Code-Division Multiplexing Using Space-Time-Coding Metasurface

Code-division multiplexing (CDM) is essential for supporting massive connectivity in wireless networks, but its implementation typically requires complex and costly radio-frequency (RF) chains. This work presents a simplified, low-cost wireless CDM system based on a space-time-coding metasurface (STCM). By modulating distinct spatial partitions of the metasurface with data streams pre-spread via orthogonal pseudo-random codes, the STCM functions as a distributed parallel modulator. This architecture enables the metasurface to perform simultaneous upconversion while directing modulated reflections from multiple spatial partitions toward either shared or distinct directions. Although these data-carrying waves may physically overlap in the far-field, the inherent orthogonality of the spread codes ensures that the receiver can successfully decorrelate and recover each multiplexed stream. Experimental validation demonstrates three core scenarios: multi-stream transmission to a single direction, single-stream to multiple directions, and multi-stream to multiple directions, all achieved without conventional RF chains. The proposed STCM-based CDM architecture enables simultaneous multi-user communication with a radically simplified RF front-end, paving the way for energy- and cost-efficient high-capacity wireless systems for future wireless networks.