  We present simulations of the 21-cm signal during the epoch of reionization.
We focus on properly modeling the absorption regime in the presence of
inhomogeneous Wouthuysen-Field effect and X-ray heating. We ran radiative
transfer simulations for three bands in the source spectrum (Lyman, UV, and
X-ray) to fully account for these processes. We find that the brightness
temperature fluctuation of the 21 cm signal has an amplitude greater than 100
mK during the early reionization, up to 10 times greater than the typical
amplitude of a few 10 mK obtained during the later emission phase. More
importantly, we find that even a rather high contribution from QSO-like sources
only damps the absorption regime without erasing it. Heating the IGM with X-ray
takes time. Our results show that observations of the early reionization will
probably benefit from a higher signal-to-noise value than during later stages.
After analyzing the statistical properties of the signal (power spectrum and
PDF) we find three diagnostics to constrain the level of X-ray, hence the
nature of the first sources.
