Observed frequency of CO and [CI]_609 μm transitions as a function of redshift. The boxes encircle both the frequency and redshift ranges covered by our ALMA band 3 observations. Boxes are color-coded by the cosmological volume probed by each transitions. Gray bands show the SPW coverage in the LSB and USB. The mean redshift of transitions entering the USB are also reported. The blue arrow points to the box representing the volume within ±1000 km s⁻¹ around the QSO CTS G18.01 systemic redshift.