  The next upgrade of the Large Hadron Collider (LHC) is planned from 2026 when
the collider will move to its High Luminosity phase (HL-LHC). The CMS detector
needs to be substantially upgraded during this period to exploit the fourfold
increase in luminosity provided by the HL-LHC. This upgrade is referred to as
the CMS Phase-2 Upgrade. A program of laboratory and beam test measurements,
and performance studies based on the detailed simulation of the detector was
carried out to support the decision of the technology of the sensors to be
adopted in the different regions of the detector for the Phase-2 Upgrade. Among
the various options considered, CMS chose to use 3D sensors with a 25 $\times$
100 $\mu$m$^2$ pixel cell in the innermost layer of the barrel and planar
sensors with a 25 $\times$ 100 $\mu$m$^2$ pixel cell elsewhere. In this paper,
we detail the simulation studies that were carried out to choose the best
sensor design. These studies include a detailed standalone simulation of the
sensors made with PixelAV and the expected performance on high level
observables obtained with the simulation and reconstruction software of the CMS
experiment.
