This letter presents a novel multiband rectenna for the ambient wireless energy harvesting of an autonomous internet of thing (IoT) sensor. A bow-tie antenna with slits is proposed to obtain four frequency bands at 840 MHz, 1.86, 2.1, and 2.45 GHz, respectively. Furthermore, a multiband rectifying circuit combined of four single ring-loop rectifiers is designed with high conversion efficiency. Finally, a power management circuit with storing and converting the harvested voltage is proposed for smoothing dc output voltage. The energy harvester can be used in practical wireless sensor applications.

Indoor positioning has attracted commercial developers and researchers in the last few decades. Global positioning system (GPS) cannot well localize in indoor environment. For clinical applications such as workflow analysis, Bluetooth Low Energy (BLE) beacons have been employed to map the positions of individuals in indoor environments. The received signal strength indicator (RSSI) fingerprinting plays a key role in the access point performance services. This paper deals with the issue of indoor localization based RSSI fingerprint using only RSSI vectors without any prior knowledge of the pose. We proposed to use machine learning (ML) combined with modified kNN algorithm to enhance the real-time performance and propose method to detect uncertainty of estimated pose. The mentioned ML algorithm is Gaussian Process Regression (GPR). In the online phase of our system, GPR gives a prediction for the location of the pose using RSSI vector. This prediction helps kNN algorithm therefore limits the searching region leading to reduce the computational cost. An analysis on the distribution of k nearest points is also presented which aims to evaluate the confidence of the estimated pose for extracting a list of trustable points. Furthermore, we also present an extrapolation process using this trust-list to have an optimized trajectory. The accuracy and timing analysis of our proposal was realized on a challenging BBIL dataset which contains very noisy RSSI signals due to a fast moving of the object. The experimental results show that our system exhibits a much better performance than traditional kNN or WkNN algorithms. The RMSE of our optimal trajectory is 1.78 m in the room with a dimension of 10 m $\times$ 25 m, which is competitive in comparison to other methods where initial pose is known.

In this letter, we propose a solution to improve efficiency and selectivity of triplexers, which operate as filters (or matching circuits) for triple-band energy harvesting. The triplexer consists of multiple higher-order open stubs of (2n+1)λ/4 length ( n is an integer, n ≥ 1), which is proven to yield high selectivity. With the high selectivity of the filters, the proposed triplexer achieves the efficiencies of 92%, 86%, and 83% at 1.85, 2.1, and 2.45 GHz, respectively. The triplexer is incorporated in a rectenna, yielding a final efficiency of 60% at 5 dBm input power.

A novel wideband circularly polarized antenna array using sequential rotation feeding network is presented in this paper. The proposed antenna array has a relative bandwidth of 38.7% at frequencies from 5.05 GHz to 7.45 GHz with a highest gain of 12 dBi at 6 GHz. A corresponding left-handed metamaterial is designed in order to increase antenna gain without significantly affecting its polarization characteristics. The wideband circularly polarized antenna with 2.4 GHz of bandwidth is a promising solution for wireless communication system such as tracking or wireless energy harvesting from Wi-Fi signal based on IEEE 802.11ac standard or future 5G cellular. A potential application of this antenna as a receiving antenna for RF-DC device to obtain DC power for a wireless sensor node from Wi-Fi signal is shown.

In this work toward a sustainable operation of a self-powered wireless sensor, we investigated a multiband Wi-Fi/3G/4G/5G energy harvester based on a novel wideband circularly polarized antenna, a quadplexer, and rectifiers at four corresponding bands. This proposed antenna consisted of four sequentially rotated dual-dipoles, fed by a hybrid feeding network with equal amplitude and an incremental 90° phase delay. The feeding network was composed of three Wilkinson power dividers and Schiffman phase shifters. Based on the sequential rotation method, the antenna obtained a −10 dB reflection coefficient bandwidth of 71.2% from 1.4 GHz to 2.95 GHz and a 3 dB axial ratio (AR) bandwidth of 63.6%, from 1.5 GHz to 2.9 GHz. In addition, this antenna gain was higher than 6 dBi in a wide bandwidth from 1.65 GHz to 2.8 GHz, whereas the peak gain was 9.9 dBi. The quad-band rectifier yielded the maximum AC–DC conversion efficiency of 1.8 GHz and was 60% at −1 dBm input power, 2.1 GHz was 55% at 0 dBm, 2.45 GHz was 55% at −1 dBm, and 2.6 GHz was 54% at 0.5 dBm, respectively. The maximum RF–DC conversion efficiency using the wideband circularly polarized antenna was 27%, 26%, 25.5%, and 27.5% at −6 dBm of input power, respectively.