**4.2. Differential bistatic radar system**

A differential bistatic radar system for detecting vital signs was also developed. It follows the approach described in Sec. 4.1, but with fully differential ICs as described in Sec. 2.2 and Sec. 2.3.3 and with significantly reduced power consumption. Here two dipole-fed circular slot antennas are chosen instead of the the Vivaldi antennas. Both the differential impulse

<sup>1</sup> Measurements on humans using the single-ended bistatic radar sensor have been approved by the ethic commission of Ulm University.

38 Will-be-set-by-IN-TECH

impulses. By this procedure, a cross-correlation curve of two fifth Gaussian derivatives is developing. The correlated curve of the above illustration example is depicted in Fig. 43. The correlation impulses from the direct coupling and the reflection at the object can be distinguished. As discussed, the reflection at the object is mirrored, because the impulse is inverted by the reflection. This signal is present at the output of the receive antenna and is sampled by the data-logger. The correlation signal is continuously repeated with a repetition rate of Δ*f* . For a separation of the correlation sweeps, the Δ*f*-signal is sampled as well by the data-logger. When the object under investigation is now moving, the part of the correlation signal coming from the object reflection is correspondingly changing its alignment to the Δ*f*-signal and the movement can be measured. The movement determination of the object is continuously done by software in the PC. First a separation of the correlation sweeps by the rising slope of the Δ*f*-signal is performed. Then both slopes of the correlation curves, the slope from the object and the slope from the direct coupling, are tracked and their positions continuously monitored. Tracking both slopes yielded best precision, compared to tracking the minimum of the correlation curve or only the slope of the correlation signal from the object

To measure the precision of the sensor a metal plate is placed in front, which is mounted on a sledge driven by an eccentric disk, moving the metal plate forward and backward with an approximately sinusoidal deviation. Fig. 44(a) shows a time domain record of a movement measurement with the metal plate placed at a distance of 19.6 cm, a deviation amplitude of approximately 1 mm and a repetition rate of around 1.35 cycles/s. The movement is clearly resolved by the measurement. In Fig. 44(b) the calculated spectrum of the measurement can be seen. The frequency maximum is very clearly visible and verifies a precision of the

In a further measurement the sensor is pointed to the abdomen of a male test person lying on the back at a distance of approximately 25 cm1. At the abdomen the largest breathing amplitude occurs. Fig. 45(a) shows a breathing measurement in case the person is breathing normally. The breathing amplitude exceeds 10 mm and the repetition rate is around 2.5 cycles/s. For a further measurement the demonstrator is placed towards a sleeping seven-week old child lying on the back at a distance of approximately 16.3 cm. Fig. 45(b) shows a rhythmic breathing period with a movement of around 1 mm in the direction of the sensor and a repetition rate of 1.5 cycles/s. These measurements show, that the sensor can be used to monitor the breathing of adults and infants lying on the back and that breathing patterns can clearly be detected using the single-ended bistatic impulse-radio UWB radar

A differential bistatic radar system for detecting vital signs was also developed. It follows the approach described in Sec. 4.1, but with fully differential ICs as described in Sec. 2.2 and Sec. 2.3.3 and with significantly reduced power consumption. Here two dipole-fed circular slot antennas are chosen instead of the the Vivaldi antennas. Both the differential impulse

<sup>1</sup> Measurements on humans using the single-ended bistatic radar sensor have been approved by the ethic commission

demonstrator in the millimeter to sub-millimeter range.

**4.2. Differential bistatic radar system**

[31].

demonstrator.

of Ulm University.

**Figure 46.** Photographs of transmit antenna with mounted differential impulse generator IC and the complete receiver with RF frontend IC and baseband circuit.

generator and the correlation receiver front-end are mounted chip-on-board at the feeding points of the dipole antennas. Fig. 46 shows the pictures of transmit and receive antennas with mounted differential ICs.

Using the same DDS clock generator and post-processing software as described in Sec. 4.1, the ability of the realized differential bistatic radar system for tracking a metal plate which moves back and forth with a sinusoidal deviation is demonstrated. The measured result in the time domain can be seen in Fig. 47(a). A deviation amplitude of around 1.5 mm can be

**Figure 47.** Measurement results of a moving metal plate in front of the bistatic radar with a distance of 35 cm in time and frequency domains.

clearly seen. Fig. 47(b) shows the calculated spectrum information of the measurement. The maximum point is clearly visible and indicates the movement frequency of the metal plate.

A common application for this radar system is the detection of vital signs. Here, an adult male with pronounced tachypnea is seated 5 cm from the radar. Fig. 48(a) shows the recorded time domain data. The breathing pattern is clearly visible and its amplitude is around 5 mm. The time domain data is Fourier transformed to frequency domain as shown in Fig. 48(b). It clearly indicates that the respiration rate is around 35/min.

between a transmitter inside tissue-mimicking liquid and a receiver placed inside or outside

RX

479

cm . The

<sup>R</sup><sup>2</sup> TX

(b) Tissue to air

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

R1

the medium. These two measurement scenarios are illustrated in Fig. 50.

<sup>R</sup><sup>0</sup> TX

attenuation of the phantom liquid.

medical applications.

found in [21].

(a) Tissue to tissue

well-suited for communication with implants.

RX

**Figure 50.** Measurement scenarios for the demonstration of impulse-based data transmission for implants using an energy detector. An amplifier is applied on the transmit side to cope with the high

As discussed in Sec. 3.1, many different approaches for IR-UWB receivers are feasible. Non-coherent detection is chosen here because of the challenges regarding synchronization in coherent receiver setups and because of the dispersive properties of human tissue, which lead to unpredictable shapes of the received signals. Even though non-coherent detectors are suboptimal, they are insusceptible to dispersive effects of the channel, and therefore

The simplest non-coherent receiver concept is an energy detector, which basically consists of a squaring device and an integrator (see Sec. 2.3). Besides its simplicity, the advantage of an energy detector is that possible narrow-band interferers within the operating frequency band could be suppressed by using the comb filter approach described in Sec. 3.2. This would improve the SNR additionally. Hence, the energy detector is chosen as the receiver in the demonstration setup for communication with implants. In this demonstration, on-off keying is used as the modulation scheme and a unidirectional transmission with a data rate of 100 Mbit/s is selected. This data rate and a unidirectional transmission are sufficient for most

The transmitter in Fig. 50 consists of a UWB impulse generator (IG) and an antenna. The transmit data are generated externally by a bit pattern generator, clocked by a signal generator. The receiver topology is the same as shown in Sec. 2.3.2, but in a single-ended configuration. The output signal is amplified and displayed on an oscilloscope. For further signal processing in the digital domain, a comparator circuit with a properly adjusted threshold voltage can be applied. A more detailed description of the transmitter and receiver structures used can be

To demonstrate the transmission in an environment similar to human tissue with a highly dispersive and lossy behavior, two antennas are immersed in a tissue-mimicking liquid. This liquid consists mainly of sugar and water. The properties of the liquid are similar to skin tissue with a relative dielectric constant *ε*<sup>r</sup> of 28 at 7 GHz and an attenuation of about 22 dB

influence of the dispersive behavior on the impulse shape is illustrated in Fig. 51 in time and frequency domain. There, the output signal after the transmission through the phantom liquid is shown in comparison to the input signal. Due to increased losses for higher frequencies, the impulse shape is significantly broadened and the amplitude is decreased by approximately

60 dB for the used path distance of 23.5 mm between the two immersed antennas.

**Figure 48.** Time domain and frequency domain measurements of vital signs of a male test person standing in front of the bistatic radar.

In a further measurement, surface estimation of a container filled with a sugar solution whose properties are similar to those of human tissue was performed by moving the radar up and down in 2 cm steps along both the x- and y-axis. For demonstrating that the radar system is capable for this application, only the lower part of the container is scanned. The trilateration-based imaging algorithm derived in Sec. 3.4 is applied. The photograph of the container is shown in Fig. 49(a), and a cloud of estimated surface points representing the front of the target can be seen in in Fig. 49(b). The result clearly indicates the distance from the radar sensor to the container and the planar surface structure.

**Figure 49.** Photograph of the liquid container and measured cloud of estimated surface points.

### **4.3. Communcation with implants**

In this section we address another application of UWB technology in medicine, the communication with implants. Impulse-based UWB technology is a promising solution for future implanted medical devices demanding data rates in the Mbit/s range and a low power consumption. Here, we present a demonstration system for uni-directional data transmission between a transmitter inside tissue-mimicking liquid and a receiver placed inside or outside the medium. These two measurement scenarios are illustrated in Fig. 50.

40 Will-be-set-by-IN-TECH

(a) Time domain (b) Spectrum

In a further measurement, surface estimation of a container filled with a sugar solution whose properties are similar to those of human tissue was performed by moving the radar up and down in 2 cm steps along both the x- and y-axis. For demonstrating that the radar system is capable for this application, only the lower part of the container is scanned. The trilateration-based imaging algorithm derived in Sec. 3.4 is applied. The photograph of the container is shown in Fig. 49(a), and a cloud of estimated surface points representing the front of the target can be seen in in Fig. 49(b). The result clearly indicates the distance from the radar

**Figure 48.** Time domain and frequency domain measurements of vital signs of a male test person

(a) Photograph (b) Measurement

**Figure 49.** Photograph of the liquid container and measured cloud of estimated surface points.

In this section we address another application of UWB technology in medicine, the communication with implants. Impulse-based UWB technology is a promising solution for future implanted medical devices demanding data rates in the Mbit/s range and a low power consumption. Here, we present a demonstration system for uni-directional data transmission

35

standing in front of the bistatic radar.

**4.3. Communcation with implants**

0 2 4 6 8 10

Time / s

sensor to the container and the planar surface structure.

40

45

Extension / mm

50

55

**Figure 50.** Measurement scenarios for the demonstration of impulse-based data transmission for implants using an energy detector. An amplifier is applied on the transmit side to cope with the high attenuation of the phantom liquid.

As discussed in Sec. 3.1, many different approaches for IR-UWB receivers are feasible. Non-coherent detection is chosen here because of the challenges regarding synchronization in coherent receiver setups and because of the dispersive properties of human tissue, which lead to unpredictable shapes of the received signals. Even though non-coherent detectors are suboptimal, they are insusceptible to dispersive effects of the channel, and therefore well-suited for communication with implants.

The simplest non-coherent receiver concept is an energy detector, which basically consists of a squaring device and an integrator (see Sec. 2.3). Besides its simplicity, the advantage of an energy detector is that possible narrow-band interferers within the operating frequency band could be suppressed by using the comb filter approach described in Sec. 3.2. This would improve the SNR additionally. Hence, the energy detector is chosen as the receiver in the demonstration setup for communication with implants. In this demonstration, on-off keying is used as the modulation scheme and a unidirectional transmission with a data rate of 100 Mbit/s is selected. This data rate and a unidirectional transmission are sufficient for most medical applications.

The transmitter in Fig. 50 consists of a UWB impulse generator (IG) and an antenna. The transmit data are generated externally by a bit pattern generator, clocked by a signal generator. The receiver topology is the same as shown in Sec. 2.3.2, but in a single-ended configuration. The output signal is amplified and displayed on an oscilloscope. For further signal processing in the digital domain, a comparator circuit with a properly adjusted threshold voltage can be applied. A more detailed description of the transmitter and receiver structures used can be found in [21].

To demonstrate the transmission in an environment similar to human tissue with a highly dispersive and lossy behavior, two antennas are immersed in a tissue-mimicking liquid. This liquid consists mainly of sugar and water. The properties of the liquid are similar to skin tissue with a relative dielectric constant *ε*<sup>r</sup> of 28 at 7 GHz and an attenuation of about 22 dB cm . The influence of the dispersive behavior on the impulse shape is illustrated in Fig. 51 in time and frequency domain. There, the output signal after the transmission through the phantom liquid is shown in comparison to the input signal. Due to increased losses for higher frequencies, the impulse shape is significantly broadened and the amplitude is decreased by approximately 60 dB for the used path distance of 23.5 mm between the two immersed antennas.

<sup>−</sup><sup>80</sup> <sup>−</sup><sup>60</sup> <sup>−</sup><sup>40</sup> <sup>−</sup><sup>20</sup> <sup>0</sup> <sup>20</sup> <sup>40</sup> <sup>60</sup> <sup>80</sup> <sup>−</sup>0.1

(a) Tissue to tissue

Fig. 50 using a data rate of 100 Mbit/s.

medium of about 22 dB

antenna beacon

dielectric medium

*4.4.2. Surface estimation results*

Time / ns

consisting of an impulse generator and an amplifier is switched on.

cylindrical array of radar sensors is emulated.

Transmitted bits Received signal −35 −30 −25 −20 −15 −10

cm , an amplifier is used. For localization, the radar sensor outside of the

IG

CLK, ADC

TX/RX control unit

Voltage / dBV

**Figure 52.** Measured output voltage of the energy detector according to the measurement scenarios in

container is operating in receive mode measuring the ToA of the transmitted impulse.

variable pos.

**Figure 53.** Measurement setup for surface estimation and subsurface transmitter localization. The radar sensor is moved in front of the liquid container to scan its surface. For localization, the upper part

In 3D surface measurements the performance of the trilateration-based imaging algorithm derived in Sec. 3.4.1 is verified. As a target object a plastic dummy of a female torso of about 60 cm height as pictured in Fig. 54(a) has been chosen. In order to increase the target's reflectivity the surface of the dummy has been treated with highly conductive copper laquer. Radar scans of the 3D surface are performed by moving the radar sensor up and down in 1 cm-steps along the *y*-axis and by rotating the target object in 5°-steps. As a result, a

Fig. 54(b) shows the output of the imaging algorithm, a cloud of estimated surface points representing the front side of the target. An interpolation of these points is necessary to obtain a surface model needed for the subsurface localization application. The interpolated surface

The performance of the proposed algorithm regarding its accuracy is compared to the well-established imaging algorithms "Seabed" [29] and "Envelope of Spheres" [16] by evaluating the surface measurement of a metal sphere with a known diameter of 35 cm. A

is illustrated in Fig. 54(c) showing a good agreement with the original target object.

<sup>1</sup> <sup>3</sup> <sup>5</sup> <sup>7</sup> <sup>9</sup> <sup>11</sup> <sup>13</sup> <sup>15</sup> <sup>−</sup><sup>40</sup>

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

/ cm

481

Distance R2

PC algorithms

(b) Tissue to air

−0.05 0 0.05 0.1 0.15 0.2 0.25

Voltage / V

**Figure 51.** Typical received signal after the transmission of an impulse through tissue-mimicking liquid. The distance between transmitter and receiver is *R*<sup>0</sup> = 23.5 mm.

A data transmission is still achievable in these cases as illustrated by the reception of a typical bit pattern in Fig. 52(a). To this end, an additional amplifier with 12 dB gain is inserted after the impulse generator in order to compensate for the high losses. This measurement setup demonstrates the transmission from a deeply implanted device to a reading device placed directly on the human body, e.g. applicable for capsular endoscopy. In a final scenario, the communication of a less deeply implanted device with a base station outside the human body is considered. There, one antenna is immersed in the phantom liquid and a second one is located outside in free space (see Fig 50(b)). In this setup, the distance of the immersed antenna to free space is fixed to 15 mm, and the location of the outer antenna is varied. Similar measurement results as before are obtained here. In Fig. 52(b) the output voltage of the energy detector is plotted against the distance *R*<sup>2</sup> between the medium surface and receiver in air. The observed maximum distance of the base station for a reception is 15 cm, then the attenuation limit is reached.

### **4.4. Surface estimation and subsurface localization measurements**

### *4.4.1. Measurement setup*

For the evaluation of the surface estimation and subsurface localization algorithms presented in Sec. 3.4, measurements are performed using a similar bistatic UWB radar sensor as described in Sec. 4.1 [22] and the miniaturized antenna optimized for radiation in human tissue, presented in Sec. 2.1.3. The measurement setup is illustrated in Fig. 53. In this setup we use one single radar sensor which is emulating a whole sensor array by being moved along the *x*- and *y*-axis in front of the target. The target object is a container filled with the same tissue-mimicking liquid already used in the measurements of Sec. 4.3. A control unit providing clock signals for the radar sensor is connected to a computer where the signal processing and visualization is performed. In the measurements for subsurface transmitter localization, the upper elements of the setup in Fig. 53 are switched on. The miniaturized antenna placed inside of the tissue-mimicking liquid is now transmitting a 5th derivative of a Gaussian pulse generated by the impulse generator (IG). Because of the high loss in the

**Figure 52.** Measured output voltage of the energy detector according to the measurement scenarios in Fig. 50 using a data rate of 100 Mbit/s.

medium of about 22 dB cm , an amplifier is used. For localization, the radar sensor outside of the container is operating in receive mode measuring the ToA of the transmitted impulse.

### *4.4.2. Surface estimation results*

42 Will-be-set-by-IN-TECH

−140

−120

−100

PSD / dBm/MHz

**Figure 51.** Typical received signal after the transmission of an impulse through tissue-mimicking liquid.

A data transmission is still achievable in these cases as illustrated by the reception of a typical bit pattern in Fig. 52(a). To this end, an additional amplifier with 12 dB gain is inserted after the impulse generator in order to compensate for the high losses. This measurement setup demonstrates the transmission from a deeply implanted device to a reading device placed directly on the human body, e.g. applicable for capsular endoscopy. In a final scenario, the communication of a less deeply implanted device with a base station outside the human body is considered. There, one antenna is immersed in the phantom liquid and a second one is located outside in free space (see Fig 50(b)). In this setup, the distance of the immersed antenna to free space is fixed to 15 mm, and the location of the outer antenna is varied. Similar measurement results as before are obtained here. In Fig. 52(b) the output voltage of the energy detector is plotted against the distance *R*<sup>2</sup> between the medium surface and receiver in air. The observed maximum distance of the base station for a reception is 15 cm, then the attenuation

For the evaluation of the surface estimation and subsurface localization algorithms presented in Sec. 3.4, measurements are performed using a similar bistatic UWB radar sensor as described in Sec. 4.1 [22] and the miniaturized antenna optimized for radiation in human tissue, presented in Sec. 2.1.3. The measurement setup is illustrated in Fig. 53. In this setup we use one single radar sensor which is emulating a whole sensor array by being moved along the *x*- and *y*-axis in front of the target. The target object is a container filled with the same tissue-mimicking liquid already used in the measurements of Sec. 4.3. A control unit providing clock signals for the radar sensor is connected to a computer where the signal processing and visualization is performed. In the measurements for subsurface transmitter localization, the upper elements of the setup in Fig. 53 are switched on. The miniaturized antenna placed inside of the tissue-mimicking liquid is now transmitting a 5th derivative of a Gaussian pulse generated by the impulse generator (IG). Because of the high loss in the

−80

−60

−40

<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>160</sup>

(b) Frequency domain

Frequency / GHz

Input signal Output signal

<sup>0</sup> 0.5 <sup>1</sup> 1.5 <sup>2</sup> <sup>−</sup>1.2

(a) Time domain

Time / ns

The distance between transmitter and receiver is *R*<sup>0</sup> = 23.5 mm.

Input signal Output signal

**4.4. Surface estimation and subsurface localization measurements**

−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1.2

limit is reached.

*4.4.1. Measurement setup*

Normalized amplitude

In 3D surface measurements the performance of the trilateration-based imaging algorithm derived in Sec. 3.4.1 is verified. As a target object a plastic dummy of a female torso of about 60 cm height as pictured in Fig. 54(a) has been chosen. In order to increase the target's reflectivity the surface of the dummy has been treated with highly conductive copper laquer. Radar scans of the 3D surface are performed by moving the radar sensor up and down in 1 cm-steps along the *y*-axis and by rotating the target object in 5°-steps. As a result, a cylindrical array of radar sensors is emulated.

Fig. 54(b) shows the output of the imaging algorithm, a cloud of estimated surface points representing the front side of the target. An interpolation of these points is necessary to obtain a surface model needed for the subsurface localization application. The interpolated surface is illustrated in Fig. 54(c) showing a good agreement with the original target object.

The performance of the proposed algorithm regarding its accuracy is compared to the well-established imaging algorithms "Seabed" [29] and "Envelope of Spheres" [16] by evaluating the surface measurement of a metal sphere with a known diameter of 35 cm. A

of "Seabed" is degrading significantly, while the errors obtained with trilateration and the

It can be seen that the trilateration-based imaging algorithm achieves similar results as state-of-the-art surface estimation algorithms while the compexity is reduced since here no preprocessing of measurement data is needed. A more detailed description of the trilateration-based imaging algorithm and further measurement results can be found in [26].

In the measurements for the evaluation of 3D transmitter localization, a liquid container whose concave surface roughly approximates the surface of a human body is chosen. Fig. 56 shows a top and side view of the measuring setup with the transmitter placed in a glass fish bowl of 21 cm diameter. The radar sensor is scanning in 1 cm steps in *x*- and *y*-direction

x

**Figure 56.** Photographs of the 3D measurement setup with a transmitter placed in a concave liquid

**Figure 57.** Measurement results in a top and side view corresponding to the photographs in Fig. 56 showing the estimated location of the beacon behind the container surface. The dots at *z* = 0 represent the

The surface points found by the trilateration-based imaging algorithm are interpolated to create the boundary shape depicted in Fig. 57. These graphs follow the perspectives of the


/ cm y

antenna array elements

18.5 cm

( =0) z

(b) Side view

20 15 10 5 0

estimated beacon position

z

(b) Side view

/ cm

15.5 cm

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

antenna phase center

z

y

483

"Envelope of Spheres" algorithm remain in the same range.

emulating an antenna array with 14×11 elements in total.

radar transmitter

receiver

beacon

(a) Top view

estimated beacon position

20 15 10 5 0 / cm

z

(a) Top view

estimated container surface

container.

sensor positions.

/ cm x

*4.4.3. Transmitter localization results*

**Figure 54.** Surface estimation result of the proposed trilateration-based imaging algorithm for a radar measurement of a human torso dummy using a circular antenna arrangement.

spherical target was chosen here since a mathematical surface of the torso dummy model is unavailable. The error distance between estimated points and the ideal surface of the target sphere is calculated and plotted in Fig. 55 for different densities of measurement points. The graphs show the percentage of estimated points having a certain deviation in cm from the ideal surface. In order to compare the results of the different algorithms at exactly the same coordinates, the estimated points have been interpolated on an identical coordinate grid.

**Figure 55.** Percentaged distribution of the error between estimated surface points and the actual surface of a trapezoidal target using a monostatic radar setup with two different distances *d* between measuring points. The performance of the proposed algorithm is compared to state-of-the-art imaging algorithms.

While in the first measurement in Fig. 55(a) a relatively small step width of *d* = 1.5 cm between two measuring positions has been used, the measurement results in Fig. 55(b) show the errors obtained with a quadrupled step width of 6 cm. It can be seen that with a high density of measurement points there is no significant difference of estimation errors between the three compared algorithms. The deviations from the ideal surface points are in a low millimeter range. However, with an increased distance between measurement points the performance of "Seabed" is degrading significantly, while the errors obtained with trilateration and the "Envelope of Spheres" algorithm remain in the same range.

It can be seen that the trilateration-based imaging algorithm achieves similar results as state-of-the-art surface estimation algorithms while the compexity is reduced since here no preprocessing of measurement data is needed. A more detailed description of the trilateration-based imaging algorithm and further measurement results can be found in [26].

### *4.4.3. Transmitter localization results*

44 Will-be-set-by-IN-TECH

/ cm y

z / cm

z / cm


x / cm

Error / cm 0.10 0.2 0.3 0.4 0.5 0.6

(b) Antenna distance *d* = 6 cm

15 (c) Interpolated surface

> Envelope Seabed Trilateration


x / cm

measurement of a human torso dummy using a circular antenna arrangement.

Envelope Seabed Trilateration

15 (b) Scattered surface points

**Figure 54.** Surface estimation result of the proposed trilateration-based imaging algorithm for a radar

spherical target was chosen here since a mathematical surface of the torso dummy model is unavailable. The error distance between estimated points and the ideal surface of the target sphere is calculated and plotted in Fig. 55 for different densities of measurement points. The graphs show the percentage of estimated points having a certain deviation in cm from the ideal surface. In order to compare the results of the different algorithms at exactly the same coordinates, the estimated points have been interpolated on an identical coordinate grid.

Percentage of total points

**Figure 55.** Percentaged distribution of the error between estimated surface points and the actual surface of a trapezoidal target using a monostatic radar setup with two different distances *d* between measuring points. The performance of the proposed algorithm is compared to state-of-the-art imaging algorithms. While in the first measurement in Fig. 55(a) a relatively small step width of *d* = 1.5 cm between two measuring positions has been used, the measurement results in Fig. 55(b) show the errors obtained with a quadrupled step width of 6 cm. It can be seen that with a high density of measurement points there is no significant difference of estimation errors between the three compared algorithms. The deviations from the ideal surface points are in a low millimeter range. However, with an increased distance between measurement points the performance

0.10 0.2 0.3 0.4 0.5 0.6

Error / cm (a) Antenna distance *d* = 1.5 cm

/ cm y

(a) Photograph

Percentage of total points

In the measurements for the evaluation of 3D transmitter localization, a liquid container whose concave surface roughly approximates the surface of a human body is chosen. Fig. 56 shows a top and side view of the measuring setup with the transmitter placed in a glass fish bowl of 21 cm diameter. The radar sensor is scanning in 1 cm steps in *x*- and *y*-direction emulating an antenna array with 14×11 elements in total.

**Figure 56.** Photographs of the 3D measurement setup with a transmitter placed in a concave liquid container.

**Figure 57.** Measurement results in a top and side view corresponding to the photographs in Fig. 56 showing the estimated location of the beacon behind the container surface. The dots at *z* = 0 represent the sensor positions.

The surface points found by the trilateration-based imaging algorithm are interpolated to create the boundary shape depicted in Fig. 57. These graphs follow the perspectives of the

photographs in Fig. 56 allowing a rough evaluation of the localization result. By applying the localization method of Sec. 3.4.2 based on the evaluation of 3D wavefronts inside of the medium, an estimated position of the transmitter of (*x*,*y*,*z*) = (3.0 cm,-18.4 cm,18.5 cm) is calculated. Even though an exact verification of the antenna position inside of the liquid is difficult, the manually measured *z*-distance of 18.5 cm between the transmitter and the receiver agrees with the estimated beacon position. In the photographs of Fig. 56 the receiver antenna is positioned at the *x*- and *y*-position closest to the beacon antenna in the container. These known sensor coordinates of (3 cm,-19 cm) also coincide well with the estimated beacon position. In case of a convex surface like the fish bowl an even better localization can be expected when using a circular or spherical arrangement of the receivers around the medium, instead of a planar arrangement.

Regarding UWB communications, a proposal for a transmission scheme has been discussed, using a special spread spectrum method and energy detection combined with a comb filter, which improves the SNR and rejects narrowband interference. The robustness of this concept has been demonstrated for multipath propagation channels as well as for narrowband interference, noise, and synchronization errors. The approach fits well to medical applications, because small multipath delay spreading promises an easier realization of the analog time delay needed for the comb filter. A trade-off between the number of UWB impulses per

485

UWB in Medicine – High Performance UWB Systems for Biomedical Diagnostics and Short Range Communications

In addition, a new concept for an impulse-radio transmission based on code shift keying with a comb filter receiver has been introduced. In this concept, the delay of the analog delay element could be shorter than the channel impulse response. It has been shown and verified by simulation that the performance and the resistance against multipath propagation, noise, narrow band and multisensor/multiuser interference are the same as in the original approach

Simulation results have shown that particle filtering can improve the ranging and tracking performance of an impulse UWB radar substantially in scenarios with low signal-to-noise ratio and cluttering in comparison with more conventional methods. A trade-off between realization complexity and performance can be adjusted thanks to the flexibility of the

It has been shown practically that a non-coherent energy detector is a suitable receiver concept for UWB communication with implants. The energy detector operates without synchronization and is insusceptible to dispersive effects of the channel. Demonstrational measurements in tissue-mimicking liquid have been performed with a data rate of 100 Mbit/s

A 3D surface estimation algorithm based on trilateration for ultra-wideband pulse radars has been presented and derived mathematically. Since this method needs no preprocessing of measurement data its implementation is very simple. In 3D surface measurements the performance of the proposed algorithm has been verified, and comparisons with established

As a next step towards the targeted application of catheter tracking, a method for the localization of UWB transmitters buried in homogeneous dielectric media has been presented. With the aid of surface estimation algorithms a localization behind an arbitrarily shaped medium boundary is possible. For this purpose we have proposed a system consisting of an array of UWB radar sensors outside the medium and a beacon inside the medium transmitting a short UWB pulse. The external sensors serve for surface scanning and for measuring the time of arrival of the transmitted signal. The performance of the proposed localization algorithm has been verified using electromagnetic field simulations and measurements, in

Dayang Lin, Michael Mirbach, Thanawat Thiasiriphet, Jürgen Lindner, Wolfgang Menzel and

algorithms have shown a similar performance regarding estimation errors.

which a transmitter has been placed in tissue-mimicking liquid.

symbol (bit) and the data rate requirement has to be made for different applications.

with longer delays in the comb filter loop.

meeting the requirements for modern medical devices.

proposed algorithm.

**Author details**

Hermann Schumacher *Ulm University, Germany*

Due to the high attenuation of signals transmitted through tissue-mimicking liquid we can only localize positions close to the container surface. In our measurements the signal-to-noise ratio of impulses running through more than 3 cm of tissue-mimicking liquid became too low to be detected. One possible way to increase the maximum transmitter distance from the surface would be the use of a cascade of multiple amplifiers to realize a higher transmitter power output. Regarding the optimization of signal processing an approach based on compressed sensing is investigated to cope with lower signal-to-noise ratios at the receiver [34].
