  The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will
image billions of astronomical objects in the wide-fast-deep primary survey and
in a set of minisurveys including intensive observations of a group of deep
drilling fields (DDFs). The DDFs are a critical piece of three key aspects of
the LSST Dark Energy Science Collaboration (DESC) cosmological measurements:
they provide a required calibration for photometric redshifts and weak
gravitational lensing measurements and they directly contribute to cosmological
constraints from the most distant type Ia supernovae. We present a set of
cohesive DDF strategies fulfilling science requirements relevant to DESC and
following the guidelines of the Survey Cadence Optimization Committee. We
propose a method to estimate the observing strategy parameters and we perform
simulations of the corresponding surveys. We define a set of metrics for each
of the science case to assess the performance of the proposed observing
strategies. We show that the most promising results are achieved with deep
rolling surveys characterized by two sets of fields: ultradeep fields (z<1.1)
observed at a high cadence with a large number of visits over a limited number
of seasons; deep fields (z<0.7), observed with a cadence of ~3 nights for ten
years. These encouraging results should be confirmed with realistic simulations
using the LSST scheduler. A DDF budget of ~8.5% is required to design observing
strategies satisfying all the cosmological requirements. A lower DDF budget
lead to surveys that either do not fulfill photo-z/WL requirements or are not
optimal for SNe Ia cosmology.
