Paloma Garcia

An adaptive digital method of imbalances cancellation in LINC transmitters

The linear amplification using nonlinear components (LINC) technique is a well-known power amplifier linearization method to reduce adjacent channel interference in a nonconstant envelope modulation system. Its major drawback is the inherent sensitivity to gain and phase imbalances between the two amplifier branches. In this paper, a novel full-digital base band method is described which corrects any gain and phase imbalances in LINC transmitters mainly due to the unmatching of the two amplifier paths. Amplifiers are characterized by a level-dependent complex gain using a memoryless model. The method uses adaptive signal processing techniques to obtain the optimal complex coefficient to adjust gain and phase imbalances. Its main advantage is the ability to track the input signal variations and adapt to the changes of amplifier nonlinear characteristics. Other effects are included in the analysis such as quadrature modulator and demodulator imbalances and loop delay. A computer simulation has been carried out to verify the method functionality.

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.

A* Based Algorithm for Reduced Complexity ML Decoding of Tailbiting Codes

The A* algorithm is a graph search algorithm which has shown good results in terms of computational complexity for Maximum Likelihood (ML) decoding of tailbiting convolutional codes. The decoding of tailbiting codes with this algorithm is performed in two phases. In the first phase, a typical Viterbi decoding is employed to collect information regarding the trellis. The A* algorithm is then applied in the second phase, using the information obtained in the first one to calculate the heuristic function. The improvements proposed in this work decrease the computational complexity of the A* algorithm using further information from the first phase of the algorithm. This information is used for obtaining a more accurate heuristic function and finding early terminating conditions for the A* algorithm. Simulation results show that the proposed modifications decrease the complexity of ML decoding with the A* algorithm in terms of the performed number of operations.

Performance analysis of turbo decoding algorithms in wireless OFDM systems

Turbo codes are well known to be one of the error correction techniques which achieve closer results to the Shannon limit. Nevertheless, the specific performance of the code highly depends on the particular decoding algorithm used at the receiver. In this sense, the election of the decoding algorithm involves a trade off between the gain introduced by the code and the complexity of the decoding process. In this work we perform a thorough analysis of the different iterative decoding techniques and analyze their suitability for being implemented in the user terminals of new cellular and broadcast systems which are based on orthogonal frequency division multiplexing (OFDM). The analyzed iterative decoding algorithms are the max-log-MAP and the soft output Viterbi algorithm (SOVA), since both of them have a relative low computational complexity, simplifying their implementation in cost efficient terminals. Simulation results have been obtained for different encoder structures, block sizes and considering realistic channel conditions (an OFDM transmission over a wireless channel).

Memory Behavioral Modeling of RF Power Amplifiers

Power amplifiers (PAs) are the most extended and widely used nonlinear devices in communications. Their non- linearities generate distortion both in amplitude and phase of the PAs output signal. The use of wideband efficient modulations in the last generation systems, such as wideband code-division multiple access (WCDMA) or orthogonal frequency-division multiplexing (OFDM), adds new problems to the use of PAs, due to the memory effects associated with the bandwidth and the increase of distortion by the non-constant envelope. In this paper it is shown the need of taking into account the memory effects in order to get a more accurate model of PA operation. We compare the performance of the memoryless model with some well-known memory models based on memory polynomials and Volterra series using a novel calibration signal.

Two step SOVA-based decoding algorithm for tailbiting codes

In this work we propose a novel decoding algorithm for tailbiting convolutional codes and evaluate its performance over different channels. The proposed method consists on a fixed two-step Viterbi decoding of the received data. In the first step, an estimation of the most likely state is performed based on a SOVA decoding. The second step consists of a conventional Viterbi decoding that employs the state estimated in the previous step as the initial and final states of the trellis. Simulations results show a performance close to that of maximum-likelihood decoding.

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

Channel Independent Precoder for OFDM-Based Systems Over Fading Channels

In this paper we propose an independent channel precoder for orthogonal frequency division multiplexing (OFDM) systems over fading channels. The design of the precoder is based on the information redistribution of the input modulated symbols among the output precoded symbols. The proposed precoder decreases the variance of the instantaneous noise power at the receiver produced by the channel variability. The employment of an interleaver together with a precoding matrix whose size does not depend on the number of data carriers in an OFDM symbol allows different configurations of time-frequency diversity which can be easily adapted to the channel conditions. The precoder is evaluated with a modified Zero Forcing (ZF) equalizer whose maximum gain is constrained by means of a clipping factor. Thus, the clipping factor limits the noise power transfer in the receiver deprecoding block in low SNR conditions.

Adaptive digital correction of gain and phase imbalances in LINC transmitters

The linear amplification using nonlinear components (LINC) technique is a well-known power amplifier linearization method to reduce adjacent channel interference in a nonconstant envelope modulation system. Its major drawback is the inherited sensitivity to gain and phase imbalances between the two amplifier branches. In this paper, a novel adaptive full-digital base band method is described which corrects any gain and phase imbalances in LINC transmitters. Its main advantage is the ability to track the input signal variations and adapt to the changes of amplifier nonlinear characteristics.