  At millimeter-wave frequencies, diffuse scattering from rough surfaces is an
important propagation mechanism. Including this mechanism in radio propagation
modeling tools,such as ray-tracing, is a key step towards realizing accurate
propagation models for 5G and beyond systems. We propose a two-stage solution
to this problem. First, we model reflection and transmission through rough
slabs, such as doors, walls and windows with the FDTD method. Our results
indicate the influence of roughness and whether this influence is measurable,
either reducing the magnitude of the reflected and transmitted waves, or (most
importantly) generating diffuse scattering components. In the latter case, the
surface effectively acts as a secondary source, whose pattern is computed by
full-wave analysis. Then, this pattern is embedded in a ray-tracer, enabling
the computation and tracing of diffuse scattering field components. We
demonstrate this approach in the ray-tracing analysis of a 28 GHz indoor
environment.
