This chapter reports what teams learned after about twenty years of trying to see the faint radio glow from the early Universe. The target is the redshifted 21‑centimeter line from neutral hydrogen, a signal that carries information about the Cosmic Dawn (CD) and the Epoch of Reionization (EoR). That signal is much fainter than the radio emission from our Galaxy and other sources, so separating it from bright foregrounds and from instrumental effects is the central difficulty. The work summarized comes from several precursor telescopes, including the upgraded Giant Metrewave Radio Telescope (uGMRT), the Hydrogen Epoch of Reionization Array (HERA), the Low Frequency Array (LOFAR) and the Murchison Widefield Array (MWA).

To test whether analysis methods can reliably recover the tiny 21‑cm signal, groups have built end‑to‑end simulation pipelines. In these tests, researchers start with a simulated 21‑cm signal and a model of the radio sky. They pass that combined signal through a model of the telescope and then run the same data analysis they use on real observations. The goal is to check whether the pipeline recovers an unbiased estimate of the input 21‑cm signal. This approach is different from simply injecting simulated signals into real data. End‑to‑end tests let teams control noise and systematics and run many realizations for robust checks.

A key technical element is the visibility simulator. These programs forward‑model the sky through the instrument using the radio interferometric measurement equation, the formal description of how a radio array converts sky brightness into measured data. High‑fidelity simulators such as DP3, WODEN and pyuvsim aim for very accurate results but are computationally expensive. Other codes like matvis, RIMEz, fftvis and BayesLIM use approximations and hardware acceleration to speed up calculations. The trade‑off is clear: more realism costs more computing time, so teams choose tools depending on the telescope and the test they want to run.