  During the LHC Run-I (2010-2013) and Run-II (2015-2018), the selection of
interesting events for muon physics in ALICE was performed with a dedicated
muon trigger system, based on 72 single-gap bakelite Resistive Plate Chambers
(RPCs), operated in maxi-avalanche mode (ADULT front-end electronics without
amplification and a threshold of 7 mV). From Run-III (starting in 2021) on, in
order to fully profit from the increased luminosity of Pb-Pb collisions, the
ALICE experiment will run in continuous readout (triggerless) mode and the muon
trigger system (MTR) will become a Muon Identifier (MID). The read-out
electronics is being upgraded in order to support continuous readout. Moreover,
in order to increase the RPC rate capability and to mitigate possible aging
effects, it was decided to operate the detectors with a lower gain, and to
replace the ADULT front-end cards with new cards (FEERIC), equipped with an
amplification stage. Also, an upgrade of the threshold distribution system to
the front-end will allow one to tune thresholds at the single front-end card
level, while this was previously only possible at the single-RPC level.
Finally, since some of the RPCs currently installed in ALICE have integrated a
non-negligible charge with respect to their certified life-time, about 25% of
the detectors will be replaced with new ones, built with a different type of
bakelite laminates. A detailed description of the MTR upgrade and of its
current status will be presented in this contribution.
