  Quantum cascade laser frequency combs are nowadays well-appreciated sources
for infrared spectroscopy. Here their applicability for free-space optical
communication is demonstrated. The spontaneously-generated intermodal beat note
of the frequency comb is used as carrier for transferring the analog signal via
frequency modulation. Exploiting the atmospheric transparency window at 4
$\mu$m, an optical communication with a signal-to-noise ratio up to 65 dB is
realized, with a modulation bandwidth of 300 kHz. The system tolerates a
maximum optical attenuation exceeding 35 dB. The possibility of parallel
transmission of an independent digital signal via amplitude modulation at 5
MS/s is also demonstrated.
