**7. Short range super-resolution UWB-radar sensing**

In recent years short range UWB radar sensing and imaging has gained steadily increasing interest in research. The demand for a wide absolute bandwidth results from the smallest dimensions to be resolved. However, the request for increased resolution capabilities strove for innovative algorithms, new hardware equipment, and for performance which is not restricted by the bandwidth defined by the hardware. In this chapter novel and pioneering methods, algorithms and antennas are presented which were investigated within CoLOR for UWB radar applications especially in short range UWB radar applications.

## **7.1. Antenna design and measurement results**

As the use of polarization diversity allows further information about the object characteristics to be obtained, for instant about the surface structure as it is shown in this section, the antenna has to be orthogonally polarized. Apart from orthogonal polarizations, further conditions for the (object recognition) antenna design are a high gain and a common phase center (for both polarizations).

A common antenna for such an application is the so-called Vivaldi antenna. The radiation mechanism is based on exponentially tapered slots and the traveling wave principle [3]. This type of antenna has a convenient time domain behavior as shown in [59] and a relatively stable radiation pattern in the whole frequency range. In CoLOR the frequency range for the final demonstrator covers 3.5 to 10.5 GHz. A second band from 4.5 to 13.5 GHz was also used during the development process. Therefore, the objective of the antenna design is to cover the hole frequency range from 3.5 GHz to 13.5 GHz.

A 3D illustration (left) and a photograph (right) of the fabricated antenna is shown in Fig. 31, see also [45]. Combining the integration of two tapered slot line antennas on a single substrate with embedding them into Polytetrafluoroethylene (PTFE) allows the total bandwidth to be covered. The integration of two radiation elements per polarization yields a higher gain and saves space compared with an array of two separated antennas. Furthermore, it is less cost intensive and easier to manufacture. The possibility of varying the tapering of the inner and outer structure, see Fig. 32, can be used to focus the beam.

32 Will-be-set-by-IN-TECH

As this type of antenna is radiating broad-side in both directions (forward and backward) and should be used as (mono-) directional radiating antenna, the backward radiation (illuminating towards the feeding network) must be absorbed by a carbon fiber housing as shown in Fig. 29. This results in a reduction of the radiation efficiency. An alternative would be to use a reflector,

The antenna characteristics are measured with a vector network analyzer in an anechoic chamber. The input impedance matching is around -10 dB between 3.5 and 10.5 GHz and

Figure 30 shows the 2D gain over frequency and angle for both planes (E- and H-plane) of one polarization in co-polarization arrangement. The measured gain of the second polarization is

**Figure 30.** Measured gain over frequency and angle in the E-plane (left) and H-plane (right).

**7. Short range super-resolution UWB-radar sensing**

**7.1. Antenna design and measurement results**

UWB radar applications especially in short range UWB radar applications.

This solution of a planar, dual-polarized UWB antenna covers the frequency range between 3.5 and 10.5 GHz. In this frequency range, the radiation pattern of the antenna remains stable and directive. Both polarizations have the same radiation phase center, which is

In recent years short range UWB radar sensing and imaging has gained steadily increasing interest in research. The demand for a wide absolute bandwidth results from the smallest dimensions to be resolved. However, the request for increased resolution capabilities strove for innovative algorithms, new hardware equipment, and for performance which is not restricted by the bandwidth defined by the hardware. In this chapter novel and pioneering methods, algorithms and antennas are presented which were investigated within CoLOR for

As the use of polarization diversity allows further information about the object characteristics to be obtained, for instant about the surface structure as it is shown in this section, the antenna has to be orthogonally polarized. Apart from orthogonal polarizations, further conditions for the (object recognition) antenna design are a high gain and a common phase center (for both

which, however, would limit the bandwidth of the antenna.

the decoupling of the two ports is approximately 20 dB.

very similar and not specifically shown.

frequency-independent.

polarizations).

Both radiating structures are fed by a network shown in Fig. 32 (left), and for the second polarization in Fig. 32 (right), respectively. Starting with microstrip, where the connector is soldered on, an aperture coupling transforms to slotline which finally divides the power and feeds it to the two elements.

As already mentioned, the integration of the antenna elements into a dielectric reduces the effective wavelength. This affects several advantages compared to an antenna in free space. The antenna is capable of radiating a lower frequency, the far-field conditions are fulfilled in a closer distance and the shaping of the dielectric can (also) be used to focus the beam and for sidelobe suppression. For this work PTFE is chosen as dielectric. Its permittivity of *<sup>r</sup>* = 2.1 is similar to that of the substrate used (Rogers Duroid 5880 with *<sup>r</sup>* = 2.2). Furthermore, PTFE has low losses and can be easily shaped to adapt to the antenna design. The shaping and the dimensions of the PTFE structure are given in [45]. The conically shaped rod (in radiation direction) allows a smoother transition of the guided wave into free space.

The two polarizations are realized by shifting two orthogonal elements into each other as shown in Fig. 31 left. For doing this, a slot has to be cut into both elements. Thus, the metalized structures are interrupted, see Fig. 32. They have to be galvanically connected again. This is realized by introducing vias in the orthogonal antenna.

34 Will-be-set-by-IN-TECH 212 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications Cooperative Localization and Object Recognition in Autonomous UWB Sensor Networks <sup>35</sup>

**Figure 32.** Schematic illustration of the antenna elements 1 (left) and 2 (right), in [mm].

The antennas are manufactured with aid of a circuit board plotter on a Duroid RT5880 substrate of a thickness of 0.79 mm. The measured S-parameter, see Fig. 33 left, show a good impedance matching for both polarizations and antenna elements, respectively, between 3.5 GHz and 13.5 GHz (and even higher). The decoupling (*S*21, *S*12) between the two elements is over the biggest portion of the bandwidth better than -25 dB.

**Figure 34.** Measured gain [dBi] and pattern of antenna element 2 in the E-plane (left) and H-plane

deconvolution is to apply a simplified Wiener filter with the transfer function

*Href*(*f*)

*Hwiener*(*f*) = <sup>1</sup>

−0.05

**Figure 35.** Example of deconvoluted pulses normalized to the same power

Amplitude [V]

**7.3. Material characterization**

0

0.05

0.1

temporal resolution. The channel impulse response of the radar link can be extracted by deconvoluting with a reference pulse, as we assume the link as an LTI-system. However, it is well known that classical deconvolution by spectral division may drastically distort the result especially at low SNR values. A highly efficient method with low complexity to perform the


where *Href*(*f*) is the Fourier transform of a previously measured offline reference pulse. Hence, the estimate of the deconvoluted channel impulse response *hdeconv*(*t*) is then obtained as *hdeconv*(*t*) = *hwiener*(*t*) ∗ *hmeasured*(*t*) , with *hmeasured*(*t*) being the measured impulse response under test. Depending on the power level, the Wiener filter either acts as an inverse or matched filter for the deconvolution. In Fig. 35 an example of the channel impulse extraction is shown. Note that both pulses are normalized to the same power to enable visual comparison

<sup>0</sup> 0.5 <sup>1</sup> 1.5 <sup>2</sup> −0.1

A material characterization in hostile and pathless scenarios requires a remote measurement. Hence, a method known from optics, the ellipsometry has been adapted to the UWB

Time [ns]

2 <sup>|</sup>*Href*(*f*)|<sup>2</sup> <sup>+</sup> <sup>1</sup> <sup>=</sup> *Href*(*f*)<sup>∗</sup>

Cooperative Localization and Object Recognition in Autonomous UWB Sensor Networks 213

Pulse under test Wiener deconvoluted pulse under test

<sup>|</sup>*Href*(*f*)|<sup>2</sup> <sup>+</sup> <sup>1</sup> (15)

(right) .

in the plot.

**Figure 33.** Measured S-parameter (left) and antenna gain in the main beam direction (right).

The gain and the pattern of the antenna were measured in an anechoic chamber. The results for antenna element 2 are presented in Fig. 34. The E-plane corresponds in this case to the azimuth direction, see Fig. 34 left, the H-plane to the elevation one. Antenna element 1 shows a similar characteristic. The maximum gain measured is around 15 dBi at 9 GHz.

To evaluate the polarization properties, the gain for both co-polarizations (Co-Pol) and both cross-polarizations (X-Pol) in the main beam direction was measured, see Fig. 33 right. The difference (between Co-Pol and X-Pol regarding the antenna gain) provides the information about the polarization purity. The cross-polarization suppression is better than 20 dB at the low frequencies up to 10.5 GHz. Starting from 10.5 GHz, the values of the X-Pol of antenna 2 are increasing (deteriorating). This is due to the current distribution of higher modes which cannot be avoided for higher frequencies. Nevertheless the measured performance allows a successful use in polarization diversity systems even above 10.5 GHz, see [45].
