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Artificial intelligence (AI) provides opportunities to enable high-efficiency wireless communication to dynamically adapt to the local environments and user demands. In this article, a continual learning digital predistortion (DPD) algorithm is proposed to linearize radio frequency (RF) power amplifier (PA) in 6G AI-empowered wireless communication. It is achieved by employing the ability of knowledge retention and knowledge transfer to merge the new PA operating state into the previous model, which makes the model more intelligent when dealing with multiple states. In addition, an augmented self-sensing model is proposed to accurately capture the common characteristics and key dynamic characteristics among different PA operating states to improve the knowledge transfer ability during the continual learning process. The experiments were carried out on a Doherty PA with dynamic configurations associated with power, bandwidth, and signal type. The experimental results have shown that the proposed technique can continuously learn when faced with unpredicted new PA operating states and so as to linearize all known states. The proposed DPD method performs with very low complexity in the long term, which is very suitable for the emerging AI-empowered dynamic scenario. [ABSTRACT FROM AUTHOR]

The coefficients of digital predistortion (DPD) models are often extracted by the least square (LS) method, which has high computational complexity because of the operations of the large matrix. In this letter, a low complexity coefficient extraction method for instantaneous sample indexed magnitude-selective affine (I-MSA) model is proposed. The proposed method uses the tangent of the midpoint of the piecewise function to fit the curve. Therefore, the LS coefficient extraction only needs a small amount of data near the midpoint, rather than using all the data in the segmented region. Simulation and experimental results have confirmed that compared with the original LS method, the proposed method has similar modeling accuracy and linearization performance, but the computational complexity of coefficient extraction is greatly reduced. [ABSTRACT FROM AUTHOR]

A kind of novel general multiband digital predistortion (DPD) model is proposed for the linearization of the multifrequency transmitters. By the theoretical analysis, we extend the existing instantaneous sample indexed magnitude-selective affine (IMSA) function-based model to multidimensional cases, which includes the 2-D-IMSA model and the 3-D-IMSA model for concurrent dual-band and triband scenarios, respectively. The core of the new models is to use the multidimensional IMSA function to replace the complex basis function of the classic multiband model through the method of segmentation and approximation. After segmentation, the proposed models have a simple linear structure in each small area, which greatly reduces the model complexity. Experiments show that the proposed models can give similar normalized mean square error and DPD performance in comparison with existing classic models when a power amplifier is excited by two 20-MHz signals in the concurrent dual-band scenario and by three 20-MHz signals in the concurrent triband scenario. [ABSTRACT FROM AUTHOR]

Various canonical piecewise linear (CPWL) function-based models have been widely used for modeling radio frequency (RF) power amplifiers (PAs). In most literatures, the thresholds of CPWL are often selected according to the uniform distribution, and how to select the optimal thresholds is rarely discussed. This letter presents a new iterative method, which updates the thresholds after calculating the model coefficients, and finally searches the optimal thresholds. The experimental results confirmed that the method can keep good linearization performance and significantly reduce the number of thresholds and the number of model coefficients. [ABSTRACT FROM AUTHOR]

A band-limited magnitude-selective affine (MSA) function-based model is proposed for digital predistortion (DPD) of broadband power amplifiers (PAs) in 5G millimeter-wave frequency band. The new model is derived from the band-limited canonical piecewise linear function-based model by employing MSA functions, which can obviously reduce the number of multiplication operations and the model complexity. Experimental results show that the proposed model gives better normalized mean square error and DPD performance in comparison with some existing models when a broadband PA is excited by a 5G new radio (NR) signal of 400-MHz bandwidth at 28-GHz band. [ABSTRACT FROM AUTHOR]

To exploit the limited communication resources, the real-time carrier frequency of the power amplifier (PA) may hop, which will result in rapid change in PA’s nonlinear behavior and bring huge challenge to make the digital predistortion (DPD) response in a timely manner. Different from the conventional methods that attempt to shorten the DPD updating time, a novel scheme of controlling its nonlinear variation is proposed. By analyzing PA’s various behaviors using graph model and extracting a directed graph to navigate its power adjustments, the variation in nonlinear behavior induced by hopping carrier frequency can be effectively compensated, which will mitigate the DPD performance deterioration when there is not enough time for DPD update. Finally, the proposed method was validated by experiments on a 100-MHz modulated signal stimulated millimeter-wave (mmWave) PA operating at five hopping carrier frequencies from 26.4 to 28 GHz, with the step of 400 MHz. [ABSTRACT FROM AUTHOR]

This letter reports a strategy of full-digital arrays with nonreciprocal transmitting (Tx) and receiving (Rx) beam patterns, for the first time, for 5G/6G wireless communications. The proposed full-digital array contains a smaller number of Rx channels than that of the Tx channels. The unequal number of Tx and Rx channels makes the transmitting and receiving patterns asymmetrical, which would not only release the burden of data processing but also reduce the hardware cost and power consumption of the entire system. Importantly, as compared with conventional large-scale beamforming arrays, the proposed strategy will not deteriorate the range of coverage at an instantaneous time. A 64-element full-digital beamforming array prototype was designed, fabricated, and characterized for a proof-of-concept demonstration. [ABSTRACT FROM AUTHOR]

A novel wideband magnetoelectric (ME) dipole antenna with dual circular polarization and unidirectional radiation is proposed in this letter. The proposed antenna is based on two perpendicular sub-ME dipoles realized by four V-shaped patches and four V-section columns, and each of them is composed of an electric dipole and a magnetic dipole in parallel. Fed by two simple Γ-shaped feeds, 90° phase difference between the electric dipole and the magnetic dipole in each of the sub-ME dipoles is achieved, hence the dual-circularly polarized radiation is generated. Wide impedance bandwidth is obtained in virtue of the two kinds of dipoles being excited simultaneously. The 3 dB axial-ratio (AR) bandwidth is enlarged by optimizing the V-section columns and a circular cavity. Besides, the port isolation is apparently enhanced by adding slits on the patches and elaborating different spacings between each feed and its two-sided columns. Measured results show that the proposed antenna has many appealing properties, such as a unidirectional radiation pattern, an impedance bandwidth (reflection coefficients ≤–10 dB and port isolation ≥ 10 dB) of 51.1% and a 3 dB AR bandwidth of 41.9%. [ABSTRACT FROM AUTHOR]

A 2-D simplified memory polynomial (2D-SMP) model is proposed for concurrent dual-band power amplifiers (PAs) in this letter. The derivation of the proposed model is similar to that of the 2-D modified memory polynomial (2D-MMP) model. The 2D-SMP model consists of an in-band part and a cross-band modulation part without the envelope factors, which exist in the 2D-MMP model. Theoretical analysis shows that it has moderate complexity but higher modeling accuracy than the 2D-MMP model. The simple least square algorithm can be used to extract the model coefficients, and the additional binary search algorithm for the best envelope factor is not needed. The experimental results show that the 2D-SMP model can achieve higher model accuracy and better digital predistortion (DPD) performance than the 2D-MMP model when a dual-band PA is excited by a combined dual-band long-term evolution signal. [ABSTRACT FROM AUTHOR]

A 2-D-canonical piecewise linear (2-D-CPWL) function-based behavioral model is proposed for concurrent dual-band power amplifiers (PAs) in this letter. The model is derived from the Volterra-based 2-D-modified memory polynomial (2-D-MMP) model by replacing the high-order terms with CPWL and it can provide a lower complexity than the 2-D-MMP model and some existing CPWL-based dual-band models. The least-square algorithm can be used to extract the model coefficients. Experimental results show that the proposed 2-D-CPWL model can achieve nearly the same model accuracy and digital predistortion performance compared with the 2-D-MMP model, when a dual-band PA is excited by a combined dual-band long-term evolution signals. [ABSTRACT FROM AUTHOR]

This paper presents a controllable mode design method for a concurrent dual-mode power amplifier (PA). The principle of the controllable mode combination is proposed by investigating the characteristics of different highly efficient PA modes, which is analyzed from frequency response of the low-pass matching circuit. Then a dual-mode PA with Class-\bf F^-\bf 1 and Class-J mode is taken for example to verify this mode combination concept. A GaN device PA with 10-W output is designed at two frequency ranges of 1.9\sim2.0 GHz and 2.3\sim2.6 GHz. Measurement results show an output power of 40.2\sim42.9 dBm and drain efficiency of 75.6\sim80.3% and 76.6\sim68.8% in two frequency bands. It also provides a broadband operation with drain efficiency of 60\sim80.3% over 1.7\sim2.8 GHz. To evaluate its modulated signal performance with concurrent multiband and broadband applications, dual-band signals (20 MHz + 20 MHz) with 140- and 300-MHz frequency spacing and 100-MHz long-term evolution advanced (LTE-A) signals at two modes are both employed. With digital pre-distortion algorithm, adjacent channel leakage ratio lower than -47.5, -48.6 and -46.1 dBc are achieved, indicating its excellent performance in mobile communication systems. [ABSTRACT FROM PUBLISHER]

A band-limited canonical piecewise-linear function (CPWL)-based model is proposed for wideband power amplifiers (PAs). The model has a similar structure of the band-limited dynamic deviation reduction (DDR) Volterra series model but without high-order terms and long finite impulse response filters, which are replaced by CPWL. The model has lower complexity and more flexibility than the band-limited DDR Volterra series model and the model parameters can be estimated by the least-square method. Experimental results show that the proposed model nearly gives the same normalized mean square error and digital predistortion performance as the band-limited Volterra series when a wideband PA is excited by a 5-carrier long-term evolution advanced signal of 100-MHz bandwidth. [ABSTRACT FROM PUBLISHER]

This paper presents a high-efficiency GaN Doherty power amplifier (PA) with 100-MHz instantaneous bandwidth for 3.5-GHz long-term-evolution (LTE)-advanced application. A modified load modulation network, employing an enlarged peaking amplifier to carrier amplifier power ratio and moderately increased load impedance of the carrier amplifier, is proposed for enhancing efficiency and achieving improved load modulation. To increase the power ratio and alleviate the influence of slight impedance mismatch, a proposed load impedance strategy and corresponding stepped-impedance matching network are adopted for high-efficiency and wideband operation. By tuning the carrier offset line, the inconsistency of efficiency, gain, and output power in the operation band can be alleviated. Measurement results show that the Doherty PA has a drain efficiency of approximately 40% with gain fluctuation less than 0.5 dB at 9-dB back-off power, and maximum efficiency of about 60% at saturation in the signal band of 3.4–3.5 GHz. By using the digital pre-distortion (DPD) technique, the Doherty PA achieves adjacent channel leakage ratio of about -48 dBc at an average output power of 40.4 dBm with efficiency of 42.5%, when driven by 100-MHz LTE-advanced signal. To the best of the authors' knowledge, this is the first high-performance result of linearization using conventional DPD technique with 100-MHz bandwidth signals for the GaN Doherty PA at 3.5-GHz frequency band thus far. [ABSTRACT FROM PUBLISHER]

A modified canonical piecewise-linear function (CPWL)-based model is proposed for characterizing wideband power amplifiers. The model has a similar structure of the decomposed vector rotation (DVR) model but without DVR operations, in which the first-order basis function can be replaced by a second-order function by analysis. The model has lower complexity than the DVR model and the model parameter scan be estimated by least-square method. Experimental results show that the proposed model nearly gives the same normalized mean square error performance as the DVR model when a wideband Doherty power amplifier is excited by a 5-carrier long-term evolution advanced (LTE-A) signal of 100 MHz bandwidth. [ABSTRACT FROM PUBLISHER]

A nonlinear filter-based Volterra model with low complexity is proposed for modeling and characterizing wideband power amplifiers. This model has a similar form of a simplified second-order dynamic deviation reduction (DDR)-based Volterra model but without complex square root calculations and high order terms, which are replaced by a first-order nonlinear filter. The model parameters with smaller magnitude distribution range and lower precision in storing and calculation can be estimated by least-square method, and then the model has lower complexity than the second-order simplified DDR Volterra model. Experimental results verified that good performance and lower complexity can be achieved by using this model for a wideband Doherty power amplifier. Finally, the model is employed to compensate the nonlinearity of the Doherty power amplifier, and over 12 dB adjacent channel leakage ratio (ACLR) improvement is achieved for a five-carrier long-term evolution advanced (LTE-A) signal of 100 MHz bandwidth. [ABSTRACT FROM PUBLISHER]