Capacity of diagonally correlated MIMO channels

The paper investigates the case of so-called diagonally correlated MIMO channels. This particular type of correlated channel, which cannot result from the popular Kronecker assumption, is shown to provide higher ergodic capacity than uncorrelated fading channels. Interestingly, such channels present a specific symmetry of the channel matrix element amplitudes: their rows are either cyclically right- or left-shifted versions of each other, so that the absolute value of the channel matrix is circulant. The proposed analysis details exemplary physical scenarios leading to high diagonal correlations, first for the simple case of 2×2 channels, and then for arbitrary array size, n. An analytical expression for capacity is also derived. It demonstrates that the ergodic capacity of diagonally correlated channels grows exactly linearly with the number of antennas.