Advanced Biophysical Model to Capture Channel Variability for EQS Capacitive HBC

Abstract

Human Body Communication (HBC) has come up as a promising alternative to traditional radio frequency (RF) Wireless Body Area Network (WBAN) technologies. This is essentially due to HBC providing a broadband communication channel with enhanced signal security in the physical layer due to lower radiation from the human body as compared to its RF counterparts. An in-depth understanding of the mechanism for the channel loss variability and associated biophysical model needs to be developed before EQS-HBC can be used more frequently in WBAN consumer and medical applications. Biophysical models characterizing the human body as a communication channel didn't exist in literature for a long time. Recent developments have shown models that capture the channel response for fixed transmitter and receiver positions on the human body. These biophysical models do not capture the variability in the HBC channel for varying positions of the devices with respect to the human body. In this study, we provide a detailed analysis of the change in path loss in a capacitive-HBC channel in the electro-quasistatic (EQS) domain. Causes of channel loss variability namely: inter-device coupling and effects of fringe fields due to body's shadowing effects are investigated. FEM based simulation results are used to analyze the channel response of human body for different positions and sizes of the device which are further verified using measurement results to validate the developed biophysical model. Using the bio-physical model, we develop a closed form equation for the path loss in a capacitive HBC channel which is then analyzed as a function of the geometric properties of the device and the position with respect to the human body which will help pave the path towards future EQS-HBC WBAN design.

Human Body–Electrode Interfaces for Wide-Frequency Sensing and Communication: A Review

“…At lower distances and when the ground electrodes of the transmitter and receiver are parallel, the inter-device coupling capacitor, , is higher (See Figure 18 ). Higher results in a part of the signal returning through instead of through the relatively high impedance path and improving the channel quality [ 107 ]. Again, applications generally dictate the locations and orientations for the transmitter and receiver ground electrodes; meaning, the options to increase is limited.…”

“…Circuit element models the resistance of the conductive core of the body. Circuit element represents the inter-device coupling capacitance between the ground electrodes of and [ 107 ]. Parallel combination of resistance, , and capacitance, , model the termination impedance [ 106 ].…”

Bi-Phasic Quasistatic Brain Communication for Fully Untethered Connected Brain Implants

“…Bio-physical models for C-HBC were developed in our earlier works 18,34 where the effects of these capacitances on the overall channel transfer function were analyzed, which was found to be proportional to the return path capacitances at both the TX and the RX. The effect of moving the TX device within the body (for an implant) was also analyzed 18 , which showed that the return path capacitance for the implanted TX becomes almost zero, as the electric fields terminate within the body, and cannot terminate to the earth's ground as in the wearable scenario. Due to the absence of a TX return path capacitance, the RX received voltage becomes zero.…”

“…Two differential metal electrodes attached to the sides of the bowl work as the RX electrodes, which are connected to a TI BUF602 buffer configured as a 50 Ω driver for measurement instruments. The TI buffer offers ~2 pF capacitive termination at the input of the RX, which helps in establishing a wideband HBC channel as shown in earlier works 18,31,33,34 The mouse was anaesthetized with 2-3% isoflurane throughout the surgery. After shaving the hair, the animal was fixed on a stereotaxic frame, so that the head does not move during the experiment, and the head skin was sterilized.…”

“…Since there is no common reference between the TX and RX, the signal transfer mechanism becomes a function of the return path capacitances between (1) the TX and earth’s ground, and (2) the RX and earth’s ground. Bio-physical models for C-HBC were developed in our earlier works 18,34 where the effects of these capacitances on the overall channel transfer function were analyzed, which was found to be proportional to the return path capacitances at both the TX and the RX. The effect of moving the TX device within the body (for an implant) was also analyzed 18 , which showed that the return path capacitance for the implanted TX becomes almost zero, as the electric fields terminate within the body, and cannot terminate to the earth’s ground as in the wearable scenario.…”