We present MetaPolar, a fully passive mmWave metasurface tag that enables robust Infrastructure-to-Vehicle (I2V) communication using standard automotive radars. Unlike conventional backscatter or spatial-encoding approaches, MetaPolar exploits the polarization domain: engineered split-ring resonators rotate the incident polarization to generate strong cross-polarized returns, while a crossed-dipole layer independently sculpts co- and cross-polarized amplitudes and phases. Through full-wave simulation, we build a codebook mapping metasurface layouts to distinct 2 × 2 polarimetric RCS signatures. A novel semi-retroreflective beam-shaping design maintains strong polarimetric contrast across a ± 7.5° field of view, ensuring reliable decoding under realistic vehicle motion. We implement MetaPolar on low-cost FR-4 PCBs (< $3 per tag) and validate it with a custom mmWave radar platform. Experiments demonstrate 2-bit encoding per tag (scalable to 2N bits with N tags), simultaneous multi-tag reading, and seamless integration into existing radar pipelines. Our results highlight MetaPolar’s potential for cost-effective, scalable radar-readable signage in next-generation intelligent transportation systems.

Millimeter wave (mmWave) networks offer multi-gigabit data rates but suffer from severe path loss and blockage, resulting in spotty coverage. Emerging reconfigurable intelligent surfaces (RIS) can mitigate these challenges, but their reliance on active control channels, power sources, and complex runtime coordination imposes significant hardware and deployment overhead. This paper introduces FlowForm, a system that expands mmWave coverage using networks of passive metasurfaces that require no power, control, or runtime coordination. FlowForm's key innovation is a hierarchical flow topology that organizes passive metasurfaces into major flows (directional relay chains using near-field focusing) and minor flows (wide-area fan beams), enabling multi-hop passive routing and over-the-air combination of analog signals. We develop a theoretical framework establishing the optimality of this topology and a hierarchical optimization algorithm that jointly determines metasurface placement and beam configurations. FlowForm operates transparently with standard mmWave network protocols, managing channel dynamics and multi-user interference through diversity-aware design rather than runtime reconfiguration. Our experimental evaluation across five indoor environments demonstrates up to 94% average rate improvement and 114% coverage expansion using low-cost 3D-printed metasurfaces ($2 per unit), achieving performance comparable to active RIS at orders of magnitude lower cost.

Millimeter-wave (mmWave) radar has become instrumental in diverse consumer applications. Yet current radar architectures face major limitations. While full-MIMO structures are feature-rich, their cost and complexity rise rapidly with more antennas. Phased-MIMO radars promise enhanced scalability by combining large phased arrays with a small number of RF chains. Nevertheless, the phased-MIMO research thus far primarily relies on simulation or theoretical analysis. In this paper, we introduce HybRadar, a novel programmable phased-MIMO radar platform to address this experimental gap. HybRadar repurposes the phased arrays on a low-cost 802.11ad radio to create a scalable low-cost array of phased subarrays. It further incorporates transmit/receive front-end, control channel, and hardware synchronization mechanisms to enable a modular phased-MIMO system. By extending recent MIMO array synthesis models, we optimize the placement of phased subarrays to maximize the spatial resolution. Our prototype validation and case studies confirm the capability and versatility of HybRadar.