Power allocation strategy for MIMO system based on beam-nulling

In this paper, we propose a scheme called “beam-nulling” using the same feedback bandwidth as beamforming but with higher capacity. In the beam-nulling scheme, the eigenvector of the weakest subchannel is fed back and then signals are sent over a generated subspace orthogonal to the weakest subchannel. Hence, the scheme can achieve high capacity. The capacities of water-filling, equal power, beamforming and beam-nulling are compared through theoretical analysis and numerical results. It is shown that at medium signal-to-noise ratio, beam-nulling approaches the optimal waterfilling scheme. Additionally, the existing beamforming and new proposed beam-nulling can be extended if more than one eigenvector is available at the transmitter. The new extended schemes are called multi-dimensional (MD) beamforming and MD beam-nulling. Theoretical analysis and numerical results in terms of capacity are also provided to evaluate the new extended schemes.