Adaptive Predistortion Technique Research Articles

Chebyshev Polynomial-Based Adaptive Predistorter for Nonlinear LED Compensation in VLC

In recent times, there has been a surge in research on visible light communications (VLC). In VLC systems, normal light emitting diode (LED) lamps are used as transmitters by modulating the optical power of the LED with the input current. All this is done at a high frequency, so that the fluctuations are not visible to the naked eye. It is a well known fact that the LED characteristics are not linear. Such nonlinearities degrade the performance of VLC. In the literature, a predistorter using a linear adaptive scaling parameter is proposed as a predistorter, which uses the well known normalized least mean squares (NLMSs) algorithm as the learning mechanism. However, to correct a nonlinearity, we need a nonlinear mapping/predistorter. This letter proposes a Chebyshev regression-based nonlinear predistorter to correct the nonlinear characteristics of LED by learning a polynomial expansion of the input electrical signal so as to mitigate the LED nonlinearity. Simulations have been carried out to validate the performance of the algorithm against existing adaptive predistortion techniques, such as NLMS-based predistortion and post-distortion techniques, such as Volterra and Hammerstein filters.

A Novel ML-2D-LUT Based Adaptive Predistorter of High Power Amplifier Using New Improved RLS Algorithm

This paper introduces a Multilevel two dimensional Look-Up-Table (ML-2D-LUT) based adaptive predistortion (APD) technique to reduce gain distortion in the saleh modeled power amplifier (HPA). The quick initialization of the ML-2D-LUT, a new improved RLS algorithm is implemented to compensate the time-varying problem in the HPA. The proposed method distinguishes the coefficients successfully. Moreover, additional convergence in steady state and less steady state error (SSE) will be accomplished for the power amplifiers with profound impacts in the memory by using the modified step-size parameter. Simulation results demonstrate that the proposed technique offers enhanced the PA efficiency for non-constant envelope modulations and nonlinear distortions can be extraordinarily decreased with the NIRLS algorithm. The BER and MSE results conclude that the proposed technique gives better results than conventional RLS.

A Comparative Analysis of Adaptive Digital Predistortion Algorithms for Multiple Antenna Transmitters

This paper presents a comparative study of adaptive algorithms for digital predistortion (DPD) in multiple antenna transmitters. Crossover predistorter (CO-DPD) and crosstalk canceling predistorter (CTC-DPD) were proposed to overcome the deleterious effect of RF crosstalk before the power amplifiers (PA) on digital predistortion (DPD) in multiple antenna transmitters. This paper discusses the linearization performance and computational complexity of least mean square (LMS) and recursive least squares (RLS) adaptive algorithms for CO-DPD and CTC-DPD. The adaptive predistortion algorithms for a single antenna transmitter can be extended for CO-DPD, by incorporating the DPD coefficients of more than one branch in to one filter coefficient vector of the adaptive algorithm. The adaptive CTC-DPD involves predistorters for each transmitter running in parallel with the adaptive algorithms that track the coupling between the transmitters. The computational complexity of CTC-DPD is considerably lower compared to CO-DPD, as it has lesser predistorter branches. It is estimated that the number of computations needed per sample duration, for the real-time operation of the adaptive CTC-DPD is 47% lesser compared to CO-DPD for a two-antenna transmitter, when a 9th order memory polynomial with 3 memory taps was used. The linearization performances of these adaptive predistortion techniques for two and four antenna transmitters are evaluated through simulations using QPSK, 16-QAM, UMTS, and LTE signals. It is observed that CTC-DPD performs approximately identical to CO-DPD in all the examined cases. In the presence of frequency selective crosstalk, an extension of CTC-DPD which can pre-cancel such crosstalk, performs considerably better than CO-DPD, which cannot model the frequency selectivity of the crosstalk.

An Adaptive Pre-Distortion Technique to Mitigate the DTC Nonlinearity in Digital PLLs

Digital fractional-N phase-locked loops (PLLs) are an attractive alternative to analog PLLs in the design of frequency synthesizers for wireless applications. However, the main obstacle to their full acceptance in the wireless-systems arena is their higher content of output spurious tones, whose level is ultimately set by the nonlinearity of the time-to-digital converter (TDC). The known methods to improve the linearity of the TDC either increase its dissipation and phase noise or require slow foreground calibrations. By contrast, the class of digital PLLs based on a one-bit TDC driven by a multibit digital-to-time converter (DTC) substantially reduces power dissipation and eliminates the TDC nonlinearity issues. Although its spur performance depends on DTC linearity, the modified architecture enables the application of a background adaptive pre-distortion which does not compromise the PLL phase-noise level and power consumption and is much faster than other calibration techniques. This paper presents a 3.6-GHz digital PLL in 65-nm CMOS, with in-band fractional spurs dropping from -39 to -52 dBc when the pre-distortion is enabled, in-band phase noise of -103 dBc/Hz and power consumption of 4.2 mW.

LED记忆非线性自适应预失真技术研究

LED记忆非线性自适应预失真技术研究

Adaptive Digital Predistortion for Wideband High Crest Factor Applications Based on the WACP Optimization Objective: An Extended Analysis

This paper provides an extended analysis of the adaptive digital predistortion technique initially proposed and conceptually overviewed in [1]. This digital predistortion technique is suitable for wideband high crest factor applications (digital audio broadcasting, digital video broadcasting-terrestrial & wideband code division multiple access high power transmitters) and overcomes the technical deficiencies of the traditional direct learning method. Specifically, predistortion filter parameter estimation is modeled as a generic mathematical optimization problem instead of a linear regression problem. In addition, the optimization objective is derived in the frequency instead of the time domain. A hybridly pruned Volterra series with memory is used to implement the predistortion filter. Hybrid pruning leads to a small optimization vector space, whilst the predistortion filter memory makes the method well suited to wideband applications. Given that predistortion filter parameter estimation does not rely on known test signals being injected into the transmitter, the method is on-air adaptive. Implementation aspects of the technique not covered in [1], but requiring extended coverage in this paper, include selection of mathematical optimization algorithms, predistortion filter memory estimation, weighted adjacent channel power weighting function application, and on-air adaption performance. Results obtained from actual hardware are presented.

Reduction Techniques of Peak-to-Average Power Ratio in MC-CDMA Systems

The high peak-to-average power ratio (PAPR) is one of the main disadvantages of multicarrier code division multiple access (MC-CDMA) system. To reduce the PAPR of multicarrier CDMA signals, a lot of studies have been focused on the power characteristics of spreading sequences. This paper presents the main techniques to reduce the PAPR in MC-CDMA like Amplitude Clipping and Filtering, Coding, Selective Mapping, The adaptive predistortion technique and DFT Spreading. Then a comparison of all these techniques is done in order to clarify the main advantages and disadvantages of each technique. DOI: http://dx.doi.org/10.11591/ij-ict.v1i2.558

Low complexity look up table based adaptive digital predistorter with low memory requirements

With the advent of spectrally efficient wireless communication systems employing modulation schemes with varying amplitude of the communication signal, linearization techniques for nonlinear microwave power amplifiers (PAs) have gained significant interest. In this article, a low complexity, direct-learning multilevel lookup table based adaptive digital predistortion technique has been proposed to linearize a PA. A loop delay compensation scheme has been used to achieve a significant reduction in convergence time and an improvement in linearization accuracy in the presence of an unknown loop delay. Compared with the conventional predistorters, the proposed technique shows fast adaptation speed which enables the predistorter to track time-varying PA nonlinearities.

Adaptive Digital Predistortion of Wireless Power Amplifiers/Transmitters Using Dynamic Real-Valued Focused Time-Delay Line Neural Networks

Neural networks (NNs) are becoming an increasingly attractive solution for power amplifier (PA) behavioral modeling, due to their excellent approximation capability. Recently, different topologies have been proposed for linearizing PAs using neural based digital predistortion, but most of the previously reported results have been simulation based and addressed the issue of linearizing static or mildly nonlinear PA models. For the first time, a realistic and experimentally validated approach towards adaptive predistortion technique, which takes advantage of the superior dynamic modeling capability of a real-valued focused time-delay neural network (RVFTDNN) for the linearization of third-generation PAs, is proposed in this paper. A comparative study of RVFTDNN and a real-valued recurrent NN has been carried out to establish RVFTDNN as an effective, robust, and easy-to-implement baseband model, which is suitable for inverse modeling of RF PAs and wireless transmitters, to be used as an effective digital predistorter. Efforts have also been made on the selection of the most efficient training algorithm during the reverse modeling of PA, based on the selected NN. The proposed model has been validated for linearizing a mildly nonlinear class AB amplifier and a strongly nonlinear Doherty PA with wideband code-division multiple access (WCDMA) signals for single- and multiple-carrier applications. The effects of memory consideration on linearization are clearly shown in the measurement results. An adjacent channel leakage ratio correction of up to 20 dB is reported due to linearization where approximately 5-dB correction is observed due to memory effect nullification for wideband multicarrier WCDMA signals.

Adaptive predistortion of lasers using multivariable feedback algorithm

An adaptive predistortion technique for direct modulated lasers is proposed and experimentally demonstrated. The predistortion calibration is through multivariable feedback control and does not require digital signal processing or storing data in memories. Therefore, this technique suggests a potential analogue implementation with less circuit complexity. A design example is implemented using standard CMOS technology. Experimental results confirm the validity and the robustness of the feedback algorithm. Measured results show about 5–15 dB reduction of IM3 and HD2 distortion over more than 300 MHz bandwidth.

Shrinking microwave filters

An adaptive predistortion technique has been presented and verified through the design and fabrication of practical filters in both the C and Ku bands. The method allows the realization of microwave filters at a lower cost, lighter mass, smaller volume, and better performance with minimum insertion loss penalties.The concept of lossy filters has been presented from a practical perspective. A simple lossy synthesis technique using any synthesized lossless (nontransversal) filter was shown, which can be used with hyperbolic rotations for loss distribution. Moreover, the limitation on the minimum Q of lossy resonators has been studied using a one-pole filter as a fundamental building block. Lossy four-pole Chebyshev and quasi-elliptic synthesis examples were presented. A four- pole Chebyshev lossy filter in the Ku band has been synthesized, modeled, and fabricated successfully using mixed combline and microstrip technologies. The design has the advantage of having all input-output paths going through more than one resonator, which minimizes unwanted source-to-load coupling, especially at high frequencies. The lossy approach is still at its early stages of development and needs more research and development effort to become as mature as the predistorted filters.

Adaptive Predistortions Based on Neural Networks Associated with Levenberg-Marquardt Algorithm for Satellite Down Links

This paper presents adaptive predistortion techniques based on a feed-forward neural network (NN) to linearize power amplifiers such as those used in satellite communications. Indeed, it presents the suitable NN structures which give the best performances for three satellite down links. The first link is a stationary memoryless travelling wave tube amplifier (TWTA), the second one is a nonstationary memoryless TWT amplifier while the third is an amplifier with memory modeled by a memoryless amplifier followed by a linear filter. Equally important, it puts forward the studies concerning the application of different NN training algorithms in order to determine the most prefermant for adaptive predistortions. This comparison examined through computer simulation for 64 carriers and 16-QAM OFDM system, with a Saleh's TWT amplifier, is based on some quality measure (mean square error), the required training time to reach a particular quality level, and computation complexity. The chosen adaptive predistortions (NN structures associated with an adaptive algorithm) have a low complexity, fast convergence, and best performance.

Predistortion technique for cross-coupled filters and its application to satellite communication systems

This paper presents a novel adaptive predistortion technique for general cross-coupled microwave/RF filters with improved insertion loss and group-delay equalization. The method enables many potential applications of an almost abandoned technique, and permits a lower Q implementation technology to emulate the performance of a higher Q filter. 10-4-4 filters were built and tested at C- and Ku-band to verify the validity of the new method. The impact to satellite communication channels was also analyzed. Another novel concept of over-predistortion was proposed and evaluated and should lead to significant improvement for applications such as satellite transponder input multiplexers, where insertion loss can be traded off for in-band flatness, mass, volume, and even overall system performance.

Adaptive predistortion of COFDM signals for a mobile satellite channel

AbstractIn this paper, we consider the optimization of the performance of QPSK and 16‐QAM coded orthogonal frequency division multiplexing (COFDM) signals over the non‐linear and mobile satellite channel. A high power amplifier and Rician flat fading channel produces non‐linear and linear distortions; an adaptive predistortion technique combined with turbo codes will reduce both types of distortion. The predistorter is based on a feedforward neural network, with the coefficients being derived using an extended Kalman filter (EKF). The conventional turbo code is used to mitigate Rician flat fading distortion and Gaussian noise. The performance over a non‐linear satellite channel indicates that QPSK COFDM followed by a predistorter provides a gain of about 1.7 dB at a BER of 3×10−3 when compared to QPSK COFDM without the predistortion scheme and 16‐QAM COFDM provides a gain of 0.5 dB output back‐off and 1.2 dB signal to noise ratio at a BER of 3×10−5 when compared with an adaptive predistorter based on the Harmmerstein model. We also investigate the influence of the guard time interval and Doppler frequency effect on the BER performance. When the guard interval increases from 0 to 0.125T samples and the normalized Doppler frequency is 0.001, there is a gain of 0.7 and 1 dB signal to noise ratio at a BER of 6×10−4 for QPSK and 16‐QAM COFDM, respectively. Copyright © 2003 John Wiley & Sons, Ltd.

An Equalizer and Memory Mapping Predistorter for Nonlinearly Amplified Signals Transmitted by Coded OFDM Systems in Multipath Fading Channel

We present an improved channel equalizing scheme to be used in Coded OFDM systems over an ISI channel. The scheme uses Turbo Trellis Coded Modulation (TTCM) which retains a powerful error correction capability while elegantly incorporating turbo coding into the Trellis Coded Modulation (TCM). In OFDM over wireless communications, M-ary Quadrature Amplitude Modulation (M-QAM) schemes have been widely used to achieve high bandwidth efficiency. However, M-QAM signals require linear amplification to avoid spectral regrowth and severe performance degradation due to their large envelope fluctuations. Moreover, the OFDM transmitter shapes the signal with a linear pulse shaping filter. The combination of a linear filter with a High Power Amplifier (HPA) forms a nonlinear system with memory, which severely degrades performance. We propose an efficient one-tap-equalizer using a modified LMS algorithm and a novel adaptive predistortion technique to compensate for the nonlinear distortion caused by the high power amplifier cascaded with a linear filter in TTCM-OFDM systems. The performance of this method is compared with methods that have been used previously with OFDM. A computer simulation confirms that the proposed approach is superior to the conventional methods.

Effect of demodulator errors on predistortion linearization

Any errors in a linearization system for power amplifiers will affect linearization results. In this paper the effect of the demodulator errors on a few adaptive predistortion techniques is studied. The investigations are focused on the complex gain predistorter and the mapping predistorter. The analyses and simulations show that in the complex gain predistorter only a DC offset will affect the adjacent channel interference (ACI). Gain imbalance and phase imbalance of the demodulator have no effect on the ACI in the complex gain predistorter system. In the mapping predistorter system, although the predistorter was also misadjusted because of errors, the ACI will not be deteriorated. But due to these errors, linear distortions will emerge in both systems and bit error rate (BER) of both systems will be degraded. The conclusions obtained in this paper can be used to guide the predistorter design. According to these guidelines the design process can be simplified and the ACI can be improved.

On compensating nonlinear distortions of an OFDM system using an efficient adaptive predistorter

This paper presents an efficient adaptive predistortion technique compensating for nonlinear distortions caused by a high-power amplifier (HPA) cascaded with a linear filter in an OFDM system. In the proposed approach, the memoryless HPA, preceded by a linear filter with memory in OFDM systems, is modeled by the Wiener system, which is then precompensated by the proposed adaptive predistorter with a minimum number of filter taps. It is confirmed by computer simulation that the proposed approach produces a faster convergence speed than the previous adaptive predistortion technique, and provides a small output backoff as low as 5.5 dB for an OFDM system employing an HPA with a linear filter.

An adaptive data predistorter for compensation of nonlinear distortion in OFDM systems

In this paper, an adaptive data predistortion technique which can compensate for the nonlinear effect of a TWT high-power amplifier (HPA) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. It is shown that the convergence time and MSE of an adaptive data predistorter can be significantly reduced by using a broadcasting technique and by designing appropriate training signals.

Response to comments on: An adaptive DSP predistortion technique for radio systems

Response to comments on: An adaptive DSP predistortion technique for radio systems

Comments on: “An adaptive DSP predistortion technique for radio systems”

Comments on: “An adaptive DSP predistortion technique for radio systems”