A low-profile double U-slot microstrip patch antenna with a wide operating bandwidth has been developed in this article. This design produces the dual radiative resonant modes adjacent to each even at the thickness of the antenna of $0.02 {\lambda _{0}}$ . An extremely stable radiation gain that varied from 6.2 to 6.8 dBi is achieved within the whole operating band. In addition, detailed parametric studies in this article provide antenna engineers with simple approximate rules that result in an excellent first-pass design with prescribed characteristics requiring only minimal tuning. Finally, the proposed antenna with the thickness of $0.04 {\lambda _{0}}$ is fabricated and measured. Simulated and measured results are found in good agreement with each other. They have evidently illustrated that the bandwidth is increased by 50% compared to the traditional U-slot antenna. With the help of these simple design procedures and examples in this article, antenna designers could easily develop a low-profile broadband antenna.

This article presents a switched-transformer-based polar Doherty digital power amplifier (DPA) for deep back-off efficiency enhancement. A single-transformer-footprint parallel-combining transformer (PCT) power combiner is introduced for 1.3-3.5-GHz wide frequency coverage, dynamic load modulation at 0-/6-/12-/18-dB power back-off (PBO) levels, and an ultra-compact die size. Implemented in 40-nm CMOS, the DPA is powered by a 1.1-V supply and only occupies 0.7 x 1.15 mm² chip area. It achieves 21.4-dBm peak power with the power added efficiencies (PAEs) of 31.3%, 27.7%, 16.6%, and 7.7% for 0-/6-/12-/18-dB PBOs at 1.5 GHz. With 20-MHz 64QAM long term evolution (LTE) signal, the DPA achieves Pavg of 15.2 dBm and the average PAE of 25.3% with -32.5-dB error vector magnitude (EVM) at 1.5 GHz. Moreover, for 20-MHz 64 QAM WLAN signal, Pavg of 14.7 dBm with the average PAE of 20.1% is obtained with - 25-dB EVM at 2.4 GHz.

Gesture air-writing is an advanced nontouching interaction method that replaces traditional typewriter keyboards, touch screens and other input devices. Due to its low power consumption, noncontact detection and independence from light conditions, millimeter wave radar has become a valuable gesture air-writing solution. In this paper, we proposed a prototype of a gesture air-writing tracking system that is based on the 24-GHz frequency-modulated continuous-wave (FMCW) radar system-on-chip (SoC). The transmitted chirp signal of this radar chip covers up to a 4-GHz bandwidth, which provides sufficient range resolution to track hand gestures. With the development of single input, multiple output (SIMO) antennas, the air-writing symbols can be reconstructed in an observation plane. A system design and signal processing algorithm for gesture air-writing applications is proposed for the prototype system in this paper. To test the performance of the proposed method, experiments with cases of five simple gestures, air-writing numbers and letter tracking are carried out. The experimental results verify the efficiency and accuracy of the proposed method.

This paper presents a dual-band high-power digital polar Doherty power amplifier (PA). A current-mode parallel-combining transformer (PCT) power combiner is introduced for Doherty operation, which realizes simultaneous dual-band coverage, high output power, and backoff efficiency enhancement with an ultra-compact single-transformer footprint. The PA is implemented in 55-nm CMOS and occupies 1.11 mm² die area. It achieves 28.9-dBm peak Pout with 36.8% PAE in low band (LB) and 27.0-dBm peak Pout with 25.4% PAE in high band (HB). For 12-subcarrier 180-kHz bandwidth cellular narrowband Internet-of-Things (NB-IoT) signals, the peak Pavg of 24.4 dBm with average PAE of 29.5% at 850 MHz and the peak Pavg of 23.0 dBm with average PAE of 17.9% at 1.7 GHz are obtained with -21.6-dB error vector magnitude (EVM). Moreover, the PA achieves 20.8-dBm Pavg, 22.7% average PAE, and -30.5-dB EVM for 20-MHz 256-QAM WLAN signal.

This article presents a 15-bit quadrature digital power amplifier (DPA) with in-phase and quadrature (IQ)-cell-sharing and transformer-based Doherty operation, which compensates the 3-dB power loss in traditional quadrature architecture and enhances efficiency. Efficiency enhancement at 3-/6-dB power backoffs (PBOs) is obtained on the IQ complex plane to improve average efficiency. A single-transformer-footprint parallel-combining transformer (PCT) power combiner is implemented for compact die size. Fabricated in a 55-nm CMOS technology, the DPA is powered by 1.2-/2.4-V supply voltages and the core circuit only occupies 1.05 $\times $ 1.14-mm ² chip area. It achieves 29.3-dBm peak output power with power added efficiency (PAE) of 43.1%. With 10-MHz 64-quadratic-amplitude modulation (QAM) local thermal equilibrium (LTE) signal, the DPA achieves $P_{\mathrm {avg}}$ of 23.62 dBm and the average PAE of 24.4% with −25.6-dB error vector magnitude (EVM) at 0.85 GHz without using any digital predistortion (DPD) technique. Moreover, for 20-MHz 64-QAM wireless local area network (WLAN) signal, $P_{\mathrm {avg}}$ of 21.01 dBm and the average PAE of 20.1% are obtained with −25.1-dB EVM.

Automatic fall detection plays a significant role in monitoring the health of senior citizens. In particular, millimeter-wave radar sensors are relevant for human pose recognition in an indoor environment due to their advantages of privacy protection, low hardware cost, and wide range of working conditions. However, low-quality point clouds from 4D radar diminish the reliability of fall detection. To improve the detection accuracy, conventional methods utilize more costly hardware. In this study, we propose a model that can provide high-quality three-dimensional point cloud images of the human body at a low cost. To improve the accuracy and effectiveness of fall detection, a system that extracts distribution features through small radar antenna arrays is developed. The proposed system achieved 99.1% and 98.9% accuracy on test datasets pertaining to new subjects and new environments, respectively.