Optimal energy beamforming under per‐antenna power constraint.

Energy beamforming (EB) is a key technique to enhance the efficiency of wireless power transfer. In this paper, we study the optimal EB under per‐antenna power constraint (PAC), which is more practical than the conventional sum‐power constraint. We consider a multiantenna energy transmitter with PAC that broadcasts wireless energy to multiple energy receivers (ER)s. We consider sum‐energy maximization problem with PAC and provide the optimal solution structure for the general case. This optimal structure implies that sending one energy beam is optimal under PAC, which means that the rank of transmit covariance matrix is one similar to sum‐power constraint. We also derive closed‐form solutions for two special cases and propose two suboptimal solutions for general case, which performs very close to optimal beamforming. [ABSTRACT FROM AUTHOR]