What is the spectral decomposition of $L^2(G(\mathbb{Q}) \backslash G(\mathbb{A}))$ for compact quotient?

Question

I'm trying to work out explicitly the spectral decomposition of $L^2(G(\mathbb{Q}) \backslash G(\mathbb{A}))$ when $G$ is anisotropic -- it has no split tori defined over $\mathbb{Q}$. This should mean that $L^2$ decomposes discretely into a direct sum of irreducible unitary representations. I'm looking at Arthur's paper "Eisenstein Series and the Trace Formula." How should the expansion simplify in this case?