Predistortion Method Research Articles

Non-orthogonal multiple access with phase pre-distortion in visible light communication.

Non-orthogonal multiple access (NOMA) offers a good balance between throughput and fairness for visible light communication (VLC). This work presents a phase pre-distortion method to improve the symbol error rate performance of NOMA uplink with successive interference cancellation (SIC) decoding in VLC. Both theoretical analysis and experimental evaluation have shown that the proposed phase pre-distortion method improves the bit-error-rate (BER) performance for NOMA under both low and high relative power ratios. Specifically, at low relative power ratios, the proposed method can eliminate the possible BER floors and alleviate the power ratio requirement by 2 dB at the BER of 3.8 × 10-3.

Extending the Characterization Bandwidth of Dynamic Nonlinear Transmitters With Application to Digital Predistortion

This paper reports a new measurement method for wideband radiofrequency power amplifier (PA) characterization and digital predistortion. The proposed measurement procedure significantly relaxes the sampling rate requirement on the analog-to-digital converters of the feedback path. Successful PA linearizations were achieved in the presence of 20, 40, and 60-MHz LTE-A signals using a vector signal analyzer with sampling speeds equal to only 24, 40.96, and 61.44 Ms/s, respectively. Despite these very low sampling rates, a quasiperfect cancellation of the PA distortions was achieved (more than 50 dBc in terms of ACLR), in all tests, over bandwidths including up to fifth-order intermodulation distortions. Such a correction bandwidth is much wider than the observation bandwidths associated with the receiver sampling rates.

Analysis on LUT based digital predistortion using direct learning architecture for linearizing power amplifiers

In modern wireless communication system, power amplifier (PA) is an important component which is expected to be operated at the region of high power efficiency, but in this region, PA is inherently nonlinear. Thus, the linearization of high power efficient PA is necessary. In this paper, direct learning architecture (DLA) and indirect learning architecture (ILA) are firstly compared. It shows that DLA is more robust than ILA. Then a baseband digital predistortion (DPD) method with DLA is proposed for power amplifier linearization based on combined look-up tables (LUT) and memory polynomial (MP) model. The main innovation is that a LUT-based approach is proposed to calculate directly the complex-valued predistorted signal. Moreover, some interpolation techniques are introduced to reduce the LUT size. The proposed DPDs are validated experimentally. Additionally, the influences of some important parameters in experimental setup, such as the number of bits of analog-to-digital converter (ADC) and the instrument bandwidth, are analyzed.

An Improved Digital Predistortion in Wideband Wireless Transmitters Using an Under-Sampled Feedback Loop

This letter presents an improved digital predistortion (DPD) method to optimize the linearization of power amplifiers (PAs) when an under-sampled feedback loop is used in the transmitter. The proposed approach is based on an adaptive piecewise Lagrange (APL) basis digital predistorter and a variable fractional-delay (VFD) finite-length impulse response (FIR) filter. A normalized least mean square (NLMS) algorithm with a direct learning structure is used to update the DPD parameters, and the VFD FIR filter is used to periodically vary the fractional-delay of the transmitted signal. This changes the initial phase of the under-sampled feedback signal to preserve the peak power information from the full sampling rate signal. Experiment results show that there is 10 dB improvement in the normalized mean square error (NMSE) for a 70 MHz Long Term Evolution-advanced signal, even when the sampling rate of feedback is reduced from 491.52 Msps to 122.88 Msps.

A Robust Adaptive Pre-Distortion Method for Optical Communication Transmitters

Present and next generation optical communication systems are constantly being developed to operate at higher baud rates and higher modulation formats. It then becomes inevitable to consider the various linear and nonlinear effects due to the imperfect components at the transmitter. State-of-the-art digital-to-analog converters, high bandwidth driver amplifiers, and dual-polarization (DP) Mach–Zehnder modulators are far from being ideal and present distortions in the form of bandwidth limitation, transmitter I/Q skew, and nonlinear effects. In this letter, we propose a new robust pre-distortion method to mitigate the undesirable linear and non-linear distortions of all electrical and optical transmitter components simultaneously. A step-by-step derivation and implementation of the algorithm are discussed and put forward. The performance of the algorithm is experimentally assessed across DP-4QAM, DP-8QAM, DP-16QAM, DP-32QAM, and DP-64QAM up to the signaling rates of 56 GBd. Significant improvements in the required optical signal-to-noise ratio at bit-error rate (BER) of $10^{-2}$ are demonstrated.

Efficient Timing Mismatch Correction for Low-Cost Digital-Mixing Transmitter

In this paper we present a novel digital predistortion method to eliminate the nonlinear timing mismatches, delay mismatch and duty cycle mismatch, in the in-phase and quadrature upconversion signals of a power-efficient digital transmitter architecture. This transmitter architecture has a separate digital-mixing stage before the digital-to-analog conversion, which halves the number of required current cells in the radio frequency digital-to-analog converter (RFDAC), but introduces a nonlinear performance impact of timing mismatches in the digital-mixing stage. This paper analyzes the inefficiency in deploying a classical look up table (LUT) predistortion scheme for this impairment. Then a much more effective predistortion scheme based on a run-time binary-tree descent searching scheme is proposed to tackle the problem. Simulation results show that, for both 64-QAM and 256-QAM modulation schemes, the error-vector-magnitude (EVM) can be improved from around ${-}24$ dB to less than ${-}42$ dB with a power-efficient implementation, which leaves substantial design margin for other nonidealities distorting the transmitted signal. The power consumption of the predistortion block, exploiting dynamic voltage scaling, is 5.52 mW when operating at a sampling rate of 600 Mega samples per second (MSPs) and scales to 3.25 and 1.48 mW when reducing the sample rate to 400 and 200 MSPs.

3-D Fourier Series Based Digital Predistortion Technique for Concurrent Dual-Band Envelope Tracking With Reduced Envelope Bandwidth

In this paper, the design, implementation, and measurement results of a new digital predistortion (DPD) method for a concurrent dual-band envelope tracking (ET) power amplifier (PA) system is presented. The PA gain is represented using a set of 3-D orthogonal Fourier series basis functions, which accounts for the distortions introduced by the two signal bands, as well as the supply modulation voltage. Here, the new Fourier series basis is shown to substantially outperform the traditional memory polynomial approach in terms of its linearization capability for subsequent data for which it was not trained for (prediction) due to its well-defined numerical rank and stability. The new DPD system was implemented using a 10-W peak gallium–nitride (GaN) ET PA operating with a dual-band input based on long-term evolution and WCDMA signals center frequency (1.89 and 2.2 GHz) spaced by 310 MHz. The linearization results are compared to the 3-D memory polynomial in two steps: extraction and prediction. In the DPD coefficient extraction phase, tested under the same signal, the traditional memory polynomial and the Fouries series reach close results in both normalized mean square error (NMSE) and adjacent channel power ratio (ACPR). In the prediction phase, however, the proposed method provides a significant performance improvement. A performance improvement as high as 17 dB in terms of NMSE and 4.6 dB in terms of ACPR in comparison to the conventional dual-band 3-D memory polynomial method implementing ET. Furthermore, the well-defined numerical rank of the Fourier series approach allows for a substantial reduction in coefficients using principal component analysis without detrimenting performance.

Digital predistortion method combining memory polynomial and feed‐forward neural network

A baseband digital predistortion (DPD) technique based on a feed-forward neural network (FFNN) is presented. The process of memory polynomial (MP) DPD is time consuming because of the large number of mathematical calculations. The FFNN is adopted to realise the mathematical calculations in MP DPD with direct learning architecture (DLA). The training samples of the FFNN are derived from MP DPD with DLA. It guarantees the accuracy of imitating the MP DPD. Although the training of the FFNN is time consuming, the trained FFNN DPD is less time consuming than MP DPD. This solution is validated based on a power amplifier (PA) ZFL-2500 driven by a wideband code division multiple access (WCDMA) signal with 3.84 MHz bandwidth. The experimental results show that the FFNN can mimic the behaviour of the MP DPD. The proposed DPD achieves a significant improvement in linearity and is stable.

Multicarrier Successive Predistortion for Nonlinear Satellite Systems

To satisfy the aggressive demand for higher satellite throughput, industry trend is moving toward sharing the transponder amplifier by multiple carriers, each employing high-order modulations that are spectrally compact. This trend, in conjunction with the inherently nonlinear nature of the amplifier when driven efficiently closer to saturation, creates significant intermodulation distortion that needs to be appropriately compensated. This paper presents a powerful compensation technique that is capable of mitigating the nonlinear intermodulation distortion and is placed at the transmitter or gateway. It is a novel multicarrier data predistortion technique that successively modifies the transmitted symbols to drive multicarrier distortion vector toward zero. This distortion vector results from passing the transmitted symbols from the multiple carriers, intrinsically accessible at the gateway, through the nonlinear satellite channel model. The novel successive predistortion technique and methods of estimating the distortion are described in detail. It is demonstrated using extensive computer simulations that the proposed multicarrier predistortion technique is capable of achieving near-optimum performance, even when only a simple linear receiver is employed and no exchange of data is assumed between receivers.

The Modeling and Equalization Technique of Nonlinear Wireless Channel

In this paper, we consider a new method for modeling and equalizing of nonlinear wireless channel. The Vol- terra structure equalizer has high computation burden and slow convergence rate due to its large number of coefficients and the pre-distortion method is very difficult to realize in actual wireless communication systems. Here, we take another scheme instead of those above. First assume the nonlinear channel under consideration has a Wiener model representa- tion. After fully estimating the parameters of the channel which consists of a linear filter with memory and a nonlinear fil- ter without memory, we take Hammerstein model equalizer to equalize the channel. The simulation results show the fea- sibility of the program by applying it to 8PSK wireless communication channel.

一种位置可调整的实时图像预畸变方法

一种位置可调整的实时图像预畸变方法

Adaptive predistorter using NLMS algorithm for nonlinear compensation in visible‐light communication system

A predistortion method using the adaptive normalised least mean square (NLMS) algorithm is proposed for the estimation of light-emitting diode distortion. The light emission characteristic could change nonlinearly depending on conditions of the physical environment, such as temperature variation, which cannot be ignored, especially in the case of high-power emissions. Instead of using the fixed values of the conventional memory look-up-table (LUT) for nonlinearity compensation, the proposed adaptive NLMS-based predistorter immediately estimates the distortion change according to environmental changes. The simulation results show that the proposed method has better performance than the conventional predistorter employing an LUT.

Low Complexity Distributed Model for the Compensation of Direct Conversion Transmitter’s Imperfections

In modern communication systems, nonlinearity in power amplifiers (PAs) and in-phase and quadrature-phase (I/Q) imperfections in the transmitter are of enormous concern. With the increase in the importance for highly energy efficient and low complexity models, there is a need to develop low complexity digital predistortion (DPD) methods. In this paper, we present a novel memory polynomial based distributed two block model to alleviate these impairments. Various performance metrics are used to evaluate the design performance and complexity of proposed model as compared to the state of the art predistorter model. Simulation and measurement results indicate the ability of the proposed model to meet the desired design purpose with reduced complexity in terms of number of coefficients, dispersion coefficient, condition number and number of floating points operations required for computing various steps in the inverse modeling algorithm. This is achieved while maintaining reasonable performances in terms of NMSE and ACEPR. The major attribute of the model is the reduction in complexity of the system. The number of complex valued coefficients and the number of floating point operations (FLOPs) are both reduced by 17%-56%, matrix conditioning is improved by 12-33 dB and the dispersion coefficient is reduced by 16-42 dB as compared to the previously proposed joint modulator and power compensation technique.

One‐step model extraction method for direct learning digital predistortion

A one-step model extraction method is proposed for direct learning (DL) digital predistortion (DPD) system. The expected predistortion signal of the training sequence is first derived by iterative injection of the baseband distortion components, which enables one-step model extraction of the DPD model by least squares estimation afterwards. Compared with the conventional model extraction method for DL DPD, the proposed method yields comparable linearisation performance but has much lower computational complexity.

Compensation of elemental image using multiple view vectors for off-axis integral floating system.

A novel method is proposed to compensate the distortion caused by the off-axis structure in integral floating with a concave mirror. The proposed method of this paper can generate the elemental image that is integrated into the multiple predistortion images for the corresponding multiview directions, which cannot be performed by the predistortion method using a single-view vector. The fundamental concept of the proposed method is that the predistortion images can be generated for more than two view vectors, which are defined by each set of corresponding elemental lens and subimage, and captured onto the subimage. In addition, the subimage can be divided into the several segments to increase the number of view vectors, so that the predistortion images generated by more view vectors can enhance the degree of compensation. The proper number of the view vectors per subimage is discussed and analyzed based on the small-angle approximation. Experiments to verify the feasibility of the proposed method are performed.

Undersampling Digital Predistortion Compensation Technology Based on Broadband Signals

Nonlinearity of power amplifier always results in spectrum spreading on input signal. When using digital predistortion technology to compensate power amplifier's nonlinearity, the required sampling frenquence for the output signal of amplifier is times of the original signal. The undersampling predistortion method can reduce the sampling frequency requirements. However, for the broadband signal, due to the significant memory effect of the power amplifier, the performance of undersampling digital predistortion is not satisfactory. In order to improve the undesirable predistortion performamce, this paper presents two kinds of undersampling digital predistortion compensation methods. For different sampling frequencies, the predistortion performances of these two kinds of compensation methods are different.

Concurrent dual‐band digital predistortion method for wideband power amplifier with large carrier spacing two‐carrier signal

ABSTRACTA concurrent dual‐band three‐layered biased memory polynomial (DB‐TLBMP) model is proposed for the linearization of wideband power amplifiers driven by large carrier spacing two‐carrier signals. The DB‐TLBMP model is derived by extending the TLBMP model to the concurrent dual‐band mode, and the least square algorithm is used for coefficients extraction of each layer. Using a two‐carrier wideband code division multiple access signal with 40‐MHz carrier spacing at 2.65 GHz, whose peak‐to‐average power ratio is 7.74 dB, the adjacent channel leakage ratios (ACLRs) improvement of more than 17 dBc are achieved to give the ACLRs below −50 dBc at the offset of ±5 MHz and an average output power of 37 dBm. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:594–597, 2014

Performance improvement of double-sideband signals in radio-over-fiber links utilizing pre-distortion method

In this paper, we investigate the optimal carrier-to-sideband ratio (CSR) for optical double-sideband (DSB) signals in radio-over-fiber (RoF) transmission. A pre-distortion method based on spectral shaping is proposed to optimize the signal CSR and eliminate the dispersion-induced power fading. A 12 GHz RoF transmission over 29 km standard single mode fiber (SSMF) is experimentally demonstrated which reveals that pre-distorted DSB has a 4.4 dB improvement over the one without pre-distortion, and 1.2 dB sensitivity advantage over single sideband (SSB) both with optimal CSR. The pre-distortion method is also applied to multi-channel transmission. The multi-channel experiment shows that for all channels the dispersion-induced power fading effects can be simultaneously mitigated and the pre-distorted DSB signals have ~1 dB sensitivity improvement over the SSB signals.

광대역 첩 신호 발생기를 위한 RF 불균형 연구

최근 우주에서의 SAR(synthetic aperture radar) 시스템은 영상 해상도와 주파수가 높아지고 있다. 높은 품질의 영상 해상도 일수록 높은 대역폭이 요구되고 RF 구성을 사용하는 광대역 신호 발생기는 매우 복잡해지고 RF 소자의 불균형 성분이 증가한다. 그러므로 이러한 에러를 줄이고 성능을 개선하는것은 매우 중요하다. 본 연구에서, 광대역 신호 발생기의 송신 신호는 위상 잡음, 직교불균형, 비선형 증폭기의 에러 모델이 적용된다. 그리고 광대역 파형 발생기의 가능한 구조들을 정의하고 평가 방법으로 PSLR(peak side lobe ratio)과 ISLR(integrated side lobe ratio)을 측정하였다. 또한, 파형으로부터 진폭과 위상 에러를 추출하고 이차 다항식을 사용하여 비선형 소자에 따른 성능 변화를 검토 하였다. 마지막으로 고출력 증폭기의 비선형 에러를 보상하기 위한 사전왜곡방식을 적용하여 혼변조 성분에 의한 증폭기 출력의 왜곡이 15 dB 감소됨을 확인하였다. Recently spaceborne SAR systems are increasing image resolution and frequency. As a high quality image resolution, the wider bandwidth is required and a wideband signal generator with RF component is very complicated and RF impairments of device is increased. Therefore, it is very important to improve performance by reducing these errors. In this study, the transmission signal of the wideband signal generator is applied to the phase noise, IQ imbalance, ripple gain, nonlinear model of high power amplifier. And we define possible structures of wideband signal generator and measure the PSLR and ISLR for the performance assesment. Also, we extract error of the amplitude and phase from the waveform and use a quadratic polynomial curve fitting and examine the performance change due to nonlinear device. Finally, we apply a high power amplifier predistortion method for non-linear error compensation. And we confirm that distortion in the output of the amplifier by intermodulation component is decreased by 15 dB.

Adding signal for peak‐to‐average power reduction and predistortion in an orthogonal frequency division multiplexing context

In this study, the authors look into digital predistortion and its combination with the peak-to-average power ratio (PAPR) reduction technique in the case of orthogonal frequency division multiplexing (OFDM)-based systems. First of all, the authors propose a new predistortion methodology which uses the addition of signals. Two different algorithms are investigated for the predistortion signals generation. The first one is based on the traditional predistortion method, where an estimation of the power amplifiers (PAs) characteristics is needed in the first place. The second algorithm to generate the predistortion signal does not need a priori PA estimation. It is based on an iterative compensation of the error between the amplified signal and the OFDM signal. Then, the authors combine this latter algorithm with the tone reservation PAPR reduction method in order to have a global adding signal expression. Two combination scenarios of PAPR reduction and predistortion obtained by means of adding a signal are proposed. The first one combines both of them in series while the second one is parallel. Performances of the proposed predistortion algorithms and the combination scenarios are compared thanks to simulations based on IEEE802.il a/g standards for a memoryless solid state PA.