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Logo: Institute of Microwave and Wireless Systems
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Logo: Institute of Microwave and Wireless Systems
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Body Area Networks

Body area networks (BAN), also named wireless body area networks (WBAN) or body sensor networks (BSN) are wireless interconnections of computing devices worn on, attached to or implanted in the human body. Wireless links through or along the body are used for medical devices as well as for portable multimedia equipment (wearable computers / wearables) and security technology (protective equipment for fire fighters etc.). Medical applications include diagnostic (vital data monitoring) and therapeutic purposes (prosthetics, neurostimulation). For healthcare and security devices reliability is a major concern. A compact design of the wireless systems is especially needed for implants. To meet these requirements a firm knowledge of the expected communication channel and matching optimal antenna characteristics is required.

Fig. 1: At the Institute of Microwave and Wireless Systems methods for the systematic development of antennas for devices worn at or implanted in the body are investigated
Fig. 2: TE- and TM-patterns for on-body wireless links

The design of the required antennas for these system is still challenging due to the high level of integration and especially the coupling with the lossy body tissue. For off-body links with the usually used modelling approach according to the Friis transmission equation the body has to be treated as part of the antenna, ref. Fig. 1. Furhermore, as can be seen in Fig. 1 on-body links complicate the systematic development even more as antenna and channel cannot be modeled as separate building blocks.

Fig. 3: Body-centric wireless communication channels

Antenna de-embedding – On-body antenna parameters

Our methods enable the de-embedding of the antenna from the on-body channel and to define adapted on-body antenna parameters linked more closely to the propagation mechanisms [1]. To consider the presence of the human body our method utilizes the Norton surface wave theory to model electromagnetic fields of antennas located at the body. The adaption of the solutions of the Norton surface wave theory, especially the solution of Bannister [2] for the problem of antennas near the human body surface was earlier published [3]. In [1] it was shown that the on-body fields of arbitrary antennas can be modeled by a decomposition of the antennas current distribution into small electric dipoles. The radiated electromagnetic fields of the antenna can then be calculated from the superposition of the fields of all these dipoles. Recently, we proved the applicability of the method through several on-body antenna designs, where the excitation of surface waves was optimized to reach shadowed regions on the body [4-5].

Wearable antenna design

For the verification, optimization and evolution of our on-body antenna design methods we are developing antennas for typical WBAN applications. Compactness is a major design criterion for wearable antennas. Thus enabling high integrated antenna designs we utilize characteristic mode analysis (CMA) to combine and reuse mechanical and electrical parts with the antenna functionality. Furthermore we support industrial partners in the development of body-centric antenna systems.

Body-centric wireless communication channels

Considering the variety of possible applications of body area networks a rough classification of the  communication channels that we are investigating as follows:

On-body to on-body path

This case is used in the networking of sensors carried on the body. These may be e.g. sensors for the determination of the blood sugar content, oxygen blood count, step counter or heart rate monitor. Already today sensors are connected with a smartphone as an evaluation unit. The propagation scenario is shown by way of example in the lower left part of Fig 3. Since transmitter and receiver are located at the body, propagation takes place through space waves (in case of line of sight) and through body-centric surface waves. Especially the surface guided waves enable interconnection of sensors without a line of sight connection.

Implant to on-body path

This case, which is illustrated here by the example of the vital parameters transmission between a close-surface implanted cardiac pacemaker and an evaluation unit worn at the body, differs from the first case in that one antenna is located in the implant below the body surface. Thus one antenna is located on the other side of the boundary layer between tissue and air. However, the preferred excited wave type would here be also a surface-guided wave, which propagates along the body contours. 

Implant to Implant path

Future application could also require a radio link between two implants. As an example, in the upper right part of Fig. 3 a sensor and an actuator of a neurostimulator device is shown. Such systems are currently being discussed for bridging severed nerve connections (e.g., cross-section paralysis). In this case, transmitter and receiver would be implanted. Due to the different dielectric tissue parameters the wave propagation still is going to take place partly at the body-air-boundary and partly at the boundary layer between muscle tissue and bones.

Antenna de-embedding – On-body antenna parameters

Our methods enable the de-embedding of the antenna from the on-body channel and to define adapted on-body antenna parameters linked more closely to the propagation mechanisms [1]. To consider the presence of the human body our method utilizes the Norton surface wave theory to model electromagnetic fields of antennas located at the body. The adaption of the solutions of the Norton surface wave theory, especially the solution of Bannister [2] for the problem of antennas near the human body surface was earlier published [3]. In [1] it was shown that the on-body fields of arbitrary antennas can be modeled by a decomposition of the antennas current distribution into small electric dipoles. The radiated electromagnetic fields of the antenna can then be calculated from the superposition of the fields of all these dipoles. Recently, we proved the applicability of the method through several on-body antenna designs, where the excitation of surface waves was optimized to reach shadowed regions on the body [4-5].

Wearable antenna design

For the verification, optimization and evolution of our on-body antenna design methods we are developing antennas for typical WBAN applications. Compactness is a major design criterion for wearable antennas. Thus enabling high integrated antenna designs we utilize characteristic mode analysis (CMA) to combine and reuse mechanical and electrical parts with the antenna functionality. Furthermore we support industrial partners in the development of body-centric antenna systems.

Body-centric wireless communication channels

Considering the variety of possible applications of body area networks a rough classification of the  communication channels that we are investigating as follows:

On-body to on-body path

This case is used in the networking of sensors carried on the body. These may be e.g. sensors for the determination of the blood sugar content, oxygen blood count, step counter or heart rate monitor. Already today sensors are connected with a smartphone as an evaluation unit. The propagation scenario is shown by way of example in the lower left part of Fig 3. Since transmitter and receiver are located at the body, propagation takes place through space waves (in case of line of sight) and through body-centric surface waves. Especially the surface guided waves enable interconnection of sensors without a line of sight connection.

Implant to on-body path

This case, which is illustrated here by the example of the vital parameters transmission between a close-surface implanted cardiac pacemaker and an evaluation unit worn at the body, differs from the first case in that one antenna is located in the implant below the body surface. Thus one antenna is located on the other side of the boundary layer between tissue and air. However, the preferred excited wave type would here be also a surface-guided wave, which propagates along the body contours. 

Implant to Implant path

Future application could also require a radio link between two implants. As an example, in the upper right part of Fig. 3 a sensor and an actuator of a neurostimulator device is shown. Such systems are currently being discussed for bridging severed nerve connections (e.g., cross-section paralysis). In this case, transmitter and receiver would be implanted. Due to the different dielectric tissue parameters the wave propagation still is going to take place partly at the body-air-boundary and partly at the boundary layer between muscle tissue and bones.

References

[1] M. Grimm and D. Manteuffel, “On-Body Antenna Parameters,” IEEE Transactions on Antennas and Propagation, vol. 63, no. 12, pp. 5812–5821, Dec. 2015, doi: 10.1109/TAP.2015.2482499.

[2] P. R. Bannister, “New Formulas That Extend Norton’s Farfield Elementary Dipole Equations to the Quasi-Nearfield Range,” NAVAL UNDERWATER SYSTEMS CENTER NEW LONDON CT, NUSC–TR–6883, Jan. 1984.

[3] M. Grimm and D. Manteuffel, “Norton Surface Waves in the Scope of Body Area Networks,” IEEE Transactions on Antennas and Propagation, vol. 62, no. 5, pp. 2616–2623, May 2014, doi: 10.1109/TAP.2014.2307347.

[4] L. Berkelmann and D. Manteuffel, “Slot Antenna Design with Optimized On-Body Pattern for Eyewear Applications,” in IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting - AP-S/URSI, 2019.

[5] L. Berkelmann, T. Martinelli, A. Friedrich, and D. Manteuffel, “Design and Integration of a Wearable Antenna System for On- and Off-body Communication Based on 3D-MID Technology,” in 12th European Conference on Antennas and Propagation (EuCAP 2018), 2018, pp. 1–4, doi: 10.1049/cp.2018.0849.