HBT power amplifier with dynamic base biasing for 3G handset applications

In this paper, a performance optimization of a digital signal processing driven and dynamically biased 3G handset's power amplifier technique is proposed, simulated and implemented. This technique uses a new dynamically base biasing heterojunction bipolar transistor which reduces the dc power consumption at low level drive and at the same time compensates the nonlinear distortion at high power drive of the PA in the transmitter of a universal mobile telecommunications system (UMTS) system with a high integrability. With the UMTS system, at low level drive, the dc power reduction is about 60% and at high emission power, the nonlinearity of the PA is corrected to respect the adjacent channel power ratio (ACPR) and error vector magnitude constraints imposed by the UMTS. With our system, the ACPR and efficiency of the power amplifier are improved, respectively, by 5 dB and 8%.     

Receiver-side nonlinearities mitigation using an extended iterative decision-based technique

This work presents an iterative receiver cancellation technique for mitigating the inband distortion introduced by a nonlinear wideband transmitter power amplifier (PA). The proposed decision-based technique employs a Wiener-Hammerstein model that accounts for the nonlinear transfer function and memory of the PA as well as for the wireless propagation channel. As such, the mitigation technique can be seen as a generalization of existing iterative decision-based techniques assuming memoryless PA nonlinearities. For successful distortion mitigation, the iterative technique requires an estimate of the nonlinear model that characterizes the PA. We propose to perform this model identification at the receiver, embedded in an iterative decision-based scheme, avoiding the nonideal analog-to-digital feedback loop associated with transmitter-based model identification. A stochastic algorithm is proposed for the model identification providing the necessary PA model parameters required for symbol detection. In addition, we analyze the convergence properties of the proposed technique. Simulation results confirm that the proposed mitigation technique provides distortion cancellation at almost the same level to the case of perfect knowledge of the PA model. These results enable us to employ power amplifiers with more relaxed linearity requirement, moving the operation point to a region with improved power efficiency while reducing the system overall degradation.     

A new unequal three-tone signal method for AM-AM and AM-PMdistortion measurements suitable for characterization of satellitecommunication transmitters/transponders

A new method for characterization of AM-AM and AM-PM distortion of power amplifiers (PAs) using only power measurements is proposed in this paper This technique was found to be suitable for the characterization of satellite communication transmitters/transponders. It consists of using an unequal three-tone signal (UTTS) to drive either the base-station PA's transmitter or the satellite transponder's PA and to measure the level of this UTTS at the output of the transmitter/transponder. By comparison of the three tone levels at the input and output of the transmitter/transponder, one can calculate the AM-AM compression factor and AM-PM conversion coefficient of the transmitter/transponder PA using closed-form expressions     

From post-distortion to pre-distortion for power amplifiers linearization

This letter presents a new method of digital adaptive pre-distortion for linearization of power amplifiers (PA). The method is derived from a post-distortion approach which identifies the PA inverse function. This approach leads to the minimization of a quadratic function of the polynomial coefficients in the case of a polynomial predistorter form and a least square criterion. We have compared our solution to a method previously proposed by Ghaderi (1996) that was also based on the transformation of a post-distortion into a pre-distortion system. We have tested our predistorter (along with a baseband adaptation of Ghaderi's) on OFDM Hiperlan signals. Both methods significantly reduced the signal distortion and the spectral regrowth. Our less complex approach proved to be even better for small peak back off values.     

Cross Cancellation TechniqueEmploying an Error Amplifier

In this letter, we propose a new distortion cancellation mechanism for a balanced power amplifier (PA) structure using the cross cancellation technique employing an error amplifier. The proposed cross cancellation balanced linear PA is implemented in the IMT-2000 ( 2.14GHz) band. With commercial PAs with a peak power of 240 W for base-station application, the proposed system shows 18.6 dB improvement at an average output power of 40 dBm for adjacent channel leakage ratio measurement with wideband code division multiple access 4FA signal. The efficiency of the proposed structure is about 2% higher than the conventional feedforward amplifier for modulated carrier.     

非线性失真环境下的天线功率分配算法

本文研究MIMO系统中,考虑功放的非线性限幅作用后,该如何在天线之间分配功率的问题。在这种情况下,采用传统的贪婪算法,信道增益小的天线上分配的功率可能很大,超过功放的线性范围,产生严重的非线性失真。不仅无法提高系统吞吐量,反而会导致吞吐量的恶化。通过对非线性失真环境下的系统吞吐量的分析,提出了一种改进的贪婪算法,综合考虑了无线信道引起的衰落和功放限幅引起的失真等因素,能够更加合理的分配天线功率。仿真结果也表明,新算法在信号功率较大的时候比传统的算法拥有更高的吞吐率。     

Digital Predistortion for Envelope-Tracking Power Amplifiers Using Decomposed Piecewise Volterra Series

Due to dynamic changes of supply voltage, envelope-tracking (ET) power amplifiers (PAs) exhibit very distinct characteristics in different power regions. It is very difficult to compensate the distortion induced by these amplifiers by employing conventional digital predistortion techniques. In this paper, by introducing a new piecewise Volterra model based on a vector threshold decomposition technique, we first set several thresholds in the input power level according to the PA characteristics, and decompose the input complex envelope signal into several sub-signals by using these thresholds. We then process each sub-signal separately by employing the dynamic deviation reduction-based Volterra series, and finally recombine them together to produce the predistorted output. Experimental results show that by using this new decomposed piecewise digital predistorter model, the distinct characteristics of the ET system at different signal power levels can be accurately modeled, and thus, the distortion, including both static nonlinearities and memory effects, caused by the amplifier nonlinear behavior can be effectively compensated.     

A predistortion linearizer using envelope-feedback technique withsimplified carrier cancellation scheme for class-A and class-AB poweramplifiers

A predistortion linearization method using an envelope-feedback technique is proposed and implemented in this paper. This linearizer compensates the gain and phase nonlinearity of power amplifier (PA) simultaneously by controlling both variable attenuator and phase shifter with the feedback of only the difference signal between input and output envelopes. A new carrier cancellation scheme composed of a minimization circuit, log detector, and vector modulator is also presented. This circuit achieves adaptive control of the linearizer by enabling direct measurement of out-of-band power. It is well suited to a multichannel system where the allocated channels are time variant. The principle of the proposed linearizer is described and simple AM-AM distortion analysis is presented analytically and graphically based on the conceptual schematic diagram. A two-tone test for a class-A PA at 1.855 GHz with frequency spacing of 1 MHz showed intermodulation-distortion reduction of maximum 16 dB and stable operation over 5-dB output power variation up to 4-dB backoff from the saturation power level. The proposed linearizer is also applicable to class-AB PA's without further special adjustments. The adaptation circuit is fully implemented with analog integrated circuits, which can further extend its applicability with the integration technology     

Feedback quantization in crosscorrelation predistorters

Amplification of signals with fluctuating envelopes inevitably leads to distortion because of nonlinear behavior of the power amplifier (PA). Digital predistortion can counteract these nonlinear effects. In this letter, the crosscorrelation predistorter is described and the effects of quantization in the feedback path are presented. One of the effects is that the quantization noise is correlated with the signal to be quantized, resulting in reduced performance of predistortion. A technique to reduce these effects is to inject a dither signal before quantization. Because of the quantization noise and dither signal, more data has to be used to obtain estimates of the PA behavior that are accurate enough for effective predistortion.     

射频放大器互调失真检测和载波抵消的研究

采用高功率射频信号内部互调失真的包络特性检测与比较技术,对射频功率带外信号进行检测和估计。设计用于高功率射频放大器前馈技术线性化的一个带外信号检测器。探讨如何进行射频载波抵消以及检测放大器互调失真信号。     

Predistorter Based on Frequency Domain Estimation for Compensation of Nonlinear Distortion in OFDM Systems

An orthogonal frequency-division-multiplexing signal is very sensitive to the nonlinear distortion of the power amplifier (PA) as a result of its high peak-to-average power ratio. Predistortion, which is an effective countermeasure for balancing off the nonlinearity of a PA, is usually necessary for the sake of mitigating the in-band distortion and the spectrum regrowth. In general, a feedback path is required to estimate the PA characteristic, and a memoryless polynomial is used in modeling the PA characteristic or constructing the predistorter. The polynomial coefficients are solved by least-square (LS) estimation or adaptive identification algorithms in the time domain. In this paper, we examine the estimation problem from the frequency domain and propose five predistortion schemes. The advantage of using frequency-domain estimation is that it is much easier to compensate the delay effect caused by the transmission and receiving filters in the feedback path. Two different criteria are used in the proposed algorithms. The first one is based on the minimization of the square error of the PA input, which is termed as the PA-input-LS criterion. The second one is based on the minimization of the square error between the input of the predistorter and the output of the PA, which is termed as PA-output-LS criterion. We also propose an easy method to cope with the delay effect caused by the transmission and receiving filters in the feedback path. The performances of the proposed schemes are compared in the simulation. The simulation results show that by using the proposed schemes, the power efficiency of the PA can be increased by at least 7 dB in the sense of total degradation for a practical IEEE 802.11a wireless local area network system with a 64-quadrature-amplitude-modulation signal constellation.     

High Efficiency RF Power Amplifier Designed With Harmonic Real-Time Active Load-Pull

In this work, a high efficiency p-HEMT radio frequency power amplifier (PA) is designed using a new multiharmonic real-time active load-pull using the large signal network analyzer. This technique synthesizes a large set of instantaneous load mismatches to quickly find the optimal harmonic impedances, so as to achieve high PA efficiency in a shortened design cycle. At 2 GHz a demo power amplifier implemented with a p-HEMT demonstrated a power added efficiency (PAE) of 68.5% for 18.0 dBm output power, while achieving a maximum PAE of 75% below the 1 dB compression point for 18.6 dBm output power.     

A hybrid digital/RF envelope predistortion linearization system for power amplifiers

This paper presents an adaptive wide-band digitally controlled RF envelope predistortion linearization system for power amplifiers (PAs). A field-programmable gate-array-based lookup table is indexed by a digitized envelope power signal, and instantaneously adjusts the input signal amplitude and phase via an RF vector modulator to compensate for the AM-AM and AM-PM distortion. The advantages of this predistortion architecture over conventional baseband digital approaches are that a 20%-33% wider correction bandwidth is achievable at the same clock speeds, and linearization can be performed without the need for a digital baseband input signal. The timing match between the input RF signal and predistorting signal, which is one of the critical factors for performance, was investigated and adjusted to obtain optimum performance. Using three-carrier cdmaOne and wide-band multitone signals, the linearization performances for a 0.5-W GaAs heterostructure field-effect transistor, a 90-W peak-envelope-power (PEP) silicon LDMOS PA, and a 680-W PEP LDMOS PA were examined. In addition, the predistortion performance variation for different signals was studied in terms of signal envelope statistics, output powers, and PA power capacities.     

Thermal memory effects modeling and compensation in RF power amplifiers and predistortion linearizers

Memory effects, which influence the performance of RF power amplifiers (PAs) and predistortion-based linearizers, become more significant and critical in designing these circuits as the modulation signal bandwidth and operation power increase. This paper reports on an attempt to investigate, model, and quantify the contributions of the electrical nonlinearity effects and the thermal memory effects to a PA's distortion generation, as well as how to compensate for these effects in designing baseband predistortion schemes. The first part of this paper reports on the development of an accurate dynamic expression of the instantaneous junction temperature as a function of the instantaneous dissipated power. This expression has been used in the construction of an electrothermal model for the PA. Parameters for the new proposed behavior model were determined from the PA measurements obtained under different excitation conditions (e.g., small-signal and pulsed RF tests). This study led us to conclude that the effects of the transistor self-heating phenomenon are more important under narrow-band signal (e.g., enhanced data for global evolution of global system for mobile communications) than for signals with wide modulation bandwidth (CDMA2000, Universal Mobile Telecommunications System). In the second part of this paper, the newly developed model has also been used to design a temperature-compensated predistortion function to compensate for these effects. The linearized PA output spectrum and error vector magnitude show a significant performance improvement in the temperature-compensated predistortion function over a memoryless predistortion. The results of these measurements that have been conducted on a 90-W peak lateral double-diffused metal-oxide-semiconductor PA are in agreement with those obtained from simulations using the developed PA and the predistorter models implemented in an ADS environment.     

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.     

A novel low-power transceiver topology for noncontact vital sign detection including the power management technique

In this paper, power management technique utilized in the direct down-conversion non-quadrature transceiver is presented for the low-power application of vital sign detection. The simultaneous switching noise (SSN) and overshoot and undershoot of the transient waveform distortion resulting from a pulse signal will give rise to interference with a vital sign signal. The pulse width, rise/fall time, and period of pulse bias are analyzed to mitigate the interference in this investigation. Significant issues about direct-current (DC) offset and noise confronted by the presented technique are addressed based on mathematical analysis. In radio-frequency (RF) transceiver architecture including power amplifier (PA), low-noise amplifier (LNA), and mixer, the current-reused (CRU) topology is utilized to achieve low DC power consumption. The post-layout simulation results exhibit that power consumption of the transceiver using the optimized pulse bias is reduced to 40% of the power consumption for transceiver applying the DC bias. In addition, DC offset and null detection point can be alleviated by tunable phase shifter.     

基于多路实值时延神经网络的大规模MIMO系统发射机射频失真补偿方案

大规模MIMO（Multiple-Input Multiple-Output）系统具有频谱效率高、系统容量大的优势。然而由于发射机射频元件存在着技术、工艺等方面的限制,大规模MIMO系统不可避免地存在着多种射频失真问题,成为制约系统性能的重要瓶颈。本文针对大规模MIMO系统中的功率放大器 (Power Amplifier,PA)非线性、同相/正交 (In-phase/Quadrature,I/Q)支路不平衡、射频链路串扰等多种射频失真问题,提出基于多路实值时延神经网络（Real-Valued Timed-Delay Neural Network, RVTDNN）的大规模MIMO系统发射机射频失真补偿方案。该方案使用多个RVTDNN预失真网络对发射信号进行预处理,补偿发射机的射频不理想特性,提高系统性能。此外,本文还提出了基于量子遗传算法的预失真网络超参数优化方案,与传统的基于遗传算法的网络超参数优化方案相比,该方案在种群数较小时可实现超参数优化,从而降低了算法的时间复杂度。      

A new PWM controller with one-cycle response

This paper proposes a new nonlinear control technique that has one-cycle response, does not need a resetable integrator in the control path, and has nearly constant switching frequency. It obtains one-cycle response by forcing the error between the switched variable and the control reference to zero each cycle, while the on and off pulses of the controller are adjusted each cycle to ensure near constant switching frequency. The small switching frequency variation due to changes in the reference signal and supply voltage and delays in the circuit are quantified. Using double-edge modulation, the switching frequency variation is further reduced, thus, the associated signal distortion is minimized. An experimental 0-20 kHz bandwidth 95 W RMS power audio amplifier using the control method demonstrates the applicability of this control technique for high-fidelity audio applications. The amplifier has a power supply ripple rejection (PSRR) of 63 dB at 120 Hz. Additionally, the total harmonic distortion plus noise (THD+N) is less than 0.07% measured with a power supply ripple of 15%     

Low sampling rate technique based frequency-domain random demodulation for broadband digital predistortion

This paper proposes a combination technique of the frequency-domain random demodulation (FRD) and the broadband digital predistorter (DPD). This technique can linearize the power amplifiers (PAs) at a low sampling rate in the feedback loop. Based on the theory of compressed sensing (CS), the FRD method preprocesses the original signal using the frequency domain sampling signal with different stages through multiple parallel channels. Then the FRD method is applied to the broadband DPD system to restrict the sampling process in the feedback loop. The proposed technique is assessed using a 30 W Class-F wideband PA driven by a 20 MHz orthogonal frequency division multiplexing (OFDM) signal, and a 40 W GaN Doherty PA driven by a 40 MHz 4-carrier long-term evolution (LTE) signal. The simulation and experimental results show that good linearization performance can be achieved at a lower sampling rate with about 24 dBc adjacent channel power ratio (ACPR) improvement by applying the proposed combination technique FRD-DPD. Furthermore, the performance of normalized mean square error (NMSE) and error vector magnitude (EVM) also has been much improved compared with the conventional technique.