  Relativistic magnetized shocks are a natural source of coherent emission,
offering a plausible radiative mechanism for Fast Radio Bursts (FRBs). We
present first-principles 3D simulations that provide essential information for
the FRB models based on shocks: the emission efficiency, spectrum, and
polarization. The simulated shock propagates in an $e^\pm$ plasma with
magnetization $\sigma>1$. The measured fraction of shock energy converted to
coherent radiation is $\simeq 10^{-3} \, \sigma^{-1}$, and the energy-carrying
wavenumber of the wave spectrum is $\simeq 4 \,\omega_{\rm c}/c$, where
$\omega_{\rm c}$ is the upstream gyrofrequency. The ratio of the O-mode and
X-mode energy fluxes emitted by the shock is $\simeq 0.4\,\sigma^{-1}$. The
dominance of the X-mode at $\sigma\gg 1$ is particularly strong, approaching
100% in the spectral band around $2\,\omega_{\rm c}$. We also provide a
detailed description of the emission mechanism for both X- and O-modes.
