**3.1.2 Heterogeneous model**

The implantable device is inserted in three heterogeneous models: the heterogeneous human model named Hugo which is the simulation tool human model [24], a multi-layered structure and a simple experimental setup made to validate simulated heterogeneous models, the "human + hand" model. Compared to previous homogeneous model, the main advantage of these heterogeneous models is the ability to carefully model all human tissues in near antenna area to accurately take into account the near field pacemaker antenna behaviour.

The pacemaker device is implanted in the pectoral of Hugo, in a limited volume sample of 11.2 x 6.4 x 11.6 cm3 (Fig. 7 (a)). The voxel size of the human body model is the minimal voxel size of the simulation tool, i.e. 1 mm3. The whole body phantom contains 44 different tissues, whose real part of permittivity (εr') and conductivity (σ) are taken from [24] at 450 MHz. The chosen limited sample obviously includes fewer tissues than the complete body model.

An Efficient Adaptive Antenna-Impedance

Fig. 9. "Human + hand" model

simulation results to be obtained.

**3.2 Results** 

Table 2. Electrical data of human body phantom tissues

will see in the next section that it constitutes a good approximation.

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Biological tissue Permittivity (εr') Conductivity (σ, S/m) Fat tissue 5.560525 0.041934 Skin 45.753101 0.708836 Muscle 58.482101 0.851437

A simple experimental setup with a real human is also experimented. This one covers the pacemaker with his hand and puts it against his bust in exercising a strong pressure (Fig. 9). This setup has not the intention to replace an implantation in a realistic human body, but we

The antenna input impedances characterized in homogeneous and heterogeneous models are respectively shown in Fig. 10 and Fig. 11. In homogeneous models, measured results with coaxial cable are systematically compared to simulated results with and without cable. In the configuration without cable, the loop antenna is fed by a lumped port which consists typically in a voltage applied between the two extremities of the loop. This configuration was used in order to simplify the numerical problem size to solve and thereby to reduce the total simulation time. Finally, only this simplified excitation setup will be used in accurate and heavy heterogeneous models because it allows fast

In order to easily design implanted antennas, multi-layered geometries which provide an acceptable model for the human body, were firstly proposed in [18]. Based on the real human body structure of the simulation tool, the heterogeneous multi-layered model used here (Fig. 8) is made of three layers (skin, fat, muscle) that have different thickness and different electrical properties. The thickness of the skin, fat and muscle tissues are respectively 4, 20 and 10 mm. The electrical properties of these three layers are taken from electrical data of human body phantom tissues and given in Table 2. The pacemaker is implanted in the fat layer just under the skin layer. The geometrical characteristics of the heterogeneous model, i.e. pacemaker position inside the rectangular block and dimensions of both layers and whole block, have been optimized in order to be in accordance with Hugo implant impedance.

Fig. 8. Multi-layer heterogeneous model

In order to easily design implanted antennas, multi-layered geometries which provide an acceptable model for the human body, were firstly proposed in [18]. Based on the real human body structure of the simulation tool, the heterogeneous multi-layered model used here (Fig. 8) is made of three layers (skin, fat, muscle) that have different thickness and different electrical properties. The thickness of the skin, fat and muscle tissues are respectively 4, 20 and 10 mm. The electrical properties of these three layers are taken from electrical data of human body phantom tissues and given in Table 2. The pacemaker is implanted in the fat layer just under the skin layer. The geometrical characteristics of the heterogeneous model, i.e. pacemaker position inside the rectangular block and dimensions of both layers and whole block, have been optimized in order to be in accordance with Hugo

Fig. 7. Heterogeneous Hugo model

Fig. 8. Multi-layer heterogeneous model

implant impedance.


Table 2. Electrical data of human body phantom tissues

A simple experimental setup with a real human is also experimented. This one covers the pacemaker with his hand and puts it against his bust in exercising a strong pressure (Fig. 9). This setup has not the intention to replace an implantation in a realistic human body, but we will see in the next section that it constitutes a good approximation.

Fig. 9. "Human + hand" model

#### **3.2 Results**

The antenna input impedances characterized in homogeneous and heterogeneous models are respectively shown in Fig. 10 and Fig. 11. In homogeneous models, measured results with coaxial cable are systematically compared to simulated results with and without cable. In the configuration without cable, the loop antenna is fed by a lumped port which consists typically in a voltage applied between the two extremities of the loop. This configuration was used in order to simplify the numerical problem size to solve and thereby to reduce the total simulation time. Finally, only this simplified excitation setup will be used in accurate and heavy heterogeneous models because it allows fast simulation results to be obtained.

An Efficient Adaptive Antenna-Impedance

**4. Single step antenna tuning unit** 

**4.1 Brief description** 

calibration.

range of antenna impedance to the front-end radio.

Fig. 12. Description of the proposed antenna tuning unit

Tuning Unit Designed for Wireless Pacemaker Telemetry 233

be neglected. To allow maximum power transfer between transceiver circuitry and antenna, it is necessary to design a variable matching network able to match automatically the wide

To address the problem due to impedance mismatch, many antenna impedance tuning units operating iteratively and/or using directional coupler to evaluate the quality of the link were investigated [7-15]. Since the use of a bulky additional coupler into the device is totally inacceptable and since iterative matching process spends time and consumes power to set the proper state of the network, we investigate on a novel coupler less method [25] solving the problems related to the impedance mismatch in a single iteration. The proposed solution detailed in this section is the first system able to match automatically in a single process both TX and RX matching networks. It reduces the power losses in transmission and in reception

In general, the power consumption of radio communication modules is dominated by the power consumption of the power amplifier during the transmitting path and by the power consumption of the low noise amplifier during the receiving path. Since antenna impedance calibration procedure is done during the transmitting mode, in order to achieve low power antenna impedance tuning unit, it is necessary to reduce strongly the time required for the

Therefore, we propose an innovative single step antenna tuning unit concept which basic topology is illustrated in Fig. 12. A generic detector made of capacitor *Cdet*, which advantageously replaces the usual bulky coupler, is inserted between the power module and the tunable matching network. The sensed signal *v1* and *v2* are attenuated for linearity issue, down converted to a lower intermediate frequency and analyzed by a processor. As described by the flow chart in Fig. 13, the processor exploits the magnitude and the phase of the sensed signals *v1* and *v2* to first calculate the impedance *Z1* and/or *Z2* located in the left and the right port of the detector, respectively. Finally, the extraction of the antenna input impedance exploits the well known deembedding techniques to calculate *ZAnt* from *Z1* or *Z2*. The obtained antenna input impedance value is used to directly calculate the parameters of the matching network that reach the proper state of the system at a selected frequency.

contributing to the optimization of the power efficiency of the transceiver itself.

Fig. 10. Antenna impedance in homogeneous human models

Fig. 11. Antenna impedance in heterogeneous human models

A global study of the impedance characteristics shows that the sensitivity of the antenna to the human tissues results in a shift of the resonant mode. As the MICS band is in the vicinity of this resonant frequency characterized by fast impedance variation, the shift of 50 MHz in frequency involves a huge shift in impedance levels (see Fig. 10 and Fig. 11); hence, while the values of real part of impedance in heterogeneous models are between 39 and 51 Ω, those in the homogeneous models are between 185 and 260 Ω. Similar discrepancies can be seen on imaginary part of impedance. These impedance random shifts are too significant to be neglected. To allow maximum power transfer between transceiver circuitry and antenna, it is necessary to design a variable matching network able to match automatically the wide range of antenna impedance to the front-end radio.
