**2.2. ultraMEDIS sensor electronics**

260 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

standard PCs and interfaces quickly reach their capacity limits.

are contradictory and cannot be met by a physically realizable antenna.

small [9].

*Measurement rate, channel number, data handling*: Except for impedance spectroscopy, the applications mentioned above require MIMO-sensor arrays which have to run at a reasonable update rate. On the one hand, this assumes cascadable sensors in order to build multi-channel systems, and on the other it poses some challenges for the data handling resulting from the large number of measurement channels and the high measurement rate. Chapter 14 (section 2.1) adverts to some measures which avoid redundant and inefficient data. Irrespective of these measures, the data throughput will be still quite high so that

*Radiators*: The radiators represent the interface between sensor electronics and test object for applications b) – d). They have to convert guided signals into free waves and vice versa. As they are linear and time-invariant devices, they may be operated with any type of signals. Certainly, their major features are the bandwidth and the beam width which should be as large as possible if they are applied for UWB imaging. However, these characteristics describe their performance only insufficiently particularly for UWB short-range applications. Ideal UWB antennas for our purposes should provide a short and angular independent impulse response (time shape and wave front), they should convert the incoming signal completely into a free wave, and the incident fields should be converted into voltage signals avoiding any re-radiation or scattering by the antenna. These conditions

A short impulse response is needed for high range and image resolution as well as the ability to recover weak targets closely behind surfaces. Otherwise, we risk the loss of the target since a slowly decaying surface reflex distorts the target response. If that signal is too abundant, even sophisticated background removal strategies will not be able to dig it out.

The angular independent impulse response is important for the imaging algorithm. For every image pixel or voxel, it has to coherently integrate signals which are captured from different positions. In order to ensure the coherence of this integration, the propagation time to the considered pixel (voxel) must exactly be known. This involves the knowledge of the propagation speed as well as the knowledge of the deviation from a spherical wave front created by the antennas. In order to achieve a simple and manageable imaging algorithms, the involved antennas should avoid such distortions, hence they should be (electrically)

However, this contradicts the physical conditions for an efficient conversion between guided signals and freely propagating waves (see Bode-Fano limit and Chu-Wheeler limit [10]). Additionally, efficient antennas backscatter (re-radiate) half of the incident power in the ideal case. For targets in close proximity of the antennas, this leads to multiple reflections which are hardly to remove by signal processing. As we saw for the impulse behavior, the inefficient antennas behave again best regarding their re-radiations (structural antenna reflections are omitted here for shortness). Hence, one has to find a reasonable compromise between efficiency and impulse as well as scattering performance. Antenna efficiency is an important issue in connection with noise suppression and high path losses. For imaging at very close distances, noise induced measurement errors are falling below In view of the previous discussion, we mostly abstain from the use of network analyzers since they will not meet the requirements of future developments of the sensing technology even if they best fulfill the demands with respect to sensitivity, bandwidth and reliability of measurement data. A new sensor concept with comparable performance but higher measurement speed, better MIMO capability and integration friendly device layout exploits ultra-wideband pseudo-noise sequences (namely M-sequences) for the target stimulation instead of the sine waves of a network analyzer. This measurement approach was favored for our investigations. Device concepts applying sub-nanosecond pulses were rejected due to their inherent weakness concerning noise and jitter robustness. The interested reader is referred to Chapter 14 and [9] for further discussions of the pros and cons of various sensor principles.

The block schematics of the M-sequence prototype devices applied by ultraMEDIS are depicted in Figs. 4 and 6 in Chapter 14. The integrated RF key components were provided by the project HaloS while the implementation of prototype devices was performed by MEODAT GmbH and later on by ILMSENS. A special issue of an M-sequence device provides 12 GHz bandwidth. Its implementation is based on [8].

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ultraMEDIS – Ultra-Wideband Sensing in Medicine 263

**Figure 3.** 4Tx-8 Rx MIMO device for organ motion tracking in MRI tomographs. (operational band 17 MHz – 4.5 GHz; 9th order M-sequence; maximum up data rate 530.4 Hz, Ethernet data link, data

**Figure 4.** 8 Tx-16 Rx MIMO radar for microwave breast imaging (operational band 20 MHz – 6 GHz; 9th order M-sequence, USB2 interface). M-sequence units (as shown in Fig. 6 of Chapter 14) and RF-front

The exploitation of UWB microwave technologies for biomedical diagnostics requires the development of antennas and sensors tailored to this application. The integration of the antennas as a part of a complex system leads to serious compatibility and functionality constraints, which must be properly addressed for high system performance. Within ultraMEDIS, two goals were pursued: Firstly, the detection of early stage breast cancer and secondly, the development of a magnetic resonance imaging (MRI) compatible navigator

ends are separated to get more flexibility for experimental purposes.

**2.3. Antennas and sensor elements** 

*2.3.1. Introduction* 

acquisition on Linux PC)

**Figure 1.** M-sequence based impedance spectroscopy (bandwidth 17 MHz – 4 GHz; 9th order Msequence). Left: Device implementation with external coupler. Right: M-sequence device with internal coupler and rigid probe connection to improve measurement reliability.

Figure 4 of Chapter 14 (HaLoS-project) relates to the basic structure which can be found in all device modifications. Such device configurations were applied in an early project state for microwave imaging and organ motion tracking experiments. Involving a directional coupler, it is further used for impedance spectroscopy as exemplified in Fig. 1. Multichannel systems and MIMO-arrays are based on the device philosophy as illustrated in Fig. 6 of Chapter 14. Implemented examples are depicted in Fig. 2 to Fig. 4.

**Figure 2.** M-sequence two-port network analyzer (operational band 40 MHz – 8 GHz, 9th order Msequence, USB2 interface). It can be extended by an RF-switch matrix for MIMO-radar imaging.

**Figure 3.** 4Tx-8 Rx MIMO device for organ motion tracking in MRI tomographs. (operational band 17 MHz – 4.5 GHz; 9th order M-sequence; maximum up data rate 530.4 Hz, Ethernet data link, data acquisition on Linux PC)

**Figure 4.** 8 Tx-16 Rx MIMO radar for microwave breast imaging (operational band 20 MHz – 6 GHz; 9th order M-sequence, USB2 interface). M-sequence units (as shown in Fig. 6 of Chapter 14) and RF-front ends are separated to get more flexibility for experimental purposes.
