pr.probability – Maximum Mutual Information of A Matrix Representation

Let $mathbf{Z}$ be a $mtimes n$ matrix with zero-mean unit-variance i.i.d complex Gaussian entries. What is the maximum value of mutual information $I(mathbf{X}_1,mathbf{X}_2;mathbf{Y})$ such that
begin{align}
mathbf{Y}=mathbf{X}_1^mathrm{H}mathbf{X}_2+mathbf{X}_1^mathrm{H}mathbf{Z},
end{align}

where $mathbf{X}_1$ and $mathbf{X}_2$ are two $mtimes n$ arbitrary random matrices and $(.)^mathrm{H}$ denotes hermitian of matrix.