**2.1. UWB antennas**

Concepts for antennas with an ultra-wideband behavior are well-known and established [30]. However, advancements focusing on specific applications and specific performance parameters are inevitable to keep pace with the requirements of modern communication, radar, and localization systems. In particular in the medical environment, application-oriented antennas are mandatory to cope with the need for reliable systems (e. g. health monitoring systems) or with extreme environmental conditions (e. g. implanted systems). In the following, three novel UWB antennas are introduced targeting different tasks in the medical field and showing outstanding characteristics with respect to certain key antenna parameters.

### *2.1.1. Circular slot antenna excited with a dipole element*

**Figure 1.** Dipole slot antenna.

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transmit/receive turn-around times in the nanosecond regime. Integrated front-ends which

The second section deals with signal processing. As, due to the large RF bandwidth, direct analog to digital conversion and digital signal processing are not feasible (at least not at reasonable power consumption), analog signal processing is one focus. For communication, detection methods based on analog correlation require channel estimation, storing of impulse responses and also precise time synchronization. Therefore methods based on energy detection are developed which require no or little channel knowledge, having low complexity, robustness to multipath propagation and high resistance to synchronization and symbol clock errors. New modulation techniques are described, which can cope with interchip and intersymbol interference. Also a novel support by a comb filter resulting in significant SNR improvements in interference and multiuser scenarios is presented. The methods developed for communication applications can also be used in the radar context. For detection and tracking of moving targets (e.g. heart in the body) new algorithms based on particle filtering are developed for the digital signal processing part. It is shown that the accuracy, the resolution and robustness can be improved compared to conventional methods. For the objective of catheter localization, the knowledge of the shape and position of the human body surface is inevitable. A UWB imaging algorithm for the detection and estimation of this surface has been developed based on trilateration and is also described in this second section. Furthermore, building on this surface estimation algorithm, a new method for the localization of transmitters in dielectric media is presented. Taking into account the refraction effects on the boundary surface, the algorithm uses the impulse time of arrival to determine

The third section finally describes the design of bistatic UWB radar systems using the components presented in the first section. Single-ended and differential radar demonstrators are developed, with which the potential of impulse-radio UWB sensing is evaluated. Measurements aimed at applications of the developed hardware such as vital sign monitoring and communication with implants are presented. Further measurements are performed to prove the functionality of the imaging algorithms derived in the second section. For surface estimation, a single radar sensor is moved around a highly reflective target in order to emulate a whole sensor array. For the verification of subsurface transmitter localization, a transmitter is placed inside of a container filled with tissue mimicking liquid, and its position is visualized

Concepts for antennas with an ultra-wideband behavior are well-known and established [30]. However, advancements focusing on specific applications and specific performance parameters are inevitable to keep pace with the requirements of modern communication, radar, and localization systems. In particular in the medical environment, application-oriented antennas are mandatory to cope with the need for reliable systems (e. g. health monitoring systems) or with extreme environmental conditions (e. g. implanted systems). In the following, three novel UWB antennas are introduced targeting different tasks in the medical field and showing outstanding characteristics with respect to certain key

successfully address this issue are presented here for the first time.

the transmitter position inside of the dielectric medium.

with respect to the estimated container surface.

**2. Circuit and component design**

**2.1. UWB antennas**

antenna parameters.

Planar broad monpoles or dipoles are favored UWB antennas for communication systems with high data rates, e. g. potentially used in base stations for patient monitoring. However, broad monopoles fed single-endedly are prone to cable currents on the feeding line disturbing the radiation characteristic in the lower frequency range [15], while dipoles behave like a *λ*-radiator with a zero in main beam direction in the upper frequency range. Both effects lead to an undesired change of the radiation pattern in the operational frequency range reducing the effective bandwidth. For the widely-used impulse based UWB systems, this leads to a broadening of the impulse and consequently to a degraded system performance.

A practical solution to overcome the described parasitic radiation pattern performance is the combination of a circular slot antenna with a dipole feeding element as depicted in Fig. 1. The circular slot behaves like a broad monopole according to Babinet's principle. A broad dipole located in the center of the circular slot and consisting of two circular segments excites the slot antenna. The inherent symmetrical feeding of the dipole avoids the propagation of cable currents due to the virtual ground plane in between the transmission lines and results in an uniform radiation characteristic over the UWB frequency range.

The length of the exciting dipole is designed to be *λ*/2 at the center of the FCC UWB frequency range. Therefore, the dipole is smaller than pure UWB dipole antennas with a typical length of *λ*/2 at the lower edge of the FCC UWB frequency range. The perimeter of the circular slot is about *λ* at the lower edge of the FCC UWB frequency range leading to a resonance at 4.3 GHz (see simulation result for |*S*11| in Fig. 2(a)), and hence, to a return loss better than 10 dB at 3.1 GHz. Additional resonances with a low qualtiy factor are arising if the perimeter of the slot is a multiple of the wavelength (see at 6.9 GHz and 9.8 GHz in Fig. 2(a)). Therefore, a UWB behavior regarding return loss is achieved.

In order to characterize the dipole slot antenna with a single-ended coaxial line, a common UWB planar transition from coplanar stripline to a microstrip line based on [32] is used. A metallic shielding around this balun suppresses any parasitic radiation (see Fig. 1). The

at the end of the rod acts as smooth transition of the waveguide impedance to the free space

Due to the rod permittivity of 2.8, the electromagnetic field is mainly concentrated within the rod. Therefore, the antenna primarily radiates in forward direction along the rod. The unidirectionality and, hence, the gain is slightly improved – especially for lower frequencies – placing a metallic reflector at the backside of the antenna (see Fig. 3(a)). The reflector distance

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**Figure 3.** Sketch and radiation performance of the dielectric rod antenna fed by the dipole slot antenna. The radiation pattern in Fig. 3(b) shows the obtained small beamwidth of the antenna for the H-plane. The increased side lobe level at frequencies above 8 GHz is caused by a parasitic leaky wave and is hardly avoidable for rod antennas. The given radiation characteristics in Fig. 3(b) are typical for both planes and results in a high mean gain of 8.7 dBi including the return loss. Due to the fact that the return loss is better than 10 dB from 3.5 GHz to 11.8 GHz,

Besides this good electrical performance, the major benefits of this antenna in contrast to [14] and [4] are the compactness, the low weight, and the ease of fabrication. These attributes make

High data rate communication for implanted devices [5] or precise catheter localization inside the human body are futuristic topics in medicine. There, impulse-based UWB technology is advantageous compared to narrow band systems due to the low power consumption caused by the simple system architecture. However, UWB antennas optimized for radiation in human

In Fig. 4(a) a UWB antenna is proposed for radiation in human tissue, which is based on a similar concept as the dipole slot antenna in Sec. 2.1.1. An elliptical slot antenna is fed by a broad monopole located in the center of the slot. A monopole is chosen to obtain a small structure and to avoid a bulky balun for characterization purposes. Instead of a single layer

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(b) Measured radiation pattern in dB (H-plane)

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for the shown structure is *λ*/4 at 5.35 GHz leading to an optimized mean gain.

(a) Sketch with transparent rod

tissue are hardly investigated.

the influence of the return loss on the realized gain is negligible.

the antenna also interesting for all kinds of industrial applications.

*2.1.3. UWB slot antenna optimized for radiation in human tissue*

impedance.

**Figure 2.** Reflection and radiation performance of the dipole slot antenna.

measured return loss behavior of the antenna including balun is presented in Fig. 2(a) and shows the predicted UWB performance.

The measured radiation pattern demonstrates the desired uniform characteristic over the FCC frequency range, as can be seen in Fig. 2(b) for the E-plane. In the H-plane a similar pattern is obtained having a slightly narrower beam with a mean 3 dB beamwidth of 60° in contrast to 65° for the E-plane. The uniform radiation behavior in the frequency domain results in a short antenna impulse response making the antenna a suitable candidate for impulse-based communication systems.

An upgrade of this single polarized antenna for horizontal and vertical polarizations is possible, if two narrow feeding dipoles are placed in orthogonal position to each other inside the circular slot. The expense of the additional feature is a deterioration in the return loss and radiation characteristic (see [20]).

### *2.1.2. Dielectric rod antenna fed by a planar circular slot*

Emerging applications in medicine are vital sign monitoring [33], breast cancer detection [11], and tracking of inner organs for improved magnetic resonance tomography [38]. For these radar based sensing systems, directive antennas with a small beamwidth in both planes are compulsory. A promising approach to achieve this attribute is to exploit dielectric rod antennas either fed by a tapered slot antenna [14] or by a biconical dipole [4]. Both ideas show very good electrical characteristics, but suffer from necessary sophisticated fabrication and assembly.

A new antenna proposal is shown in Fig. 3(a). In this concept, the planar circular slot antenna presented in Sec. 2.1.1 acts as feed for a circular dielectric waveguide. The electrical field distribution inside the circular slot is similar to the fundamental mode *H*<sup>11</sup> of the dielectric waveguide. Hence, this mode is predominantly excited. Due to that and since the *H*11-mode possesses no cutoff frequency, in general an ultra-wideband performance is obtained. The diameter of the dielectric rod is chosen to 43 mm, which is a compromise between a good return loss behavior and single-mode operation of the dielectric waveguide. The conical shape at the end of the rod acts as smooth transition of the waveguide impedance to the free space impedance.

Due to the rod permittivity of 2.8, the electromagnetic field is mainly concentrated within the rod. Therefore, the antenna primarily radiates in forward direction along the rod. The unidirectionality and, hence, the gain is slightly improved – especially for lower frequencies – placing a metallic reflector at the backside of the antenna (see Fig. 3(a)). The reflector distance for the shown structure is *λ*/4 at 5.35 GHz leading to an optimized mean gain.

(a) Sketch with transparent rod

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Angle / degree

measured return loss behavior of the antenna including balun is presented in Fig. 2(a) and

The measured radiation pattern demonstrates the desired uniform characteristic over the FCC frequency range, as can be seen in Fig. 2(b) for the E-plane. In the H-plane a similar pattern is obtained having a slightly narrower beam with a mean 3 dB beamwidth of 60° in contrast to 65° for the E-plane. The uniform radiation behavior in the frequency domain results in a short antenna impulse response making the antenna a suitable candidate for impulse-based

An upgrade of this single polarized antenna for horizontal and vertical polarizations is possible, if two narrow feeding dipoles are placed in orthogonal position to each other inside the circular slot. The expense of the additional feature is a deterioration in the return loss and

Emerging applications in medicine are vital sign monitoring [33], breast cancer detection [11], and tracking of inner organs for improved magnetic resonance tomography [38]. For these radar based sensing systems, directive antennas with a small beamwidth in both planes are compulsory. A promising approach to achieve this attribute is to exploit dielectric rod antennas either fed by a tapered slot antenna [14] or by a biconical dipole [4]. Both ideas show very good electrical characteristics, but suffer from necessary sophisticated fabrication and

A new antenna proposal is shown in Fig. 3(a). In this concept, the planar circular slot antenna presented in Sec. 2.1.1 acts as feed for a circular dielectric waveguide. The electrical field distribution inside the circular slot is similar to the fundamental mode *H*<sup>11</sup> of the dielectric waveguide. Hence, this mode is predominantly excited. Due to that and since the *H*11-mode possesses no cutoff frequency, in general an ultra-wideband performance is obtained. The diameter of the dielectric rod is chosen to 43 mm, which is a compromise between a good return loss behavior and single-mode operation of the dielectric waveguide. The conical shape

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measurement

shows the predicted UWB performance.

communication systems.

assembly.

radiation characteristic (see [20]).

*2.1.2. Dielectric rod antenna fed by a planar circular slot*

simulation (antenna without balun)

Frequency / GHz

**Figure 2.** Reflection and radiation performance of the dipole slot antenna.

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The radiation pattern in Fig. 3(b) shows the obtained small beamwidth of the antenna for the H-plane. The increased side lobe level at frequencies above 8 GHz is caused by a parasitic leaky wave and is hardly avoidable for rod antennas. The given radiation characteristics in Fig. 3(b) are typical for both planes and results in a high mean gain of 8.7 dBi including the return loss. Due to the fact that the return loss is better than 10 dB from 3.5 GHz to 11.8 GHz, the influence of the return loss on the realized gain is negligible.

Besides this good electrical performance, the major benefits of this antenna in contrast to [14] and [4] are the compactness, the low weight, and the ease of fabrication. These attributes make the antenna also interesting for all kinds of industrial applications.

### *2.1.3. UWB slot antenna optimized for radiation in human tissue*

High data rate communication for implanted devices [5] or precise catheter localization inside the human body are futuristic topics in medicine. There, impulse-based UWB technology is advantageous compared to narrow band systems due to the low power consumption caused by the simple system architecture. However, UWB antennas optimized for radiation in human tissue are hardly investigated.

In Fig. 4(a) a UWB antenna is proposed for radiation in human tissue, which is based on a similar concept as the dipole slot antenna in Sec. 2.1.1. An elliptical slot antenna is fed by a broad monopole located in the center of the slot. A monopole is chosen to obtain a small structure and to avoid a bulky balun for characterization purposes. Instead of a single layer

#### 6 Will-be-set-by-IN-TECH 444 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications <sup>7</sup>

structure, two substrate layers with slots in the top and bottom layer metalizations are used. The monopole is arranged in the center metalization layer and is fed by a triplate line. In this way, the buried feeding is insulated from the adjacent highly lossy human tissue. The antenna dimensions need to be optimized according to the surrounding medium of the antenna. The width of the antenna is 11 mm assuming skin tissue around the antenna with a typical permittivity of 28 at 6.85 GHz. The size reduction factor compared to an antenna designed for air instead of skin tissue is 5.4 leading automatically to a miniaturized UWB antenna.

<sup>1</sup> <sup>2</sup> <sup>3</sup> <sup>4</sup> <sup>5</sup> <sup>6</sup> <sup>7</sup> <sup>8</sup> <sup>9</sup> <sup>10</sup> <sup>11</sup> <sup>12</sup> <sup>−</sup><sup>30</sup>

Sugar Solution

(a) Reflection coefficient

convenience of achieving biphase modulation.

Skin

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**Figure 5.** Reflection and radiation performance of the tissue optimized antenna.

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substrate. The technology is fully adequate for impulse-radio-ultra-wideband (IR-UWB)

Generating short time-domain impulses making efficient use of the spectral mask is the key challenge in IR-UWB systems. Approaches include the up-conversion of base band pulses to the allocated UWB frequency band using an oscillator and mixer [39] and direct generation based on damped relaxation oscillator [8]. Here impulse generators based on a quenched-oscillator concept with great circuit simplicity are presented. A cross-coupled LC oscillator is chosen as the core to introduce tunability of the waveform and the inherent

(a) Circuit schematic (b) Simulation

**Figure 6.** Complete circuit schematic of the UWB impulse generator. The dashed components (C1*b*, C2 and T7) show the extension for tunability of the impulse shape for different spectral masks and transistor

Fig. 6(a) shows the impulse generator circuit. First, disregard the components with a dashed line, which are the extention for tunability of the impulse shape. T1 and T2 form a Schmitt trigger, creating a fast rising edge at the collector of T2 when a positive input signal triggers T1 to be on. This reduces the effect of the time-domain influence of the input clock signal

level simulation: the collector potential of T3 and the collector current of T4.

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**Figure 4.** Sketch of the tissue optimized antenna and photograph including microstrip-to-coaxial transition.

In order to connect the antenna to a coaxial cable, a broadband transition from the triplate line to a coaxial line is applied [19]. The realized antenna including this transition is depicted in Fig. 4(b). The characterization of the tissue optimized antenna is performed in a tank filled with tissue-mimicking liquid approximating the permittivity and loss behavior of skin tissue. The chosen liquid is a 50% sugar solution in water [20]. Fig. 5(a) shows the return loss of the antenna being inside the sugar-water solution and compares its performance with a measurement, where the antenna is placed on both sides on human skin. Both measurement results agree very well and show a return loss of more than 10 dB above 3.8 GHz.

The radiation pattern is also measured in the tissue-mimicking liquid using two identical antennas and applying the two-antenna method. The obtained radiation pattern for the H-plane is presented in Fig. 5(b). There, the losses of the tissue-mimicking medium are compensated. Since the losses are increasing significantly with increasing frequency, measurements only up to 9 GHz are possible limited by the dynamic range of the measurement setup. Within this frequency range, a desired uniform and broad characteristic is achieved. Hence, UWB performance for a sufficiently small antenna for implants is demonstrated. For catheter localization additional miniaturization is required. A possible approach as well as more details about all presented antennas in Sec. 2.1 can be found in [20].
