CMS Pixel detector for the Phase I upgrade at HL-LHC

The CMS pixel detector is part of the complex tracking system of the CMS experiment at the LHC collider (CERN, Geneva CH). It has been designed for a stable operation and optimal performance at the LHC instantaneous luminosity L$_{max}$ =10$^{34}$ cm$^{-2}$s$^{-1}$. The future plans of the LHC collider envisage an increase of the luminosity up to 2.2 $\times$ L$_{max}$ with 7 TeV per proton beam, namely the high luminosity upgrade (HL-LHC) phase I. In order to maintain the high level of accuracy and efficiency of the tracking in this new challenging condition, a CMS pixel detector upgrade phase I program has been set up. The main goals of the upgrade activity are the material budget reduction in the tracking volume and the increase of the number of hits associated to a charged track. This paper gives an overview of the upgrade project, describes the planned R\&D activities and focuses on the expected improvements of the new CMS pixel detector system.