  We study the redshift evolution of the quasar UV Luminosity Function (LF) for
0.5 < z < 6.5, by collecting the most up to date observational data and, in
particular, the recently discovered population of faint AGNs. We fit the QSO LF
using either a double power-law or a Schechter function, finding that both
forms provide good fits to the data. We derive empirical relations for the LF
parameters as a function of redshift and, based on these results, predict the
quasar UV LF at z=8. From the inferred LF evolution, we compute the redshift
evolution of the QSO/AGN comoving ionizing emissivity and hydrogen
photoionization rate. If faint AGNs are included, the contribution of quasars
to reionization increases substantially. However, their level of contribution
critically depends on the detailed shape of the QSO LF, which can be
constrained by efficient searches of high-z quasars. To this aim, we predict
the expected (i) number of z>6 quasars detectable by ongoing and future NIR
surveys (as EUCLID and WFIRST), and (ii) number counts for a single
radio-recombination line observation with SKA-MID (FoV = 0.49 deg^2) as a
function of the Hnalpha flux density, at 0<z<8. These surveys (even at z<6)
will be fundamental to better constrain the role of quasars as reionization
sources.
