Upgrade plans and ageing studies for the CMS muon system in preparation of HL-LHC

The CMS muon system operates gas-based detectors. Upgrades of the detectors and trigger components are needed to cope with increasingly challenging conditions of the HL-LHC. New irradiation tests are performed to ensure that the muon detectors will survive the harsher conditions and operate reliably. The new CERN GIF++ (Gamma Irradiation Facility) allowed to perform aging tests. We present results in terms of system performance under large backgrounds and after accelerated aging tests. New detectors will be added to improve the performance in the critical forward region gas electron multiplier (GEM) detectors will be installed in LS2 in the region 1.6 $<$ $\mid\eta\mid$ $<$ 2.4, aiming at suppressing background triggers while maintaining high trigger efficiency. Further enhancements are foreseen with a second GEM station and with two stations of new generation RPCs, having low resistivity electrodes. These detectors will combine tracking and triggering capabilities and can stand particle rates up to few kHz/cm$^2$. In addition, to take advantage of the pixel tracking coverage extension, a new GEM detector covering up to $\mid\eta\mid$ = 2.8. is foreseen behind the new forward calorimeter.