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High frequency (HF) transmitter implemented with the multioctave broadband power amplifier (PA) is always faced with the problem of interfering harmonics because of the nonlinearity of the PA. With the harmonic‐cancelation digital predistortion (HC‐DPD) scheme, the harmonics at lower frequencies can be canceled by the injected components and the harmonics at higher frequencies can be filtered out using a low‐pass filter (LPF). However, the HC‐DPD scheme brings the problem of unwanted spurs while lowering the requirements for sampling rate. In this article, a time‐delay neural network (TDNN) model is used to replace the conventional memory polynomial (MP) model in the forward modeling. Theoretical derivation and simulation results validate the effectiveness of the TDNN model in harmonic cancelation and spur suppression. Further, a frequency‐agile neural network (FANN) model is proposed based on the TDNN model. By adding the carrier frequency to the inputs of the network, the trained model can apply to all the trained carrier frequencies and is more friendly to the frequency‐hopping scenarios. Experiments were carried out on a 2–30 MHz HF transmitter testbench. Measurement results show that the harmonic‐to‐fundamental power ratio, the adjacent channel power ratio (ACPR), and the error vector magnitude (EVM) performances were all improved by more than 20 dB. Compared with the MP model, the unwanted spurs can be suppressed by up to 22 dB. In addition, the number of total model coefficients of the proposed FANN model is only 22% of that of the TDNN model under the frequency‐hopping scenario. [ABSTRACT FROM AUTHOR]

High frequency (HF) transmitters implemented with the multi‐octave power amplifier (PA) are always faced with the problem of interfering harmonics because of the nonlinearity of the PA. In this paper, the harmonic cancelation digital predistortion (HC‐DPD) scheme is proposed and well discussed to cancel the in‐band intermodulation distortion (IMD) as well as out‐of‐band harmonics at the same time. With the digital filter‐less scheme, the bulky and lossy filter banks can be removed. Nonlinear models are first utilized to approximate the nonlinear behavior of the HF PA and compared in terms of the model accuracy. Then, the learning architecture, model identification, and feedback compensation of the HC‐DPD scheme are detailed discussed. Finally, a series of simulations and experiments demonstrated the effectiveness of the proposed nonlinear models and linearization algorithms. [ABSTRACT FROM AUTHOR]

This paper proposes a single and dual‐band compatible digital predistortion (SDBC‐DPD) for broadband transmitters with carrier aggregation. Since complicated spectrum scheduling, the stimuli of the power amplifiers (PAs) can be single‐band and dual‐band. The proposed SDBC‐DPD scheme can well linearize a dual‐band signal in a conventional 1‐D‐DPD way utilizing a spectrum splicing and segmentation method. A series of experiments were carried out on a dual‐band test bench based on a 3.0–4.0 GHz PA. The stimuli with different bandwidths and sampling rates were used in the experiments. Measurement results show that the SDBC‐DPD can well mitigate the nonlinearity in the dual‐band signals and needs a much lower sampling rate for model identification. [ABSTRACT FROM AUTHOR]

This article presents a $D$ -band Doherty power amplifier (PA) featuring a low-loss and compact slotline-based eight-way power combiner to improve the saturated output power and power back-off (PBO) efficiency. The proposed combiner consists of grounded coplanar waveguides (GCPWs), slotlines, and SLOT-GCPW coupling structures. Instead of the conventional transformer structure, the proposed SLOT-GCPW coupling structure effectively reduces insertion loss, increases the common-mode rejection, and encounters a comfortable symmetrical compact layout. More importantly, the proposed combiner can also be applied to achieve impedance inverters for active load modulation in a compact size. Based on the proposed multiway combining Doherty PA prototype, a high-efficiency and high-output $D$ -band PA is implemented in a 130-nm SiGe bipolar CMOS (BiCMOS) with ${f_{t}/f_{\max }}$ of 350/450 GHz. The extremely compact power combiner leads to a small PA core area of $1.1 \times 0.53$ mm2. The PA achieves a peak small-signal gain of 21.8 dB with 3-dB bandwidth from 107 to 135 GHz. At 110/120/130 GHz, the PA achieves 22.7/22.6/22.4 dBm ${P_{\mathrm{ sat}}}$ with 18.7%/17.2%/16.1% peak power-added efficiency (PAE) and 12.1%/11.7%/9.8% PAE at 6-dB PBO from ${P_{\mathrm{ sat}}}$. Modulation measurement with single-carrier 16-quadrature amplitude modulation (QAM) and 64-QAM signals has been performed. For a 2-GHz 16-QAM signal, the PA achieves 13.74 dBm ${P_{\mathrm{ avg}}}$ and 7.88% average collector efficiency with 11.6% error vector magnitude (EVM) at 131.5 GHz. The PA also supports a 2-GHz 64-QAM signal, achieving 13.76 dBm ${P_{\mathrm{ avg}}}$ and 7.92% average collector efficiency with 11.9% EVM at 131.5 GHz. [ABSTRACT FROM AUTHOR]

In this article, a uniform neural network (NN) digital predistortion (DPD) model of radio frequency (RF) power amplifiers (PAs) is proposed for dynamic applications, which is suitable for RF PAs under various operating conditions without updating the coefficients. With the development of communication systems, it is difficult for the DPD to track the nonlinearity of the PA as the operating condition varies frequently. As one of the most promising achievements in recent years, the NN has shown excellent generalization ability, which is applicable to the DPD for scalable applications. In this situation, a uniform neural network model (UNNM), whose structure is a two-stage network, is proposed for scalable output power, scalable bandwidth, or simultaneous scalable power and bandwidth. The experiments are carried out on two sub-6 GHz broadband GaN Doherty PAs (DPAs). The experimental results show that the proposed model can achieve comparable performance without coefficient update in the scalable output power range of about 5 dB and the bandwidth range of 100 MHz, which outperforms the conventional fixed model with better than 3 dB power range and 40 MHz bandwidth range. [ABSTRACT FROM AUTHOR]

A novel dual-band printed diversity antenna is proposed and studied. The antenna, which consists of two back-to-back monopoles with symmetric configuration, is printed on a printed circuit board. The effects of some important parameters of the proposed antenna are deeply studied and the design methodology is given. A prototype of the proposed antenna operating at UMTS (1920-2170 MHz) and 2.4-GHz WLAN (2400-2484 MHz) bands is provided to demonstrate the usability of the methodology in dual-band diversity antenna for mobile terminals. In the above two bands, the isolations of the prototype are larger than 13 dB and 16 dB, respectively. The measured radiation patterns of the two monopoles in general cover complementary space regions. The diversity performance is also evaluated by calculating the envelope correlation coefficient, the mean effective gains of the antenna elements and the diversity gain. It is proved that the proposed antenna can provide spatial and pattern diversity to combat multipath fading. Index Terms--Diversity antenna, diversity gain, dual-band antenna, envelope correlation coefficient, mean effective gain (MEG).

A novel electromagnetic-bandgap (EBG) structure in a fork-like shape is investigated. This structure has an extremely compact size. A comparison has been carried out between the new structure and the conventional mushroom-like EBG structure. Simulations and experimental results have verified that the area of the fork-like structure is less than 40% of the latter. The presented structure also provides an additional degree of freedom to adjust the bandgap position, which is applied to design a novel reconfigurable multiband EBG structure. Several application examples have been demonstrated, including a double-element microstrip antenna array with low mutual coupling, notch-type antenna duplexer, and steerable array with a linearly discrete beamsteering of 20[degrees] in steps of 10[degrees] at 2.468 GHz. Index Terms--Beamsteerer, electromagnetic-bandgap (EBG) structures, microstrip antenna arrays, periodic structures, reconfigurable structures, surface wave.

The ground plane affects the characteristics of a planar inverted-F antenna (PIFA) significantly. Using the idea of the high-impedance surface to construct the ground plane, a new type of compact PIFA with a photonic bandgap type (PBG-type) ground plane is proposed. Both the traditional PIFA and the proposed antenna are analyzed using the finite-difference time-domain (FDTD) method in detail. It is found that the two antennas have similar directivity pattern characteristics, but the size of the latter can be reduced obviously. The results also show that the bandwidth of a traditional PIFA is not greatly affected by the thickness of the metallic ground plane and that the shapes of the directivity patterns are not greatly affected by the ground plane under the small size condition. The operating-frequency band of the proposed antenna can be adjusted by changing the dielectric substrate thickness of the ground plane. They are suitable for mobile communication applications. Index Terms--finite-difference time-domain (FDTD) method, high-impedance surface, mobile antennas, mobile communication, photonic bandgap, planar inverted-F antenna (PIFA).

In this article, a high-efficiency broadband millimeter-wave (mm-Wave) integrated power amplifier (PA) with a low-loss slotline-based power combing technique is proposed. The proposed slotline-based power combiner consists of grounded coplanar waveguide (GCPW)-to-slotline transitions and folded slots to simultaneously achieve power combining and impedance matching. This technique provides a broadband parallel–series combining method to enhance the output power of PAs at mm-Wave frequencies while maintaining the compact area and high efficiency. As a proof of concept, a compact four-to-one hybrid power combiner is implemented in a 130-nm SiGe BiCMOS back-end-of-line (BEOL) process, which leads to a small die area of 126 $\mu \text{m}\,\,\times $ 240 $\mu \text{m}$ and a low measured insertion loss of 0.5 dB. The 3-dB bandwidth is over 80 GHz covering the whole G-band (140–220 GHz). Based on this structure, a high-efficiency mm-Wave PA has been fabricated in the 130-nm SiGe BiCMOS technology. The three-stage PA achieves a peak power gain of 30.7 dB, 3-dB small-signal gain bandwidth of 40 GHz from 142 to 182 GHz, a measured maximum saturated output power of 18.1 dBm, and a peak power-added efficiency (PAE) of 12.4% at 161 GHz. The extremely compact power combining methodology leads to a small core area of 488 $\mu \text{m}\,\,\times $ 214 $\mu \text{m}$ and an output power per unit die area of 662 mW/mm2. [ABSTRACT FROM AUTHOR]

This article presents a highly linear Doherty power amplifier (DPA) based on phase mismatch. When output phase mismatch (OPM) is introduced, i.e., the phase shift of the output impedance transformer deviates from 90°, the power-combining network (PCN) will exhibit a certain amplitude-to-phase (AM–PM) characteristic. When input phase mismatch (IPM) is introduced, i.e., the main and auxiliary branches are not phase-aligned, the AM–PM of the PCN can be further finely tuned. By choosing proper OPM and IPM, the AM–PM of the overall DPA can be compensated by that of the PCN while maintaining reasonable back-off and saturated performances. Moreover, the PCN with phase mismatch shows gain expansion, and thus the amplitude-to-amplitude (AM–AM) distortion of the DPA can also be improved to some extent. A fully integrated DPA is implemented in a $0.25~\mu \text{m}$ gallium nitride (GaN)-HEMT process to validate the proposed method. The fabricated DPA realizes an AM–PM of 2° and an AM–AM of 0.3 dB at 6.3 GHz, with a saturated power of 41.1 dBm and a 6 dB back-off drain efficiency (DE) of 45%. Applying a 200 MHz signal with a 7.8 dB peak-to-average power ratio (PAPR), a raw adjacent channel power ratio (ACPR) of −42 dBc and an average DE of 37.4% are measured at the output power of 33.1 dBm. When the carrier frequency is swept from 6.1 to 6.5 GHz, a raw ACPR below −39 dBc and an average DE better than 37% are maintained. [ABSTRACT FROM AUTHOR]

This letter presents a high precision wideband active vector-modulating phase shifter with high dynamic range gain control ability. Compared with conventional switching-filter phase shifter and vector-modulation phase shifter, this $\Delta $ – $\Sigma $ modulated vector phase shifter can provide quasi-continuous phase and gain control capability. According to the experimental results, the dynamic range of gain control is greater than 35.5 dB. The output reflection is lower than −10 dB through the 18–50-GHz band, and the input reflection is lower than −10 dB from 18 to 45 GHz. The phase shifter occupies 700 $\mu \text{m}\,\,\times $ 900 $\mu \text{m}$ on 0.13- $\mu \text{m}$ SiGe bipolar complementary metal-oxide-semiconductor (BiCMOS) technology, and the core size is only $400\,\,\mu \text{m}\,\,\times 500\,\,\mu \text{m}$. After digital calibration, the proposed phase shifter has achieved the highest phase and gain control precision compared with the state-of-the-art digital controlled phase shifters. [ABSTRACT FROM AUTHOR]

In this article, a fully integrated dual-band gallium nitride (GaN) Doherty power amplifier (DPA) based on multi-resonant circuits (MRCs) is presented. A lumped T-type network in combination with the output capacitances of the main and auxiliary transistors is able to realize the quarter-wavelength transformer (QWT) in DPA, and its component values are totally decided by the operating frequency, given a certain characteristic impedance. MRCs are proposed to exhibit the required component values at two different frequencies. By replacing each component in the T-type network with an MRC, a dual-band QWT can be achieved. An integrated dual-band DPA is implemented in a commercial 0.25- $\mu \text{m}$ GaN-HEMT process to validate the proposed method. The fabricated DPA shows a saturated output power of 43.8–44.7/44.4–44.8 dBm, a 6-dB back-off drain efficiency (DE) of 41%–46%/41.5%–47%, and a saturated DE of 49%–52%/56%–59% in 3.35–3.6/4.8–5.2 GHz, with a compact size of $2.8\times3.5$ mm2. Applying a 100-MHz OFDM signal with a 7.8-dB peak-to-average power ratio (PAPR), an average efficiency of 37%/40% and an average power of 37/37 dBm are measured at 3.5/5.0 GHz, and the adjacent channel power ratio (ACPR) is better than −49 dBc after digital predistortion (DPD). To the best of our knowledge, the proposed DPA is the first demonstration of fully integrated dual-band GaN DPAs. [ABSTRACT FROM AUTHOR]

Conventional digital predistortion (DPD) techniques are facing the challenge of complexity in the fifth generation (5G) and beyond concurrent dual-band terminal transmitters. This article proposes novel low-complexity behavior models for the DPD of terminal power amplifiers (PAs). Derived from the conventional memory polynomial (MP) model, the new models utilize magnitude-selective affine (MSA) functions as nonlinear basis functions to greatly decrease the model complexity. The proposed paralleled envelope indexed 2-D MSA (PI-2-D-MSA) model and combined envelope indexed 2-D MSA (CI-2-D-MSA) model take both the in-band intermodulation and cross-band modulation into account and can well compensate for nonlinear distortion in dual-band terminal PAs. Moreover, the proposed models are extended to nonuniform sampling rate (NSR) scenarios, where the aliasing effect for the low sampling rate band is analyzed. Simulations and experimental measurements under the uniform sampling rate and the NSR scenarios were carried out on a 1.7–2.8-GHz wideband Doherty PA and a 4.4–5.0-GHz class-AB terminal PA and demonstrated the linearization performances of the proposed models. The total numbers of floating-point operations (FLOPs) of the PI-2-D-MSA and the CI-2-D-MSA models were 11% and 8% of that of the prior 2-D-MP model, respectively. Even though their linearization accuracy deteriorated, the relaxed linearization standard for terminal transmitters can still be achieved. [ABSTRACT FROM AUTHOR]

In this brief, a 24–29.5 GHz millimeter-wave (mm-wave) voltage-combined Doherty power amplifier (DPA) based on compact low-loss combiner is proposed for the fifth-generation (5G) applications. By maximizing the use of parasitic parameters in the circuit, a compact low-loss combiner is proposed to achieve the required impedance transformation with only one additional series capacitor. A detailed design procedure is also introduced in the brief. Based on the proposed combiner, a fully integrated Doherty PA is implemented in 0.13- ${\mu }\text{m}$ SiGe BiCMOS process with a core area of only 0.29 mm2. The DPA achieves a 27% −3-dB small-signal gain fractional bandwidth and a 15.9 dB small signal gain at the center frequency of 27 GHz. The −1-dB Psat bandwidth covers 24–29.5 GHz (20% fractional bandwidth), and the maximum output power is 20.8 dBm ± 0.4 dB. From 24 to 29.5 GHz, the designed DPA provides 23.2% to 28.4% power-added efficiency (PAE) at saturated output power, and 18.3% to 21.9% PAE at 6 dB of output power back-off (OPBO). [ABSTRACT FROM AUTHOR]

In this article, a novel broadband millimeter-wave (mm-wave) front end (FE) based on the asymmetric switch network structure is proposed for multistandard fifth-generation (5G) applications. The asymmetric switch network consists of a bidirectional impedance transformation network and a T-type topology single-pole single-throw (SPST) switch. In the TX-mode, the bidirectional impedance transformation network serves as an output matching network to achieve a continuous-mode Class-F−1 power amplifier (PA) to expand the bandwidth. In the RX-mode, the OFF impedance of the PA can be transformed into a high-impedance region by the bidirectional impedance transformation network. Based on the proposed structure, a broadband mm-wave front end is implemented in the 0.13- $\mu \text{m}$ SiGe BiCMOS process, which occupies $906\,\,\mu \text{m}\,\,\times 997\,\,\mu \text{m}$ including pads. In the TX-mode, the −1-dB $P_{\mathrm {sat}}$ bandwidth of the FE covers 21–44 GHz with the maximum output power of 18.1–19 dBm and the peak power added efficiency (PAE) of 20.2%–26.8%. In the RX-mode, the FE achieves an 84.6% and −3-dB small-signal gain fractional bandwidth from 20.2 to 49.8 GHz with 38-mW power consumption. It achieves a measured noise figure (NF) of 7.62–11.64 dB over the frequency range of 24–48 GHz. To the best of our knowledge, this is the first FE that covers all the present 5G mm-wave bands without reconfiguration. [ABSTRACT FROM AUTHOR]

This paper proposes a new feeding topology with internal 180° phase reversal for wideband series‐fed slot array antennas. As a demonstration, a serial fed, half‐wavelength separated, 7‐element array is designed at 5.8 GHz. Comparing with conventional series‐fed arrays, which require one‐wavelength separation, the phase difference between adjacent elements varies less with frequency. Thus, the proposed array has more stable beam pointing and wider 1 dB gain bandwidth. A prototype was fabricated and measured to validate the performance of the feeding topology. A wide operating bandwidth of 22.4% for S11 < −10 dB, a 1 dB gain bandwidth of 14% and bi‐directional peak gain of 10.01 dBi are achieved with the array length of 2.7λ0. The proposed feeding topology shows its potentials in realizing signal coverage on some special occasions, such as coal mines and tunnels. [ABSTRACT FROM AUTHOR]

High-frequency (HF) transmitter implemented with the multioctave power amplifier (PA) is faced with the problem of high-order harmonics owing to the nonlinearity of the PA. In this letter, a complexity-reduced harmonic-cancellation digital predistortion (CR-HC-DPD) is proposed and a narrow pulse sampling (NPS) method is adopted for model identification. Compared with the previous HC-DPD scheme, the proposed CR-HC-DPD approach greatly decreases the number of sampling points used for modeling and, in turn, reduces the computational complexity. Measurement results on a class-AB GaN PA with a 600-Hz two-tone signal demonstrated that the CR-HC-DPD scheme canceled all the harmonics for being below −60 dBc with less than 5% complexity compared with the HC-DPD scheme. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article presents a pattern-reconfigurable saber-like antenna with low wind drag for the onboard communication application, under the requirement of upper hemispherical coverage. The compact antenna mainly consists of a segmented loop and a $\Pi $ -shaped polygon, which are closely integrated for compact size. By controlling the OFF/ON states of a p-i-n diode embedded in the segmented loop, the proposed antenna can be operated at dipole/monopole modes. An upper hemispherical coverage area can be achieved since broadside and omnidirectional patterns are obtained at dipole and monopole modes, respectively. To validate the design strategy, a prototype was fabricated and tested. A good agreement between the simulated and measured results is achieved. The measured −8 dB reflection coefficient common bandwidth of two modes is from 2.16 to 2.63 GHz with the relative bandwidth of 19.6% while keeping a smaller size of $30.5\times 32.5\times 0.6$ mm3 ($0.24\,\,\lambda _{0} \times 0.26 \lambda _{0}\times 0.005 \lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at the center frequency). We portend that the proposed design scheme, with merits of low wind drag, metal ground compatibility, and upper hemispherical coverage, has the potential for airborne application. [ABSTRACT FROM AUTHOR]

In this letter, a broadband millimeter-wave (mm-wave) continuous-mode Class-F power amplifier (PA) based on the deembedded transistor model is proposed for the fifth-generation (5G) applications. To design the output matching network at the intrinsic current-generator plane, an $S$ -parameter fitting method is proposed to extract the parasitic parameters of nonpackaged transistors over a wide frequency range. The PA is implemented in the 0.13- $\mu \text{m}$ SiGe BiCMOS process, which occupies only 0.42 mm2. By controlling the even and odd harmonics impedance, the PA achieves a 78.2% −3-dB small-signal gain fractional bandwidth from 21.1 to 48.2 GHz. The −1-dB $P_{\mathrm {sat}}$ bandwidth covers 21–43 GHz (68.8% fractional bandwidth) and the saturated output powers are 17.2–18.1 dBm. Meanwhile, the designed PA provides a 24.1%–32.1% peak power-added efficiency (PAE) over the band. [ABSTRACT FROM AUTHOR]

A positive feedback gain‐enhanced technique for power amplifiers has been introduced in this Letter. With the proposed circuit design method for optimal inductive feedback, a 180 GHz high‐gain single‐stage cascode power amplifier in a 130 nm SiGe process is implemented. The measurement results show a power gain of 10 dB and 20 GHz bandwidth with 110 mW dc power are achieved, and 6.5 dBm of output power referred to P1dB and 3.5% collector efficiency are attained. The chip area of the power amplifier is only 0.50 × 0.48 mm2. [ABSTRACT FROM AUTHOR]

This article presents a wideband orthogonal-mode dual-antenna pair with a shared radiator for fifth-generation (5G) multiple-input multiple-output (MIMO) metal-rimmed smartphones. The wideband decoupling property of the dual-antenna pair is realized by the combination of the orthogonal monopole/dipole modes in the lower band and the orthogonal slot/open-slot modes in the higher band. With the orthogonal-mode design scheme, the dual-antenna pair shows a wide impedance bandwidth of 3.3–5.0 GHz and a high isolation of more than 21.0 dB across the entire band without using any external decoupling structures. By arranging four such dual-antenna pairs at two side edges of the smartphone, an $8 \times 8$ MIMO system is fulfilled. Both the simulation and measurement results show that the proposed $8 \times 8$ MIMO system could offer an isolation of better than 12.0 dB and an envelope correlation coefficient of lower than 0.11 between all ports. The measured average antenna efficiencies are 74.7% and 57.8% for the two antenna elements of the dual-antenna pair. We portend that the proposed design scheme, with merits of shared radiator, wide bandwidth, and metal rim compatibility, has the potential for the application of future 5G smartphones. [ABSTRACT FROM AUTHOR]

In this letter, a robust multisampling rate digital predistortion (DPD) scheme is proposed to linearize ultra‐broadband power amplifiers (PAs) under the limited DAC sampling rate. In the ultra‐broadband scenarios, it is no longer affordable to employ DACs with 3‐5 times over‐sampling rate, which will inevitably lead to spectrum aliasing for the sake of high order basis function and compromise the DPD performance. In order to improve the modeling accuracy, the input signal with low sampling rate will be up‐sampled firstly to generate nonaliasing basis functions. Then, after filtering and down‐sampling operation, the coefficients are extracted with low sampling rate. Moreover, the conventional behavioral model is modified with the linear‐spline basis functions to improve the stability and complexity. Measurements on Doherty PA and class‐AB PA with 100 MHz, 200 MHz, and 500 MHz bandwidth input signal at 2.1 GHz carrier frequency show the EVM can be improved to 0.71%, 0.94%, and 1.86% respectively. [ABSTRACT FROM AUTHOR]

In this letter, a dual-beam periodic leaky-wave antenna with a low beam-squinting character is presented. It has a simple and low-profile configuration, which consists of 15 slot radiating elements and one microstrip feedline. The slot radiating element creates nulls in the pattern at forward and backward endfire. By properly arranging the radiating elements with a small separation, the higher order spatial harmonics are suppressed. Fed by an air-substrate microstrip line working on the non-dispersion transverse electromagnetic (TEM) mode, the antenna has a moderate phase constant leading to a stable endfire array factor. By integrating the almost unchanged array factor and element pattern, the dual-beam radiation pattern with reduced beam squinting is obtained. To verify the design strategy, a prototype was designed, fabricated, and measured. Over the operating bandwidth from 2.7 to 3.2 GHz, the antenna exhibits a small measured beam squint of 6.8°. [ABSTRACT FROM AUTHOR]

In this article, a novel nested-mode power amplifier (NMPA) is proposed for highly efficient multi-octave applications. By introducing a group of nested guaranteed regions (GRs) at the fundamental and harmonic frequencies as the target design space, the difficulties caused by frequency overlapping in harmonically tuned multi-octave power amplifiers (PAs) can be overcome, leading to a self-consistent theory of multi-octave PAs. Furthermore, the methodology of constructing the proposed nested GRs is demonstrated based on a simplified numerical model. According to the methodology, the desired nested GRs can be constructed by following several well-defined closed-form steps. In addition, a feasible design methodology of the NMPA is established based on a simplified deembedding technique of packaged devices. Based on the proposed theory and design method, a 1–3100-MHz NMPA is designed and measured. According to the measured results, the designed PA exhibits 57.5–73.9% drain efficiency and 39–42.6-dBm output power across the band. Moreover, the linearity and linearizability of the designed NMPA are verified using up to 20-MHz-bandwidth-modulated signals across the designed band accompanied with a digital predistortion (DPD) procedure. To the best of our knowledge, this is the state-of-the-art performance of multi-octave broadband PAs. [ABSTRACT FROM AUTHOR]

In this article, a modified beam-oriented digital predistortion (BO-DPD) utilizing the over-the-air diversity feedbacks (OTA DFBs) is proposed for linearizing the hybrid beamforming (HBF) large-scale array. The proposed DPD can solve the hardware implementation issue of conventional DPD architecture in HBF array by estimating and linearizing the main beam signal while avoiding the complex feedback configurations. Different from the far-field OTA measurements, the OTA feedback receivers are placed symmetrically inside the transmit array according to its geometric structure. Theoretical analysis proves that it is possible to derive the main beam signal from the OTA feedback information directly by adjusting the phase shifter of each branch using a relative simple procedure which allows the predistorter to be identified based on the estimated main beam signal. Furthermore, an iterative algorithm for eliminating uncorrelated coupling interference from other subarrays in the feedback signal is proposed. Measurement results on a $1 \times 4$ transmitter and a $2 \times 3$ transmitter as well as simulations on a $4 \times 64$ array are presented to benchmark the proposed DPD technique against the existing techniques. Better than 15-dB adjacent channel power ratio (ACPR) improvement has been realized with the proposed DPD. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this letter, a novel high-gain leaky-wave endfire antenna is proposed. By utilizing an air substrate parallel strip line, which is periodically disturbed by the radiating elements, such as the feedline, the phase constant of the antenna approaches the phase requirement of the Hansen–Woodyard (H–W) condition. By optimizing the size of the radiating element, which is dipole element, an approximately constant aperture distribution is obtained by the antenna. Thereby, with the characters similar to the H–W endfire antenna around the center frequency and moderate phase constant over the operating bandwidth, the antenna achieves a high gain, narrow beamwidth, and stable endfire radiation pattern. With the radiating aperture length of two wavelengths, the antenna generates the measured gain of 12.5 dBi at the center frequency of 5 GHz, operating bandwidth of 20.8%, and measured 1 dB gain bandwidth of 16.2%. Fabricated by metal, the antenna may be an attractive substitute for the long-distance communication applications, especially for the systems where cost and power capacity are important. [ABSTRACT FROM AUTHOR]

The thermoacoustic response of carbon nanotube (CNT) thin films at frequencies ranging from 300 Hz up to 100 kHz has been studied in a variety of gaseous mediums. Theoretical derivations show that the sound pressure generated by CNT thin films is approximately proportional to the inverse of the heat capacity of the gas within the audible frequency range of human hearing, which is consistent with the experimental results in argon, air, and helium. For large size CNT films, a decrease in sound pressure is observed within a higher frequency range in air, which is attributed to the destructive interference of sound waves in the near field zone according to the theoretical calculations. [ABSTRACT FROM AUTHOR]

A broadband and high-gain endfire antenna array fed by an air-substrate parallel strip line is proposed in this communication. It has a simple and planar structure consisting of multiple dipole radiating elements and one parallel strip line. By utilizing the center-fed dipole working between the first and third resonances as the radiating element, the antenna array could obtain stable radiating capability over a broad bandwidth. Fed by an air-substrate parallel strip line working on the TEM mode, the antenna array achieves a moderate phase constant, which is helpful for realizing stable endfire radiation. The radiating element separation is optimized to eliminate the high-order spatial harmonics, ensuring single beam radiation. Fabricated by thin copper plates, the merits of the antenna array include lightweight, low cost, and easy fabrication. According to the measured results, the antenna array has a good match and a stable endfire radiation pattern over the operating band from 4 to 10.5 GHz with the measured gain varying from 10.1 to 13.5 dBi. [ABSTRACT FROM AUTHOR]

In this paper, a Ka/Q dual-band Doherty power amplifier (DPA) with simplified offset lines is implemented in a 0.1- $\mu \text{m}$ gallium arsenide (GaAs) process. It is found that the dual-band transmission lines (TLs) employed in the sub-6-GHz dual-band DPA are not suitable to be used as the offset lines in millimeter-wave (mm-wave) dual-band DPAs due to their large sizes and insert losses. An in-depth analysis reveals that the phase requirement of the offset lines can be relaxed, and the DPA exhibits a reasonable performance in a certain phase-shift range. A novel design method is proposed to realize offset lines using simple TLs, which can satisfy the phase-shift ranges in dual bands by choosing a proper electrical length. To enhance the gain of the DPA, a reversed uneven power splitter is adopted to deliver more power to the main power amplifier (PA). The fabricated DPA achieves an output power of 25.4/25.2 dBm, a peak power-added efficiency (PAE) of 33%/25%, and a 6-dB back-off PAE of 22%/17% at 29/46 GHz, respectively, with a compact size of 2.2 1.4 mm2. Applying a 20-MHZ 64-quadratic-amplitude modulation (QAM) signal with a 7.7-dB peak-to-average power ratio (PAPR), the measured average output power, PAE, and error vector magnitude (EVM) at 29/46 GHz are 18.4/19 dBm, 17%/15%, and 1.5%/1.1% after linearization, respectively. To the best of our knowledge, the proposed DPA is the first demonstration of mm-wave dual-band DPAs that do not require any additional switching or reconfiguration. [ABSTRACT FROM AUTHOR]

This communication presents a periodic leaky-wave antenna array with the advantages of a high endfire gain at the center frequency and a stable endfire radiation beam over the entire operating band. The antenna has a narrow width and consists of multiple monopoles located on the microstrip line. It can be fabricated by line-cutting copper strips. The antenna has a high endfire gain because it is a leaky-wave antenna designed based on the Hansen–Woodyard condition. With a length of $5.5\lambda _{0}$ and a width of $0.33\lambda _{0}$ , a measured endfire gain of 12.9 dBi is achieved at the center frequency of 5 GHz. Good matching and stable endfire radiation beams are achieved over the entire operating band from 4.5 to 5.5 GHz. Good agreement between the measurement and the simulation has been obtained. [ABSTRACT FROM AUTHOR]

A planar air-filled terahertz antenna array based on channelized coplanar waveguide (CCPW) is proposed and fabricated using silicon bulk micromachining technology. Planar air-filled four-unit array composed of two silicon layers is verified at 300 GHz. Periodical field blocking stubs are loaded in the gaps of the CCPW alternatively to break the fringing field symmetry, generating an in-phase radiating aperture. The fabrication technology has two features: first, the silicon dielectrics are completely plated with gold, generating a purely air-filled antenna with high performance and second, the hierarchical micromachining method enables a single silicon layer to be divided into two sublayers, resulting in reduced antenna layers. The numerical result shows a simulated −10 dB impedance bandwidth of 71.1 GHz (25.95%, 238.4–309.5 GHz) and stable fan-shaped beams with broadside gains and radiation efficiencies higher than 10.6 dBi and 87.9%, respectively. Experiment of the fabricated prototype exhibits a bandwidth that totally covers the simulation and stable beams with broadside gains higher than 8.6 dBi. To the best of author’s knowledge, this is the first time that a planar air-filled terahertz antenna array is proposed. Moreover, confirmation of the strategy demonstrates the antenna’s distinguished suitability to realize antenna-in-package solution. [ABSTRACT FROM AUTHOR]

Abstract: The feeding network is an essential part of the series‐fed microstrip array antenna. It needs additional space on the substrate or even extra layer of the substrate. However, the traditional feeding network has no contribution to the gain, which will result in an unnecessary waste of space. In this letter, a novel 1‐to‐3 feeding network combining with radiated patches is presented. To the best of the authors’ knowledge, the feeding network with radiation contribution is reported for the first time. The modified feeding network can feed antenna and radiate energy at the same time. To verify the design strategy, a series‐fed microstrip array antenna with the proposed radiated feeding network is designed. Compared with the series‐fed microstrip array antenna with traditional feeding network, a 15.3% higher gain is obtained by the proposed antenna. Finally, a prototype of the proposed antenna is fabricated and tested. Measured results show that the antenna works in a band from 9.41 to 10.18 GHz. The measured peak gain is 19.75 dBi at 10 GHz. A good agreement between simulated and measured results is achieved. [ABSTRACT FROM AUTHOR]

In the massive multiple-input multiple-output systems, the antennas should have the ability of beamforming. Under this requirement, the sliding aperture technique is adopted. In this paper, the principle of the sliding aperture technique is first demonstrated. This technique refers to the customization of each beam by part of the aperture, resulting in low scan loss over a wide angular range. Then, with the help of II-shaped transmit element, a linear multibeam transmitarray antenna based on the sliding aperture technique is designed. To the best of the authors’ knowledge, the linear transmitarray is reported for the first time. The fabricated prototype shows that the proposed antenna can generate eight beams simultaneously covering the ±42° azimuth area with low scan loss and the beam intersection level better than −4 dB. The proposed antenna has been simulated, fabricated, and measured, and the experimental results are in good agreement with the simulation results. [ABSTRACT FROM AUTHOR]

In this paper, we propose a beam-oriented digital predistortion (BO-DPD) technique for power amplifiers (PAs) in hybrid beamforming massive multiple-input multiple-output (MIMO) transmitters, which can achieve linearization of the transmitted signal in the main beam direction and address the DPD implementation issue in the hybrid beamforming array. In massive MIMO hybrid beamforming transmitters, the conventional DPD to linearize each PA is impractical to implement for that the number of digital chains is less than the number of PAs; however, the BO-DPD can resolve this issue by constructing and linearizing the “virtual” main beam signal rather than single PAs. According to the beamforming weights for array elements, the feedback path combines all PA’s outputs to estimate the main beam signal for DPD processing. In addition, an average estimation method is also presented to broaden the linearized angle range around the main beam direction. A $4 \times 16$ (four subarrays and 16 antennas in each subarray) uniform linear array (ULA) simulation along with a $1 \times 2$ ULA at 2.5 GHz and a $2 \times 2$ ULA at 3.5-GHz experimental tests was performed. The experimental results show that the proposed technique and linearization angle range broadening method achieve up to 15-dB adjacent channel power ratio improvement and 10° linearization angle range in the main beam direction. [ABSTRACT FROM AUTHOR]

A wideband microstrip patch antenna is proposed in this communication for pattern diversity application. Broadside and omnidirectional radiation patterns are generated by the odd and even modes of a single circular patch. A compact feeding network based on coplanar waveguide is designed to provide in-phase and out-of-phase excitations, which can excite the two modes in wide bandwidth. The size of the feeding network is only 35 \times 14.5 mm2. A prototype of the proposed antenna is fabricated and tested. The measured overlapped bandwidth ( \vert \text{S}_{11}\vert < -10 dB and \vert \text{S}_{22}\vert < -10$ dB) achieves 775 MHz (2.165–2.94 GHz) or 31% corresponding to the center frequency. The Port isolation is below −28 dB in the entire bandwidth. [ABSTRACT FROM PUBLISHER]

Abstract: A design methodology of utilizing the harmonic impedance tuning for the envelope tracking (ET) power amplifier (PA) is proposed in this letter. Owing to the assistance of harmonic tuning, the PA efficiency can be enhanced obviously. Experimental results show that PA prototype exhibits higher than 60% efficiency in the 7‐dB backoff power region and 36% average efficiency including envelope amplifier (EA) when driven by a 7‐dB PAPR long‐term evolution (LTE) signal. [ABSTRACT FROM AUTHOR]

In this paper, the scanning performance at a fixed frequency of the 2-bit linear array is presented. Then, for the first time, a microstrip line that supports a traveling wave is used as bit phase sources with any bit number to construct reconfigurable fixed-frequency beam steering bit arrays. Using the \text TM1n0\,\,(n = 0.5 ) open cavity as the radiating element, a ten-element reconfigurable 2-bit fixed-frequency beam steering array operating at 5 GHz is designed and fabricated to verify the idea. By sampling two feed points with a separation distance of a quarter wavelength on one side of the microstrip line, the phase states of 0° and −90° are obtained; other two counterpart points on the other side of the line can offer the phase states of −180° and −270°, producing the required 2-bit unit. Five prototypes representing five array configurations show a measured scan range of 94° between −48° and +46° at 5 GHz, and the measured five beams show good quality and consistency: the peak gains range from 10.01 to 12.33 dBi with a gain variation of only 2.32 dB and the 3 dB beam widths vary between 13° and 17°. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel approach for digital predistortion, based on direct learning architecture (DLA), to reduce the computational complexity. In power amplifier (PA) behavioral models, the coefficients of a Volterra polynomial or a simplified Volterra polynomial are extracted by calculating the inverse of a time-varying matrix, which is resource-consuming, time-consuming, and power-consuming due to its matrix dimension and inverse operation in a field-programmable gate array. To speed up the computation and save hardware resources, we propose a low computational complexity DLA with covariance matrix that uses the constant covariance matrix to replace the time-varying input signal filled matrix based on a stationary ergodic random process. To verify the proposed method, it was applied to a wideband Doherty gallium nitride (GaN) PA at 2.6 GHz with a 40-MHz orthogonal frequency division multiplexing signal, and to a dual-band Doherty GaN PA at 1.9 and 2.6 GHz with two 20-MHz long-term evolution signals. Experimental results show that the proposed algorithm achieves almost the same performance as the traditional approach, with less than one fifth of the computational quantity. [ABSTRACT FROM PUBLISHER]