  The study of reionization history plays an important role in understanding
the evolution of our universe. It is commonly believed that the intergalactic
medium (IGM) in our universe are fully ionized today, however the reionizing
process remains to be mysterious. A simple instantaneous reionization process
is usually adopted in modern cosmology without direct observational evidence.
However, the history of ionization fraction, $x_e(z)$ will influence cosmic
microwave background (CMB) observables and constraints on optical depth $\tau$.
With the mocked future data sets based on featured reionization model, we find
the bias on $\tau$ introduced by instantaneous model can not be neglected. In
this paper, we study the cosmic reionization history in a model independent
way, the so called principle component analysis (PCA) method, and reconstruct
$x_e (z)$ at different redshift $z$ with the data sets of Planck, WMAP 9 years
temperature and polarization power spectra, combining with the baryon acoustic
oscillation (BAO) from galaxy survey and type Ia supernovae (SN) Union 2.1
sample respectively. The results show that reconstructed $x_e(z)$ is consistent
with instantaneous behavior, however, there exists slight deviation from this
behavior at some epoch. With PCA method, after abandoning the noisy modes, we
get stronger constraints, and the hints for featured $x_e(z)$ evolution could
become a little more obvious.
