Capacity optimizing power loading scheme for spatially constrained antenna arrays: Channels with narrow angular spread

This paper proposes a capacity optimizing power loading scheme for spatially constrained antenna arrays. The proposed power loading scheme is designed mainly for MIMO systems that are operating over channels with narrow angular spread values. The design problem is approached from a physical wave field perspective, in particular using a modal expansion for free space wave propagation. When a signal is transmitted from a spatially constrained antenna array, there will be an infinite number of modes excited at the transmit antenna region and only a finite number of those excited modes will have sufficient power to carry information to the other end of the channel. We show that in a non-isotropic scattering environment, correlation between adjacent modes is significantly higher than the correlation between well separated mode orders. Based on this finding, the power loading scheme is designed to eliminate the correlation between adjacent modes by allocating zero power to every second effective mode at the transmitter region. The capacity performance of the proposed scheme is evaluated for 1-D and 2-D antenna array geometries and results are compared with the traditional equal power loading scheme. We show that the proposed scheme gives more scope for capacity improvements for 1-D arrays at low angular spread values than for 2-D arrays.