Nonlinear High Power Amplifier Research Articles

Analysis and Compensation of Power Amplifier Nonlinearity in MIMO Transmit Diversity Systems

The nonlinearity of high-power amplifiers (HPAs) has a crucial effect on the performance of multiple-input-multiple-output (MIMO) systems. In this paper, we investigate the performance of MIMO orthogonal space-time block coding (OSTBC) systems in the presence of nonlinear HPAs. Specifically, we propose a constellation-based compensation method for HPA nonlinearity in the case with knowledge of the HPA parameters at the transmitter and receiver, where the constellation and decision regions of the distorted transmitted signal are derived in advance. Furthermore, in the scenario without knowledge of the HPA parameters, a sequential Monte Carlo (SMC)-based compensation method for the HPA nonlinearity is proposed, which first estimates the channel-gain matrix by means of the SMC method and then uses the SMC-based algorithm to detect the desired signal. The performance of the MIMO-OSTBC system under study is evaluated in terms of average symbol error probability (SEP), total degradation (TD) and system capacity, in uncorrelated Nakagami-m fading channels. Numerical and simulation results are provided and show the effects on performance of several system parameters, such as the parameters of the HPA model, output back-off (OBO) of nonlinear HPA, numbers of transmit and receive antennas, modulation order of quadrature amplitude modulation (QAM), and number of SMC samples. In particular, it is shown that the constellation-based compensation method can efficiently mitigate the effect of HPA nonlinearity with low complexity and that the SMC-based detection scheme is efficient to compensate for HPA nonlinearity in the case without knowledge of the HPA parameters.

Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link

We introduce analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder high-power amplifier (HPA). A Volterra filter is proposed which is novel in its implementation of this analytical characterization and modeling of intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms with pilot-based training so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are provided to compensate for nonlinear ISI and ACI. The first set performs adaptive channel inversion and then applies equalization. The second set of solutions performs adaptive channel identification and then applies cancellation. The effectiveness of the proposed analysis and techniques is demonstrated via extensive simulations for high-order QAM and APSK modulations. Also included is the coded performance with selected LDPC codes designed for the DVB-S2 standard. Finally, computational complexity is assessed and performance impact is quantified when complexity is reduced by decreasing the number of Volterra coefficients.

MRRC 안테나 기법을 이용한 MC-DS-CDMA 전송방식의 성능 분석

MC-DS-CDMA 전송 방식에서는 서로 직교성이 있는 여러 개의 부반송파들을 사용함으로서 주파수 다이버시티 효과를 얻을 수 있어 우수한 성능의 전송이 가능하다. 본 논문에서는 파일럿 심볼을 이용하여 채널을 추정하는 MC-DS-CDMA 시스템에 MRRC 안테나 다이버시티 기법을 적용할 때의 시스템 성능을 분석한다. 이러한 다중 반송파 전송 방식에서는 채널 추정 에러, 비선형 전력 증폭기 및 송수신간 반송 주파수 편차 등에 의하여 시스템 성능이 열화 된다. 그러므로 위와 같은 영향들을 모두 고려하여 송신 신호가 다중경로 페이딩 채널을 통하여 전송될 때의 데이터 비트 오류율 성능을 분석한다. MC-DS-CDMA is technique where a single data symbol is transmitted at multiple subcarriers which are orthogonal to each other. With this technique, frequency diversity can be achieved. The performance of PSAM MC-DS-CDMA system with the application of MRRC antenna diversity is analyzed. The BER performance of this system over multipath fading environment is evaluated, considering the channel estimation error, carrier frequency offset, and the nonlinear high power amplifier.

Enhanced multiple access schemes for future broadband satellite communications

A new structure of next generation integrated communication system was proposed, which is composed of space segment based on satellites and terrestrial segment. Moreover, the characteristics of enhanced multiple access schemes based on orthogonal frequency division multiplexing (OFDM) technique were analyzed for satellite links. However, OFDM is a doubtful candidate as its higher peak-to-average power ratio (PAPR) that causes the distortion of high power amplifier (HPA). Furthermore, different schemes were evaluated and compared in terms of the HPA nonlinearity and the link level performance in detail. And the pilot-aided channel estimation and equalization techniques were also considered for analyzing the problem. Simulation results show that the bit error rate (BER) and block error rate (BLER) performance of orthogonal frequency division multiple access (OFDMA) outperforms that of single carrier-frequency division multiple access (SC-FDMA) for the satellite links in the proposed structure, though discrete Fourier transform-spread OFDM DFT-S OFDM has low PAPR, especially the BER performance of OFDMA is 3.6 dB larger than that of SC-FDMA at the target BER.

Link supportability analysis of digital channelised satellite communication system using min–max optimisation and variable neighbourhood search algorithm

In a frequency-division multiple access satellite communication (SATCOM) system, how to effectively reduce the non-linearity of high-power amplifier to improve the transponder power utility and system capacity, which is the key goal of link supportability analysis, is a challenging and essential step in system design. In this study, the authors creatively model the link supportability analysis problem as a min-max optimisation for a digital channelised SATCOM system, and propose an effective variable neighbourhood search (VNS) algorithm for the established optimisation. By minimising the maximum of carrier powers output from the satellite transponder, the established min-max optimisation model aims at reducing the difference among carrier powers in the satellite transponder as much as possible to avoid carrier power being irregular so that the authors can optimally set the transponder's operating point to improve the transponder power utility, and minimise each terminal's uplink effective isotropic radiated power (EIRP) so that as many links as possible could be supported in the system and thus the system capacity could be enhanced. Practical examples are also presented to verify the proposed approach's superior performance in improving a digital channelised SATCOM system's transponder power utility and the system capacity.

Theoretical Characterization of Memory Polynomial Models With Gaussian Inputs

We consider distortion of a Gaussian signal by a memory polynomial system and prove a theorem about a linear representation of the output. As an application example, we analyze an OFDM system with a nonlinear high power amplifier at the transmitter.

Design and Performance Analysis of Pre-Distorter Including HPA Memory Effect

OFDM(Orthogonal Frequency Division Multiplexing) signals suffer serious nonlinear distortion in the nonlinear HPA(High Power Amplifier) because of high PAPR(Peak Average Power Ratio). Nonlinear distortion can be improved by a pre-distorter, but this pre-distorter is insufficient when the PAPR is very high in an OPFDM system. In this paper, a DFT(Discrete Fourier Transform) transform technique is introduced for PAPR reduction. It is especially important to consider the memory effect of HPA for more precise predistortion. Therefore, in this paper, we consider two models, the TWTA(Traveling-Wave Tube Amplifier) model of Saleh without a memory effect and the HPA memory polynomial model that has a memory effect. We design a pre-distorter and an adaptive pre-distorter that uses the NLMS(Normalized Least Mean Square) algorithm for the compensation of this nonlinear distortion. Without the consideration of a memory effect, the system performance would be degraded, even if the pre-distorter is used for the compensation of the nonlinear distortion. From the simulation results, we can confirm that the proposed system shows an improvement in performance.

Analytical performance of orthogonal frequency division multiplexing systems impaired by a non-linear high-power amplifier with memory

The performance of modern communication transmitters utilising multi-carrier modulation techniques is highly sensitive to non-linear distortions arising mainly from the high-power amplifier (HPA). In addition, the wideband characteristics of multi-carrier signals result in frequency-dependent distortions, typically known as memory effects. This study aims to analytically evaluate the impact of the distortion induced by non-linear HPA with memory on a multi-carrier signal exemplified by orthogonal frequency division multiplexing (OFDM). The behavioural model of HPA considered for theoretical analysis is a memory polynomial model, which is a truncated form of the Volterra series. In the theoretical framework developed, this study shows that memory in HPA considerably degrades the performance of OFDM in terms of symbol error rate (SER), and the distortion itself can be canonically characterised by a complex attenuation component and a non-linear noise component. Closed-form expressions for SER in additive white Gaussian noise channel are derived and the SER for fading channel is approximated by the adaptive Gauss-Kronrod quadrature method. Simulation results are shown for a realistic HPA, based on the Wiener-Hammerstein model, and compared with the analytical results to validate the proposed analysis.

Peak-to-average power ratio reduction using second order cone programming based tone reservation for terrestrial digital video broadcasting systems

High peak-to-average power ratio (PAPR) is a critical issue in any multicarrier systems using orthogonal frequency division multiplexing, as terrestrial digital video broadcasting (DVB). It can result in low power efficiency and large performance degradation of systems, due to the nonlinearity of high-power amplifier (HPA). A PAPR reduction method based on tone reservation technique with second-order cone programming (SOCP) approach in terrestrial DVB systems is proposed. The authors first demonstrate the superiority of the SOCP optimisation algorithm compared with an iterative gradient-based algorithm, using the current DVB-T parameters: significant PAPR reduction gains can be achieved with only a very small set of subcarriers in the useful bandwidth, making the proposed method more promising in terms of spectral efficiency. Moreover, the proposed solution presents a very good trade-off between PAPR reduction gain and mean transmitted power increase. An overall study, taking into account the limitation of the power level of the dedicated subcarriers and the evaluation of the performances in presence of a nonlinear HPA, is presented. These performances are given in terms of adjacent channel power ratio and bit error rate. The resulting PAPR reduction gain demonstrates that the relevance of the proposed method for the future DVB-T standard is straightforward.

System design and analysis of MIMO SFBC CI-OFDM system against the nonlinear distortion and narrowband interference

The CI-OFDM (carrier interferometry orthogonal frequency division multiplexing) system has been widely studied in the multi-carrier communication system. This CI-OFDM system spreads each information symbol across all N sub-carriers using orthogonal CI spreading codes. The CI-OFDM system can show the advantage of PAPR (peak to average power ratio) reduction and frequency diversity gains without any loss in the communication throughput. On the other hand, a great attention has been devoted to MIMO (multiple input multiple output) antenna array systems and space-time-frequency processing. In this paper, focusing on the two Tx (transmit) / one Rx (receive) antennas and two Tx / two Rx antennas configuration, we evaluate the performance of MIMO OFDM and MIMO CI- OFDM system. SFBC (space frequency block coding) coding is applied into both MIMO OFDM system and MIMO CI-OFDM system. For CI-OFDM realization, digital implemented CI-OFDM structure is used in which CI code spreading operation and carrier allocation are separately processed by simple IFFT type operation. From the simulation results, it is shown that MIMO SFBC CI-OFDM reduces PAPR significantly compared with MIMO SFBC-OFDM system. The out-of band re-growth of signal spectrum in MIMO SFBC CI-OFDM system is much smaller than MIMO SFBC OFDM. In the NBI (narrow band interference) channel MIMO SFBC CI-OFDM system achieves considerable BER improvement, compared with the MIMO SFBC-OFDM system in which error floor occurs in most of SNR range. So, it can be expected that MIMO SFBC CI-OFDM system is very useful for the high speed communication system in the situation of nonlinear HPA (high power amplifier) and NBI channel.

Power Backoff Reduction Techniques for Generalized Multicarrier Waveforms

Amplification of generalized multicarrier (GMC) signals by high-power amplifiers (HPAs) before transmission can result in undesirable out-of-band spectral components, necessitating power backoff, and low HPA efficiency. We evaluate variations of several peak-to-average power ratio (PAPR) reduction and HPA linearization techniques which were previously proposed for OFDM signals. Our main emphasis is on their applicability to the more general class of GMC signals, including serial modulation and DFT-precoded OFDM. Required power backoff is shown to depend on the type of signal transmitted, the specific HPA nonlinearity characteristic, and the spectrum mask which is imposed to limit adjacent channel interference. PAPR reduction and HPA linearization techniques are shown to be very effective when combined.

IMD 저감 방식을 기반으로 하는 OFDM 통신 시스템

Orthogonal Frequency Division Multiplexing(OFDM) 시스템은 주파수 선택적 페이딩에 강하고 높은 대역 효율을 갖는 통신 시스템이다. OFDM 신호는 높은 PAPR로 인하여 비선형 증폭기에서 왜곡이 생긴다. 그래서 비선형 왜곡 성분인 IMD(Inter-Modulation Distortion)를 저감하는 것이 중요하다. IMD 저감 방식은 비선형 왜곡에 대하여 PAPR(Peak-To-Average Power Ratio) 저감 방식보다 BER 성능을 좋게할 수 있지만, 송신기에 FFT(Fast Fourier Transform)가 추가되어 시스템 복잡도를 증가시킨다. 본 논문에서는 IMD 저감 방식을 기반으로 하는 OFDM 통신 시스템의 BER 분석과 복잡도를 저감하기 위한 연구를 하였다. SPW(Sub-Block Phase Weighting)을 적용한 IMD 저감 방식에서 계산량 감소를 위한 새로운 방식을 제안하고 기존의 방식과 계산 복잡도를 비교한다. SPW 방식은 입력 데이터를 여러 개의 서브 블록으로 나누고 위상 회전 벡터를 곱해줌으로써 PAPR을 감소하거나 IMD를 저감한다. 제안된 방식은 BER 성능 면에서 기존 방식과 비슷하지만, PAPR처럼 시간 영역에서 IMD의 전력량을 계산하므로 시스템의 계산 복잡도와 시스템 크기를 줄일 수 있는 장점이 있다. OFDM system has very good high spectral efficiency and the robustness to the frequency-selective fading. Because of the high PAPR, OFDM signals can be distorted in nonlinear HPA(High Power Amplifier). So, to overcome the nonlinear distortion, it is very important to reduce the IMD value. With respect to the BER performance, IMD reduction method is better than the PAPR reduction method. However, IMD reduction method has much more system complexity because of the additional FFT processor in transmitter. In this paper, we study the OFDM communication system based on the IMD reduction method using SPW method. A new IMD reduction method is proposed to reduce the computational complexity. SPW method is to divide the input OFDM data into several sub-blocks and to multiply phase weighting values with each sub-blocks for the reduction of PAPR or IMD. Unlike the conventional method, the system size and computational complexity can be reduced.

Predistortion and nonlinear detection for OFDM signals in the presence of nonlinear high power amplification

AbstractOrthogonal frequency division multiplexing (OFDM) is characterised by a large peak‐to‐average‐power ratio and an orthogonal signal design. As a consequence, OFDM is particularly vulnerable with respect to nonlinear distortions caused, for example by a nonlinear high power amplifier (HPA). In the presence of nonlinear high power amplification, OFDM suffers from crosstalk between all subcarriers. Furthermore, the power density spectrum is significantly widened.In this article, we simultaneously use a memoryless predistorter (PD) at the transmitter side and a nonlinear detector at the receiver side. The PD reduces the out‐of‐band power, whereas the nonlinear detector improves the bit error rate (BER). Since maximum‐likelihood detection is prohibitive, a novel reduced‐state symbol detector (RSSD) derived from the maximum‐likelihood detector is proposed. For a QPSK/OFDM system in the presence of a memoryless solid‐state power amplifier, it is shown that the PD is not only useful for spectral shaping, but also in order to reduce the computational complexity of the nonlinear detector and to provide more robustness, concerning an incomplete knowledge of the characteristics of the nonlinearity at the receiver. The overall raw bit error performance is shown to be close to that of a linear QPSK/OFDM system. Copyright © 2007 John Wiley & Sons, Ltd.

A New PAPR Reduction Technique of OFDM System with Nonlinear High Power Amplifier

High peak-to-average power ratio (PAPR) of the transmit signal is a major drawback of multicarrier transmission such as orthogonal frequency division multiplexing (OFDM). Most of recent works have focused on minimizing the PAPR of OFDM signal to enhance performance. However, after relating this issue with the specific characteristics of high power amplifier (HPA), we propose a new scheme based on iterative partial transmit sequence (I-PTS) scheme that uses the amount of OFDM signal's nonlinear distortion to be caused by high power amplifier (HPA) as a new measure. The proposed scheme can have simple subblock combining method by applying iterative PTS and then, a new measure which is called adaptive nonlinear estimator (ANE), is achieved by estimating the mean square error (MSE) between the OFDM signals before and after the nonlinear HPA. The one with the minimum MSE among OFDM candidate signals is selected for transmission. From the results, it can be seen that a proposed scheme based-iterative PTS can be easily implemented with low combining complexity, still maintaining a better BER performance, compared to the conventional PTS approach.

PAPR reduction of an OFDM signal by use of PTS with minimum nonlinear distortion

A new partial transmit sequence (PTS) scheme that uses the amount of OFDM signal's nonlinear distortion to be caused by high power amplifier (HPA) is proposed. However, since it may not be practical to know in prior the exact characteristics of HPA, a new measure which is called adaptive nonlinear estimator (ANE), is achieved to estimate OFDM signals after the nonlinear HPA by applying gradient algorithm. According to combining ANE and iterative PTS (I-PTS) with simple subblock combining method, the proposed scheme can transmit the OFDM signal with minimum nonlinear distortion among OFDM signal candidates. From the results, it can be seen that a proposed scheme based-iterative PTS can achieve better performance with lower computational complexity when compared to that of conventional PTS.

Experimental Evaluation of Predistortion Techniques for High-Power Amplifier

Modern communication systems suffer from the nonnegligible distortion caused by the inherent nonlinearity of high-power amplifiers (HPAs). As such, many methods have been proposed to mitigate this nonlinear effect. However, their performances are seldom compared, and their practicalities are rarely evaluated. In this paper, experimental setups to investigate two popular types of HPA systems have been developed: 1) the traveling wave tube amplifiers and 2) the solid-state power amplifiers. Experimental data were collected to evaluate four predistortion compensation techniques including the polynomial, rational function (RF), neural networks, and neurofuzzy methods. Using different performance measures, the experimental evaluation shows that the neuro approaches have a higher accuracy for some measures, but the tradeoff is a much higher computation cost. Overall speaking, the RF method seems to have the best balance between the compensation accuracy and complexity among the methods

Companding System Based on Time Clustering for Reducing Peak Power of OFDM Symbol in Wireless Communications

This paper proposes a novel compressing and expanding (companding) system for OFDM wireless communications that minimizes the compression distortion and so reduces the peak power of OFDM symbols. OFDM systems suffer from large variations in instantaneous peak power. Such transients distort the signals when they are passed through a nonlinear high power amplifier (HPA) prior to transmission. Existing companding systems are far from perfect since the receiver can not accurately estimate the degree of compression applied by the transmitter and thus can not regenerate the original signal by expansion; the key problems are the band pass filter (BPF), HPA, and the noise component enhanced by the channel compensation filter. In the proposed companding system, each symbol is divided into segments, and series of consecutive segments are grouped into clusters. Each cluster is multiplied by a weight equal to the inverse of the largest instantaneous power within the cluster. The receiver estimates the weight used for each time cluster. The weights for all clusters are averaged to mitigate the weight estimation error. As a result, the proposed expander can accurately estimate the weights used and thus well suppress the compression distortion.

PRE-DISTORSIÓN DIGITAL ADAPTATIVA PARA LINEALIZACIÓN DE AMPLIFICADORES EN SISTEMAS OFDM

Gran parte de los nuevos estandares para sistemas de comunicacion digital, concebidos para superar la creciente demanda por mayores velocidades de transmision y niveles de eficiencia espectral, consideran el uso de modulaciones de amplitud no constante tales como M-QAM en formatos de OFDM. Una de las limitaciones mas criticas para el funcionamiento de estos sistemas es la distorsion no lineal tipicamente introducida por los amplificadores de alta potencia presentes en la cadena de transmision. Este articulo resume la propuesta de un algoritmo de procesado de senal para la estimacion adaptativa de la caracteristica inversa de ganancia compleja de un amplificador no lineal en un esquema de pre/post-distorsion. Algunos resultados de simulaciones son presentados a fin de evaluar la efectividad del proceso de linealizacion propuesto y considerar posibles lineas futuras de investigacion.

Threshold IBO of HPA in the Predistorted OFDM Communication System

Orthogonal frequency division modulation (OFDM) system is very useful for the multi-path channel and highly bandwidth-efficient system. However, OFDM signal shows the high peak to average power ratio (PAPR) so that nonlinear distortion can happen in the high power amplifier (HPA). This paper studies BER performance variation according to input back off (IBO) values when the predistorter is used for the compensation for the HPA nonlinearity. In the case of the solid state power amplifier (SSPA) of nonlinearity parameter p=0.5 and IBO=0 dB, the system with predistorter is poorer than the one without predistorter. But this situation is inverted, if IBO is increased from 0 dB to 3 dB. So, we can find that there is a threshold value of IBO for the predistorter to effectively compensate for the nonlinear SSPA when the sub-carrier number and p value are changed. This threshold IBO value is proportional to the nonlinearity parameter p value but it is nearly independent of the sub-carrier number because amplitude and distribution are normalized. The OFDM system with predistorter can be improved only when thee IBO value is higher than the threshold value.

Adaptive control approach to nonlinearity predistortion of high power amplifier

A new time-domain adaptive predistortion scheme is presented for compensating nonlinearity of high power amplifiers (HPA) in OFDM systems. A power series model with memory (PSMWM) is proposed to approximate the HPA expressed by a complex Wiener-Hammerstein model (WHM) which can describe the input-output relationship of unknown HPA with linear dynamics. By using the PSMWM, the compensation input to HPA is calculated in a real-time manner so that the linearization from the predistortor input to the HPA output can be attained even if the nonlinear input-output relation of HPA is uncertain and changeable. The effectiveness of the proposed adaptive compensation scheme is validated by numerical simulation for 64QAM–OFDM systems and compared with other methods.