Multi-Dimensional Physical-Layer Security via Digitally Mediated Space-Time Modulation.

Modern sensing and communication systems increasingly operate in congested electromagnetic environments, where suppressing the observability of radiated signals at unintended receivers is essential for secure operation. Space-time modulation offers a powerful means of shaping radiation for secure transmission by exploiting temporal degrees of freedom, but its achievable control is often limited by hardware-bound modulation dimensions, with further expansion incurring substantial system complexity. Here, we propose a new digitally mediated space-time modulation paradigm that significantly expands multi-dimensional radiation-control capability with minimal hardware overhead. By coherently coupling digital baseband processing with analog time modulation, the proposed architecture enables simultaneous multi-beam transmission of distinct waveforms while effectively reducing waveform observability in unintended spatial regions. Numerical simulations and experimental measurements of a multi-target prototype verify that complex radiation behaviours and intrinsic physical-layer security can be achieved using simple periodic modulation, providing a versatile strategy for radiation-domain control in next-generation sensing and communication systems.