Spectral Regrowth Research Articles

Analysis on spectral regrowth of chirp bandwidth expansion technique in high‐resolution SAR system

AbstractThis article presents an analysis of the root causes of the spectral regrowth due to the higher‐order intermodulation in a chirp bandwidth expansion scheme using a single‐sideband modulator and frequency multipliers. The spectral regrowth of synthetic aperture radar waveform was reduced by minimizing the gain and phase imbalances between the I and the Q channels. Further, the gain and phase imbalance requirements were defined on the basis of the evaluation of the impulse response function. The 150‐MHz bandwidth of a chirp signal at 2.4 GHz was expanded to 600 MHz at 9.6 GHz. By adjusting the dc offset and the gain imbalance between the I and the Q channels, the carrier level was reduced from −28.7 to −53.4 dBm and the sidelobe levels caused by the spectral regrowth were reduced by approximately 8–9 dB. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:400–404, 2014

A Volterra-Based Procedure for Multi-Port and Multi-Zone GaN FET Amplifier CAD Simulation

This paper reports a systematic method for the computer-aided-design (CAD) simulation of GaN FET power amplifiers (PAs). The core of the proposal is a Volterra-based behavioral model (BM) with multi-spectral and multi-node capabilities, which black-box structure is formally derived from a circuit-level representation of the PA and accounts for both short and long-term memory effects. Starting with the equivalent circuit of a typical FET device with thermal power feedback, the structures of the kernels for the gate, drain and thermal nodes are developed and are shown to be dependent on the frequency response of the PA terminating impedances and thermal filter. The model has been applied to simulate the nonlinear response of a typical PA circuit, showing the ability of the proposed model to provide an accurate prediction of multi-spectral, multi-node characteristics, including AM/AM-AM/PM conversion, spectral regrowth, intermodulation, and temperature rise, under diverse input signal waveforms and bandwidths. These results have been successfully compared with commercial CAD tools based on harmonic balance or envelope simulation.

A High-Power Packaged Four-Element $X$-Band Phased-Array Transmitter in ${\hbox{0.13-}}\mu{\hbox {m}}$ CMOS for Radar and Communication Systems

This paper presents a four-element X-band phased-array transmitter in 0.13-μm CMOS. The design is based on the all-RF architecture and contains a 5-bit phase shifter (lowest bit is used as a trim bit), 4-bit gain control (to reduce the rms gain error), and power amplifiers capable of delivering a PSAT of 13.5 dBm per channel at 8.5-10.5 GHz. The chip can be used in the linear mode for communication systems and in the saturated mode for frequency-modulated continuous-wave radar systems, and therefore, extensive analysis is done on the phase shifter distortion versus input power. Spectral regrowth and error vector magnitude measurements indicate that the chip can support at least 20-MSym/s quadrature phase-shift keying and binary phase-shift keying modulation at an output power of +5 dBm per channel. Packaging techniques based on chip-on-board and quad flat no-lead (QFN) modules have been implemented with the four-channel chip, and both show a nearest neighbor coupling of -30 dB at 8-10 GHz, limited by bond-wire coupling. The chip dimensions are 2.9×3.0 mm2 and it consumes 870 mW from 2- and 3-V power supplies.

Analysis and Modeling of the Non-ideal Performance in a Polar Transmitter Caused by Limited Bandwidth and Inaccurate Pulsewidth in a ΔΣ Envelope Modulator

In this paper, the spectral regrowth issues of the separated broadband signals’ magnitude and phase components have been analyzed using stochastic theory for polar transmitters. The analysis shows that limited bandwidth of the low-pass envelope reconstruction filter in the ΔΣ supply modulator can affect the transmitting signals’ fidelity of the polar transmitter, which can be measured by the system specifications of error vector magnitude (EVM) and adjacent channel power ratio (ACPR). The relations between the cut-off frequency of the filter and the specifications of EVM and ACPR have been derived to determine the minimum required filter bandwidth. The relations are verified by simulation examples of wideband WCDMA and WLAN 802.11a signals. In the time-domain, the limited bandwidth of the low-pass filter is verified to be responsible for the quasi-memory effects observed in the input envelope amplitude to output envelope amplitude (AM–AM) relation of the ΔΣ supply modulator. Through numerical examples, the effects of inaccurate pulsewidth caused by non-zero transistor’s rise and fall time have been found to be one of the factors causing nonlinear AM–AM relation of supply modulators. The ΔΣ envelope modulator restrained by limited envelope bandwidth and inaccurate pulsewidth can be characterized as a nonlinear system with memory, which is modeled by ARMAX Hammerstein systems in this paper. The validity of this model is verified by a simulation example of a WCDMA supply modulator.

Accurate analysis of spectral regrowth of nonlinear power amplifier driven by cyclostationary modulated signals

In this paper we present an exact analytical expression to calculate the spectral regrowth at the output of a nonlinear power amplifier (PA) using the higher order cumulants and Poisson summation f...

Design of a Direct Conversion Transmitter to Resist Combined Effects of Power Amplifier Distortion and Local Oscillator Pulling

This work elucidates how the combined effects of power-amplifier distortion and local-oscillator (LO) pulling adversely impact a wireless direct-conversion transmitter (DCT) that adopts a time-varying envelope modulation. An analytical model is developed to evaluate the deterioration of DCT output signal quality, including error vector magnitude, adjacent channel power ratio, and spectral regrowth. Additionally, an integrated approach for PA linearization and anti-LO pulling is designed based on an open-loop digital-predistortion method and the proposed analog feedback compensation mechanism. Experimental results demonstrate that a quadrature-modulation-based DCT that incorporates the proposed approaches can significantly improve the LO spectral purity, while achieving a high linearity and efficiency performance simultaneously. These attributes are highly desired for third-generation systems such as cdma2000 1x and W-CDMA.

Performance evaluation of time-multiplexed and data-dependent superimposed training based transmission with practical power amplifier model

The increase in the peak-to-average power ratio (PAPR) is a well known but not sufficiently addressed problem with data-dependent superimposed training (DDST) based approaches for channel estimation and synchronization in digital communication links. In this article, we concentrate on the PAPR analysis with DDST and on the spectral regrowth with a nonlinear amplifier. In addition, a novel Gaussian distribution model based on the multinomial distribution for the cyclic mean component is presented. We propose the use of a symbol level amplitude limiter in the transmitter together with a modified channel estimator and iterative data bit estimator in the receiver. We show that this setup efficiently reduces the regrowth with the DDST. In the end, spectral efficiency comparison between time domain multiplexed training and DDST with or without symbol level limiter is provided. The results indicate improved performance for DDST based approaches with relaxed transmitter power amplifier requirements.

다중 대역 전송 시스템을 위한 전치왜곡 알고리즘

New digital predistortion technique is proposed for power amplifier linearization in multi-band transmission systems. We consider a system where muli-band signals are combined and amplified by a single power amplifier (PA). In this system, the PA output is distorted by the nonlinear cross-products between different band signals as well as their own nonlinear self-products. To compensate these nonlinear effects, we propose a multiple PD structure. Each PD removes the nonlinear cross-products and self-products to mitigate the spectral regrowth for the corresponding band. Since the PD parameters for different bands are linked together, it is difficult to find the PD parameters separately. Thus, we propose an iterative method for finding the PD parameters jointly. For demonstration of the proposed method, multi-band characteristics of PA are extracted from a commercial power amplifier. Computer simulation was executed based on the PA parameters. The simulation results show that the proposed method can effectively linearize the PA and remove spectral regrowth at each signal band.

A nonlinear filtered‐x prediction error method algorithm for digital predistortion in digital subscriber line systems

SUMMARYDigital predistortion of nonlinear systems is an important topic in many practical applications. This paper considers direct predistortion of a Volterra system by connecting in tandem an adaptive Volterra predistorter. The coefficients of the predistorter can be recursively estimated using the nonlinear filtered‐x least mean squares (NFxLMS) algorithm. In this paper, the prediction error method (PEM) is used to derive a novel nonlinear filtered‐x PEM (NFxPEM) algorithm. A simulation study on Volterra systems shows that the NFxPEM algorithm more significantly suppresses spectral regrowth and converges much faster than the NFxLMS algorithm. Also, the NFxPEM algorithm is used in this paper to design more efficient digital predistorter—as compared with the NFxLMS algorithm—for digital subscriber line systems. Copyright © 2012 John Wiley & Sons, Ltd.

A Neural Approach for Compensation of Effects of PAPR Causing BER Degradations in Hiperlan2

Emergent telecommunication systems use spectral efficient multilevel modulation formats together with multicarrier schemes such as Orthogonal Frequency division multiplexing (OFDM). OFDM transmission is an efficient way to deal with multipath. However one of its major drawbacks is its sensitivity to nonlinear distortions due to its greatly variable envelope and high peak to average power ratio (PAPR). Nonlinear distortions are mainly introduced by high power amplifiers (HPA), cause in band distortions. Misalignments in IQ modulators and demodulators causes imbalance, which results in loss of orthogonality and create intercarrier interference and spectral regrowth in an OFDM system. Nonlinear distortions also cause intermodulation effect. This degrades Bit error rate (BER) performance of the system. This paper introduces the self organization map(SOM) and parameterless self organization map(PLSOM) based and Hiperlan/2 receiver structure used for the compensation of nonlinear distortions. Simulation results presented in this paper clearly indicate the improved performance of the proposed system. Simulations are run for AWGN channel model and for two HPA models namely Travelling wave tube amplifier (TWTA) and Solid state power amplifier (SSPA), with and without SOM and PLSOM neural network blocks.

An Outphasing Scheme for Reducing Spectral Regrowth of Multi-Tone Signal in LINC Transmitter

This paper suggests an outphasing scheme to reduce adjacent channel spectral regrowth triggered by the gain and phase mismatch between two signal paths in linear amplification with nonlinear component (LINC) systems. The error vector magnitude and power spectral density of the output signal considering path mismatch are described analytically using path mismatch factor. An outphasing scheme is proposed to reduce the spectral regrowth. The proposed outphasing scheme reshapes the phases of the separated signals in LINC systems to reduce the changes of the phases. Its performance is verified by performing simulations with multi-tone signals. The result shows that the scheme can reduce the spectral regrowth of the multi-tone signals significantly compared to the conventional outphasing scheme for LINC systems with path imbalance.

EVM measurements using orthogonal separation at the output of a non-linear amplifier

A method is developed to obtain the error vector magnitude (EVM) of non-linear amplifiers by mean of orthogonalising the output of the amplifier to extract its uncorrelated non-linear terms. This method allows for extraction of in-band distortion power and spectral regrowth power from the measured RF waveforms which provides invaluable insight to the EVM behaviour of the device. From a frequency domain analysis, the EVM is attributed to the in-band distortion power and spectral regrowth power where it is determined that the in-band distortion power contributes more to the EVM degradation. The amplifier used for verification is modelled from Avago's MGA83563 measured amplitude distortion (AM–AM) and phase distortion characteristics (AM–PM). Finally, the relationship between EVM and gain compression characteristic is derived from the proposed orthogonal model.

Power amplifier- memory-less non linear modeling

The power amplifier’s nonlinearity broadens the input signal’s bandwidth. This is known as spectral re-growth which is undesirable since it causes interference with adjacent channels. It also causes distortions within the signal bandwidth, which affects the bit error rate at the receiver. Most recent transmission schemes, such as wideband code division multiple access (WCDMA) or orthogonal frequency division multiplexing (OFDM), are especially vulnerable to the nonlinear distortions due to high fluctuations in their power levels. To ensure that linearity is maintained to a high degree, it is necessary to examine the nature of amplifier distortion. Audio amplifier distortion is of concern for many years. The conventional feedback techniques used at audio frequencies are not applicable to many RF amplifiers due to following problems: 1. Stability at high bandwidth.2. Cost for high gains in RF stages. Memory less or Instantaneous Nonlinear Model assumes that the PA has no memory effects, that is, it has no knowledge of past events and hence the present o/p signal is only a function of present i/p signal. The model is further simplified by restricting the analysis to signal and distortion contained within the first harmonic zone, such models are referred to as band pass memory-less nonlinear models and are further simplified since they do not need to model even order nonlinearities. A memory less nonlinear model can be modeled in polar (amplitude and phase) or Cartesian (I and Q) form. Key words: Nonlinearity, memory-less, distortion, intermodulation.

Adaptive Square-Rooting Companding Technique for PAPR Reduction in OFDM Systems

This paper addresses the problem of peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It also introduces a new PAPR reduction technique based on adaptive square-rooting (SQRT) companding process. The SQRT process of the proposed technique changes the statistical characteristics of the OFDM output signals from Rayleigh distribution to Gaussian-like distribution. This change in statistical distribution results changes of both the peak and average power values of OFDM signals, and consequently reduces significantly the PAPR. For the 64QAM OFDM system using 512 subcarriers, up to 6 dB reduction in PAPR was achieved by square-rooting technique with fixed degradation in bit error rate (BER) equal to 3 dB. However, the PAPR is reduced at the expense of only -15 dB out-ofband spectral shoulder re-growth below the in-band signal level. The proposed adaptive SQRT technique is superior in terms of BER performance than the original, non-adaptive, square-rooting technique when the required reduction in PAPR is no more than 5 dB. Also, it provides fixed amount of PAPR reduction in which it is not available in the original SQRT technique.

가변 스펙트럼 할당을 지원하는 광대역 전력 증폭기를 위한 디지털 전치왜곡기

A new predistortion technique for dynamic spectrum allocation systems such as cognitive radio (CR) is proposed. The system model considered in this paper occupies a small band at a time, but the center frequency can be changed in the wide range of frequency. In this scenario. the front-end filter may not eliminate the harmonics of the power amplifier (PA) output. The proposed PD reduces the spectral regrowth of the fundamental signal at the carrier frequency () and removes the harmonics (, , ...) at the same time. The proposed PD structure is composed of multiple predistorters (PDs) centered at integer multiples of . The PD at is for removing spectral regrowth of the fundamental signal, and the others are for harmonic reduction. In the proposed PD structure, parameters of PDs are found jointly. Simulation results show that the spectral regrowth can be reduced by 20dB, and the 2nd and 3rd harmonics can be reduced down to -70dB from the power of the fundamental signal.

Predistortion of a 100‐W base station high‐power amplifiers

AbstractThis article presents the design of a fifth‐order RF predistorter for use in a base‐station high‐power amplifier (HPA).The predistorter uses two intermodulation (IM) signal generators, with identical configuration, to individually generate the third‐ and fifth‐order inband intermodulation (IM3 and IM5) signals for cancellation of the third‐ and fifth‐order intermodulation distortion products (IMDP3 and IMDP5) at the output of the HPA. The IM signal generators are constructed using two Schottky diodes as non‐linear devices to generate the inband IM3 and IM5 signals. The proposed predistorter is designed and optimized using computer simulation. A hardware prototype is built and tested in a practical 100‐W HPA using a standard IS‐95 CDMA signal at 2.2 GHz. Measured results show that the spectral regrowth of the signal at the HPA output can be significantly suppressed over a wide range of output power levels. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:861–867, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25855

Development of non-linear transfer functions for power amplifiers

A behavioural-level model for non-linear power amplifiers is developed based on measured gain compression (AM-AM) and phase distortion (AM-PM) characteristics. The measured envelope-dependent gain compression component must be transformed to its instantaneous form for analysis. An improved method is proposed to perform this transformation using the measured 1 dB gain compression (P1 dB) and third-order intermodulation product (IP3) of a non-linear power amplifier. The authors also show that the phase distortion component depends only on the magnitude of the input signal and does not require a similar transformation. Using the proposed model, the simulated spectral regrowth and error vector magnitude (EVM) show a good match to the measured results of a 2.412 GHz Wireless Local Area Network (WLAN) GaAs field-effect transistor (FET) power amplifier.

An Efficient Measure for Nonlinear Distortion Severity due to HPA in Downlink DS-CDMA Signals

This paper deals with the nonlinear distortion (NLD) effects of high power amplifiers (HPAs) on direct sequence-code division multiple access systems. Such a distortion drastically degrades the system performance in terms of bit error rate (BER) degradation and spectral regrowth. Much effort has been conducted to minimize NLD. A key requirement to do so is to define a certain measure for the HPA nonlinearity, which when reduced often allows NLD to also be reduced. Several measures were proposed such as peak-to-average power ratio, instantaneous power variance, and cubic metric. In this paper, we show that such measures are not closely related to NLD and their reduction does not always lead to optimum performance. Hence, we introduce an efficient measure, namely, nonlinearity severity (NLS), to characterize NLD effects, as an alternative to the existing measures. The NLS is characterized by having direct link to the system performance as it is formulated based on the signal characteristics contributing to BER performance and spectral regrowth. Additionally, a major advantage of the NLS measure is that it is linked to the IBO level allowing the possibility of improving performance at all IBO levels of interest.

Predistortion of Hammerstein and Wiener systems using the Nonlinear Filtered-x Prediction Error Method algorithm

Adaptive predistortion of nonlinear systems described using Hammerstein and Wiener models is considered in this paper. The adaptive predistorter is modeled as a Wiener or Hammerstein system, respectively. The parameters of the linear and nonlinear blocks of the predistorter are estimated simultaneously using the Nonlinear Filtered-x Prediction Error Method (NFxPEM) algorithm. The NFxPEM algorithm is derived under the assumption that the parameters of the Wiener and Hammerstein predistorters are changing slowly during adaptation. Simulation study shows that the suggested predistorter using the NFxPEM algorithm can well compensate nonlinear distortion and effectively reduce spectral regrowth. Moreover, the suggested NFxPEM algorithm achieves much better results as compared to the Nonlinear Filtered-x Least Mean Squares (NFxLMS) algorithm.

On Adaptive Identification Algorithm for PA Linearization

According to the nonlinear amplification characteristic with memory effection of power amplifier(PA) in WCDMA systems, we have analyzed the structure of Wiener digital predistortion system in which Hammerstein power amplifier model with memory is adopted. LR algorithm and LMS algorithm were used respectively to identify and update parameters of Wiener predistortion system. An LR / LMS identification algorithm is proposed for syetem adaptive Identification. The method has a very good suppression of signal spectral regrowth. Theoretical analysis and computer simulation verified the effectiveness and practicability of the algorithm.