Wake-up radio is a promising approach to mitigate the problem of idle listening, which incurs additional power consumption for the Internet of Things (IoT) wireless transmission. Radio frequency (RF) energy harvesting technique allows the wake-up radio to remain in a deep sleep and only become active after receiving an external RF signal to ‘wake-up’ the radio, thus eliminating necessary hardware and signal processing to perform idle listening, resulting in higher energy efficiency. This review paper focuses on cross-layer; physical and media access control (PHY and MAC) approaches on passive wake-up radio based on the previous works from the literature. First, an explanation of the circuit design and system architecture of the passive wake-up radios is presented. Afterward, the previous works on RF energy harvesting techniques and the existing passive wake-up radio hardware architectures available in the literature are surveyed and classified. An evaluation of the various MAC protocols utilized for the novel passive wake-up radio technologies is presented. Finally, the paper highlights the potential research opportunities and practical challenges related to the practical implementation of wake-up technology for future IoT applications.

Source localization is a key technology in array signal processing. However, it has a serious problem that the direction of arrival (DOA) estimation accuracy is reduced at low signal-to-noise ratio (SNR). Thus, this Letter proposes an improved near-field multiple signal classification (INF-MUSIC) algorithm to improve DOA estimation accuracy at low SNR. Firstly, the fourth-order cumulant is used to construct a Hermitian matrix with only DOA information. Secondly, the spatial spectrum has physical property that is a breakpoint and approaches a larger value at the DOA, thus first derivative of spatial spectrum is used to improve DOA estimation accuracy at low SNR. Finally, the corresponding ranges are estimated one by one by the onedimensional MUSIC algorithm. The simulation results show that the INF-MUSIC algorithm makes the DOA estimation accuracy improved by 2° at low SNR.

This brief presents a dual-band tunable concurrent passive phase shifter monolithic microwave integrated circuit utilizing a two-stage dual-branch phase tuning network topology. The designed phase shifter with two independent phase tuning bands operates at 5.9 GHz and 16 GHz. The dual-band phase shifter is fabricated in a 0.25 μm GaAs process and occupies a small chip area of 0.98 mm ² (including pads). Measurement results show that transmission phases in the two frequency bands can be tuned concurrently. Phase tuning range is greater than 105° in either band. Insertion losses of less than 2.8 dB and 3.5 dB are exhibited in 5.8~6 GHz and 15.8~16.2 GHz, respectively. And amplitude imbalances within the whole phase tuning range in 5.8~6 GHz and 15.8~16.2 GHz are less than 1.1 dB and 1.3 dB, respectively. The return losses are greater than 10 dB in both frequency bands. The phase shifter is employed in a four-element dual-band phased array network. The phased array is able to independently tilt its main beam toward -20° ~ +20° in either band.