Cosmic line background as a function of frequency. The solid red line, the dashed yellow line, and the long-dashed brown line, show the predictions of our simulation assuming the DL14 relation, the L18 relation, and the flat z >  4 SFRD (see Sect. 2.7), respectively. The CO, [CI], and continuum background predicted by SIDES are shown as a dot-dashed blue line, a two-dot-dashed dark blue line, and a solid gray line, respectively. The dot-dashed green line, the short-dash-long-dashed purple line, and the dotted blue line are the [CII] forecast from the Chung et al. (2020) model, the Yue & Ferrara (2019) model, and the Yang et al. (2022) model, respectively.