Wearable 868 MHz LoRa Wireless Sensor Node on a Substrate-Integrated-Waveguide Antenna Platform

Nowadays, wireless sensor networks at sub-GHz frequencies are becoming more and more ubiquitous, owing to their impressive link budget. One of the most widespread standards is LoRa, employing Chirp Spread Spectrum modulation to achieve a relatively high data rate at low transmit powers. The latter is ideal for body-centric communication, but the challenge for wearable devices consists in keeping the antenna dimensions sufficiently small. By exploiting substrate-integrated-waveguide technology, a compact wearable 868 MHz LoRa sensor node has been successfully integrated onto a compact textile antenna platform. The wearable LoRa unit operates in a fully autonomous manner, including an integrated battery, transceiver, microprocessor and memory. This paper documents the construction and the radiation patterns of the wearable node, autonomously deployed on the human body and measured in an anechoic chamber. The measurements were performed without any cables attached to the wearable node and, hence, accurately characterize realistic off-body propagation at sub-GHz frequencies. Finally, a body-to-body wireless link is measured between two persons equipped with wearable nodes in the anechoic chamber, considering different body orientations.