  Cosmic microwave background (CMB) measurements are fundamentally limited by
photon statistics. Therefore, ground-based CMB observatories have been
increasing the number of detectors that are simultaneously observing the sky.
Thanks to the advent of monolithically fabricated transition edge sensor (TES)
arrays, the number of on-sky detectors has been increasing exponentially for
over a decade. The next-generation experiment CMB-S4 will increase this
detector count by more than an order of magnitude from the current
state-of-the-art to ~500,000. The readout of such a huge number of exquisitely
precise sub-Kelvin sensors is feasible using an existing technology:
frequency-domain multiplexing (fMux). To further optimize this system and
reduce complexity and cost, we have recently made significant advances
including the elimination of 4 K electronics, a massive decrease of parasitic
in-series impedances, and a significant increase in multiplexing factor.
