Upgrades of the CMS muon system in preparation of HL-LHC

The present CMS muon system operates three different detector types in the barrel drift tubes (DT) and resistive plate chambers (RPC), along with cathode strip chambers (CSC) and another set of RPCs in the forward regions. In order to cope with increasingly challenging conditions various upgrades are planned to the trigger and muon systems. New detectors will be added to improve the performance in the critical forward region large-area triple-foil gas electron multiplier (GEM) detectors will already be installed in LS2 in the pseudo-rapidity region $1.6 < \eta < 2.4$, aiming at suppressing the rate of background triggers while maintaining high trigger efficiency for low transverse momentum muons. For the High Luminosity (HL)-LHC operations, the muon forward region should be enhanced with another large area GEM based station, called GE2/1, and with two new generation RPC stations, called RE3/1 and RE4/1, having low resistivity electrodes. These detectors will combine tracking and triggering capabilities and can stand particle rates up to few kHz/$\mathrm{cm^{2}}$. In addition to take advantage of the pixel tracking coverage extension, a new detector, ME0 station, will be installed behind the new forward calorimeter, covering up to $\eta$ = 2.8.