  In this white paper, we lay out a US roadmap for high-redshift 21 cm
cosmology (30 < z < 6) in the 2020s. Beginning with the currently-funded HERA
and MWA Phase II projects and advancing through the decade with a coordinated
program of small-scale instrumentation, software, and analysis projects
targeting technology development, this roadmap incorporates our current best
understanding of the systematics confronting 21 cm cosmology into a plan for
overcoming them, enabling next-generation, mid-scale 21 cm arrays to be
proposed late in the decade. Submitted for consideration by the Astro2020
Decadal Survey Program Panel for Radio, Millimeter, and Submillimeter
Observations from the Ground as a Medium-Sized Project.
