  CMB-S4 will map the cosmic microwave background to unprecedented precision,
while simultaneously surveying the millimeter-wave time-domain sky, in order to
advance our understanding of cosmology and the universe. CMB-S4 will observe
from two sites, the South Pole and the Atacama Desert of Chile. A combination
of small- and large-aperture telescopes with hundreds of thousands of
polarization-sensitive detectors will observe in several frequency bands from
20-300 GHz, surveying more than 50 percent of the sky to arcminute resolution
with unprecedented sensitivity. CMB-S4 seeks to make a dramatic leap in
sensitivity while observing across a broad range of largely unprotected
spectrum which is increasingly being utilized for terrestrial and satellite
transmissions. Fundamental aspects of CMB instrument technology leave them
vulnerable to radio frequency interference (RFI) across a wide range of
frequencies, including frequencies outside of their observing bands.
Ground-based CMB instruments achieve their extraordinary sensitivities by
deploying large focal planes of superconducting bolometers to extremely dry,
high-altitude sites, with large fractional bandwidths, wide fields of view, and
years of integration time. Suitable observing sites have historically offered
significant protection from RFI, both naturally through their extremely remote
locations as well as through restrictions on local emissions. Since the
coupling mechanisms are complex, safe levels or frequencies of emission that
would not interfere with CMB measurements cannot always be determined through
straightforward calculations. We discuss models of interference for various
types of RFI relevant to CMB-S4, mitigation strategies, and the potential
impacts on survey sensitivity.
