autocorrelation of multiple signals
You apply the same formula, but instead of using a scalar $x(n)$, you will have $x(n) \in \mathbb{C}^{M \times 1}$, where $M$ is the number of channels, and $x^{H}(n)$ is the $1 \times M$ matrix whose entries are the complex conjugate of the entries in $x(n)$. As a result $R \in \mathbb{C}^{M \times M}$ is an Hermitian matrix, and its eigenvalues are all real.