  We investigate the main limitations which prevent the continuous-variable
quantum key distribution protocols from achieving long distances in the
finite-size setting. We propose a double-modulation protocol which allows using
each state for both channel estimation and key distribution. As opposed to the
standard method, we optimize the parameters of the protocol and consider
squeezed as well as coherent states as a signal. By optimally combining the
resources the key rate can approach the theoretical limit for long distances,
and one can obtain about ten times higher key rate using ten times shorter
block size than in the current state-of-the-art implementation.
