Studies on the capacity of MIMO systems in mobile-to-mobile environment

The use of multiple input and multiple output (MIMO) antennas is a promising technique for increasing the capacity of wireless communication systems. Huge capacities are guaranteed only when channels between different transmitting and receiving antenna pairs are statistically independent. In this paper, we investigate the capacities of MIMO systems in a mobile-to-mobile environment by employing a "two ring" model which is simple extension of previous "one ring" model. With this model, we investigate the spatial correlation and information theoretic channel capacity of MIMO systems that use omni-directional antennas as well as MIMO systems with directional antennas. We show that MIMO systems with directional antennas yield good spatial decorrelation between antenna pairs even in a scarce scattering environment, while MIMO systems with omni-directional antennas suffer capacity degradation either when antennas are located closely each other, or when there is not enough multipath richness. Directional antennas guarantee the independence between different antenna pairs by not sharing scatterers, and the capacity is maximized by exploiting the independence between antenna pairs.