I. INTRODUCTION

S MART devices in and around the body are rapidly becom- ing an integral part of our lifestyle, with applications such as wearable and implantable remote health-monitoring devices redefining the healthcare sector. Unfortunately, these developments also imply an abundantly present communication of sensitive data around ourselves [1], [2]. Implantable medical devices such as pacemakers and insulin pumps have been shown to be vulnerable to attacks resulting in fatal consequences (see Fig. 1) [3], [4]. Thus, an in-depth study, of the in-body to out-of-body (IBOB) channel characteristics is essential in developing a communication architecture that is both efficient and secure. Previous works include the development of channel models [5]- [9] and efficient transceivers [10]- [15] for IBOB communication. In this study, we present for the first time a quantitative study of physical layer security as a function of operating frequency using a figure of merit (FoM) (see Fig. 1) to assess the channel quality. A thorough analysis is performed through EM simulations and experiments of the effect of human body tissues on transmitted signals and signal leakage away from the body for IBOB communication using frequently used ISM bands-400 MHz, 900 MHz, 2.4 GHz, and human body communication (HBC) [16]- [19] at 21 MHz.

II. THEORETICAL ANALYSIS AND SIMULATIONS

A. Key Parameters for IBOB Communication

Size-constrained implantable devices are required to have a low transmit power to ensure long device usage. Furthermore, critical information communicated from an implantable to an on-body hub needs to be protected to prevent eavesdropping by a skilled attacker. Thus, lowering the transmit power for longevity and minimizing signal leakage out of the body to ensure physical layer security are the two key factors that help us define an FoM for secure and efficient IBOB communication. Furthermore, the parameters defined must also be independent of the antenna or coupler parameters being used for the simulations and experiments to ensure that the FoM is strictly dependent on the physical properties of the signal and its interaction with the body tissues.

B. Figure of Merit for IBOB Communication

In the subsequent discussions, PL X refers to the path loss of the channel with the Rx at a distance X away from body.

1) Physical Layer Security:

To ensure physical layer security of the IBOB Communication channel, a high signal strength decay is required as we move away from the body. We define the parameter Leakage Loss at a distance X away from the body (LL X ) as a measure of signal decay as shown by the following equation:

(1)

This parameter (LL X ) effectively captures the amount of signal decay as we move the Rx away from the body channel as illustrated by Fig. 1. The higher the value of LL X , more signal gets decayed away from the body providing a more physically secure channel.

2) Communication Power: Signal decay due to the body tissues is an essential factor in determining the transmit power of the system. Higher absorption of signal by body tissues implies we need a higher transmit power to ensure that the Rx sensitivity is enough for successful communication. As a measure of signal decay due to body tissues, we perform experiments and simulations in two different circumstances as shown in Fig. 1 (cases I and II). In case I, the Tx is present inside the body and the Rx is placed close to the body (0 cm away from body). The path loss in this case is termed as PL 0-Body . In case II, the Tx and Rx are placed at the same distance away from each other as in case I but without the presence of body or in other words, in free space. The path loss here is termed as PL 0-Air . The FoM parameter PL Body is defined as shown by the following equation:

(

The parameter PL Body captures the difference in channel loss that occurs due to the presence of body tissues affecting the transmitted signal. Hence, a low PL Body is desired for effective data transmission. A negative PL Body value indicates that the channel loss reduces due to the presence of body, and the body in such a scenario helps the received signal quality instead of adding to channel loss.

The FoM is defined by the following equation:

The higher the value of FoM, the better a given frequency band is for IBOB communication. In this study, equal priority is given to both the parameters ((LL X ) and PL Body ). However, we may also use a weighted FoM to assign higher priority to one of the two parameters as per the requirements of the communication system.

III. SIMULATIONS

We perform finite-element method (FEM)-based electromagnetic simulations on Ansys high-frequency structure simulator (HFSS) to validate the FoM defined.

A. Simulation Setup

A simplified crossed-cylindrical model of the human body made up of skin and muscle tissues is used for the simulations, as shown in Fig. 2(a). This simplified model is used to reduce the computational complexity as well as simulation time. This simplified structure has been validated by comparing the EM field distribution around the model with that generated by a complex human model-VHP Female v2.2 by Neva Electromagnetics [20] that provided identical results. The simulations have been performed at 400 MHz, 900 MHz, and 2.4 GHz, which are part of the frequently used ISM bands, as well as at 21 MHz, which has been the standard defined by IEEE 802.15.6 for HBC.

The Tx for HBC simulations [see Fig. 2(b)] is a galvanic mode voltage coupler [8], which is embedded inside the human model. The Rx used is a capacitive mode voltage coupler that provides a low-loss HBC channel at 21 MHz. The Rx is moved away from the body to measure the channel loss at various points as shown by Fig. 2(c).

A monopole antenna with lumped port excitation is used for RF simulations as the Tx and Rx to reduce the design complexity as illustrated by Fig. 2(d). The antenna was designed to have S11 < -10 dB to ensure efficient transmission of signals. However, the FoM calculations are independent of the antenna parameters and depend strongly on the signal and its interaction with the body tissues. The Rx is moved away from the Tx for two cases, as described in Section II-B2: 1) the Tx inside the body and 2) the Tx in air or free space.

B. Simulation Results

The simulation results are illustrated in Fig. 3. The values of the critical parameters ((LL X ), PL Body ) are highlighted in the figures. The simulation results show how the signal decays away from the body for the different communication methodologies. We observe that for HBC [see Fig. 3(d)], the effect of body on the signal changes as the body starts helping the channel loss instead of hurting it by acting like a wire to provide a low-loss channel when compared to when no-body is present. Thus, as explained in Section II-B2, the value of PL Body becomes negative. Higher conductivity of body tissues for larger frequencies (2.4 GHz) results in a higher attenuation inside the body, thus degrading the FoM. A comparison of the FoM described by ( 3) is provided by the table in Fig. 4. The table shows that FoM for HBC is at least an order of magnitude higher than that observed for RF communication.

A. Experimental Setup

The experiments are performed in a standard lab environment with a hanging meat box setup [see Fig. 5(c)] to model the human body. A hanging setup is essential in accurately observing the HBC results [8]. The galvanic transmitter for HBC is a 3/5 stage ring oscillator on FR4 PCB transmitting at 21 MHz as shown in Fig. 5(a). The RF transmitter [see Fig. 5(a)] is designed using a 434-MHz transceiver module by Cytron Technologies. The transmitter is controlled using a TIVA C Launchpad microcontroller using the UART serial communication protocol. The transmitters are embedded inside layers of pork meat to emulate an implanted device. Pork meat is used due to the close resemblance in their dielectric properties with human tissues. The receiver [see Fig. 5(b)] is an RF Explorer handheld Spectrum Analyzer. For the HBC measurements, we use a coupler with a high-impedance termination obtained using a broadband buffer-BUF602ID from Texas Instruments [see Fig. 5(b)]. The RF signals are obtained using an ANT500 antenna by Great Scott Gadgets. The path loss is measured as we move away from the meat box setup with the Tx inside the meat layers (case I) and in air (case II) to observe the decay in signal strength.

B. Experimental Results

The experimental results are illustrated in Fig. 5(d) and(e). We observe that the signal decay in HBC [see Fig. 5(e)] away from the body is much higher than that observed for RF transmission at 434 MHz [see Fig. 5(d)]. Furthermore, the FoM for HBC and 434-MHz RF communication is compared in the table in Fig. 6. The difference in the FoM values observed in simulation and experiments rises from the variation in experimental and simulation conditions. The experiments conducted in a laboratory environment results in higher attenuation of signals, resulting in a faster decay. Furthermore, a thin layer of meat used in the experiments allows for high dipole coupling between the Tx and Rx, resulting in a low path loss for HBC measurements with the Rx close to the Tx, thus further increasing the measure of signal decay. However, the trends shown in the experimental results as well as the FoM values match with the simulation results and the FoM trends where HBC is observed to provide a much better performance than RF communication techniques for IBOB communication.

V. CONCLUSION

A quantitative study for physical layer security in IBOB communication as a function of frequency is presented for the first time in literature. An FoM is developed to compare the performance of the IBOB channel for the different frequencies, which shows that HBC operating at 21 MHz provides order(s) of magnitude better performance for IBOB communication compared to typical RF-based communication methodologies.