Digital Predistortion Technique Research Articles

A novel direct matching network synthesis technique and its application to broadbandclass‐Jpower amplifier

A novel direct matching network (MN) synthesis method improving the conventional simplified real frequency technique (SRFT) is presented in this article. By straightforwardly optimize the characteristic impedance (physical width) and electrical length (physical length) of each distributed element in a preselected configuration, the proposed method has one more degree of design freedom by comparison with the SRFT, and therefore increases design flexibility and matching effects. To demonstrate its effectiveness, a broadband class-J power amplifier (PA) is devised for which both the input and output MNs are realized using the proposed method. The simultaneous manipulation of fundamental and second harmonic impedances is successfully realized by defining a novel target function that indicates the degree of proximity for the realized impedances to the optimal transistor impedances. Comprehensive equations and complete design procedures of the new technique are given. The measured class-J PA implemented for verification achieves an output power of 39.2 to 42.3 dBm and power-added efficiency of 61.8% to 71.6% over the frequency range of 2.5 to 3.8 GHz using a 10-W GaN HEMT. A 20-MHz LTE-A signal is employed to validate the linearization capability of this class-J PA. An adjacent channel leakage ratio level around −43.8 dBc is achieved after utilizing digital predistortion technique.

A Methodology and a Metric for the Assessment of the Linearizability of Broadband Nonlinear Doherty Power Amplifiers

This letter proposes a novel methodology and a metric for the assessment of the linearizability of broadband nonlinear power amplifiers (PAs). Validation of the proposed methodology and the proposed metric on two linearized Doherty PAs (DPAs) using digital predistortion (DPD) technique was carried out and it demonstrated the suitability and appropriateness of the new metric. Different from the iterative optimization procedure between the PA circuit design and the DPD algorithm compensation, the proposed method aims to provide a quantitative criterion on the PA linearizability based on its frequency-dependent amplitude modulation (AM)/AM and AM/phase modulation (PM) characteristics. To verify the effectiveness of the metric, two DPAs were characterized before and after a DPD-based linearization. The average correlation coefficients between the values of the metric and the adjacent channel power ratio after DPD were 0.8763 and 0.9156, and that between the values of the metric and the error vector magnitude after DPD were 0.8618 and 0.7200 for PA1 and PA2, respectively, indicating that the evaluation method and the linearizability metric can be used as a good measure to assess the linearizability of broadband PAs.

A Robust and Scalable Harmonic Cancellation Digital Predistortion Technique for HF Transmitters

High frequency (HF) transmitter implemented with the multioctave broadband power amplifier (PA) enables long-distance communication and has strong survivability. However, it is faced with the problem of harmonics because of the nonlinearity of the PA. We first consider canceling the harmonics using a harmonic cancellation digital predistortion (HC-DPD) technique, instead of lossy and bulky analog filter banks. A uniform complex-valued Hilbert transform-based polynomial (HTBP) function is proposed for the modeling of the harmonics, as well as the fundamental component in the real-valued input and output signals. In order to validate this idea, detailed experiments were performed for carrier frequencies located at 5 and 10 MHz, respectively. After the proposed HC-DPD, the harmonics were all canceled to satisfy the linearization requirements. In particular, the improvement of the second harmonic is more than 40 dB, which is nearly 20 dB better than that of the memoryless polynomial (MLP) model. Furthermore, the experimental results demonstrate that the proposed technique is scalable, in which the model coefficients extracted at a specific power level, signal bandwidth, and carrier frequency can be extended to other scenarios. Both the second and third harmonics were canceled for being below −45 dBc after the HC-DPD.

Constant response window channel estimation method and LUT-based digital predistortion applied to OFDM VLC links

In this work, we investigate a LUT-based digital predistortion technique applied to a Visible Light Communication system. We study the performance of the predistortion technique for OFDM signals with up to 2048 subcarriers, 16-QAM modulation, and three levels of DC bias considering the voltage–current LED response of a commercially available device. In addition, we propose a new and simpler pilot-assisted approach for the OFDM channel estimation, named Constant Response Window, the performance being tested with up to 64-QAM modulation and 2048 IFFT size OFDM signals. BER and MSE results show that the combination of the LUT-based digital predistortion technique with the new estimation method presents an improved performance over the Least Squares and the Minimum Mean Square Error channel estimation techniques in the considered scenario.

Linearization of RF Power Amplifiers in Wideband Communication Systems by Adaptive Indirect Learning Using RPEM Algorithm

This paper proposes a new approach of digital predistortion (DPD) technique based on the adaptive indirect learning architecture (ILA) by using a recursive prediction error minimization (RPEM) algorithm for linearizing radio frequency (RF) power amplifiers (PAs) in emerging wideband communication systems. In the proposed RPEM-based linearization approach, the forgetting factor varies with time and is less sensitive to noise. Therefore, the predistorter (PD) parameter estimates become more consistent and accurate in steady state so that the mean square errors can be reduced. Both the error vector magnitude (EVM) and the adjacent channel power ratio (ACPR) are used to evaluate the DPD technique in RF PAs employing the proposed linearization. The efficiency validation of the proposed method is based on a simulated PA Wiener model. The simulation results have clarified the improvement of the proposed adaptive ILA-based DPD with RPEM algorithm in terms of both EVM and ACPR.

Multibeam Digital Predistortion for Millimeter-Wave Analog Beamforming Transmitters

In this letter, a novel multibeam digital predistortion (MB-DPD) technique is proposed to linearize multiple beams simultaneously targeted at the multiuser scenario of millimeter-wave (mmWave) analog beamforming transmitters. It is achieved by effectively constructing the predistorted signal for each beam to remove both the intrinsic nonlinear distortion of power amplifier (PA) and the multibeam interference incurred by all beams except the targeted main beam. To validate the proposed technique, a two-beam test on an analog beamforming transmitter was carried out and the measurement results have shown that the proposed method can effectively realize the multibeam linearization, providing a promising solution for mmWave multiuser applications.

Bandwidth Enhancement of Frequency Dispersive Doherty Power Amplifier

This letter presents a modified Doherty power amplifier (DPA) combining network with wide bandwidth (BW) and high power utilization factor (PUF). For the first time, the impact of the excessive phase dispersion of matching networks on the performance of conventional broadband DPA is analytically revealed. For dispersion compensation, an optimized adaptive combining load is introduced to enhance the achievable DPA BW and PUF. For verification, a 42-dBm equal-cell DPA is prototyped to cover 1.4–2.55 GHz. Measurement results indicate that, over the prescribed frequency range, a PUF of over 0.95, back-off efficiency (6 dB) of 48%–58%, and saturation efficiency of 62%–74% can be achieved. Moreover, under WCDMA excitation, the adjacent channel leakage ratio (ACLR) was found to be better than −32 dBc with average efficiency of over 45% without applying digital predistortion techniques.

Energy-efficient power amplifiers and linearization techniques for massive MIMO transmitters: a review

Highly efficient power amplifiers (PAs) and associated linearization techniques have been developed to accommodate the explosive growth in the data transmission rate and application of massive multiple input multiple output (mMIMO) systems. In this paper, energy-efficient integrated Doherty PA monolithic microwave integrated circuits (MMICs) and linearization techniques are reviewed for both the sub-6 GHz and millimeter-wave (mm-Wave) fifth-generation (5G) mMIMO systems; different semiconductor processes and architectures are compared and analyzed. Since the 5G protocols have not yet been finalized and PA specifications for mMIMO are still under consideration, it is worth investigating novel design methods to further improve their efficiency and linearity performance. Digital predistortion techniques need to evolve to be adapted in mMIMO systems, and some creative linearity enhancement techniques are needed to simultaneously improve the compensation accuracy and reduce the power consumption.

Millimeter Wave SOI-CMOS Power Amplifier With Enhanced AM-PM Characteristic

This paper proposes a novel inter-stage load-pull characterization method to enhance the linearity of millimeter wave integrated power amplifiers (PAs) by minimizing their amplitude-to-phase (AM-PM) distortion without worsening their AM-AM or efficiency performances. The proposed method identifies the optimal solution for the inter-stage matching network which enables the synthesis of a driver stage AM-PM characteristic that is complementary to that of the power stage; consequently, reducing the overall AM-PM distortion of the PA. The proposed technique is applied to design a proof-of-concept 28–31 GHz PA demonstrator using 45 nm silicon-on-insulator CMOS technology. The measurement results obtained under continuous wave excitation at 29 GHz demonstrate an excellent AM-PM characteristic, with phase distortions as low as 0.2° at the 1-dB compression power level of 13.9 dBm and less than 1° at an output power level of up to 16 dBm (very close to the saturation power of 16.6 dBm). This enhanced AM-PM linearity improves the linearizability of the PA. This was confirmed by testing the PA with a 64 quadrature amplitude modulated test signal with an instantaneous bandwidth of 800 MHz. Without applying any digital pre-distortion (DPD) technique, the PA delivers a power added efficiency (PAE) of 8.7% at an average output power ( $P_{avg}$ ) of 9.4 dBm while maintaining an error vector magnitude (EVM) of −25 dB. However, after applying a very simple memoryless DPD function with only four coefficients, the PA can operate at a higher $P_{avg}$ of 11.1 dBm with a much better PAE of 12.2% while still maintaining an acceptable EVM of −25.2 dB. Thanks to the proposed technique, the PAE of the proposed PA can be improved by 40% with a very simple application of a low cost and low complexity DPD technique.

Stabilization of a Modified LMS Algorithm for Canceling Nonlinear Memory Effects

The least-mean-square (LMS) is promising in a system whose signal statistics are time-varying, and it is preferred in adaptive digital predistortion techniques. In a nonlinear memory system, it is found that the high nonlinearity, the finite sampling rate and the finite number of digits will introduce colored noise that is dependent on signals. In this paper, the long-term stability of the conventional LMS in the presence of signal-dependent noise is analyzed using perturbation analysis. It is revealed that the cumulative effect of the signal-dependent noise may lead to a divergence of the conventional LMS, and a properly selected leaky coefficient is needed to suppress the divergence. Unfortunately, the compensation performance may be harmed by enlarging the leaky coefficient. To address this problem, a modified LMS (MLMS) algorithm is proposed. By adjusting the updating strategy in the iterations, the long-term stability of the algorithm is guaranteed, without harming the linearization performance. Numerical simulations verify that the MLMS outperforms the conventional LMS both in stability and compensation performance.

Improved estimation for Saleh model and predistortion of power amplifiers using 1‐dB compression point

This paper proposes an improved estimation approach for modelling RF power amplifiers (PAs) using the Saleh behavioural model. The proposed approach is appropriate for solid-state PA technologies. The 1-dB compression point of the PA is included in the estimation approach to improve the estimation of the Saleh coefficients. Thus, expressions are derived to describe the relationship between the parameters of the Saleh model and the manufacturing specifications of PAs: gain ( G ), third-order intercept point (IP 3 ) and 1-dB compression point ( P 1dB ). This method is a simple estimation of a memoryless amplitude-to-amplitude (AM/AM) nonlinearity to benefit RF designers evaluating the PA distortion using the PA parameters: P 1dB , G , and IP 3 , before conducting experimental validation. The linearisation method using digital predistortion (DPD) is derived as a function of G , IP 3 , and P 1dB , for mitigating the AM/AM nonlinear distortion. Finally, the modelling and DPD techniques are both evaluated using the experimental results of the GaAs PA.

Linearization of a Directional Modulation Transmitter Using Low-Complexity Cascaded Digital Predistortion

Directional modulation (DM) implemented at the transmitter side is a physical layer security technology based on the multi-antenna structure. Current DM methods paid little attention to the issue of the nonlinear distortion of power amplifiers (PAs), which may lead to poor amplifier efficiency and worsen the signal’s error vector magnitude (EVM). For the first time, we consider linearization of a DM transmitter using the digital predistortion (DPD) technique. We modify the paralleled DM DPD model to a novel cascaded model based on the characteristics of the DM transmitter. This modified method applies a common DPD to mitigate the nonlinear distortion of all paths and an auxiliary DPD to fine-tune each path, so as to significantly reduce the complexity. In order to validate this idea, two-element uniform linear array (ULA) and four-element ULA experiments were performed, in which 16QAM 20-MHz bandwidth signals with a peak-to-average power ratio (PAPR) of about 8.0 dB were generated for commercial part PAs. Experimental results show that the modified approach has almost the same linearization performance as the paralleled DM DPD technique but reduces the computational complexity significantly.

Performance Evaluation of Digital Pre-Distortion Technique using Low-Complexity and Low-Precision ADC

This paper analyzes the Digital Pre-Distortion linearization technique using a low-precision Analog-to-Digital Converter (ADC). The output of a power amplifier exhibits various spurious emissions, spectral regrowth and intermodulation distortion (IMD) products due to its non-linear behavior. So, to preserve the performance of power amplifier, linearization becomes mandatory. Digital Pre-Distortion does the training on the output of the power amplifier (distorted signal) and generates exactly the inverse characteristics to that of power amplifier. Their cascading results into a linear response. In practical systems, the output of power amplifier has to go through an analog-to-digital converter for digital processing and a low-resolution ADC results in the degradation of the signal and affects the DPD performance. But a low-resolution ADC not only reduces the computational complexity in the digital processing but it also provides lower power consumption and costs less because less hardware would be required. In this work, the aim is to find the precision up to which ADC resolution can be reduced without affecting the DPD performance in a significant manner. This paper evaluates the performance of two DPD systems - Full-band DPD and Sub-band DPD and from simulations, it is observed that for a full-band DPD, 1-bit ADC can be reliably used and for a sub-band DPD, single bit to 4-bits ADC can be used.

Single-Receiver Over-the-Air Digital Predistortion for Massive MIMO Transmitters With Antenna Crosstalk

In this article, a novel digital predistortion (DPD) technique considering antenna crosstalk and using over-the-air (OTA) measurements is proposed for massive multiple-input multiple-output (mMIMO) transmitters. In the proposed DPD for hybrid beamforming with regular matrix array, all subarrays share an integrated linear crosstalk model (ICTM) block, a signal decomposition (SD) block, and a single receiver (SR) to reduce hardware complexity and cost greatly. Subarray coupling coefficients can be identified for the ICTM block without direct measurements. Using the SD block, receiving-end signals can be distinguished as feedback signals for each subarray from one feedback signal from SR. The receiving-end signals for each subarray can be linearized by a modified dual-input predistorter as per the distinguished feedback signals. For the special case of fully digital beamforming, the proposed DPD is the same as for hybrid beamforming. Simulation of eight-subarray hybrid beamforming transmitter is given to validate how the proposed DPD scales to mMIMO scenarios. Experimental results of three-subarray hybrid beamforming transmitter are provided, which indicate that the proposed DPD can achieve the similar linearization performance with less complexity, compared with the state-of-the-art dual-input with crosstalk and mismatch model (DI-CTMM) DPD.

Full-Angle Digital Predistortion of 5G Millimeter-Wave Massive MIMO Transmitters

In this paper, a full-angle digital predistortion (DPD) technique is proposed to linearize fifth-generation (5G) millimeter-wave (mmWave) massive multiple-input-multiple-output (mMIMO) transmitters with low implementation complexity. It is achieved by compensating the differences of power amplifiers (PAs) in different transmitter chains first and then adopting a common digital block to linearize the whole subarray. Based on this operation, all the transmitter chains can be efficiently linearized simultaneously, providing the merits of full-angle linearization including the main beam and sidelobes. To validate the proposed idea, an mmWave full-digital beam-forming transmitter has been developed, which is operated at the center frequency of 24.75–28.5 GHz to meet the 5G candidate frequency bands. Experimental results show that the proposed method can effectively linearize the mmWave mMIMO transmitter in all directions, which provides a promising linearization solution for 5G mMIMO beam-forming systems.

Behavioral modeling and digital pre-distortion techniques for RF PAs in a 3 × 3 MIMO system

AbstractModern telecommunications are moving towards (massive) multi-input multi-output (MIMO) systems in 5th generation (5G) technology, increasing the dimensionality of the systems dramatically. In this paper, the impairments of radio frequency (RF) power amplifiers (PAs) in a 3 × 3 MIMO system are compensated in both the time and the frequency domains. A three-dimensional (3D) time-domain memory polynomial-type model is proposed as an extension of conventional 2D models. Furthermore, a 3D frequency-domain technique is formulated based on the proposed time-domain model to reduce the dimensionality of the model, while preserving the performance in terms of model errors. In the 3D frequency-domain technique, the bandwidth of the system is split into several narrow sub-bands, and the parameters of the model are estimated for each sub-band. This approach requires less computational complexity, and also the procedure of the parameters estimation for each sub-band can be implemented independently. The device-under-test consists of three RF PAs including input and output cross-talk channels. The proposed techniques are evaluated in both behavioral modeling and digital pre-distortion (DPD) perspectives. The experimental results show that the proposed DPD technique can compensate the errors of non-linearity and memory effects in the both time and frequency domains.

Direct digital predistortion technique for the compensation of laser chirp and fiber dispersion in long haul radio over fiber links

Analog Radio-over-Fiber (A-RoF) communication technology constitutes a promising technique for next generation radio access networks thanks to its relatively low bandwidth requirements. Within this context, an efficient predistortion technique is proposed, which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel. The radio frequency signal is firstly put in digital form through an analog to digital converter, then the predistortion operation is realized, and finally the resulting signal is put again into analog form. A comprehensive analysis on the theoretical basis of the proposed approach is presented, together with the approximations introduced, which makes it practically realizable. The improvements on the quality of the received signal due to the proposed solution are illustrated with reference to scenarios of applicative interest. The performance of proposed technique is evaluated in terms of adjacent channel leakage ratio and error vector magnitude when long term evolution signal is applied at the input.

Optimizing Optical Interconnects: Digital Predistortion and Postequalization Techniques in Optical Communications

For the past few decades, we have witnessed explosive growth in data consumption in our everyday lives. While consumers rely more and more on wireless connections to obtain all these data, backbone data networks and infrastructures now depend largely on optical connectivity.

Ubiquitous Digital Predistortion and Postcorrection [From the Guest Editors' Desk

The articles in this special section that focuses on recent advances in applications for digital predistortion (DPD) and digital post correction (DPC) techniques. The issue surveys emerging DPD and DPC techniques in wireless, wireline, and optical communications and revisits the principles these techniques have in common.

Linearization of multiband OFDM RoF system employing frequency-separated model based digital pre-distortion

To mitigate the nonlinear distortion of a multiband orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system, a frequency-separated model-based digital pre-distortion (DPD) technique is proposed and experimentally demonstrated herein. Frequency-separated model DPD, when used as a pre-equalizer, mitigates nonlinearity in a transmitter. The nonlinear characteristic of an RoF system depends on the frequency band. Therefore, the proposed equalizer separates the input by frequency band and mitigates nonlinearities. A least mean square algorithm is used to estimate the variables of the proposed equalizer. Using the proposed DPD technique, performance enhancement was verified experimentally relative to the adjacent carrier leakage ratio (ACLR). The proposed technique obtained ACLR values of 27 to 30 dB. The results demonstrate that signal performance improved in terms of error vector magnitude from 6.3% to 3.5% at an optical modulation index value of 63%.