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— Presentation at ISAP 2016 —
December 5, 2016
The International Symposium on Antenna and Propagation (ISAP 2016) was held in Okinawa, Japan, from October 24 to 28, 2016. This conference focused on the progress of research and development in antennas, propagation, electromagnetic-wave theory, and related fields. At ISAP 2016, there were 638 presentations and a paper entitled "Direction-of-Arrival Estimation with Lüneburg Lens and Metamaterial Absorber" was presented.
Fig. 1 Concept of DOA estimation
Localization of electromagnetic wave sources from electronic and industrial equipment is an important issue in electromagnetic comparability (EMC) field. One of the popular technique for a small electronic device is near-field scanning. However, for a large object, it is too difficult to scan the whole space of interest because of the longer measurement times required. Direction-of-arrival (DOA) estimation is a good choice for a large object and many techniques have been developed. Most of these techniques are based on antenna arrays with a sophisticated algorithm like multiple signal classification (MUSIC) or estimation of signal parameters via rotational invariance technique (ESPRIT). To achieve high resolution in the incident angle, the complicated signal processing algorithms and high performance hardware are required.
A Lüneburg lens with photonic sensors or power detectors is another DOA technique without any sophisticated algorithms. The lens can separate the multiple incident waves with different DOAs. A sensor array is used to detect the incident wave which was concentrated at the focal point. The proposed technique is mainly composed of a Lüneburg lens and a frequency tunable metamaterial absorber (Fig. 1). The absorber is employed as a compact and high-density 2D sensor array whose elements are located at the focal points near the lens surface for the incident waves with different DOAs.
In addition, a correlation algorithm was used for increasing the accuracy of DOA estimation. The absorber obtains a diffraction image of the lens. The spot size of the focal point due to the diffraction limit was determined by the diameter of the lens and the frequency of the incident wave. The correlation coefficients between the measured distribution and theoretical images were computed. The estimated direction of the incident wave is the angle which has the largest correlation coefficient. Fig. 2 shows the theoretical diffraction image and the measured distribution at an incident angle of 10 degrees at 2.5 GHz.
The incident angle was estimated by the correlation algorithm. The estimation error observed for various angles is shown in Fig. 3. It shows that it is possible to estimate DOAs by this technique from 2 to 3 GHz, with the estimation error less than ± 2.0 degrees. The accuracy was possibly limited by the gain variation among the sensor elements and reflection at the edge of the absorber, which could be improved by calibration.