  We review the measurements of dark energy enabled by observations of the Deep
Drilling Fields and the optimization of survey design for cosmological
measurements. This white paper is the result of efforts by the LSST DESC
Observing Strategy Task Force (OSTF), which represents the entire
collaboration, and aims to make recommendations on observing strategy for the
DDFs that will benefit all cosmological analyses with LSST. It is accompanied
by the DESC-WFD white paper (Lochner et al.). We argue for altering the nominal
deep drilling plan to have $>6$ month seasons, interweaving $gri$ and $zy$
observations every 3 days with 2, 4, 8, 25, 4 visits in $grizy$, respectively.
These recommendations are guided by metrics optimizing constraints on dark
energy and mitigation of systematic uncertainties, including specific
requirements on total number of visits after Y1 and Y10 for photometric
redshifts (photo-$z$) and weak lensing systematics. We specify the precise
locations for the previously-chosen LSST deep fields (ELAIS-S1, XMM-LSS, CDF-S,
and COSMOS) and recommend Akari Deep Field South as the planned fifth deep
field in order to synergize with Euclid and WFIRST. Our recommended DDF
strategy uses $6.2\%$ of the LSST survey time. We briefly discuss synergy with
white papers from other collaborations, as well as additional mini-surveys and
Target-of-Opportunity programs that lead to better measurements of dark energy.
