Compensation of power amplifier nonlinear distortion in spatial modulation systems

Joint waveform and beamforming design is demonstrated as an effective approach to improve the wireless energy transfer (WET) performance. Meanwhile, the emerging technique of reconfigurable holographic surface (RHS) without the half-wavelength limitation is able to achieve a higher spatial gain. Hence, the RHS-assisted WET system is designed to enhance WET performance by optimizing the waveform and beamforming. However, the ideal linear power amplifier is always conceived, while the nonlinearity of the power amplifier is ignored. In this paper, a more practical nonlinear power amplifier (NPA)-based RHS-assisted WET system is proposed, where the energy harvesting (EH) model by considering the channel aging effect is derived to adapt to the time-varying channel. In order to improve the WET performance, a predistortion technique is adopted to tackle the nonlinear input-output characteristic of NPAs, while the waveform and beamforming are jointly optimized. Simulation results demonstrate that the RHS outperforms the phased array benchmark having the same size on the WET performance, when the NPAs work in the unsaturated region. Furthermore, the impact of the NPAs on the WET performance by considering channel aging effect is demonstrated, while the waveform design is invalid when the NPAs work near the saturation region.

Network densification is essential to enhance the areal capacity of future wireless networks, for which the user-centric network is a promising solution. In the perspective of network scalability, user association and cooperative beamforming (BF) among the base stations (BSs) should be designed elaborately to manage strong interference from nearby BSs. However, practical systems with hardware impairments including nonlinearity of power amplifiers may disturb these techniques and make the network difficult to be scalable. The nonlinearity of power amplifiers affects not only radio frequency (RF)-level signal but also performance according to the number of user equipments (UEs), resulting in necessity of re-designing network-level operations including BS-UE association. This paper proposes a deep learning-based cooperative BF framework for distributed networks with nonlinear power amplifiers. To optimize the cooperative BF for complicated nonlinear systems, we adopt unsupervised learning approach with constraints. To reduce the communication overhead of the network, we propose a novel neural network structure, from which the BSs can perform the cooperative BF in distributed manner. In particular, the information exchange between the central unit and the local BSs is reduced by designing neural network so that the local channel state information are utilized only in the local BSs while the central unit utilizes only covariance matrices. We show that the proposed scheme achieves a higher effective sum rate compared to the baseline schemes by adjusting the user association, BF, and power allocation to control interference and nonlinear distortion with reduced communication overhead.

Multiple-input multiple-output (MIMO) millimeter-wave (mm-wave) systems are vulnerable to hardware impairments due to operating at high frequencies and employing a large number of radio-frequency hardware components. In particular, nonlinear power amplifiers (PAs) employed at the transmitter distort the signal when operated close to saturation due to energy efficiency considerations. In this paper, we study the performance of an MIMO mm-wave hybrid beamforming scheme in the presence of nonlinear PAs. First, we develop a statistical model for the transmitted signal in such systems and show that the spatial direction of the inband distortion is shaped by the beamforming filter. This suggests that even in the large antenna regime, where narrow beams can be steered toward the receiver, the impact of nonlinear PAs should not be ignored. Then, by employing a realistic power consumption model for the PAs, we investigate the tradeoff between spectral and energy efficiency in such systems. Our results show that increasing the transmit power level when the number of transmit antennas grows large can be counter-effective in terms of energy efficiency. Furthermore, using numerical simulation, we show that when the transmit power is large, analog beamforming leads to higher spectral and energy efficiency compared to digital and hybrid beamforming schemes.

This paper presents a detailed analysis of the impact of power amplifier (PA) non-linearity on the performance of single carrier (SC) and OFDM systems in 60 GHz spectrum. The efficiency of the considered systems was analyzed by estimating values of the power amplifier output backoff needed to meet the requirements on the transmit spectrum mask (TSM) compliance and error vector magnitude (EVM). Different models of the PA including an experimentally measured model for the 60 GHz PA were considered. It was estimated that for many types of modulations and PA models, SC system can have up to 1.5 dB higher output power than OFDM system. However, for the experimental PA model accounting for amplitude modulation-phase modulation distortion in addition to amplitude modulation amplitude modulation distortion, the difference between the OFDM and SC output power levels was minimal, and OFDM even had approximately 1 dB better performance than SC for the BPSK modulation. A potential improvement in the transmit power due to application of digital predistortion techniques was estimated for both systems to be in the range from 1.5 to 3.4 dB.

Nowadays energy efficiency (EE) of wireless communication systems has become a hot issue, yet the nonlinear effect and inefficiency of power amplifier (PA) have posed practical challenges for system designs to achieve high EE. However, most of previous work only considered linear PA. In this paper, we studies EE optimization concerning with I-way Doherty PA which has widespread use with I = 1 and I = 2 in orthogonal frequency division multiplex (OFDM)-based cognitive radio (CR) systems. The aim is to maximize EE with nonlinear PA subject to the total power budget, the interference constraint and the minimum rate requirement. Other than traditional methods, the problem is hard to solve directly due to the nonlinearity of PA, and a bisection search method tailored for nonlinear PA is adopted to achieve the sub-optimal solution. Numerical results demonstrate that if the PA's nonlinearity is not considered, the EE performance of OFDM-based CR systems can be severely overestimated by 34% at P max out = 80 W for 1-way Doherty PA. Meanwhile, the EE performance can be enhanced by approximate 32% if 2-way Doherty PA is utilized instead of 1-way Doherty PA.

Performance of conventional baseband digital predistortion method for power amplifiers (PAs) degrades severely as the accuracy of loop delay compensation (LDC) decreases. The existing LDC error free method based on cumulative distribution function (CDF) neglects the phase nonlinearity of PA and therefore further application is limited. An improved CDF predistortion method is proposed in this paper, which considers not only amplitude nonlinearity but also phase nonlinearity of PAs. Experiment based on High Frequency (HF) PA is done, and the results show that the proposed method works well in compensating the amplitude and phase nonlinearities of HF PA, and the performance in third-order intermodulation (3IM) of the PA output can be improved by 23 dB, but the improvement of the conventional CDF method is 8 dB.

Improved spectral and energy efficiency are two key requirements for the fifth generation (5G) wireless communication systems. The inefficiency and non-linearity of traditional power amplifiers (PA) pose significant challenges to meet these requirements, especially for multicarrier modulation signals (MCM) with high peak to average power ratio (PAPR). In this paper, we study methods to improve the energy efficiency of PAs and compensate for the distortion of PA non-linearity. A low complexity envelope tracking (ET) method where the supply voltage of the PA consists of two or more discrete levels is presented. To reduce the distortion due to PA nonlinearity, a power amplifier model with power supply variation is introduced and a digital predistortion technique based on a Volterra series is used. Finally, an integrated architecture combining crest factor reduction (CFR), DPD, and ET is presented. Simulations and measurements from a hardware platform demonstrate the energy efficiency gains attained by the proposed techniques.

The paper tackles the problem of efficient reduction of the out-of-band (OOB) radiation in multi-carrier systems when considering the presence of a nonlinear power amplification. It is important to provide “green” transmitters producing less electromagnetic “pollution” out of its nominal frequency band and obtaining at the same time high energy efficiency of the used power amplifier. One of the known methods assumes appropriate modulation of dedicated subcarriers by specially calculated symbols reducing OOB radiation. The solution proposed in this paper combines two approaches, among which the first, called cancellation carriers, is used traditionally for subcarrier sidelobes minimization, and the second, called tone reservation, for peak to average power ratio reduction. The novelty of this work is in joint minimization of both aforementioned metrics in the near optimal way while keeping low computational complexity. The result is seen as a decreased power of the OOB radiation observed after signal passing through the power amplifier. The obtained results confirm that the proposed algorithm outperforms the approaches in which those metrics are minimized separately. Additionally, a number of simulations has been carried out to investigate the influence of the proposed algorithm parameters on the resultant out-of-band radiation performance.

The performance of turbo-coded amplitude-phase shift keying (APSK) in an aeronautical telemetry system with a non-linear power amplifier is investigated. The AM/AM curves for four L-band power amplifiers are modeled and used to simulate the performance the turbo-coded APSK system over a nonlinear channel. Spectral efficiency and bit error rate performance as a function of back-off are quantified and compared with a system based on the existing aeronautical telemetry standard using uncoded SOQPSK. The result show that the -50 dBc bandwidth can be matched with 2.5 to 4.1 dB of backoff, the -60 dBc bandwidth can be matched with 4.3 to 6.6 dB of backoff, and the 99% bandwidth can be matched with 0.8 to 1.6 dB of backoff. The bit error rate results show that each power amplifier has an optimal backoff and that this backoff is 3 dB for the four amplifiers used in this study. When put together, this system is capable of outperforming an uncoded system based on SOQPSK by matching (or exceeding) the detection efficiency while simultaneously achieving a spectral efficiency in excess of three times that of SOQPSK.

Power consumption and dissipation of satellite payloads for space telecommunication systems are mainly due to power amplifiers. To increase system capacity with limited bandwidth, multicarrier operations are required. Linearity is specified to limit the signal distortion by the nonlinear power amplifiers. In order to minimize power consumption under those linearity requirements, accurate measurements have to be performed. The NPR is well known as the figure of merit for the intermodulation distortion performance of amplifiers in telecommunications. In this paper, the development of an accurate large bandwidth NPR measurement system is presented. It enables characterizations up to Ka band (17-21 GHz and 27-31 GHz for space telecommunications applications with typically 250 MHz bandwidth). Measurements made on power amplifiers have been used to optimize operating conditions.

The nonlinear power amplifier (NPA) in a cognitive radio (CR) network can degrade sensing performance. While spectrum sensing primarily focuses on the receiver, modeling the transmitter's NPA is crucial to understand its impact on detection. This paper investigates spectrum sensing performance in CR networks assisted by intelligent reconfigurable surfaces (IRS), considering the impact of NPA distortions. A novel multiple‐input single‐output (MISO) IRS‐aided CR system, termed MICR, is proposed. The sensing performance of the proposed MICR system is analyzed in terms of the probabilities of false alarm and detection, considering the effect of IRS elements. The results are presented using receiver operating characteristic (ROC) curves. Furthermore, the impact of various system parameters such as the number of samples, number of IRS elements, IRS position, number of transmitting antennas, and phase shifts is analyzed. Additionally, the effects of line‐of‐sight (LoS) and non–line‐of‐sight (NLoS) propagation are evaluated. The ROC performance using different signal detectors is analyzed. The impact of interference, IRS overhead, and sensing energy on the proposed MICR system is also investigated. Extensive simulations demonstrate that IRS significantly improves the sensing performance of the CR network, both with and without NPA.

In this paper, an efficient method to characterize electrical memory effects arising from the interaction between the static nonlinearity and the frequency response is proposed using a multi-tone measurement. A transmitter system with nonlinear power amplifier (PA) is modeled with two box Wiener structure that consists of a static Saleh model and a linear filter. A nonlinear least square method with a multi-tone measurement is used to extract frequency response and nonlinear characteristics. The proposed method is verified with a simulation and experiment with a 10W class AB gallium-nitride (GaN) HEMT PA.

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.

Polynomial predistortion techniques for power amplifier (PA) non-linearities based on indirect learning architecture (IDLA) are widely used. The benefit of the IDLA leaves unnecessary the assumption of a model for PA, corresponding parameters estimation and inverse construction. In this paper, a novel scheme based on IDLA for PA predistortion is proposed. It has better power spectral density (PSD) and relative mean square error (RMSE) performances than the conventional IDLA-based methods especially when the PA non-liearity is more severe. Simulations and theoretical analysis verify the good performances of the proposed scheme.

With the rapid development of wireless communication techniques, the main limitations of wireless communications lie in spectrum resource constraints and nonlinearity of high amplifier power (HPA). Albeit exploring high-order modulation improves the spectrum efficiency significantly, however, the nonlinear distortion seriously degrades the performance especially when nonlinearity sensitive high-order non-constant envelop modulation signal pass through the saturate regime of HPA. Due to the good performance in linearization, adaptive baseband digital pre-distortion has been widely developed and applied. Particularly, the optimization of the parameters for adaptive pre-distortion algorithm has a positive impact on linearization. In this paper, an improved algorithm is proposed to determine the optimum step by taking maximum error vector magnitude EVM improvement as the objective function. And thus the nonlinearity of power amplifier is compensated with indirect learning architecture and optimum step adaptive algorithm. Simulation results demonstrate that the constellations are well compensated and out-band power spectrum regenerations are significantly suppressed, which yields a satisfactory system performance linearization.