Pseudorandom Noise Sequence Time-Modulated Reflective Metasurfaces for Target Recognition

In this article, pseudorandom noise (PN) sequence time-modulated reflective metasurfaces are proposed to provide a low-cost, low-complexity, simple solution for target recognition. By covering the target to be recognized with a time-modulated metasurface modulated by a PN time sequence, the metasurface scatters the interrogating harmonic wave into a binary PN-modulated wave. As a result, the radar detector with the foreknowledge of the modulating PN sequence may recognize the target by applying the properties of strong autocorrelation and weak cross correlation of PN sequences and performing cross correlation between the modulating PN sequence and the picked-up signal after downshifting its frequency without actually decoding and reading the scattered information. A 1-bit reconfigurable metasurface unit cell operating at 2.4 GHz is proposed and full-wave simulated to achieve out-of-phase reflective responses by controlling the ON and OFF states of the p-i-n diode for PN time modulation. The target recognition is demonstrated by experiment in two different scenarios where only one metasurface and multiple metasurfaces are present for recognition. The experimental results show that by providing an appropriate threshold in the cross-correlation detecting results, the proposed method can recognize the targets well in both scenarios.