Nanosecond-level time-domain coding metasurface for radar signal generation

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.