Improving degrees of freedom of wireless channels using superdirectivity

In this paper, we show the spatial degrees of freedom (DoF) in free space can be improved by using superdirectivity and such a gain is theoretically unlimited regardless of the physical size of the transmit and receive antenna arrays. Our conclusion is based on calculating the exact power consumption in the presence of mutual coupling and on calculating the exact spatial correlation of the noise field using the fluctuation dissipation theorem. As a result, high current excitation and high signal-to-noise ratio are available at the transmitter and the receiver, respectively. They were not believed to be possible in many previous works that did not consider such exact behaviors. We also analyze the effect of scattering in the near field region and discuss some solutions for many practical problems arising in implementing superdirectivity.