  The CMB Stage 4 (CMB-S4) experiment is a next-generation, ground-based
experiment that will measure the cosmic microwave background (CMB) polarization
to unprecedented accuracy, probing the signature of inflation, the nature of
cosmic neutrinos, relativistic thermal relics in the early universe, and the
evolution of the universe. To advance the progress towards designing the
instrument for CMB-S4, we have established a framework to optimize the
instrumental configuration to maximize its scientific output. In this paper, we
report our first results from this framework, using simplified instrumental and
cost models. We have primarily studied two classes of instrumental
configurations: arrays of large aperture telescopes with diameters ranging from
2-10 m, and hybrid arrays that combine small-aperture telescopes (0.5 m
diameter) with large-aperture telescopes. We explore performance as a function
of the telescope aperture size, the distribution of the detectors into
different microwave frequencies, the survey strategy and survey area, the
low-frequency noise performance, and the balance between small and large
aperture telescopes for the hybrid configurations. We also examine the impact
from the uncertainties of the instrumental model. There are several areas that
deserve further improvement. In our forecasting framework, we adopt a simple
two-component foregrounds model with spacially varying power-law spectral
indices. We estimate delensing performance statistically and ignore possible
non-idealities. Instrumental systematics, which is not accounted for in our
study, may influence the design. Further study of the instrumental and cost
models will be one of the main areas of study by the whole CMB-S4 community. We
hope that our framework will be useful for estimating the influence of these
improvement in future, and we will incorporate them in order to improve the
optimization further.
