Millimeter wave (mmWave) promises multi-gigabit wireless data rates, but the physics works against reliable indoor coverage. mmWave signals suffer from severe path loss and vulnerability to even minor obstructions. A single step sideways or a person walking across the room can kill the connection entirely.

Conventional fixes like denser access points, active relays, or reconfigurable intelligent surfaces (RIS) all require power supplies, control channels, and coordination protocols, with costs that scale poorly. A single active mmWave RIS device can easily exceed thousands of dollars.

A UC San Diego team led by PhD student Wuqiong Zhao in Prof. Xinyu Zhang's group at the Center for Wireless Communications is taking a radically different approach. Their system, FlowForm, replaces active infrastructure with coordinated networks of entirely passive metasurfaces–3D-printed reflective tiles that cost roughly $2 each and require no power, no control, and no runtime coordination. The key insight is that organizing many cheap passive surfaces into a coordinated network can rival the performance of active systems.

FlowForm introduces a hierarchical "major–minor flow" topology inspired by how tributaries feed into rivers. Major flows are focused relay chains that shuttle signals across long distances and around obstacles, forming the communication backbone. Minor flows are wide fan beams that branch off to blanket user areas with coverage from multiple angles. The team proves mathematically that this two-tier structure emerges naturally from the physics of passive relaying, and is near-optimal for maximizing both coverage and robustness.

Once fabricated and mounted on walls or ceilings, the metasurfaces behave like natural reflectors to standard mmWave radios. No firmware changes, no modifications to standard network protocols, no awareness of the metasurface network required.

The team validated FlowForm across five real indoor environments using a mmWave software-defined radio testbed they built. FlowForm nearly doubles average link rates and more than doubles coverage area in challenging environments, matching idealized active RIS performance at a fraction of the cost.

The work will be presented at ACM SIGCOMM '26, August 17–21 in Denver, CO. Learn more at the project page: flowform.wqzhao.org.