Joint Polynomial and Look-Up-Table Predistortion Power Amplifier Linearization

Digital predistortion at baseband is an efficient and low-cost method for the linearization of a power amplifier (PA) in a wireless system employing a nonconstant-envelop modulation scheme, so as to reduce the adjacent channel interference. The polynomial and the look-up table (LUT) predistortion schemes are two commonly used approaches. However, in each of the two approaches, to reach a satisfactory adjacent channel power ratio (ACPR) in the PA output signal, people usually end up with a complex system having the involved algorithms converge rather slowly. In this brief, we propose a low-complexity joint-polynomial-and-LUT predistortion PA linearizer, where the two mutually dependent predistortion schemes can skillfully help each other. Simulation results show that the proposed joint linearizer can reduce the algorithm convergence time while achieving an excellent ACPR.     

Comparison of advanced coding and modulation schemes for multicarrier satellite communication systems

Digital satellite communication systems use coherent quaternary PSK (QPSK) as the preferred modulation scheme. In recent years, however, the search for bandwidth and power-efficient digital modulation methods has become an active research area. In this paper the performances of combined coding and modulation schemes employing multilevel signal alphabets are considered, in the framework of a system model which reasonably approximates the INTELSAT SCPC system. The main goal of the research was to investigate whether the efficiency of coded modulation schemes and their savings in transmitted power could be maintained in a system environment dominated by interference more than by the additive white Gaussian noise. A second goal was to compare the performance of signals with constant envelope, such as multilevel PSK, with that of signals allowing variations in amplitude, such as multilevel PSK, with that of signals allowing variations in amplitude, such as multilevel QAM. In the paper, we propose a channel model which is reasonably close to the real system, and a method for performance evaluation leading to an estimate of the asymptotic power gain of the modulation systems. The channel model and the method are applied to the analysis of coded 16- and 32- PSK and QAM.     

Direct sequence spread spectrum Walsh-QPSK modulation

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel     

射频功放单元ADEF线性化技术的理论分析及模型仿真

本文从调制的角度，对由调幅-调幅转换、调幅-调相转换产生射频功放单元互调干拓的机理进行了分析，推导了调幅-调幅转换系数与信号对三阶互调干扰抑制比（C/I3）的函数关系式。并在这个理论基础上，采用ADEF，即自适应双包络反馈线性化技术对射频功放单元的非线性特性进行补偿。最后，建立了自适应双包络反馈线性电路的模型并进行了仿真，结果表明，由于此种技术的采用，C/I3的值达到了60dB左右。     

FPGA Implementation of a Power Amplifier Linearizer for an ETSI-SDR OFDM Transmitter

Most satellite digital radio （SDR） systems use orthogonal frequency-division multiplexing （OFDM） transmission, which means that variable envelope signals are distorted by the RF power amplifier （PA）. It is customary to back off the input power to the PA to avoid the PA nonlinear region of operation. In this way, linearity can be achieved at the cost of power efficiency. Another attractive option is to use a linearizer, which compensates for the nonlinear effects of the PA. In this paper, an OFDM transmitter conforming to European Telecommunications Standard Institute SDR Technical Specifications 2007-2008 was designed and implemented on a low-cost field-programmable gate array （FPGA） platform. A weakly nonlinear PA, operating in the L-band SDR frequency, was used for signal transmission. An adaptive linearizer was designed and implemented on the same FPGA device using digital predistortion to correct the undesired effects of the PA on the transmitted signal. Test results show that spectral distortion can be suppressed between 6-9 dB using the designed linearizer when the PA is driven close to its saturation region.     

Ka波段高精度模拟预失真线性化器

模拟预失真技术是改善行波管放大器非线性失真的一种有效方法,但补偿精度较低的缺点是制约其进一步发展的关键因素。增益相位独立调节技术和补偿曲线形状调节技术是提升模拟预失真补偿精度的重要技术。提出了一种适用于Ka波段行波管放大器的高精度模拟预失真器,该预失真器采用双路矢量合成式结构,在29~31 GHz 范围内,通过调节二极管偏置电压可以同时实现补偿曲线形状调节和增益相位扩张量独立调节,有效提升了补偿精度。与行波管放大器的联合测试结果表明,在30 GHz 时,该预失真器可以将行波管放大器的增益压缩从5.3 dB 减小到1.2 dB,相位偏移从62°减小到6.5°。线性化后的行波管放大器的非线性失真明显降低,在输出功率回退5 dB 时,三阶互调系数提高了9.3 dB。     

Impact of a New TWTA Linearizer Upon QPSK/TDMA Transmission Performance

In transmitting a band-limited PSK/TDMA signal via satellite, the signal is distorted due to the nonlinearities of an earth station HPA and a satellite TWTA. To reduce the impairments due to these nonlinearities, a new TWTA linearizer, called a soft-limiter type linearizer (SL-LRZ), has been developed. The overall amplitude characteristic of a TWTA associated with an SL-LRZ is linear up to saturation and has a constant envelope beyond saturation. As to the AM/PM conversion characteristic, it has almost zero phase shift, irrespective of the operating point. The TWTA with SL-LRZ gives better bit error rate (BER) performance and less out-of-band emission (OBE) compared to cases where TWTA is operated by itself or with a conventional linearizer. In this paper, the impact of the new TWTA linearizer upon the transmission performance of a QPSK/TDMA signal is described. The transmission performance over a nonlinear satellite channel has been evaluated experimentally using a 120 Mbit/s QPSK modem and an INTELSAT V channel simulator.     

On the representation of CPM signals by linear superposition ofimpulses in the bandpass domain

A novel signal generation concept for continuous phase modulations (CPMs) with modulation index 1/2 based on real impulses is presented. With this concept, bandpass CPM signals can be generated directly in one step instead of the two consecutive steps, namely, the generation of the complex envelope and the modulation of the carrier by the complex envelope, which are necessary in conventional signal generators. Mathematical expressions for both the real impulses and the bandpass CPM signals are derived and a simple modulator structure is discussed. Examples for the real impulses are given. Among these are the well-known CPM schemes of minimum shift keying (MSK), sinusoidal frequency shift keying (SFSK), and Gaussian minimum shift keying (GMSK). As an example, the validity of the novel signal generation concept is shown for the latter CPM scheme     

Novel linearizer using balanced circulators and its application tomultilevel digital radio systems

A miniaturized RF predistortion linearizer for a GaAs field-effect transistor power amplifier applicable to 256 quadrature-amplitude modulation (QAM) digital microwave systems is presented. This linearizer, which is based upon the cuber linearizer technique, utilizes circulators and a pair of diodes in the distortion generator to obtain high signal component isolation, and in the variable phase shifter to compensate for temperature variations. This allows miniaturization and easy adjustment of the circuit. Results show that a distortion reduction of more than 10 dB is obtained over a 300 MHz bandwidth. It was verified that distortion reduction can be achieved over a temperature range from 0 to 50°C. The fundamental characteristics of the linearizer and its effect on the 256-QAM signal are shown. The results show an improvement of more than 6 dB in the output back-off of the amplifier     

Measurement and simulation of memory effects in predistortionlinearizers

Detrimental effects of memories in linearizers are described, and two efficient techniques are presented to measure them. These techniques allow a real-time adjustment and correction of the linearizer and/or amplifier circuits and an overall improvement of the intermodulation performance over a wide envelope frequency band. This is especially important for broadband applications, where high efficiency and linearity are required, and where memory effects, if not detected and properly corrected, may significantly degrade the system's performance. Experimental results for a 20-W linearized L-band power amplifier are presented to illustrate the effects of memory. The two techniques-a dynamic AM/AM and AM/PM measurement system and a pulse power measurement system-are described. A simulation approach that can be used to predict the influence of memory effects in practical systems is illustrated for nonlinear amplifiers. Adaptive linearization techniques are discussed     

Linearity Improvement of a Power Amplifier Using a Series LC Resonant Circuit

A radio frequency power amplifier microwave monolithic integrated circuit with a series LC resonant circuit as well as a bias control circuit for wide-band code division multiple access application is presented. The linearizer that consists of a series LC resonant circuit and base-emitter junction of a bias transistor operates as a diode rectifier circuit. A comparison between the circuits with and without the linearizer has been demonstrated. The power amplifier (PA) with the series LC resonant linearizer exhibits adjacent channel leakage ratio-1 (ACLR1) of -37.2 dBc at output power of 27 dBm, a 5.6 dB improvement compared to the circuit without the linearizer. The bias control circuit reduces consumed average dc current from 83 mA to 57 mA for efficiency improvement. The linearized PA exhibits 1-dB compression point (P₁dB) of 29.3 dBm, power-added efficiency of 45.7%, and power gain of 20.6 dB at low quiescent current of 37 mA with a 3.4 V single supply.     

Quantization analysis and design of a digital predistortionlinearizer for RF power amplifiers

Significant improvements in terms of reduced power consumption and increased bandwidth are obtained if a digital predistortion linearizer is implemented with an application specific digital signal processor. This paper investigates the quantization effects in different parts of a table based complex gain predistortion linearizer. The analysis can be used to optimize the predistortion linearizer with respect to word length based on the knowledge of the RF amplifier gain characteristic, the probability density function for the modulation scheme and the maximum allowable adjacent channel interference level. A predistorter chip is described that has been designed using the analysis. The chip has been fabricated and tested. Compared with a standard digital signal processing (DSP) solution it provides seven times higher bandwidth but consumes only 10% of the power     

High-Power EMI on RF Amplifier and Digital Modulation Schemes

We present the performance of an RF amplifier and digital modulation techniques in the presence of high-power electromagnetic interference (EMI) to provide existing and next generation communication systems with critical information. An advanced measurement setup comprised of a large-signal network analyzer is used to characterize the adverse effects of EMI on the device characteristics of an RF power amplifier and the performance of digital modulation schemes. Furthermore, our analysis incorporated hybrid numerical tools, such as the hybrid S-parameter method to carry out an extensive EMI analysis of digital modulation schemes in the presence of complex structures, such as cylindrical cavities. Our studies yield critical information for the communication systems. For instance, our analysis suggests that digital modulation schemes are more susceptible to EMI than the RF power amplifier that processes the modulated signals. Power levels of the order of megawatts are required to have a notable impact on the device characteristics of an RF amplifier in the presence of a missile-like body, whereas, much lower power levels are sufficient to degrade the performance of a digital modulation scheme as long as it is within the bandwidth of the modulated signal. Our analysis further indicates that nonconstant envelope digital modulation schemes are more susceptible to EMI.      

High-efficiency class-A power amplifiers with a dual-bias-controlscheme

A new scheme for power amplifiers is proposed, which can provide both high efficiency and linearity. The proposed amplifier operates in a virtual class-A mode under dual-bias control to maximize the power-added efficiency along with its inherent class-A linearity. The dynamic dual-bias control involves controlling both bias current and voltage of the amplifier with a varying envelope of input RF signals. The efficiency of the proposed amplifier is theoretically evaluated and compared with that of other conventional amplifier schemes. Based on theoretical analyses, several promising schemes for dual analog and digital bias control are proposed and discussed     

DS-CDMA chip waveform design for minimal interference underbandwidth, phase, and envelope constraints

This paper investigates the effect of chip waveform shaping on the error performance, bandwidth confinement, phase continuity, and envelope uniformity in direct-sequence code-division multiple-access communication systems employing offset quadrature modulation formats. An optimal design methodology is developed for the problem of minimizing the multiple-access interference power under various desirable signal constraints, including limited 99% and 99.9% power bandwidth occupancies, continuous signal phase, and near-constant envelope. The methodology is based on the use of prolate spheroidal wave functions to obtain a reduced-dimension discrete constrained optimization problem formulation. Numerous design examples are discussed to compare the performance achieved by the optimally-designed chip waveforms with other conventional schemes, such as offset quadrature phase-shift keying, minimum-shift keying (MSK), sinusoidal frequency-shift keying (SFSK), and time-domain raised-cosine pulses. In general, it is found that while the optimized chip pulses achieved substantial gains when no envelope constraints were imposed, these gains vanish when a low envelope fluctuation constraint was introduced. In particular, it is also shown that MSK is quasi-optimal with regard to the 99% bandwidth measure, while the raised-cosine pulse is equally good with both the 99% and 99.9% measures, but at the expense of some envelope variation. On the other hand, SFSK is quasi-optimal with regard to the 99.9% bandwidth occupancy, among the class of constant-to-low envelope variation pulses     

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     

射频预失真器与基带预失真算法结合对行波管功率放大器线性化改善的影响

为了保证自适应算法的效果,一般会将行波管输出功率进行一定回退,但是会降低行波管的工作效率。通过在行波管前端加入射频预失真器,在额定输入功率范围内,线性化后的行波管的非线性失真现象得到了一定程度改善,然后再进行基带自适应算法的实现。X波段行波管在输出功率回退较少的同时,达到较好的非线性失真改善效果。     

Multicarrier-Based QAPM Modulation System for the Low Power Consumption and High Data Rates

Low power consumption and high data rate are the most important requirements for the communication system. Especially, very low power consumption modulation method is required for the short range communication systems such as the medical implantable communication devices or capsule endoscope, and so on. For the higher data rate, we like to combine the OFDM system into the QAPM since the OFDM system has higher bandwidth efficiency than a single-carrier system. In this paper, we like to propose a QAPM (Quadrature amplitude position modulation) method combined with the orthogonal frequency-division multiplexing (OFDM) system. Next, we analyze the performance of three low-power-consumption modulation schemes: the phase shift position modulation (PSPM), phase silence shift keying (PSSK), and QAPM using orthogonal frequency-division multiplexing (OFDM) system in the multi-path channel. These schemes have lower bandwidth efficiency and the higher power efficiency than the existing phase-shift keying (PSK) and quadrature amplitude modulation (QAM) schemes. It can be shown that they can achieve greater power efficiency because every modulation symbol has a zero-envelope period as in pulse-position modulation (PPM) techniques. Finally, we compare the performances of the PSPM, PSSK, and QAPM modulation combined with the OFDM system with regard to bit error rate performance and throughput.     

Experimental Results on Constant Envelope Signaling with Reduced Spectral Sidelobes

Considerable recent attention has been given to the use of pulse modification to reduce spectral sidelobes in constant envelope data transmission within the "offset" modulation format that includes offset QPSK and MSK. Analytic techniques have been developed which predict probabilistically (without dependence on simulation) the power spectral density that results from pulse modification combined with hard limiting where necessary to achieve constant envelope. Here the computational results are compared with experimental results. Agreement between theory and experiment is seen to be good. The implications for bandwidth efficiency and communication efficiency are discussed.     

Increasing the talk-time of mobile radios with efficient lineartransmitter architectures

Power efficiency of the RF transmitter stage in mobile handsets has a direct impact on battery capacity as well as talk-time between battery recharge cycles. Unfortunately the power amplifier (PA) linearisation methods in common usage today to meet the linearity specification of standards such as GSM EDGE, TETRA and UMTS result in low DC to RF efficiency. This paper discusses the relationship between linearity and transmitter efficiency and reviews various linearisation schemes: Cartesian loop, polar loop, 2nd harmonic injection, envelope elimination and restoration (EER), predistortion methods, and synthesis techniques (e.g. LINC and CALLUM). The paper also shows that to increase the power efficiency (and thus talk-time) hybrid solutions to power amplifier linearisation are required and these are also discussed