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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

Fig 1: Body-centric communication channels

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. The traditional free-space antenna theory, with an angle and polarity-dependent antenna gain, gives insufficient insights for the development of such body centric systems so far, as the wave propagation partially proceeds as surface bound waves along the body contour.

Research at Institute of Microwave and Wireless System

 

At the Institute of Microwave and Wireless System methods for the systematic development of antennas for devices worn at or implanted in the body are investigated. Therefore channel models for all elementary cases are being developed. Through de-embedding the antenna from the channel, optimal antenna properties for all transmission scenarios can be investigated. Measurements with appropriate body phantoms and numeric simulations are used to verify the developed methods and to develop antennas for example applications.

Body-centric wireless communication channels

Considering the variety of possible applications of body area networks a rough classification of the  communication channels can be made 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 1. 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. Thus for antennas for on-body paths a characterization regarding the excitation of space waves and surface waves on the body would be valuable. In addition a physical channel characterization, which is also based on the two wave types, is desirable for this case. Using both models a target-oriented antenna development can take place.

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.  Again a corresponding antenna characterization, which is generally mapped to this case, in conjunction with a corresponding channel model is required.

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 1 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. This case also differs from both mentioned above.

Antenna de-embedding – On-body antenna parameters

To evaluate the properties of an antenna in terms of on-body propagations the electric current distribution on the antenna is calculated by the FDTD method. At this point even the interaction with the human body is modeled by an anatomical or simplified human body model. Using the Norton surface wave theory, the contribution of each current element to the on-body antenna far field can be modeled. Separating the corresponding antenna far field in its TM- and TE-components, even the derivation of on-body antenna parameters are realized. This enables a comparison of different antenna types in terms of on-body communications and a discussion of the underlying wave species.

Propagation link calculation

The combination of on-body parameters and channel models enable a systematic segmentation and combination of various propagation scenarios. By this approach a wide class of scalable pathloss models can be realized, and smart antenna concepts can be developed which aim for specific propagation qualities.

Research at Institute of Microwave and Wireless System

 

At the Institute of Microwave and Wireless System methods for the systematic development of antennas for devices worn at or implanted in the body are investigated. Therefore channel models for all elementary cases are being developed. Through de-embedding the antenna from the channel, optimal antenna properties for all transmission scenarios can be investigated. Measurements with appropriate body phantoms and numeric simulations are used to verify the developed methods and to develop antennas for example applications.

Body-centric wireless communication channels

Considering the variety of possible applications of body area networks a rough classification of the  communication channels can be made 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 1. 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. Thus for antennas for on-body paths a characterization regarding the excitation of space waves and surface waves on the body would be valuable. In addition a physical channel characterization, which is also based on the two wave types, is desirable for this case. Using both models a target-oriented antenna development can take place.

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.  Again a corresponding antenna characterization, which is generally mapped to this case, in conjunction with a corresponding channel model is required.

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 1 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. This case also differs from both mentioned above.

Antenna de-embedding – On-body antenna parameters

To evaluate the properties of an antenna in terms of on-body propagations the electric current distribution on the antenna is calculated by the FDTD method. At this point even the interaction with the human body is modeled by an anatomical or simplified human body model. Using the Norton surface wave theory, the contribution of each current element to the on-body antenna far field can be modeled. Separating the corresponding antenna far field in its TM- and TE-components, even the derivation of on-body antenna parameters are realized. This enables a comparison of different antenna types in terms of on-body communications and a discussion of the underlying wave species.

Propagation link calculation

The combination of on-body parameters and channel models enable a systematic segmentation and combination of various propagation scenarios. By this approach a wide class of scalable pathloss models can be realized, and smart antenna concepts can be developed which aim for specific propagation qualities.