  In this paper we report a continuous-variable quantum key distribution
protocol using multimode coherent states generated on subcarrier frequencies of
the optical spectrum. To detect the quadrature components of bosonic field we
propose a coherent detection scheme where power from a carrier wave is used as
a local oscillator. We compose a mathematical model of the proposed scheme and
perform its security analysis in the finite-size regime using fully quantum
asymptotic equipartition property technique. We calculate a lower bound on the
secret key rate for the system under the assumption that the quantum channel
noise is negligible compared to detector dark counts, and an eavesdropper is
restricted to collective attacks. Our calculation shows that the current
realistic system implementation would allow distributing secret keys over
channels with losses up to 9 dB.
