Author(s): Erda Wen, Daniel F. Sievenpiper, Patrick P. Mercier
The wireless connections between wearable devices, which allow them to interact with each other or to communicate sensed data, are established via body area networks (BANs). However, the commonly used far-field radio frequency (RF) protocols operating at sub-6-GHz, such as Bluetooth, are extremely inefficient and unstable due to significant loss and reflection of the energy flux on the body surface. As a result, the radio system usually takes a large proportion of the power consumption and becomes the bottleneck of achievable battery life.
To address this, we propose the magnetic human body communication (mHBC) approach that adopts magnetic-dominant near-field coupling with resonant coils. A radiating near-field coupling model and numerical simulations are presented to show that the proposed method offers low path loss across the body and exhibits extra robustness to antenna misalignment compared to far-field RF schemes. To overcome the pitfalls in conventional vector-network-analyzer-based measurement configurations, we propose a standardized setup applied to broadband channel loss measurement with portable instruments. Two types of PCB coils, designed for large devices such as smartphones and small devices such as earbuds, respectively, are built and measured. The mHBC link for the ear-to-ear non-line-of-sight (NLOS) path measures up to -23.1 dB and -31.2dB with large and small coils, respectively, which is 50 dB (100000x) more efficient than conventional Bluetooth channels utilizing antennas of similar sizes. Ear-to-pocket and pocket-to-pocket channels also show at least 16 dB higher transmission than Bluetooth. We conclude that for coils with dimensions of several centimeters, working between 100 MHz and 200 MHz minimizes the channel loss while providing a bandwidth above 1 MHz. The extremely high efficiency of the mHBC method offers a solution to the energy problem for miniaturized wearables, and potentially leads to new wearable device designs.