  Mobile operators are poised to leverage millimeter wave technology as 5G
evolves, but despite efforts to bolster their reliability indoors and outdoors,
mmWave links remain vulnerable to blockage by walls, people, and obstacles.
Further, there is significant interest in bringing outdoor mmWave coverage
indoors, which for similar reasons remains challenging today. This paper
presents the design, hardware implementation, and experimental evaluation of
mmWall, the first electronically almost-360 degree steerable metamaterial
surface that operates above 24 GHz and both refracts or reflects incoming
mmWave transmissions. Our metamaterial design consists of arrays of
varactor-split ring resonator unit cells, miniaturized for mmWave. Custom
control circuitry drives each resonator, overcoming coupling challenges that
arise at scale. Leveraging beam steering algorithms, we integrate mmWall into
the link layer discovery protocols of common mmWave networks. We have
fabricated a 10 cm by 20 cm mmWall prototype consisting of a 28 by 76 unit cell
array, and evaluate in indoor, outdoor-to-indoor, and multi-beam scenarios.
Indoors, mmWall guarantees 91% of locations outage-free under 128-QAM mmWave
data rates and boosts SNR by up to 15 dB. Outdoors, mmWall reduces the
probability of complete link failure by a ratio of up to 40% under 0-80% path
blockage and boosts SNR by up to 30 dB.
