Programmable metasurfaces (PMSs) exhibit great potentials in target detection techniques, because they can take actions to change channel propagation characteristics which introduces further degrees of freedom for system optimizations. However, responses of most traditional PMSs are sensitive to incident angles of impinging electromagnetic waves, resulting in a failure of angular estimation to dynamic targets coming from different directions. Herein, by proposing a fully resonant structure and introducing a mode-alignment technology, we report an isotropic angle-insensitive PMS whose phase response is stable with respect to different incident angles in both elevation- and azimuth-planes. A radar scheme that uses such a PMS as a periscope is also demonstrated, to detect drones in a non-line-of-sight (N-LOS) scenario which usually happens in an urban environment. Our proposed scheme enables those targets even falling in shadow areas caused by high buildings to be successfully detected and tracked, which shows promising potentials in N-LOS target detections.

The time-domain coding metasurface (TDCM) offers a rapid and efficient approach for manipulating frequency spectra of electromagnetic waves. To date, not only finite-order harmonics can be generated and coded discretely, frequency modulation of continuous waves has also been investigated. However, limited phase-tuning speed still constrains the modulation bandwidth and practical applications. Here, we report a TDCM capable of nanosecond-level phase tuning as fast as 20 ns within a full 360° tuning cycle. Unlike conventional varactor-based approaches, the proposed TDCM adopts a reconfigurable PIN-diode array, reducing transition time between adjacent states to only several nanoseconds. Furthermore, this approach can be extended to Ku- and even millimeter-wave bands, overcoming the frequency constraint of varactors. To validate its effectiveness, we built a C-band frequency-modulated continuous-wave radar prototype with the metasurface as the signal generator. A high-quality 10-MHz-bandwidth FMCW signal was generated, enabling accurate measurement of a flying drone’s range and speed.

The time-domain coding metasurface (TDCM) offers a rapid and efficient approach to manipulate frequency spectra of electromagnetic waves within their transmission channels. To date, not only finite-order harmonics can be generated and coded in a discrete manner, the method of frequency modulation for continuous temporal waves is also investigated. However, due to the achievable speed of phase tuning, the modulation bandwidth and applicable scope of the current methods are still limited with respect to the requirement of practical systems. Here, we provide theoretical evaluations of the quality of generated frequency-modulated continuous wave (FMCW) radar signals with respect to the phase tuning speed, as well as a time-coding metasurface that can achieve a phase tuning speed smaller than 20 ns to support the generation of high quality FMCW signals with a bandwidth larger than 10 MHz. Different from varactors used in conventional approaches, the phase tuning of the proposed TDCM is based on the reconfiguration of a PIN-diode array. With this measure, the phase transition time between adjacent phase states can be reduced to a nano-second level, which is crucial for ultrahigh-speed coding. Furthermore, this design approach is also convenient to be rolled out to X- and even higher frequency bands, which overcomes the frequency spectra constraint when varactors are used. To validate its effectiveness, we have built a C-band radar prototype in which the metasurface operates as a transmitter to measure the range and speed of a flying drone.

With the increasing use of digital array radars, radar signal processing systems are faced with the challenge of real time processing for a mass of data. A kind of field programmable gate array (FPGA) realization of signal processing is proposed for two-dimensional phased array digital multi-beam radar in this paper. This scheme can realize the real-time signal processing of 6 beams at the same time. Each beam processes data from 8400 range gates. The details of implementation including digital multi-beamforming (DBF), pulse compression and moving target detection (MTD) are described. The results show that the design is correct and feasible.

This paper illustrates the improved signal processing system architecture and processing flow for a two-dimensional (2- D) phased array digital multi-beam radar. 2D phased array digital multi-beam radar is required to achieve electronically scanning in both azimuth and elevation, which demands higher processing performance of the system. To improve the real-time performance of the system, we modify the signal processing system architecture. In this paper, we will focus on DSP to demonstrate the utility of the improved system architecture and processing flow. And a new detection strategy which combines dual-threshold variable index constant false alarm (DT-VI-CFAR) and 2-D Cell Average CFAR (CACFAR) is proposed. Practical validation shows that this detection strategy significantly reduces the complexity compared to traditional 2-D CA-CFAR, while improving the detection probability compared to traditional DT-VI-CFAR.