LoRa is a low-power long-range IoT standard that uses the chirp spread spectrum technique, and we have strived to further extend its coverage by utilizing the direct device-to-device (D2D) links to construct a multi-hop relay network. In LoRa, the spreading factor (SF) is an important parameter, which not only provides great flexibility between the data rate and sensitivity but also presents a new dimension for multiple accesses. Our approach to improving the capacity of a multihop LoRa network is to attempt to off-load the data traffic into several subnets by utilizing this multiple-access dimension. Each subnet rooted at a sink node is allocated a specific SF on the basis of network clustering. This enables packet transmission in parallel with multiple SFs to become feasible. To allow such parallel transmissions, our considerations are: 1) ensuring the connectivity of all subnets; 2) off-loading the traffic according to the number of nodes, data rates, and network topologies of each subnet; and 3) shortening the airtime of each subnet by reducing the hop count. Toward these objectives, we present a tree-based SF clustering algorithm (TSCA) to conduct SF allocation in a multihop LoRa network. The TSCA focuses on balancing the airtime between the subnets while ensuring connectivity. Furthermore, we use simulations to show that our approach can significantly increase the network performance compared with other approaches. We additionally deploy a real-chip experiment to evaluate the feasibility of parallel transmission in practice. INDEX TERMS Low-power wide area network (LPWAN), LoRa, spreading factor (SF) allocation algorithm, tree-based spreading factor clustering algorithm (TSCA), multi-hop network.
Output voltage regulation is an essential technology for achieving stable wireless power supply. A receiver-side switching voltage regulator is useful for realizing output voltage regulation. However, this paper shows that the switching voltage regulator degrades the transfer efficiency to below 50% in a wireless power transfer system that consists of a class-D power inverter and series-resonant transmitting and receiving resonators. Such efficiency degradation is caused by the instability of an operating point where the efficiency is >50%. The input resistance value of the switching voltage regulator at a stable operating point is much higher than the optimum value for maximizing the efficiency. To stabilize the high-efficiency operating points, this paper formulates a stability condition and derives its sufficient condition. The sufficient condition facilitates a system design method using a K-impedance inverter that allows for the optimum input resistance value to lie in the range of allowable input resistance values. In addition, we introduce an input-voltage-based efficiency maximization method for the system with the receiver-side switching voltage regulator. By combining these two methods, efficiency maximization is realized with the receiver-side switching voltage regulator. The proposed methods were verified by both simulations and measurements.
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