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Pseudo-Random Sequence (PRS) Space-time-modulated Metasurfaces: General Concept and Fundamental Operations

This paper presents the general concept and fundamental operations of a new class of space-time-modulated metasurfaces, viz., space-time metasurfaces modulated by PseudoRandom Sequence (PRS) waveforms. The metasurfaces may be either purely PRS time-modulated or space-time-modulated by a set of orthogonal PRSs. The PRSs are quasi-random periodic sequences with binary values (±1) and exhibit noise-like wideband spectra. The metasurfaces are assumed to operate in the slow time modulation regime, where the time-variance and frequency-dispersion effects are safely decoupled. In this regime, the bit duration of the PRS is much larger than the transient time associated with temporal dispersion, i.e., than the time for the metasurface to reach the steady regimes in each modulation state. PRS space-time-modulated metasurfaces provide many functionalities that complement those of their deterministic-modulation counterparts. In contrast to these metasurfaces, they massively transform the temporal spectrum of the incident waves, hence providing novel encoding and decoding capabilities. The resulting metasurfaces can perform fundamental operations such as spectrum spreading, interference suppression, and row/cell selection, which are described in detail and which lead to applications such as electromagnetic stealth, secure communication, direction of arrival estimation, and spatial multiplexing.