A study of the capacity for different element spacing on compact MIMO platforms

The multiple-input multiple output (MIMO) technique is an enabling technology for meeting the capacity demands in future wireless communication systems. For military tactical communication use, one of the interesting frequency bands is around 300 MHz. The frequency is low enough to provide good coverage, e.g., in urban peer-to-peer scenarios where line-of-sight (LOS) rarely exist. However, the size of the MIMO antennas arrays increases with the wavelength. At 300 MHz, an element separation of about 0.5 m is often used. For larger platforms, such as vehicles, arrays with quite many elements are feasible. However, on small platforms (handheld devices being one extreme), the number of possible elements can be very restricted. In this work, the impact of reducing the antenna element separation is studied in terms of reduced capacity for a circular antenna array in a 4times4 (four transmit antennas and four receive antennas) and a 2times2 MIMO system. This is investigated by using a double-directional channel description derived from measurements in a suburban area. It is shown that the degradation is graceful. For example, for a 2times2 system, the penalty of using a radius of 0.125 m is only about 10% compared to the capacity for a radius of 0.25 m. This opens up for MIMO antenna configurations with spatially separated antenna elements on very compact platforms. For these results, it is assumed that the mutual coupling between the antenna elements is taken care of by other techniques and is therefore not treated in this work.