Understanding linearity in wireless communication amplifiers

This paper investigates the linearity of active devices and amplifiers that have modulated input signals. We describe an implementation of a mathematical technique for calculating spectral regrowth due to the nonlinear amplification of modulated signals typically used in wireless communication systems. This technique only requires knowledge of the single-tone gain and phase data as a function of input power and can be applied to any modulated signal given (a) the device or amplifier characteristics do not change significantly over the bandwidth of the input signal and (b) the modulation frequencies are much less than the carrier frequency. Verification of the mathematical technique is presented using examples of measured and calculated spectra for π/4-differential quadrature phase shift keying (DQPSK) personal handy phone system (PHS) and code division multiple access (CDMA) Offset-QPSK personal communication system (PCS) modulation schemes. This calculation technique is essential in determining fundamental tradeoffs between device performance (linearity, output power, gain, and power-added efficiency) and device bias conditions and load impedance     

Genetic algorithm optimization of a hybrid analog/digital predistorter for RF power amplifiers

This study presents a genetic algorithm optimization of a hybrid analog/digital predistorter, in order to reduce the intermodulation distortion (IMD) caused by the nonlinear properties of the radio frequency (RF) power amplifier (PA). Designed predistorter based on polynomial work function and the coefficient of the polynomial is optimized in order to reduce IMD by spectrum monitoring. The design procedure and validation of predistorter have been carried out by Agilent-ADS2005A. In order to validate the predistorter two different modulation schemes as CDMA and 16-QAM have been used. Also in order to verifying the linearization a test power amplifier circuit has been examined including Motorola MOSFET MRF9742 showing the nonlinear characteristics with memory. Simulations have been shown that adjacent channel power ratio (ACPR) improvements were acceptable for both CDMA and 16-QAM modulation schemes.     

LINC power amplifier combiner method efficiency optimization

Linear amplification using nonlinear components (LINC) is a method of vector summing two constant amplitude phase-modulated signals to achieve power amplification. The theoretical efficiency of the LINC power amplifier has been reported as 100% since highly efficient nonlinear constant amplitude amplifiers can be used. However, the 100% efficiency performance is only possible at one or two loads along the power output curve. The bulk of the papers regarding LINC has focused on clever implementations of the signal vector decomposition as well as methods to achieve highly linear signal separation. There has been little regard in the literature to the signal combiner implementation necessary to achieve the high power-added efficiency (PAE) of the LINC radio frequency (RF) power amplifier. Efficiency is not an intrinsic property of the combiner implementations, however, the combiner method is the single biggest contributor to efficient performance of a LINC RF power amplifier. This paper develops an analysis method that determines the efficiency of the LINC power amplifier as a function of the amplitude modulation statistics. This can be employed to design the RF communication system amplitude modulation characteristics and to tradeoff and optimize the RF transmitter PAE     

High Average-Efficiency Multimode RF Transmitter Using a Hybrid Quadrature Polar Modulator

This paper presents a hybrid quadrature polar modulator (HQPM) to drive the power amplifier (PA) highly efficiently in a wireless RF transmitter required for multimode operation. For enhancing the transmit efficiency, a switching-mode PA realized as Class-E design is used in the transmitter. The HQPM consists of a quadrature modulator for processing the RF modulated carrier and a Class-S modulator for processing the supply-voltage signal. The quadrature modulator and the Class-S modulator deliver the output signals with proportional envelope variation before being inserted into the RF-input terminal and the supply-voltage terminal of a Class-E PA, respectively, causing the double envelope modulation to distort the modulated RF signal at the PA output. Therefore, a digital predistorter is embedded in the HQPM for compensation. The proposed HQPM-based transmitter can help reducing the average dc and input RF powers and the output feedthrough levels so as to enhance power added efficiency and adjacent channel power rejection remarkably.     

基于面积等效原理的数字射频调制策略

为了驱动后级开关功放,全数字发信机需要利用数字射频调制器将输入信号转换为相应的脉冲序列信号.针对现有脉宽调制（PWM）和Delta-Sigma调制（DSM）策略在调制性能及系统实现方面存在的不足,本文利用面积等效原理提出一种新的数字射频调制策略.理论分析和仿真结果表明,本文策略相比PWM和DSM,不仅简化了硬件实现难度,而且能够获得更优的带内SNR和编码效率性能.     

A Discrete‐Amplitude Pulse Width Modulation for a High‐Efficiency Linear Power Amplifier

A new discrete‐amplitude pulse width modulation (DAPWM) scheme for a high‐efficiency linear power amplifier is proposed. A radio frequency (RF) input signal is divided into an envelope and a phase modulated carrier. The low‐frequency envelope is modulated so that it can be represented by a pulse whose area is proportional to its amplitude. The modulated pulse has at least two different pulse amplitude levels in order that the duty ratios of the pulse are kept large for small input. Then, an RF pulse train is generated by mixing the modulated envelope with the phase modulated carrier. The RF pulse train is amplified by a switching‐mode power amplifier, and the original RF input signal is restored by a band pass filter. Because duty ratios of the RF pulse train are kept large in spite of a small input envelope, the DAPWM technique can reduce loss from harmonic components. Furthermore, it reduces filtering efforts required to suppress harmonic components. Simulations show that the overall efficiency of the pulsed power amplifier with DAPWM is about 60.3% for a mobile WiMax signal. This is approximately a 73% increase compared to a pulsed power amplifier with PWM.     

非线性功放对数字调制信号的影响研究

功放的非线性会带来调制信号的性能恶化,基于simulink建模,对比仿真了恒包络调制MSK和非恒包络调制BPSK,在功放线性和非线性区域时的误码性能,以及在带宽受限条件下,相邻信道干扰对信号误码性能影响.仿真结果表明,功放饱和对非恒包络调制信号影响更大,尤其是信道间隔较小的情况下,非恒包络调制信号的误码性能恶化严重.该研究为数字通信系统的工程设计提供了一种可借鉴分析方法.     

Characterizing the effects of nonlinear amplifiers on linearmodulation for digital portable radio communications

The design and performance of linear modulation techniques and RF power amplifiers are discussed. Various types of power amplifier are used in the comparison, using simulation, of out-of-band radiation and power efficiency. The designs focus on high-frequency silicon bipolar power amplifiers suitable for low-power portable radio applications. Two different 4-QAM (4-level quadrature amplitude modulation) schemes are compared. Power efficiency as high as 40% can be achieved using a dynamically biased amplifier operating near saturation. The effects of nonlinear distortions from power amplifiers on adjacent channel interference as well as bit and block error rate performance are also investigated     

QAM and QPRS digital broadband cable systems

The performance of high-level modulation schemes in non-linear broadband digital communication systems is investigated. The tolerance of 16-, 64-, 256-QAM and 49-, 225-QPRS schemes to these non-linearities is given in terms of their phase margins. Our research results show that high level QAM schemes are less sensitive to non-linearities when operated in offset mode. Comparison of non-linearly amplified broadband QAM and QPRS schemes reveals that QPRS schemes need less carrier-to-noise ratio (CNR) than corresponding QAM schemes for identical operating conditions. Also, when the non-linearities are not severe, the degradation in performance is controlled by AM/PM characteristic of the broadband amplifier.     

射频功率放大器数字预失真技术及其发展趋势

3G无线通信系统对功率放大器的设计提出了更加严格的要求。为了有效地利用宝贵的频谱资源,功率放大器既要保证高线性度以减小带外发射功率以及误码率,又要有较高的效率以减少电源消耗。在众多功放线性化高效率技术中,数字预失真技术被认为是最有效的解决方案,受到越来越多的关注。本文对数字预失真技术作了详细介绍,结合实例分析了当前主要的几种预失真模型和对应的预失真方案,并展望了Doherty放大器技术与数字预失真结合的应用方向。     

Amplitude modulated RF fields stemming from a GSM/DCS‐1800 phone

Experiments which investigate possible effects arising from handheld phones employing amplitude modulated Radio Frequency (RF) fields, must deploy an excitation field which matches the true situation. Amplitude modulation arising from digital systems such as Global System for Mobile communication (GSM) or Digital Enhanced Cordless Telecommunications (DECT) is more complicated than just bursting of RF fields with a certain repetition rate and a certain duty cycle. In the GSM system the RF field is amplitude modulated with both a 217 Hz and an 8 Hz repetition rate. Furthermore, when the GSM land mobile network allows Discontinuous Transmission (DTX) the amplitude modulation rate can be as low as 2 Hz. Also the transmitted power may change in a complex manner. Extremely Low Frequency (ELF) magnetic fields connected with the current drawn from the battery of a handheld phone are investigated and measured, as a function of both time and frequency, and it is shown that the behavior of the current drawn is nearly the same as the one derived for the amplitude modulation of the RF power. This paper does not explain all details of the GSM system, but investigates the system in order to establish an experimental setup, which matches the real fields stemming from GSM mobile transmitters. For the purpose of comparison with digital cordless systems employing Time Division Multiple Access (TDMA) some figures of power and ELF magnetic field values are included.     

Traveling-wave tube amplifier performance evaluation and design optimization for applications in digital communications with multilevel modulations

In this paper, we demonstrate the use of a power margin as a figure-of-merit for evaluating the performance and optimizing the design of traveling-wave tube amplifiers (TWTAs) used in digital communication applications with multilevel modulations. The power margin is a system-level measure that balances both device efficiency and nonlinear distortion and provides a more direct prediction of the system-level performance of power amplifiers than device-level measures such as device efficiency or error-vector-magnitude. We calculate the power margin for M quadrature amplitude modulation for an existing TWTA to demonstrate the setting of an optimal amplifier operating drive level according to the criterion of the maximum power margin. The power margin can also be used to compare the performance of different traveling-wave tube (TWT) configurations. We compare the calculated power-margin performance for helix TWT circuits optimized with different optimization goal functions using the helix TWT design code CHRISTINE. The goal functions used in the optimization of the TWT circuits include AM/PM optimization, complex gain optimization, efficiency optimization, and a new digital goal function optimization. The digital goal function is shown to provide an enhanced power margin compared to the other three goal functions and demonstrates the potential of TWT device design optimization from a system perspective.     

A compact large signal model of LDMOS

Lateral double-diffused MOSFETs (LDMOS) are becoming more popular in RF power amplifiers for wireless communication applications. The understanding of non-linearity in LDMOS is critical in order to design ultra-linear power amplifiers to meet the stringent needs of current wireless systems. We have developed a compact large signal model that can predict accurately the transconductance of the device and its higher order derivatives. Such large signal model is needed in the accurate simulation of non-linear circuits. Our device measurements show that the higher order derivatives of the transconductance are very sensitive to the gate bias. The model has been applied to simulate the gain and third order intermodulation distortion in a RF LDMOS amplifier, and the simulated results agree well with the experimental measurements.     

Fast complex memory polynomial-based adaptive digital predistorter

Today's 3G wireless systems require both high linearity and high power amplifier (PA) efficiency. The high peak-to-average ratios of the digital modulation schemes used in 3G wireless systems require that the RF PA maintain high linearity over a large range while maintaining this high efficiency; these two requirements are often at odds with each other with many of the traditional amplifier architectures. In this article, a fast and easy-to-implement adaptive digital predistorter has been presented for Wideband Code Division Multiplexed signals using complex memory polynomial work function. The proposed algorithm has been implemented to test a Motorola LDMOSFET PA. The proposed technique also takes care of the memory effects of the PA, which have been ignored in many proposed techniques in the literature. The results show that the new complex memory polynomial-based adaptive digital predistorter has better linearisation performance than conventional predistortion techniques.     

A New Two-Branch Amplification Architecture and its Application with Various Modulated Signals

This paper proposes a new two-branch amplification architecture that combines baseband signal decomposition with RF front-end optimization. In the proposed architecture, the filtered modulated signals are separated into two components that are then amplified independently and combined to regenerate an amplified version of the original signal. A branch with an efficient amplifier transmits a low-varying envelope signal that contains the main part of the information. Another branch amplifies the residual portion of the signal. The baseband decomposition and parameters of the RF part are optimized to find the configuration that gives the best power efficiency and linearity. For M-ary quadrature amplitude modulation （M-QAM） signals, this technique is limited in terms of power efficiency. However, for filtered continuous phase modulation （CPM） signals, especially for minimum shift keying （MSK） and Gaussian MSK （GMSK） signals, high power efficiency can be achieved with no significant impact on the overall linearity. The results show that this technique gives better performance than the single-ended ctass-B amplifier.     

卫星通信中不同调制信号的预失真研究

卫星通信技术近年来发展较快,承载的通信业务也较多,导致频谱资源紧张,功率资源受限,迫切需要采用高阶调制、多载波传输等方式来缓解通信资源的紧张,QPSK、8PSK、16QAM等更高阶调制方式在卫星通信中将会获得越来越广泛的应用。这类信号直接通过功放后,信号产生失真,影响接收端信号的正确解调,预失真技术是近年来补偿功放非线性的最有效手段之一。在研究这几种高阶调制信号特点的基础上,对几种调制方式的信号分别进行预失真处理,并分析各信号的抗噪声性能,总结出16QAM信号在卫星通信中的应用会缓解目前通信资源紧缺问题。     

Effect of conducted EMI on the DC performance of operationalamplifiers

Electromagnetic interference can affect the performance of operational amplifiers even when the frequency is well above the usable frequency range of the amplifier. This can result in an apparent change in offset voltage. This effect is investigated experimentally with a number of device types. An attempt was made to compare the DC performance of various operational amplifiers in the presence of conducted impulsive EMI, as might be experienced in mixed analogue and digital circuitry. This Letter describes the test procedure and notes the results obtained     

High-efficiency power amplifier using dynamic power-supply voltagefor CDMA applications

Efficiency and linearity of the microwave power amplifier are critical elements for mobile communication systems. This paper discusses improvements in system efficiency that are obtainable when a DC-DC converter is used to convert available battery voltage to an optimal supply voltage for the output RF amplifier. A boost DC-DC converter with an operating frequency of 10 MHz is demonstrated using GaAs heterojunction bipolar transistors. Advantages of 10 MHz switching frequency and associated loss mechanisms are described. For modulation formats with a time-varying envelope, such as CDMA, the probability of power usage is described. Gains in power efficiency and battery lifetime are calculated. An envelope detector circuit with a fast feedback loop regulator is discussed. Effects of varying supply voltage with respect to distortion are examined along with methods to increase system linearity     

Hybrid frequency-polarization shift-keying Modulation for optical transmission

In this paper, we propose a new modulation scheme based on combining frequency and polarization modulated signals, which we will refer to as hybrid frequency-polarization shift keying (FPolSK). The FPolSK modulation is basically an extension of the conventional M-PolSK modulation over orthogonal domains. This expansion enables representing signal constellation points over multidimensional space, which ensures increasing the geometric distances between these points, and in turn, improving the system power efficiency. On the other hand, compared with M-FSK modulation, FPolSK improves the bandwidth efficiency by employing less number of orthogonal frequencies to represent information symbols. Moreover, FPolSK is extremely useful for implementing communication systems that have limitations in power and bandwidth usage. This advantage comes from the fact that FPolSK inherently enables selecting the appropriate number of orthogonal frequencies that convey with system constraints. The contribution in this paper is threefold. First, we propose a design for the transmitter and the receiver of the FPolSK technique. Second, we perform analysis for the system power and bandwidth efficiencies. Third, we derive an expression for the system power spectral density (PSD). A performance comparison between the FPolSK modulation technique and previously developed techniques is also presented in this paper. Our results reveal that the proposed modulation scheme performs better than M-PolSK, M-DPSK, and M-FSK modulation schemes in terms of both power and bandwidth efficiencies. We have also found that same bandwidth efficiency can be obtained using different FPolSK modulation formats, and the PSD of the FPolSK modulation does not contain discrete components that are considered as a waste of power. Finally, the effects of the laser phase noise and fiber dispersion on the performance of the proposed modulation are also discussed in detail.     

功放设计中的检测及保护电路

射频功放作为通信系统的最末级,因大功率而极易损害.由于功率放大器是一种相对比较昂贵,且为比较脆弱的器件,因此在设计功率放大器时,应重点关注如何保护功率放大器,以避免静电、浪涌、过热过温、过压、过流、过载造成功放故障或者失效.本文详细分析了射频功率放大器的失效因为,并列举了工程上常采用的保护方式,通过模型分析,构建了功放的检测电路及保护电路,从而达到保护射频功放的目的.