FlowForm: Scalable Passive Metasurface Network for mmWave Coverage Expansion
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.