Bio-Physical Modeling, Characterization, and Optimization of Electro-Quasistatic Human Body Communication

Abstract

Human Body Communication (HBC) has emerged as an alternative to radio wave communication for connecting low power, miniaturized wearable and implantable devices in, on and around the human body. HBC uses the human body as the communication channel between on-body devices. Previous studies characterizing the human body channel has reported widely varying channel response much of which has been attributed to the variation in measurement setup. This calls for the development of a unifying bio-physical model of HBC supported by in-depth analysis and an understanding of the effect of excitation, termination modality on HBC measurements. This paper characterizes the human body channel up to 1MHz frequency to evaluate it as a medium for broadband communication. A lumped bio-physical model of HBC is developed, supported by experimental validations that provides insight into some of the key discrepancies found in previous studies. Voltage loss measurements are carried out both with an oscilloscope and a miniaturized wearable prototype to capture the effects of noncommon ground. Results show that the channel loss is strongly dependent on the termination impedance at the receiver end, with up to 4dB variation in average loss for different termination in an oscilloscope and an additional 9 dB channel loss with wearable prototype compared to an oscilloscope measurement. The measured channel response with capacitive termination reduces low-frequency loss and allows flat-band transfer function down to 13 KHz, establishing the human body as a broadband communication channel. Analysis of the measured results and the simulation model shows that instruments with 50Ω termination impedance (Vector Network Analyzer, Spectrum Analyzer) provides pessimistic estimation of channel loss at low frequencies. Instead (1) high impedance (2) capacitive termination should be used at the receiver end for accurate voltage mode loss measurements of the HBC channel at low frequencies. The experimentally validated bio-physical model shows that capacitive voltage mode termination can improve the low frequency loss by up to 50dB, which helps broadband communication significantly.

Wireless Body Sensor Communication Systems Based on UWB and IBC Technologies: State-of-the-Art and Open Challenges

“…Establishing a proper procedure and measurement setup for measuring IBC channel characteristics, while keeping the overall IBC signal path intact, is a very challenging task [ 156 , 157 ], since introducing any kind of measuring equipment into the IBC channel modifies the return signal path and usually influences the measurement results. The use of different measurement equipment (signal generator, oscilloscope, network and spectrum analyzer), effects caused by different cables and connections, differences when using transformers or battery powered devices, the effect of the load resistance, and ground strategies were extensively studied in [ 13 , 153 , 158 , 159 , 160 ]. It was shown that using balun transformers and commercial equipment with 50 Ω input impedance resulted in higher measured gain than in a realistic IBC channel due to the improper ground isolation, and with lower gain at low frequencies due to the low frequency termination [ 159 , 160 ].…”

“…The use of different measurement equipment (signal generator, oscilloscope, network and spectrum analyzer), effects caused by different cables and connections, differences when using transformers or battery powered devices, the effect of the load resistance, and ground strategies were extensively studied in [ 13 , 153 , 158 , 159 , 160 ]. It was shown that using balun transformers and commercial equipment with 50 Ω input impedance resulted in higher measured gain than in a realistic IBC channel due to the improper ground isolation, and with lower gain at low frequencies due to the low frequency termination [ 159 , 160 ]. Also, devices with large physical size (like commercial network and spectrum analyzers) created a larger than expected return path, whether they were isolated with baluns or not, thereby increasing the measured channel gain [ 15 , 41 , 160 , 161 , 162 , 163 ].…”

Design of Galvanic Coupling Intra-Body Communication Transceiver Using Direct Sequence Spread Spectrum Technology

“…Battery-powered devices should be used in IBC channel test. Avoid using devices with big ground, otherwise it will lead to an optimistic estimation of channel loss [29]. To avoid the common ground of the transmitter and receiver, three ways of mutually independent peripheral circuit power supply are set up: mobile power supply (portable battery), isolation transformer power supply, and uninterrupted power supply.…”

“…When we did the human channel measurements, the power of unrelated equipment was pulled out in order to decrease the common ground interference. The power supply ways used in the experiments, which all has a small ground plane, to emulate a more realistic IBC scenario and avoid an optimistic estimation of channel loss [29]. In summary, the power supply method used in the experiment can simulate the real human communication scene.…”

User Recognition Based on Human Body Impulse Response: A Feasibility Study

“…For practical wearable applications, channel modeling for battery-powered devices has been investigated in terms of path loss according to the frequency [40]. The previous study in [41] investigated the channel loss for important measurement issues in the HBC, in terms of ground sharing for a wearable measurement system, dependence on the excitation conditions for capacitive coupling and galvanic coupling, and impedance conditions for the measurement terminations. These works on capacitive coupling have contributed to the development of measurement setups dedicated to the human body channel and presented their unique channel properties by analyzing measured data.…”