  We present an optimization scheme for the focal plane of a multi-frequency
CMB B-modes experiment with a fixed number of detectors and apply it to the
specific case of LSPE experiment. Optimal focal planes are identified on the
ground of different figures of merit defined in terms of the forecasted
uncertainty $\sigma_r$ on the tensor--to-scalar ratio $r$ and the expected map
variance from foreground and instrumental noise residuals. We then perform a
forecast analysis in order to assess the precision achievable in B-modes
measurements.
