Toward Understanding the Return Path Capacitance in Capacitive Human Body Communication

Human Body Communication (HBC) utilizes the human body as a conductive channel to enable ultra-low-power secure communication between devices on and around the body. In Capacitive HBC, although the body provides the forward path for communication, the closed loop return path is formed through parasitic coupling of the device's ground plane with the environment. This return path capacitance (C-G) is a critical factor in determining the overall HBC channel response. Hence an indepth understanding C-G and the key factors affecting its value will enable optimized design of HBC systems. In this brief we provide a detailed analysis of the return path capacitance for wearable devices of different shapes (disc, rectangular) through derivations, simulations as well as experiments. FEM simulation results show that the parasitic capacitance is almost equal to the self capacitance found through theoretical derivations, when the distance of the device to earth's ground is significantly larger than the device dimensions. Independent experiments using an LCR meter and wearable devices, show close correspondence with theory and simulations. The return path capacitance (C-G) value, which is in the range of a few pFs for a watch sized wearable, is primarily dependent on the size of the device and almost independent of the distance from earth's ground.