  This white paper addresses key challenges for the design of next-decade
Cosmic Microwave Background (CMB) experiments, and for assessing their
capability to extract cosmological information from CMB polarization. We focus
here on the challenges posed by foreground emission, CMB lensing, and
instrumental systematics to detect the signal that arises from gravitational
waves sourced by inflation and parameterized by $r$, at the level of $r \sim
10^{-3}$ or lower, as proposed for future observational efforts. We argue that
more accurate and robust analysis and simulation tools are required for these
experiments to realize their promise. We are optimistic that the capability to
simulate the joint impact of foregrounds, CMB lensing, and systematics can be
developed to the level necessary to support the design of a space mission at $r
\sim 10^{-4}$ in a few years. We make the case here for supporting such work.
Although ground-based efforts present additional challenges (e.g., atmosphere,
ground pickup), which are not addressed here, they would also benefit from
these improved simulation capabilities.
