  In this paper, we propose a joint beamforming algorithm for a multiuser
wireless information and power transfer (MU-WIPT) system that is compatible
with the conventional multiuser multiple input multiple output (MU-MIMO)
system. The proposed joint beamforming vectors are initialized using the well
established MU-MIMO zero-forcing beamforming (ZFBF) and are further updated to
maximize the total harvested energy of energy harvesting (EH) users and
guarantee the signal to interference plus noise ratio (SINR) constraints of the
co-scheduled information decoding (ID) users. When ID and EH users are
simultaneously served by joint beamforming vectors, the harvested energy can be
increased at the cost of an SINR loss for ID users. To characterize the SINR
loss, the target SINR ratio,u, is introduced as the target SINR (i.e., SINR
constraint) normalized by the received SINR achievable with ZFBF. Based on that
ratio, the sum rate and harvested energy obtained from the proposed algorithm
are analyzed under perfect/imperfect channel state information at the
transmitter (CSIT). Through simulations and numerical results, we validate the
derived analyses and demonstrate the EH and ID performance compared to both
state of the art and conventional schemes.
