Radio-Frequency Front-End-Free Hybrid Code-Division and Space-Division Wireless 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 wireless CDM system based on a space-time-coding metasurface (STCM). By modulating distinct spatial partitions of the metasurface with data streams prespread 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: multistream to a single direction, distinct single-stream to distinct directions, and multistream to multiple directions (each direction gets multiple streams), all achieved without conventional RF chains. The proposed STCM-based hybrid CDM and space-division multiplexing (SDM) architecture enables simultaneous multiuser communication with a radically simplified RF front-end, paving the way for simplified, high-capacity wireless systems for future wireless networks.