  Abreg: We investigate the impact of polarized foreground emission on the
performances of future CMB experiments in measuring the tensor-to-scalar ratio
r. We design a component separation pipeline, based on the Smica method, aimed
at estimating r and the foreground contamination from the data with no prior
assumption on the frequency dependence or spatial distribution of the
foregrounds. We derive error bars accounting for the uncertainty on foreground
contribution. We use the current knowledge of galactic and extra-galactic
foregrounds as implemented in the Planck Sky Model (PSM), to build simulations
of the sky emission for various experimental setups. Our method, permits us to
detect r = 0.1 at more than 3 sigma from B-modes only with Planck data, and r =
0.001 at 6 sigma for the most ambitious designs of the future EPIC probe. We
find that all-sky experiments permit a proper measurement of the reionization
bump despite the large scale foreground emission and are nearly insensitive to
contamination from point sources and lensing if their statistical contribution
can be modelled accurately. Investigating the observation of a small but clean
part of the sky, we show that diffuse foregrounds remain a concern for a
sensitive ground-based experiment with a limited frequency coverage when
measuring r < 0.1, but are dealt with efficiently by a deep field space mission
which is in return quite sensitive to lensing. Our results do not significantly
depend on the overall level and frequency dependence of the diffused foreground
model.
