  The electron cyclotron emission (ECE) in fusion devices is non-trivial to
model in detail at frequencies well below the fundamental resonance where the
plasma is optically thin. However, doing so is important for evaluating the
background for microwave diagnostics operating in this frequency range. Here we
present a general framework for estimating the ECE levels of fusion plasmas at
such frequencies using ensemble-averaging of rays traced through many
randomized wall reflections. This enables us to account for the overall vacuum
vessel geometry, self-consistently include cross-polarization, and quantify the
statistical uncertainty on the resulting ECE spectra. Applying this to ITER
conditions, we find simulated ECE levels that increase strongly with frequency
and plasma temperature in the considered range of 55-75 GHz. At frequencies
smaller than 70 GHz, we predict an X-mode ECE level below 100 eV in the ITER
baseline plasma scenario, but with corresponding intensities reaching keV
levels in the hotter hybrid plasma scenario. Benchmarking against the SPECE
raytracing code reveals good agreement under relevant conditions, and the
predicted strength of X-mode to O-mode conversion induced by wall reflections
is consistent with estimates from existing fusion devices. We discuss possible
implications of our findings for ITER microwave diagnostics such as ECE,
reflectometry, and collective Thomson scattering.
