Nonlinear Power Amplifier Research Articles

Widely-Linear Nyquist Criteria for DFT-Spread OFDM of Constellation-Rotated PAM Symbols

In this paper, the Nyquist criteria for ordinary linear modulation are generalized to the discrete-Fourier transform-spread (DFT-spread) orthogonal frequency-division multiplexing (OFDM) of constellation-rotated PAM symbols. Unlike the conventional Nyquist criteria with a linear receiver, the new criteria are to remove intersymbol interference in the output of a widely-linear (WL) receiver. It is shown that not only the folded spectrum, but also the shifted and reverse-folded spectrum is crucial in the criteria, where the shift offset is determined by the rotation angle. As a design example, combinations of a constellation rotation angle and a square-root raised cosine pulse are proposed. It is shown that the proposed design can achieve better performance both in spectral efficiency and in peak-to-average power ratio than the conventional DFT-spread OFDM of π/2-BPSK symbols. The optimal one-tap frequency-domain equalizer is also derived under the WL zero-forcing (ZF) constraint. It is shown that, when the rotation angle is properly chosen, the proposed one-tap WL-ZF equalizer significantly outperforms the one-tap linear ZF equalizer in output signal-to-noise ratio. The proposed equalizer also outperforms the conventional equalizer in error vector magnitude measured at the output of a nonlinear power amplifier.

Advanced SLM scheme based on discrete forest optimization algorithm for PAPR minimization in UFMC waveform

Inherent multicarrier transmission mechanism of the universal filtered multicarrier (UFMC) waveform engenders the problem of high peak-to-average power ratio (PAPR). Since it is impossible for a nonlinear high power amplifier (HPA) to execute a distortionless amplification unless the PAPR of transmission signal is below an acceptable level, eliminating the aforementioned PAPR drawback in UFMC waveform is so critical for smooth communication. With this in mind, we developed a new selective mapping (SLM) scheme based on discrete forest optimization algorithm (DFOA) for the UFMC waveform. The related scheme was created by embedding the DFOA into the conventional SLM with the intention of optimizing the values of phase factors by which the phase rotation process is carried out in frequency domain to reduce the PAPR of eventual time domain signal attained from the SLM output. It is confirmed via the simulations that, remarkable PAPR improvements are achieved through the DFOA-SLM scheme in the UFMC signal thanks to the DFOA-supported search for the optimal sequence of phase factors instead of classical random search strategy inherent in the conventional SLM method.

Sum-Rate of Cell Free Massive MIMO Systems With Power Amplifier Non-Linearity

Cell free (CF) massive multiple input multiple output (mMIMO) has been suggested as a key solution to meet the high data rate demands of future wireless communications. Studying the performance of these systems in practical scenarios such as in the presence of hardware impairments is of vital importance. In this paper, we study the effect of power amplifier non-linearity on the uplink and downlink sum-rate of CF mMIMO systems. We derive closed form expressions for the uplink and downlink achievable sum-rates of orthogonal frequency division multiplexing (OFDM) based CF mMIMO systems. Our results show that in the uplink the sum-rate does not increase unlimitedly as the number of access points (AP) increases, being upper bounded, contrarily to the ideal linear case. In fact, the rate of each user is limited by the distortion signal of its power amplifier. However, in the downlink, the sum-rate of the system with non-linear power amplifiers is not bounded and increases unlimitedly with the number of APs. Our results also indicate that for the same signal to distortion ratio (SDR) at the power amplifier output or the same normalized saturation level of the power amplifiers, the relative downlink sum-rate degradation is lower than the relative uplink sum-rate degradation (both with respect to their corresponding values in the ideal linear case). In fact, our results confirm that the user side power amplifier non-linearity has higher impact on the system performance than the power amplifier non-linearity on the AP antennas.

Massive MIMO as an Extreme Learning Machine

This work shows that a massive multiple-input multiple-output (MIMO) system with low-resolution analog-to-digital converters (ADCs) forms a natural extreme learning machine (ELM). The receive antennas at the base station serve as the hidden nodes of the ELM, and the low-resolution ADCs act as the ELM activation function. By adding random biases to the received signals and optimizing the ELM output weights, the system can effectively tackle hardware impairments, such as the nonlinearity of power amplifiers and the low-resolution ADCs. Moreover, the fast adaptive capability of ELM allows the design of an adaptive receiver to address time-varying effects of MIMO channels. Simulations demonstrate the promising performance of the ELM-based receiver compared to conventional receivers in dealing with hardware impairments.

АНАЛИЗ ПАРАМЕТРИЧЕСКОЙ ЧУВСТВИТЕЛЬНОСТИ СПОСОБА ОПТИМИЗАЦИИ СТРУКТУРЫ СПЕКТРАЛЬНО-ЭФФЕКТИВНЫХ РАДИОСИГНАЛОВ

Currently, increasing the speed of information transmission in wireless communication networks is a topical issue. The growing demand for spectral efficiency in radio communication systems gives rise to the introduction of signals with amplitude-phase modulation. Despite all its advantages, signals with vector modulation are characterized by a change in the envelope, which makes it impossible to use highly efficient nonlinear power amplifiers. Energy efficiency is clearly a critical factor in portable radio communications systems, where power supplies are often used in small packages, and the presence of energy losses due to heat dissipation is a significant drawback that requires additional cooling systems. In this connection, when developing modern radio communications, radio engineers are faced with the contradictory task of achieving maximum energy and spectral efficiency. One of the approaches to reduce nonlinear distortion of vector modulation is the use linearization schemes for nonlinear power amplifiers. This work is devoted to increasing the energy and spectral efficiency of signals with amplitude-phase modulation. The article discusses the restrictions imposed on the input signals for the method for optimizing of structure of spectrally effective radio signals with vector modulation, the formation of the output signal, the influence of phase shifters phase imbalance on p/2, the index of balance modulation, and the analysis of parametric sensitivity. The possibility of using quadrature modulators to form components with phase modulation and constant envelope is considered. This method was simulated and the effect of phase imbalance on the shape of the output signal constellation was revealed.

Generalized Iterative Dichotomy PAPR Reduction Method for Multicarrier Waveforms

5th generation systems (5G) will be based on Orthogonal Frequency Division Multiplexing (OFDM) multicarrier modulation technique. This modulation has a very high Peak to Average Power Ratio (PAPR). This high PAPR is a drawback when nonlinear Power Amplifiers (PA) are operated near the saturation region, for energy efficiency purposes, due to in-band and out-of-band impairments. In this paper, a new PAPR reduction technique called Iterative Dichotomy PAPR (IDP) is proposed. This method is suited for any multicarrier modulation and is studied here using realistic PA model. For evaluating the signal distortions, two main metrics have been considered to evaluate the performance of IDP technique: bit error rate (BER) and power spectrum density (PSD). The analytical expressions of the IDP technique for any dichotomy order $M$ is firstly described and subsequently the energy efficiency for different $M$ values is compared. We have shown that the proposed IDP method reduces the PAPR by $2.2dB$ for $M=2$ and $4dB$ for $M=4$ . Regarding the energy efficiency, for the same Error Vector Magnitude (EVM) and the same PA power consumption, the output signal power obtained with the IDP based technique outperforms classical OFDM by 20%, 50% and 80% for $M=2$ , $M=3$ and $M=4$ , respectively.

Radio Frequency Fingerprint Identification for Narrowband Systems, Modelling and Classification

Device authentication is essential for securing Internet of things. Radio frequency fingerprint identification (RFFI) is an emerging technique that exploits intrinsic and unique hardware impairments as the device identifier. The existing RFFI literature focuses on experimental exploration but comprehensive modelling is missing. This paper systematically models impairments of transmitter and receiver in narrowband systems and carries out extensive experiments and simulations to evaluate their effects on RFFI. The modelled impairments include oscillator imperfections, imbalance of inphase (I) and quadrature (Q) branches of mixers and power amplifier (PA) nonlinearity. We then propose a convolutional neural network-based RFFI protocol. We carry out experimental measurements over three months and demonstrate that oscillator imperfections are not suitable for RFFI due to their unpredictable time variation caused by temperature change. Our simulation results show that our protocol can classify 50 and 200 devices with uniformly and randomly distributed IQ imbalances and PA nonlinearities with high accuracy, namely 99% and 89%, respectively. We also show that the RFFI has some tolerance on different receiver imbalances during training and classification. Specifically, the accuracy is shown to degrade less than 20% when the residual receiver's gain and phase imbalances are small. Based on the experimental and simulation results, we made recommendations for designing a robust RFFI protocol, namely compensate carrier frequency offset and calibrate IQ imbalances of receivers.

A Nonlinear MIMO-OFDM Based Full-Duplex Cooperative D2D Communications System

Multiple input multiple output (MIMO) based orthogonal frequency division multiplexing (OFDM) has widely been used in a wireless communications system for its robustness to frequency-selective channels and better spectral efficiency. The introduction of full-duplex (FD) and device-to-device (D2D) communications, which are potential candidates of fifth-generation (5G) and beyond, further improve the spectral efficiency of MIMO-OFDM based systems. This paper proposes a novel MIMO-OFDM based FD Cooperative-D2D (C-D2D) communications system wherein a cellular user (CU) acts as an FD relay to facilitate seamless communications between D2D transmitter and receiver. The complete analytical framework is presented to analyze the end-to-end performance of the proposed system. Specifically, the closed-form expression for symbol error rate (SER) of the proposed system is derived in the presence of multipath Rayleigh fading channel and residual self-interference (RSI) due to the FD mode. Further, the performance of the system is analyzed in the presence of a nonlinear high power amplifier (HPA) which introduces the nonlinear distortion. Finally, simulation results are presented to verify the derived expressions. The results show that there is higher SER in the presence of nonlinear HPA due to nonlinear distortion, which can be reduced by increasing input back off (IBO). The presence of RSI also increases the SER at CU, which is further reduced by employing self-interference cancellation (SIC) techniques. A complete end-to-end analytical framework is designed to compare the performance of the proposed system with the conventional system with linear HPA and no RSI.

Continuous-Phase Modulation for DFT-Spread Localized OFDM

Massive machine-type communications to support a large number of low-cost, low-power, and/or low-rate radio-frequency transceivers such as Internet-of-Things (IoT) devices is a major usage scenario of the 5G mobile communication system. To save cost and power, highly nonlinear power amplifiers (PAs) are expected to be used in the devices. However, the envelope fluctuation of the input of a PA may make the output suffer from severe nonlinear distortion. In this paper, a continuous-phase modulation is proposed for discrete-Fourier transform (DFT)-spread localized orthogonal frequency-division multiplexing (OFDM) of π/2-BPSK symbols. In particular, the combination of a spectrum shaping vector and an extra rotation angle is proposed, which enables the use of a receiver that is fully compatible with the current standard. It is shown that the proposed design significantly outperforms the conventional designs in terms of effective signal-to-distortion ratio and error vector magnitude at the cost of a negligible increase in out-of-band emission.

Optimal Parameter Design of Continuous Phase Modulation for Future GNSS Signals

Continuous phase modulation (CPM) is a power and spectrum efficient modulation that intrinsically possesses constant envelope, phase continuity as well as less out of band (OOB) radiation, especially appropriate for stringently bandwidth-constrained systems employing non-linear power amplifiers. Such advantages render CPM as a promising modulation candidate with application to satellite navigation. CPM comprising of a few parameters has large amounts of modulation subclasses. This paper is devoted to optimal parameter design of CPM for future GNSS signals. Parameter design approaches endowing CPM with similar spectrum profiles as legacy binary offset carrier (BOC) and binary phase shift keying (BPSK) modulations are also proposed. Several optimal subclasses of CPM family are suggested as unimodal or bimodal GNSS signal solutions. The simulation results indicate that the designed CPM signals behave same or similar main lobes with BPSK and BOC signals, and proposed optimal subclasses of CPM family with high spectral efficiency are comparable or superior than legacy modulations, typical GMSK, and filtered multitone (FMT) signal using the squared root raised-cosine (SRRC) basic pulse in terms of tracking accuracy, multipath mitigation and compatibility. In addition, some constructive comments on CPM design are presented, which provides a reference for CPM applications in GNSS.

Enhanced Uplink Coverage for 5G NR: Frequency-Domain Spectral Shaping With Spectral Extension

This paper describes and investigates a novel concept of frequency-domain spectral shaping (FDSS) with spectral extension for the uplink (UL) coverage enhancement in 5G New Radio (NR), building on discrete Fourier transform spread orthogonal frequency-domain multiplexing (DFT-s-OFDM). The considered FDSS concept is shown to have large potential for reducing the peak-to-average-power ratio (PAPR) of the signal, which directly impacts the feasible maximum transmit power under practical nonlinear power amplifiers (PAs) while still meeting the radio frequency (RF) emission requirements imposed by the regulations. To this end, the FDSS scheme with spectral extension is formulated, defining filter windows that fit to the 5G NR spectral flatness requirements. The PAPR reduction capabilities and the corresponding maximum achievable transmit powers are evaluated for a variety of bandwidth allocations in the supported 5G NR frequency ranges 1 and 2 (FR1 and FR2) and compared to those of the currently supported waveforms in 5G NR, particularly π/2-BPSK with FDSS without spectral extension and QPSK without FDSS. Furthermore, an efficient receiver structure capable of reducing the noise enhancement in the equalization phase is proposed. Finally, by evaluating the link-level performance, together with the transmit power gain, the overall coverage enhancement gains of the method are analyzed and provided. The obtained results show that the spectrally-extended FDSS method is a very efficient solution to improve the 5G NR UL coverage clearly outperforming the state-of-the-art, while being also simple in terms of computational complexity such that the method is implementation feasible in practical 5G NR terminals.

PAPR reduction in OFDM system using combined MCS and DHMT precoding

Orthogonal Frequency Division Multiplexing (OFDM) has become a preferable scheme for most high data rate wireless communication standards. However, the non-linear power amplifier effect experienced in the OFDM system has increases the peak-to-average power ratio (PAPR). This paper proposed a Median Codeword Shift (MCS) as a new solution to alleviate the effect of high PAPR. MCS takes advantage of the codeword structure and bit position changes through the manipulation of the codeword structure and permutation process to achieve a low PAPR value. Additionally, the enhanced version of MCS is also being proposed by merging MCS with the Discrete Hartley matrix transform (DHMT) precoding method to boost the PAPR reduction. Simulation results show that MCS is capable of minimizing PAPR of conventional OFDM with 24% improvement and at the same time outperform Selective Codeword Shift (SCS) with a 0.5 dB gap. A remarkable result was also achieved by MCS-DHMT with a 15.1% improvement without facing any bit error rate (BER) degradation.

PAPR reduction in OFDM system using combined MCS and DHMT precoding

Orthogonal Frequency Division Multiplexing (OFDM) has become a preferable scheme for most high data rate wireless communication standards. However, the non-linear power amplifier effect experienced in the OFDM system has increases the peak-to-average power ratio (PAPR). This paper proposed a Median Codeword Shift (MCS) as a new solution to alleviate the effect of high PAPR. MCS takes advantage of the codeword structure and bit position changes through the manipulation of the codeword structure and permutation process to achieve a low PAPR value. Additionally, the enhanced version of MCS is also being proposed by merging MCS with the Discrete Hartley matrix transform (DHMT) precoding method to boost the PAPR reduction. Simulation results show that MCS is capable of minimizing PAPR of conventional OFDM with 24% improvement and at the same time outperform Selective Codeword Shift (SCS) with a 0.5 dB gap. A remarkable result was also achieved by MCS-DHMT with a 15.1% improvement without facing any bit error rate (BER) degradation.

Performance Analysis of Power Amplifier Nonlinearity on Multi-Tone SWIPT

We study how the nonlinearity of the high power amplifier (HPA) affects multi-tone simultaneous wireless information and power transfer (SWIPT). Considering the nonlinear HPA model, we analyze the symbol-error rate performance of the peak-to-average power ratio (PAPR) modulation for the multi-tone SWIPT. Numerical results demonstrate that the HPA has a significant impact on both energy harvesting and information decoding while providing insights which is important to take into account the nonlinearity of the HPA when evaluating the end-to-end efficiency of the multi-tone SWIPT.

Full-Duplexing With SDR Devices: Algorithms, FPGA Implementation, and Real-Time Results

In this paper, we present a novel nonlinear digital self-interference canceller algorithm, its implementation details on a software-defined radio (SDR) platform, and performance results of real-time full-duplex experiments on both device and link level. The canceller algorithm is based on an augmented Hammerstein model, with a nonlinear part modeling the transmitter non-idealities followed by a linear filter to model the self-interference (SI) channel. The nonlinear part includes a spline-based model for the nonlinear power amplifier, a polynomial model for baseband nonlinearities, as well as models for I/Q mismatch and LO leakage. The canceller is implemented on an FPGA as a part of an OFDM transceiver testbed for real-time measurements. Extensive real-time measurements show excellent performance: (1) the digital canceller, together with an RF isolator, can suppress the SI to within 1-2 dB's of the receiver noise floor, with total SI suppression of up to 103 dB; (2) digital cancellation of up to 46 dB is evidenced, which is among the highest real-time cancellations in literature; (3) system-level measurements with OFDM signals demonstrate the benefit of utilizing the proposed canceller in a two-way communication scenario, showing up to 90 % increase in sum-rate compared to half-duplex communication.

Joint MIMO Communication and MIMO Radar Under Different Practical Waveform Constraints

A joint transceiver coexistence design of multiple-input multiple-output (MIMO) communication and MIMO radar is addressed under different practical waveform constraints. Without radar waveform constraints, the conventional optimal waveforms can result in large modulus variation and poor range resolution. To tackle these problems, the constant-modulus and/or similarity constraints are added in this work. The proposed constant-modulus waveform constraint is favoured by the cheap and efficient nonlinear power amplifier, and the proposed similarity constraint provides a flexible control on modulus variation, range resolution and transmit power, at the cost of a small extra computational complexity.

Large-Signal Modeling of GaN HEMTs Using Hybrid GA-ANN, PSO-SVR, and GPR-Based Approaches

This article presents an extensive study and demonstration of efficient electrothermal large-signal GaN HEMT modeling approaches based on combined techniques of Genetic Algorithm (GA) with Artificial Neural Networks (ANN), and Particle Swarm optimization (PSO) with Support Vector Regression (SVR). Another promising Gaussian Process Regression (GPR) based large-signal modeling approach is also explored and presented. The GA-ANN addresses the typical problem of local minima associated with the backpropagation (BP) based ANN. The GA successfully aids in the determination of optimal initial values for BP-ANN and enables it to find a unique optimal solution after subsequent of iterations with higher rate of convergence. This is also achieved using PSO-SVR with lower optimization variables. The developed modeling techniques are demonstrated and used to simulate the gate and drain currents of a 2-mm GaN device. All the models are relatively simple, practical, and easy to implement. The gate and drain currents models are embedded in an equivalent large-signal circuit's model and built in Advanced Design System (ADS) software. The implemented model is validated by large-signal measurements and very good fitting results have been obtained. The model also showed an accurate simulation for a nonlinear power amplifier with very good computational speed and convergence.

A Simplified Accuracy Enhancement to the Saleh AM/AM Modeling and Linearization of Solid-State RF Power Amplifiers

The Saleh behavioral model exhibits high prediction accuracy for nonlinearity of traveling-wave tube power amplifiers (TWT-PAs). However, the accuracy of the Saleh model degrades when modeling solid-state power amplifiers (SSPAs) technology. In addition, the polynomial expansion of the Saleh model consists of only odd-order terms as analyzed in this work. This paper proposes a novel model accuracy enhancement for the Saleh amplitude-to-amplitude (AM/AM) model when applied to radio frequency (RF) SSPAs. The proposed model enhancement accounts for the second-order intermodulation distortion, which is an important nonlinearity challenge in wideband wireless communications. The proposed static AM/AM model is a three-parameter rational function, which exhibits low complexity compared to the state-of-the-art behavioral models. A transpose architecture of finite-impulse digital filter is used to quantify the memory effect in SSPAs. A least-squares method is used for extracting all the model parameters. A linearization technique using a three-parameter digital predistortion model is also calculated to compensate for the AM/AM nonlinear distortion in SSPAs. Finally, the identification and evaluation of the enhanced Saleh model is presented based on measurements of RF SSPAs.

Very low‐complexity filter lookup‐table design with non‐uniform spacing for SOA linearisation

Non-linearities introduced by power amplifiers in communication systems have been a subject of interest for industry and academia for a long time. On one hand, the search for increased range intrinsically incites operators to boost input power. On the other hand, this leads to stronger non-linear phenomena in amplifiers, which in turn increase the bit error rate. The situation led to the emergence of digital predistortion algorithms that compensate for the power amplifier's non-linearities and improve the overall performance of the link. A rich corpus of literature exists on this topic but it mainly addresses wireless microwave networks. The case of digital predistortion specifically tailored for optical communications is much less explored. This study focuses on predistortion via filter-lookup-tables and tackles the issue of non-uniform spacing. The work is mainly aimed at coherent optical-orthogonal frequency-division multiplexing systems including low-cost semiconductor optical amplifiers.

Precoding Design and Power Control for SINR Maximization of MISO System With Nonlinear Power Amplifiers

In practical multi-antenna systems, the design of precoding and power control is not straightforward even for multi-input single-output (MISO) systems because of the distortion caused by nonlinear power amplifiers. Through analysis of the signal-to-interference-plus-noise ratio (SINR), we found that the equal gain transmission scheme can achieve the maximum SINR of a nonlinear MISO system only at the point of local optimal transmit power. Then we derived an approximated optimal transmit power for the nonlinear MISO system. To maximize the SINR in all transmit power region, we propose a precoding and power control algorithm for a MISO system with nonlinear power amplifiers under power constraint. By simulation, we show that the proposed scheme can achieve the local optimum SINR, and outperform other baseline precoding techniques.