal basis. This basis is composed of Zernike polynomials, which are very robust and stable when they are applied to classical identification techniques. Computer simulations in a typical DVB-T OFDM transmission confirm the predistorter computed with the Zernike polynomial model provides improvements in adjacent channel power ratio with stability and moderate complexity in the DPD coefficient estimation.

Application of an Impedance Tuning Network for Mobile DVB-H Terminals

This paper presents the application of a reconfig- urable impedance tuning network for a mobile DVB-H terminal. These kind of networks are becoming widely used in wireless communication systems to improve its efficiency. On the other hand, antenna design in DVB-H terminals is challenging due to the size limitation and as a result, the antenna presents large mismatches. To address this issue, we propose the use of a reconfigurable tuning network to reduce the mismatch (or the antenna size for a given mismatch) between the antenna and the RF front-end in a DVB-H terminal. This technique offers some advantages over other proposed alternatives and improves the antenna realized gain, as well as the efficiency of the DVB-H receiver. I. INTRODUCTION Reconfigurable tuning networks are called to play an impor- tant role in the development of new multistandard RF front- ends in wireless communication systems based on software defined radio. The benefits and applications of this kind of networks are promising. One of its main applications is to match the output impedance of the power amplifier stage to the antenna input impedance for allowing the maximum power transfer. This idea may also be exploited in reception, to get an optimum power transfer from the receiver antenna to the reception chain, typically leaded by a low noise amplifier (LNA) or a filter and a LNA. This application is specially interesting for mobile DVB-H terminals, where the antenna design is stimulating due to the frequency band (470�702 MHz) defined in the European DVB-H standard (1). In this bandwidth, DVB-H has a wavelength of 640 mm in the lower frequencies, while a handset is usually much shorter. As a result, the antenna presents large mismatches which lead to a decrease in the receiver efficiency. The idea of using impedance tuning networks to fix this problematic was already stated in (2). In this work, the design of the impedance tuning network for DVB-H applications is presented, although only improvement in terms of VSWR for an audio ear piece and an FM antenna are shown. Results are not presented for a real DVB-H antenna, and no improvement in terms of realized gain are reported. Nevertheless, its perfor- mance with a real DVB-H will probably be very satisfactory in terms of matching and realized gain improvement. The classical way to face up the problem here stated, is based on the design of tunable antennas (3)-(10), extensively

Analysis of the Antenna Stochastic Effective Gain in Mobile Environments

Multipath propagation effect usually degrades the wireless system performances. Regarding the antenna element, this degradation implies the variation of some important para- meters, such as the gain, input impedance and efficiency. Instead of being deterministic quantities, those parameters are rather random, according to the stochastic nature of multipath and the variability of the environment. Therefore, mobile antenna performances should be evaluated as random processes, which will depend on channel parameters. In this paper we focus on the gain of mobile antennas, dealing with its stochastic nature. The starting point is the Mean Effective Gain (MEG), as the first moment statistics of the Stochastic Effective Gain. On one hand, the MEG has provided a useful method to account the mobile channel impact on the antenna performance. On the other hand, it has several limitations, such as it does not provide information about bounds, or time evolution along a path. In this work, we propose a Monte-Carlo method to compute the pdf of the effective gain and evaluate higher order moments in order to achieve a better understanding about the antenna gain behaviour. This study will be carried out on a typical radiating structure, such as a standard half-wave dipole , using typical channel parameters presented in the literature.

Radio-over-fiber linearization with optimized genetic algorithm CPWL model

This article proposes an optimized version of a canonical piece-wise-linear (CPWL) digital predistorter in order to enhance the linearity of a radio-over-fiber (RoF) LTE mobile fronthaul. In this work, we propose a threshold allocation optimization process carried out by a genetic algorithm (GA) in order to optimize the CPWL model (GA-CPWL). Firstly, experiments show how the CPWL model outperforms the classical memory polynomial DPD in an intensity modulation/direct detection (IM/DD) RoF link. Then, the GA-CPWL predistorter is compared with the CPWL model in several scenarios, in order to verify that the proposed DPD offers better performance in different optical transmission conditions. Experimental results reveal that with a proper threshold allocation, the GA-CPWL predistorter offers very promising outcomes.

Modeling and Digital Predistortion based on Zernike polynomial functions for LTE-RoF system

A novel modeling technique is proposed to improve the Radio over Fiber (RoF) modeling and the Predistorter identification. This method is based on an orthogonal basis composed by Zernike polynomials, whose features guarantee numerical stability, a problem present in the classical polynomial model based on Volterra series. The measurements have been carried out with a LTE downlink signal with a bandwith of 3 MHz and QPSK modulation. The DFB laser has been biasing with 40, 70 and 100 mA to compare the distortion effects in each case. The measurements confirm that the RoF and PA modeling has an important improvement in terms of the ACEPR. In order to the DPD, the results show that the new technique reach an improvement of 2.57 dB in order to the ACPR, as well as an increase in the channel power.

RF Wideband Power Amplifier Performance Improvement with Reconfigurable Matching Networks

An optimized process for linearity and efficiency performance using a reconfigurable matching network (RMN) for power amplifiers (PAs) is presented. Long-term evolution (LTE) waveforms modeled as multisine excitations are used to simulate a general purpose PA, assuming that the antenna is perfectly matched. Selecting RMN state for linearity the adjacent channel power ratio (ACPR) is improved while obtaining efficiency enhancement. Moreover, it is shown that power added efficiency (PAE) is highly increased selecting properly RMN state while linearity also improves. With this architecture both linearity and efficiency are improved simultaneously.

High Power Amplifier Linearization Using Zernike Polynomials in a LTE Transmission

Digital predistortion (DPD) is a highly cost- effective technique to linearize high power amplifiers (HPAs) in wireless applications. The polynomial-type behavioral models are widely used both for HPAs and predistorters, but they often suffer from the numerical instability when high order nonlinearities are included. In this paper, we propose a new polynomial predistorter model considering the use of a orthogonal polynomial model based on a Zernike polynomial basis which is very robust and stable when classical identification techniques are applied. Computer simulations in a LTE uplink transmission confirm the predistorter computed with the Zernike polynomial model provides improvements in adjacent channel leakage ratio (ACLR) with stability and moderate complexity in the DPD coefficient estimation.

Improving Digital Predistortion Mismatch Sensitivity Using Tunable Matching Networks

Digital predistortion (DPD) is a well-known method to reduce non-linear distortion in RF power amplifiers (PAs). In this paper, we present a study on the sensitivity of this linearization technique under large mismatch conditions, and how this sensitivity can be improved using a tunable matching network (TMN). Under different load conditions, PA behavior may change significantly making more complicated the DPD solution convergence, reducing its performance or even disabling its correct operation. Using a TMN the load impedance variation effect can be reduced, observing improvements in adjacent channel power ratio (ACPR) and regions of DPD convergence in the Smith chart.