  The calculation of the radiative energy loss encountered by a fast charged
particle which undergoes multiple scattering is being investigated. A detailed
derivation of the Landau-Pomeranchuk-Migdal coherent effect in QED is given,
focusing on the specific feature of the Coulomb interaction. As a result the
radiation intensity per unit length in the coherent regime is shown to be
proportional to $\sqrt \omega$ for a photon energy $\omega$ times a logarithmic
enhancement which is determined exactly.
