Low SNR regime capacity analysis of MIMO systems using directional antennas

In this paper, we evaluate the capacity of multiple-input multiple-output (MIMO) systems using directional antennas in non-isotropic scattering environments. Our analytical analysis approach results in closed-form approximate formulas in low signal to noise ratio (SNR) regime (in contrast to previous research works which have investigated the same problem by employing experimental or simulation frameworks). We propose an upper bound for the introduced approximate error. Our results show that MIMO systems equipped with directional antennas may have much higher capacity in non-isotropic scattering environments compared to MIMO systems equipped with omni-directional antennas. We also evaluate our results by some numerical examples in communication systems employing square microstrip patch multi-element antennas (MEA)s as well as horn MEAs.